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LABORATORY SCALE DEMONSTRATION OF CIS PROCESS

S. SHIMIZU, K. ONUKI, H. NAKAJIMA, Y. IKEZOE and S. SATO

Japan Atomic Energy Research Institute, Tokai-mura+ Ibaraki-ken, 319-11, Japan

(Received forpublication 27 May 1987)

Abstract--For the purpose of demonstrating the chemical practicability of CIS Process, a closed loop demonstration apparatus was constructed and operated. The loop consists of four key reactions of six reactions in CIS Process. Methanol was converted to methane and oxygen by the apparatus with conversions of 63-80%. 12 cyclic operations were successfully carried out with no serious troubles. Indications of side reactions, deposition of carbon-like compounds and formation of sulfurqike precipitates, were recognized.

INTRODUCTION

A thermochemical hydrogen production cycle compris- ed of the following chemical reactions (CIS Process) has been studied in our laboratory since 1983 [1].

S O 2 ( aq ) .+ xI 2 (aq) + 2H20(1) = 2HIx(aq) +

H2SO4(aq) (1)

CH3OH(1) + HI,(aq) = CH3I(1) + H20(1) + 0.5 (x

- 1) I2(aq) (2)

CHsI(g) + HI(g) + 0.5 (x + 1) I2(g) + nHzO(g) =

CH4(g) + 0.5 (x + 1) I2(g ) + nH20(g) (3)

CH4(g) + H20(g) = CO(g) + 3He(g) (4)

components in a laboratory apparatus is unfavorable because (a) it becomes sizable, (b) operation of the compressor is difficult at low gas throughputs, and (c) safety measures complicate the apparatus [4].

The objectives of the demonstration are - - to ascertain whether the specified reactant conver-

sion could be reached and maintained using circulat- ing materials repeatedly,

- - to check the possible formation of by-products, - - to obtain knowledge for the design of the larger scale

apparatus which will be operated continuously. The apparatus was constructed of quartz and glass

equipment which were connected with the PTFE tubes. For the sake of simplifying the problem, no metal reactor was used, although a material screening test has been performed for the main reaction [5]. In this communication, a brief description of the apparatus and the results of cyclic operations are presented.

CO(g) + 2H2(g) = CH3OH(g) (5)

H2SO4(g) = H20(g) + SO2(g) + 0.5 O2(g ) (6)

The theoretical thermodynamic efficiency of the cycle was estimated [2]. Also, we verified the two key reactions, i.e. reactions (2) and (3), experimentally [3].

Based on these studies, we have tried to design and construct a closed loop demonstration apparatus. In the present study, though, it was preferred to demonstrate a cycle composed of reactions (1), (2), (3), and (6), by which methanol was converted to methane and oxygen, instead of demonstrating the whole cycle. The reasons of the preference are as follows. (A) Since steam reforming reaction, reaction (4), and methanol synthesis, reaction (5), are both well known and are already realized on the industrial scale, chemical practicability of the whole cycle can be evaluated from the closed loop operation of the four reactions. (B) Methanol synthesis is carried out at pressures at least more than 50 atm. The use of such high pressure

DESCRIPTION OF THE APPARATUS

The apparatus consists of seven sections, (1) Bunsen reaction section, (2) methyl iodide synthesis section, (3) methane synthesis section, (4) concentration section of sulfuric acid, (5) decomposition section of sulfuric acid, (6) holding section of circulating materials (settler) and (7) analysis section. The Bunsen reaction section has two reactors, a packed column and a semi-batch reactor. The methyl iodide synthesis is carried out in a batch reactor. Flow reactors are used for methane synthesis and decomposition of sulfuric acid The concentration of sulfuric acid is performed by distillation. The flow diag- ram of the apparatus is shown in Fig. 1. In the ligurc, main equipment is numbered from 1 upwards, and a short description is also given. In one cyclic operation, (I. 1 tool of methanol is to be converted to 0.1 mol of methane and 0.05 mol of oxygen. The production rate of methane was designed to be 1 1/h S.T.P. (20°C, I atm). Reactions arc performed in the apparatus as follows.

