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Ammonia Synthesis at Reduced Pressure via Reactive Separation † ‡ † † Mahdi Malmali, Yongming Wei, Alon McCormick, and Edward L. Cussler*, † Department of Chemical Engineering and Materials Science, University of Minnesota, 421 Washington Ave. SE #151, Minneapolis, Minnesota 55455, United States ‡ State Key Laboratory of Chemical Engineering, Membrane Science and Engineering R&D Lab, Chemical Engineering Research Center, East China University of Science and Technology, 130 Meilong Rd., Shanghai 200237, China

ABSTRACT: Ammonia is normally made at high temperature and pressure using a promoted iron catalyst. High temperatures are needed to get fast kinetics; the high pressure is used to ensure high conversion. Alternatively, ammonia can be made at high temperature but lower pressure if the product ammonia is rapidly separated. Here, we have systematically studied the effect of temperature and pressure on the rates of reaction. We then have qualitatively investigated the absorptive separation of ammonia using calcium chloride in a reaction−separation process. Rapid separation reduces the constraint of reversible reaction and enables us to obtain appropriate reaction rates at relatively lower pressure. The effect of different operating conditionsreaction temperature, pressure, absorption temperature, and gas transporton production rates is carefully measured, and this elucidates the potential and the limits of this type of low-pressure ammonia synthesis.

■ INTRODUCTION ammonia-based fertilizer is needed. Thus, by working on Those in the global chemical enterprise now agree that the ammonia, we are investigating a possible chemical synthesis valuable both as a carbon-neutral liquid fuel4,5 and as a future of the chemical industry is dependent on becoming more 6 sustainable.1 Such a goal includes developing energy sources fertilizer. that are not based on nonrenewable fossil fuels, and that do not We have begun to investigate making ammonia from wind using a small ammonia plant operating at the West Central release large quantities of carbon dioxide. This goal implies 6 improving the efficiency of energy collected from the sun and Research and Outreach Center (WCROC) in Morris, MN. the wind. Solar energy is continuing to get less expensive, but is This plant lets us explore what parts of a conventional process still more expensive than fossil fuels. Wind energy is more work well, and what parts are potential problems. The plant immediately attractive and is being rapidly developed in areas of uses a reactor, a condenser, and a compressor, all of which are high population density, especially in northern Europe.2 scaled down parallel to a conventional large-scale plant. The reaction rates observed in the plant are consistent with parallel Downloaded via UNIV OF UTAH on February 24, 2020 at 17:13:25 (UTC). However, both wind and solar energy are periodic and, laboratory experiments with the same catalyst, and with values hence, may not be available at times of highest demand. No one 7−10 wants light only in the middle of a sunny day; similarly, no one from the literature. Analysis of this process gives three wants power only when the wind is blowing. Moreover, wind characteristic times: one for the reaction, one for the See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles. and solar productions are distributed geographically, with condensation, and one for the pump recycling unreacted practical limitations in supplying the electrical grid in remote gases. As the process is currently operated, the reaction time is locations. Thus, both of these resources must be coupled to largest, so that the small process is currently controlled by the methods of energy storage, especially as liquid fuels. Methanol chemical kinetics. However, the analysis also shows how and ammonia are two such fuels. Methanol is attractive because process changes could increase productivity until one of the it meshes well with the existing infrastructure for distributing other characteristic times became largest, and hence the limit of fuel. Ammonia is interesting because it can be made from production. Thus, this plant supplies a test for process changes totally renewable resources: nitrogen produced from pressure that may make sense for a small-scale plant, even when they are ff swing adsorption of air, hydrogen produced from electrolysis of known to have limited e ectiveness in a large fossil-fuel-based water, and electricity obtained from stranded solar arrays or synthesis. wind turbines. A major advantage of ammonia over methanol as In this paper, we explore the potential of process changes to a fuel is that ammonia does not require a carbon source for the reduce the pressure needed for production. In seeking these synthesis. We are especially attracted by ammonia because much of the Received: May 16, 2016 potential wind power is far from areas of high population. As a Revised: July 24, 2016 result, wind is sometimes called “stranded energy”.3 At the Accepted: July 25, 2016 same time, much of the stranded wind energy is where Published: July 25, 2016

