GB2300855A - Methanol synthesis - Google Patents

Abstract

The process for producing methanol by passing a synthesis gas mixture of carbon oxides and hydrogen over a copper-containing catalyst is potentially subject to the problem that, when it is to be shut down, the synthesis plant has to be isolated from the supply of fresh synthesis gas and if the synthesis reactor is then maintained at its normal operating pressure, its contents can reach equilibrium and the resulting temperature increase can lead to methanation. The invention remedies this situation by a shut-down procedure which (a) comprises releasing gas from the synthesis reactor at a position (44) above the catalyst bed and/or (b) comprises feeding desulphurised natural gas and/or carbon dioxide (42, 242, 46) into the synthesis reactor at a rate sufficient to substantially suppress exothermic reactions.

Description

METHANOL SYNTHESIS THIS INVENTION relates to methanol synthesis and in particular to a process of methanol synthesis characterised by a shut-down procedure.

Methanol synthesis is usually carried out by passing a mixture of carbon oxides and hydrogen over a copper-containing catalyst such as coppertzinc oxide/alumina at a pressure typically 50-100 bar abs, possibly higher, with control of temperature at usually 220-270 C by indirect heat exchange and/or cool gas injection and/or interbed cooling. A further measure of temperature limitation arises from controlling the space velocity of the reacting gas at a level higher than permits chemical equilibrium to be reached, so as to benefit from faster reaction when the partial pressure of unreacted carbon oxides and hydrogen is still reasonably high.The process is operated on a recycle basis, in a loop including the successive steps of cooling reacted gas, separating methanol as liquid, purging part of the remaining gas, adding fresh gas to the rest of the remaining gas and feeding the mixture to the catalytic reactor. Further details of methanol are available from standard textbooks such as Kirk-Othmer's Encyclopedia of Chemical Technology.

When such a process is to be shut down, for example in the event of failure of a gas circulator, the synthesis plant has to be isolated from the supply of fresh synthesis gas. If the synthesis reactor is then maintained at its normal operating pressure, its contents can reach equilibrium and the resulting temperature increase can lead to methanation, catalysed by the copper catalyst or by iron as particles resulting from decomposition of carbonyl or the conStructional' steel of the plant.

The problem is potentially more serious when the synthesis reactor is of the tube-cooled type, since then gas passing upward through tubes surrounded by catalyst is heat-exchanged with the over-heated reacted gas in the catalyst and is thus over-hot as it enters the catalyst. There thus arises a thermal positive feedback damaging to the copper catalyst and potentially hazardous to operators.

ACCORDING TO THE INVENTION in its first aspect a process for producing methanol is characterised by a shut-down procedure which comprises releasing gas from the synthesis reactor at a position above the catalyst bed.

This aspect of the invention also provides a methanol synthesis reactor having a gas take-off valve at a position above the catalyst bed and, yet further, a methanol synthesis plant having such a valve and a piping connection therefrom to for example a vent or flare stack or gasholder or furnace burners or gas fractionator or chemical user such as hydrodesulphurisation.

The first aspect process, reactor and plant are especially effective when the reactor is tube-cooled, since the gas from the loop cools the gas already in the catalyst and leaves without passing over the catalyst.

IN ITS SECOND ASPECT the invention provides against the risk of over-heating by utilising gases available in the methanol production process, namely desulphurised natural gas and/or carbon dioxide. This process is characterised by a shut-down procedure which comprises feeding desulphurised natural gas and/or carbon dioxide into the synthesis reactor at a rate sufficient to substantially suppress exothermic reactions and/or purging the reactive gases from the synthesis plant, suitably through the normal plant blow-down or nonreactives purge or, indeed, through the outlet envisaged for the invention in its first aspect.

This procedure operates by any of the following mechanisms: sweeping reactive gases from the catalyst; cooling by mixing; decreasing the partial pressures of reactant gases; cooling by reverse shift reaction of carbon dioxide; limiting methanation by le Chatelier effect of added methane; deactivating iron by oxidising effect of carbon dioxide.

Ways of carrying it out depend on the details of the synthesis plant and on the extent to which modifications may be considered worth while. Thus the added gas(es) may be fed at the start of shut-down if they are available at synthesis pressure, for example if carbon dioxide is normally fed to the loop, or if a compressor is available.

If the added gas(es) are available only at the inlet pressure of a steam reforming section of a methanol production process, they can be fed only when part depressurisation has taken place.

The processes of the first and second aspects can be used together. It is advantageous to leave the synthesis catalyst in an atmosphere of methane and/or carbon dioxide while the process is shut down. In order to purge the whole synthesis plant of reactive gas, feeding of natural gas and/or carbon dioxide is preferably includes letdown through the normal purge outlet.

The invention is illustrated by the accompanying drawings in which: Fig. 1 shows the process as used in conjunction with a tube-cooled synthesis reactor; and Fig. 2 shows the process as used in conjunction with a quenchcooled synthesis reactor.

Referring to fig.l, synthesis reactor 10 contains catalyst bed 12 in which are disposed vertical tubes 14 (only 2 shown) branched from header 16 leading upwardly from feed point 18. The outlet of catalyst bed 12 is at 20, leading via heat recovery exchanger 22, feed gas warmer 24 hot side and water-cooled condenser 26 to methanol separator 28, from which liquid methanol is taken as bottoms 30. Unreacted synthesis gas is taken overhead and divided into purge stream 34 and recycle stream 36. Stream 36 is mixed at 38 with fresh synthesis gas ('make-up gas') from a source such as hydrocarbon steam reforming or partial oxidation or coal gasification, followed by purification and, if necessary, by compression. The mixture is fed to circulator 40, passes point 42 to be described, then flows through feed gas warmer 24 cold side to reactor feed point 18.

