US3430451A

US3430451A - Compression of gaseous streams containing carbon monoxide

Info

Publication number: US3430451A
Application number: US652182A
Authority: US
Inventors: Dinshaw D Mehta; Axel Christensen
Original assignee: Chemical Construction Corp
Current assignee: Chemical Construction Corp
Priority date: 1967-07-10
Filing date: 1967-07-10
Publication date: 1969-03-04
Anticipated expiration: 1986-03-04
Also published as: FR1572827A; DE1767977B2; GB1192327A; DE1767977A1

Description

March 4, 1969 D. D. MEHTA ET AL COMPRESSION OF GASEOUS STREAMS CONTAINING CARBON MONOXIDE Filed July 10, 1967 DINSHAW D. MEHTA AXEL CHRISTENSEN I N VENTORS.

United States Patent 3,430,451 COMPRESSION 0F GASEOUS STREAMS CON- TAINING CARBON MONOXIDE Dinshaw D. Mehta, New York, N.Y., and Axel Christensen, Stamford, Conn., assignors to Chemical Construction Corporation, New York, N.Y., a corporation of Delaware Filed July 10, 1967, Ser. No. 652,182 US. Cl. 62-85 Int. Cl. FZSb 47/00 16 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTION Field of the invention The invention pertains to the compression of gaseous streams consisting of or containing carbon monoxide, such as methanol synthesis gas, which are to be compressed in centrifugal compressors which are constructed or fabricated of structural elements consisting of carbon steel as a material of construction. Such structural elements may include the compressor casing, rotor or wheels.

Description 0 f the prior art Compression of methanol synthesis gas, which contains carbon monoxide, has been carried out in the prior art by the use of piston-type reciprocating compressors. With the advent of large-scale plants which produce large tonnages of methanol per stream day, it has proven necessary for optimum economy to provide centrifugal compressors for the compression of the methanol synthesis gas to synthesis pressures of about 350 kg./sq. cm. These centrifugal compressors have been provided with internal elements such as rotor blades which are fabricated of stainless steel, which resists attack by carbon monoxide and does not form carbonyl compounds by reaction with carbon monoxide in service. However, the use of stainless steel is a costly expedient and also introduces fabrication problems not encountered with carbon steel elements SUMMARY OF THE INVENTION The present invention relates to the compression of gas streams which consist of or contain carbon monoxide. The invention is particularly directed to the compression of methanol synthesis gas prior to high pressure methanol synthesis. The typical methanol synthesis gas principally contains carbon monoxide, carbon dioxide and hydrogen in suitable proportions for catalytic synthesis of methanol by reaction of the hydrogen with the carbon oxides.

In the present invention, a gaseous stream containing carbon monoxide, such as methanol synthesis gas, is compressed in a rotating centrifugal compressor which has at least one carbon steel structural element. Thus, the compressor casing, rotor, wheels or blades or all of these elements may be fabricated of carbon steel. Within the context of the present invention, the term centrifugal compressor will be understood to include various types of rotating compressors such as conventional centrifugal compressors, axial compressors etc., and the improvement of the present invention does not relate to prior art piston-type reciprocating compressors. In accordance with the present invention as relating to centrifugal compressors, an improved method of preventing the reaction of carbon monoxide with carbon steel structural elements of the compressor during compression is provided. The reaction of carbon monoxide with carbon steel leads to carbonyl formation, with consequent corrosion and weakening of the structural element, and entrainment of iron carbonyl in the process stream. This reaction is prevented by injecting a liquid oil such as hydrocarbon oil, mineral oil or silicone oil into the feed gas stream being passed to the rotating centrifugal compressor. The liquid oil must have a flash point above the maximum temperature developed in the gas stream during compression, in order to avoid decomposition of the oil or reaction of the oil with the gas stream. The injected oil provides a thin film or coating on the internal structural elements of the compressor, which effectively prevents the reaction of carbon monoxide in the gas stream with the carbon steel structural elements of the compressor. The resulting compressed gas stream discharged from the centrifugal compressor is passed to a conventional gas-liquid separator, for separation of liquid oil from the compressed gas stream. In most instances, the compressed gas stream will be cooled prior to separation of the liquid oil component.

