Pergamon

E H n

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hr. J. Hydrogen Energy, Vol. 21, No. 10, pp. 865-870, 1996 Copyright c 1996 International Association for Hydrogen Energy

Elsevier Science Ltd

PII: SO360-3199(96)00024-9 Printed in Great Britain. All rights reserved 036@3199/96 $15.00+0.00

ADIABATIC UT-3 THERMOCHEMICAL PROCESS FOR HYDROGEN PRODUCTION

M. SAKURAI, E. BILGEN,* A. TSUTSUMI and K. YOSHIDA

Department of Chemical System Engineering, The University of Tokyo, 7-3-I Hongo, Bunkyo-ku, Tokyo 113, Japan

(Receiced,forpublication 26 Februarv 1996)

Abstract-The UT-3 thermochemical hydrogen producing cycle is a four step process developed at the University of Tokyo. In the process, only solid and gas reactants/products are used and the maximum temperature is 1033 K. In this paper, a new UT-3 process has been developed in which all four reactions are carried out adiabatically using a heat carrier vector (steam or an inert gas). The new process has been evaluated using the ASPEN-PLUS process code. The first law efficiency of the cycle has been determined as 48.9% and the second law efficiency as 53.2%. The process can be realized using conventional materials for the reactors, which can be packed bed, honeycomb or fluidized bed type. In the latter case, it is found that the flow rate of the heat carrier fluid is sufficient to fluidize all four reactors. For a plant producing 30,000 Nm’ per hour or 2.68 x IO6 GJ H, per year, the plant sizing has been carried out, its operation is discussed and its advantages are presented. Copyright (0 1996 International Association for Hydrogen Energy

NOMENCLATURE

Exergy (Jjmol) Enthalpy (Jjmol) Number of moles in a given stream Pressure (Pa) Thermal energy (Jjmol) Gas constant (Jjmol K) Entropy (Jjmol K) Thermodynamic temperature (K) Mechanical energy (Jjmol)

Greek Symbols

i Exergetic efficiency Difference

rl Thermal efficiency

Subscripts e Excess i Summation index m Module number 0 Surroundings, dead state

I. [NTRODUCTION

The UT-3 thermochemical process of the University of Tokyo involves five compounds and is carried out in four steps [ 11.

*Author to whom correspondence should be addressed

CaBr,(s)+ H,O(g)-+CaO(s)+ 2HBr(g) (1)

CaO(s)+BrZ(g)+CaBr,(s)+0.502(g) (2)

FejOl(s) + 8HBr(g)+3FeBr,(s)+4HzO(g) + Br,(g)

(3)

3FeBr,(s)+4H,0(g)-+Fe,0,(s)+6HBr(g)+H1(g).

(4)

Suitable reaction temperatures for the above equations have been determined as 1033, 845, 493 and 833 K, respectively. As can be noted, the process involves only solid and gas reactants if run without the condensation of H,O.

This process has been studied extensively, including its reaction and kinetic measurements [2, 31, bench scale tests [4, 51. development of solid reactants and kinetic measurements [6, 71 and engineering evaluations [8, 91. As a result, the process flow sheet has been modified over the years. In the latest version, a loop flow process was developed [5], which is taken as a base in the present study. Engineering evaluation shows that the combined thermal efficiency of a system consisting of a high tem- perature gas cooled reactor (HTGR), the UT-3 process and an electric power generating system can reach 45- 48% depending on the membrane recovery rate [9]. The efficiency of the thermochemical process alone is not given. However, it is estimated at 39.2% from the results of technical reports by the Toyo Engineering Corp.

865

866 M. SAKURAI er ul.

(1995). Costing results show that the thermochemical hydrogen cost can be as low as 35 US$( 1995)/GJ hydro- gen. To improve this and for further development of technological applications, laboratory studies and engin- eering work still continues in various institutes and com- panies. For example, in a recent process flow sheet developed by the Toyo Engineering Co. (1995), from now on called the Toyo flow sheet, the energy supply to endothermic reactions is done using heat exchangers, which may be costly and difficult to operate. Heat exchanger duties supplying heat to the endothermic reac- tors are 730 kJ/mol H, and the others in the process amount to 4983 kJ/mol H,. Power consumption for the circulation of gas products is 137 kJ/mol H,. The type of reactor which can be used is limited to certain types, such as packed bed or honeycomb reactors.

The aim of this study is to improve the process flow sheet by using adiabatic equipment, a suitable heat carrier fluid and to evaluate its performance.

2. DESCRIPTION OF THE CHEMICAL PROCESS

A simplified flow sheet of the process is shown in Fig. 1. It has four reactors, Rl-R4, connected in series in a loop where gaseous substances circulate. Hydrogen and oxygen are separated as products, by separators Sl and S2. Control of maximum reaction temperatures is ensured by use of three heat exchangers, HXl-HX3 so that necessary cyclic operation can be maintained [3].

