JPS585889B2 - Methanol synthesis method - Google Patents

Description

[Detailed description of the invention] This invention relates to a methanol synthesis method.

This invention relates to a synthesis method that converts almost all of the carbon dioxide supplied to a methanol synthesis circulation system into methanol without escaping it outside the system.

When producing methanol using natural gas as a raw material, usually about % mole of carbon dioxide is added to 1 mole of methane in the natural gas, and steam reforming is performed as shown in the following formula (1). The reaction produces synthesis gas for methanol.

Using this synthesis gas, methanol is catalytically synthesized as shown in (2).

As shown on the left side of the above equation (2), it is appropriate that the ratio R of the methanol gas is about 2.

This method of appropriately adjusting methanol synthesis gas stoichiometrically is called a stoichiometric process.

The carbon dioxide added in the stoichiometric process is recovered and utilized by scrubbing the flue gas in steam reformers and other facilities with alkaline solvents such as monoethanolamine and others.

On the other hand, advances in catalysts for methanol synthesis have made it possible to lower the methanol synthesis pressure, and so-called medium-low pressure methanol synthesis is becoming popular.

In this medium-low pressure method of methanol synthesis, the energy loss associated with the depressurized release of a small amount of the process stream in the pressurized synthesis system is not as large as in the high pressure method, so carbon dioxide is not supplemented to the raw natural gas. , a process is implemented in which synthesis gas is produced by adding a steam reforming reaction treatment and excess hydrogen is released from the synthesis system.

This mode is called a non-stoichiometric process.

In the above-mentioned stoichiometric method, equipment consisting of an absorption tower, a regeneration tower, a compressor, and various other equipment is required for carbon dioxide recovery, and the construction cost is naturally high.

Furthermore, in order to recover carbon dioxide using this device, large amounts of water vapor, cooling water, and electricity are required, which increases operating costs, and in addition, there is corrosion of the device due to the absorption liquid.
It is difficult to expect stable operation over a long period of time.

On the other hand, in non-stoichiometric methods, although the synthesis pressure is low, the release of excess hydrogen is a clear loss of energy, and excess hydrogen may reside in the precious synthesis system space. This is one of the causes of reduction in equipment capacity.

It is an object of the present invention to provide a new method in which the disadvantages of these conventional methods are alleviated and other advantages are added.

The method of the present invention involves bringing the exhaust gas stream of a methanol synthesis circulation system into contact with crude methanol obtained from the methanol synthesis circulation system, dissolving carbon dioxide in the exhaust gas in the crude methanol, and then expelling carbon dioxide from the crude methanol. It is characterized by supplying the methanol to the upstream of the methanol synthesis circulation system.

In the method of this invention, useful carbon dioxide is recovered and utilized from the exhaust gas stream from the methanol synthesis system, which was conventionally only used as fuel. Utilizes the absorption of carbon dioxide by crude methanol and the dissipation treatment of the absorbed carbon dioxide.

In the method of the invention, the absorption treatment of carbon dioxide with crude methanol is carried out in the temperature range from -50 to 10°C and the pressure range from 40 to 200 kg/cm'G, and the exhaust gas stream undergoing the cleaning absorption treatment and the cleaning The quantitative relationship (by weight) of the crude methanol streams is in the range of 1:3 to 1:5.

In the method of this invention, the crude methanol diffusion treatment that has absorbed carbon dioxide is carried out at a pressure range of 0 to 40 kg/cm'G and a temperature range of 0 to 50°C.

In the method of this invention, in order to bring crude methanol into contact with natural gas or reformed gas for carbon dioxide diffusion treatment, the flow rate ratio (by weight) of crude methanol to gas is
is within the range of 1:1 to 30:1.

Regarding the temperature and pressure in carbon dioxide absorption treatment using crude methanol, if the carbon dioxide concentration in the exhaust gas stream is high, the temperature may be 10°C or lower, but in order to increase the amount of recovered carbon dioxide, The temperature is preferably in the range of -50°C.

In principle, the absorption pressure is approximately equal to the pressure within the methanol synthesis circulation system, and the absorption pressure is approximately the same as the pressure within the methanol synthesis circulation system, and the
It is selected within the range of g/cm'G.

The quantitative relationship between the exhaust gas flow and the crude methanol flow in carbon dioxide absorption treatment is such that by adjusting the amount of crude methanol retained in the absorption tower 49, the crude methanol flow is brought into contact with an amount of 3 to 5 times the exhaust gas flow rate. It will be done.

