Process for the production of methanol

Details Process for the production of methanol Process for the production of methanol

1998-01-14

2001-07-10

Richter, Johann (Department: 1621)

Chemistry: fischer-tropsch processes; or purification or recover

Plural zones each having a fischer-tropsch reaction

C518S702000, C518S703000, C518S704000, C518S705000, C518S713000, C518S700000, C518S726000, C518S706000

Reexamination Certificate

active

06258860

ABSTRACT:

This invention relates to the production of methanol.
Methanol is synthesised in large volumes annually by conversion of a carbonaceous feedstock, more usually a hydrocarbon-containing feedstock, such as natural gas, naphtha, or other oil fraction, into a mixture of carbon oxides and hydrogen. Such a mixture of gases is often referred to as synthesis gas.
The conversion of a hydrocarbon-containing feedstock into synthesis gas can be effected by any appropriate technique, for example by steam reforming, by partial oxidation, by secondary/autothermal reforming, or by a combination of two or more of these processes. Conversion of the carbon oxides and hydrogen to methanol occurs according to the following reactions:
CO+2H
2
CH
3
OH
CO
2
+3H
2
CH
3
OH+H
2
O
These reactions can be carried out by contacting the synthesis gas with a suitable methanol synthesis catalyst under an elevated synthesis gas pressure, typically in the range of from about 30 bar up to about 100 bar, more usually in the range of from about 50 bar up to about 100 bar, and at an elevated methanol synthesis temperature, typically from about 210° C. to about 270° C. or higher, e.g. up to about 300° C. As an example of a suitable methanol synthesis catalyst there can be mentioned a catalyst comprising a reduced zinc oxide/copper oxide mixture.
In order to obtain maximum usage of carbon oxides and hydrogen it is desirable to ensure that as near a stoichiometric amount of hydrogen is present in the synthesis gas to match the respective contents of the carbon oxides. To this end it may be desirable to add CO
2
to the hydrocarbon-containing feedstock or to the synthesis gas, particularly when natural gas is used as feedstock for the production of the synthesis gas, so as to achieve the desired stoichiometry.
A conventional methanol synthesis plant can be considered to comprise three distinct parts, namely:
1. a synthesis gas production section, which produces a mixture of carbon oxides and hydrogen from a hydrocarbon feedstock by a suitable process, such as partial oxidation, secondary/autothermal reforming, or steam reforming, or a combination thereof;
2. a methanol synthesis section, in which crude methanol is produced from the carbon oxides and hydrogen; and
3. a distillation section, in which the final refined methanol product is produced from the crude methanol.
Whilst it is desirable to maximise the efficiency of conversion of hydrocarbon feedstock to methanol, it is generally recognised that the cost of providing process plant and equipment to achieve the highest possible efficiencies within the overall process have hitherto been considered to be economically prohibitive. Hence existing methanol synthesis plant design has generally been a compromise between efficiency and plant investment costs.
In a conventional methanol synthesis plant there are several streams that are considered to be byproduct or waste gas streams from the process. Although these streams contain significant quantities of carbon oxides and hydrogen, and/or methanol, they are conventionally discharged to the plant fuel gas system. Amongst these streams are methanol synthesis loop purge gas, compressor seal gases, flash gases from crude methanol processing, and distillation overhead gases originating from gases dissolved in crude methanol. In addition there may be liquid byproduct streams which also contain significant quantities of methanol. These liquid streams include a heavy byproduct/water stream containing some methanol, often described as fusel oil, and a light byproduct stream, often described as secondary methanol.
These losses of methanol and methanol precursors in the waste streams mean that the synthesis gas production and methanol synthesis sections of the plant must be designed for a higher methanol capacity than necessary for a given refined methanol production rate.
It has hitherto generally been considered uneconomic to recover the potentially valuable constituents from all of these various waste gas and liquid streams, although several schemes have been proposed or implemented to recover such constituents from an individual stream. For example, scrubbing of flash gases or synthesis loop gases with water has been proposed. In addition it has been proposed to obtain additional production of methanol directly from synthesis loop purge gases by passing the purge gases through a purge gas converter containing a second charge of a methanol synthesis catalyst. Another proposal utilises recovery of hydrogen from synthesis loop purge gases with recycle of the recovered hydrogen back to the synthesis loop with additional imported carbon dioxide.
In DE-A-3220995 there is proposed a process for producing methanol from a synthesis gas containing hydrogen and carbon oxides in which the proportion of hydrogen is greater than the stoichiometric proportion required for methanol synthesis and in which unreacted synthesis is partially recirculated in a synthesis loop to the synthesis stage and partly removed as waste gas so that at least a part of the waste gas is returned to the process. In this process the waste gas is separated at least into a CO-rich stream and a residual gas stream and the CO-rich stream is returned to the synthesis gas.
DE-A-3244302 proposes a process for production of methanol in which fresh synthesis gas and recycled synthesis gas recirculated in a loop are taken to a reactor and partially converted to methanol as they flow through a layer of catalyst under methanol synthesis conditions and in which crude methanol is condensed out of the stream leaving the reactor and unreacted synthesis gas is returned in a loop to the reactor inlet. This process is characterised in that an additional methanol synthesis reactor is provided, to which synthesis gas is taken, and which is operated without recirculation, and that synthesis gas which has not reacted in this reactor is taken as fresh synthesis gas to the reactor operated with recirculation of synthesis gas.
Further background to the invention is provided by the following papers:
(a) “The Commercial Proving of the Tube Cooled Converter and its Use as a Purge Gas Reactor” by Simon Early of John Brown—Davy Process Technology, London, United Kingdom presented at 1994 World Methanol Conference, Geneva, Switzerland, Nov. 29-Dec. 1, 1994; and
(b) “Methanol Reactor Design Choices” by P. E. J. Abbott, of ICI Katalco, Billingham, United Kingdom at 1992 World Methanol Conference, Monte-Carlo, Monaco, Dec. 8-10, 1992.
It would be desirable to provide a process for the production of methanol enabling the recovery of significant proportions of the potentially valuable materials normally lost in the gaseous or liquid waste streams from a methanol synthesis plant and their utilisation for the production of further methanol without adding significantly to the cost of producing refined methanol therein.
It would also be desirable to provide a process which would enable the production of methanol from an existing methanol synthesis plant to be significantly increased by addition of a relatively modest amount of additional equipment.
It is accordingly an object of the present invention to provide a process for the production of methanol which enables significant additional methanol production to be achieved economically from an existing methanol synthesis plant, simultaneously with an increased feedstock to methanol conversion efficiency or yield, without requiring modification to existing process equipment items nor increasing the process duties of these items, by the utilisation of byproduct or waste streams from the existing plant.
It is a further object of the present invention to provide an improved methanol synthesis process enabling methanol production to be economically achieved together with an enhanced feedstock utilisation efficiency.
According to the present invention there is provided a process for the production of methanol which comprises:
(A) converting a carbonaceous feedstock in a synthesis gas production plant into a synthesis gas comprising hydrogen and at l

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