Reactions 14 are carried out in reactors RI-R4. The order of reactions is as shown in Fig. 1, i.e. the hydrogen generation step involves reactions 1 and 4 and the oxygen generation step, reactions 3 and 2, respectively. The flow of gas products is in the clockwise direction in Fig. 1. The bromination is carried out in R2 and R3, producing CaBr,(s) and FeBr,(s), respectively. Hydrogen and oxy- gen are separated as products, each in a separator (Sl and S2). Water to decompose is fed into the loop after the separation of O2 at separator S2. Nuclear heat is supplied at heat exchanger HXOl. The first reaction

requires the highest temperature of 1033 K to ensure 100% conversion. as Fig. 2 shows. Other reaction tem- peratures are required to maintain the necessary cyclic operation of the process [3]. Kinetic studies show that 100% conversion of the first reaction at 1033 K takes approximately 1 hour. Reaction 2 requires about 10 min, the third and fourth reactions take less than I hour each at the temperatures given above. Therefore, one complete cycle takes approximately 1 hour at the end of which reactor R2 contains CaBr,(s), reactor R3 FeBr,(s) and similarly, reactor Rl contains CaO(s) and reactor R4 Fe,O,(s). The reactors are switched (R2+Rl, R3-R4) and the direction of the cycle is reversed. Therefore, the process is operated in cycles, continuously producing hydrogen and oxygen.

Heat exchangers HXI, HX2 and HX3 are used to regulate the reaction temperatures of reactions 4, 3 and 2 respectively. Sl and S2 are composite membrane sep- arators, which are being developed. For example, to sep- arate hydrogen, a zirconium-silica membrane is being developed, which has a maximum operating temperature of 773 K [IO]. A similar scheme has been employed recently in the flow sheet of the non-adiabatic UT-3 pro-

‘.O I 0.8

_ 0.6 L

x 0.4

0.2

0.0 I 920 940 960 980 1000 1020 1040

T[Kl Fig. 2. Experimental results showing the effect of temperature

on the conversion of the first reaction, equation 1.

Fig. 1. Flow sheet of the adiabatic UT-3 thermochemical process (numbers in diamonds are the stream numbers used in Table 2).

ADIABATIC UT-3 THERMOCHEMIC4L PROCESS 867

cess [9]. Depending on the final characteristics of the composite membranes, it is possible to enrich the hydro- gen and oxygen contents in the streams and to have the necessary favorable concentrations at the separators.

3. PROCESS EVALUATION

Following the design characteristics of earlier UT-3 thermochemical plant designs, the process is run at 2.0 MPa, the line pressure of hydrogen is assumed to be 2.5 MPa, that of oxygen 1.8 MPa, the capacity factor is 80% and the mean temperature difference of the heat exchangers is AT = 30 K [S]. Design characteristics are summarized in Table 1.

ASPEN-PLUS is used for the evaluation of the process. Mass balance and stream composition results of the simulation with excess steam are shown in Table 2. It is noted that the results are the same when H,O+N, is used instead of steam alone as a heat carrier fluid. For example, the composition of stream 1 becomes (12276 H,O+24310 N2) and reactor Rl still operates adia- batically. The power required by the process is evaluated as 225.4 MW, as shown in Table 1. Process heat matching

Table 1. Design characteristics of adiabatic UT-3 process

Item Value Unit

Hydrogen production rate Hydrogen production rate Process system pressure Helium inlet/outlet temp. from

HTGR Hydrogen line pressure Oxygen line pressure Capacity factor Thermal power of the process

372 mol/s 30,000 Nm3/h

2.0 MPa 1123/973 K

2.5 MPa 1.8 MPa

0.80 225.4 MW

and power requirements are presented in Tables 3 and 4, respectively. Table 3 shows that after heat matching, a thermal energy of 109.4 MJ/s at the 587.8/493.0 K level remains. This energy is used to produce steam, which is expanded in a Rankine turbine to produce power. The resulting power of 19.6 MJ/s (or MW) is used to supply power to equipment as presented in Table 4. The excess power is 0.7 MW.