© 2016 American Chemical Society 8922 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article process changes, we have decided not to focus on the reaction breakthrough of the absorber’s packed bed, we have a perfect chemistry, feeling that this has been so carefully studied over separation of ammonia from the gas mixture. Hence, before the last century that the chances of big improvements at any breakthrough, the absorption resistance is effectively zero. − scale are small.11 14 Instead, we are investigating three other However, after breakthrough, the production rate can become aspects of this process. First, we are investigating synthesis at dominated by the increasing absorption resistance. lower pressure, because such pressure could reduce process The presence of three resistances in series is observed in complexity and make a small-scale process less expensive and many rate processes, where the amount produced is propor- less hazardous. Second, we are investigating ammonia-selective tional to the overall driving force divided by the total absorbents to reduce the need for recycle. Third, we are resistance.23 The total resistance is the sum of the resistances beginning to explore processes that avoid recycle altogether. of reaction, absorption, and recycle. Phrased in other terms, the In this paper, we limit the discussion just to the first aspect: total resistance is a harmonic average of the speeds of these studies of lower pressure synthesis. After a summary of the three steps, so that the slowest speed has the biggest effect on process itself, we report measurements of chemical kinetics to the rate of ammonia production. If the reactor is operating at a show that at low pressures and a range of temperatures, we get temperature that is too low, the reactor resistance will be the same results as others working under these conditions. We dominating: if the absorber is undersized, the absorption then report the measurements of ammonia production in a resistance can be largest, and if the recycle rate is too low to reaction-separation process. Calcium chloride absorbents are take full advantage of the other unit operations, the recycle employed, where absorption occurs not just on the surface of resistance will be most important. We will use this model later the solid, but also by diffusion into the solid. Absorption and in this paper to discuss the experiments and, hence, to explore desorption15 of ammonia on chlorides of alkaline-earth metals how the reaction−separation process can be made more have been studied in numerous applications, such as chemical productive. heat pumps,16 ammonia storage,4,17,18 indirect hydrogen Linearized Chemical Reaction Rate Constant. Before storage,19,20 and enhanced ammonia synthesis.21 We use the we make this analysis, we want to review how the linearized measured combination of reaction and absorption to estimate reaction rate constant can be estimated from earlier studies. the feasibility of ammonia synthesis at lower pressures. The reaction rate is most often correlated using the Temkin− Overall Production Rate. Here, we employ a model that Pyzhev8,9,24 equation: we developed in our earlier work6 for the reaction−separation process with recycle, which guides the analysis of our ⎛ 1.5 ⎞ ⎛ ⎞ experiments. This theory helps us to understand the complex PPNH P r =−k ⎜ 22⎟ k ⎜ A ⎟ behavior of the reaction−separation process, and we feel that 1⎜ ⎟ 2⎜ 1.5 ⎟ ⎝ PA ⎠ ⎝ P ⎠ this theoryspecifically, the time constants of the reaction, H2 (2) separation, and transportmakes it easier to compare the where k1 and k2 are forward and reverse reaction rate constants, findings of our reaction−separation process. In the following, respectively; and PN , PH , and PA are the partial pressures of we give more details about the model that eases the 2 2 nitrogen, hydrogen, and ammonia, respectively. This equation comprehension of the theory and the model. More details on is rewritten by defining the following variable: this model can be found elsewhere.6 In this simplified model, we assume that the process is in * PPAA− steady state, and that the reactor and absorber are each well- X = P* mixed. (The steady-state assumption is accurate only at the N2 (3) beginning of our experimental test.) The well-mixed Linearization using the Taylor series for small values of X assumption implies that the average nitrogen concentration in simplifies this expression: the reactor is equal to the nitrogen concentration at the exit.22 While this is untrue if most of the nitrogen entering the reactor ⎛ **1.5 ⎞⎡ * ⎤ ⎜ PPNH ⎟ 9 PN reacts in a single pass, it is much closer to being almost true r = k 22⎢ + 2⎥()XX−* 1⎜ * ⎟⎢ * ⎥ when only a small fraction of the nitrogen reacts per pass, ⎝ PA ⎠⎣ 4 PA ⎦ which will be the case here. However, this will not interfere ⎛ ⎞⎡ ⎤ with our findings, as will be discussed later. The model predicts * P* ⎜ PA ⎟⎢ N2 1 ⎥ fi + k −−*()XX the rate of ammonia production (a)tobede ned as 2⎜ 1.5 ⎟⎢ * ⎥ ⎝ P* ⎠⎣ PA 4 ⎦ H2 xx* − 0 a = AA ′ − * = kXR()−* X (4) 11++1 xA kVRR k abt PS m (1) ′ where kR has the dimension of moles of ammonia per catalyst * 0 ′ where xA and xA are the mole fractions of ammonia at reaction volume per time. The kR parameter is related to kR because equilibrium and in the absorber, respectively; k and k are the ⎡ ⎤ R ab PP* − x linearized chemical reaction rate constant and the absorption X = AA= 4⎢x* − A ⎥ ffi P* ⎣ A (1− x* )2 ⎦ mass-transfer coe cient, respectively; Pt is the operating N2 A (5) pressure; S is the surface area of the absorber; and m is the fl Hence, when the conversion is close to zero, which is the case total molar ow rate. In this result, the term (1/kRVR) is the resistance to ammonia production due to the chemical reaction. in our analysis, then − * Similarly, (1/kabPtS) and (1 xA )/m are the resistances of the kk′ ≈ 4 (6) absorber and of the recycle loop, respectively. RR ’ ′ In this model, the absorber s performance is assumed to where kR is the corrected linearized reaction rate constant after behave as a first-order rate process. In reality, before the the change of variables. Equation 4 shows that the linearized

