GB2192703A

GB2192703A - Gas treatment method and apparatus

Info

Publication number: GB2192703A
Application number: GB08617528A
Authority: GB
Inventors: Timothy David Atkinson; Eric Stephen Moore
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 1986-07-17
Filing date: 1986-07-17
Publication date: 1988-01-20
Also published as: GB8617528D0; GB2192703B

Abstract

A method of forming gas mixture of hydrogen and nitrogen typically suitable for use as an ammonia synthesis gas includes passing crude hydrogen through an array 2 of heat exchangers 4, 6 and 8 to refrigerate the hydrogen. The crude hydrogen is then introduced into a nitrogen wash column 20 in which it is washed with a nitrogen stream liquefied by passage through the heat exchanger 4, 6 and 8. Carbon monoxide, methane and other such impurities are thereby removed from the hydrogen and a gas mixture of nitrogen and hydrogen is returned from the top of the column 28 through the heat exchargers 8, 6 and 4. To provide refrigeration for the process, a stream of nitrogen is compressed in compressor 40, expanded in expansion turbine 44, liquefied by passage through heat exchanger 8 and then pumped by pump 48 into a mixer 31 in which it is mixed with the mixture of nitrogen and hydrogen. Evaporation of the liquid nitrogen component of the mixture helps to provide additional refrigeration for the process. <IMAGE>

Description

SPECIFICATION Gas treatment method and apparatus This invention relates to a gas treatment method and apparatus. In particular, it relates to a method of forming a gaseous mixture of hydrogen and nitrogen, for example an ammonia synthesis gas mixture (i.e. a gas mixture comprising nitrogen and hydrogen in essentially the stoichiometric proportions required for the formation of ammonia) from a crude (ie. impure) hydrogen stream that includes at least one of carbon monoxide and methane as an impurity.

One well-established process of forming synthesis gas comprises performing first a partial oxidation of a hydrocarbon feedstock, second a "shift" reaction on the partial oxidation products, and then removing carbon dioxide to form an intermediate crude hydrogen stream that typically contains more than 90% by volume of hydrogen and may also contain one or both of carbon monoxide and methane as impurities. The crude hydrogen stream may also incude small amounts of nitrogen and argon. The crude hydrogen stream is typically produced at--relatively a high pressure, say, above 25 atmospheres.To form the synthesis gas from the intermediate crude hydrogen stream, the stream is subjected to a nitrogen wash process whereby carbon monoxide and methane impurities are in effect washed out of the hydrogen by means of liquid nitrogen and then further nitrogen is added to the gas mixture, in order to adjust the proportions of nitrogen and hydrogen in the mixture to those required for the synthesis of ammonia.

Typically a cryogenic air separation plant is employed to provide oxygen for the partial oxidation step and nitrogen for the nitrogen wash and nitrogen make-up.

The nitrogen wash plant can be integrated with the air separation plant such that the air separation plant meets the refrigeration requirements for the nitrogen wash plant. Alternatively, the nitrogen wash plant may stand alone, not relying on thy air separation plant for its refrigeration requirements.

The present invention is addressed at providing a method and apparatus for forming a gaseous mixture of nitrogen and hydrogen (eg.

an ammonia synthesis gas) which employ nitrogen wash column and which do not depend for their refrigeration upon an external source.

The present invention provides a method and apparatus that are able to meet this need while employing only one or more expansion machines, thereby minimising or eliminating the need for compression power for refrigeration, and in some examples, becoming a net generator of power.

According to the present invention there is provided a method. of forming a gaseous mixture of nitrogen and hydrogen, comprising the steps of reducing the temperature of a crude hydrogen stream including at least one of carbon monoxide and methane, by heat exchange and then introducing the crude hydrogen stream into a nitrogen wash column, liquefying a first stream of gaseous nitrogen by heat exchange, introducing the liquefied nitrogen into said nitrogen wash column and washing said crude hydrogen stream therewith, withdrawing a stream of liquid from the bottom region of the column, withdrawing a stream of vapour comprising hydrogen and nitrogen and essentially free of methane and carbon-mo- noxide from the top region of said nitrogen wash column, expanding a second gaseous nitrogen stream to produce refrigeration, liquefying it by heat exchange, pumping it- into said vapour stream and mixing it therewith, and passing the resulting mixed stream and the, said stream of liquid in countercurrent heat exchange relationship to said crude hydrogen and first and second gaseous nitrogen streams.

The partial pressure of nitrogen in said mixed stream is preferably less than the pressure to which said second stream of nitrogen is expanded.