One novel feature of the process is represented by item 42, which is a feed point for desulphurised natural gas and/or carbon dioxide.

Point 42 is at the position shown because circulator 40 is out of action and its failure is the cause of the shut-down. It could be elsewhere, for example at 46.

A second novel feature is represented by items 44 and 48-52.

Upper outlet 44, which may be already present or may be added as a simple modification or may be provided by using an existing gas inlet, leads via flow restriction orifice 48 (optional) and valve 50 to line 52 leading to a flare stack or other destination as mentioned above. In normal process operation valve 50 is closed, but for depressurisation it is opened. The extent of opening is controlled to avoid such rapid gas flow that the catalyst pellets in bed 12 are lifted, or is limited by orifice 48. (Gas inlet 46 is not used during depressurisation, but may be used to supply methane or carbon dioxide after depressurisation).

Referring to fig.2, reactor 210 contains 3 catalyst beds 212. It is fed by synthesis gas at upper inlet 213 and quench gas inlets 215.

Beds 212 may be each supported on a grid, leaving a clear space under each, or may be part of a single charge of catalyst, interrupted for part (but not all) of its cross-section by cages ('lozenges') in which gas from the next-above bed mixes with quench gas. (Items 246, 250 and 252 will be referred later). Reacted gas leaves reactor 210 at 220 and is divided at 221 into 2 sub-streams, one of which passes to heat recovery exchanger 222, the other to feed gas heater 224. The resulting cooled streams are reunited at 225 and passed via water-cooled condenser 226 to methanol separator 228, from which liquid methanol is taken as bottoms 230. Unreacted gas is taken overhead and divided at 232 into purge stream 234 and recycle stream 236. Stream 236 receives at 238 a feed of fresh synthesis gas as in fig.l. The mixture is passed through circulator 240 to point 241, where it is divided into (a) a main stream to be heated to catalyst inlet temperature in the cold side of heat evxchanger 224 and passed to feed point 213 of reactor 210; and (b) a quench stream to be passed direct to quench inlets 215.

Item 242 represents one of the novel features Qf the invention. It is used as an inlet for methane and/or carbon dioxide in the event of failure of circulator 240. Then gas flow between points 241 and 242 is towards exchanger 224, the opposite of its direction during normal operation of the process. Item 246 is an alternative inlet for methane and/or carbon dioxide, with valve 247 open and valve 250 closed.

Items 246, 250 and 252 are in a line not used in normal operation of the process, but added or adapted for the invention. With valve 247 closed, valve 250 is opened to allow gas to leave reactor 210 and flow to a flare stack or gas user as described above. As a result hot gas from 224 no longer enters the catalyst and the upper 2 beds 212 are cooled with quench gas.

Claims (11)

1. A process for producing methanol by passing a synthesis gas mixture of carbon oxides and hydrogen over a copper-containing catalyst, with control of temperature by indirect heat exchange and/or cool gas injection and/or interbed cooling, characterised by a shut-down procedure which (a) comprises releasing gas from the synthesis reactor at a position above the catalyst bed and/or (b) comprises feeding desulphurised natural gas and/or carbon dioxide into the synthesis reactor at a rate sufficient to substantially suppress exothermic reactions.

2. A process according to claim 1 in which for the indirect heat exchange cool incoming synthesis gas passes upwards through bottomfed tubes in heat exchange with hot gas reacting over the catalyst, whereby such incoming gas in normal operation becomes heated and passes downwards through the catalyst bed and at shut-down cods the gas in the catalyst and passes out without entering the catalyst.

3. A process according to claim 1 or claim 2 in which the desulphurised natural gas acts by one or more of sweeping reactive gases from the catalyst, cooling by mixing, decreasing the partial pressures of reactant gases and limiting methanation by le Chatelier effect of added methane.

4. A process according to any one of the preceding claims in which the carbon dioxide acts by one or more of sweeping reactive gases from the catalyst, cooling by mixing, decreasing the partial pressures of reactant gases, cooling by reverse shift reaction and deactivating iron by oxidising it.

5. A process according to any one of the preceding claims operated on a recycle basis, in a loop including the successive steps of cooling reacted gas, separating methanol as liquid, purging part of the remaining gas, adding fresh gas to the rest of the remaining gas and feeding the mixture to the catalytic reactor.

6. A process according to claim 5 in which the shut-down procedure comprises feeding desulphurised natural gas and/or carbon dioxide and synthesis gas is let down through the outlet used in normal operation for purging.

7. A process according to any one of the preceding claims which comprises leaving the synthesis catalyst in an atmosphere of methane and/or carbon dioxide while the process is shut down.

8. A methanol synthesis reactor having a gas take-off valve at a position above the catalyst bed.

9. A methanol synthesis plant having a synthesis reactor according to claim 8.

10. A methanol synthesis plant having means to feed desulphurised natural gas and/or carbon dioxide to the synthesis reactor.

11. A methanol synthesis process, plant or reactor, substantially as described in the accompanying drawings and specific description.