chemengineering
Article CFD Design of Hydrogenation Reactor for Transformation of Levulinic Acid to γ-Valerolactone (GVL) by using High Boiling Point Organic Fluids
Alon Davidy
Tomer Ltd., Tel-Aviv 6473424, Israel; [email protected]; Tel.: +972-03-904-9118
Received: 29 December 2018; Accepted: 21 March 2019; Published: 27 March 2019
Abstract: Levulinic acid (LA) has been ranked as one of the “Top 10” building blocks for future bio-refineries as proposed by the US Department of Energy. It is considered one of the most important platform molecules for the production of fine chemicals and fuels based on its compatibility with existing processes, market economics, and industrial ability to serve as a platform for the synthesis of important derivatives. Hydrogenation of LA to produce γ-valerolactone (GVL) is an active area of research due to the potential of GVL to be used as a biofuel in its own right and for its subsequent transformation into hydrocarbon fuels. This paper contains a new design for a simple, cost effective, and safe hydrogenation reactor for the transformation of levulinic acid to γ-valerolactone (GVL) by utilizing high boiling point organic fluid. The hydrogenation reactor is composed of a heating source—organic fluid (called “DOWTHERM A” or “thermex”) and the catalytic reactor. The advantages of high boiling temperature fluids, along with advances in hydrocracking and reforming technologies driven by the oil and gas industries, make the organic concept more suitable and safer (water coming in contact with liquid metal is well understood in the metallurgical industry to be a steam explosion hazard) for heating the hydrogenation reactor. COMSOL multi-physics software version 4.3b was applied in this work and simultaneously solves the continuity, Navier-Stokes (fluid flow), energy (heat transfer), and diffusion with chemical reaction kinetics equations. It was shown that the heat flux supplied by the DOWTHERM A organic fluid could provide the necessary heat flux required for maintaining the hydrogenation process. It was found that the mass fractions of hydrogen and levulinic acid decreased along the reactor axis. The GVL mass fraction increased along the reactor axis.
Keywords: bio-refinery; levulinic acid (LA); hydrogenation reactor; γ-valerolactone (GVL); alternative fuels; CFD; diphenyl mixture; Navier-Stokes equation; energy equation; process simulation; catalysis; kinetics
1. Introduction The utilization of biomass for the production of fuel and chemicals has become an important research topic because of the growing concerns regarding the depletion of fossil carbon reserves and the environmental impact (such as air pollution) of our continued dependence on these resources [1]. Levulinic acid (LA) is ranked as one of the “Top 10” building blocks for future bio-refineries as proposed by the US Department of Energy [2]. It is considered as one of the most important platform molecules for the production of fine chemicals and fuels [3] based on its compatibility with existing processes, market economics, and industrial ability to serve as a platform for the synthesis of important derivatives. Hydrogenation of LA to produce γ-valerolactone (GVL) is an active area of research due to the potential of GVL to be used as a biofuel in its subsequent transformation into hydrocarbon fuels [4].
ChemEngineering 2019, 3, 32; doi:10.3390/chemengineering3020032 www.mdpi.com/journal/chemengineering ChemEngineering 2019, 3, 32 2 of 14 ChemEngineering 2018, 2, x FOR PEER REVIEW 2 of 14
GVL is considered as a sustainable liquid since it is renewable [5]. It has several very attractive GVL is considered as a sustainable liquid since it is renewable [5]. It has several very attractive physical and chemical properties [6]. Its vapor pressure is remarkably low, even at elevated physical and chemical properties [6]. Its vapor pressure is remarkably low, even at elevated temperatures and it does not hydrolyze at neutral pH. GVL is a safe material for large-scale use and temperatures and it does not hydrolyze at neutral pH. GVL is a safe material for large-scale use can be utilized for the production of energy by adding it to gasoline [5]. Bereczky et al. showed that and can be utilized for the production of energy by adding it to gasoline [5]. Bereczky et al. showed GVL significantly reduced the exhaust concentration of CO, unburned fuel, and smoke. The smoke that GVL significantly reduced the exhaust concentration of CO, unburned fuel, and smoke. The smoke reduction was particularly notable in light of the very recent suggestion that black carbon was the reduction was particularly notable in light of the very recent suggestion that black carbon was the second most important greenhouse gas in the atmosphere next to carbon dioxide [7]. Licursi et al. second most important greenhouse gas in the atmosphere next to carbon dioxide [7]. Licursi et al. [8] [8] have studied and optimized cascade strategy for the catalytic valorization of levulinic acid. Their have studied and optimized cascade strategy for the catalytic valorization of levulinic acid. Their study study has been carried out applying water as the only reaction solvent and heterogeneous has been carried out applying water as the only reaction solvent and heterogeneous commercial commercial catalytic systems, which are more economical, available, and reproducible. Figure 1 catalytic systems, which are more economical, available, and reproducible. Figure1 shows the shows the conversion processes of LA to a GVL and GVL to 2-Butanol and 2-Pentanol biofuels. conversion processes of LA to a GVL and GVL to 2-Butanol and 2-Pentanol biofuels.