8923 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article

Figure 1. Schematic diagram of the apparatus. The gas mixture is recirculated between the reactor and absorber using a piston pump. ′ reaction rate constants kR and kR obtained here are functions of controllers were connected to a National Instrument Labview * ′ concentration. When xA approaches xA , kR decreases to zero; card (Austin, TX), with the data being recorded every second. ′ fl however, when xA approaches zero, kR increases to its Labview controlled the mass ow controllers (Model maximum value, representing only the forward reaction rate. SLA5850S, Brooks Instruments, Hatfield, PA) installed in the apparatus inlet, which injected known volumetric flows of ■ EXPERIMENTAL SECTION reactant gases. The inlet and outlet of the system were equipped with needle valves. Materials. Anhydrous CaCl in granular form (>7 mm) 2 Procedure. Three grams of catalyst, loaded in the reactor at with 93% purity (CAS No. 10043-52-4, Lot No. SLBL2770 V) room temperature, was reduced by pumping hydrogen gas was purchased from Sigma−Aldrich (St. Louis, MO). The through the catalyst bed while slowly increasing the reactor reactant gases N and H with ultrahigh purity were purchased 2 2 temperature. Hydrogen gas was pumped through the catalyst from Matheson (New Brighton, MN). We employed a bed at the flow rate of 0.5 standard liters per minute (SLPM). prereduced nonstoichiometric ferrous oxide catalyst (wustite) The hydrogen reacted with the coating layer, forming water with promoters (AmoMax-10 RS, Clariant, Charlotte, NC). fi The catalyst is provided in the form of irregularly shape vapor. A multiramp temperature pro le heated the catalyst very − slowly to ensure that the water vapor concentration during the granules, with a nominal size range of 1.5 3 mm, and is ∼ stabilized with an oxygen-rich protective layer. activation is <1000 ppm. The temperature ramp required 27 h for the temperature to increase from room temperature to Apparatus. The experimental apparatus, a schematic fl diagram of which is shown in Figure 1, was built using 723 K. After the reactor reached 723 K, hydrogen ow Swagelok (Chaska, MN) 316 stainless steel tubing and fittings continued through the reactor for at least 24 h. After this initial reduction, the apparatus was always kept under nitrogen. of 6 mm inner diameter. The reactor was 0.15 m long. Catalyst fi particles were ground from their produced size until smaller We performed two sets of experiments. During the rst set, “ ” (<1 mm) before loading the reactor. The absorber was a 0.3-m- which we refer to as reaction experiments , we were only long stainless steel tube, with an inner diameter of 0.022 m, interested in the kinetics of ammonia synthesis, so the absorber bundled with heating tape. The absorber and heating tape was bypassed and the gas mixture circulated only through the ff bundle were insulated with silica woven insulation (AVS reactor. We investigated the e ect of operating pressure on the Industries, New Castle, DE). A variable piston pump (Model reaction rate constants. In the second set of experiments, which PW2070N, PumpWorks, Inc., Minneapolis, MN) circulated the we refer to as “reaction−separation experiments”, the gas gas mixture between the reactor and the absorber. mixture was circulated between the reactor and absorber using A ceramic heater (Model CRFC-36/115-A, Omega, the pump. This set of experiments was similar to the Stamford, CT), equipped with a multiramp proportional− conventional Haber−Bosch process, except that we replaced integral−differential (PID) controller (Omega, Model the condenser with an absorber. In both sets of experiments, we CN96211TR) controlled the temperature of the reactor. The utilized the pressure reading data log to calculate the ammonia inlet and outlet gas temperatures and the temperature on the reaction rates and conversion. outer surface of the reactor were measured using Type K The reaction experiments utilized a three-way valve to direct thermocouples connected to a signal conditioner (Omega, the circulation loop toward the bypass line, without the Model DRG-SC-TC). The system pressure was recorded using absorber. The catalyst remained unchanged throughout the a pressure transducer (Model 50426877, WIKA, Lawrenceville, experiments. The hydrogen and nitrogen were fed to the GA) with a 0−10 V dc output. The experiments were carried reactor with the ratio of 3:1. We stopped the flow and closed out in a recirculation batch mode. All instruments and the inlet valve once the system reached to the desired pressure.

8924 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article

After pump started to recirculate the gas mixture with the flow of ammonia reaction at each operating temperature. The rate of rate of 0.7 mL/s, we began to record pressure data. These reaction at 740 K is much faster than the rate at 660 K, and we 25 conditions are summarized in Table 1. shall determine if these rates are consistent with the literature. The reaction rate is initially fast, but quickly slows at longer Table 1. Conditions for Reaction and Reaction−Separation times, because the reverse reaction rate accelerates as the partial Experiments pressure of ammonia increases. The half-lives of reaction, as an alternative representation of Experimental Conditions the reaction rates, are shown as an Arrhenius plot versus reaction reaction−separation temperature in Figure 2b. To determine the half-life from the − fi fi reaction temperature (K) 620−740 700 pressure time pro les, the equilibrium conversion and nal fi absorption temperature (K) 460 pressure at each speci c temperature were calculated, following 25 initial pressure (bar) 74 55 the analysis of Gillespie et al. The time at which the half-life H:N ratio 3:1 3:1 operating pressure occurred then was extracted. The half-lives pump flow rate (mL/s) 0.7 0.7 range from 0.15 h to 12 h. This variation in the half-life of fi reaction/reaction-absorption time (h) 3 3 reaction is evident in the pressure pro les as well. For instance, desorption temperature (K) 600 in the test at 740 K, the pressure reaches equilibrium after ∼ purge flow (SLPM N ) 100 2000 s, while the pressure at 660 K continues to decrease 2 ffi desorption time (h) ∼20 until 5000 s. This a rms the common conclusion for this system, that higher temperature is advantageous for reaction rate, while lower temperature favors the thermodynamic − Unless otherwise mentioned, the reaction separation experi- equilibrium. ments were at 700 K. The three-way ball valve included the The operating pressure also affects the reaction kinetics, but absorber in the recirculation loop. The absorber was initially to a lesser degree than the temperature does. The data in Figure fi ∼ lled with 70 g of fresh anhydrous CaCl2 and was always kept 3, for a variety of temperatures and pressures, illustrate this. at temperatures greater than 460 K. The absorber was purged with dry nitrogen for 24 h at 673 K before starting the first test. The absorber and catalyst were unchanged throughout the experiments. An initial pressure of 55 bar was used, and the apparatus was fed with hydrogen and nitrogen gas mixture with the ratio of 3:1. The pump rates of 0.7 mL/s recirculated the gas mixture for 3 h. After each test, the absorber was regenerated by heating to 600 K, decreasing the system pressure to 2−3 bar, and pumping nitrogen through the absorber at 0.1 SLPM for 20 h. Details of these experiments are given in Table 1. ■ RESULTS Reaction Experiments without Absorption. To under- stand the reaction kinetics with our catalyst, we carried out several experiments to investigate the effect of operating Figure 3. Arrhenius plot of reverse rate of reaction versus 10 000/T. temperature and pressure. For example, Figure 2a reports the Results indicate a strong dependency of the reverse rate constant on pressure versus time for the catalytic reaction of ammonia temperature, but a smaller variation with pressure. initially at 74 bar and reaction temperatures of 660 and 740 K. The slopes of the pressure profiles represent the apparent rates This figure reports the reverse reaction rate constant in the form of an Arrhenius plot. We chose reverse reaction rate constants rather than our more directly measured forward rate constants, because most previous literature8,26 on catalytic reaction of ammonia is rather focused on disassociation of ammonia, so more data are available on reverse rate constants. The reverse reaction rate constant is simply proportional to the forward reaction rate constant

