GB2145704A - A method of decomposing SO3 gas - Google Patents

Abstract

A method of decomposing SO3 gas without a catalyst comprises contacting the SO3 gas with a silicon carbide rod, preferably heated by running an electrical current through the rod, at a temperature about 1250K.

Description

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GB 2 145 704 A

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SPECIFICATION

A method of decomposing S03 gas

5 Hydrogen, a valuable raw material for the petroleum and petrochemical industries, is expected to become by 5 early in the next century an important renewable-based, transportable fuel either by itself or in some hydrocarbon form such as methanol or gasoline. Hydrogen can be produced through the decomposition of water by means of thermochemical cycles which reduce the higher temperature requirements of the 3000K (degrees Kelvin) straight thermal decomposition process to the 1200K levels that can be generated in nuclear

10 fission orfusion reactors or in high intensity, focused solar reflectors. 1q

A purely thermochemical process for producing hydrogen is the sulfur-iodine cycle being developed by the General Atomic Company. The essential steps of the sulfur iodine cycle are represented by the following reactions:

15 2H20 + S02 + xl2—»H2S04 + 2HIX (370-390K) 15

2HIX—> H2 + xl2 (393K)

H2S04—> H20 + S02 + 1/2 02 (1144K)

Other thermochemical processes include the Westinghouse sulfur cycle (which is partly electrochemical):

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2H20 + S02 H2 + H2S04 (-350K)

H2S04-»H20 + S02 + 1/2 02 (-1100K)

and the sulfur-bromine cycle (which is partly electrochemical), being developed at ISPRA, Italy:

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2 H20 + S02 + Br2^> H2S04 + 2 HBr (320-370K)

2 HBr-n> Br2 + H2 (-350K)

H2S04^. H20 + S02 + 1/2 02 (1000-1100K)

30 The dominant energy requirements, heat versus temperature, are necessary in these processes for the 30

H2S04 concentration and vaporization, conversion of H2S04 into S03 + H20, and the high temperature S03 decomposition steps.

The S03 decomposer is the critical process unit in nearly all the viable thermochemical plants to produce hydrogen. These plants can be driven by high temperature gas-cooled reactors, solar collectors orfusion 35 reactors, utilizing sodium, potassium or helium as heat transfer fluids to supply the large heat demand of the 35 S03 decomposer. Catalysts are required in the composer if the temperature is to be kept at levels of about 1070-1120K. The key requirement is to supply heat to the surfaces where the endothermicSO reaction occurs. This S03 decomposition produces S02 and 02 for the thermochemical production of hydrogen.

Measured S03 kinetics and equilibrium show this high temperature S03 decomposition reactor to be 40 surface kinetics (heterogeneous) controlled at lower temperatures, below 1050K, and homogeneous at 40

higher temperatures, above 1180K. For non-catalytic surface the conversion from S03 to S02 plus 02 is about 20-30% over the temperature range 1080K to 1180Kfor 0.3 to 1 second residence time ar around 1.5 atm.

total pressure, but the conversion is.about 80% at the higher temperature of 1250K. The low conversion at lower temperatures lead to large recycle H2S04 flows and thus much larger and more expensive equipment. 45 Increased residence time improves the kinetics but increases the size of the equipment. Increased total 45

pressure decreases the equipment size but unfavorably shifts the equilibrium, the decreased conversion increases equipment size. Catalytically enhanced kinetics greatly improve the conversion at lower temperatures to the range of 65-80%. However, the use of catalysts greatly increases the capital costs,

particularly if very expensive platinum catalysts must be utilized. Operation at temperatures of 1250K '50 eliminates the need for catalysts, but imposes very serious materials problems on the heat source which 50 provides thermal heat directly to the decomposer.

The design of the chemical reactor with fast kinetics and large associated heat effects is very difficult. A design of least cost and greatest simplicity is desired. Catalytic decomposers heated by internal heat exchangers appear to be too large to be cost competitive with other hydrogen production technologies. The 55 most obvious choice, a packed bed reactor, does not appear feasible because heat transfer from in-bed heat 55 exchangers to the packed bed of catalysts is very inefficient and requires extremely large temperature gradients between the heat exchanger fluid and the packed bed. Costly, high heat transfer media flow rates are also required, and large radial temperature gradients appear within the bed between the internal heat exchanger tube elements. Fluidization of the bed of catalysts greatly reduces the temperature differences 60 between the heat transfer fluid and the catalyst surface. However, substantial pumping power is required to 60 fluidizethe bed, resulting in a higher operational cost design.