Figure 1. Conversion of levulinic acid to GVL and levulinic acid to 2-Butanol and 2-Pentanol. Figure 1. Conversion of levulinic acid to GVL and levulinic acid to 2-Butanol and 2-Pentanol. 1.1. GVL Production Process Catalysts 1.1.The GVL hydrogenation Production Process of Catalysts LA to GVL has been described by using both homogeneous and heterogeneousThe hydrogenation catalysts [9]. of Filiz LA et to al. GVL [10] studiedhas been the described catalytic hydrogenation by using both of homogeneous LA over zirconia and supportedheterogeneous ruthenium catalysts catalysts. [9]. Filiz They et al. prepared[10] studie fourd the different catalytic Ru/ZrO hydrogenation2 catalysts of LA with over different zirconia pre-treatmentssupported ruthenium and used catalysts. different zirconiumThey prepared supports. four Theydifferent discovered Ru/ZrO that2 catalysts one of thewith catalysts different pre-treatments and used different zirconium supports. They discovered that one of the catalysts ChemEngineering 2018, 2, x FOR PEER REVIEW 3 of 14 ChemEngineering 2019, 3, 32 3 of 14 produced a yield of more than 99% of GVL under mild conditions. This catalyst was also robust producedand could abe yield recycled of more at thanleast 99%four oftimes GVL without under mild any conditions.loss in activity This or catalyst selectivity. was also It was robust shown and thatcould the be activity recycled can at leastbe attributed four times to withoutthe presence any loss of small in activity ruthenium or selectivity. particles It together was shown with that acidic the sitesactivity on canthe catalyst be attributed [10]. Figure to the presence2 shows ofthe small schema rutheniumtic of the particles hydrogenation together reactor with acidic of LA sites to GVL. on the catalyst [10]. Figure2 shows the schematic of the hydrogenation reactor of LA to GVL.
Figure 2. Schematic of the conversion process of levulinic acid (LA) to γ-valerolactone (GVL). Figure 2. Schematic of the conversion process of levulinic acid (LA) to γ-valerolactone (GVL). Ftouni et al. [11] compared the stability of different supported Ru-based catalysts under typical Ftouni et al. [11] compared the stability of different supported Ru-based catalysts under typical LA hydrogenation conditions in dioxane. It was shown that Ru/ZrO2 performed better than Ru/C LA hydrogenation conditions in dioxane. It was shown that Ru/ZrO2 performed better than Ru/C and Ru/TiO2. The former catalyst materials displayed high activity, selectivity, and stability upon and Ru/TiO2. The former catalyst materials displayed high activity, selectivity, and stability upon repetitive recycling. The hydrogenation reaction was studied at the hydrogen pressure of 30 bar and repetitive recycling. The hydrogenation reaction was studied at the hydrogen pressure of 30 bar temperature of 423 K in dioxane as the solvent. All catalysts showed excellent yields of GVL when and temperature of 423 K in dioxane as the solvent. All catalysts showed excellent yields of GVL used fresh, but only the Ru/ZrO2 catalyst could maintain these high yields upon multiple recycling. when used fresh, but only the Ru/ZrO2 catalyst could maintain these high yields upon multiple The widely used Ru/TiO2 catalyst already showed quick signs of deactivation after the first catalytic recycling. The widely used Ru/TiO2 catalyst already showed quick signs of deactivation after the test. The partial deactivation was due to the partial coverage of the Ru nanoparticles. In contrast, the first catalytic test. The partial deactivation was due to the partial coverage of the Ru nanoparticles. zirconia support displayed high morphological and structural stability even after five recycling tests. In contrast, the zirconia support displayed high morphological and structural stability even after In the fresh Ru/ZrO2 catalyst, Ru was found to be fully atomically dispersed on the fresh catalyst even five recycling tests. In the fresh Ru/ZrO2 catalyst, Ru was found to be fully atomically dispersed on at 1 wt % Ru loading, with some genesis of Ru nanoparticles being observed upon recycling. Further the fresh catalyst even at 1 wt % Ru loading, with some genesis of Ru nanoparticles being observed studies with the Ru/ZrO2 catalyst showed that dioxane could be readily replaced by more benign upon recycling. Further studies with the Ru/ZrO2 catalyst showed that dioxane could be readily solvents including GVL itself. The addition of water promotes the selective hydrogenation reaction. replaced by more benign solvents including GVL itself. The addition of water promotes the selective1.2. CFD hydrogenation Simulations of Biofuel reaction. Production
1.2. CFDWensel Simulations et al. [12 ]of performed Biofuel Production a computational fluid dynamics (CFD) simulation on the bio-refinery process for succinic acid and co-product. The finite volume method (FVM) technique was employed in theWensel CFD simulation. et al. [12] Theperformed simulation a computational included kinetic, fluid stoichiometric, dynamics (CFD) mass, andsimulation energy balanceon the bio-refineryequations in process order to for simulate succinic the acid effects and of co-product. inlet temperature The finite impeller volume speed, method diameter, (FVM) and technique spacing, wasinlet employed temperature, in andthe CFD fermentor simulati volumeon. The on thesimulation fermentor included cooling kinetic, jacket heat stoichiometric, transfer area. mass, Predicted and energydissolved balance carbon equations dioxide concentrations in order to simulate in the fermentor the effects had goodof inlet agreement temperature with thoseimpeller reported speed, in diameter,the literature. and Thespacing, effects inlet of the temperature, microfiltration and recirculation fermentor volume rate, microfiltration on the fermentor stage cooling numbers, jacket and heatabsorber transfer sorbent area. particle Predicted diameter dissolved on the dimensionalcarbon dioxide requirements concentrations and power in the consumption fermentor had have good also agreementbeen considered. with those Yields reported and estimated in the literature. volume and Th areae effects requirements