Development of a Process Simulation Model for the Analysis of the Loading and Unloading System of a CNG Carrier Equipped with Novel Lightweight Pressure Cylinders

applied sciences

Article Development of a Process Simulation Model for the Analysis of the Loading and Unloading System of a CNG Carrier Equipped with Novel Lightweight Pressure Cylinders

Rodolfo Taccani 1,*, Gabriele Maggiore 2 and Diego Micheli 1 1 Department of Engineering and Architecture, University of Trieste, 34127 Trieste, Italy; [email protected] 2 Cenergy srl, c/o Area Science Park, Basovizza, 34149 Trieste, Italy; [email protected] * Correspondence: [email protected]

Received: 31 August 2020; Accepted: 23 October 2020; Published: 27 October 2020

Abstract: Natural gas is becoming increasingly important to meet the growing demand for energy, guaranteeing a reduction in polluting emissions. Transportation in form of Compressed Natural Gas (CNG) could be an alternative to the traditional transportation by pipeline or, as liquefied gas, by ships, but the ratio between the mass of transported gas and the container weight is currently too low. One of the many projects focusing on the development of innovative lightweight pressure cylinders is GASVESSEL, which proposes composite cylinders with a diameter of more than 3 m: loaded on a ship, they could allow transporting quantities of CNG as big as 10,000 tons. The related loading and unloading processes affect both the overall time required for transport and the quantity of transported gas; therefore, they have an impact on the economic feasibility of the whole project. In this paper, a newly developed process simulation model is presented that allows assessing the duration of the loading and unloading processes, the mass of transported CNG, and the amount of power and energy required by the process. The model is useful to support the design of the system considering different plant components and operating strategies. It is applied to the analysis of the loading and unloading of a ship that meets the GASVESSEL project specifications. The results show that the duration of the process is of the order of magnitude of 100 h, depending on ambient temperature, and that the energy consumption can vary in the range of 150–180 kJ for a unit mass of CNG. Finally, the model is used to simulate the same process with hydrogen, an energy carrier that allows meeting, together with the use of fuel cells, the requirements of zero local emissions. The results show increments of both the final loading temperature and compressor power with respect to the CNG case.

Keywords: CNG transportation; CNG carrier; CNG loading model

1. Introduction Natural Gas (NG) has a central role, in the global energy scene, in the shift to an economy with lower pollutant emissions. It is currently in second place, behind oil and together with coal, when it comes to global energy consumption, with a percentage close to 24% [1]. Globally, it has seen from 2010 the second highest demand growth rate, after renewable sources. According to [1], in 2040, NG should exceed coal by about six percentage points, in order to satisfy global energy needs. The traditional systems for the supplying of NG are mainly two: the transportation of Liqueﬁed Natural Gas (LNG) by ships and the conveyance into pipelines. An alternative system is the transportation using pressure cylinders. This method is often referred to as Compressed Natural Gas (CNG). The advantages of CNG, with respect, in particular, to LNG, can be summarized as follows:

Appl. Sci. 2020, 10, 7555; doi:10.3390/app10217555 www.mdpi.com/journal/applsci Appl. Sci. 2020, 10, 7555 2 of 23

The exploitation of stranded gas fields or flare and associated gas can be economically viable [2,3]; • It is an economic way of transporting small quantities of gas and exploiting small reserves [3]; • It can be an economic way of transporting ng over short distances, between 800 and 3000 km, • or 250–5000 km, making also use of cooling [2]; The containment of costs due to the lack of liquefaction and regasification plants; • The return time of the investment can be shorter; • The largest investment (85%) for cng concerns the ship, which is a mobile value that can be used • in other places, while for an lng project, 60% of the cost falls within liquefaction and regasification plants [4], which cannot be used in other places; CNG does not have the problem of the loss of load due to the boil off. • CNG technology consists of compressing the gas at high pressure, from 200 up to 300 bar. The volume reduction is 200–300 times compared to those in standard conditions [2,3]. However, the density of CNG is at most equal to about half of that of the LNG (218 kg/m3 for methane at 3 300 bar and 20 ◦C versus 415 kg/m for LNG at 1 bar and 162 ◦C), but the energy consumption of the compression process is about half of that required by liquefaction and regasification. Once compressed, the gas is stored in tanks placed on the ship and transported near a gas pipeline, where it is discharged at high pressure. Therefore, the most important phases of this transport technology are the following:

Gas treatment; • Compression and possible cooling; • Loading and transport with ships; • Reception and unloading by decompression. • The safety of this technology can be a problem, but since the discharge can take place offshore, some kilometers away from the coast, the risks remain confined and do not concern entire populated areas, as is the case for the LNG technology regasification terminals. In any case, the risk related to the storage of a flammable substance at high pressure was one of the problems that in the past did not allow an important development of this technique. Technological development with regard to storage tanks has allowed this transport system to become more efficient and safer. In particular, the introduction of composite materials has brought greater safety and, at the same time, a considerable weight saving [2]. The first CNG transport project using ships dates back to the 1960s and involved the use of a series of vertical cylinders [2]. This project was abandoned due to the very high cost of pressure vessels. Subsequently, the renewed interest in the technology has led to the development of some alternatives. In the last decade of the last century, Cran and Stenning designed a new type of pressure tank called Coselle [2,3] (from the English “coil”, winding, and “carousel”, bobbin). Later, new technologies emerged such as the VolumeOptimized TRANsport and Storage (VOTRANS) [2,3] of EnerSea Transport, the Pressurized Natural Gas (PNG) [2] of Knutsen OAS, the Gas Transportation Module (GTM) [2,3] of TransCanada, the Composite Reinforced Pressure Vessel (CRPV) [2] of Trans Ocean Gas (TOG), and the Compressed Energy Technology (CETech) [3]. Tables1a and1b, adapted from [ 3], with the only addition in the last column of Table1b of the GASVESSEL project [ 4], shows a summary of the various technologies. As far as the ratio between the gas transported and the weight of the containers is concerned, only the weight of the tanks without valves, pipes, and flanges is considered. The first and only CNG ship currently in service, the “Jayanti Baruna”, was launched in January 2016 in Indonesia and has a capacity of 0.67 MNm3 @250 bar of gas, allowing linking East Java with the island of Lombok. The gas is stored in an 832 vertical cylinder (height 12 m, diameter 615 mm, thickness 19 mm), and the ratio container/cargo weight is 0.15 [4]. Therefore, the currently too low ratio between the mass of transported gas and the vessels–cargo weight can be considered one of the reasons why CNG technology currently has such a limited diffusion. Appl. Sci. 2020, 10, 7555 3 of 23

Table 1. CNG vessel technologies, part a,b.

(a) GEV Coselle GEV Optimum VOTRANS Knutsen PNG coiled X-80 steel pipe API 5LX80 steel X-80 steel cargo containment X-80 steel cylinders forming a carousel pipes tank cylinders vertical tanks modules or arrangement coiled pipe in holds horizontal in holds vertically stacked horizontal pipes gas press. (bar) 200–275 250 125 250 gas temp. ( C) ambient ambient 30 ambient ◦ − cargo/container 0.12–0.18 0.4 0.35–0.39 0.21 weight ratio advanced for 5.5 development stage advanced concept advanced concept MNm3 capacity (MNm3) 1.5–15 5.5–12.5 2–30 2–35 (b) TOG TransCanada CeTech GASVESSEL composite HDPE and composite composite or X-80 steel composite containment ﬁberglass cylinders reinforced steel pipes reinforced steel (MEGC) GTMs container or modular GTMs stacked arrangement cassettes with vertical vertical/horizontal vertical (cylinders) cylinders gas press. (bar) 250 206 150–250 300 gas temp. ( C) ambient/ 30 ambient ambient/ 30 ambient ◦ − − cargo/container 0.7 (composite) 0.34/0.47 1.5 1.5 weight ratio 0.24 (X-80 steel) development stage concept concept concept advanced concept 7–15 (composite) capacity (MNm3) 7 0.3–3 12 8–35 (steel)