687

688 S. SHIMIZU et al.

Vent CH

15o (~)

CH4/N

I~0°cll I I I < . I

,k HAS04 phase N

HI× phase

~OzlNz

@

HI×

Fig. 1. Scheme of the demonstration apparatus. (1) Bunsen reactor I. (2) Bunsen reactor II. (3) CH3I synthesizer. (4) CH4 synthesizer. (5) H2SOa concentrator. (6) H,SO4 decomposer. (7) Mixed acid settler. (8) Gas-liquid separator. (9) CH ~1 vaporizer. (1{I) Feed pump, Ill,. (11) Feed pump, H-SO4. (12) Fced

pump, 65 wt% H 2 S O 4.

Bunsen reaction [6, 7]

The Bunsen reaction is performed in two reactors at room temperature. The first one is a packed column (10 mm I.D. × 300 mmL) in which glass springs of 5 mmO.D. × 7 mmL are packed. Sulfur dioxide and oxygen from the sulfuric acid decomposer are carried to reactor I by nitrogen and flow down in it contacting with HIx solution. HIx solution flowing down the column is returned to the settler continuously. A large amount of sulfur dioxide is absorbed in the solution by the treat- ment. Then, the gas is separated from the liquid and enters reactor II. Reactor II is of gas washing bottle type and has a capacity of 200 ml, where the gas bubbles up through HIx solution.

Methyl iodide synthesis

Methyl iodide is produced from methanol and HIx solution by heating the mixture to 90°C, for 2 h under reflux. Methyl iodide produced is isolated from the mixture by distillation.

Methane synthesis

Methane synthesizer consists of a methyl iodide vaporizer, a HIx solution vaporizer, and a reactor. Methyl iodide is boiled and mixed with the nitrogen carrier. The partial pressure of methyl iodide is control- led by passing through a condenser at 15°C. HI, solution

is vaporized on a quartz wool bed of ca 330°C. and mixed with the nitrogen-methyl iodide mixture at the entrance of the reactor. The reactor is a quartz tube of 2.2 I and is kept at ca 480°C. The exit gas is cooled to below 100°C, and the condensed HI, solution flows down to the settler, where methane is separated from the liquid and carried to the analysis section by nitrogen.

Concentration and decomposition o f sulfuric acid

Sulfuric acid produced by the Bunsen reaction is heated to ca 145°C and concentrated to 65 wt%. Hydriodic acid and iodine in the sulfuric acid are removed by distillation and returned to the settler. The sulfuric acid decompos- er is made of a quartz tube of 8 mmI.D, x 300 mmL. Catalyst of 1 wt% Pt on a-A1203 beads (2 mmD) is packed in the central portion of the tube for length of 100 mm, where temperature is kept at 850°C. The sulfuric acid of 65 wt% is pumped to the lower portion of the tube, where temperature is controlled at 400°C. It is vaporized there continuously, flows up through the tube, and is decomposed in the catalyst zone. Nitrogen is used as a carrier gas.

Analysis

The gaseous product, CH4 or O2, is carried to the analysis section by nitrogen. The gas is washed with

CIS PROCESS

alkaline solution (NaOH) and dried by cooling to -40°C. Then, the flow rate is moni tored by a mass flow meter (Hastings, ALL-1KP). The concentrat ions of methane and oxygen are moni tored by an IR spec- t rophotometer (Shimadzu, IR-435) and an oxygen meter using solid zirconia electrolyte (Toray, LC- 700H), respectively. These data are transferred to a personal computer (NEC, PC-9801E) where the flow rate and the total volume of the product are calculated. The circulating solution is characterized by the density and the concentrat ions of acid, iodine and sulfate. These parameters are measured by a densimeter ( P A A R , DMA35) and titration.