of the microfiltration for the units recirculation of operation wererate, microfiltration stage numbers, and absorber sorbent particle diameter on the dimensional ChemEngineering 2018, 2, x FOR PEER REVIEW 4 of 14 ChemEngineering 2019, 3, 32 4 of 14 requirements and power consumption have also been considered. Yields and estimated volume and area requirements for the units of operation were obtained for the baseline process. Their work obtainedrepresents for the the first baseline reported process. indust Theirrial-scale work represents bio-succinic the acid first reportedprocess model. industrial-scale Gorshkova bio-succinic et al. [13] acidperformed process a model. three phase Gorshkova CFD etmodel al. [13 for] performed trickle bed a threereactors. phase Their CFD model model was for extended trickle bed to reactors. include Theirreactions, model mass was transfer, extended and to includeheat transfer. reactions, The massstationary transfer, gas andand heatliquid transfer. inside the The porous stationary particles gas andwere liquid modeled inside separately the porous from particles the bulk were gas modeled and liquid separately phases from flowing the bulk outside gas andthe liquidparticles phases with flowingconvective outside and thediffusive particles mass with transfer convective between and diffusivethe inner mass and outer transfer fluids. between It was the assumed inner and that outer the fluids.catalytic It was reactions assumed took that place the inside catalytic the reactions catalyst pa tookrticles. place The inside process the catalystmodeled particles. in this work The processwas the modeledhydrogenation in this workof octane was the(C8H hydrogenation16) in a Ni/Al2 ofO3 octanereactor. (C The8H16 reaction) in a Ni/Al is highly2O3 reactor. exothermic, The reaction resulting is highlyin theexothermic, evaporation resulting and condensation in the evaporation of the andcomponents. condensation All ofsub the models components. were implemented All sub models in wereFluent implemented software (version in Fluent 12.1). software Numerical (version tests 12.1).were Numericalcarried out teststo show were that carried the CFD out tomodel show allows that thefor CFDthe investigation model allows of for local the variations investigation in the of reactor, local variations which are in caused, the reactor, for example, which are by caused, bed drying for example,or the effects by bed of irregular drying or liquid the effects feed. of irregular liquid feed. TheThe structure structure of of this this paper paper is as is follows: as follows: The thermodynamicThe thermodynamic properties properties of high of temperature high temperature fluids arefluids described are described in detail inin Sectiondetail in2.1 Section. Section 2.1. 2.2 describesSection 2.2 the describes properties the and properties benefits of and DOWTHERM benefits of ADOWTHERM organic fluid. A Section organic 2.3 fluid. describes Section the multiphysics2.3 describes analysesthe multiphysics of hydrogenation analyses reactor.of hydrogenation Section3 presentsreactor. theSection COMSOL 3 presents (version the COMSOL 4.3b) results (version for the 4.3b) temperature results for andthe temperature conversion. and The conversion. described workThe described contains a work new designcontains for a anew hydrogenation design for a reactor hydrogenation for the transformation reactor for the of transformation levulinic acid toof γlevulinic-valerolactone acid to (GVL) γ-valerolactone by using high (GVL) boiling by pointusing organichigh boiling fluids. point This organic is most likelyfluids. the This first is timemost thatlikely organic the first fluid time has beenthat organic proposed fluid to supply has been the requiredproposed heat to neededsupply tothe sustain required the hydrogenationheat needed to reactionsustain forthe producinghydrogenation GVL. reaction for producing GVL.
1.3.1.3. Critical Critical Heat Heat Flux Flux of of Water Water ThereThere are are inherent inherent benefits benefits of of two-phase two-phase cooling. cooling. A A poorly poorly designed designed two-phase two-phase coolingcooling system,system, however,however, can can fail fail catastrophically catastrophically due due to to critical critical heat heat flux. flux. Figure Figure3 3shows shows the the transition transition from from natural natural convectionconvection to to nucleate nucleate boiling boiling (A), (A), and and critical critical heat heat flux flux (B) (B) for for water water [ 14[14,15].,15]. Critical Critical heat heat flux flux (B) (B) is is markedmarked by by an an excessive excessive rise rise in in device device temperature temperature thatthat cancan resultresult inin the the loss loss of of heat heat transfer transfer to to the the hydrogenationhydrogenation reactor reactor external external surface surface (D). (D).
Figure 3. A typical boiling curve. Figure 3. A typical boiling curve. At the point of dry-out, the depletion of the liquid layer leads to a significant decrease in heat transfer at the reactor surface, which will result in a decrease of heat flux to the hydrogenation reactor ChemEngineering 2019, 3, 32 5 of 14 surface. However, apart from the dry-out condition, a similar decrease in heat transfer can also occur at lower vapor qualities during the subcooled or saturated nucleate boiling regimes. This condition is achieved at high heat fluxes when the rapid nucleation of bubbles results in the formation of dry areas that start to cover more and more of the surface. Finally, a vapor film separates the liquid from the reactor surface and film boiling has been initiated. This causes a drastic reduction in heat transfer since the vapor has a much lower thermal conductivity than the liquid. The condition is referred to as the departure from nucleate boiling (DNB) where the point of maximum heat flux is called the critical heat flux (CHF). The organic fluid has inherent benefits over water as its high boiling temperature improves the heat transfer to the hydrogenation reactor surface (the heat flux is more effective at the liquid phase), and has low viscosity (it minimizes the pressure losses).
