GB2066841A - Synthesis Gas Production - Google Patents

Abstract

A method of producing synthesis gas from steam and a hydrocarbon feed by reformation which method comprises contacting the hydrocarbon feed directly with water prior to reformation by passing the feed through a heat exchanger co- or counter-currently to a reformation products stream; the water being arranged to flow co- or counter- currently to the feed stream. The synthesis gas so produced can then be converted to methanol, ammonia and other chemical products. <IMAGE>

Description

SPECIFICATION Synthesis Gas Production The present invention relates to the production of synthesis gas from steam and a hydrocarbon feed by reformation.

Conventionally in methanol plants and the like, gaseous or light liquid hydrocarbons are reformed in the presence of steam to produce synthesis gas (a mixture of hydrogen and carbon monoxide) which is then reacted to form methanol or other products. The generation of steam is a considerable charge on the process. In a known method of reducing this charge, heat from the reactor products is recovered against an indirect stream of water, which is circulated in direct counter-current flow against the vaporised hydrocarbon feed, thus heating and humidifying it. Heat and mass transfer are restricted with such a method by there being two temperature differences in series, one being between the reactor products and the water, and the other being between the water and the feed gas.

In accordance with the present invention, there is provided a method of producing synthesis gas from steam and a hydrocarbon feed by reformation which method comprises contacting the hydrocarbon feed directly with water prior to reformation by passing the feed through a heat exchanger co or counter-currently to a reformation products stream; the water being arranged to flow co- or counter-currently to the feed stream.

"Reformation products" as used herein includes reformer flue gases, and the heat thereof may be removed either directly or indirectly via a heat conducting fluid.

Preferably the heat exchanger is a vertical tube heat exchanger used counter-currently, and the water contains volatile and/or non-volatile plant effluents. Preferably also the reformation products stream is the synthesis gas stream and is reacted in a synthesis reactor, before itself entering the heat exchanger, to form such products as methanol, ammonia, ethylene oxide, ethanol, or a dialkyl ether, e.g. dimethyl ether or diethyl ether.

These synthesis products are often used in the manufacture of other products, such as urea and methyl tertiary butyl ether.

Two embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a schematic flow sheet showing the production of synthesis gas leading to the methanol production using a saturator in a conventional process.

Figures 2 and 3 are schematic flow sheets illustrating two methods of the present invention; and Figures 4 and 5 are cooling curves for the conventional process and the process of Figure 2, respectively.

Figure 1 shows typical current practice; the irrelevant items having been omitted.

Hydrocarbon feed gas 1 is fed to a saturator 2 through which water 3 for vaporisation is being circulated by pump 4. The humidified feed, together with any make-up steam 5 which may be required, is fed to steam reformer 6 to form synthesis gas before being reacted in synthesis reactor 7 to form methanol. Heat from the reactor products is removed by the use of heat exchanger 8 using the circulating water 3, before the products are passed to a fractionation process to recover the methanol. A circulating water purge 9 is provided, if necessary, and make-up water 10 can be added as required.

For a large methanol plant, the saturator would be about 10 feet ( )diameter and 80 feet high, with a design pressure of about 450 pounds per square inch gauge ( About 70 tons ( ) per hour of water would be evaporated into the feed gas, but in order to secure the adequate removal of heat away from the gas stream it would be necessary to circulate from 500 to 600 tons ( ) per hour of water.

Figures 2 and 3 show two possible embodiments of the invention in which water flow is respectively with and against the flow of feed gas; again only the relevant items being shown. In both embodiments, the functions of the conventional saturator 2 and of the heat exchanger 8 have been combined in a vertical tube heat exchanger 11. Here the reactor products are cooled on the shell side of the exchanger against the hydrocarbon feed gas 1 on the tube side whilst the water 3 for vaporisation is also fed to the tube side of the heat exchanger 11.

By this method evaporative cooling as well as heat exchange occurs in this same unit, and humidification of the feed gas 1 is also achieved.

In either case, provided that the water is pure and uniformly distributed, the pump capacity need theoretically be no more than the amount of water evaporated; a seven or eight-fold reduction from conventional methods. In practice however, a margin to allow for maldistribution and to avoid the build-up of impurities would be added.