However, the problems relating to loading and unloading processes must also be considered. These processes affect both the overall time required for transport and the quantity of transported gas and, therefore, they have an impact on the economic feasibility of the whole project. Based on these considerations, in this paper, it is first shortly described the GASVESSEL project, which focuses on the development of innovative lightweight composite pressure cylinders with a diameter of more than 3 m: loaded on a ship, they could allow transporting quantities of CNG as big as 10,000 tons [4]. Then, a newly developed process simulation model is presented and realized in the Matlab® environment, which allows assessing the duration of the loading and unloading processes, the mass of transported CNG, and the amount of power and energy required by the process. The model is useful to support the design of the system considering different plant components and operating strategies. Almost all the calculation models of the filling and emptying processes of CNG tanks found in the literature refer to the use of CNG to power road vehicles with spark ignition engines, replacing petrol. In fact, today, this is the most widespread use of CNG (the share is about 1.6% in total vehicle feet worldwide [5]), while the use as a supplying system of NG on ships is currently under development. The gas is taken from the low-pressure distribution network, and then, it is compressed in the storage tanks of the filling station. Then, the vehicle’s cylinder is filled by the pressure difference. These models are based on the first law of thermodynamics, appropriate assumptions about components efficiency, and on the choice of a suitable Equation of State (EOS) for the calculation of the thermodynamic properties of NG, which are treated as a real gas. The main problems are related to the so-called Fast Filling Process (FFP). They concern the reduction of the filling time without an excessive reduction of the fill efficiency. The fill efficiency is the ratio of the actual filled mass to the capacity at the rated pressure and ambient temperature. The underfilling is due to the heat generated by the re-compression of CNG in the cylinder. Another problem is the required compressor input work, Appl. Sci. 2020, 10, 7555 4 of 23 which is then partially wasted during FFP. In turn, the filling time, underfilling, and compression work also depend on the gas composition, ambient temperature, storage type, and compressor performance. The first FFP model was presented by Kountz. In [6], with reference to a CNG storage cylinder, he found a rise of 40 ◦C of the storage gas temperature. It is also shown by Kountz that the fast fill of CNG cylinders, at 20 ◦C ambient temperature, typically achieves a fill efficiency of 80%. In [7], two methods are compared for storing fuel in CNG stations, namely buffer storage and three pressure levels cascade storage systems. The authors found that the filling time is lower and the charged mass is higher with buffer storage, whilst the compressor work is lower with cascade storage. In [8], the authors studied an FFP based on a three-stage compressor and both buffer and cascade configurations, which are referred to as a single storage system (SSS) and cascade storage system (CSS). The optimum amounts of inter-stage pressure have been found for every configuration using the particle swarm optimization (PSO) algorithm. They too found that SSS allows a low refueling time and high filling ratio, while CSS allows reducing the energy consumption. The influence of NG composition is investigated in [9] using the AGA8 EOS to calculate the thermodynamic properties of NG with methane percentage in the field 87–98.6%. They found that the composition has a small effect on the filling time but significant effects on the charged mass, with variations around 20%. In another work [10], the same authors, using a detailed model of a reciprocating compressor with automatic valves, found that natural gas composition, with methane percentage varying from 79% to 98%, has significant effects on compressor performance. The consuming work per cycle and the mean discharge temperature increases with methane percentage. In [11], the authors investigate the FFP of NG in Type-III cylinders, which have a metal liner and a full-wrapped composite reinforcement, for heavy-duty vehicles with and without active heat removal in order to improve the fill efficiency. The active heat removal system involves placing cooling coils inside the cylinder through which a coolant is circulated. The passive heat removal system involves pre-chilling thermal heat sinks in the cylinder. In [5], the results are presented of the operational and safety tests of a CNG home fast refueling station based on a one-stage hydraulic compressor. A problem somewhat closer to that covered by the present paper is addressed in [12]. A mathematical model is developed to simulate the filling process of high-pressure tankers for marginal gas wells. Then, the model is validated against experimental data obtained from a marginal gas well. In this paper, the newly developed model is applied to the analysis of the loading and unloading of a ship that meets the GASVESSEL project specifications. Finally, the model is used to simulate the same process with hydrogen, which is an energy carrier that allows meeting, together with the use of fuel cells, the requirements of zero local emissions.

2. The GASVESSEL Project The four-year project GASVESSEL [4], funded by the EU H2020 research and innovation programme, started in June 2017 and was proposed by a consortium formed by thirteen partner companies from eight countries (Belgium, Cyprus, Germany, Greece, Italy, Norway, Slovenia, and Ukraine). The main design data of the pressure vessel and of the ship considered in the project are reported in Table2. The pressure vessels arrangement in the CNG carrier ship is vertical. To reduce ﬁre hazard, the holds are saturated with nitrogen. The pressure vessels are made of austenitic stainless steel liner (5 mm) externally reinforced with a composite made of carbon ﬁber and impregnated with epoxy resin (35 mm). Appl. Sci. 2020, 10, 7555 5 of 23

Table 2. GASVESSEL project [4]: CNG vessel and ship design main speciﬁcations.

Pressure Vessel Main Speciﬁcations: gas cylinder outer diameter 3.4 m gas cylinder length 22.5 m volume (water) 175 m3 vessel mass capacity, referred to CH4 @300 bar, 20 ◦C 38 ton design pressure 300 bar CNG ship design: length, overall 205 m max breadth 36 m height to main deck 22.0 m design draft 7.5 m n◦ vessels in the CNG ship 272 total vessels volume (water) 44800 m3 total gas mass @ 300 bar 20 ◦C (*) 9771 ton total gas volume 13.762 MNm3 cargo gas/container weight ratio 1.5 * referred to pure methane.

3. Plant Arrangements and the Loading and Unloading Processes The schematics of the kind of CNG plant considered in this study are shown in Figure1, in both the loading (a), and unloading (b), configurations. The compressor, with its control valves and CNG cooler, is installed on board the ship. The two valves mounted before and after the compressor introduce a variable pressure drop, allowing the machine to operate in non-flooding conditions. The cooler always presents after the compressor to maximize the compressed gas density. During the loading phase, the suction is connected to the storage terminal and delivers to the pressure vessels. During the unloading phase, the suction is connected to the pressure vessels, with the interposition of a heater, while the delivery is connected to the pipeline. The heater is used only in the unloading configuration, because the temperature of the pressure vessels can reach process values as low as 60 C, due to the − ◦ expansion of the gas contained in it. In both configurations, a by-pass allows the compressor to be excluded from the circuit when the flow occurs spontaneously due to the pressure difference. In fact, the loading and unloading processes are divided into two subsequent free and forced phases. During the free phases, the gas flow is due to the pressure difference between the pressure vessels and the storage terminal or the pipeline, respectively. During the forced phases, the compressor provides the pressure vessels with complete filling in the loading process and the most complete as possible emptying in the unloading one. By acting on the lamination valve placed on the intake, the compressor can operate even when the pressure of the upstream tanks is higher than that of the downstream pipeline. For example, this is necessary when the flow in the free discharge phase falls below 20 kg/s; therefore, it is more convenient to operate the compressors to speed up the process. Since the required volume flow rates and compression ratios are not excessively low or high, they can be obtained with both centrifugal and volumetric compressors. In a centrifugal compressor, the maximum flow is limited by choking, the minimum flow is limited by surge, and the compression ratio is a function, other than of stages number and geometry, of gas molecular weight and compressibility. In a volumetric compressor with automatic valves, the flow rate is proportional to the speed of rotation, while the stage compression ratio is independent from it and less sensitive, with respect to the centrifugal ones, to changes of the composition of the fluid. On the other hand, the composition of NG depends on the field of origin, but the variation of the mean molecular weight, density, and other physical properties is not so large as to prefer the use of volumetric compressors for this reason. Appl. Sci. 2020, 10, x FOR PEER REVIEW 6 of 24

°C, due to the expansion of the gas contained in it. In both configurations, a by-pass allows the compressor to be excluded from the circuit when the flow occurs spontaneously due to the pressure difference. In fact, the loading and unloading processes are divided into two subsequent free and forced phases. During the free phases, the gas flow is due to the pressure difference between the pressure vessels and the storage terminal or the pipeline, respectively. During the forced phases, the compressor provides the pressure vessels with complete filling in the loading process and the most complete as possible emptying in the unloading one. By acting on the lamination valve placed on the intake, the compressor can operate even when the pressure of the upstream tanks is higher than that of the downstream pipeline. For example, this is necessary when the flow in the free discharge phase

Appl.falls Sci.below2020 , 1020, 7555kg/s; therefore, it is more convenient to operate the compressors to speed up6 ofthe 23 process.

(a) loading plant

(b) unloading plant

Figure 1. Schematic of the loading (a), and unloading (b), CNG plants.

Reciprocating compressors have very high adiabatic efficiency, but as compression ratio drops, Since the required volume flow rates and compression ratios are not excessively low or high, adiabatic efficiency drops. However, with NG, the drop is limited, and adiabatic efficiency can be they can be obtained with both centrifugal and volumetric compressors. 93% at a compression ratio of 3 and 91% at a compression ratio of 1.4 [13]. In centrifugal compressors, In a centrifugal compressor, the maximum flow is limited by choking, the minimum flow is the polytropic efficiencies range from 70% to 85%, and efficiencies approaching 90% are possible. limited by surge, and the compression ratio is a function, other than of stages number and geometry, The corresponding values of adiabatic efficiency depend on the compression ratio, but they will be in of gas molecular weight and compressibility. In a volumetric compressor with automatic valves, the general lower than those of reciprocating compressors. flow rate is proportional to the speed of rotation, while the stage compression ratio is independent In terms of costs, generally, a reciprocating compressor will have a lower CAPital EXpenditure from it and less sensitive, with respect to the centrifugal ones, to changes of the composition of the (CAPEX), since centrifugal ones use complex geometry parts, but they will have a higher Operating fluid. On the other hand, the composition of NG depends on the field of origin, but the variation of EXpenditure (OPEX), because a centrifugal compressor has fewer wearing parts, resulting in lower the mean molecular weight, density, and other physical properties is not so large as to prefer the use operating costs in terms of replacement parts, repairs, and downtime (high reliability). Moreover, of volumetric compressors for this reason. with centrifugal compressors, there are no vibrations. Reciprocating compressors have very high adiabatic efficiency, but as compression ratio drops, In conclusion, at the end of this analysis, it was decided to evaluate the adoption of centrifugal adiabatic efficiency drops. However, with NG, the drop is limited, and adiabatic efficiency can be compressors, considering the lower operating costs and the higher reliability as main arguments. 93% at a compression ratio of 3 and 91% at a compression ratio of 1.4 [13]. In centrifugal compressors, 4.the Model polytropic Equations efficiencies range from 70% to 85%, and efficiencies approaching 90% are possible. The corresponding values of adiabatic efficiency depend on the compression ratio, but they will be in generalThe lower equations than ofthose the of model reciprocating describe thecompressors. behaviors of all the components of the systems reported in Figure1, i.e.,

Vertical cylindrical tanks; • Regulation valves; Cooler/heater; • Centrifugal compressors; • Ducts. • NG cannot be treated as an ideal gas under the thermodynamic conditions considered here. Various approaches are described in the literature for the calculation of its properties as real gas, with particular reference to the ﬁlling of small tanks for automotive applications [8,13]. In this work, the gas properties in all the components of the loading and unloading plant are obtained from the National Institute of Standards and Technolgy (NIST) database by using the open source software tool Coolprop, which allows considering both pure components, as CH4, and various NG mixtures, Appl. Sci. 2020, 10, x FOR PEER REVIEW 7 of 24

In terms of costs, generally, a reciprocating compressor will have a lower CAPital EXpenditure (CAPEX), since centrifugal ones use complex geometry parts, but they will have a higher Operating EXpenditure (OPEX), because a centrifugal compressor has fewer wearing parts, resulting in lower operating costs in terms of replacement parts, repairs, and downtime (high reliability). Moreover, with centrifugal compressors, there are no vibrations. In conclusion, at the end of this analysis, it was decided to evaluate the adoption of centrifugal compressors, considering the lower operating costs and the higher reliability as main arguments.