2 k1 Kp = k2 (7) and has the following stoichiometry: 1 3 N +⇔HNH 2 222 3 (8)

where Kp is the equilibrium reaction constant calculated from 25 Figure 2. Effect of reaction temperature on (a) kinetics of reaction and Gillespie et al., which includes fugacities that can alter the (b) half life of reaction. Higher temperature increases the rates of equilibrium reaction constant by a few percent. reaction; however, at higher temperature, the mole fraction of The reverse reaction rate constants in Figure 3 exponentially ammonia at equilibrium decreases. increase with temperature, but change less with pressure.

8925 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article fi Speci cally, the rate constant increases by a factor of 10 000 Hence, the absorption rates into the CaCl2 structure can when the operating temperature is increased from 600 K to 730 change. Thus, we repeated the reaction−absorption and then K; however, the rate constant at 650 K varies by only a factor of desorption cycles until we obtained reproducible results. The 2 when the operation pressure is increased from 20 bar to 69 data are illustrated in Figure 5a for selected cycles. The total bar. At 740 K, there is an order-of-magnitude difference in the pressure, our measure of reaction−separation, is linear with reaction rate constants at 20 and 69 bar, which is certainly time during the first cycle. The pressure changes increase significant but less than the variation with temperature. Lower notably in subsequent cycles, consistent with a structural operating pressures give larger reaction rate constants while change in the CaCl2. The initial production rates (slopes) are higher operating pressures give lower ones. The tests at 20 and relatively fast, and the rates then go through a transition region; 28 bar show less variation, probably because the times to reach finally, they slow to a rate very similar to that of the first cycle the operating pressures are smaller and more reproducible. for all cycles. The initial rates get faster after desorption of each Nevertheless, the experimental errors for reaction engineering cycle, and take longer to slow down. If we plot the reciprocal of in this range are typical (cf. Figure 4). the apparent ammonia capacity after 10 000 s versus the reciprocal of the cycle number, we get a straight line, as shown in Figure 5b. The time period of 10 000 s chosen here is arbitrary, but it illustrates some of the implications of Figure 5a. The intercept in Figure 5b corresponds to the inverse capacity for an infinite number of cycles (that is, for the maximum change in the solid geometry). In practice, we always used samples subjected to seven or more cycles, which are thus within ∼10% of the maximum absorption that is obtainable at the operating temperature. Effect of Reaction Temperature. To determine whether the chemical reaction is the rate-limiting step, we carried out tests with reaction temperatures of 680, 700, 720, and 740 K, while keeping all other conditions unchanged. The pressure−time graphs for these tests are shown in Figure 6. The results show

Figure 4. Comparison of the reverse reaction rate constants with literature. Reverse reaction rate constants are independent of operating pressure.

These reaction rate measurements are compared with those reported in the literature27,28 in Figure 4. This shows an Arrhenius plot of the Temkin reverse reaction rate constant k2, both as reported in the literature and as found from this study. Our work displays a systematic negative deviation, most likely due to experimental error associated with the time to start our experiment. Again, these data and those of others all show that the reaction rate constants vary much less with pressure than with temperature. Reaction−Separation Experiments. Effect of Absorbent History. Earlier work says that the absorption−desorption of Figure 6. Effect of reaction temperature on the production rates pf ff ammonia on CaCl2 absorbent can generate a nanoporous reaction/absorption. The small e ect on the overall kinetics of the structure with a Brunauer−Emmett−Teller (BET) surface area reaction temperature shows that reaction kinetics is not the rate- that is up to 10 times larger than was there originally.20,29 limiting step of the combined reaction−absorption.

ff Figure 5. E ect of absorption history of the CaCl2 on the production rate of reaction/absorption: (a) the production rate increases until the seventh cycle, and (b) the accessible volume of the CaCl2 increases gradually.