The invention also provides apparatus for forming a gaseous mixture of nitrogen and hydrogen, comprising heat exchange means for reducing the temperature of a crude hydrogen stream, including at least one of carbon monoxide and methane and for liquefying a first stream of gaseous nitrogen; a nitrogen wash column having a first inlet for the cooled crude hydrogen stream, and a second inlet for said liquefied nitrogen, a first outlet for the withdrawal of a vapour stream comprising hydrogen and nitrogen affording communication between the top of the nitrogen wash column and a first conduit for returning said vapour stream through said heat exchange means, and a second outlet, for the withdrawal of-a liquid stream affording communication between the bottom of the nitrogen wash column and a second conduit for returning- the vapour stream through said heat exchange means; means for expanding a second gaseous stream of nitrogen to produce refrigeration; means for introducing said expanded nitrogen stream into said heat exchange means, whereby said nitrogen is liquefied, a pump for pumping said liquefied second stream of nitrogen into said first conduit, and means for-mixing said second stream with said vapour--frac- tion.

Refrigeration for the heat exchange means is provided by evaporation of the liquid nitrogen component of the mixture of nitrogen and hydrogen as it returns through the heat exchanger means. Evaporation of the liquid nitrogen content of this mixture takes place over a discrete- temperature range.

The gaseous mixture of nitrogen and hydrogen may be used for the synthesis of ammo nia.

By arranging for substantially all the liquefied second stream to flow into said pump, and not employing a recycle of part of such liquid through the heat exchange means, the need for a recycle compressor is avoided.

The method and apparatus according to the invention will now be described by way of example with. reference to the accompanying drawing which is a flbw diagram illustrating a plant for forming ammonia synthesis gas from a crude hydrogen stream.

The plant shown in the drawing includes a heat exchanger array 2. ,The array 2 com prises heat exchangers 4, 6 and 8,. which may be separate from one another or may be part of a single heat exchanger assembly. The heat exchanger array 2 is employed to precool a crude hydrogen stream upstream of its purification in a nitrogen wash column 20. The heat exchanger array 2 is also- used in the liquefaction of a stream of nitrogen that is employed to 'wash' the impure hydrogen stream in the. nitrogen wash column 20.

Streams from the bottom and top of the col umn 20-are returned through the heat exchanger array 2 in countercurrent heat exchange with the nitrogen and crude hydrogen streams.

Crude hydrogen, which in one example of the invention comprises about 98% by volume of hydrogen, about 2% by volume of carbon monoxide, and traces of nitrogen, methane and noble gases its passed into a conduit 10 which extends from warm end 3 of the heat .exchanger array 2 through the cold end 5 of the array 2, and terminates in an inlet 22 to the nitrogen wash column 20. The crude hydrogen stream may for example enter the conduit 10 at a pressure of about 30 atmos pheres absolute, and allowing for pressure drop through the heat exchangers 4, 6 and 8, it is this pressure that sets the operating pressure for the nitrogen wash- column 20. The invention is not limited to forming (anmonia) synthesis gas from an impure hydrogen stream at a pressure in the order of 30 atmospheres.

Indeed the crude hydrogen stream may be received at higher or lower pressures, although it is preferred that the nitrogen wash column operates at as high a pressure as possible. The temperature- of the crude hydrogen stream as it. enters the conduit 10 is typically about ambient temperature or a little above ambient temperature. As the crude hydrogen stream flows through the heat exchangers 4, 6 and 8 it is progressively cooled and it may for example.leave the heat exchanger 4 at a temperature of about 160K, the- heat exchanger 6 at a temperature of about 120K, and the cold end 5 of the heat exchanger array as vapour at a temperature of about 92K, at which temperature it enters the bottom of the nitrogen wash column 20through the inlet 22.

A gaseous nitrogen stream, typically at a pressure slightly in excess of that of the crude hydrogen stream enters a conduit 12 which extends- from upstream of the warm end 3 of the heat exchanger array 2 through the cold end 5 of the array 2, and terminates in an inlet 24 near the top of the nitrogen wash column 20. The nitrogen stream as it enters the conduit 12 is typically at a temperature in the order of or slightly above ambient temperature. The nitrogen stream is preferably taken from a cryogenic air separation plant (not shown) and is typically essentially pure containing less than 20 ppm (by volume) of impurities, save for argon. The nitrogen stream as it passes through the heat exchanger array 2 is progressively cooled to a temperature of about 92K.The nitrogen stream, now a subcooled liquid, then passes through valve 26 in the conduit 12 and enters the top of the nitrogen wash column 20 through the inlet 24.