Operational procedure The operations are carried out consecutively in the

order of (1) methyl iodide synthesis, (2) concentrat ion of sulfuric acid, (3) decomposit ion of sulfuric acid, (4) Bunsen reaction and (5) methane synthesis. Here, reactions (3) and (4) are carried out sequentially at the same time. The order was determined so as not to largely change the composit ion of the circulating solution, preventing iodine precipitation in the PTFE tube. Since the concentrat ion of iodine in HIx solution is very close

689

0 ( ~

2.0

1.0

0.10 tool

CH4

02

0 I I I I I I I I I I f I

0 I 2 3 4 5 6 7 8 9 IO II 12 No. of cycle

Fig. 2. Results of the cyclic operation of the apparatus.

to saturation, it is likely to precipitate by operations which cause the hydriodic acid concentrat ion lower or the iodine concentrat ion higher.

Table 1. Characteristics of circulating materials

Initial charge After operation

HIx solution density 2.342 g cm -3 2.513 g cm 3 volume 266 ml 208 ml [H + ] 4.06 N 4.13 N [I2] 5.20 M 5.82 M [SO4- -] 0.19 M 0.24 M

Sulfuric acid solution density 1.300 g cm -3 1.368 gcm 3 volume 56 ml 38 ml [H +] 10.6 N 11.4 N [12] 0.056 M 0.16 M [SO- 4] 5.25 M 5.41 M

Concentrated sulfuric acid density 1.502 g cm- 3 1.512 g cm 3 volume 33 ml 27 ml [H +] 19.6 N 20.1 N

Solid phase iodine 99 g 96 g

Total amounts of chemicals H2504 65 g 51 g HI 130 g 100 g 12 450 g 410 g H20 220 g* 150 g

* 20 g of water was added at the end of 10th cyclic operation.

690 S. SI11MIZU et al.

RESULTS

Before the cyclic operation of the apparatus, some preliminary experiments were carried out, and the following results are obtained using the apparatus. The yield of methyl iodide to methanol, which largely depended on the heating and stirring conditions of the reaction mixture in distillation, was 90% at best, when 4 ml of methanol was reacted with 60 ml of the Hit solution for 2 h at 87°C and product separation was done by distillation. The yield of methane to methyl iodide was 96% in the methane synthesis. The concentration of methane was kept about 10 mol% in the nitrogen carrier. The expected production rate of 1 I-CH4/h S.T.P. was attained. Also, the expected oxygen flow rate was reached in the catalytic decomposition of sulfuric acid. The concentration of oxygen was about 5 mol% in the nitrogen carrier.

Following these experiments, cyclic operations were performed 12 times with no serious trouble. In each cycle, 4 ml of methanol (ca 0.1 mol) was introduced. The yields of methane and oxygen are shown in Fig. 2. The total volumes of methane and oxygen produced by 12 runs were 20.9 1 and 10.4 1 at 20°C and 1 arm, respectively. The stoichiometric ratio of the yields of methane and oxygen was maintained, i.e. CH4/O2 = 2/1, by controlling the amount of sulfuric acid to be decom- posed. Since the initial charge of sulfur was 0.66 tool, and 20.9 1, i.e. 0.87 tool of methane was produced, the sulfur in the system transformed its chemical state 1.3 times in average at the end of all the operations. The yield of methane to methanol increased from 63% to 80% with increasing number of cycles. This increase is partly due to the improvement of distillation condition to isolate methyl iodide from the circulating HI, solu- tion, and partly due to the accumulation of unseparated methyl iodide in the HIx solution. Some indications of side reactions were recognized after the cyclic opera- tions. A small amount of sulfur-like precipitates was observed at the settler and at the sulfuric acid vaporizer. A possible source of the sulfur is the reaction of hydriodic acid and sulfuric acid in the concentrated solution [6-8]. Also, black carbon-like deposits were observed at the HI~ solution vaporizer and at the catalyst supporting quartz wool of the sulfuric acid decomposer. Carbon might be originated from the methyl iodide accumulated in the circulating solution during cyclic operations.

