Pergamon ht. J. Hwfrogm tkr,~.v. Vol. 20, No. ,4, pp. 29:’ WI+ 1YYi

Copyrtght ‘I‘ 1995 Internatmml Associatmn for Hydrogen Energ) Elsevier Science Ltd

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IMPROVEMENT OF Ca-PELLET REACTIVITY IN UT-3 THERMOCHEMICAL HYDROGEN PRODUCTION CYCLE

M. SAKURAI, A. TSUTSUMI and K. YOSHLDA Department of Chemical Engineering, University of Tokyo, 7 Hongo. Bunkyo-ku, Tokyo 113, Japan

Abstract--The UT-3 thermochemical hydrogen production cycle consists of four gas-solid reactions. In these reactions, hydrolysis of CaBrz is the slowest reaction. Aiming at the improvement of this reaction rate, the mechanism of hydrolysis was carefully analysed, and a model for explaining the reaction progress was proposed. Then. a nr’\h C.t-pellet. which was produced by removing an obstacle out of the reaction process. was tested.

cg,, MCaBr, M CaO Rcao w

w, x

NOMENCLATURE

Mole concentration of CaO in a pellet (mol pellet ‘) Molecular weight of CaBr, Molecular weight of CaO Weight fraction of CaO in a pellet Weight of reacted pellet (kg) Weight of unreacted pellet (kg) Conversion defined by equation (5)

INTRODUCTION

It is important to produce hydrogen from water by using a clean heat source such as solar and nuclear energy, because hydrogen is a clean energy carrier. The thertno- chemical production process is expected to be a method which may produce hydrogen from water at high effi- ciency. The UT-3 thermochemical hydrogen production cycle [l] consists of four reactions, containing Ca, Fe and Br compounds, expressed as

<‘aBrz (s) + H,O (g) m CaO (s) + 2HBr (g) (1)

CaO (sl + Br, (g) 7:38731( CaBr, (s) + 40, (g)

(2)

Fe,O, (s) + 8HBr (g) B 3FeBr, (s)

+ 4H,O k) + Br, k) (3)

3FeBr, (s) + 4H,O (g) 823-8731( Fe,O, (s) + 6HBr (g) + H, (g). (4)

A bench-scale hydrogen production plant. named MASCOT, based on the above cycie was constructed, and both hydrogen and oxygen were generated smoothly in 11 cycles [2].

All the reactions in the UT-3 cycle are gas--solid reactions. Because the hydrolysis of CaBr, [equation (l)] is the slowest step in this cycle, the mechanism of this reaction in a Ca-pellet was clarified. Considering the proposed reaction model, Ca-pellets were newly prepared and the reactivity was compared with that of previous pellets.

ANALYSIS OF HYDROLYSIS

E.\-periwentcrl

Solid reactants of the UT-3 cycle were made into spherical pehets from mixtures of reactive compounds and binding inert materials. Ca-pellets were made by the alkoxide method [3]; aqueous ethanol was added to a binary ethanol solution of calcium ethoxide, CWW% and titanium tetra-isopropoxide, Ti(OC3H7)4r and hydrolysis took place immediately, forming a mixture of CaO and CaTiO, precursors. After filtration and pelletization, the compounds were calcined at 1327 K for 2 h. The whole process is shown in Fig. I. In this experiment, a 1.77: 1 mole ratio between Ca and Ti was used and the diameter of peilets was about 5 mm. A quartz thermobalance of 31 mm i.d. was used for kinetic measurements. as shown in Fig. 2. A Ca-pellet was put in a platinum wire basket and was suspended by a quartz spring in a thermobalance. The change of weight during the reaction was detected through the eiongcttion of the quartz spring measured by a differential transformer. The reactor was heated by an electric furnace at constant temperature. In this study, the reaction temperatures of

298 M. SAKURAI et al

CaO CaTi(a Pellet

Fig. 1. Flow chart of the alkoxide method.