The principal advantage of the present invention is that the reaction of the carbon monoxide component of the gaseous stream with carbon steel structural elements of the centrifugal compressor is effectively prevented. Thus, centrifugal compressors having carbon steel casings or other structural elements fabricated of carbon steel may be employed in the compression of gaseous streams containing carbon monoxide, and the prior art requirement of stainless steel elements for this service has now been obviated. The fabrication or machining of carbon steel elements is considerably simpler and less costly than the production of these elements when stainless steel is the material of construcion. In addition, due to the injection of the liquid oil into the feed gas stream to the centrifugal compressor, the resultant coating of the compressor internals results in the in situ lubrication of rotating elements and support, which provides lenghened service life for the unit, and the oil coating on the centrifugal compressor internals such as the rotating wheels and blades effectively prevents erosion and cavitation effects. Finally, an important advan age is that the entrainment or carryover of iron carbonyl into the process gas stream being compressed is effectively prevented. This is highly important in such instances as the compression of methanol synthesis gas, since the carryover or deposition of iron carbonyl into the methanol synthesis catalyst beds is effectively pre vented. At the operating temperatures of the methanol catalyst beds, generally in the range of 330 C. to 400 C., iron carbonyl will decompose in o iron and carbon monoxide. The presence of iron in the catalyst bed will promote a methanation reaction, which is most difficult to control and which leads to a condition generally known as a runaway reaction, which has been responsible for severe mechanical damages to catalyst baskets of several methanol plants.

It is an object of the present invention to provide an improved method for compression of gaseous streams containing carbon monoxide in centrifugal compressors.

Another object is to prevent carbonyl formation during the compression of gaseous streams containing carbon monoxide in centrifugal compressors which are fabricated wi.h a carbon steel structural element.

A further object is to provide an improved method for the compression of methanol synthesis gas in centrifugal compressors which have at least one carbon steel structural element in contact with the methanol synthesis gas stream.

An additional object is to prevent the reaction of carbon monoxide with carbon steel during the compression of a gaseous stream containing carbon monoxide in a centrifugal compressor having an internal element fabricated of carbon steel.

Still another object is to prevent the carryover of iron carbonyl into the methanol synthesis catalyst bed, in instances when methanol synthesis gas is compressed in centrifugal compressors having at least one carbon steel structural element.

These and other objects and advantages of the present invention will become evident from the description which follows.

DESCRIPTION OF THE DRAWINGS AND PREFERRED EMBODIMENTS Referring now to the drawing, a flowsheet of a preferred embodiment of the invention is presented. Stream 1 consists of a gaseous stream containing carbon monoxide, and in this embodiment of the invention, stream 1 consists of methanol synthesis gas principally containing carbon monoxide, carbon dioxide and hydrogen. Stream 2 consisting of a liquid oil such as a hydrocarbon oil, mineral oil or silicone oil is injected into stream 1. The oil stream 2 will have a flash point above 230 C., in order to avoid subsequent oil decomposition or reaction with the synthesis gas stream during compression. The resulting mixed gas-liquid stream 3 is passed into the inlet port or wheel of rotating centrifugal compressor 4, and the gas stream is compressed in unit 4. The liquid oil component of stream 3 serves to coat the carbon steel internals of unit 4 with a thin film or coating of liquid oil, which effectively prevents the reaction of the carbon monoxide component of the gas stream with the carbon steel internals or casing of unit 4, and thus prevents carbonyl formation. Unit 4 is driven by drive shaft 5, which extends to steam turbine 6 in this embodiment of the invention. The steam turbine 6 provides the motive power for operation of the centrifugal compressors such as unit 4. High pressure steam stream 7 is expanded through the blades or wheels of unit 6 in accordance with conventional steam turbine practice, and low pressure steam is removed from unit 6 via stream 8,

which may be passed to a vacuum or surface condenser.

Alternative drive units such as an electric motor or combustion gas turbine may be employed instead of or in addition to unit 6.