4. Model Equations The equations of the model describe the behaviors of all the components of the systems reported in Figure 1, i.e., • Vertical cylindrical tanks; • Regulation valves; Cooler/heater; • Centrifugal compressors; • Ducts. NG cannot be treated as an ideal gas under the thermodynamic conditions considered here. Various approaches are described in the literature for the calculation of its properties as real gas, with particular reference to the filling of small tanks for automotive applications [8,13]. In this work, the gas properties in all the components of the loading and unloading plant are obtained from the Appl. Sci. 2020, 10, 7555 7 of 23 National Institute of Standards and Technolgy (NIST) database by using the open source software tool Coolprop, which allows considering both pure components, as CH4, and various NG mixtures, asas those those found found in in [14 [14].]. For For CH CH4 calculations,4 calculations, the the considered considered EOS EOS is is described described in in [15 [15],], while while in in the the case case ofof mixtures, mixtures, the the calculations calculations employ employ a model a model that that applies applies mixing mixing rules rules to the to Helmholtzthe Helmholtz energy energy of the of components.the components. The departure The departure function function from from ideal ideal mixing mixing is consistent is consistent with thewith GERG the GERG model model [16]. [16].

4.1.4.1. The The Tank Tank Model Model TheThe time-dependent time-dependent variables variables of of the the tank tank model model are ar (seee (see Figure Figure2, where 2, where the tankthe tank is referred is referred to as to a Receiveras a Receiver Vessel): Vessel):

• GasGas pressure, pressure,pr ;pr; • • GasGas temperature, temperature,T rT; r; • • MassMass of of gas, gas,m rm; r; • • w CylinderCylinder wall wall temperature, temperature,T wT. . •

Figure 2. Scheme representing the variables of the receiving tank. Figure 2. Scheme representing the variables of the receiving tank. Some hypotheses have been made to simplify the equations characterizing this element: The potential energy linked to any difference in altitude between the storage terminal and the • tank is neglected; The kinetic energy of the gas inside the tank is neglected; • The pressure and temperature of the gas inside the tank does not depend on the spatial coordinates • (perfect mixing hypothesis); The mass flow rate is calculated, in the case of free loading/unloading, as an isenthalpic expansion • through an orifice while, if the compressors is used, it is taken from the machine characteristic curve, once the compression ratio is known; The heat exchange from the control volume of the tank to the wall is described by a convective heat • flow toward the wall, which is considered a concentrated mass: this implies that the cylinder wall temperature does not depend on the spatial coordinates but assumes a uniform average value. The equations needed to calculate the values of the tank model variables are: Conservation of mass: •

dm . r = m (1) dt i Energy balance: •

2 ! d(mr ur) . vi dQr · = mi hi + + (2) dt 2 i dt Appl. Sci. 2020, 10, 7555 8 of 23

Mass ﬂow rate: • (a) Free loading cases: v u  +  tu ! 2 ! k 1 k .  p k p k  p k 1 k  r r  r 2 − mi = Cd A 2p ρ i   if > (3) · · i· k 1 p − p  p k + 1 − i i i v t  k+1  k .  2 k 1  p 2 k 1 = ρ  −  r − mi Cd A k pi i  if (4) · · · · k + 1 pi ≤ k + 1

(b) Forced loading cases: centrifugal compressor curves (see Section 4.4)

Cylinder wall temperature: • ! dT 1 dQ dQ w = r amb (5) dt mwCpw − dt − dt where assuming that natural convection is dominant, the internal heat ﬂux of the vessel is:

dQr = α S (Tr Tw) (6) dt − i i − and the external heat ﬂux between the vessel surface and the atmosphere is:

dQamb = αeSe (Tw T ). (7) dt − amb

The internal convective heat exchange coeﬃcient, αi, is obtained by the value calculated for the 2 Nusselt number, while for the external one, it is assumed αe = 4.5 W/m /K[17]. The equations have been rearranged in order to calculate at each time step the mass of CNG and the temperature in the tank. Then, after the calculation of the gas density as the ratio between the mass and the tank internal volume, the pressure and all the other thermodynamic properties of interest are calculated by means of Coolprop.

4.2. The Regulation Valves Model The type of formulation used to tie the pressure loss through the valve with the mass ﬂow rate is the so-called universal gas sizing method [18,19]. Applying this method, the mass ﬂow rate is given by Equation (8), where pressures must be introduced in psia. r ! . 59.64 po m = 1.06 Cg √pi ρi sin 1 (8) · · · C1 · − pi

The valve manufacturers supply the gas flow-sizing coefficient, Cg, in conditions of maximum opening. The coefficient C1 = Cg/Cv can be assumed equal to 25 if the value of the fluid flow size coefficient Cv is unknown. The correlation between the coefficient and percentage of opening depends on the type of characteristic of the valve, which can be a linear, equal-percentage, or quick opening.

4.3. The Cooler/Heater Model The heat power transferred in the heat exchangers is given by Equation (9):

. Q = m(h ho). (9) hex i − The pressure drop is assumed to be one percentage point of the pressure at the heat exchanger inlet. Appl. Sci. 2020, 10, 7555 9 of 23

4.4. The Centrifugal Compressor Model The centrifugal compressor is modeled by means of its characteristic curves. They are provided by the manufacturer and eventually manipulated to be represented in the form of compression ratio, β, . as a function of the corrected mass flow rate, m √T01/p01, for different values of the corrected rotational speed, n/ √T01, as shown in Figure3. The corresponding values of the polytropic e fficiency, ηp, are also Appl.provided Sci. 2020 by, 10 the, x FOR manufacturer. PEER REVIEW 10 of 24

FigureFigure 3. 3. CompressorCompressor characteristic characteristic curves. curves.

4.5. TheThen, Ducts the Model input parameters of the compressor model are the stagnation temperature and pressure at suction, T01 and p01, respectively, and the compression ratio. The last one is determined by (i) the For the calculation of the two pipes present in the plant diagrams, one upstream and one tank pressures, (ii) the pressure drop in the ducts, and (iii) the pressure losses introduced by the valves. downstream of the compressor, the following hypotheses have been introduced: Once the compression ratio is known, the mass ﬂow rate and the polytropic eﬃciency can be obtained •from One-dimensional the characteristic flow; curves. Then, the absorbed power is given by Equation (10): • Perfect gases, with specific heat at constant pressure function of temperature only; . n 1 • n − 1 Uniform gas properties in the Pduct.= m ZRT01 β n 1 . (10) · n 1 − ·ηp With these hypotheses, the mass and momentum− conservation equations for the duct are: The temperature of the gas leaving the compressor is calculated given the corresponding enthalpy 𝑑𝑝 𝑘𝑅𝑇 and pressure values. = (𝑚 −𝑚 ) (11) 𝑑𝑡 𝐴𝐿 4.5. The Ducts Model = (𝑝 +𝑝) − (12) () For the calculation of the two pipes present in the plant diagrams, one upstream and one where the Darcy friction factor 𝜆 is derived from the Moody diagram. downstream of the compressor, the following hypotheses have been introduced:

5. ModelOne-dimensional Validation ﬂow; • Perfect gases, with speciﬁc heat at constant pressure function of temperature only; • The validation of the model was carried out by comparison with experimental literature data. In [20], the authors consider the filling of the cylinder shown in Figure 4: it has an aluminum liner with a reinforcement in composite material (epoxy resin with carbon fiber). The diameter is 396 mm, the length 830 mm, and the internal volume 70 L. The service pressure is 207 bar, and it is fed by a supply unit of 450 L at a pressure of 400 bar. Appl. Sci. 2020, 10, 7555 10 of 23

Uniform gas properties in the duct. • With these hypotheses, the mass and momentum conservation equations for the duct are:

dpi kRTi . . = m mo (11) dt AL i −

. . 2 dmo A λRTimo = (pi + po) (12) dt L −DA(pi + po) where the Darcy friction factor λ is derived from the Moody diagram.