The characteristics and the amounts of circulating materials are shown in Table 1 before and after the cyclic operations. As seen in the table, the densities and the concentrations were nearly identical before and after the operation, while the volumes decreased greatly after the operation. It should be noted that 20 ml of water was added to the circulating solution at the end of the 10th cyclic operation in order to compensate water loss during the cyclic operations. The amount of each chemical is also shown in the table, which was calculated assuming that the chemicals in the solution were H2SO~, HI, I2 and H2 O, The amount of each chemical de-

creased 10--20 mol%. The chemicals is considered to have escaped from the apparatus with the carrier gas [9]. In the case of sulfur dioxide, a certain amount of SO2 in the carrier gas (ca 0.1 mol%) was detected by a gas detector.

CONCLUDING REMARKS

Chemical practicability of the CIS Process was confirmed by operating the closed loop demonstration apparalus. All the reactions and separation steps proceeded with no serious problems. The following points can be pointed out as the concluding remarks.

(1) Methanol could be converted to methane and oxygen by the apparatus with the conversion of 63-80%. This value is rather good considering that each reaction was carried out only once in one cycle.

(2) Sulfuric acid and HIx solution should be free from carbon compounds when they are vaporized, to prevent the possible deposit of carbon-like compounds. In order to realize this, it is important to separate methyl iodide and/or unreacted methanol completely from HIx solu- tion, since the yield of this step is low to that of methane synthesis. It is necessary, then, to obtain data of vapor-liquid equilibrium of HI-I2-CH3I-H20 system.

(3) It is important to separate hydriodic acid and sulfuric acid as completely as possible to prevent sulfur formation. It might be attained by carrying out Bunsen reaction with more excess iodine at 95°C [10].

(4) The dissipation of chemicals out of the cycle will be decreased if carrier gas is not used in an industrial plant.

The operation of the closed loop demonstration apparatus was discontinued, since the apparatus is being remodeled in order to deal with a less complicated cycle of the iodine-sulfur family for thermochemical hyd- rogen production.

REFERENCES 1. S. Sato, S. Shimizu, H. Nakajima, K. Onuki, and Y.

Ikezoe, Proc. 5th Worm Hydrogen Energy Conf., Vo[. 2, pp. 457-465. Toronto, Canada (July 1984).

2. K. Onuki, S. Shimizu, H. Nakajima, Y. Ikezoe and S. Sato, Int. J. Hydrogen Energy 9,391-396 (1984).

3. K. Onuki, S. Shimizu, H. Nakajima, Y. Ikezoe and S. Sato, Proc. 6th World Hydrogen Energy Conf. Vol. 2, pp. 723--731. Vienna, Austria (July 1986).

4. D. van Velzen, H. Langenkamp, G. Schuetz, D. Lalonde, J. Flamm and P. Fiebelmann, Int. J. Hydrogen Energy 5,131-139 (1980).

5. JAERI, unpublished data (1985). 6. T. Kumagai, C. Okamoto and S. Mizuta, DENK1

KAGAKU 52,812-819 (1984). 7. S. Shimizu, K. Onuki, H. Nakajima and S. Sato, DENKI

KAGAKU53, 114-118, (1985). 8. S. Mizuta and T. Kumagai, Int. J. Hydrogen Energy 10,

651-659 (1985).

CIS PROCESS 691

9. S. Mizuta and T. Kumagai, Proc. 5th World Hydrogen Energy Conf., Vol. 2, pp. 421431. Toronto, Canada (July 1984).

10. J. H. Norman, K. J. Mysels, D. R. O'Keefe, S. A. Stowell and D. G. Williamson, General Atomic Report GA-A14746 (Dec. 1977).