Utilizing a packed bed catalytic reactor driven by a fusion reactor, the catalyst thermal requirements are 1050K and the blanket temperature must be 1350K in order for the flowing coolant to maintain the catalyst at the required temperature. With a fluidized bed design the blanket temperature can be lowered to 1100K in 65 order for the coolant to still maintain the catalyst at the 1050K temperature. Utilizing the catalytic cartridge g5

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S03 decomposer driven by a heat pipe from the blanket as described in U.S. Patent Application Serial No. 208,218, filed November 18,1980, the blanket temperature is reduced to the catalyst temperature of 1050K.

Fusion reactors offer some unique advantages as drivers for thermochemical hydrogen plants. Thermal heat from the blanket of a tandem mirror fusion reactor can be utilized. One particular tandem mirror blanket 5 concept is a lithium-sodium, liquid metal 50% atomic mixture in the cauldron blanket module. Helium or sodium can be used as the heat transfer fluid to carry heat ourside the nuclear island to process exchangers within the thermochemical hydrogen production cycle. Either a direct condensing vapor heat exchange loop or a heat pipe driven loop can be utilized. Problems with this design, however, include the safety problems of the isolation of liquid metals from the process stream and the permeation of radioactive tritium into the 10 product stream. A severe operational problem is the requirement that the blanket be at a temperature as high as the highest temperature required in the thermochemical process. This high temperature operational requirement for a direct heat source to the thermochemical cycle presents serious materials problems, since most conventional alloys are unsuitable for operation over about 1150K.

The tandum mirror reactor consists of a long central cell, about 200 m in length, in which power producing 15 deuterium-tritium (DT) plasma is confined by straight magnetic field lines produced by simple circular superconducting coil modules. The power producing plasma of the central cell is electrostatically confined at its ends by the plasma inyin-yang end plugs, each of which is a minimum-B stabilized mirror. The tandem mirror utilizes a positive potential of the plugs with respect to the central cell to repel and prevent escape of central cell ions. A further improvement on the basictandem mirror is the addition of thermal barriers, i.e., 20 large magnetic mirrors, at the ends of the central cell but inside of the end plugs.

The tandem mirror reactor provides a configuration of relatively simple blanket modules along the central cell length which can be utilized as a source of process heat. The tandem mirror reactor produces energy by fusing deuterium and tritium to produce energetic neutrons and alpha particles. The neutron kinetic energy is captured in the moderating blanket surrounding the reacting plasma, producing thermal energy. The 25 alpha particles lose some energy by heating the plasma through collisions before leaving the central cell through the ends. The alpha particles are captured by the direct converter located beyond the end cells which produces two forms of energy, an electrical DC component and a thermal component. The direct converter produces about 13% of the reactor energy output. The availability of both thermal energy and DC electricity from open-ended fusion machines is a unique advantage compared to closed systems that can be 30 utilized for thermochemical processes, e.g., the Westinghouse sulfur cycle and ISPRA sulfur bromine cycle are partly electrochemical. Process heat for the thermochemical processes is provided by both the blanket and the thermal component of the direct converter, while surplus electrical energy from the direct converter beyond that required to drive the reactor can be utilized to satisfy electrical demands.

The thermochemical production of hydrogen has a significant effect on the reactor design, particularly the 35 blanket modules which surround the plasma to convert the neutron kinetic energy to thermal energy. The blanket moderating fluids or solids must run hot enough to provide the highest temperature thermal requirements of the hydrogen fuel production process, typically 1200Kor higher, which imposes serious materials problems. A blanket for electrical production can run at a much lower temperature, typically 750K-900K. One high temperature blanket module configuration is a lithium-sodium-caldron blanket 40 resembling a pool boiler. A pool of liquid lithium-sodium mixture surrounds the plasma cell. It acts as neutron moderator heat transfer fluid (and also as a tritium producer). Heat is removed by vaporizing the soidum. The sodium vapor travels upwards into the dome region of the cauldron, condenses on heat exchanger tubes and returns as liquid droplets to the pool. The heat exchanger in the dome transfers the thermal energy out of the module.

45 A modified cauldron design with heat pipes transferring heat from the moderator to the heat exchanger eliminates a problem of excessive void fraction. Liquid lithium or LiPb is substituted for LiNa since the sodium performs no function. The heat pipe working fluid is sodium or potassium.

An alternative blanket concept is the flowing Li20 microsphere blanket. By generating heat and breeding tritum in the microspheres as they flow through the blanket, the hot microspheres transfer heat to a process 50 working fluid in the module heat exchanger.