5. Model Validation The validation of the model was carried out by comparison with experimental literature data. In [20], the authors consider the ﬁlling of the cylinder shown in Figure4: it has an aluminum liner with a reinforcement in composite material (epoxy resin with carbon ﬁber). The diameter is 396 mm, the length 830 mm, and the internal volume 70 L. The service pressure is 207 bar, and it is fed by a Appl.supply Sci. 2020 unit, 10 of, x 450 FOR L PEER at a pressureREVIEW of 400 bar. 11 of 24

Figure 4. Geometry of the type III CNG cylinder considered for the model validation [20]. Figure 4. Geometry of the type III CNG cylinder considered for the model validation [20]. The average temperature reported in [20] has been compared with the simulation results. The average temperature reported in [20] has been compared with the simulation results. Temperature data are of particular interest, because the safety of the cylinders in the composite Temperature data are of particular interest, because the safety of the cylinders in the composite material is veriﬁed when going to study the temperature rise that occurs during the compression of material is verified when going to study the temperature rise that occurs during the compression of the gas. the gas. The ambient temperature is not given in the paper, and therefore, it was assumed to be 25 ◦C, Thesince ambient the measurements temperature were is not performed given in inthe a summerpaper, and period. therefore, The initial it was cylinder assumed temperature to be 25 °C, is since8 C, − ◦ theand measurements its initial pressure were isperformed 10 bar. The in a results summer obtained period. are The shown initial incylinder Figure temperature5. The ﬁnal temperatureis −8 °C, and itsreached initial bypressure the numerical is 10 bar. model The results is 41 ◦ C,obtained while the are experimental shown in Figure temperature 5. The final is 39temperature◦C. The error reached is just byover the 5%, numerical but we mustmodel consider is 41 °C, that while not the all theexperiment errors areal attributable temperature to is the 39 model,°C. The as error the temperatureis just over 5%,of the but external we must environment consider that is not not all exactly the errors known. are attributable In any case, to numerical the model, simulations as the temperature provide anof theacceptable external comparison environment with is experimentalnot exactly known. results. In any case, numerical simulations provide an acceptable comparison with experimental results.

Figure 5. Comparison between experimental [20] and numerical results.

6. The Case Study and Simulation Results

6.1. The Case Study The model has been applied to simulate the loading and the unloading processes of the CNG tanker that is being designed under the GASVESSEL project; see Table 2. Appl. Sci. 2020, 10, x FOR PEER REVIEW 11 of 24

Figure 4. Geometry of the type III CNG cylinder considered for the model validation [20].

The average temperature reported in [20] has been compared with the simulation results. Temperature data are of particular interest, because the safety of the cylinders in the composite material is verified when going to study the temperature rise that occurs during the compression of the gas. The ambient temperature is not given in the paper, and therefore, it was assumed to be 25 °C, since the measurements were performed in a summer period. The initial cylinder temperature is −8 °C, and its initial pressure is 10 bar. The results obtained are shown in Figure 5. The final temperature reached by the numerical model is 41 °C, while the experimental temperature is 39 °C. The error is just over 5%, but we must consider that not all the errors are attributable to the model, as the temperature of the external environment is not exactly known. In any case, numerical simulations provide an Appl. Sci. 2020, 10, 7555 11 of 23 acceptable comparison with experimental results.

Figure 5. Comparison between experimental [[20]20] and numerical results.

6. The Case Study and Simulation Results 6.1. The Case Study 6.1. The Case Study The model has been applied to simulate the loading and the unloading processes of the CNG The model has been applied to simulate the loading and the unloading processes of the CNG tanker that is being designed under the GASVESSEL project; see Table2. tanker that is being designed under the GASVESSEL project; see Table 2. The on-board compression station consists of two double-stage centrifugal compressors that can work in parallel. The polytropic efficiency varies from 0.5 near the choke point up to the maximum value, which is supposed to be 0.73. The outlet temperature of both the intercooler and cooler is set to 40 ◦C. Regulation is carried out by means of a throttling valve and variation of speed. The objective is to optimize the mass flow rate, keeping the absorbed power below the maximum design value of 10 MW. The loading process is carried out from a storage terminal constituted of a Floating Production Storage and Offloading (FPSO) unit, whose water volume is supposed to be twice the ship water volume. The FPSO initial pressure is 240 bar. The initial pressure in the ship gas cylinders is 30 bar, and the loading process ends when it reaches the design value of 300 bar. The forced loading phase starts when the mass flow rate due to the free loading falls below 20 kg/s. The unloading process is carried out in a pipeline at a constant pressure of 120 bar. The initial pressure in the ship gas cylinders is supposed to be the same reached at the end of the loading process, 300 bar, neglecting the possible variations due to the ship engines consumption and to the heat exchange with the surroundings. The unloading process ends when it reaches the final value of 30 bar. Such a final value has been imposed considering that the remaining gas must be sufficient to guarantee the fuel for the return journey. The forced unloading phase starts when the mass flow rate due to the free unloading falls below 20 kg/s. In these conditions, the pressure in the gas cylinders remains for a while higher than that in the pipeline: this requires acting on the lamination valve to make the suction pressure lower than 120 bar, so allowing the normal operating conditions of the compressors. Two calculation hypotheses have been taken into account: i.e., adiabatic or not adiabatic pressure vessels. The first is useful as a term of comparison, while the second is more realistic. In the not adiabatic case, the environmental temperature is another variable of the model: the temperature of nitrogen filling the holds has been set to 40 ◦C and 5 ◦C in summer and winter conditions, respectively. Appl. Sci. 2020, 10, 7555 12 of 23

The results discussed below have been obtained considering, as a term of comparison for further analysis, the compression of pure methane.

6.2. Simulation Results: Adiabatic Processes Calculations have been carried on by setting the terms relating to the heat exchange in the pressure vessels to zero (see Equations (6) and (7) of the tank model). Appl. Sci. 2020, 10The, x FOR results PEER obtained REVIEW with reference to the loading process are summarized in Figures6–11. 13 of 24 They show the trend as a function of time of pressure and temperature in the storage terminal (Figure6) and in the pressure vessels (Figure7), the mass ﬂow rate (Figure8), the mass of CNG in the pressure Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 24 vessels (Figure9), the compression ratioTemperature (Figure 10), and the compressorPressure power (Figure 11), respectively. 45 350 Temperature Pressure 35 300 45 350 25 250 35 300 15 200 25 250 5 150 15 200 -5 100 5 150 CNG pressure (bar)

CNG temperature (°C) -15 50 -5 100 -25 0 CNG pressure (bar)

CNG temperature (°C) -15 0 1020304050 -25 Time (h) 0 0 10203040 Time (h) Figure 6. Storage terminal CNG conditions (adiabatic loading). Figure 6. Storage terminal CNG conditions (adiabatic loading). Figure 6. Storage terminalTemperature CNG conditionsPressure (adiabatic loading).

80 Temperature Pressure350 70 300 80 350 60 250 70 300 50 200 60 250 40 150 50 200 30 100 40 150

20 50 CNG pressure (bar)

CNG temperature (°C) 30 100 10 0

20 0 1020304050 CNG pressure (bar)

CNG temperature (°C) 10 Time (h) 0 0 10203040 Figure 7. Pressure vessels CNG conditions (adiabatic loading). Time (h) Figure 7. Pressure vessels CNG conditions (adiabatic loading).

Figure 7. Pressure450 vessels CNG conditions (adiabatic loading). 400 450350 400300 350250 300200 250150 200100

CNG flow rate (ton/h) CNG flow 15050 1000

CNG flow rate (ton/h) CNG flow 50 0 10203040 0 Time (h) 0 10203040 Figure 8. CNG flow rateTime (adiabatic (h) loading).

Figure 8. CNG flow rate (adiabatic loading). Appl. Sci. 2020, 10, x FOR PEER REVIEW 13 of 24

Temperature Pressure

45 350 35 300 25 250 15 200 5 150 -5 100 CNG pressure (bar)

CNG temperature (°C) -15 50 -25 0 0 10203040 Time (h)

Figure 6. Storage terminal CNG conditions (adiabatic loading).

Temperature Pressure

80 350 70 300 60 250 50 200 40 150 30 100

20 50 CNG pressure (bar)

CNG temperature (°C) 10 0 0 10203040 Time (h)

Appl. Sci. 2020, 10, 7555Figure 7. Pressure vessels CNG conditions (adiabatic loading). 13 of 23

450 400 350 300 250 200 150 100

CNG flow rate (ton/h) CNG flow 50 Appl. Sci. 2020, 10, x FOR PEER REVIEW 0 14 of 24 0 10203040 Time (h) Appl. Sci. 2020, 10, x FOR PEER REVIEW8000 14 of 24 Figure 8. 7000 CNG ﬂow rate (adiabatic loading). Figure 8. CNG flow rate (adiabatic loading). 80006000 70005000 60004000 50003000 4000 CNG mass (ton) 2000 30001000

CNG mass (ton) 20000 1000 0 10203040 Time (h) 0 0 10203040 Time (h) Figure 9. Mass of CNG in the pressure vessels (adiabatic loading). Figure 9. Mass of CNG in the pressure vessels (adiabatic loading).

Figure 9. Mass5 of CNG in the pressure vessels (adiabatic loading).

Compression(-) ratio 21

Compression(-) ratio 10 0 10203040 0 Time (h)

Figure0 10. Compression 10203040 ratio (adiabatic loading). Figure 10. CompressionTime ratio (h) (adiabatic loading).

Figure10 10. Compression ratio (adiabatic loading).

108

compress. power (MW) 42

compress. power (MW) 20 0 10203040 0 Time (h)

0 10203040 Figure 11. CompressorTime power (h) (adiabatic loading).

The entire loading phaseFigure needs 11. Compressorabout 35 h. powerThe singular (adiabatic point loading). that can be observed at time 11 h in the flow rate, compression ratio, and compressor power curves corresponds to the passage from the freeThe entireloading loading to the phase forced needs loading about phase 35 h. with The thesingular start ofpoint the thattwo can compressors. be observed Therefore, at time 11 the h inforced the flow loading rate, phasecompression takes 24 ratio, h. The and singularities compresso rvi powersible during curves this corresponds phase, between to the passage11 h and from 25 h, the free loading to the forced loading phase with the start of the two compressors. Therefore, the forced loading phase takes 24 h. The singularities visible during this phase, between 11 h and 25 h, Appl. Sci. 2020, 10, x FOR PEER REVIEW 14 of 24

8000 7000 6000 5000 4000 3000

CNG mass (ton) 2000 1000 0 0 10203040 Time (h)

Figure 9. Mass of CNG in the pressure vessels (adiabatic loading).

