Reactor layout for methanol production from low quality synthesis gas

Abstract

A reactor layout for a process of methanol production from low quality synthesis gas, in which relatively smaller adiabatic reactors can be operated more efficiently, some of the inherent disadvantages of adiabatic reactors for methanol production are avoided. This is done by controlling the outlet temperature in the pre-converter by rapid adjustment of the recycle gas, i.e. by manipulating the gas hourly space velocity in the pre-converter.

Claims

1. A reactor layout for carrying out a process for methanol production from synthesis gas, said reactor layout comprising: one or more methanol pre-converters/guard reactors, in which the synthesis gas is partially converted to methanol over a heterogeneous catalyst to generate an effluent gas without impurities, a feed-effluent heat exchanger for pre-heating the synthesis gas and for cooling the effluent gas from the one or more methanol pre-converters/guard reactors, and a methanol synthesis loop comprising: a main methanol converter for converting a mixture of recycle gas and the cooled effluent gas from the one or more methanol pre-converters/guard reactors, a main feed-effluent heat exchanger, a cooler or a series of coolers, a high pressure gas-liquid separator, which splits the inlet flow into raw methanol and recycle gas, a recycle compressor, a loop purge, wherein the loop purge is drawn from the recycle gas before the recycle compressor, and means for defining and controlling a maximum allowable outlet temperature using a cold recycle gas stream which is injected into the one or more methanol pre-converters/guard reactors via a control valve.

2. The reactor layout according to claim 1, which has fresh feed gas splitting located on the cold side, before the feed-effluent heat exchanger.

3. The reactor layout according to claim 1, which enables on-stream catalyst replacement in the one or more pre-converters/guard reactors.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) The invention is illustrated further with reference to the drawings, in which:

(2) FIG. 1 is a block diagram showing the methanol synthesis process according to the invention, and

(3) FIG. 2 is a drawing illustrating an example of a process layout.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

(4) In the process of the invention for methanol production from synthesis gas, one or more pre-converter(s) for methanol synthesis is/are followed by a conventional methanol synthesis loop. In the present context, a pre-converter is a methanol converter that only receives feed synthesis gas in full load operation. The main methanol converter in the conventional methanol synthesis loop converts a mixture of recycle gas and the effluent gas from the pre-converter. The pre-converter may be fed with a fraction of the recycle gas from the synthesis loop in part load condition, whereas a safe gas space velocity across the reactor is required to avoid overheating of the catalyst in the reactor; see FIG. 1.

(5) In the conventional methanol synthesis loop, the methanol converters are often boiling water reactors (BWR), i.e. tubular reactors with catalyst loaded into several tubes surrounded by water on the shell side. The boiling water efficiently removes the heat liberated by the methanol synthesis reaction and thus ensures an almost isothermal reaction path at conditions close to the maximum rate of reaction. This not only ensures a high conversion per pass and thus a high catalyst utilisation as well as a low recirculation, but also a low by-product formation.

(6) Adiabatic converters are widely used for methanol synthesis, and a synthesis loop with adiabatic reactors as methanol converters normally comprises a number (e.g. 2-4) of fixed bed reactors arranged in series with cooling between the reactors.

(7) Further, the methanol converters can be quench reactors. A quench reactor consists of a number of adiabatic catalyst beds installed in series within one pressure shell. In practice, up to around five catalyst beds can be used. The reactor feed is split into several fractions and then distributed to the synthesis reactor between the individual catalyst beds.

(8) With reference to FIG. 2, the following is an example of a process layout according to the invention:

(9) Pressurized synthesis gas A is pre-heated in a feed-effluent heat exchanger 2 and passed through one of the methanol pre-converters/guard reactors 1, in which synthesis gas is partially converted to methanol over a heterogeneous methanol synthesis catalyst. Traces of impurities are also removed in the reactor. The extent of conversion is dictated by the pre-converter outlet temperature. In part load condition, in which the gas space velocity drops from the design point (designed for full load condition), the outlet temperature will rise. A maximum allowable outlet temperature is defined and controlled 10 using a cold recycle gas stream which is injected to the pre-converter inlet via the control valve 11. An alternative control approach would be to fix the gas space velocity in the pre-converter in part load, i.e. to compensate the synthesis gas flow drop by the recycle gas. The latter approach may be practiced if the poisonous content of the make-up gas is negligible, meaning that the deactivation due to poisoning is not a dominant mechanism. Nevertheless, relying on the adiabatic reactor outlet temperature is a reliable strategy to protect the guard bed and control the product quality (low by-product formation).

(10) Two or more pre-converters/guard reactors are installed in parallel whereas only one pre-converter is on stream. The other pre-converter(s)/guard reactor(s) is/are isolated by the inlet valve 9. The effluent from the pre-converter/guard reactor is cooled to a temperature, which is suitable for the main converter, in the feed-effluent heat exchanger 2, and subsequently it enters a conventional methanol synthesis loop. In this example, the conventional methanol synthesis loop is presented by a main methanol converter 3, a main feed-effluent heat exchanger 4, a cooler or a series of coolers 5, a high pressure gas-liquid separator 6, which splits the inlet flow into raw methanol B and recycle gas, and a recycle compressor 7. The loop purge C is drawn from the recycle gas before the recycle compressor.

(11) The process layout described above is only one example of a useful layout, and a number of variations are possible, such as: any other type of main methanol converter, any type of pre-converter/guard reactor, MUG (make-up gas) splitting on the cold side, i.e. before the feed-effluent heat exchanger 4, the number of pre-converters/guard reactors, on-stream catalyst replacement in pre-converters/guard reactors, use of low-activity catalysts in the pre-converters/guard reactors and a high-activity catalyst in the main converter, and catalyst replacement in the pre-converters/guard reactors during operation.