The compressed synthesis gas together with entrained liquid oil is discharged from unit 4 via stream 9 at an elevated temperature generally in the range of 130 C. to 200 C. Stream 9 is cooled in heat exchanger 10, usually by heat exchange with boiler feed water or cooling water or both, and is discharged via stream 11 at a reduced temperature generally in the range of 30 C. to 80 C. Stream 11 now contains entrained water droplets produced by the condensation of water vapor from the gas phase at elevated pressure and reduced temperature, as well as entrained liquid oil. Although stream 11 may be passed directly to further compression, it will usually be preferable to remove the entrained liquid phase from stream 11, and add a second stream of fresh liquid oil to the resulting gas stream. Stream 1] is therefore passed into gas-liquid separator 12, which is a conventional baffled or cyclonic unit for the separation of gas and liquid phases. A liquid phase stream 13 containing water and oil is removed from separator 12. In most instances, stream 13 will be passed to an oil separator, not shown, where the oil and water phases separate from each other. The lower water phase is discharged to a sewer or other disposal, while the oil phase may be reprocessed to remove sludge etc. and then recycled.

A liquid-free intermediate pressure synthesis gas stream 14 is withdrawn from unit 12, and is preferably combined with a second liquid oil stream 23, which is similar or identical in composition to stream 2 described supra. The resulting mixed gas-liquid stream 24 is passed into the inlet port or wheel of rotating centrifugal compressor 15, and the gas stream is further compressed in unit 15. The liquid oil component of stream 24 serves to coat the carlton steel internals of unit 15 with a thin film or coating of liquid oil, which effectively prevents the reaction of the carbon monoxide component of the gas stream with the carbon steel internals or casing of unit 15, and thus prevents carbonyl formation. Unit 15 is driven by drive shaft 16, which is connected with drive shaft 5 in this embodimen. of the invention, and rotates together with shaft 5. Al ernatively, shaft 16 may be driven by a separate power source, such as an additional steam turbine, combustion gas turbine or electric motor.

The final fully compressed synthesis gas together with entrained liquid oil is discharged from unit 15 via stream 17 at an elevated temperature generally in the range of 130 C. to 200 C. Stream 17 is cooled in heat exchanger 18, usually by heat exchange with boiler feed water or cooling water or both, and is discharged via stream 19 at a reduced temperature generally in the range of 30 C. to C. An entrained liquid phase principally consisting of water and oil is removed from stream 19, by passing s'ream 19 into gas-liquid separator 20, which is similar in configuration and function to unit 12 described supra. A liquid phase stream 21 containing water and oil is removed from unit 20, and will usually be combined with stream 13 for subsequent processing as described supra. The liquidfree high pressure synthesis gas stream 22 is withdrawn from unit 20 and passed to methanol synthesis.

Numerous alternatives within the scope of the present invention will occur to those skilled in the art. As mentioned supra, unit 6 may alternatively be replaced by functionally equivalent drive units such as an electric motor or combustion gas turbine, or a plurality of units may be employed in tandem. Stream 1 may consist of any gas stream consisting of or containing carbon monoxide, such as synthesis gas during various stages of gas reforming, methanol synthesis gas, Fischer-Tropsch synthesis gas etc. As mentioned supra, in some instances stream 11 may be directly passed into unit 15 and unit 12 and stream 23 may be omitted. Alternatively, only a partial separation of the liquid phase may take place in unit 12, in which case stream 14 may be passed directly into unit 15, with the omission of stream 23. In some instances, stream 9 may be at desired pressure for subsequent processing and unit 15 may be omitted. In this case,

stream 9 may be treated for the removal of entrainedv liq-uid oil, by passing to a separation unit such as unit 12, or unit 10 may be provided for cooling of the gaseous stream prior to passing to unit 12. More than two compression units such as

units

4 and 15 may be provided in series or parallel or both. in which case liquid oil will usually be injected into the gas stream prior to each unit. In some instances of multi-stage series compression, the temperature rise in the later or final stages may be small, and the intermediate gas stream temperature may be sulficiently low, for example a temperature below C., that carbon monoxide reaction and carbonyl formation will not occur. In this case, oil injection in the later stage or stages of compression may be omitted.

An example of an industrial application of the method of the present invention to the compression of methanol synthesis gas for a 2000 tons/day methanol plant will now be described.

Example The method of the present invention was applied to the compression of a methanol synthesis gas which was received from gas reforming at 37 C. and 18.4 kg./Sq. cm., and had the following initial composition:

Content in Synthesis Gas Component Mols/hour Mol percent Carbon monoxide 3, 08). IL 5 Carbon dioxide. 2, 801 13. 2 Hydrogen, 14, 836 69. 8 Methane 4&8 2. 1 Nitrogen.-. 2t) 0. 1 Water vapor 74 0. 3

Following are pertinent operating conditions and compositions of principal process streams.