2. Materials and Methods
2.1. High Temperature Fluids Table1 contains a list of heat transfer fluids that boil at temperatures exceeding 100 ◦C. Most of these fluids are flammable, extremely corrosive, explosive, or harmful (or all four). Three of these fluids may be suitable as a high-temperature coolant such as DOWTHERM (boiling point 258 ◦C), ethylene glycol (boiling point 197 ◦C), and propylene glycol (boiling point 187 ◦C). DOWTHERM A (diphenyl mixture) was developed by the Dow Chemical Company. Propylene glycol and ethylene glycol are organic compounds that are widely used as automotive antifreezes [16].
Table 1. Boiling points of various fluids at 1 atm [16].
Fluid Boiling Point (◦C) Acetic acid anhydride 139 Alcohol 97–117 Aniline 184 Butyric acid n 162 Carbonic Acid 182 Dowtherm 258 Glycerin 290 Ethylene bromide 131 Ethylene glycol 197 Iodine 184 Jet Fuel 163 Kerosene 150–300 Mercury 359 Napthalene 218 Nitric Acid 120 Nitrobenzene 210 Nonane-n 150 Octane-n 125 Olive oil 300 Petroleum 210 Propionic Acid 141 Propylene Glycol 187 Toluene 110 Turpentine 160 Xylene-o 142
2.2. Properties and Benefits of Utilization of an Organic Fluid
DOWTHERM A (also known as “thermex”) is a mixture composed of biphenyl (C12H10) and diphenyl oxide (C12H10O). These two compounds have practically the same vapor pressures, therefore, this mixture can be considered as a single compound. Diphenyl mixture fluid may be applied in systems employing either liquid phase or vapor phase heating. Its normal application range is 60 ◦F ChemEngineering 2018, 2, x FOR PEER REVIEW 6 of 14
2.2. Properties and Benefits of Utilization of an Organic Fluid
DOWTHERM A (also known as “thermex”) is a mixture composed of biphenyl (C12H10) and diphenyl oxide (C12H10O). These two compounds have practically the same vapor pressures, therefore, this mixture can be considered as a single compound. Diphenyl mixture fluid may be appliedChemEngineering in systems2019, 3 ,employing 32 either liquid phase or vapor phase heating. Its normal application6 of 14 range is 60 °F to 750 °F (15 °C to 400 °C), and its pressure range is from atmospheric to 152.5 psig (10.6to 750 bar)◦F [17]. (15 ◦ TheC to diphenyl 400 ◦C), andmixture its pressure possesses range unsurpassed is from atmospheric thermal stability to 152.5 at temperatures psig (10.6 bar) of [75017]. °FThe (400 diphenyl °C). The mixture maximum possesses recommended unsurpassed film thermaltemperat stabilityure is 800 at °F temperatures (425 °C). It has of 750 practically◦F (400 ◦noC). corrosiveThe maximum action recommendedon the common film structural temperature materials is 800 ◦suchF (425 as◦ C).steel, It hascast practically iron, copper no corrosivebrass, bronze, action etc.on theEven common at the structuralhigh temperatures materials suchinvolved, as steel, the cast equipment iron, copper usually brass, exhibits bronze, excellent etc. Even service at the highlife. Originaltemperatures equipment involved, in many the equipment hydraulic usually systems exhibits prolongs excellent after service30 years life. of Original continuous equipment service. in Stainlessmany hydraulic steel and systems low prolongsalloy steels, after Monel 30 years alloys, of continuous etc. are service.also employed Stainless in steel equipment and low alloyand instruments.steels, Monel This alloys, mixture etc. are has also a employed freezing inpoint equipment of 53.6 and°F (12 instruments. °C) and its This low mixture viscosity has aprevents freezing start-uppoint of system 53.6 ◦F problems. (12 ◦C) and It itsis considered low viscosity as preventsa combustible start-up fluid, system however, problems. its vapors It is considered do not pose as a a seriouscombustible flammability fluid, however, hazard its at vapors room dotemperature, not pose a seriousbecause flammability its saturation hazard concentration at room temperature, is far less thanbecause the itslower saturation flammability concentration limit [17]. is far It has less a than relatively the lower high flammability flash point limitof 236 [17 °F]. (113 It has °C) a relatively(SETA), ahigh fire flashpoint pointof 245 of °F 236 (118◦F °C) (113 (C.O.C.),◦C) (SETA), and a an fire au pointto-ignition of 245 ◦temperatureF (118 ◦C) (C.O.C.), of 1110 and°F (599 an auto-ignition°C) (ASTM, E659-78).temperature The oflower 1110 flammable◦F (599 ◦C) limit (ASTM, is 0.6% E659-78). (volume) The at lower 175 °C, flammable while the limit upper is 0.6% limit (volume) is 6.8% at at 190175 °C.◦C, Ignited while the diphenyl upper limit mixture is 6.8% burns at 190 slowly◦C. Ignited and is diphenyl practically mixture explosion burns slowlyproof as and experience is practically of operatingexplosion diphenyl proof as experienceheating installations of operating has diphenylproven that heating there installationsis practically has no provenfire hazard that from there it is withpractically proper no equipment fire hazard and from operation. it with proper Abagnale equipment et al. [18] and discussed operation. the Abagnale usage of et DOWTHERM al. [18] discussed A asthe a working usage of fluid DOWTHERM in the Organic A as Rankine a working Cycle fluid (ORC). in the It Organic has been Rankine mentioned Cycle in (ORC).[19] that It this has fluid been hasmentioned been successfully in [19] that applied this fluid in hasheat been pipes. successfully The use appliedof an organic in heat fluid pipes. enables The use operation of an organic near atmosphericfluid enables pressure operation and near the atmospheric use of plain pressurecarbon steel and thefor the use piping of plain system. carbon It steel was for proposed the piping to applysystem. this It fluid was proposedas coolant to for apply UAV this engine fluid [20]. as coolant for UAV engine [20].
