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Ind. Eng. Chem. Res. 1996, 35, 1785-1787
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RESEARCH NOTES Noncatalytic Partial Oxidation of Sour Natural Gas Versus Catalytic Steam Reforming of Sweet Natural Gas H. K. Abdel-Aal* and M. A. Shalabi Chemical Engineering Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
The work presented in this paper is carried out to validate the technical feasibility of a noncatalytic partial oxidation process of sour natural gas. Basic reactions along with thermodynamic data are given, and proposed schemes are outlined. Comparison is made between proposed and existing processes of handling sour natural gas. Table 1. Basic Reactions with Thermodynamic Dataa
Introduction The current technology used in the manufacture of synthesis gas for ammonia/urea production and methanol synthesis is based mainly on the steam reforming of natural gas. This is a catalytic process in which the feed gas has to be sulfur-free to avoid catalyst poisoning. As a result, acidic-gas removal is a prerequisite step for the steam reforming process. In the natural gas processing industry, the H2S is simply separated from the natural gas by one of the many physiochemical separation methods. This is a costly operation involving the use of amine solvents for the chemisorption of acidic gases followed by regeneration of the solvent.
∆Gb
∆Hc (kcal)
1700 °F 2200 °F 1700 °F 2200 °F CH4 + 3/2O2 f CO + 2H2O CH4 + O2 f CO + H2O + H2 CH4 + 2H2O f CO2 + 4H2 3H2S + 3/2O2 f SO2 + H2O + 2H2S 5. 2H2S + SO2 f 3/2S2 + 2H2O 6. 3H2S + 3/2O2 f 3/2S2 + 3H2O 7. H2S + 1/2O2 f H2O + S1 1. 2. 3. 4.
-148.6 -154.2 -124.3 -125.2 -105.3 -114.6 -64.8 -65.4 -17.8 -32.7 46.4 47.0 -101.5 -96.2 -124.6 -124.5 -6.3 -10.1 10.1 9.9 -107.8 -106.3 -114.0 -114.6 -1.3 -4.8 13.9 13.8
a Source: ref 6. b Change in Gibbs free energy for reaction. Negative change indicates spontaneous reaction possible. c Heat of reaction. Negative ∆H indicates exothermic.
and by:
Proposed Method In this paper, it is proposed to bypass the costly gas sweetening process of sour natural gas feedstocks. The proposed method makes use of what is known as “noncatalytic partial oxidation” of the sour natural gas. The method is highlighted as follows: Basic Reactions. (a) It will be assumed for the sake of illustration that the sour feed gas consists mainly of CH4, to represent natural gas, and H2S/CO2, to represent acidic gases. (b) The main reactions for the partial oxidation of methane are given by:
2CH4 + 2H2O f 2CO + 6H2 Total (net):
4CH4 + 2O2 f 4CO + 8H2
(3)
(d) Similarly, the basic reactions representing the combustion of H2S gas with O2 (partial oxidation) are represented by:
3H2S + 3/2O2 f SO2 + H2O + 2H2S
(4)
2H2S + SO2 f 3S + 2H2O
(5) (6)
CH4 + 1/2O2 f CO + 2H2
(1)
S + O2 f SO2
CH4 + O2 f CO + H2O + H2
(2)
(e) Thermodynamic data (Gibbs free energy, ∆G, and heat of reaction, ∆H), for two different operating temperatures, namely, 1700 and 2200 °C, for the partial oxidation reactions are included in Table 1. For the reactions involving CH4, the values of ∆G are in the range of -17.8 to -148.6 kcal at 1700 °F, while for the combustion reactions of H2S, the values are in the range of -1.3 to -101.5 kcal at the same conditions. The negative values of ∆G indicate that spontaneous reactions are possible at the indicated temperature. (f) By using air to supply O2, we are in effect introducing N2 for the ammonia synthesis. In the case of methanol, however, pure O2 must be used. (g) At this stage, the composition of the gas produced by the partial oxidation of sour natural gas is repre-
(c) The reaction given by eq 1 is considered to proceed in two well-defined steps: first, complete reaction of the oxygen with some of the methane to give water and carbon dioxide, followed by slower reaction of the carbon dioxide and the water with the excess of methane to give mixtures of carbon monoxide and hydrogen, so that the sum total of the reactions occurring is
CH4 + 2O2 f CO2 + 2H2O followed by:
CH4 + CO2 f 2CO + 2H2 * Author to whom all correspondence should be addressed.
