catalytic production of elemental sulfur from the

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Department ofCivil and Environmental Engineering, Department ofChemical ... As was mentioned, the recovery of sulfur from coal before combustion usually.
© 1996 OPA (Overseas Publishers Association)

Chem. Eng. Comm., 1996, Vol. 143, pp.73-89 Reprints available directly from the publisher Photocopying permitted by license only

Amsterdam B. V. Published in The Netherlands under license by Gordon and Breach Science Publishers SA Printed in Malaysia

CATALYTIC PRODUCTION OF ELEMENTAL SULFUR FROM THE THERMAL DECOMPOSITION OF H 2S IN THE PRESENCE OF CO 2 Downloaded by [University of Cincinnati Libraries] at 11:57 01 June 2012

DEBORAH SORIANO', TIM C. KEENER', and SOON-1AI KHANG Department of Civil and Environmental Engineering, Department of Chemical Engineering, University of Cincinnati, Cincinnati, Ohio 45221-0071 (Received May 17, 1994, infinalform August 25,1995)

The feasibility of using a cobalt-molybdenum (Co-Mo) sulfide catalyst that was prepared from a commercial Co-Mo oxide catalyst for the production of elemental sulfur from hydrogen sulfide (H,S) and carbon dioxide (CO,) in a packed bed catalytic reactor was studied. It was demonstrated that the desired sulfide catalyst could be prepared by first reducing, then sulphiding the corresponding oxide. The results showed that the prepared catalyst was capable of producing elemental sulfur from the thermal decomposition of H,S in the presence of CO 2 over a temperature range of 465-700°C and at atmospheric pressure. A specific rate coefficient was calculated as well as the Arrhenius parameters for the non-equilibrated reaction. The H 2S decomposition reaction was found to be a second order reaction and have an activation energy of 114.4 kI/mol (27.3 kcal/rnol).

KEYWORDS

Sulfur

H,S

CO,

Catalyst

Co-Mo

Decomposition

INTRODUCTION As part of the 1990 Clean Air Act, legislation was invoked to prevent environmental degradation due to acid rain precipitation. The precursor for the formation of acid rain is sulfur dioxide (SO,) which, for the most part, is produced by coal-fired power plants .' For this reason, most coal-fired plants are being equipped with air pollution control systems such as scrubbers in order to scrub out the SO, from the flue gas to curb S02 emissions, Since these air pollution control systems add to the operating costs of a coal-fired power plant, there has been an increasing incentive to recover the sulfur being emitted into the atmosphere in the form of elemental sulfur from either the coal before combustion or the flue gas. The sale of the recovered elemental sulfur, which is marketable since there are many industrial uses for it,? would offset the cost of producing "clean energy". The sulfur recovered from coal before combustion is usually in the form of gaseous hydrogen sulfide (H,S).3 There are presently many processes that have been developed for the treatment of H,S. Of these treatment methods, only a few are based on 'Current address: Brais, Martres et Associes Inc., 75, Rue de Port-Royal Est, Montreal, (Qq, Canada, H3L 3Tl. 'To whom correspondence should be addressed. 73

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a catalytic reaction process while the rest are sorbent based processes. Among the catalytic reaction based processes, the most common is the Claus process which reacts H 2S with S02 to form elemental sulfur as follows: (I)

Another process utilizes oxygen (02) to oxidize H 2S according to the following reaction:

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2H 2S+02+2S2+2H 20

(2)

A third process has the advantage of recovering hydrogen (H 2)..The sulfur is produced from the thermal decomposition of H 2S: (3)

In these cases, the catalyst is used to improve the rate of reaction and thus improve the overall efficiency. Catalysts make up a considerable amount of the total cost, therefore, it is useful to evaluate additional catalysts which could be more effective and/or less expensive.

BACKGROUND AND INCENTIVE As was mentioned, the recovery of sulfur from coal before combustion usually generates H 2S. The thermal decomposition of H 2S to form elemental sulfur has been studied to a great extent in the literature/' " 14 This reaction has the form:

(3) The use of carbon dioxide (C0 2) in the elemental sulfur production process would be an advantage since CO 2 is a known greenhouse gas. In addition, it is readily available since it is the main combustion product in coal-fired power plants. The use of CO 2 to aid in the decomposition of H 2S has been patented by Bowman's and extensively reported by Towler and Lynn.!" They concluded that CO 2 reacts with H 2 formed by the H 2S decomposition reaction according to the water-gas shift reaction: (4)