The-nitrogen wash column 20 is a liquidvapour contact column in which a stream of liquid flows downwardly through the column and comes intro intimate mass exchange relationship- with ascending vapour. Typically, the column 20 is provided with a plurality of spaced, horizontal liquid-vapour contact trays 21. The inlet 24 is positioned above the level of the uppermost tray, and the inlet 22 is positioned below the level of the lowermost tray..

Liquid nitrogen flows from the inlet 24 down the column tray-by-tray and comes into intimate contact with crude hydrogen vapour entering the bottom of the column 20 through the inlet 22. Since hydrogen is more volatile than nitrogen, which in turn is more volatile than carbon monoxide and methane, the liquid nitrogen performs the function of stripping the carbon monoxide and methane impurities from the vapour ascending the column. Thus, the vapour leaving the uppermost tray comprises hydrogen together with a proportion of nitrogen that enters the vapour phase from the liquid phase, but is essentually free of methane and carbon monoxide impurities. A liquid collects at the bottom of the column which comprises nitrogen, carbon monoxide, methane, and some hydrogen which dissolves in the liquid phase as the liquid descends -the column. In one example of the invention, the column 20 operates at a pressure of about 30 atmospheres, the liquid that collects at the bottom of the column is at a temperature of about 82K, and a vapour comprising about 90% by volume of hydrogen and 10% by volume of nitrogen leaves the top of the column 20 at its dewpoint (a temperature of about 83K).

The vapour mixture leaves the top of the nitrogen wash column 20 through an outlet 23 and enters a conduit 30 which terminates in mixing means 31, the operation of which shall be described below. A fluid stream from the mixer 31 is returned through the heat exchangers 8, 6 and 4 in sequence, countercurrently to the aforesaid nitrogen and crude hydrogen streams.

The liquid that collects at the bottom of the column is withdrawn through the outlet 32 and passes along conduit 36, flowing through an expansion valve 34 effective to reduce its pressure to about 2.5 to 3 atmospheres, or other pressure a little above the pressure at which it is to be supplied. The resulting expanded fluid then enters the heat exchanger 8 and flows in sequence through the heat exchangers 8, 6 and 4 countercurrently to the aforesaid nitrogen and crude hydrogen streams.

This fuel gas stream may leave the cold end 3, of heat exchanger array 2 at a slightly elevated pressure and a temperature of about ambient and may be employed as a fuel gas.

It is to be appreciated that in order to precool the crude hydrogen stream and in'order to liquefy the nitrogen stream, net refrigeration needs to be supplied to the heat exchangers 4, 6 and 8. In addition, there will inevitably be some inleak of heat from the ambient environment, even though the heat exchangers 4, 6 and 8 and other parts of the apparatus operating at below ambient temperature are located in a thermally-insulated housing (not shown) sometimes called a 'cold box'. There will thus be an additional requirement for net refrigeration in order to balance the heat leak.

Although some refrigeration is generated as a result of the expansion through valve 34 of the liquid withdrawn from the nitrogen wash column 20, this refrigeration is not adequate to meet the total requirement for refrigeration of the process. In accordance with the invention a second stream of nitrogen is employed in providing refrigeration for the process and in making-up the proportion by volume of nitrogen in the stream. leaving the- top of the nitrogen wash column to approximately the proportion required for the synthesis of ammonia (ie 1 mole of nitrogen for each three moles of hydrogen).Accordingly a second gaseous nitrogen stream, typically at the same temperature and pressure as the stream that enters the conduit 12, flows into a conduit 38 which terminates in the inlet to a booster compressor 40 which raises the pressure of the nitrogen to about 36 atmospheres. The outlet of the booster compressor 40 communicates with a cooler (not shown) for removing the heat of compression from the compressed nitrogen. The nitrogen leaves the aftercooler and enters a conduit 42 which at its other end terminates in the inlet to an expansion turbine 44 and which extends through the heat exchanger 4 from its warm end to its cold end. The nitrogen is thus progressively lowered in temperature as it flows through the heat exchanger 4 and may for example leave the heat exchanger 4 at a temperature of about 160K.

The expansion turbine 44 is effective to ex- pand the nitrogen to a pressure of, for example, about 11 atmospheres. The expansion is accompanied by a reduction in temperature of the nitrogen, which may, for example, leave the turbine 44 at a temperature of about 120K. The expansion turbine 44 is preferably coupled to the booster compressor whereby the work done by the expanding nitrogen is used to drive the compressor. Thus the-rotors (not shown) of the two machines are mounted on the same shaft 45. It is not essential to the invention to employ a booster compressor 40, but its use makes possible enhanced production of refrigeration by the expansion turbine 44.