Next, it is desirable that the diffusion pressure of the carbon dioxide absorbed in the crude methanol stream be at a low pressure, but it is related to the supply pressure of the raw natural gas, and when atmospheric pressure natural gas is used, the diffusion process is performed at a low pressure. When carried out and high pressure natural gas is supplied, the rate is 20 to 40 kg/cm' in relation to the steam reformer pressure.
The pressure is preferably within the range of 15 to 20 kg/cm'G in the dispersion treatment using steam reformed gas.

The temperature of the dispersion treatment is set at a lower limit of 0° C. because the dispersion air stream is washed with water to collect methanol accompanying the raw material natural gas or steam-reformed gas, and an upper limit of 50° C. in consideration of economic efficiency.

The flow rate ratio in the dispersion treatment may be selected appropriately from within the above range in consideration of economic efficiency, although the entire amount of raw material natural gas or steam reformed gas may be used, or a portion thereof may be used. .

Next, the method of the present invention will be explained with reference to FIG.

Raw material natural gas is introduced into the lower part of the stripping tower 2 through a pipe 1,
It ascends through the stripping tower and comes into contact with the descending crude methanol that has absorbed carbon dioxide, and the carbon dioxide in the crude methanol is expelled.

The natural gas that has reached the upper part of the stripping tower has accompanying methanol removed by contact with washing water supplied to the upper part of the stripping tower through a pipe 3, and is then sucked into a compressor 5 through a pipe 4 and compressed.

The compressed natural gas flows into the heater γ through a pipe 6, where it is heated for the purpose of effective desulfurization, and flows through a pipe 8 into a desulfurization tower 9, where sulfur compounds in the natural gas are removed. be done.

A required amount of steam is added to the desulfurized natural gas through a pipe 11, and the desulfurized natural gas flows through a pipe 10 into a steam reforming furnace 12.

In the steam reforming furnace, the carbon dioxide added in the stripping tower 2, the steam from the pipe 11, and the natural gas are expressed by the following equation (3).
It reacts endothermically and is converted to synthesis gas for methanol, as shown in Figure 2.

Since the reformed gas flowing out from the steam reforming furnace through the pipe 13 is at a high temperature, it is introduced into the waste heat recovery device 14, where it is used for steam generation and methanol distillation, and then passes through the pipe 15 to the final stage. It then reaches the cooler 16, where it is cooled down to room temperature.

The cooled reformed gas is supplemented with carbon dioxide through the pipe 18, flows through the pipe 1T to the separator 19, and after the condensate is separated, it is sucked into the compressor 20, whereby the pressure is increased to the pressure required for methanol synthesis. Ru.

The pressurized reformed gas is passed through pipe 2 as methanol synthesis gas.
1 into the circulator 22 together with the methanol synthesis recycle gas stream which is joined via pipe 23 and is further compressed.

The synthesis gas from the circulator 22 flows into the preheater 25 through a pipe 24, is heated, flows into the synthesis tower 27 through a pipe 26, and a part of the synthesis gas is converted into methanol as shown in the following equation (4). be done.

The effluent from the synthesis tower 2T is separated and supplied to a preheater 25 and a heat recovery device 30 through a pipe 28, and waste heat is recovered.
It passes through a pipe 31 to a condenser 32 where it is cooled to ambient temperature and the methanol in the effluent is condensed.

The effluent containing condensed methanol enters a separator 34 via a line 33 where it is separated into gas and liquid and the gas stream is fed as a circulation stream via a line 35 to the circulator 22, although a portion of the gas stream is By means of pipe 36, it is not allowed to enter the synthesis system as an exhaust gas stream, but after pressure adjustment it passes to preheater 3T, where it is heated and then flows through pipe 39 into converter 40 together with the water vapor added by pipe 38. , where the following formula (
As shown in 5), carbon monoxide in the exhaust gas stream is converted to carbon dioxide and hydrogen.

This carbon monoxide conversion step can be omitted if desired.

The gas flow from the converter 40 passes through a pipe 41 to the preheater 3γ, where it is cooled, and then passes through a pipe 42 to the cooler 4.
3 and cooled to room temperature.

The gas stream then passes through a pipe 44 to a separator 45, where the condensate is separated, and then passes through a pipe 46 to a cooler 47, where it is precooled by heat exchange with the gas stream from the absorption tower 49.
will be introduced at the bottom of the page.

On the other hand, the liquid stream separated in the separator 34, that is, crude methanol, passes through the precooler 51 and the subcooler 53 via pipes 50, 52, and 54, and is supplied to the upper part of the absorption column 49 and descends in the column.

In the absorption column 49, the carbon dioxide from the exhaust gas stream which is highly soluble in methanol dissolves well in the crude methanol.