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Figure 1. Simplified flow diagram for synthesis gas production from sour natural gas: (a) conventional method (gas sweetening using DGA: diglycol amine); (b) noncatalytic partial oxidation with sulfuric acid production. Table 2. Detailed Comparison between Proposed and Existing Methods of Handling Sour Natural Gas parameters
existing (steam reforming) (1) Preparation of Raw Gas requires using amine solvents
proposed (partial oxidation) not required
treatment of sour natural gas to remove H2S processing of H2S to recover S
Claus process
use of steam use of water use of catalysts generation of waste heat energy
yes yes yes yes
no yes no yes
(3) Posttreatment of Synthesis Gasa no takes place by burning S to SO2, converting SO2 to SO3, then H2SO4 (catalytic)
yes takes place by Westinghouse method using aqueous SO2 solution (noncatalytic)
(i) not required (ii) modified version
(2) Preparation of Synthesis Gas
removal of SO2 by water scrubbing conversion to H2SO4
for case (1) NH3/urea production for case (2) CH3OH production a
(4) End Products (Two Possibilities) H2, N2, CO2 mixture and S H2, CO mixture and S
H2, N2, CO2 mixture and H2SO4 H2, CO mixture and H2SO4
A number of treatment steps are carried out for both methods to remove CO2 and traces of CH4.
sented by the following compounds:
case (1) for NH3/urea:
H2, N2, CO, CO2, SO2
case (2) for CH3OH:
H2, CO, CO2, SO2
(h) For case (1), the next step will involve the transfer of CO into CO2 using what is known as “water shift conversion reaction”. The options exist at this stage. Either to remove the SO2 before going to the shift conversion reaction or to remove it after. This is dependent on the activity of the iron oxide catalyst with regards to SO2 gas. The main reaction taking place in
the water shift conversion is given by:
CO + H2O f CO2 + H2
(7)
This reaction is mildly exothermic and is favored by low temperature but unaffected by pressure. Excess steam also forces this reaction to completion. A single stage converts 80-95% of the residual CO to CO2 and H2. Because the reaction is exothermic, the reactor temperature rises; this enhances the reaction rate but has an adverse effect on the equilibrium. When high concentrations of CO exist in the feed, the shift conversion is usually conducted in two or more stages, with interstage cooling to prevent an excessive temperature rise. The
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Ind. Eng. Chem. Res., Vol. 35, No. 5, 1996 1787
Figure 2. Direct-oxidation (Claus) process for low concentrations of H2S in sour natural gas.
first stage may operate at higher temperatures to obtain high reaction rates, and the second, at lower temperatures to obtain good conversion. For case (2), on the other hand, the process requires CO as a feedstock. (i) Irrespective of the water shift conversion step, SO2 removal from the synthesis gas stream could be accomplished to produce sulfuric acid, H2SO4, using a novel approach. This implies first the absorption of SO2 in water, followed by electrolysis under the conditions of 298 K and 0.17 V. This process is described in the literature as S-cycle or Westinghouse cycle for the thermochemical production of hydrogen (1, 2). It is represented by the equation:
2H2O(l) + SO2(aq) f H2SO4(aq) + H2(g)
(8)
In this proposal, the Westinghouse cycle is a very attractive alternative for producing H2SO4 directly from SO2 formed in the partial oxidation process. Comparing it with the conventional “contact process” for sulfuric acid manufacture, some pronounced advantages can be counted. In the contact process, provision has to be made for sulfur burning into SO2 first. This is followed by the expensive catalytic stepsusing V2O5sto convert SO2 into SO3. Then the final step of producing H2SO4 by absorption of SO3 in sulfuric acid takes place. Much savings are anticipated in using the Westinghouse process in terms of less equipment operating at much lower temperature and eliminating the use of catalysts. Proposed Schemes. (a) A comparison between the conventional process of handling sour natural gas feed streams in practice in many oil-producing countries and the proposed one is illustrated by the block diagrams given in Figure 1a,b. The novel concept of producing H2SO4 noncatalytically is to be compared with the current technology using sulfur as a raw material and V2O5 as a catalyst for the conversion of SO2 into SO3. (b) Another option of the proposed partial oxidation scheme of sour natural gas is to produce S instead of SO2. This is basically the case of acid gas with low H2S content (say, less than 10 mol %). In the literature (3), such cases is handled by a version of the original Claus process. It is known as the direct-oxidation process for low concentration of H2S. Equation 5 describes the main reaction which occurs catalytically using alumina catalyst. Figure 2 describes this option, in which acidic gases are partially oxidized to give synthesis gas plus elementary sulfur. (c) A detailed comparison between the proposed scheme and the existing one is presented in Table 2. Important comments drawn from this comparison as well as other conclusions relevant to the proposed process are given next.