2.3.2.3. Multiphysics Multiphysics Analyses Analyses of of the the Hydrogenation Hydrogenation Reactor Reactor ThisThis section section deals deals with with the the numerical numerical analysis analysis of of the the reactor. reactor. Figure Figure 4 shows the geometry of the hydrogenationhydrogenation reactor. reactor.
FigureFigure 4. 4. 3D3D plot plot of of hydrogenation hydrogenation reactor. reactor. The hydrogenation reactor is made of the catalyst bed material. It was assumed that the radius of The hydrogenation reactor is made of the catalyst bed material. It was assumed that the radius the reactor was 0.05 m and the height of the reactor was 0.4 m. The hydrogenation chemical reaction of the reactor was 0.05 m and the height of the reactor was 0.4 m. The hydrogenation chemical occurred inside the catalytic reactor where the heat was supplied through the organic fluid to drive reaction occurred inside the catalytic reactor where the heat was supplied through the organic fluid the endothermal reaction system. The heat flux was supplied at the inner and outer radius of the reactor. Levulinc acid and hydrogen were mixed in stoichiometric amounts and entered through the inlet of the hydrogenation reactor. This model investigates the hydrogenation of LA to GVL. COMSOL multi-physics software version 4.3b was applied in this work and simultaneously solved the continuity, Navier-Stokes (fluid flow), energy (heat transfer), and diffusion with chemical reaction ChemEngineering 2019, 3, 32 7 of 14 kinetics transport equations. The model kinetics are described in detail in Section 2.3.1. Section 2.3.2 describes the continuity and fluid flow. Sections 2.3.3 and 2.3.4 describe the energy transport and diffusion equations.
2.3.1. Model Kinetics Inside the hydrogenation reactor, levulinic acid and hydrogen react together to form GVL and water [21]: Levulinic Acid + H2 → GVL + H2O (1) The rate constant of the levulinic reaction is temperature dependent according to [21]:
E k = A exp − (2) RT where A represents the frequency factor. It was assumed that its value was 6.2 × 103 (1/s); E is the activation energy with a value of 48.0 (kJ/mole) [18]; R is the gas constant (8.3143 J/(mole·K)); and T represents the temperature (K). It should be noted that this reaction is endothermic (consumes heat). The production rate (mole/(m3·s)) of the GVL is given by:
RGVL = kcLA (3)
2.3.2. Fluid Flow and Continuity Equations Since the hydrogenation reaction occurs at a hydrogen pressure of 30 bar [11], it was assumed that the flow of the hydrogen and levulinic acid inside the hydrogenation reactor was laminar. The flow was incompressible. The flow of species through the bed is described by Navier-Stokes equation [22]:
∂u ρ − ∇ · η ∇u + (∇u)T + ρu · ∇u + ∇p = F (4) ∂t Since the flow is incompressible, the density of fluid is constant. Thus, the continuity equation for the reacting species is [22]: ∇ · u = 0 (5) where η denotes the dynamic viscosity of the fluid (Pa·s); u the velocity (m/s); ρ the density of the fluid (kg/m3); p the pressure (Pa); and F is a body force term (N/m3).
2.3.3. Energy Transport Equation The energy balance equation applied to the reactor domain considers heat transfer through convection and conduction [21]:
∂T ρc + ρc u · ∇T − ∇ · (−k∇T) = Q (6) p ∂t p
In Equation (6), cp denotes the specific heat capacity (J/(kg·K)); k is the thermal conductivity of the fluid in (W/(m·K)); and Q is a sink or source term (W/m3). The last term is calculated by the following equation: Q = RLA · ∆H (7) where ∆H is the heat of reaction in (J/mole). The value of the heat reaction was taken from [23]. ChemEngineering 2018, 2, x FOR PEER REVIEW 8 of 14
ChemEngineering 2019, 3, 32 8 of 14 where ΔH is the heat of reaction in (J/mole). The value of the heat reaction was taken from [23].