Compression(-) ratio 1

0 0 10203040 Time (h)

Appl. Sci. 2020, 10, 7555 Figure 10. Compression ratio (adiabatic loading). 14 of 23

compress. power (MW) 2

0 0 10203040 Time (h)

Figure 11. Compressor power (adiabatic loading). Figure 11. Compressor power (adiabatic loading). The entire loading phase needs about 35 h. The singular point that can be observed at time 11 h in Thethe entire flow rate,loading compression phase needs ratio, and about compressor 35 h. The power singular curves correspondspoint that can to the be passage observed from at the time 11 h free loading to the forced loading phase with the start of the two compressors. Therefore, the forced in the flow rate, compression ratio, and compressor power curves corresponds to the passage from loading phase takes 24 h. The singularities visible during this phase, between 11 h and 25 h, are due the free toloading the regulation to the of forced the compressor loading with phase variation with ofthe the start rotational of the speed, two starting compressors. at 10,000 rpmTherefore, up the forced loadingto 11,500 phase rpm, to takes maintain 24 theh. The flow singularities rate within the vi rangesible between during the this pumping phase, and between flooding 11 limits. h and 25 h, The last singularity, at about 28 h, corresponds to the switching-off of one of the two compressors, due to the reduction of mass flow rate that occurs toward the end of the process. The loaded mass is about 6750 ton. The overall mean flow rate is about 192 ton/h but, when compressors are running, the mean value is 133 ton/h. The maximum compression ratio is 3.3, obviously at the end of the forced loading phase. The fuel consumption is 36 ton ( 0.5% of the ' total cargo), and the specific energy consumption is 96 kJ/kg. Particular attention requires, for the reasons discussed in paragraph 1, the temperature trend in the pressure vessels. Starting from the initial chosen value of 40 ◦C, at first, the temperature decreases by about 10 ◦C. As described in [7], it is due to the Joule–Thompson cooling effect that undergoes the gas in an isenthalpic expansion through an orifice. When it enters in a nearly empty cylinder, this cold gas mixes with and compresses the gas originally in the tank. The result is that the combined mixed gas temperature initially reduces. When the compression and conversion of supply enthalpy energy to cylinder internal energy overcomes the Joule–Thompson cooling effect, which becomes smaller as the cylinder pressure increases, the mixed gas temperature in the cylinder begins to increase [13]. It reaches 78 ◦C at the end of the process in the considered case, with an increment of 38 ◦C. This result is consistent with those found in [6], and it corresponds to a fill efficiency of 85%, in comparison with an ideal loading at the initial temperature of 40 ◦C. The results obtained with reference to the adiabatic unloading process are summarized in Figures 12–16. They show the trend as a function of time of pressure and temperature in the pressure vessels (Figure 12), the mass flow rate (Figure 13), the mass of CNG in the pressure vessels (Figure 14), the compression ratio (Figure 15), and the compressor power (Figure 16), respectively. The entire unloading process needs about 46 h, divided into 12 h for the initial free phase and 34 h for the subsequent forced phase, as clearly shown by the compressor power diagram. The 10 h more than the loading phase are due to the compressor working at higher compression ratio values. As in the previous case, the singularities in the flow rate, compression ratio, and compressor power curves during the forced phase are due to the speed regulation of the compressor. Appl. Sci. 2020, 10, x FOR PEER REVIEW 15 of 24 are due to the regulation of the compressor with variation of the rotational speed, starting at 10,000 rpm up to 11,500 rpm, to maintain the flow rate within the range between the pumping and flooding limits. The last singularity, at about 28 h, corresponds to the switching-off of one of the two compressors, due to the reduction of mass flow rate that occurs toward the end of the process. The loaded mass is about 6750 ton. The overall mean flow rate is about 192 ton/h but, when compressors are running, the mean value is 133 ton/h. The maximum compression ratio is 3.3, obviously at the end of the forced loading phase. The fuel consumption is 36 ton (≃ 0.5% of the total cargo), and the specific energy consumption is 96 kJ/kg. Particular attention requires, for the reasons discussed in paragraph 1, the temperature trend in the pressure vessels. Starting from the initial chosen value of 40 °C, at first, the temperature decreases by about 10 °C. As described in [7], it is due to the Joule–Thompson cooling effect that undergoes the gas in an isenthalpic expansion through an orifice. When it enters in a nearly empty cylinder, this cold gas mixes with and compresses the gas originally in the tank. The result is that the combined mixed gas temperature initially reduces. When the compression and conversion of supply enthalpy energy to cylinder internal energy overcomes the Joule–Thompson cooling effect, which becomes smaller as the cylinder pressure increases, the mixed gas temperature in the cylinder begins to increase [13]. It reaches 78 °C at the end of the process in the considered case, with an increment of 38 °C. This result is consistent with those found in [6], and it corresponds to a fill efficiency of 85%, in comparison with an ideal loading at the initial temperature of 40 °C. The results obtained with reference to the adiabatic unloading process are summarized in Figures 12–16. They show the trend as a function of time of pressure and temperature in the pressure vessels (Figure 12), the mass flow rate (Figure 13), the mass of CNG in the pressure vessels (Figure Appl.14), Sci.the2020 compression, 10, 7555 ratio (Figure 15), and the compressor power (Figure 16), respectively. 15 of 23

Pressure Temperature 350 100

300 75

250 50

200 25

150 0

100 -25 CNG pressure (bar) 50 -50 CNG temperature (°C)

0 -75 0 1020304050 Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 24 Time (h) Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 24

Figure 12. 450Pressure vessels CNG conditions (adiabatic unloading). Figure 12. Pressure vessels CNG conditions (adiabatic unloading). 400450 350400 300350 250300 200250 150200

CNG flow rate (ton/h) CNG flow 100150

CNG flow rate (ton/h) CNG flow 10050 500 00 1020304050 0 1020304050Time (h) Time (h) Figure 13. CNG flow rate (adiabatic unloading). Figure 13. CNG ﬂow rate (adiabatic unloading). Figure 13. CNG flow rate (adiabatic unloading). 8000

70008000

60007000

50006000

40005000

30004000

CNG mass (ton) 20003000 CNG mass (ton) 10002000

10000 00 1020304050 0 1020304050Time (h)

Figure 14. Mass of CNG in the pressureTime vessels (h) (adiabatic unloading). Figure 14. Mass of CNG in the pressure vessels (adiabatic unloading). Figure 14. Mass of CNG in the pressure vessels (adiabatic unloading). 5 5 4 4 3 3 2 2

compression(-) ratio 1

compression(-) ratio 1 0 00 1020304050 0 1020304050Time (h)

Time (h) Figure 15. Compression ratio (adiabatic unloading). Figure 15. Compression ratio (adiabatic unloading). Appl. Sci. 2020, 10, x FOR PEER REVIEW 16 of 24

450 400 350 300 250 200 150

CNG flow rate (ton/h) CNG flow 100 50 0 0 1020304050 Time (h)

Figure 13. CNG flow rate (adiabatic unloading).

8000

7000

6000

5000

4000

3000

CNG mass (ton) 2000

1000

0 0 1020304050 Time (h)

Appl. Sci. 2020, 10, 7555 16 of 23 Figure 14. Mass of CNG in the pressure vessels (adiabatic unloading).

compression(-) ratio 1

0 0 1020304050 Time (h) Appl. Sci. 2020, 10, x FOR PEER REVIEW 17 of 24 Figure 15. Compression ratio (adiabatic unloading). Figure 15. Compression ratio (adiabatic unloading). 10

2 Compress. power (MW) 0 0 1020304050 Time (h)

FigureFigure 16.16. Compressor powerpower (adiabatic(adiabatic unloading).unloading). The unloaded mass is about 6150 ton. The residual mass left in the cylinders is 600 ton higher The entire unloading process needs about 46 h, divided into 12 h for the initial free phase and 34 than that at the start of the loading process, at the same pressure of 30 bar: this is due to the very low h for the subsequent forced phase, as clearly shown by the compressor power diagram. The 10 h more temperature, about 60 C below zero, reached at the unloading end. In fact, such a low temperature than the loading phase◦ are due to the compressor working at higher compression ratio values. As in cannot be tolerated because it could lead to the condensation of propane and butane, compromising the the previous case, the singularities in the flow rate, compression ratio, and compressor power curves normal functionality of the compressor. To avoid this, the heater (Figure1) must be able to guarantee a during the forced phase are due to the speed regulation of the compressor. gas temperature of at least 4 C. The unloaded mass is about◦ 6150 ton. The residual mass left in the cylinders is 600 ton higher The mean flow rate is about 135 ton/h. The fuel consumption is equal to 51 ton ( 0.7% of the total than that at the start of the loading process, at the same pressure of 30 bar: this is due' to the very low cargo), and the specific energy consumption is 151 kJ/kg. temperature, about 60 °C below zero, reached at the unloading end. In fact, such a low temperature Taking into account both the loading and unloading processes, the adiabatic overall fuel cannot be tolerated because it could lead to the condensation of propane and butane, compromising consumption is 1.14% of the total cargo, corresponding to about three pressure vessels. the normal functionality' of the compressor. To avoid this, the heater (Figure 1) must be able to 6.3.guarantee Simulation a gas Results: temperature Not Adiabatic of at least Processes 4 °C. The mean flow rate is about 135 ton/h. The fuel consumption is equal to 51 ton (≃ 0.7% of the totalThe cargo), heat and exchange the specific between energy the pressureconsumption vessels is and151 kJ/kg. the nitrogen of the holds has been calculated at eachTaking time step.into Theaccount overall both heat the transfer loading coe andfficient unloading has been processes, obtained calculatingthe adiabatic the internaloverall andfuel externalconsumption convective is ≃ 1.14% heat of exchange the total coecargo,fficients correspondi accordingng to to about Section three 4.1, pressure while the vessels. heat conduction through the composite wall of the GASVESSEL cylinders has been obtained on the basis of the specifications6.3. Simulation given Results: in TableNot Adiabatic3. Processes From the temperature trends in the pressure vessels obtained in the previous case of adiabatic loadingThe and heat unloading, exchange it canbetween be deduced the pressure that the vessels heat exchange and the with nitrogen the environment of the holds should has allowbeen improvingcalculated at both each the time filling step. of The the cylinderoverall heat during transfer the loadingcoefficient phase has andbeen the obtained emptying calculating during thethe unloadinginternal and phase. external In fact, convective the heat heat exchange exchange with coef theficients environment according contrasts to Section both 4.1, the while heating the of heat the accumulatedconduction through gas in thethe firstcomposite case and wall the of coolingthe GASV ofESSEL the residual cylinders one has in thebeen second. obtained However, on the basis it is of the specifications given in Table 3.