Thermochemical cycles, the tandem mirror fusion reactor, blanket configurations, the interface with thermal reactors, fluidized bed decomposer designs, and associated problems, are described in UCRL-84212, "Interfacing the Tandem Mirror Reactorto the Sulfur Iodine Process for Hydrogen Production", T,.R. Galloway, Lawrence Livermore National Laboratory, June 1980; UCRL-84285, "The Process Aspects of 55 Hydrogen Production Using the Tandem Mirror Reactor", T.R. Galloway, Lawrence Livermore National Laboratory, September 1980; UCRL-84632, "Some Chemical Engineering Challenges in Driving Thermochemical Hydrogen Processes with the Tandem Mirror Reactor", T.R. Calloway et al, Lawrence Livermore National Laboratory, November 1980; and UCID-18909, Vol. I and II, "Synfuelsfrom Fusion-Producing Hydrogen with the Tandem Mirror Reactor", R.W. Werner (editor), Lawrence Livermore National Laboratory, 60 January 1981, which are herein incorporated by reference.

The invention relates to a high temperature method of decomposing S03 without a catalyst by contacting S03 with silicon carbide heating elements at a temperature of about 1250K; the silicon carbide rods may be heated by electrical heating.

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In the drawings:

Figure 7 is a schematic diagram of the interface between a thermochemical hydrogen cycle and a tandem mirror reactor.

Figure 2 is a schematic diagram of the H2S04 processing system.

5 Figure 3 is a heating curve for the H2S04 process step.

Figure 4 is a schematic diagram of a joule boosted S03 decomposer.

Figure 5 is a graph of sulfuric acid decomposition as a function of temperature.

A method and apparatus for driving a high temperature S03 decomposer in a thermochemical hydrogen production process by means of a tandem mirror fusion reactor, is illustrated schematically in Figure 1. The 10 S03 decomposer 10 operates at about 1250Kto decompose S03 into S02 + 02 and form s the critical high temperature component with the remaining thermochemical process units 12 for producing hydrogen. Thermal energy at about 900K is removed from the blanket 14 and direct converter 16 of a tandem mirror fusion reactor by means of a heat transfer fluid which flows along flow paths 18 and 20, respectively, and along flow path 22 into thermochemical process units 12 for directly heating the lower temperature process 15 components. A portionof the heat transfer fluid is taken from path 22 and flowed along path 24 through a turbine 26 which drives an electrical generator 28, and then returns along path 30 to the reactor blanket 14. Part of the electrical energy produced is provided along path 32 for driving the reactor, including pumps and other mechanical equipment in the system. The remainder of the electrical energy produced is taken by path 34 and added to electric energy produced in the direct converter in path 36 to the S03 decomposer 10 for 20 joule heating the decomposer to sufficiently high temperatures for producing S03 decomposition reactions. By the means of joule boosting the decomposer, i.e., converting lower temperature thermal energy to electrical energy and then reconverting the electrical energy to higher temperature thermal energy in the decomposer, the high temperature requirements of the S03 decomposer (1250K) are satisfied while the reactor blanket can operate at a much lower temperature (900K).

25 The major components of the H2S04 processing unit are shown schematically in Figure 2, illustrating the heat requirements of each component. The reactor blanket operates at about 900K and supplies all the thermal energy demand up to 900K directly while all the thermal energy demand about 900K is supplied by electrical heating. Sulfuric acid, H2S04, passes through multi-effect evaporators 40 operated between 500K and 680Kand then through boiler 42 which operates at680K. TheS03 gas produced, pumped by turbine 43, 30 is passed through preheater 44 which is a heat exchanger which raises the S03 temperature to about

950K-1050Kand then into the joule boosted S03 decomposer 46 which operates at a temperature of 1250K supplied by electrical heater 48 which is driven by joule boosting by electrical energy produced from the fusion reactor. The product gas from the decomposer 46 exists at high temperature (1250K) and is circulated through the heat exchanger of preheater 44 where it functions as the working fluid, decreasing in 35 temperature to about 730K while increasing the S03 gas temperature for input into decomposer 46. The product gases are further cooled by passing through decomposer cooler 50 in which in other process fluids are increased in temperature by heat exchange with the product gases and then passed through vapor/liquid splitter 52 to remove S02 + 02 at a temperature of about 418Kfor use in the thermochemical hydrogen production cycles.

40 The heating curve for the H2S04 process step is illustrated in Figure 3. The lower temperature requirements including evaporation and boiling of H2S04 and preheat of the S03 gas are provided by blanket heat. Additional preheating is provided in the heat exchanger (HX curve) of preheater 44 (Figure 2) from S02 quench to raise the S03 temperature to about 1050K. Electrical heating (resistive or ohmic heating)

according to the invention is utilized to raise the S03 temperature from 1050Kto 1250K, thereby producing 45 S03 decomposition reactions.

