Production of methanol

Details Production of methanol Production of methanol

1999-12-13

2001-07-03

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Liquid phase fischer-tropsch reaction

C518S702000, C518S704000, C518S707000

Reexamination Certificate

active

06255357

ABSTRACT:

This invention relates to methanol and in particular to the production thereof from a hydrocarbon feedstock.
Methanol is conventionally produced by subjecting a hydrocarbon feedstock to steam reforming, separation of the excess of steam, and then compression of the reformed gas to the desired synthesis pressure. The resultant synthesis gas, consisting of hydrogen, carbon oxides, methane and possibly a small proportion of nitrogen, is then added as “make-up gas” to a synthesis loop where it is mixed with recycled unreacted gas, heated to the desired synthesis inlet temperature and then passed over a synthesis catalyst. The effluent from the synthesis reactor is then cooled to condense methanol and the unreacted gas is recycled. A purge is generally taken from the loop to prevent a build-up of inerts.
The steam reforming step is conventionally effected by passing a hydrocarbon feedstock, in admixture with steam, at a pressure in the range 10-40 bar abs. over a catalyst, usually nickel on a support such as calcium aluminate cement or alumina, disposed in externally heated tubes. The tubes are heated such that the reformed gas leaves the catalyst at a temperature of the order of 700-900° C. The synthesis is generally effected at a pressure in the range 50-120 bar abs. The recycled unreacted gas typically forms 60-85% of the gas entering the synthesis reactor. A considerable amount of power is required to compress the make-up gas to the synthesis pressure and to recycle the unreacted gas, and also the compressor represents a considerable capital cost.
A methanol synthesis process with no compression of the synthesis gas after reforming has been proposed in U.S. Pat. No. 5,472,986. In this reference a hydrocarbon feedstock is compressed to a sufficiently high pressure prior to reforming by an adiabatic partial oxidation using enriched air. Instead of utilizing a synthesis loop, a plurality of synthesis stages is employed with separation of synthesized methanol after each synthesis stage: since the adiabatic partial oxidation process gives a hydrogen-deficient synthesis gas, hydrogen is recovered from the unreacted synthesis gas remaining after methanol synthesis and is recycled to the inlet of the first synthesis stage. However, since the recovered hydrogen is inevitably at a significantly lower pressure than the desired synthesis pressure, re-compression of the recovered hydrogen is necessary before recycle.
It has been proposed in U.S. Pat. No. 5,177,114 to employ a “single pressure” process where there is no compression of the gas after reforming and no recirculation of the unreacted gases. As in U.S. Pat. No. 5,472,986, the hydrocarbon feedstock is compressed to a sufficiently high pressure before reforming by adiabatic partial oxidation using air or enriched air and the synthesis gas is passed through a series of synthesis stages with separation of the synthesised methanol between the synthesis stages.
In the process of U.S. Pat. No. 5,177,114, there are typically two or three synthesis stages. After separation of the methanol from the last synthesis stage, the remaining gas is used to fuel a gas turbine driving the feedstock and air compressors. The carbon efficiency, i.e moles of methanol per gram atom of hydrocarbon carbon, of the process exemplified in this reference was said to be 60.5%, even though a high reforming pressure of 120 atmospheres was employed. Furthermore, since the methanol synthesis gas contains a relatively high proportion of inerts, mainly nitrogen resulting from the use of air or enriched air in the adiabatic partial oxidation step, a relatively large volume of methanol synthesis catalyst is required.
A “single-pressure” methanol synthesis process, utilizing a synthesis loop, is disclosed in U.S. Pat. No. 4,910,228 wherein the hydrocarbon feedstock is subjected to steam reforming in a heat-exchange reformer and the reformed gas is then subjected to partial oxidation with oxygen. The resultant hot partially oxidized reformed gas is then used to heat the heat exchange reformer. In this process, the heat exchange reformer is operated at such a pressure that the partially oxidized reformed gas was at a pressure equal to or above the inlet pressure of the loop circulator. The power requirements (to produce the compressed feedstock, oxygen, and circulation etc.) were supplied by combustion of part of the loop purge and by steam raised in the methanol synthesis stage.
We have now devised a “single-pressure” process using a heat exchange reformer that does not require a hydrogen recovery, air enrichment, or oxygen production unit. In the process of the present invention, the synthesis gas is produced by steam reforming in a pressurized heat exchange reformer wherein heat for reforming is supplied to the gas undergoing reforming from a) the products of combusting a fuel comprising the unreacted synthesis gas remaining after the series of synthesis stages, and preferably also b) from the reformed gas. The heat exchange reformer is preferably a modification of the type described in the aforesaid U.S. Pat. No. 4,910,228.
Accordingly the present invention provides a process for the production of methanol comprising converting a hydrocarbon feedstock at a pressure above the desired synthesis pressure into a synthesis gas mixture containing hydrogen, carbon oxides and steam at an elevated temperature and pressure, cooling said mixture to condense water from the mixture, separating the condensed water, and passing the resultant gas mixture, with no further compression and no recycle of unreacted gas, at an elevated temperature through a series of at least two methanol synthesis stages with separation of synthesized methanol from the gas mixture after each stage, and combusting the remaining unreacted gas with compressed air, wherein the hydrocarbon feedstock is converted into said synthesis gas mixture by passing a mixture of said hydrocarbon feedstock and steam through a steam reforming catalyst disposed in reformer tubes heated by the products of the combustion of said unreacted gas and, preferably, also by the reformed gas after it has left the reforming catalyst.
In one type of heat exchange reformer, the catalyst is disposed in tubes extending between a pair of tube sheets through a heat exchange zone. Reactants are fed to a zone above the upper tube sheet and pass through the tubes and into a zone beneath the lower tube sheet. The heating medium is passed through the zone between the two tube sheets. A heat exchange reformer of this type is described in GB 1 578 270.
In order to obtain a reformed gas with a reasonably low methane content, it is necessary that the reformed gas leaves the catalyst at a relatively high temperature, for example in the range 850-1100° C. For efficient operation, heat has to be recovered from this high temperature reformed gas. While, when using a heat exchange reformer of the type described in the aforementioned GB 1 578 270, heat can be recovered from the hot reformed gas by steam raising, reactants pre-heating etc., in the present invention it is preferred to use at least some of this high grade heat to supply part of the heat required for the endothermic reforming reaction. Thus by employing a different type of heat exchange reformer and effecting heat exchange between the reformed gas and the gas undergoing reforming as it passes through the catalyst, high grade heat in the reformed gas can be recovered directly as part of the heat required for the endothermic reforming process and the reformed gas can be partially cooled. Hence it is preferred to employ a heat exchange reformer wherein heat is supplied to the catalyst from the products of combustion of the unreacted gas remaining after methanol synthesis and from the reformed gas that has left the catalyst.
The preferred type of heat exchange reformer is a double-tube heat exchange reformer wherein the reformer tubes each comprise an outer tube having a closed end and an inner tube disposed concentrically within the outer tube and communicating with the annular space between the

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