8926 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article that the initial overall rate is almost the same, independent of Figure 8a, may indicate that two competing mechanisms are the temperature. Later, the rates at higher catalyst temperatures controlling the absorption of ammonia. First, the diffusion ffi are systematically slower. This implies that the overall kinetics coe cients in the solid CaCl2 increase with temperature. are not controlled by catalytic reaction. It is also likely that, with Second, the capacity of the absorbent decreases with temper- small reactor residence time and effective absorber operation, ature: for instance, the equilibrium absorption capacity of the fi the reverse reaction for ammonia synthesis is not signi cant CaCl2 at 325 and 470 K at an ammonia partial pressure of 1 bar 20 because the ammonia mole fraction is kept low. is 4 and 1.5 mol of ammonia per mole of CaCl2, respectively. Effect of Initial Pressure. The initial rate of reaction is As Figure 8a shows, there is no significant effect of temperature proportional to the system operating pressure, as shown in on absorption: the rate is greatest at 460 K, smaller at 510 K, Figure 7. The operating pressure has two synergistic effects on and somewhat larger at 600 K. The apparent production rate per capacity versus the absorber temperature shown in Figure 8b is approximately constant, implying that diffusion and absorption capacity counterbalance. Earlier experiments on the absorption of ammonia without reaction in magnesium chloride also show only a weak absorption dependence on temper- ature.30 Effect of Recycle Flow Rate. A key route to reducing the total resistance to ammonia synthesis is to increase the fluid velocity in the system, as shown in Figure 9. While the flow of gas has an insignificant effect on the forward or reverse chemical rate constants, it may affect both the absorption resistance and the recirculation resistance. At the beginning of the test, the packed-bed absorber works as a perfect separator. Thus, the absorption time constant is equal to zero until the breakthrough point. We can then infer that the film mass- Figure 7. Higher initial operating pressure enhances the production transfer resistance around the particle is not important. We rates of absorption and apparent capacity of the absorbent. expect that the resistance to absorption will gain more importance once the breakthrough occurs. As a result, we fl rates of the reaction−separation experiments. First, the catalytic suspect that the resistance of the recycle ow, which is − * fl reaction rate is higher at higher operating pressure. Second, the represented by the term (1 xA /m)ineq 1,inuences the larger ammonia concentration in the gas stream leaving the overall rate significantly. This is true: faster pumping accelerates reactor increases the driving force for the absorption of the process, as Figures 9a and 9b show. Even increasing the fl ammonia into CaCl2 crystals. At longer times, the behavior is ow rates by a factor of 12 does not yet suggest an asymptote in more complex, for example, when the conversion curves for Figure 9b. initial pressures of 55 and 69 bar cross. These complexities If we could maintain almost-complete removal of the probably come from factors like the onset of reverse reaction ammonia in the absorber, increasing the recycle flow decreases rate, increased mass-transfer resistance, and changes in the single-pass conversion of ammonia. In other words, faster absorbent structure. This more-complex behavior is beyond recycle flow accelerates forward reaction and decelerates reverse our current objective. Instead, we will focus on maintaining the reaction, because of the lower ammonia concentration in the high initial rate seen here with appropriate choice of absorbent reactor. However, the fast recirculation rates required may and recycle operation. compromise the reactor and absorber temperatures. In our Effect of Absorber Temperature. The effect of absorber case, increasing the pump flow rates to more than 3 mL/min temperature on the rate of ammonia production, displayed in reduced the reactor temperature because of the heat loss to the

Figure 8. Absorption temperature effect on production rates: (a) pressure versus time graphs at different absorption temperatures and (b) apparent rates versus absorption temperatures. Higher absorption temperature increases the production rate constant but decreases the driving force. The ′ apparent rates and capacities shown are calculated for the initial 1000 and 10 000 s, respectively. (In panel (b), r [mmol NH3/s] represents the production rate, and C is the apparent absorbent capacity [mmol NH3/mmol CaCl2].)

