GB2087867A - Process for production of methanol - Google Patents

Abstract

In a process comprising production of synthesis gas in a reforming stage and the utilisation of the synthesis gas in a synthesis reaction stage, in particular a process for production of methanol, a purge gas is removed from the synthesis reaction stage at an elevated pressure and is expanded to recover energy therefrom prior to combustion with an oxygen-containing gas. Exhaust gases from the combustion are expanded to recover energy, and then sent to the reforming stage for either pre-heating reformer feed streams or providing heat necessary for the reforming of a feed stream with steam.

Description

SPECIFICATION Process for production of methanol BACKGROUND OF THE INVENTION This invention relates to an improvement in a process for the production of a crude reaction product such as liquid phase methanol. More specifically, this invention provides a method of utilizing purge gas commonly associated with the synthesis stage of the overall process so as to improve the efficiency of the overall process.

A typical methonal synthesis process includes reforming, energy recovery, compression and synsthesis reaction stages. A purge gas is normally removed from the synsthesis reaction stage and then used as a fuel in the reforming stage. According to the present invention, the purge gas is initiallly passed through an expander to provide mechanical energy, then used as a fuel for production of electrical energy and ultimately used as a source of heat to generate steam, preheat feed streams, or heat reactants in the reforming stage. Since this method uses the potential energy associated with pressurized purge gas as well as the chemical energy of the purge gas, more efficient use of the total energy associated with the purge gas is achieved.

SUMMARY OF THE INVENTION In accordance with an illustrative embodiment demonstrating features and advantages of the present invention there is provided an improvement in a process for the production of a crude reaction products which process includes reforming, energy recovery, compression and synthesis reaction stages.

A purge gas at an elevated pressure is removed from the synthesis reaction stage and is passed through an expander to recover energy from the purge gas line pressure. The expanded gas is combusted to yield a heated exhaust gas which is expanded and thereafter sent to the reforming stage.

BRIEF DESCRIPTION OF THE DRAWINGS The above brief description as well as further objects, features and advantages of the present invention will now be more fully appreciated by reference to the following description of presently preferred but nonetheless illustrative embodiments in accordance with the present invention when taken in connection with the accompanying drawings, wherein: Figure 1 is a schematic representation of a methanol synthesis process incorporating the present invention; and Figure 2 is a schematic representation of an alternative arrangement of the present invention.

BRIEF DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to Fig. 1, there is shown a shematic diagram illustrative of a typical process 10 for production of a crude reaction procuct. In the preferred embodiment methanol is produced form natural gas. However, it is to be understood that other feedstocks such as naptha can be used, and other crude reaction products such as ammonia can be obtained.

A stream of natural gas is passed through line 1 2 into a preheater 14 disposed within the convection section 1 6 of primary reformer 1 8. Thereafter the preheated natural gas is passed through line 20 to desculfurizer 22. A stream of hydrogen gas is passed through line 24 and added to the preheated natural gas at a point 26 upstream of desulfurizer 22. The combined stream of natural gas and hydrogen is removed from desulfurizer 22, and added to a stream of steam flowing through line 27.

The combined stream of natural gas, hydrogen, and a steam is then passed into another preheater 28 disposed in convection section 16. The preheated combined stream is then passed via line 29 through a series of reformer tubes 30 filled with catalyst, such as a nickel-containing material, within the furnace section 32 of primary reformer 1 8. As the combination stream passes through the tubes 30 the natural gas, hydrogen and steam, reacts to yield a synthesis gas stream including carbon monoxide, carbon dioxide, and hydrogen gas. A portion of the natural gas usually does not react with a portion of the steam and therefore the gas stream obtained from the reforming stage will include carbon monoxide, carbon dioxide, hydrogen steam and unreacted natural gas.This gas exits reformer 1 8 at a temperature of approximately 1 500 degrees F. and is passed through line 34 into a waste heat boiler 36. In boiler 36 some of the heat is removed from the synthesis gas stream and absorbed by water circulated through boiler 36, thereby generating steam which is removed from boiler 36 through line 38. A portion of the steam removed through line 38 is sent through line 27 for combination with the preheated natural gas and hydrogen stream. The remainder of the steam is passed through line 40 to an expander 42; after expansion the steam is sent through line 44, and supplemented by make-up water introduced through line 46.

Thereafter the expanded steam and make-up water will pass through a feedwater heater 48 and then be recirculated to boiler 36 via line 49.

After passing through waste heat boiler 36 the synthesis gas is removed at a temperature of approximately 450 degrees F. and then passes through feedwater heater 48 wherein it gives off another portion of its heat to the boiler feedwater passing in an indirect heat exchange relation therethrough. Upon exiting heater 40 the temperature of the synthesis gas is approximately 425 degrees F. The synthesis gas is then passed via line 50 through a second waste heat boiler 52 for additional heat recovery. Although not shown, it is to be understood that this boiler could generate steam that would later be used for purification of the crude reaction product. The synthesis gas is removed from the second waste heat boiler 52 at a temperature of approximately 340 degrees F.The synthesis gas is next sent via line 53 through a cooler 54 to further reduce the temperature of the synthesis gas to approximately 110 degrees F. It should be understood that a fan could also be used to cool the synthesis gas to this level, in which case the fan could be disposed upstream or downstream of cooler 54.