Table 3. GASVESSEL project: CNG vessel wall properties [4].

Thickness Density Specific Heat Thermal Conductivity

(m) (kg/m3) (J/kg/K) (W/m/K) liner 0.005 7800 500 60 carbon fiber epoxy 0.035 938 1494 1

From the temperature trends in the pressure vessels obtained in the previous case of adiabatic loading and unloading, it can be deduced that the heat exchange with the environment should allow improving both the filling of the cylinder during the loading phase and the emptying during the unloading phase. In fact, the heat exchange with the environment contrasts both the heating of the accumulated gas in the first case and the cooling of the residual one in the second. However, it is evident that the improvement will be less significant in the loading phase during the summer and in the unloading phase during the winter. The results obtained with the simulations under these two conditions, which can be considered of greater operational interest, are presented below. Appl. Sci. 2020, 10, 7555 17 of 23 evident that the improvement will be less signiﬁcant in the loading phase during the summer and in the unloading phase during the winter. The results obtained with the simulations under these two conditions, which can be considered of greater operational interest, are presented below.

Table 3. GASVESSEL project: CNG vessel wall properties [4].

Appl. Sci. 2020, 10, x FOR PEER REVIEWThickness Density Speciﬁc Heat Thermal Conductivity 18 of 24 (m) (kg/m3) (J/kg/K) (W/m/K)

Appl. InSci. the2020 case, 10,liner x ofFOR summer PEER REVIEW loading, 0.005 the loaded 7800 mass was 7290 500 ton, with an increment 60 of 8%18 with of 24 respect tocarbon the adiabatic fiber epoxy case. However, 0.035 the duration 938 of the process 1494 is extended by 1 45%, from 35 h to aboutIn 51 the h. Thecase increment of summer of loading,16 h is almost the loaded entirely mass due was to the 7290 greater ton, masswith loadedan increment during of the 8% forced with phaserespectIn (39 theto h the caseinstead adiabatic of summerof 24 case. h) at loading, However, high compression the the loaded duration ratios. mass of was the process 7290 ton, is withextended an increment by 45%, from of 8% 35 with h to respectaboutFigure 51 to h. the The17 adiabatic shows increment the case. trend of 16 However, has is a almost function the entirely duration of time due of of theto pressure the process greater and is extendedmass temperature loaded by 45%,during in the from the pressure 35forced h to vessels.aboutphase 51(39 By h. h comparing Theinstead increment of the24 h) temperature of at 16 high h is compression almost trend entirely with ratios. those due of to Figure the greater 7, it can mass be loaded observed during that thethe forcedinitial coolingphaseFigure (39 of hthe instead17 CNG shows ofin 24the h) vesselstrend at high as disappears, compressiona function thatof ratios. time the maximumof pressure temperature and temperature is 70 °C in insteadthe pressure of 78 °C,vessels. andFigure that By comparing 17it occurs shows at the theabout trend temperature half as athe function duration trend of with timeof the ofthose process. pressure of Figure Most and temperature7,importantly, it can be observed in the the final pressure that temperature the vessels. initial reducesBycooling comparing toof 60the °C, theCNG giving temperature in thethe vesselscited trend increment disappears, with those of th that ofe loaded Figure the maximum7 mass., it can The be temperature observedcompressor that poweris the 70 initial°C trend instead cooling reported of of78 inthe°C, Figure CNGand that 18 in theisit quiteoccurs vessels similar at disappears,about to thathalf ofthe that the duration theadiabatic maximum of thecase process. of temperature Figure Most 17 until importantly, is 70 time◦C instead 35 h; the then, offinal 78 the temperature◦C, reduction and that ofitreduces occurstemperature to at 60 about °C, leads giving half to the the a duration finalcited riseincrement of of the the process. of power, the loaded Mostfollowing mass. importantly, Thethe compressorrise the of finalthe rotationalpower temperature trend speed reported reduces and compressiontoin 60Figure◦C, giving18 is ratio. quite the The similar cited maximum increment to that ofvalu the ofe theadiabatic of loadedthe last case mass.one of increases Figure The compressor 17 withuntil respecttime power 35 h;to then,the trend adiabatic the reported reduction case in fromFigureof temperature 3.3 18 to is 4.3. quite leads similar to toa thatfinal of rise the adiabaticof the power, case offollowing Figure 17 theuntil rise time of 35the h; rotational then, the reduction speed and of temperaturecompression leads ratio. to The a final maximum rise of the valu power,e of followingthe last one the increases rise of the with rotational respect speed to the and adiabatic compression case ratio.from 3.3 The to maximum 4.3. value of the lastPressure one increases withTemperature respect to the adiabatic case from 3.3 to 4.3. 350 80

300 Pressure Temperature 70 350 80 250 60 300 70 200 50 250 60 150 40 200 50 100 30 150 40

CNG pressure (bar) 50 20

100 30 CNG temperature (°C) 0 10

CNG pressure (bar) 50 0 10203040506020 CNG temperature (°C) 0 Time(h) 10 0 102030405060

Time(h) Figure 17. Pressure vessels CNG conditions (summer loading). Figure 17. Pressure vessels CNG conditions (summer loading). Figure 17. Pressure vessels CNG conditions (summer loading). 10

108

24 compress. power (MW) compress. power 02 0 102030405060 compress. power (MW) compress. power 0 Time (h) 0 102030405060 Figure 18. Compressor power (summer loading). Time (h) Figure 18. Compressor power (summer loading).

As a whole, the mean flowFigure rate 18. decreases Compressor from power 192 (summerto 143 ton/h, loading). the fuel consumption increases from 36 to 51 ton (≃ 0.6% of the total cargo), and the specific energy consumption reaches 134 kJ/kg, insteadAs of a whole,96 kJ/kg. the mean flow rate decreases from 192 to 143 ton/h, the fuel consumption increases fromAs 36 farto 51as tonthe (heat≃ 0.6% exchange of the total is concerned, cargo), and the the peak specific power energy dispersed consumption by the singlereaches cylinder 134 kJ/kg, is equalinstead to of 32 96 kW, kJ/kg. and the energy dispersed in the hold is 658 kWh. Considering that there are 256 cylindersAs far in asthe the hold, heat which exchange are filled is concerned, at the same the time, peak there power is a totaldispersed maximum by the dispersed single cylinder power ofis 8.2equal MW to and 32 kW,a total and dispersed the energy energy dispersed of 168.448 in the kWh. hold is 658 kWh. Considering that there are 256 cylinders in the hold, which are filled at the same time, there is a total maximum dispersed power of 8.2 MW and a total dispersed energy of 168.448 kWh. Appl. Sci. 2020, 10, 7555 18 of 23