1 Ambient.

Stream 22 was further compressed to methanol synthesis pressure of 360 kg./sq. cm. in two subsequent centrifugal compressors operated in series, with oil injection prior to the third compressor only, since the gas stream temperature prior to and within the fourth compressor remained below 90 C. The

compressor units

4 and 15, and the two subsequent compressors, were fabricated with carbon steel casings, which were not subject to carbon monoxide attack and carbonyl formation, due to the presence of the injected oil.

We claim:

1. In the method of compression of a gaseous stream containing carbon monoxide in which said gaseous stream is passed into a rotating centrifugal compressor at an initial pressure, said centrifugal compressor having at least one carbon steel structural element in contact with said gaseous stream, said gaseous stream is compressed in said centrifugal compressor to a final pressure which is higher than the initial pressure, and the resulting gaseous stream is discharged from said centrifugal compressor at said final pressure, the improved method of preventing carbon monoxide reaction with said structural element and carbonyl formation which comprises injecting a liquid oil into said gaseous stream at said initial pressure,

said liquid oil having a flash point above the maximum temperature of said gaseous stream within said centrifugal compressor, and separating said liquid oil from the resulting gaseous stream discharged from said centrifugal compressor at said final pressure.

2. The method of claim 1, in which said gaseous stream is a methanol synthesis gas principally containing carbon monoxide, carbon dioxide and hydrogen.

3. The method of claim 2, in which said liquid oil has a flash point above 230 C.

4. The method of claim 1, in which said liquid oil is a hydrocarbon oil.

5. The method of claim 1, in which said liquid oil is a mineral oil.

6. The method of claim 1, in which said liquid oil is a silicone oil.

7. The method of claim 1, in which said resulting gaseous stream discharged from said centrifugal compressor is cooled prior to separation of said liquid oil.

8. In the method of compression of a gaseous stream containing carbon monoxide in which said gaseous stream is passed into a first rotating centrifugal compressor at an initial pressure, said first compressor having at least one carbon steel structural element in contact with said gaseous stream, said gaseous stream is compressed in said first'compressor to an intermediate pressure which is higher than the inital pressure, the resulting intermediate pressure gaseous stream is discharged from said first compressor, said intermediate pressure gaseous stream is cooled to a lower temperature, the resulting cooled gaseous stream is passed into a second rotating centrifugal compressor at intermediate pressure, said second compressor having at least one carbon steel structural element in contact with said gaseous stream, said gaseous stream is compressed in said second compressor to a final pressure which is higher than the intermediate pressure, and the resulting gaseous stream is discharged from said second compressor at said final pressure, the improved method of preventing carbon monoxide reaction with the carbon steel structural elements of said first compressor and said second compressor, and thereby preventing carbonyl formation, which comprises injecting a liquid oil into said gaseous stream at said initial pressure, said liquid oil having a fiash pont above the maximum temperature of said gaseous stream within said first and second centrifugal compressors, and separating liquid oil from the resulting gaseous stream discharged from said second compressor at said final pressure.

9. The method of claim 8, in which said gaseous stream is a methanol synthesis gas principally containing carbon monoxide, carbon dioxide and hydrogen.

10. The method of claim 9, in which said liquid oil has a flash point above 230 C.

11. The method of claim 9, in which a liquid phase containing oil and condensed liquid water is separated from said cooled intermediate pressure gas stream, and thereafter a second stream of liquid oil is added to the cooled intermediate pressure gas stream prior to passing said gas stream into said second rotating centrifugal compressor.

12. The method of claim 8, in which said liquid oil is a hydrocarbon oil.

13. The method of claim 8, in which said liquid oil is a mineral oil.

14. The method of claim 8, in which said liquid is a silicone oil.

15. The method of claim 8, in which said resulting gaseous stream discharged from said second compressor at said final pressure is cooled prior to separation of said liquid oil.

16. The method of claim 8, in which additional oil is injected into the cooled intermediate pressure gaseous stream prior to passing said gaseous stream into said second rotating centrifugal compressor.

No references cited.

ROBERT A. OLEA-RY, Primary Examiner. WILLIAM E. WAYNER, Assistant Examiner.

US. Cl. X.R.