The synthesis gas removed from cooler 54 at approximately 110 degrees F. is at a pressure of approximately 400 psi. This gas is then sent via line 55 to compressor 56 in order to raise the pressure of the synthesis gas to approximately 1 500 psi. Some hydrogen is bled from the gas stream at a point downstream of compressor 56, and is sent via line 24 to point 26 for addition to the feed stream. Thereafter the synthesis gas is passed through line 58 to a second stage of compression within compressor 60. The synthesis gas is compressed to approximately 1 600 psi as it passes through compressor 60. The compressed synthesis gas is then introduced to reactor 62 wherein the carbon monoxide, carbon dioxide, and hydrogen gas react to form a product gas which includes methanol.The gaseous phase product is removed from reactor 62 through line 64 and then passed through partial condenser 66 wherein most of the methanol included in the product gas is condensed to a liquid phase. Thereafter the entire stream of product is introduced to separator drum 68. Within drum 68 the liquid methanol is separated from the gaseous portion of the crude reaction product and is removed through line 70. The liquid methanol is then compressed in liquid compressor 72 and is ultimately removed as crude liquid product through line 74. The gaseous portion of the crude reaction product is removed from drum 68 through line 76.A portion of the gaseous product is recirculated through line 78 and combined with incoming compressed synthesis gas at a point 80 upstream of compressor 60; the portion of the gaseous product passed through line 78 represents approximately 90% of the gaseous product removed from drum 68, and is commonly referred to as "recycle" gas. The remaining portion of the gaseous product removed from drum 68 is passed through line 82; this portion of the gaseous product is commonly referred to as "purge" gas. The purge gas comprises hydrogen, carbon monoxide, car- bon dioxide, steam, gaseous methanol, and unreacted natural gas. Normally the purge gas is sent to the furnace section of primary reformer 18, and is combusted as a fuel to provide heat necessary to promote the reaction of the natural gas with steam within reformer tubes 30.

According to the present invention the purge gas is sent through a line 82 to an expander 84 wherein the purge gas is expanded from a pressure of approximately 1400 psi to a pressure of approximately 210 psi. Expander 88 drives one or more compressor services, such as compressor 86 associated with gas turbine 88. The expanded purge gas is then sent to the gas turbine combustor 90 wherein it is burned with an oxygen-containing gas such as air. A heated exhaust gas is removed from gas turbine combustor 90 through line 92 and is expanded in gas turbine expander 94 which drives electrogenerator 96, thereby producing electrical energy. Expanded exhaust gas is removed from expander 94 through line 98.

Some or all of the exhaust gases can be sent through line 100 into preheater 102. Within preheater 102 the exhaust gases come in indirect heat exchange contact with the air charge which has been compressed in compressor 86, and sent through line 104 to combustor 90, thereby preheating the air charge before its introduction to the combustor. The exhaust gases are then routed through line 106 back into line 98 whereat they combine with the remainder of the exhaust gases passing through line 98.

The exhaust gases can thereafter be sent directly to the primary reformer, or all or a portion of the gases can first be passed through a waste heat boiler 108. The exhaust gases give up some of their heat to generate steam within boiler 108 which is removed through line 110 and then used to drive a steam expander 11 2. Steam expander 112 drives an electrogenerator 114 which generates electrical energy. The expanded steam is then recirculated through line 11 6 back into boiler 108. The exhaust gases, if not passed through waste heat boiler 108, or after being passed through boiler 108 are then routed through line 118 to the reforming stage of the process. A branch line 11 9 connects between line 11 8 and the furnace section of primary reformer 18. Since the exhaust gases are rich in oxygen, all or a portion of the exhaust gas can be used for supplying preheated oxygen into the furnace section of reformer 1 8 for combustion therein with a fuel introduced through line 1 20. The remaining portion of the exhaust gases flowing through line 11 8 are introduced into the convection section 1 6 in order that they may give up additional heat to the streams flowing through preheater 14, 28. The exhaust gas is ultimately removed from reformer 1 8 through line 1 22.

Referring to Fig. 2, an alternative embodi ment of the invention is shown, wherein expander 84 drives liquid compressor 72 rather than compressor 86. It is to be understood that the expander 84 can be used to drive other compressor services, bearing in mind that the principal aspect of the invention relates to the use of the potential energy of the pressurized purge gas as well as its chemical energy so as to improve the efficiency of the overall process.

In order to illustrate the advantages of the present invention, the following example is provided: EXAMPLE In this example the purge gas is removed from drum 68 at a pressure of approximately 1400 psig, and is reduced to a pressure of approximately 210 psig. In the case of a 500 MT/day methanol plant, the purge gas has a net heating value of approximately 310 BTU per standard cu. ft. This gas may be burned in gas turbine 88 to produce approximately 40 megawatts of electricity at electrogenerator 96. The gas turbine exhaust will be at a temperature of approximately 1000 degrees F., which could be used to generate an additional MW at elecrogenerator 114.