The expanded nitrogen passes out of the turbine 44 into a conduit 46 which passes from through the heat exchanger 8 from its warm to cold end (but which by-passes the heat exchanger 6) and terminates in the inlet to a pump 48. As the expanded nitrogen stream passes through the heat exchanger 8, so it is progressively reduced in-temperature until it reaches the temperature at which it liquefies, which temperature, in the example of a stream of nitrogen at a pressure of about 11 atmospheres, is about 104K. Since the expanded nitrogen is at a pressure below its critical pressure liquefaction takes place -at the one temperature (e.g. 104K). Once all the nitrogen is liquid, it is sub-cooled by continuing passage thorugh the heat exchanger 8.The liquefied nitrogen may, for example, leave the cold end of the heat exchanger 8- at a temperature of about 92K. It then flows into the pump 48 which is effective to pump the liquid through a conduit 50 that terminates in the mixer 31. The mixer 31 may comprise any suitable device for satisfactorily entraining a liquid nitrogen stream into the nitrogen-hydrogen vapour mixture that leaves the top of the nitrogen wash column 20. It may for example comprise a simple venturi with the -conduit 50 terminating at the throat of the venturi. The flow of the nitrogen-hydrogen mixture through the venturi thus helps to induce entry of the liquid nitrogen into the venturi.

As the liquid nitrogen enters the venturi so it tends to atomise and become thoroughly entrained in the vapour phase. If desired, in order to enhance the mixing of the liquid nitrogen and the nitrogen-hydrogen vapour an more sophisticated mixing device 31 may be employed in addition or as an alternative tithe venturi.

The relative flow rates of the liquid nitrogen stream through the conduit 50 and the nitrogen-hydrogen vapour stream into the mixer 31 are preferably chosen such that the respective mole fractions of hydrogen and nitrogen in the mixture that is formed in the mixer31 is that required for the synthesis of ammonia (i.e. 3 moles of hydrogen to 1 mole of nitrogen). It is possible, however, to operate with a deficit of nitrogen in the mixture-and-to top it up with nitrogen after the mixture has been warmed to ambient temperature (or above) by return through the heat exchanger array 2.

The mixture leaves the mixing device 31 at a temperature of about 83K and a pressure of about 28 atmospheres. Since the nitrogen comprises 25 mole per cent of the mixture, its partial pressure is 7 atmospheres, that is a pressure less than that at which the nitrogen liquefies in the conduit 46. Accordingly, the liquid nitrogen in the mixture returning via con duit 52 through the heat exchangers 8, 6 and 4 begins to boil at a lower temperature than that at which the liquid in the conduit 52 con denses. Moreover, since the boiling or evapo rating nitrogen is part of an essentially two component mixture, its change of phase takes place not at one temperature, but over a tem perature range.It is thus'possible to use the nitrogen-hydrogen stream returning through the heat exchanger array 2 from the cold end to its warm end to provide sufficient refrigeration over the- entire operating temperature range of the heat exchange array 2 to obviate the need to supply refrigeration from an exter nal source.It is to be appreciated that the refrigeration is generated by operation of the expansion turbine 44, by virtue of the mixing of the liquid-nitrogen-with the nitrogen-hydro gen stream in the mixer 31, and by virtue of the evaporation of the liquid nitrogen returning through the conduit 52 at a partial pressure less thanks that at which the second nitrogen stream liquefies in the conduit 46, this iast phenomenon making it possible for the return ing nitrogen-hydrogeh stream to provide ade quate refrigeration at the cold knd of heat ex changer 8.

There is an appreciable amount of freedom in setting the net refrigeration generated by the expansion of the second nitrogen -stream and its subsequent pumping, after liquefaction, into the mixer 31 for mixing with the nitrogen hydrogen mixture leaving the tdp of the nitro gen wash column 20. The amount of refrige ration generated will depend on the inlet and outlet pressures of the expansion turbine 44.

Other pressures than the aforementioned inlet pressure. of 36 atmospheres and the outlet pressure of 1'1 atmospheres-may be selected.

Normally, the said turbine inlet and outlet pressures will be selected so that the refrige ration generated balances the demand for re frigeration of the process.

As aforementioned, the return of the stream of nitrogen hydrogen mixture through the heat exchangers 8, 6 and 4 results in-the warming of this- stream, and it leaves the warm end 3 of-the heat exchanger array 2, at about ambi ent temperature.

In operation of the plant shown in the draw ing, means may be provided for making small variations in the amount of liquid nitrogen that is introduced into the column 20. In the event that a minor decrease in the said amount of liquid nitrogen is desired, liquid nitrogen is passed through conduit 54 from a region of the conduit 12 upstream of the valve 26 into the- mixer 31 or conduit 50, a valve 56 being set to control the rate of flow. In the event that a minor increase in the amount of liquid nitrogen is desired, liquid nitrogen is passed through conduit 58 from the conduit 50 into the conduit 12 at a region thereof upstream of the valve 26, the rate of flow being controlled by a valve 60 in the conduit 58.