8927 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article

Figure 9. (a) Effect of pumping flow rates on production rates, and (b) initial rates of absorption in different pumping flow rates. Results indicate that film resistance surrounding the particle is not important. The production rates shown are calculated from the initial 1000 s. gas flowing out of the reactor, so none of those data are shown relatively fast ammonia synthesis rates (unchanged for up to here. This must be a concern in future process design. 5000 s). This suggests that high production rates at pressures as Figure 10 shows the production rates at different time low as 25 bar are viable when ammonia is removed from the intervals for different pumping rates. Results suggest that during system efficiently. The absorption separates the synthesized ammonia from the reaction environment and reduces the reverse reaction. However, while we have developed a reaction−separation process with enhanced rates and conversion, we do not understand the detailed performance of each unit operation. We have investigated the impact of different operating conditions. For instance, we find that the reaction temperature has a minor effect on the process, while increased recycle pumping shows significantly increased production. In the following paragraphs, we discuss this performance in more detail. To do so, we employ the model summarized in the theory above to understand the rate constants for each unit at different operating conditions. The complete reaction conditions and calculated times for each unit are summarized in Table 2. The Figure 10. Effect of recycle flow rate on the production rate in ff first column in this table identifies the experimental category. di erent time intervals of the test. Absorber column breakthrough − leads to the formation of the asymptote at different flow rates; larger Columns 2 5 identify the reaction conditions, including the recycle flow results in faster absorber breakthrough. operating pressure, reaction temperature, absorption temper- ature, and pumping flow rate. Columns 6 and 7 give the the initial 1000 s, the transport resistance in eq 1, represented measured time constants for the reaction and the recycle − * obtained from eq 1, respectively. When the absorber is as (1 xA /m), is much larger than the reaction resistance in eq fi removing all ammonia from the gas flow before the 1, which is de ned as [1/(kRVR)]. Because the regenerated packed bed absorber is removing the ammonia completely, the breakthrough of the bed, the absorption time constant is absorption resistance is zero; therefore, the recycle flow is zero. The values of these three rate constants merit careful controlling the reaction−separation process, which leads to the consideration, because they are the key by which the process linear increase in the production rate. At times between 1000 can be improved. The longest time constant corresponds to and 4000 s, the absorber’s packed bed breakthrough starts to slowest step in the reaction−separation and, hence, controls the appear, so the absorber is no longer capable of complete overall rate of the process under the conditions studied. removal of the ammonia from the gas stream. By contrast, at Table 2 shows that the recycle flow is the rate-controlling longer times in the range of 4000−5000 s, we observe smaller step before breakthrough of the packed bed absorber. The time production rates at higher recycle flows; this lower production constant for the recycle is 100 times larger than the reaction rate is attributed to the appearance of the breakthrough point time constant. Thus, our reactive separation process can be and (partial) exhaustion of the bed. Note that at higher recycle improved significantly by faster recycle flows. However, this is flow, we had larger production rates, which led to faster loading not a trivial change, because controlling the operating of the absorbents. This is consistent with our earlier parameters of the reaction−separation systems becomes observation of the slower slopes at ∼5000 s. difficult. These results still guide speculation about the design of a ■ DISCUSSION small, efficient process which includes the synthesis and The results shown above confirm the viability of the reaction− simultaneous removal of the ammonia at reduced pressure. In absorption process for the enhanced production of ammonia at the widely verified rate equations, there are three factors that significantly lower operating pressures. For example, in one set directly affect the reaction rate: the temperature, which affects of measurements, we obtained more than 80% conversion with all the reaction rate constants; the partial pressures of reactants,

8928 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article

a Table 2. Examples of Process Conditions and Time Constants

fl × −3 pressure (bar) reaction temperature (K) absorption temperature (K) pumping ow rate (L/h) 1/(kRVR)( 10 h/mol) 1/m (h/mol) Absorption History Test 55 700 460 2.5 5.8 0.18 Reaction Temperature Test 55 680 460 2.5 16 0.18 55 700 460 2.5 5.8 0.18 55 720 460 2.5 2.2 0.18 55 740 460 2.5 1.1 0.18 Pressure Test 55 700 460 2.5 5.8 0.18 69 700 460 2.5 5.8 0.14 83 700 460 2.5 5.8 0.12 Absorption Temperature Test 55 700 460 2.5 5.8 0.18 55 700 510 2.5 5.8 0.18 55 700 600 2.5 5.8 0.18 Pumping Flow Rate Test 55 700 460 1 5.8 0.46 55 700 460 2.5 5.8 0.18 55 700 460 10 5.8 0.04 aThe largest time constant, for the absorption, is the step which most affects the overall rate.

Figure 11. Schematic diagram of a conventional reactor (top) and a reaction−separation reactor with absorption (bottom). In the reaction− separation design, green, blue, red arrows shown represents the bed in process, transition, and regeneration, respectively. which influence the forward reaction rate; and the partial Table 3. Experimental Conditions of the Nielsen Experiment pressure of the ammonia, which influences the reverse rates. If we can remove ammonia from the reaction chamber efficiently, parameter value then the disassociation of ammonia through the reverse P 304 bar T 765 K reaction no longer proceeds rapidly. Thus, we can obtain 3 respectable rates of reaction at reduced pressure if we keep the Vcat 2.5 cm concentration of ammonia low with an efficient separation. Lcat 130 mm We now compare the performance of a conventional reaction space velocity 94200 1/h with a reaction−separation process. One attractive design for outlet % NH3 19.7 the reaction−separation would be mixed catalyst and absorber in the same bed. However, the catalyst can be poisoned by water and other impurities on the absorbent. Another attractive ammonia per hour. We assume that the absorber works perfect design would be a conventional reactor divided into short and the gas stream leaving the absorption sections has no segments with absorption beds between them. Our results ammonia. show that faster pumping increase production, but higher fl ffi To illustrate this, we use the rate equation proposed by recycle ows may be di cult in practical applications. Still, the Nielsen et al.27 (eq 9) for the synthesis of ammonia. We choose new process has promise. this form of rate equation because the reaction rate does not To explore this promise, we compare the reactor used by diverge to infinity when the ammonia partial pressure is zero Nielsen et al. with that proposed here using the specifics shown 27 ’ (although we acknowledge that the authors tested this rate only schematically in Figure 11. From Neilsen s extensive down to P = 0.2 bar): experiments, we choose the one summarized in Table 3, A ⎛ 2 ⎞ which is capable of producing 1.8 moles of ammonia per hour. 0 2 PA kPK⎜⎟− 2 Na2 3 In our simplified reaction−separation process, we can set the ⎝ PH ⎠ r = 2 mole fraction of ammonia at the outlet of each catalytic section ⎡ ⎛ ⎞⎤2α ⎜⎟PA to <1% and then determine the required initial operating ⎢1 + K ω ⎥ ⎣ 3⎝ P ⎠⎦ pressure, which satisfies the production rate of 1.8 moles of H2 (9)