Since gas turbines are usually supplied with excess air in order to keep the temperature of the exhaust gases below an acceptable upper limit imposed by materials which gas turbine components are made (approximately 2000 degrees F.) the exhaust gas is rich in oxygen.

This oxygen rich preheated stream can be introduced to the furnace section 32 for combustion with a fuel provided via line 120, or can be passed into convection section 1 6 for heating feed streams passing through preheaters 14, 28.

A latitude of modification, change and substitution is intended in the foregoing disclosure and in some instances some features of the invention will be employed without a corresponding use of other features. Accordingly, it is appropriate that the appended claims be construed broadly and in a manner consistent with the spirit and scope of the invention herein.

Claims (9)

1. An improvement in a process for the production of a crude reaction product including reforming, energy recovery, compression and synthesis reaction stages, comprising the steps of a) removing a purge gas at an elevated pressure from said synthesis reaction stage, b) passing said purge gas through an expander so as to recover energy from said purge gas line pressure, c) combusting said expanded purge gas with an oxygen containing gas to obtain a heated exhaust gas, d) expanding said heated exhaust gas to recover energy therefrom, and e) passing said expanded heated exhaust gas to said reforming stage for use thereat.

2. The improvement of claim 1 further comprising the step of passing a portion of said expanded heated exhaust gas through an indirect heat exchanger for preheating said oxygen containing gas prior to combustion of said oxygen containing gas with said purge gas.

3. The improvement of claim 1 further comprising the step of passing said expanded heated exhaust gas through a waste heat boiler prior to the passage of said gas to said reforming stage.

4. The improvement of claim 1 wherein said step of passing said expanded heated exhaust gas to said reforming stage comprises introduction of said gas as a reactant to a reformer furnace for combustion with a fuel therein.

5. The improvement of claim 1 wherein said step of passing said expanded heated exhaust gas comprises introduction of said gas into the convection section of a reformer furnace for preheating reformer feedstreams therein.

6. The improvement of claim 1 further comprising the steps of a) passing expanded heated exhaust gas through an indirect heat exchanger for preheating said oxygen containing gas prior to combustion of said oxygen containing gas with said purge gas, b) passing said heated exhaust gas into a waste heat boiler for heating a vaporizable fluid flowing therethrough, prior to the passing of said heated exhaust gas to said reforming stage, and wherein said step of passing said expanded heated exhaust gas to said reforming stage comprises passing a portion of said gas to a furnace section of a reformer for combustion with a fuel therein, and passing the remainder of said gas to a convection section of said reformer for heating reformer feedstreams therein.

7. An improvement in a process for the production of methanol from natural gas including reforming, energy recovery, compression and sysnthesis reaction stages comprising the steps of: a) removing a purge gas at an elevated pressure from said synthesis reaction stage, b) passing said purge gas through an expander so as to recover energy from said purge gas line pressure, c) combusting said expanded purge gas with an oxygen containing gas to obtain a heated exhaust gas, d) expanding said heated exhaust gas to recover energy therefrom, and e) passing said expanded heated exhaust gas to said reforming stage for use thereat.

8. A process for the production of a crude reaction product comprising the steps of a) preheating a stream including natural gas and steam, b) passing said preheated stream through a plurality of catalyst line reforming tubes disposed in a furnace section of a reformer, c) combusting a fuel with an oxygen containing gas within said furnace section of said reformer to heat said stream passing through said plurality of tubes, a portion of said natural gas reacting with a portion of said steam within said tubes to form carbon monoxide, carbon dioxide and hydrogen gases, d) removing a gaseous stream including carbon monoxide, carbon dioxide, hydrogen, natural gas and steam from said reforming tubes at an elevated temperature, e) passing said gaseous stream through heat exchange means to reduce the temperature of said stream and recover heat energy therefrom, f) compressing said gaseous stream, g) passing said compressed gaseous stream into a synthesis reactor, said carbon monoxide reacting with said hydrogen, and said carbon dioxide reacting with said hydrogen to yield methanol, h) removing a resultant gaseous product stream from said synthesis reactor, said product including methanol, steam, carbon dioxide, carbon monoxide, hydrogen and natural gas, i) cooling said resultant gaseous product stream such that a major portion of said methanol condenses to a liquid, j) separating said condensed methanol from the gaseous remainder of said cooled resultant product stream, k) recirculating a major portion of said gaseous remainder for addition to said gaseous stream prior to the introduction of said gaseous stream to said sysnthesis reactor, I) passing the remaining minor portion of said gaseous remainder through an expander, the pressure of said remainder being reduced as it passes therethrough, m) combusting said expanded gaseous remainder with an oxygen containing gas to yield a heated exhaust gas, n) expanding said heated exhaust gas, and o) passing said expanded heated exhaust gas into said reformer.

9. The process of claim 8 wherein said step of passing said expanded heated exhaust gas into said reformer comprises passing said exhaust gas into said furnace section for combustion with a fuel therein.

1 0. The process of claim 8 wherein said step passing said expanded heated exhaust gas into said reformer comprises passing said exhaust gas into a convection section of said reformer.