4. EXERGY ANALYSIS AND EFFICIENCIES

The chemical process is made up of various modules and streams where chemical reactions involving com- position, temperature and pressure changes take place. Following [ll], the first and second laws of ther- modynamics, for an open and steady flow system, can be written for each module as

Qm-Wm= AH (5)

AH= C~H-~PZH (6) ““L 1”

s,=AS-($ (7) cl

Using equations (5) and (7) the entropy production in a module, S,,,, can be calculated as

s,=AS-!5g!S. 0

The exergy loss in module m is calculated from the Gouy- Stodola Theorem and defined as

E,,, = T,S,,,. (10)

Table 2. Mass balance and stream compositions in mol/s of the adiabatic UT-3 process

Stream No. H, 02 HBr Brz H*O State T (K) P (MPa)

2 3 4 5 6 7 8 9

10 11 12 13 14 I5 I6 17 18

372 372 372

I86 186 186

744 744

2976

2976 2976 2976

372 372

31,372 37,000 3’7,000 35,512

35,512 35,512 35,512 3’7,000 3’7,000 37,000

3’7,000 372 372

3’7,372

g g g g g g g g g g g g g g g I g g

1033.0 2.0 957.0 2.0 833.0 2.0 724.5 2.0 303.0 0.019 636.0 2.5 724.5 2.0 493.0 2.0 493.0 2.04 576.4 2.0 845.0 2.0 864.9 2.0 303.0 0.0095 656.0 1.8 864.9 2.0 300.0 2.0 527.8 2.0 861.7 2.0

868 M. SAKURAI et al.

Table 3. Process heat matching in the UT-3 adiabatic process

Stream supplying heat Stream absorbing heat Heat Temp. (K) Energy (MJ/s) Heat Temp. (K) Energy (MJ/s) exch. initial final available remaining exch. initial final needed remaining

HX2 724.5 493.0 321.0 128.7 HX3 546.4 715.6 378.8 186.5 HXI 957.0 833.0 186.5 0 HX3 715.6 845.0 186.5 0 HX2 587.8 493.0 128.7 109.4 HX4 300.0 527.8 19.3 0

Table 4. Power requirements in the UT-3 adiabatic process

Equipment Pressure (MPa)

inlet outlet Temp. (K)

inlet outlet Flow Rate

(mob) Power WV

Cl 0.019 2.5 303 636 372 10.3 c2 2.0 2.04 493 493 38488 2.9 c3 0.0095 1.8 503 656 186 5.7

Total 18.9

The total exergy loss of the process is the summation of all exergy losses for each module.

E,,,, = 1 Em. m

The overall thermal efficiency (or efficiency) of the hydrogen producing culated as

? = Q(Hd+Qe CQi

(11)

the first law process is cal-

(12)

where Q(H2) is the high heating value of hydrogen, Qe is the excess heat in the process and ZQi is the total thermal energy input supplied to the process. Q(H2) = 285.9 kJ/H,, the total thermal energy supplied to the process is that from HXOl by HTGR.

The overall exergetic efficiency of the process is cal- culated as

W-I,) + W,) + E, &= =,

where E(H,) is the exergy content of hydrogen, E(0,) that of oxygen, Ee is the exergy of the excess power in the process and ZEi is the total exergy supplied to the process. E(H,) = 235.200 kJ/H,, E(0,) = 3.948 kJ/02. The total exergy, CE,, supplied to the process is the exergy content of the nuclear energy, supplied by He from 1123 to 913 K.

Using the results of the process evaluation, the thermal efficiency is calculated by equation (12) as

r? = W’W8W+ 3823 = 48 90/ 225405

. 0

and using equation (13) the exergetic efficiency is

&= (372)(235.2)+(186)(3.948)+683 = 53,20/

167114 0

where 3823 kJ/s is the excess energy and 683 kJ/s the corresponding excess power.

5. RESULTS AND DISCUSSION

The results show that the adiabatic UT-3 cycle is more efficient than others reported earlier. The thermal eficiency of the adiabatic cycle is 48.9%, which is higher than the estimated efficiency of the Toyo flow sheet. This shows its superiority. An exergy efficiency of 53.2% con- firms the excellent use of exergy in the process. The energy and exergy duties of various equipment are shown in Table 5. It can be noted that exergy changes in the reac- tors are small compared to those in the heat exchangers. The exergies from HXl and HX2 are available, which are used in HX3 and HX4 and in power generation. The net exergy produced by the process is the sum of the exergies of hydrogen and oxygen, which are 87.5 and 0.4 MJ/s, respectively.