As a whole, the mean flow rate decreases from 192 to 143 ton/h, the fuel consumption increases Appl.from Sci. 36 2020 to 51, 10 ton, x FOR ( PEER0.6% REVIEW of the total cargo), and the specific energy consumption reaches 13419 kJ of/kg, 24 ' instead of 96 kJ/kg. In the case of winter unloading, the unloaded mass is 7025 ton, with an increment of about 14% Appl. Sci.As 2020 far,as 10, the x FOR heat PEER exchange REVIEW is concerned, the peak power dispersed by the single cylinder is19 equal of 24 into 32comparison kW, and the with energy the adiabatic dispersed case. in the The hold residual is 658 kWh.mass Consideringis reduced of that 265 there ton with are 256 respect cylinders to the in adiabaticthe hold,In the whichcalculation, case of are winter filled due atunloading, to the the same higher time,the final unloaded there temper is a ma totalature.ss is maximum Figure7025 ton, 19 dispersed withshows an the increment power trend ofas 8.2of a functionabout MW and 14% of a time of pressure and temperature in the pressure vessels: the minimum temperature is −25 °C, which intotal comparison dispersed with energy the of adiabatic 168.448 kWh.case. The residual mass is reduced of 265 ton with respect to the is reached at about two-thirds of the unloading time, while the final temperature is −17 °C (remember adiabaticIn the calculation, case of winter due unloading,to the higher the final unloaded temper massature. is Figure 7025 ton, 19 shows with an the increment trend as a of function about 14% of that in the adiabatic case, the minimum was −60 °C at the end of the process). The unloading process timein comparison of pressure with and thetemperature adiabatic in case. the pr Theessure residual vessels: mass the is minimum reduced oftemperature 265 ton with is − respect25 °C, which to the lasts 55 h, 44 of which are due to the forced phase. The increments with respect to the adiabatic case isadiabatic reached calculation,at about two-thirds due to the of the higher unlo finalading temperature. time, while Figurethe final 19 temperature shows the trend is −17 as °C a (remember function of are of 9 and 10 h, respectively. The mean flow rate is about 119 ton/h against 135 ton/h. The fuel thattime in of the pressure adiabatic and case, temperature the minimum in the pressurewas −60 vessels:°C at the the end minimum of the process). temperature The unloading is 25 ◦C, whichprocess is consumption goes from 51 ton to 67 ton (≃1% of the total cargo), and the specific energy− consumption lastsreached 55 h, at 44 about of which two-thirds are due of to the the unloading forced phase. time, The while increments the final temperaturewith respect isto the17 ◦adiabaticC (remember case goes from 151 kJ/kg to 183 kJ/kg. The compressor adsorbed power, reported in Figure− 20, shows a arethat of in 9 the and adiabatic 10 h, respectively. case, the minimum The mean was flow60 ◦rateC at is the about end of119 the ton/h process). against The 135 unloading ton/h. The process fuel trend very similar to those of the adiabatic case− of Figure 16. consumptionlasts 55 h, 44 ofgoes which from are 51 dueton to to 67 the ton forced (≃1% phase. of the Thetotal incrementscargo), and withthe specific respect energy to the adiabaticconsumption case Considering the overall balances of the non-adiabatic process, the total fuel consumption is goesare of from 9 and 151 10 kJ/kg h, respectively. to 183 kJ/kg. The The mean compressor flow rate adsorbed is about power, 119 ton reported/h against in 135Figure ton /20,h. shows The fuel a ≃1.45% of the total cargo, corresponding to about 3.5 pressure vessels. trendconsumption very similar goes to from those 51 of ton the to adiabatic 67 ton ( 1% case of of the Figure total cargo),16. and the specific energy consumption ' goesConsidering from 151 kJ /kgthe tooverall 183 kJ /balanceskg. The compressorof the non-adia adsorbedbatic process, power, reportedthe total infuel Figure consumption 20, shows is a ≃ Temperature Pressure trend1.45% very of the similar total tocargo, those correspondi of the60 adiabaticng to caseabout of 3.5 Figure pressure 16. vessels.350

45 Temperature Pressure 300 60 350250 30 45 300200 15 250 30 150 0 200100 15 150 -15 50 CNG pressure (bar) CNG temperature (°C) 0 -30 1000

-15 0 10203040506050 CNG pressure (bar) CNG temperature (°C) Time (h) -30 0 0 102030405060 Time (h) Figure 19. Pressure vessels CNG conditions (winter unloading). Figure 19. Pressure vessels CNG conditions (winter unloading).

Figure 19. Pressure10 vessels CNG conditions (winter unloading).

8 10

6 8

4 6

2 compress. power (MW) compress. power 4

0 2 compress. power (MW) compress. power 0 102030405060

0 Time (h) 0 102030405060 Figure 20. Compressor power (winter unloading). Figure 20. CompressorTime power (h) (winter unloading). Considering the overall balances of the non-adiabatic process, the total fuel consumption is 1.45% ' 7.of Future the total Application: cargo, corresponding The Hydrogen to about Case 3.5 pressure vessels. Figure 20. Compressor power (winter unloading). The use of hydrogen as an energy carrier is a very topical issue, because it allows meeting, 7.together Future withApplication: the use of The fuel Hydrogen cells, the requirements Case of zero local emissions. One aspect that limits the diffusionThe useon aof large hydrogen scale of as hydrogen an energy is carrierits low isvolumetric a very topical energy issue, density. because Pure it hydrogen allows meeting, reaches togetheracceptable with storage the use densities of fuel cells,only theat cryogenic requirements temperatures, of zero local at emissions.−253 °C, at Onehigh aspect pressure, that orlimits both the at diffusion on a large scale of hydrogen is its low volumetric energy density. Pure hydrogen reaches acceptable storage densities only at cryogenic temperatures, at −253 °C, at high pressure, or both at Appl. Sci. 2020, 10, 7555 19 of 23 Appl. Sci. 2020, 10, x FOR PEER REVIEW 20 of 24

7.high Future pressure Application: and low Thetemperature. Hydrogen The Case solution with the higher level of technology maturity is the storage at high pressure, at the same pressure levels considered for LNG. TheIn useconsidering of hydrogen the asloading an energy and carrierunloading is a very processes topical of issue, hydrogen, because it it must allows also meeting, be taken together into Appl.withaccount Sci. the 2020 use that, 10 of , thex fuel FOR energycells, PEER theREVIEWdensity requirements of H2 is much of zero less local dependent emissions. on Onetemperature aspect that than limits that theof methane, diff20usion of 24 onas a shown large scale in Figure of hydrogen 21. Moreover, is its low the volumetric compressor energy efficiency density. changes Pure hydrogen with molecular reaches weight, acceptable in highstorageparticular pressure densities with and the only low volumetric at temperature. cryogenic ones. temperatures, The It is solutionmainly due at with 253to thethe◦C, highervalve at high loss level pressure, power, of technology which or both is at lowermaturity high with pressure is low the − storageandmolecular low at temperature. high weight. pressure, For The example,at solutionthe same as with pressurereported the higher levelsin [21], level considered the of maximum technology for LNG. adiabatic maturity efficiency is the storage drops at from high pressure,93%In with considering at CH the4 sameto a thestill pressure loadingvery good levels and 88% consideredunloading with H2 at forprocesses a LNG.compression of hydrogen, ratio of 3it butmust from also 91% be totaken 70% intoat a accountcompressionIn consideringthat the ratio energy theof 1.4. loading density and of H unloading2 is much processes less dependent of hydrogen, on temperature it must also than be taken that of into methane, account asthat shown theSimulations energy in Figure density carried 21. of Moreover, Hout2 is with much H the2 less on compressor dependentthe same plant onefficiency temperature considered changes thanfor the thatwith loading of molecular methane, and unloading asweight, shown ofin in particularFigureCNG 21show. with Moreover, that the the volumetric thetemperature compressor ones. reached It e isffi mainlyciency at the changesdue end to ofthe with the valve loading molecular loss power,is weight,much which higher, in particular is loweras can with withbe seen low the molecularvolumetriccomparing weight. ones. Figure It For is7 with mainly example, Figure due as to22, reported the with valve referenc in loss [21], power,e to the the maximum whichadiabatic is lower adiabaticcase. withIn addition, efficiency low molecular the drops maximum weight. from 93%Forrequired example,with CH compressor4 as to reported a still powervery in [good21 becomes], the 88% maximum withhigher H 2and adiabaticat aex compressionceeds effi theciency maximum ratio drops of from3design but93% from value with 91% of CH to10 4 70%MW,to a at still as a compressionveryshown good in 88%Figure ratio with 23.of H1.4. 2 at a compression ratio of 3 but from 91% to 70% at a compression ratio of 1.4. Simulations carried out with H2 on the same plant considered for the loading and unloading of CNG show that the temperature reached at the end of the loading is much higher, as can be seen comparing Figure 7 with Figure 22, with reference to the adiabatic case. In addition, the maximum required compressor power becomes higher and exceeds the maximum design value of 10 MW, as shown in Figure 23.

FigureFigure 21. 21.Energy Energy density density ofof CHCH44 vs.vs. H 2..

Simulations carried out with H2 onPressure the same plantTemperature considered for the loading and unloading of CNG show that the temperature350 reached at the end of the loading140 is much higher, as can be seen comparing Figure7 with Figure 22, with reference to the adiabatic case. In addition, the maximum 300 120 required compressor power becomes higher and exceeds the maximum design value of 10 MW, 250 100 as shown in Figure 23. Figure 21. Energy density of CH4 vs. H2. 200 80 Pressure Temperature 350150 14060 pressure (bar) 2 300100 12040 temperature (°C) H 2 H 25050 10020

2000 800 0 5 10 15 20 25 150 60 Time (h) pressure (bar)

2 100 40 temperature (°C) H 2

Figure 22. Pressure vessels conditions (H2 loading).H 50 20

0 0 0 5 10 15 20 25 Time (h)

Figure 22. Pressure vessels conditions (H2 loading). Figure 22. Pressure vessels conditions (H2 loading). Appl.Appl. Sci. Sci.2020 2020, 10, 10, 7555, x FOR PEER REVIEW 2021 of of 23 24

2 compress. power (MW) compress. power 0 0 5 10 15 20 25 Time (h)

Figure 23. Compressor power (H2 loading). Figure 23. Compressor power (H2 loading). Taking into account the entire process, the specific energy consumption is almost ten times higher than thatTaking of methane, into account as reported the entire in Table process,4. the specific energy consumption is almost ten times higher than that of methane, as reported in Table 4. Table 4. Loading process: methane vs. hydrogen. Table 4. Loading process: methane vs. hydrogen. Methane Hydrogen Methane Hydrogen loading time [h] 35 21 finalloading pressure time [bar] [h] 300 35 300 21 finalloaded pressure mass [ton] [bar] 7609 300 724 300 loadedloaded volume mass [Nm[ton]3] 10,601,922 7609 8,157,685 724 storedloaded energy volume @20 ◦ C[Nm3] [MWh] 130,278 10,601,922 31,136 8,157,685 compressors energy [kWh] 180,575 163,636 stored energy @20 °C [MWh] 130,278 31,136 fuel consumption [%] 0.5 1.7 specificcompressors energy consumption energy [kWh] [kJ/kg] 180,57596 945 163,636 num.fuel of consumption pressure vessels [%] used 1 0.5 3.5 1.7 specific energy consumption [kJ/kg] 96 945 8. Conclusions num. of pressure vessels used 1 3.5