The continuous consumption of H 2 has the effect of shifting the equilibrium of the H 2S decomposition reaction to the right thus allowing more H 2S to decompose and therefore, more elemental sulfur to be produced. An added advantage was the production of carbon monoxide (CO) in addition to H 2 since CO and H2 are the main constituents of synthesis gas (or syn gas) which is a high quality fuel and has many industrial uses. l ? The studies by Towler and Lynn 16 also included preliminary testing of a molybdenum sulfide (MoS 2) catalyst to demonstrate its effect on elemental sulfur production. They reported significant conversion increases. Since MoS 2 was the only catalyst tested for this process, there is a need for testing other catalysts to determine if they can also be used for the decomposition of H2S with CO 2 present. Cobaltrnolybdenum (Co-Mo) catalysts are typically used for hydrodesulfurization and are therefore readily available from commercial manufacturers. Since Co-Mo is also a catalyst for

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DECOMPOSITION OF HYDROGEN SULPHIDE

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the water-gas shift reaction, 18 this catalyst was tested as a possible candidate for this process. The objective of this research was to test the feasibility of using a Co-Mo sulfide catalyst to produce elemental sulfur from an HzS-CO z gas mixture. This included conducting tests to obtain a method of preparation for the sulfided form of the Co- M 0 catalyst from the oxide form in which the catalyst was purchased. In addition, the prepared catalyst was tested in a reactor to determine if elemental sulfur could be produced over a range of temperatures below which sintering of the catalyst was not likely to occur. If this was determined to be feasible, the conversion was measured and compared with theoretical values obtained from a thermodynamic analysis.

EXPERIMENTAL SETUP Materials

c.P. grade liquid phase hydrogen sulfide (99.5%), Coleman Instrument grade carbon dioxide (99.99%), oxygen-free nitrogen, (N z), and zero grade hydrogen « I ppm impurities) were purchased from Wright Brothers Inc., a distributor for Matheson Gas Products. The commercially manufactured cobalt-molybdenum catalyst (Crosfield 465, 1/20" extrudate) was obtained compliments ofCrosfield Catalysts. The catalyst was reported to contain 5% cobalt oxide (CoO) and 20% molybdenum oxide (Mo0 3 ) by weight with the balance consisting of y-alumina. Its average apparent bulk density was 41 Ib/tt:' (0.658 g/cm 3), the specific area was 255 m 2/g, and the pore volume was 0.58 em 3 / g. In addition, carbon disulfide (CS 2 ) (99.95% purity) was obtained from Fisher Scientific Inc. Thermogravimetric Analysis

A thermogravimetric analyzer (Du Pont Instruments 951 TGA) was used to establish the method of preparation for the Co-Mo sulfided catalyst from the Co-Mo oxide that was obtained from the manufacturer. A schematic diagram of the TGA is shown in Figure 1. Sample sizes of approximately 40 mg consisting of several Co-Mo oxide catalyst pellets were used. Experimental Apparatus

A schematic diagram of the experimental apparatus is shown in Figure 2. Flow rates of CO 2 , H 2S, H 2 , and N 2 were controlled by rotameters. In order to obtain a mixture of the reactants, the gases were first passed through a stainless steel manifold before entering into the reactor. The bypass loop was used when the gases were being turned on in order to obtain a stabilized flow before switching the flow to the reactor. All of the tubing that came into contact with the H 2S gas was stainless steel tubing in diameter. The reactor consisted of a 30" long, I" J.D. quartz tube reactor with a rubber gasket and stainless steel flange on each end. The first 12" of the reactor was heated by

±"

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D. SORIANO et al. Weighing pan Containing catalyst pellets

r-------

Purging gas

Reactant Gas

To gas analyzer

c.=.::>----I Furnace

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TGA Unit FIGURE I

Schematic diagram of thermogravimetric analyser.

Rotameter

~ Gas Purifier

Reactor System

Hydrogen

Gas treatment

GC Ouanz wool

Catalyst bed

Hydrogen Sulfide

Strawberry Tree AID Ctlmcr1,,'r

FIGURE 2

Diagram of packed bed catalytic reactor.

a horizontal tubular furnace while the rest of the length of the reactor was packed with quartz wool and exposed to ambient air. The catalyst bed was centered in the heated section of the reactor and supported on both sides by a plug of quartz wool. In order to control and monitor the temperature inside the catalyst bed, a shielded K type Watlow thermocouple was inserted through the entrance of the reactor and positioned!" off the centerline and approximately half way along the length of the bed. This thermocouple was connected to a data acquisition system to record the temperature history inside the reactor. A second K type thermocouple, with an exposed junction, was used to measure the surface temperature of the reactor. It was connected to a Watlow temperature controller which turned the furnace on and off according to a set point temperature which was adjusted to obtain the desired temperature in the reactor.

DECOMPOSITION OF HYDROGEN SULPHIDE

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For odor control, the product gases were incinerated at the exit of the reactor by a Bunsen burner and vented in a fumehood.