2.3.42.3.4. Diffusion Diffusion Transport Transport Equation Equation The mass mass transfer transfer in in the the reactor reactor domain domain is give is givenn by bythe theconvection convection and anddiffusion diffusion equation. equation. The diffusionThe diffusion equation equation for levulinc for levulinc acid (LA) acid (LA)is written is written in Equation in Equation (8): (8): ∂c ∂c LA +∇⋅() −D ∇cc +u =− R (8) LA∂t+ ∇ · (−D LA∇c LA+ c LAu) = −R LA (8) ∂t LA LA LA LA where DLA denotes its diffusion coefficient of LA in the solution in (m2/s) and RLA denotes the where DLA denotes its diffusion coefficient of LA in the solution in (m2/s) and RLA denotes the reaction term in (mole/(m3·s)). Equation (8) assumes that the species LA is diluted in a solvent. The reaction term in (mole/(m3·s)). Equation (8) assumes that the species LA is diluted in a solvent. diffusion equation for the gamma valerolactone (GVL) is written in Equation (9): The diffusion equation for the gamma valerolactone (GVL) is written in Equation (9): ∂c GVL +∇⋅() −D ∇cc +u = R (9) ∂c ∂t GVL GVL GVL GVL GVL + ∇ · (−D ∇c + c u) = R (9) ∂t GVL GVL GVL GVL where DGVL denotes its diffusion coefficient of GVL in the solution in (m2/s), and RGVL denotes the wherereaction DGVL term denotesin (mole/(m its diffusion3·s)). The coefficientconversion of of GVL LA inwas the calculated solution inaccording (m2/s), andto Equation RGVL denotes (10). the reaction term in (mole/(m3·s)). The conversion of LA− was calculated according to Equation (10). ccLA,, in LA out X = (10) LA cLA,in − cLA,out XLA = cLA, in (10) cLA,in where cLA, in denotes the LA concentration at the entrance to the reactor and cLA, out denotes the LA where cLA,in denotes the LA concentration at the entrance to the reactor and cLA,out denotes the LA concentration at the exit of the reactor.
3. Results This sectionsection presents presents the the model model results results for the for temperature the temperature and concentration and concentration fields. A parametricfields. A parametricstudy was performedstudy was toperformed analyze theto influenceanalyze th ofe theinfluence input temperatureof the input on temperature the hydrogenation on the hydrogenationreactor performance. reactor performance.
3.1. Model Validation Several works works have have dealt dealt with with burners burners as asa heat a heat source source for maintaining for maintaining the reaction the reaction [24,25]. [24 This,25]. workThis workfocused focused on the onorganic the organic fluid heat fluid source. heat The source. numerical The numericalresults for a results heat flux for of a 2730 heat (W/m²) flux of 2 are2730 presented (W/m ) arein this presented section. in It this was section. assumed It wasthat assumedthe temperature that the of temperature the reactants of theentering reactants the ◦ hydrogenationentering the hydrogenation reactor was reactor 200 °C. was Figure 200 C. Figure5 shows5 shows the the3D 3Dtemperature temperature field field inside inside thethe hydrogenation reactor.
Figure 5. 3D3D plot plot of of the the hydrogenatio hydrogenationn reactor temperature field. field. ChemEngineering 20192018,, 32,, 32x FOR PEER REVIEW 9 of 14
As seen in Figure 5, the temperature at the bottom section of the reformer is higher than the As seen in Figure5, the temperature at the bottom section of the reformer is higher than the temperature at the upper side. This is due to two reasons: first, the endothermic reactions absorb the temperature at the upper side. This is due to two reasons: first, the endothermic reactions absorb the heat, and second, the thermal conductivity of the solution (levulinic acid and hydrogen) has a lower heat, and second, the thermal conductivity of the solution (levulinic acid and hydrogen) has a lower value. The temperature at the internal and external surfaces of the reactor was much higher than the value. The temperature at the internal and external surfaces of the reactor was much higher than the temperature inside the bulk of the reactor, which is because they are exposed to heat supplied by temperature inside the bulk of the reactor, which is because they are exposed to heat supplied by organic fluid. The lower thermal conductivity led to a higher temperature gradient across the radial organic fluid. The lower thermal conductivity led to a higher temperature gradient across the radial axis axis of the reactor. Figure 6 shows the 3D levulinic acid concentration field inside the hydrogenation of the reactor. Figure6 shows the 3D levulinic acid concentration field inside the hydrogenation reactor. reactor.
Figure 6. 3D plot of the levulinic acid (LA) concentration field.field.
Figure 66 indicatesindicates that that the the LA LA almost almost converte convertedd completely completely (100% (100% conversion). conversion). A similar A similar value valuehas been has beenreported reported in [26–28]. in [26– 28Figure]. Figure 7 show7 showss the the 3D 3D GVL GVL concentration concentration field field insideinside thethe hydrogenation reactor.
Figure 7. 3D3D plot plot of of the γ-Valerolactone-Valerolactone (GVL)(GVL) concentration field.field.
Figure 88 indicatesindicates that the the LA LA al almostmost transformed transformed completely completely to toγ-valerolactoneγ-valerolactone (GVL). (GVL). The Theconcentration concentration of GVL of GVLat the at top the of top the of hydr the hydrogenationogenation reactor reactor had the had highest the highest values. values.
3.2. Calculated Results for Lower Temperatures ChemEngineering 2018, 2, x FOR PEER REVIEW 10 of 14
The numerical results for the heat flux input of 2730 (W/m²) are presented in this section. It was ChemEngineeringassumed that2019 the, 3temperature, 32 of the reactants entering to the hydrogenation reactor was 10010 of°C. 14 FigureChemEngineering 8 shows 2018 the, 2 ,3D x FOR temperature PEER REVIEW field inside the hydrogenation reactor. 10 of 14
The numerical results for the heat flux input of 2730 (W/m²) are presented in this section. It was assumed that the temperature of the reactants entering to the hydrogenation reactor was 100 °C. Figure 8 shows the 3D temperature field inside the hydrogenation reactor.
Figure 8. 3D3D plot plot of of the hydrogenatio hydrogenationn reactor temperature field.field.