bromination and hydrolysis were 873 and 973 K, respect- ively, and the concentrations of reactant gas were 0.33 mol rnT3 in bromination and 11.28 mol rnv3 in hydroly- sis, respectively. Ca compounds were cyclically exposed to bromination and hydrolysis. The progress of these reactions, therefore, was described by using only the conversion of bromination, which was represented by the mole fraction of CaO, X, converted into CaBr,, as

x= W - w,). Mcao w, . &a,. Wcasn - MC,,) ’ (3

1. Bn bottle 5. Roller pump 9. Differential transformer 2. Gas mixer 6. Evaporator 10. Qum spring 3. Water bath 7. Quartz tuba 11. Recorder 4. Flow meter 6. Electric furnace 12. Absorption bulb

Fig. 2. Experimental set-up for kinetic study.

In the hydrolysis, the conversion X decreases with the progress of reaction.

The amount of CaBr, in the different radius position in a pellet was measured during the hydrolysis to elucidate the mechanism of reaction within the pellet.

Results of analysis

When bromination was terminated, it was found that the final conversion was radially almost the same in the pellet [4]. At six different values of total conversion, X, in the hydrolysis, each Ca-pellet was divided into three regions, (A), (B) and (C), from the outside, and the amount of CaBr, in each region was measured by using a Br-ion meter. Table 1 shows that the conversions in all regions were almost the same, implying that hydrolysis proceeds homogeneously in the pellet.

IMPROVEMENT OF THE Ca-PELLET

Reaction model

A reaction model of bromination has already been proposed, as shown in Fig. 3 [4]. In a single pellet, a number of agglomerates about 3 pm in diameter, that are classified into two types, one including CaO particles about 0.5 pm in diameter and the other including CaTiO, particles about 0.5 pm in diameter, are homogeneously distributed. The pores below 0.5 pm in diameter (defined as micro pores) exist within the agglomerates, while the pores above 0.5 pm in diameter (defined as macro pores) are crevices among agglomerates and the bromination takes place in the CaO agglomerates uniformly. When the pores below 0.5 pm in diameter in the CaO agglom- erate are plugged up, bromination is terminated. Figure 4 shows a comparison between computed and observed time histories of bromination.

Ca-pellet

Fig. 3. Concept of a reaction model.

Ca-PELLET REACTIVITY IN A HYDROGEN PRODUCTION CYCLE 299

Table 1. Change in CaBr, concentration in a Ca-pellet during hydrolysis

No. (A) region

X

(B) region (C) region Total conversion

I 0.59 0.59 0.60 0.59 2 0.49 0.50 0.50 0.50 3 0.46 0.43 0.46 0.45 4 0.36 0.39 0.33 0.36 5 0.14 0.25 0.26 0.22 6 0.07 0.14 0.18 0.13

0.8

y- 0.6

x0.4 I P

0.2 t# c Exp.Jl.01) - - -Cak.(t.o~)

O.Oo I% 2.

EXP (2.01) -. - Calc.(2.01,

4 6 8 10 time [ min ]

Fig. 4. Comparison between computed and observed time hIstories of bromination K’aOKaTiO, = 0.45, 0.76, 1.01, 2.01).

A new method for pellet preparation

Considering that hydrolysis starts from the point where bromination terminates, it is expected that subdividing CaO agglomerates to increase the reaction surface area will be quite effective for increasing the rate of hydrolysis. A concept of subdivision of, CaO agglomerates is shown in Fig. 5. Thus, a foaming a@nt is added in the precursor

1 subdivision

Fig. 5. Image of the subdivision of a CaO agglomerate

23 Filtration

Fig. 6. Flow chart of a new method for the preparation of a pellet.

of the reactant and by volatikkg this agent in the calcination process the CaO agglomerates are expected

precursors of the reactive and biitder compounds were prepared stprtrately by using the a&oxide method. The precursor of the reactant was filtrated and then dispersed

Pore diameter [ pm ]

Fig. 7. Comparison of pore size distributions between the conventional pellet and the new one.