The advantages of the adiabatic UT-3 process over the non-adiabatic one, affecting its efficiency, cost and operation are:

Table 5. Energy and exergy duties in various equipment of the adiabatic UT-3 process

Equipment Energy (MJ/s) Exergy (MJ/s)

Reactor Rl 0 8.0 Reactor R2 0 9.3 Reactor R3 0 36.9 Reactor R4 0 2.7 Heat exchanger HXOI 225.4 167.1 Heat exchanger HX 1 186.5 124.4 Heat exchanger HX2 321.0 164.3 Heat exchanger HX3 378.8 218.4 Heat exchanger HX4 19.3 6.8

ADIABATIC UT-3 THERMOCHEMICAL PROCESS 869

(1) Intermediate heat exchangers to supply heat to the endothermic reactions are not required. In general, heat exchangers operating in a corrosive atmosphere are very expensive and also have high flow resistance, resulting in large power requirements for circulating the product gases. (2) Total heat exchanger duty is lower. In fact, the energy duty of heat exchanger HXOl supplying heat to the endo- thermic reactions is 605.9 kJ/mol H? and the others in the process, HXl to HX4, amount to 2434.4 kJ/mol H,. The duties of the same heat exchangers in the non- adiabatic Toyo flow sheet are 730 and 4983 kJ/mol H,, respectively; they represent about 47% economy in heat exchange, which is directly related to the cost of heat exchangers. (3) Total power consumption for the circulation of gas products and for compression of product gases is 50.7 kJ/mol H,. This can be compared to 137 kJ/mol H, required in the non-adiabatic UT-3 process [8]. The power economy is approximately 63%. (4) Reaction energies are supplied or absorbed by direct contact heat exchange between the vector (steam or steam + nitrogen) and the solid reactants. Thus, the reac- tors can be fabricated using common materials where the containment of corrosive fluids becomes easier [12].

The reactor type may be packed bed, honeycomb type or fluidized bed. F’or the first type, pellets have been developed [6, 71 and the necessary improvements are being done. For the second, honeycomb reactors using Ca and Fe compounds are being developed, which will be considered for the UT-3 process [13]. For the last, suitable pellets should be studied and developed. This study shows that the flow rate of the circulating gas through the reactors is sufficient to obtain the necessary velocities for fluidization in all four reactors. Therefore, this represents an interesting alternative, since the advan- tages of fluidized bed reactors are numerous [ 141.

Combined production of hydrogen and power using HTGR is one of the aims of the ongoing efforts [8]. Therefore, modulation of the hydrogen and power pro- duction rates is an important operational aspect of the project. For this reason, a parametric study has been carried out to find the nuclear power needed in HXOI and the thermal efficiency for various hydrogen production rates. The production level is modulated for the same process plant, which is sized for 30,000 Nm’ per hour hydrogen production. The operation of the process is performed using the same amount of heat carrier fluid, but by taking some part of the reactor modules off duty and reducing the feed water into the process. For exam- ple, to halve the hydrogen production rate, half of the reactors are shut down and 186 mol H,O/s is fed to the process. The results are presented in Fig. 3 where the thermal efficiency is calculated by equation (12) with Qc = 0, i.e. without credit to excess thermal energy or power. It is seen that when the process is run at the maximum temperature of 1033 K, the process efficiency decreases for decreasing hydrogen production rate and as a result, the nuclear heat supply to HXOl increases.

=220 * 8 ' 11 * j " 18 ' c I !t

10 t- 15 20 25 30

PRODUCTION H, [ Nm3/h x 10" ]

Fig. 3. Nuclear heat required and process thermal efficiency (calculated using equation (12) with Q. = 0) as a function of

hydrogen production rate.

The increase of nuclear heat, the decrease in thermal efficiency of the chemical process and the hydrogen pro- duction rate are related by nonlinear relations. For exam- ple, for 50% less hydrogen production, nuclear heat demand is increased by 4.7% for a thermal efficiency drop of 52%. Power duties of compressors Cl and C3 are reduced by 50%. Heat duties of heat exchangers HXl to HX3 are about 20% larger and an excess heat is available for power production. When the excess heat, Qe, is taken into account, the results showed (not presented in the figure) that the overall thermal efficiency of the combined cycle remained approximately constant while producing power and less hydrogen. For example, for 15,000 m3 per hour hydrogen production, the nuclear heat demand is 236 MW and an excess power of 27.5 MW is produced. Therefore, the modulation of the hydrogen and power production in the same plant can be accomplished easily using modular reactors. Over- sizing of heat exchangers, HXl to HX4, is not required since the temperature levels and differences become more favorable for heat transfer when the hydrogen production rate is reduced.

6. CONCLUSIONS

An adiabatic UT-3 thermochemical process has been conceived and evaluated. It is a high efficiency, inherently low cost thermochemical process. In comparison with non-adiabatic processes,

(i) Energy and exergy economies have been increased approximately by 20%.

(ii) Total heat duty of heat exchangers has been reduced approximately by one half.

(iii) Total power of the equipment has been reduced by more than half.

(iv) The reactors can be fabricated using conventional materials.

(v) Any reactor type can be used, including fluidized bed.

(vi) Hydrogen and power production can be modulated

870 M. SAKURAI L’I ul

by using modular reactors and by adjusting flow rate of water feed.

8.

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