FIG. 2 shows the solubility of each component gas in the carbon dioxide and other exhaust gas streams in methanol.

Methane, hydrogen, carbon monoxide, and nitrogen also dissolve in methanol, but in small amounts.

In order to remove the heat of absorption in the absorption tower 49, a methanol subcooler and a circulation pump are provided as necessary.

In order to cool the supercooler and supercooler 53, the waste heat of the flue gas of the steam reformer 12, the reformed gas waste heat recovery device 14
Alternatively, it is possible and effective to use an absorption refrigeration system that uses the heat recovery device 30 as a heat source.

The exhaust gas stream from the top of absorption tower 49 passes through pipe 48 and its cooled state is recovered in cooler 4T before being used as fuel or other suitable uses.

Crude methanol that has absorbed carbon dioxide passes through pipes 55, 56, and 57, passes through a precooler 51, and a heater 58 installed as necessary, and is supplied to the upper part of the stripping tower 2 after being depressurized.
It comes into contact with the natural gas flowing down and rising in the tower, and carbon dioxide is expelled.

The crude methanol at the bottom of the stripping column 2 is supplied to a distillation device (not shown) through a pipe 59 and distilled to produce methanol product.

In the stripping tower 2, the crude methanol that has been heated under reduced pressure is simply transferred to the stripping tower 2 without performing ejection treatment of carbon dioxide using natural gas.
It is possible and effective to supply the reformed gas to the upper part and let it descend through the tower to release carbon dioxide, or to supply the reformed gas leaving the cooler 16 to the lower part of the stripping tower 2 and perform ejection treatment with the reformed gas. It is.

In these carbon dioxide separations, the carbon dioxide is recycled to the methanol synthesis system through pipe 18 rather than through pipe 4.

As an example of the method of this invention, Table 1 shows the pressure, temperature, composition and flow rate at key points in the process.

Next, the effects produced by the method of the present invention are listed below.

(1) It becomes possible to reduce the unit consumption of natural gas.

(2) It becomes possible to reduce the capacity of the synthesis gas production equipment.

(3) By effectively utilizing the pressure within the process, no separate power is required to recover carbon dioxide.

(4) The utility is small because in-process waste heat can be used as a carbon dioxide recovery heat source.

(5) Since the carbon dioxide absorption treatment is carried out at a low temperature, corrosion of the device is alleviated, and inexpensive carbon steel can be used as the device material.

(6) It is possible to reduce the capacity of the device for recovering carbon dioxide, which should be separately supplied from flue gas and other sources.

This separate make-up carbon dioxide recovery device may be omitted if carbon dioxide is recovered only from the exhaust gas stream for use in methanol synthesis.

(7) Methane in the exhaust gas stream is also recovered in the carbon dioxide absorption process.

The effect of (6) above can be expressed numerically as follows.

Methane content: 95 vol. If the stoichiometric process is adopted as a methanol production facility of 1000 T/day when the raw material is ordinary natural gas of about 1 5 2
It is necessary to separately supply 0 00 Ntn'/day of carbon dioxide to the process, but if this invention is used, 51 000 Nrn'/day of carbon dioxide can be recovered from the exhaust gas stream of the methanol synthesis system, and carbon dioxide can be separately supplied to the process. The amount of carbon dioxide that should be extracted and supplied from road gas, etc. is 101000N
m'/day.

Note that even if a raw material gas other than natural gas is used, this invention can be used as long as the carbon to hydrogen ratio of the raw material gas is equivalent to natural gas, but in this case, this invention should not be applied. Of course.

[Brief explanation of drawings]

FIG. 1 is a flow sheet of an example of a process for implementing the present invention, and FIG. 2 is a graph showing the solubility of various gases in methanol.

Claims (1)

[Claims] 1. Water vapor is added to the exhaust gas stream of the methanol synthesis circulation system under pressure in the range of 40 to 200 kg/cm'G to convert carbon monoxide in the exhaust gas stream to carbon dioxide and cool it, followed by methanol synthesis. 4. Crude methanol produced and separated in the circulation system and under methanol synthesis pressure. Carbon dioxide in the exhaust gas stream is dissolved in crude methanol by contacting it under a pressure/temperature condition in the range of 0 to 200 kg/cm'G/-50 to 10°C, and then the crude methanol in which carbon dioxide has been dissolved is Carbon dioxide in the crude methanol is ejected by contacting the raw material natural gas or steam reformed gas with the whole or a portion of the raw material natural gas or steam reformed gas under pressure and temperature conditions within the range of 0 to 40 kg/cm'G and 0 to 50°C. A methanol synthesis method characterized by transferring it into gas or steam reformed gas.