300 M. SAKURAI et al.

Fig. 8. SEM photograph of a conventional pellet.

Fig. 9. SEM photograph of a new pellet.

Ca-PELLET REACTIVITY IN A HYDROGEN PRODUCTION CYCLE

0.0 s I I I 1 I 0 2 4 6 8 10

time [ min ]

0 20 40 60 80 100 120

time [ min ]

Fig. 12. Solid conversion profiles of hydrolysis bl cyclic Fig. 10. Comparison of solid conversion profiles of bromination

between the conventional pellet and the new one. operation

again in iso-propanol. The foaming agent was added in this slurry and dissolved. In this study, the molar ratio between this agent and CaO, the reactant, was 1: 1. Water was added to deposit lauric acid again, then this precursor was mixed with the precursor of binder compounds. This mixture was filtrated and pelletized. The pellet was calcined at 1327 K for 2 h. A flow chart of this method is shown in Fig. 6. The formation of CaO and CaTiO, was confirmed by X-ray diffraction. The molar ratio between Ca and Ti was 1.71: 1 and the diameter of pellets was about 5 mm. Comparison of the pore size distribution and structure between new and old pellets was made by a mercury porosimeter and a scanning electron micro- scope (SEM). Furthermore, the reactivity of hydrolysis was compared by using the thermobalance shown in Fig. 2.

SEM photographs of pellets prepared by the two methods are shown in Figs 8 and 9. It is observed that the subdivision of CaO agglomerates is realized, as expected, in the new method.

The brominations of the two kinds of pellets were almost the same, as shown in Fig. 10. On the other hand, the rate of hydrolysis was improved remarkably in the new pellet. Figure 11 shows the time history of the first-cycle hydrolysis of the new pellet (C&o = 1.30 x 10m4 mol pellet-‘) in comparison with the old pellet Gao = 1.29 x 1O-4 mol pellet- ‘). It is noticeable that the reaction rate of hydrolysis becomes three times faster. Figure 12 shows the time history of the cyclic operation using the new pellet. It is found that the reactivity of hydrolysis is well maintained in the cyclic operation.

CONCLUSIONS

Results and discussion

Figure 7 shows a comparison of pore size distributions between the two methods. It was found that pore volume below 0.5 pm (micro pore) was almost the same, but pore volume above 0.5 pm (macro pore) in this new method was about four times greater than that in the old method.

experiments and model analysis, a new method for

The hydrolysis of CaBr, in the UT-3 thermochemical

preparation of Ca-pellets was proposed. The foaming

cycle was analysed. Based on results obtained from

agent was introduced to split the CaO agglomerates into finer agglomerates in the preparation. The reactivity of hydrolysis of the new pellet was improved remarkably, and it was well maintained in cyclic operatior+

1.” / -Conventional method -----New method

Acknowledyement-This study was partially supported by a Grant-in-Aid for Scientific Research on Priority Areas from the Ministry of Education, Japan.

REFERENCES

i I. H. Kameyama and K. Yoshida, Proc. 2nd World Hydrogen

Energy Co@, Zurich, Switzerland, pp. 829-850 (1978). 0 50 100 150 200 250 2. M. Sakurdi, M. Aihara, N. Miyake, A. Tsutsumi and K.

time [ min ] Yoshida, Int. J. Hydrogen Energy 17, 587--592 (1992).

3. M. Aihara, H. Umida, A. Tsutsumi and K. Yoshida, Inr. J,

Fig. 11. Comparison of solid conversion profiles of hydrolysis Hydrogen Energy 15, 7-11 (1990).

between the conventional pellet and the new one. 4. M. Sakurai, A. Tsutsumi and K. Yoshida, submitted to 10th

World Hydrogen Energy Cot@. Cocoa Beach, FI (1994).