A joule boosted S03 decomposer as illustrated in Figure 4, utilizes electrical energy to meet the high temperature thermal demands, thereby interfacing with a lowertemperature thermal heat source. The joule boosted decomposer comprises a cylindrical vessel 60 having a pair of headers 62 and 64 located near opposite ends to define a chamber 66 therebetween. A plurality of heating elements or rods 68 pass through 50 the headers 62 and 64 through insulators (not shown) and are aligned along the axis of the vessel 60. One header may be floating to provide for thermal expansion or bellows may be placed at the ends of the heating elements. An inlet port 70 allows S03 gas to flow into chamber 66 to contact the heating elements 68 which are maintained at a sufficiently high temperature to promote decomposition reactions into S02 + 02. The product gases are removed through outlet port 72 which is located substantially opposite to inlet port 70, 55 thereby producing a cross flow of the S03 gas across the heating elements 68. The ends of the heating elements 68 extend into the end portions 74 and 76 of the vessel 60. A coolant gas is circulated through the end regions 74 and 76 by means of inlets 78 and outlets 80 in order to cool the ends of the heating element 68 down to about 600K. Electrical connections 82 at the ends of the rods 68, e.g., aluminum impregnated electrical feed connected, allow an electrical current to pass through the rods 68 from electrical lines 84 and 60 86 which pass through the ends of the vessel 60 and are connected to the rods 68 by contacts 82.

The heating elements or rods 68 are heated to a temperature sufficient to promote S03 decomposition reactions. From the sulfuric acid decomposition data on alumina substrates shown in Figure 5, the rods are heated to about 1250K to produce an 80% conversion. According to the invention, the heating rods 68 are preferably silicon carbide rods which can tolerate the corrosiveness of S03 gas because a protective Si02 65 scale develops on the surface. These silicon carbide heating elements normally have a rough-textured

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surface, providing large surface area to promote decomposition reactions. Alternating current must be used to avoid polarization problems and non-uniform heating.

In one particular configuration the chamber 66 is 3 m long and 3 m in diameter. The vessel 60 is fabricated from Incology 800H and the process gas pressure is 7 atm.The interior of the vessel 60 is insulation-lined 5 and the walls are cooled. The silicon carbide heating rods are about 5.5 cm in dimater and slightly over 3 m 5 long. The end regions 74 and 76 of the vessel 60 are each about 0.5 m long. The silicon carbide heating rods are configured in a hexagonal array with a 6.1 cm spacing normal to the flow and 5.3 cm spacing in the flow direction. The heating rods further can be formed with small corrugations in the surface to benefit the gas-solid heat transfer coefficient. The heating elements can be operated up to 130 KW/m2 at 1250Kbut 10 preferably will operate at 63KW/m2 to achieve longer lifetime. The cross flow geometry for the decomposer iq heating elements has significant heat transfer advantages. Cross flow around the 5.5 cm diameter elements is more effective owing to reformation and growth of the boundary layer and the separation and wake formation aft of the cylinder. These wakes provide turbulence which enhances the heat transfer. The silicon carbide rods are non-catalytic; however, it may be possible to increase impurity dopants to provide some 15 catalytic action. 15

Changes and modifications in the specifically described embodiments can be carried out without departing from the scope of the invention, which is intended to be limited only by the scope of the appended claims.

In our co-pending Application No. 8213463, Publication No. 2099805A, there is described and claimed a 20 joule-boosted method of driving a high temperature step of a thermochemical process for hydrogen 20

production involving the S02/H20/S03 cycle from a magnetic fusion reactor having a blanket operating at a substantially lower temperature than the high temperature step, comprising: removing lower temperature thermal energy from the blanket of the reactor; converting the lower temperature thermal energy from the blanket to electrical energy; and converting the electrical energy to thermal energy at a sufficient 25 temperature to meet the heat requirements of the high temperature step. 25

Claims (3)

1. A method of decomposing S03 gas without a catalyst comprising contacting the S03 gas with silicon

30 carbide rod at a temperature of about 1250K. 30

2. A method as claimed in claim 1, wherein the silicon carbide rod is heated by running an electrical current through the rod.

3. A method as claimed in claim 1 or 2, substantially as hereinbefore described and illustrated.

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Printed in the UK for HMSO, D8818935, 2/85, 7102.

Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1AY, from which copies may be obtained.