8929 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article

Figure 12. Comparison of rate of ammonia synthesis and ammonia mole fraction along the length of the reactor. We need ∼17 catalytic sections, followed by absorption, to maintain the concentration of ammonia at <1% in this design. where Pi is the partial pressure of component i; Ka and K3 are divided by the total initial charge in the system) is shown. The equilibrium and reaction rate constants; and α and ω are the black data points (square markers) represent data from the parameters of ammonia reaction rate. In our case, we used α = reaction test at 723 K and an initial operating pressure of 55 0.64 and ω = 1.564. Other constants of this equation, in bar; red data points (circle markers) represent data from the accordance with these parameters, can be found elsewhere.27 As result for the reaction test at 723 K and an initial operating the equation shows, the concentration of ammonia product pressure of 55 bar; blue data points (rectangular markers) does have a significant influence on the rate of ammonia represent data from the result for the reaction−separation test, production. where the reaction occurs at 723 K, absorption occurs at 460 K, Our data show that such a process is capable of producing and the initial operating pressure is 55 bar, and the pump was 1.8 mol of ammonia per hour at 25 bar, i.e., at pressures ∼12 also set to a value of 0.7 mL/s. The data points are averaged for times lower than the pressure used in the Nielsen experiment. each 50 s. The slopes of these curves represent the apparent Figure 12 shows the rate of ammonia production and mole rates of ammonia synthesis. fraction of ammonia along the length of the reactor for two When the absorber is introduced to the batch−recycle−loop example designs. system, the rate of production remains comparable to the initial As shown, the rate of ammonia production in conventional rate seen in the absence of the absorber. The absorber is reactor rapidly decreases at the entrance of the reactor. By the managing to keep the product concentration above the catalyst time the mole fraction of ammonia reaches 5%, the rate of low, so the rate is comparable to just the forward rate. Doing reaction decreased significantly. Alternatively, if we alternate so, it permits us to attain rates comparable to just the forward rate. Figure 13 clearly indicates that, after a short period, the catalytic and absorption sections, the reaction rate in our − simplified reaction−separation process is high while operating reaction separation test system is showing faster apparent at 25 bar. In principle, we can make ammonia rapidly and at rates, compared to the reaction test system. While during the ̅ lower pressure. We look forward to exploring the practical initial stage (t = 100 s/mol), the reaction test shows the fastest utility of the science that we have now demonstrated. rate at 90 bar, the reaction test at 55 bar displays a slower rate, similar to that observed in the reaction−separation test. The proof-of-the-concept for the viability of enhanced fi ammonia synthesis at reduced pressure via reaction−separation However, the apparent rates change signi cantly after a while (t̅> 100 s/mol). Comparison of reaction test at initial pressures process is displayed in Figure 13. The dimensionless pressure − (P/P ) versus normalized time (t,̅ which is defined as time of 90 and 55 bar with the reaction separation test at an initial 0 pressure of 55 bar shows that the rates of the reaction tests decrease very quickly while the apparent rate of the reaction− separation test remains unchanged. For instance, at t̅= 2500 s/ mol, the apparent rate of the reaction−separation test is four times larger than the apparent rate of the reaction test, although the operating pressure is almost half. At t̅> 5000 s/mol, the reaction tests approach equilibrium while the apparent rate of the reaction−separation test is still unchanged at pressures as low as 25 bar. While the concept of enhanced ammonia synthesis using metal chlorides has been demonstrated in our earlier work,25 here, we show the viability of ammonia synthesis at much reduced pressure with comparable rates.

■ CONCLUSIONS In this study, we have reported the application of a reactive− Figure 13. Comparison of reaction and reaction−separation tests at absorption process for enhanced production of ammonia at different operating pressures. Greater apparent reaction rate is reduced operating pressure. Absorption on calcium chloride observed via the reaction−separation test at lower operating pressures. provides an efficient separation for the removal of ammonia,