3.2. Calculated Results for Lower Temperatures As seen from Figure 8, the temperature at the bottom section of the reformer was higher than the temperatureThe numerical at the results upper for side. the heat This flux is due input to tw ofo 2730 reasons: (W/m first,2) are the presented endothermic in this reactions section. absorb It was Figure 8. 3D plot of the hydrogenation reactor temperature field. theassumed heat, and that second, the temperature the thermal of theconductivity reactants of entering the solution to the (levulinic hydrogenation acid and reactor hydrogen) was 100 has◦ C.a lower value. The temperature at the internal and external surfaces of the reactor was much higher FigureAs8 showsseen from the 3DFigure temperature 8, the temperature field inside at the the hydrogenation bottom section reactor. of the reformer was higher than than the temperature inside bulk of the reactor because they were exposed to heat supplied by the the temperatureAs seen from atFigure the upper8, the side. temperature This is due at theto tw bottomo reasons: section first, of the endothermic reformer was reactions higher than absorb the organic fluid. The lower thermal conductivity led to a higher temperature gradient across the radial temperaturethe heat, and at second, the upper the side.thermal This conductivity is due to two of reasons: the solution first, the(levulinic endothermic acid and reactions hydrogen) absorb has the a axis of the reactor. Figure 9 shows the 3D levulinic acid concentration field inside the hydrogenation heat,lower and value. second, The temperature the thermalconductivity at the internal of theandsolution external(levulinic surfaces of acid the and reactor hydrogen) was much has a higher lower reactor. value.than the The temperature temperature inside at the bulk internal of the and reactor external because surfaces they of were the reactor exposed was to muchheat supplied higher than by the temperatureorganic fluid. inside The lower bulk thermal of the reactor conductivity because led they to a were higher exposed temperature to heat gradient supplied across by the the organic radial fluid.axis of The the lowerreactor. thermal Figure conductivity 9 shows the led3D tolevulinic a higher acid temperature concentration gradient field across inside the the radial hydrogenation axis of the reactor. Figure9 shows the 3D levulinic acid concentration field inside the hydrogenation reactor.
Figure 9. 3D plot of the levulinic acid (LA) concentration field.
Figure 9 indicates that the conversion of LA to GVL was about 32.5%. 4. Discussion Figure 9. 3D plot of the levulinic acid (LA) concentration field.field.
TheFigure application9 9 indicates indicates of thatthat biomass thethe conversionconversion for the production of of LA LA to to GVL GVLof fuel was was and about about chemicals 32.5%. 32.5%. has become an important research topic due to growing concerns about the finite nature of fossil carbon reserves and the environmental4. Discussion impact of our continued dependence on these resources. Levulinic acid (LA) is The application of biomass for the production of fuel and chemicals has become an important research topic due to growing concerns about the finite nature of fossil carbon reserves and the environmental impact of our continued dependence on these resources. Levulinic acid (LA) is ChemEngineering 2019, 3, 32 11 of 14
4. Discussion The application of biomass for the production of fuel and chemicals has become an important research topic due to growing concerns about the finite nature of fossil carbon reserves and the environmental impact of our continued dependence on these resources. Levulinic acid (LA) is ranked as one of the “Top 10” building blocks for future bio-refineries as proposed by the US Department of Energy. It is one of the most important platform molecules for the production of fine chemicals and fuels based on its compatibility with existing processes, market economics, and industrial ability to serve as a platform for the synthesis of important derivatives. Hydrogenation of LA to produce γ-valerolactone (GVL) is an active area of research due to the potential of GVL to be used as a biofuel in its subsequent transformation into hydrocarbon fuels. This paper contains a new design for a simple, cost effective, and safe hydrogenation reactor for the transformation of levulinic acid to γ-valerolactone (GVL) by utilizing high boiling point organic fluid. The hydrogenation reactor was composed of the heating source, an organic fluid (called “DOWTHERM A” or “Thermex”) and the catalytic reactor. The advantages of high boiling temperature fluids, along with advances in hydrocracking and reforming technologies driven by the oil and gas industries, make the organic concept even more suitable and safe (water coming in contact with liquid metal can cause steam explosion) for heating the hydrogenation reactor. DOWTHERM A is a mixture composed of biphenyl (C12H10) and diphenyl oxide (C12H10O). These two compounds have practically the same vapor pressures, therefore this mixture can be considered as a single compound. The diphenyl mixture fluid can be applied in systems employing either liquid phase or vapor phase heating. Its normal application range is 60 ◦F to 750 ◦F (15 ◦C to 400 ◦C), and its pressure range is from atmospheric to 152.5 psig (10.6 bar) [11]. The diphenyl mixture possesses unsurpassed thermal stability at temperatures of 750 ◦F (400 ◦C). The maximum recommended film temperature is 800 ◦F (425 ◦C) and it has practically no corrosive action on common structural materials such as steel, cast iron, copper brass, bronze, etc. Even at the high temperatures involved, the equipment usually exhibits excellent service life. Original equipment in many hydraulic systems can be prolonged even after 30 years of continuous service. Stainless steel and low alloy steels, Monel alloy, etc. have also been employed in equipment and instruments. This mixture has a freezing point of 53.6 ◦F (12 ◦C) and its low viscosity prevents start-up system problems. It is considered as a combustible fluid, however, its vapors do not pose a serious flammability hazard at room temperature because its