Figures 4 and 5 show the cooling curves for current practice and for the method of the invention for a large methanol plant. In Figure 4, a minimum temperature difference of 20C is used with 26.6 Gigacals/hr of heat being transferred, whilst in Figure 5 a 90C temperature difference with 29.2 Gigacals/hr of transferred heat are used. "Effluent Gas" refers to the gaseous reactor products stream.

A further improvement, when using the cocurrent upwards flow of water, is to entrain the water into the gas stream without pumping it, either by a Venturi nozzle, or by prolonging the heat exchanger tubes close to the water level so that they can act as gas-lifts. With a downward flow of water, it can be fed onto the top tube-sheet of the exchanger directly. These and similar methods permit the circulating pump to be eliminated or at least further reduced in size.

It will be clear that other designs of heat exchanger giving a counter-current flow of feed and reactor products, with co- or counter-current flow of water with respect to the feed, may be used, e.g. by a plate type exchanger, and that orientations other than vertical, e.g. horizontal, may be used.

Advantages of the improved methods hereinbefore described include, but are not limited to, the following:- 1) The elimination of the direct-contact saturator, its instruments, piping and associated equipment (See Figures 1, 2 and 3); 2) The reduction of the size of or elimination of the circulating water pumps, and of their associated equipment, thus saving capital and operating costs; 3) The reduction of heat exchanger surface, since higher temperature differences are available (See Figures 4 and 5); 4) The improvements of heat recovery, thus giving lower operating costs (See Figures 4 and 5); 5) The reduction of the size of or elimination of the boiler feed-water treatment plant, since the water which is vaporised may approach or exceed that chemically converted in the synthesis reaction;; 6) The use of impure effluent streams as process water, thus eliminating effluent treating costs, recovering the fuel values in the effluents, and thus permitting lower fractionation costs, since it is no longer necessary to fractionate the methanol and/orother products so exactly.

(These improvements are better adapted to handling volatile impurities than with a saturator, since the feed gas and process water are exposed to a higher temperature and steam partial pressure here. Where feed gas and process water flow through the tubes, the improvements are also better adapted to handling non-volatile impurities, since scale formed in the tubes is easily removed, which is not possible from a saturator filted with trays or packing);; 7) A closer approach to equilibrium (if feed gas be on the inside of the tubes) than in a plate or packed-type saturator, since wetted-wall columns (such as those effectively constituted by the tubes of a vertical heat exchanger) are inherently more efficient, and may have a height per theoretical stage equal to little more than the internal diameter, (say 1 to 2 inches ( )) as compared with packed or plate columns having typically, heights per theoretical stage of 1 to 2 feet( 8) A minor but valuable advantage of cocurrent flow of feed gas and process water, over counter-current flow is an inherently better heat recovery, since sensible heat to the water is supplied at the lowest possible temperature level at all times; ; 9) A further advantage of the described methods is that by recovering more heat from the reactor products stream, much of the heat which must at present be transferred to the fractionation section for use as reboil is no longer available; thus to a great extent decoupling the product synthesis and the fractionation sections and giving greater stability of the plant as a whole against upsets and fluctuations.

Claims (10)

Claims

1. A method of producing synthesis gas from steam and a hydrocarbon feed by reformation, which method comprises contacting the hydrocarbon feed directly with water prior to reformation by passing the feed through a heat exchanger co- or counter-currently to a reformation products stream; the water being arranged to flow Co- or counter-currently to the feed stream.

2. A method as claimed in claim 1 wherein the heat exchanger is a vertical tube heat exchanger the feed and the products stream being countercurrent.

3. A method as claimed in claim 1 or claim 2 wherein the water contains volatile and/or nonvolatile plant effluents.

4. A method as claimed in any one of the preceding claims wherein the reformation products stream is the synthesis gas stream and is reacted in a synthesis reactor, before itself entering the heat exchanger.

5. A method as claimed in claim 4 wherein the synthesis reactor products stream entering the heat exchanger comprises methanol or ammonia.

6. A method as claimed in claim 1 substantially as hereinbefore described with reference to any one of Figures 2, 3 or 5 of the accompanying drawings.

7. Synthesis gas when produced by a method as claimed in any one of the preceding claims.

8. Methanol, ammonia, ethylene oxide, ethanol or a dialkyl ether when produced from synthesis gas as claimed in claim 7.

9. Urea when produced from ammonia as claimed in claim 8.

10. Methyl tertiary butyl ether when produced from methanol as claimed in claim 8.