8. ConclusionsCNG technology can be advantageous over LNG for transporting by ship small quantities of gas over short distances, up to 3000–5000 km. However, these advantages are connected to the developmentCNG technology of advanced can technologies be advantageous for the over design LNG and for construction transporting of by lightweight ship small and quantities safe pressure of gas vesselsover short for CNG distances, carriers. up The to vessels 3000–5000 loading km. and However, unloading these processes advantages also must are be connected optimized to from the thedevelopment points of view of advanced of timing andtechnologies energy consumption, for the design because and theyconstruction have an impactof lightweight on the economic and safe feasibilitypressure ofvessels the whole for CNG CNG transportcarriers. The project. vessels loading and unloading processes also must be optimizedThe simulation from the modelpoints developedof view of fortiming this and purpose energy proved consumption, to be a useful because tool they for the have study an impact of the loadingon the economic and unloading feasibility system. of the The whole application CNG transport of the model project. to a CNG carrier with the specifications of theThe GASVESSEL simulation project model madedeveloped it possible for this to purpose highlight proved the followingto be a useful points, tool dependingfor the study on of the the calculationloading and hypotheses unloading on system. the heat The exchange application of the of tanksthe model with to the a hold:CNG carrier with the specifications of the GASVESSEL project made it possible to highlight the following points, depending on the The time required for the loading process varies between 35 and 51 h, and for the unloading •calculation hypotheses on the heat exchange of the tanks with the hold: process, it varies between 46 and 55 h; • The time required for the loading process varies between 35 and 51 h, and for the unloading The temperature in the pressure vessels varies at the end of the loading process between 78 and • process, it varies between 46 and 55 h; 60 C, while at the end of the unloading process, it varies between 60 and 17 C. In the first • The◦ temperature in the pressure vessels varies at the end of the loading− process− between◦ 78 and case, the relatively high temperature reduces the fill efficiency of the vessels, while in the second 60 °C, while at the end of the unloading process, it varies between −60 and −17 °C. In the first case, the low temperature is worrying for the proper functioning of the compressor, so that a gas case, the relatively high temperature reduces the fill efficiency of the vessels, while in the second heater is required; case, the low temperature is worrying for the proper functioning of the compressor, so that a gas The energy consumption can vary in the range of 150–180 kJ for a unit mass of CNG; • heater is required; The total fuel consumption of the processes varies between 1.14% and 1.45% of the total cargo. •• The energy consumption can vary in the range of 150–180 kJ for a unit mass of CNG; • The total fuel consumption of the processes varies between 1.14% and 1.45% of the total cargo. Appl. Sci. 2020, 10, 7555 21 of 23

The next research goals are performing some experimental tests on the tanks that will allow the model to be ﬁne-tuned. In the future the model will be integrated in a ship overall energy simulation model. As regards the aspects related to safety, the rules and regulatory requirements will be applied currently being issued by the certiﬁcation bodies, such as those by the American Bureau of Shipping [22].

Author Contributions: R.T. conceived the presented idea and was responsible of the research. R.T. and D.M. designed the model, and G.M. designed the computational framework and performed the data presentation. All authors have read and agreed to the published version of the manuscript. Funding: The project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 723030. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature

Acronyms CAPEX CAPital EXpenditure CNG Compressed Natural Gas FPSO Floating Production Storage and Offloading LNG Liquefied Natural Gas NG Natural Gas OPEX Operating EXpenditure Symbols A flow area [m2] C1 Cg/Cv [-] Cd outflow coefficient [-] 3 0.5 Cg gas flow sizing coefficient [lb/h/(psia lb/ft ) ] · Cp constant pressure specific heat [J/kg/K] 3 0.5 Cv fluid flow sizing coefficient [lb/h/(psia lb/ft ) ] · D diameter [m] h enthalpy [J/kg] k specific heat ratio [-] L length [m] m mass [kg] . m mass flow rate [kg/s] n polytropic exponent [-] p pressure [Pa] P power [W] Q heat [J] R gas constant [J/kg/K] S surface [m2] T temperature [K] u internal energy [J/kg] v velocity [m/s] Z compressibility factor [-] α convective heat exchange coefficient [W/m2/K] β compression ratio [-] % density [kg/m3] η efficiency [-] λ Darcy friction factor [-] Subscripts O1 stagnation condition at compressor inlet amb ambient Appl. Sci. 2020, 10, 7555 22 of 23 hex heat exchanger r receiving vessel i inlet, internal o outlet e external p polytropic w cylinder wall

References

1. ExxonMobil. Outlook for Energy: A Perspective to 2040; ExxonMobil Corporation: Irving, TX, USA, 2019. 2. Poli, R.; Scaroni, P. Tranpsorting Natural Gas by Sea, Encyclopaedia of Hydrocarbons; Istituto della Enciclopedia Italiana G. Treccani: Roma, Italy, 2005. 3. Tractebel Engineering, S.A. CNG for Commercialization of Small Volumes of Associated Gas; World Bank: Washington, DC, USA, 2015. 4. GASVESSEL-Compressed Natural Gas Transport System. Available online: www.gasvessel.eu (accessed on 30 August 2020). 5. Kuczynski, S.; Liszka, K.; Laciak, M.; Olijnyk, A.; Szurlej, A. Experimental Investigations and Operational Performane Analysis on Compressed Natural Gas Home Refueling System (CNG-HRS). Energies 2019, 12, 4511. [CrossRef] 6. Kountz, K.J. Modeling the fast fill process in natural gas vehicle storage cylinders. In Proceedings of the Spring National Meeting of the ACS, San Diego, CA, USA, 13–18 March 1994; pp. 462–469. 7. Farzaneh-Gord, M.; Deymi-Dashtebayaz, M.; Rahbari, H.R. Studying effects of storage types on performamce of CNG filling stations. J. Nat. Gas Sci. Eng. 2011, 3, 334–340. [CrossRef] 8. Khamforoush, M.; Moosavi, R.; Hatami, T. Compressed natural gas behaviour in a natural gas vehicle fuel tank during fast filling process: Mathematical modeling, thermodynamic analysis, and optimization. J. Nat. Gas Sci. Eng. 2014, 20, 121–131. [CrossRef] 9. Farzaneh-Gord, M.; Reza-Rahbari, H.; Deymi-Dashtebayaz, M. Effects of Natural Gas Compositions on CNG Fast Filling Process for Buffer Storage System. Oil Gas Sci. Technol. 2014, 69, 319–330. [CrossRef] 10. Farzaneh-Gord, M.; Niazmand, A.; Deymi-Dashtebayaz, M. Effects of natural gas composition on CNG (compressed natural gas) reciprocating compressors performance. Energy 2015, 90, 1152–1162. [CrossRef] 11. Zhang, G.; Brinkerhoff, J.; Li, R.; Forsberg, C.; Sloan, T. Analytical and numerical study on the fast refill of compressed natural gas with active heat removal. J. Nat. Gas Sci. Eng. 2017, 45, 552–564. [CrossRef] 12. Cen, K.; Song, B.; Jia, W.; Li, W.; Han, T. Numerical modeling of the dynamic filling process of high-pressure tankers for marginal gas wells. J. Nat. Gas Sci. Eng. 2019, 72, 103029. [CrossRef] 13. Farzaneh-Gord, M. Real and ideal gas thermodynamic analysis of single reservoir filling process of natural gas vehicle cylinders. J. Theor. Appl. Mech. 2011, 41, 21–36. 14. Mokhatab, S.; Poe, W.A.; Mak, J. Handbook of Natural Gas Transmission and Processing; Elsevier: Amsterdam, The Netherlands, 2015. 15. Setzmann, U.; Wagner, W. A New Equation of State and Tables of Thermodynamic Properties for Methane Covering the Range form the Melting Line to 625 K at Pressure up to 1000 MPa. 6. J. Phys. Chem. Ref. Data 1991, 20, 1061–1151. [CrossRef] 16. Kunz, O.; Wagner, W. The GERG-2008 Wide-Range Equation of State for Natural Gases and Other Mixtures: An Expansion og GERG-2004. J. Chem. Eng. Data 2012, 57, 3032–3091. [CrossRef] 17. Woodfield, P.L.; Monde, M.; Takano, T. Heat transfer Characteristics for Practical Hydrogen Pressure Vessels Being Filled at High Pressure. J. Therm. Sci. Technol. 2008, 3, 241–253. [CrossRef] 18. Aspen Tech Inc. Aspen HYSYS v8.8 Unit Operation Guide; Aspen Tech Inc.: Bedford, MA, USA, 2019. 19. Kirmaen, J.; Niemela, I.; Pyotsia, J.; Simula, M.; Hauhia, M.; Riihilathi, J.; Lempinen, V.; Koukkuluoma, J.; Kanerva, P. Flow Control Manual; Metso Automation Inc.: Helsinki, Finland, 2011. 20. Kim, C.J.; Cho, S.M.; Kim, E.J.; Yoon, K.B. The study on the internal temperature change of type 3 and type 4 composite cylinder during filling. In Proceedings of the 5th International Conference on Hydrogen Safety (ICHS 2013), Brussels, Belgium, 9–11 September 2013. Appl. Sci. 2020, 10, 7555 23 of 23

21. Phillippi, G. Basic thermodynamics of reciprocating compression. In Proceedings of the 45th Turbomachinery & 32nd Pump Symposia, Houston, TX, USA, 12–15 September 2016. 22. American Bureau of Shipping. Guide for Vessels Intended to Carry Compressed Natural Gases in Bulk; American Bureau of Shipping: Houston, TX, USA, 2020.

Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional aﬃliations.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).