8930 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article even at temperatures close to the reaction temperature. Both (5) Wojcik, A.; Middleton, H.; Damopoulos, I.; Van Herle, J. our analysis and model predictions indicate that both reaction Ammonia as a Fuel in Solid Oxide Fuel Cells. J. Power Sources 2003, and separation rates are fast and recycle flow is the rate-limiting 118, 342. step in our proposed process. (6) Reese, M.; Marquart, C.; Malmali, M.; Wagner, K.; Buchanan, E.; McCormick, A.; Cussler, E. L. Performance of a Small-Scale Haber Process. Ind. Eng. Chem. Res. 2016, 55, 3742. ■ AUTHOR INFORMATION (7) Dyson, D. C.; Simon, J. M. Kinetic Expression with Diffusion Corresponding Author Correction for Ammonia Synthesis on Industrial Catalyst. Ind. Eng. *Tel.: 612-625-1596. Fax: 612-626-7246. E-mail: cussler@umn. Chem. Fundam. 1968, 7, 605. edu. (8) Temkin, M.; Pyzhev, V. Kinetics of Ammonia Synthesis on Promoted Catalysts. Acta Physiochim. USSR 1940, 12, 327. Notes (9) Annable, D. Application of the Temkin Kinetic Equation to The authors declare no competing financial interest. Ammonia Synthesis in Large-Scale Reactors. Chem. Eng. Sci. 1952, 1, 145. ■ ACKNOWLEDGMENTS (10) Horiuti, J.; Takezawa, N. The Mechanism of Catalyzed Synthesis of Ammonia in the Presence of Doubly Promoted Iron This work was primarily supported by the Minnesota Catalyst. J. Res. Inst. Catal., Hokkaido Univ. 1960, 170−187. Environment and Natural Resources Trust Fund as recom- (11) Stoltze, P. Surface Science as the Basis for the Understanding of mended by the Legislative-Citizen Commission on Minnesota the Catalytic Synthesis of Ammonia. Phys. Scr. 1987, 36, 824. Resources (LCCMR), and by the MNDrive, an initiative of the (12) Ertl, G. Primary Steps in Catalytic Synthesis of Ammonia. J. Vac. University of Minnesota. Joshua Prince is also acknowledged Sci. Technol., A 1983, 1, 1247.  for assisting with the rate constant calculations. (13) Ertl, G. Surface Science and Catalysis Studies on the Mechanism of Ammonia Synthesis: The P. H. Emmett Award Address. Catal. Rev.: Sci. Eng. 1980, 21, 201. ■ NOMENCLATURE (14) Catalytic Ammonia Synthesis: Fundamentals and Practice; a = rate of ammonia production (mol/h) Springer Science & Business Media: New York, 2013. (15) Jones, M. O.; Royse, D. M.; Edwards, P. P.; David, W. I. F. The C = apparent absorbent capacity (mmol NH3/mmol CaCl2) k = absorption initial rate Structure and Desorption Properties of the Ammines of the Group II k = forward rate of reaction constant Halides. Chem. Phys. 2013, 427, 38. 1 (16) Lebrun, M.; Spinner, B. Models of Heat and Mass Transfers in k2 = reverse rate of reaction constant  ffi 2 Solid gas Reactors Used as Chemical Heat Pumps. Chem. Eng. Sci. kab = absorption mass-transfer coe cient (mol/m Pa h) 1990, 45, 1743. kR = linearized catalytic reaction rate constant (mol/L h) (17) Sharonov, V. E.; Aristov, Y. I. Ammonia Adsorption by MgCl , K = equilibrium constant in Nielsen rate expression 2 a CaCl2 and BaCl2 Confined to Porous Alumina: The Fixed Bed Kp = reaction equilibrium constant Adsorber. React. Kinet. Catal. Lett. 2005, 85, 183. K3 = reaction rate constant (18) Liu, C. Y.; Aika, K. Ammonia Absorption into Alkaline Earth m = number of moles of gas mixture per hour in the recycle Metal Halide Mixtures as an Ammonia Storage Material. Ind. Eng. flow (mol/h) Chem. Res. 2004, 43, 7484. Pi = partial pressure of component i (19) van Hassel, B. A.; Karra, J. R.; Santana, J.; Saita, S.; Murray, A.; S = total surface area of the absorber Goberman, D.; Chahine, R.; Cossement, D. Ammonia Sorbent r = rate of reaction Development for On-Board H2 Purification. Sep. Purif. Technol. ′ 2015, 142, 215. r = apparent production rate (mmol NH3/s) 3 (20) Sørensen, R. Z.; Hummelshøj, J. S.; Klerke, A.; Reves, J. B.; VR = volume of catalyst (m ) x* = ammonia mole fraction at equilibrium Vegge, T.; Nørskov, J. K.; Christensen, C. H. Indirect, Reversible A High-Density Hydrogen Storage in Compact Metal Ammine Salts. J. xi = mole fraction of component i in the recycle stream 0 Am. Chem. Soc. 2008, 130, 8660. xA = mole fraction of ammonia in the absorber (21) Himstedt, H. H.; Huberty, M. S.; McCormick, A. V.; Schmidt, L. Subscripts D.; Cussler, E. L. Ammonia Synthesis Enhanced by Magnesium i = component i Chloride Absorption. AIChE J. 2015, 61, 1364. A = ammonia (22) Kumar, A.; Daoutidis, P. Nonlinear Dynamics and Control of H = hydrogen Process Systems with Recycle. J. Process Control 2002, 12, 475. N = nitrogen (23) Baldea, M.; Daoutidis, P. Dynamics and Nonlinear Control of α, ω = parameters of the ammonia reaction rate Integrated Process Systems; Cambridge University Press: Cambridge, U.K., 2012. (24) Guacci, U.; Traina, F.; Ferraris, G. B.; Barisone, R. On the ■ REFERENCES Application of the Temkin Equation in the Evaluation of Catalysts for (1) Russo, M. V. The Emergence of Sustainable Industries: Building the Ammonia Synthesis. Ind. Eng. Chem. Process Des. Dev. 1977, 16, on Natural Capital. Strateg. Manag. J. 2003, 24, 317. 166. (2) Henderson, A. R.; Morgan, C.; Smith, B.; Sørensen, H. C.; (25) Gillespie, L. J.; Beattie, J. A. The Thermodynamic Treatment of Barthelmie, R. J.; Boesmans, B. Offshore Wind Energy in EuropeA Chemical Equilibria in Systems Composed of Real Gases. I. An Review of the State-of-the-Art. Wind Energy 2003, 6, 35. Approximate Equation for the Mass Action Function Applied to the (3) Piwko, R.; Osborn, D.; Gramlich, R.; Jordan, G.; Hawkins, D.; Existing Data on the Haber Equilibrium. Phys. 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8931 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932 Industrial & Engineering Chemistry Research Article

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8932 DOI: 10.1021/acs.iecr.6b01880 Ind. Eng. Chem. Res. 2016, 55, 8922−8932