EXPERIMENTAL PROCEDURES

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Catalyst Preparation

A standard method of preparation for Co-Mo sulfide does not exist in the literature since preparation of sulfide catalysts is not usually accomplished by any standard proced ure but under special conditions of preparation. \9 In most cases, however, active sulfide catalysts.are often prepared by converting the respective oxides to sulfides. For this reason, the TGA was used to obtain a suitable method of preparation for the sulfide catalyst. First, enough catalyst pellets were placed on the weighing pan at the end of the balance rod to cover the surface of the pan. The weight of the pellets was approximately 40 mg. The pellets were purged under N 2 at a temperature of 160°C to remove the moisture inside the pellet pores. The purging step was stopped when the weight being recorded reached a steady value. Next, the pellets were reduced using a pure flow of hydrogen at a temperature of 500°C. Once again, the H 2 flow was stopped when the recorded weight reached a steady value. Finally, the pellets were sulfided using a pure flow of hydrogen sulfide at a temperature of 500°C. The flow was also stopped when the recorded weight reached a steady value. For each step, the weight change and temperature above the surface of the pellets were recorded. Catalytic Reactor

The reactor was packed such that one third of the reactor volume that was heated in the furnace contained the catalyst bed in order to maintain an isothermal reaction zone (one-third preheating, one-third reaction and one-third post reaction zones). The catalyst bed weight was approximately 45-50 g. Initially, N 2 was flowed through the bed at a flow rate of 0.02832 m 3/hr (I ft 3/hr (cfh)) at a temperature of 200°C in order to purge the catalyst. After the catalyst had been completely purged, while maintaining the N 2 flow, the cool end of the reactor was opened to remove the moisture that had evaporated from the catalyst bed in the heated section of the reactor and condensed in the cool section. After the moisture was removed, a plug of quartz wool was placed in the reactor just at the exit of the furnace to act as a condenser to collect the sulfur produced. The bed was then heated to a temperature of 500°C and H 2 passed through the reactor at a flow rate ofO.002832m 3/hr (0.1 cfh) in order to reduce the catalyst. Once the catalyst had been reduced, flow to the reactor was stopped and the temperature was adjusted to the desired reactor temperature. One of the key considerations for sulfur-recovery systems is to operate the catalytic reactor at as Iowa temperature as possible that is still above the condensation temperature for the sulfur concentration which is produced.i" Sulfur has a boiling point temperature of 444.6°C @ 1 atmosphere pressure."! Operation at higher temperatures may result in sintering of the catalyst. Both of these would result in blockage of the catalyst pores and reduction of catalytic activity. Therefore, in order to neglect sintering effects as well as

D. SORIANO et al.

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sulfur condensation, the majority of the experiments were carried out at temperatures of 465,490,515, and 575°C; a test was also conducted at 700°C in order to confirm the temperature dependence of the rate constant. Once the desired temperature was reached in the reactor, H 2S was flowed through the reactor at a flow rate of 0.005664 rrr' /hr (0.2 cfh) until the catalyst was sulfided. Afterwards, while maintaining the H 2S flow, the CO 2 flow was turned on and adjusted to 0.005664 rn' /hr (0.2 cfh) to obtain an equimolar reactant mixture of H 2S and CO 2, It was important to sulfide the catalyst at the same temperature that the decomposition was to take place since the decomposition took place immediately after sulphidation of the catalyst. Adjustment of the temperature after sulphidation would have resulted in a non-isothermal reaction. The reaction was run for three hours while the data acquisition system recorded the temperature history within the catalyst bed. After the experiment was over, the bed was cooled down to room temperature under a flow of N 2. Then, while maintaining the N 2 flow, the cool end of the reactor was opened to remove the quartz wool plug and the sulfur that had collected on the inside walls of the reactor. This was accomplished using carbon disulfide, CS 2, to dissolve the sulfur on the walls as well as on the quartz wool plug. The CS 2 solution containing the sulfur was collected in a beaker and the CS 2 evaporated in order to weigh the sulfur product.

THERMODYNAM IC ANALYSIS Thermodynamicanalyses for the reaction ofH 2S and CO 2 were performed by using the JANAF Thermochemical Tables [22] and the STANJAN program obtained from Professor Wm. C. Reynolds (Department of Mechanical Engineering, Stanford University, Stanford, CA 94305-3030). From the findings of Kaloidas and Papayannakos, II S2 was assumed to be the only elemental sulfur species present. In reality, other allotropes will exist in small amounts making these calculations conservative. Our preliminary calculations showed that more S2 was formed than S8 over the temperature range 400-1000°C. In addition, the mole fraction ratio of S2 to S8 decreased with temperature. This is in agreement with Gamson and Elkins '? and Rau, Kutty, and Carvalho/ ' who have concluded that at higher temperatures (i.e. T> 700°C), 52 is the predominant sulfur species. The results indicated that the following individual reactions were possible for the reaction of H 2S and CO 2: 2 H 2 S