The ratio of the flow rates of first nitrogen stream entering conduit 12, second nitrogen stream entering conduit 38 and crude hydrogen stream entering conduit 10 may, for example, be about 1:1.8:8.2.

In other examples of the invention, the turbine 44 rather than being coupled to the booster-compressor 40, may be employed to generate electrical power.

Claims

1. A method of forming a gaseous mixture of nitrogen and hydrogen, comprising the steps of reducing the temperature of a crude hydrogen stream, including at least one of carbon monixide and methane, by-heat exchange and then introducing the crude hydrogen stream into a nitrogen wash column, liquefying a first stream of gaseous nitrogen by heat exchange, introducing the liquefied nitrogen into said nitrogen wash column and washing said crude hydrogen stream therewith, withdrawing a stream of liquid from the bottom region of the column, withdrawing a stream of vapour comprising hydrogen and nitrogen and essentially free of methane and carbon monoxide from the top region of said nitrogen wash column, expanding a second gas nitrogen stream to produce refrigeration, liquefying it by heat exchange, pumping it into said vapour stream and mixing it therewith, and passing the resulting mixed stream and the said stream of liquid in countercurrent heat exchange relationship to said crude nitrogen and first and second gaseous nitrogen streams.

2. A method as claimed in claim 1, in which the partial pressure of nitrogen in said mexed stream is less than the pressure to which said second stream of nitrogen? is expanded.

3. A method as claimed in claim 1 or claim 2, in which said second nitrogen stream is expanded in an expansion turbine.

4. A method as claimed in claim 3, in which said expansion turbine is employed to drive a booster compressor that raises the pressure of the stream of gaseous nitrogen upstream of the expansion turbine.

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5. Method as claimed in claim 3, in which said expansion turbine is employed to generate electrical power.

6. A method as claimed in any one of the preceding claims, in which, a portion of the liquefied second stream of nitrogen is pumped into the first liquefied stream of nitrogen.

7. A method as claimed in any one of claims 1 to 5, in which a portion of the first stream of nitrogen is taken from downstream of where it liquefies and is mixed with the said vapour stream of nitrogen and hydrogen or with the second stream of nitrogen downstream of the pumping means employed to introduce the second stream of nitrogen into the said vapour stream of nitrogen and hydrogen.

8. A method as claimed in any one of the preceeding claims, in which the gaseous mixture of nitrogen and hydrogen is employed in the synthesis of ammonia.

9. A method of forming an ammonia synthesis gas, substantially as herein described with reference to the accompanying drawing.

10. Apparatus for forming a gaseous mixture of nitrogen and hydrogen, comprising heat exchange means for reducing the temperature of a crude hydrogen stream including at least one of carbon monoxide and methane, and for liquefying a first stream of gaseous nitrogen; a nitrogen wash column having a first inlet for the cooled crude hydrogen stream, and a second inlet for said liquefied nitroqen, a first outlet for the withdrawal of a vapour stream comprising hydrogen and nitrogen affording communication between the top of the nitrogen wash column and a first conduit for returning said vapour fraction through said heat exchange means, and a second outlet, for the withdrawal of a liquid stream affording communication between the bottom of the nitrogen wash- column and a second conduit for returning the vapour through said heat exchange means; means for expanding a second gaseous- stream of nitrogen to produce refrigeration; means for introducing said expanded nitrogen stream into said heat exchange means, whereby said nitrogen is liquefied; a pump for pumping said liquefied second stream of nitrogen into said first conduit; and means for mixing said second stream with said vapour fraction.

11. Apparatus as claimed in claim 10, in which said expansion means comprises an expansion turbine.

12. Apparatus as claimed in claim 11, in which said expansion turbine is adapted to drive a booster-compressor which has an inlet for said second stream of gaseous nitrogen and an outlet in communication with the inlet of the expansion turbine.

13. Apparatus as claimed in claim 1-1, in which the expansion turbine is coupled to electrical power operating means.

14. Apparatus as claimed in any one of claims 10 to 13, additionally including means for passing liquid nitrogen from downstream of said pump into said second inlet two the nitrogen wash column.

15. Apparatus as claimed in any one of claims 10 to 13, additionally including means for passing liquid nitrogen from upstream of said second inlet to the nitrogen wash column to said mixer or a region intermediate said mixer and said pump.

16. Apparatus for forming an ammonia synthesis gas substantially as herein described with reference to the accompanying drawing.