saturation concentration is far less than the lower flammability limit [11]. It has a relatively high flash point of 236 ◦F (113 ◦C) (SETA), a fire point of 245 ◦F (118 ◦C) (C.O.C.), and an auto-ignition temperature of 1110 ◦F (599 ◦C) (ASTM, E659-78). The lower flammable limit is 0.6% (volume) at 175 ◦C, while the upper limit is 6.8% at 190 ◦C. The ignited diphenyl mixture burns slowly and is practically explosion proof as experience of operating diphenyl heating installations has proven that there is practically no fire hazard from it with proper equipment and operation. DOWTHERM A has been considered as a working fluid in the Organic Rankine Cycle (ORC) and has been successfully applied in heat pipes. The usage of organic fluid enables operation near atmospheric pressure and the use of plain carbon steel for the piping system. COMSOL multi-physics software version 4.3b was applied in this work and simultaneously solves the fluid flow, heat transfer, and diffusion with chemical reaction kinetics equations. This paper presented the model results for the temperature and concentration fields. A parametric study was performed to analyze the influence of the input temperature on performance of the hydrogenation reactor. The numerical results for a heat flux of 2730 (W/m2) were presented in this paper. It was assumed that the temperature of the reactants entering the hydrogenation reactor was 200 ◦C. The results indicate that the LA almost converted completely (100% conversion) where similar values have been reported in the literature. It was found that the temperature at the bottom section of the reformer was higher than the temperature at the upper side due to two reasons: first, the endothermic reactions absorb the heat, and second, the thermal conductivity of the solution (levulinic acid and hydrogen) has a lower value. The temperature at the internal and external surfaces of the reactor was much higher than the temperature inside the bulk of the reactor because they were exposed to heat ChemEngineering 2019, 3, 32 12 of 14 supplied by the organic fluid. The lower thermal conductivity led to a higher temperature gradient ChemEngineeringacross the radial 2018,axis 2, x FOR ofthe PEER reactor. REVIEW It was shown that the heat flux supplied by the DOWTHERM 12 of 14 A organic fluid could provide the necessary heat flux required for maintaining the hydrogenation levulinic acid decreased along the reactor axis and the GVL mass fraction increased along the reactor process. It was found that the mass fractions of the hydrogen and levulinic acid decreased along the axis. reactor axis and the GVL mass fraction increased along the reactor axis.
5.5. Conclusions Conclusions AA new new design for aa hydrogenationhydrogenation reactorreactor to to transform transform levulinic levulinic acid acid to toγ -valerolactoneγ-valerolactone (GVL) (GVL) by byusing using high high boiling boiling point point organic organic fluids isfluids proposed is proposed in this section. in this The section. heat carried The heat by the carried DOWTHERM by the DOWTHERMA and supplied A and to the supplied hydrogenation to the hydrogenation reactor can be reactor produced can be by produced using solar by energyusing solar or by energy using orother by heatusing sources other heat such sources as a furnace such burneras a furnace or solar burner power or station. solar Thepower Hydrogen station. requiredThe Hydrogen for the requiredhydrogenation for the reaction hydrogenation can be produced reaction by can using be Methane produced Steam by Reformingusing Methane System. Steam Figure Reforming 10 shows System.proposed Figure GVL 10 production shows prop system.osed GVL production system.
Figure 10. Proposed γ-Valerolactone (GVL) production system. Figure 10. Proposed γ-Valerolactone (GVL) production system.
Funding:Funding: ThisThis research research received received no no external external funding. funding. ConflictsConflicts of Interest: The author declares no conflict conflict of interest.interest. No No funding funding sponsors sponsors had had any any role role in in the the numericalnumerical analyses, analyses, or or interpretation interpretation of of data; data; in inthe the writ writinging of ofthe the manuscript, manuscript, and and in the in thedecision decision to publish to publish the the results. results. Appendix A Preliminary Results for Fire Dynamics Simulator of GVL Burner Appendix A: Preliminary Results for Fire Dynamics Simulator of GVL Burner A computational model for the simulation of GVL burner was developed by using Fire Dynamics A computational model for the simulation of GVL burner was developed by using Fire Simulator software (FDS) version 5.0 [29,30]. FDS simulation can provide much detailed information on Dynamics Simulator software (FDS) version 5.0 [29,30]. FDS simulation can provide much detailed the GVL burner, including the local and transient gas velocity, gas temperature, species concentration, information on the GVL burner, including the local and transient gas velocity, gas temperature, solid wall temperature, fuel burning rate, radiative heat flux, convective heat flux and HRR. species concentration, solid wall temperature, fuel burning rate, radiative heat flux, convective heat The thermomechanical properties of the GVL are listed in Table A1[7]. flux and HRR. The thermomechanical properties of the GVL are listed in Table A1 [7]. Table A1. Thermomechanical properties of GVL [7]. Table A1. Thermomechanical properties of GVL [7]. Material Property Value Material Property Value Lower Heating value 25,000 (kJ/kg) Lowerdensity Heating value 25,0001040 (kJ/kg) (kg/m 3) density 1040 (kg/m³)
The temperature field at t = 42.1 s is shown in Figure A1. ChemEngineering 2019, 3, 32 13 of 14
ChemEngineeringThe temperature 2018, 2, x FOR field PEER at REVIEWt = 42.1 s is shown in Figure A1. 13 of 14
Figure A1. Temperature field (◦C) inside the burner at t = 42.1 s.
The maximal temperatureFigure A1. Temperature at time = 42.1 field s approaches(°C) inside the to burner 570 ◦C. at t = 42.1 s.
The maximal temperature at time = 42.1 s approaches to 570 °C. References
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