technical feasibility of an improved process. The economic part has to be considered later. The thermodynamic data reported earlier for the proposed combustion reactions and the information given in Table 2 are indexes in support of the technical feasibility of the partial oxidation process of sour natural gas. In the proposed approach, it is anticipated to manufacture a gas mixture consisting of H2/N2 via the noncatalytic partial oxidation of sour natural gas, for NH3/urea production (by using air) or H2/CO for CH3OH synthesis (by using O2), along with H2SO4 as a byproduct. The immediate advantages of such an approach as compared with the catalytic reforming of sweet natural gas are (a) elimination of the costly process of natural gas desulfurization, (b) elimination of catalysts used in the steam reforming process, and (c) simplification of the process flow equipment used in the industry. Since heat transfer occurs at very high temperature within the proposed process boundary, provision has to be made for utilizing the heat-liberated. This is normally taken care of by using waste heat boilers (WHB). The proposal offers a new approach to produce H2SO4 via a noncatalytic process known as the S-cycle, in which byproduct hydrogen adds extra merit to the overall process, thus increasing the yield of synthesis gas. It is understood that a knowledge of the ideal performance of the proposed process aids in the determination of the overall feasibility. In principle, ideal performance is assumed when all stoichiometric relationships for the proposed process reactions are satisfied under the most favorable thermodynamic conditions, and equilibrium is reached at all points in the process. The answer to this question as well as others such as thermodynamic favorability is beyond the scope of this paper. What matters is the viability of the proposal based on the given information which could have a great impact on the Master Gas System of Saudi Arabia. Finally, experimental work is pursued currently by the authors to test sour natural gas of different compositions. A kinetic and theoretical study is anticipated as well for the proposed system using an approach similar to the work reported earlier (4, 5). Literature Cited (1) Ohata, T. Solar Hydrogen Energy Systems; Pergamon Press Ltd.: New York, 1979. (2) Abdel-Aal, H. K. Opportunities of Open-loop Thermochemical Cycles: A Case Study. Int. J. Hydrogen Energy 1984, 9, 767772. (3) Grekel, H.; Kunkel, L. V.; McGalliard, R. Package Plants for Sulfur Recovery. Chem. Eng. Prog. 1965, 61, 70-73. (4) Abdel-Aal, H. K.; Shalabi, M. A. Non-Petroleum Routes to Petrochemicals. Int. J. Hydrogen Energy 1992, 17, 359-367. (5) Shalabi, M. A.; Baldwin, R. M.; Bain, R. L.; Gary, G. H.; Golden, J. O. Non-Catalytic Coal Liquefaction in a Donor Solvent: Rate of Formation of Oil, Asphaltenes, and Preasphaltenes. Ind. Eng. Chem. Process Des. Dev. 1979, 18, 474. (6) Paskall, H. G. Capability of Modified-Claus Process. Report published by Western Research, Alberta, Canada, 1983.
Received for review January 10, 1995 Revised manuscript received January 18, 1996 Accepted February 9, 1996X IE950040P
Discussion and Conclusions This proposal presents what could be considered “a prefeasibility study” geared mainly to report on the
Abstract published in Advance ACS Abstracts, March 15, 1996. X
