GB2167409A - Heat recovery from a methanol synthesis process - Google Patents

Abstract

A method for the heat recovery from a reaction system for methanol synthesis is described, in which an outlet gas from a methanol synthesis reactor 102 is divided into two streams. One stream is subjected to heat exchange 103 with a starting synthetic gas 1,4 and the other stream is subjected to heat exchange 104 with part of a purge gas 5 which is separated in a separator 106 located downstream of the reactor 102. The two streams are combined and passed into the separator 106, and the heated purge gas 6 is passed into an expansion unit 108 to recover the heat of the purge gas. <IMAGE>

Description

SPECIFICATION Heat recovery from a methanol synthesis process This-invention relates to a method for the efficient heat recovery from a reaction system for methanol synthesis.

In recent years, synthesis of methanol has been dependent largely on low-pressure processes using copper-base catalysts. Accordingly, there is an increasing demand of energy saving in the methanol synthesis processes and thus, various methods of heat recovery have been proposed.

Starting materials used for methanol synthesis may be various types of hydrocarbons covering from natural gases to heavy oils or coal. Depending on the type of starting material, a variety of methods are used for preparation of a starting gas, including a reforming method using water vapor, a partial oxidation method using oxygen, and the like.

However, the starting gases for the methanol synthesis obtained by any methods inevitably contain gases (CH4, N2, air and the like) which are inert to the reaction for methanol synthesis.

These inert gases are accumulated in a circulation system of the methanol synthesis and serve to lower the reaction efficiency, so that it is necessary to withdraw a certain amount of such gases, as a purge gas, from the circulation system.

The purge gas consists chiefly of H2, CH4, and CO and is usually utilized as a fuel in an effective manner. However, if hydrocarbons, which are obtained through a number of steps of desulfurization, reforming and compression and are thus imparted with a high added value, are used as the starting material, it is not economically advantageous to simply employ the purge gas, as a fuel, after reduction in pressure.

In processes in which hydrocarbons having high molecular weights and high carbon contents, such as heavy oils, coal and the like, are used as the starting material, it has been proposed that hydrogen in the purge gas is separated and recovered by methods such as a low temperature gas separation method, an organic membrane separation method and a pressure swing method. The resulting residue is effectively re-utilized as part of the starting gas.

However, in processes using, as the starting material, light hydrocarbons such as natural gas, a starting synthetic gas prepared contains an excess of hydrogen over a stoichiometric ratio necessary for the methanol synthesis reaction. Accordingly, the separation and recovery of hydrogen from the purge gas is not economical and has not been reduced to practice.

The purge gas has a pressure substantially same as the pressure of the circulation system (50 kg/cm2 - 150 kg/cm2) necessary for the reaction of methanol synthesis. Attention has been paid to the above fact that there has been proposed a method for recovering power by subjecting the purge gas, prior to the use as a fuel, to pressure reduction in an expansion unit.

Because the purge gas is generally taken off from a circulation gas which has been obtained by cooling an outlet gas of a reactor for methanol synthesis by which produced methanol is condensed, and separating the unreacted gas from the gas, the purge gas temperature is close to ndrmal temperatures. Accordingly, if the purge gas is passed into the expansion unit as it is, the recovered power is very small. To overcome the above drawback, there have been proposed several methods of increasing the power to be recovered in which the purge gas is preheated prior to introduction into the expansion unit.

For instance, Japanese Laid-open Patent Application No. 56-40624 describes a method in which a purge gas is preheated by means of a discharge gas from a compressor for a synthesized gas. In recent years, however, the compressor itself is designed to attain a high efficiency, the synthetic reaction tends to proceed at low pressure, and a reforming system which is a process of preparing starting gases for synthesis, so that the temperature discharged from the compressor is generally 180"C or below. Accordingly, the temperature of the purge gas preheated by heat exchange with the discharge gas is also 180"C or below.

Another method is disclosed in Japanese Patent Publication No. 59-3971. In the method, heat of a gas from a reformer is transferred to a purge gas for methanol synthesis in a heat exchanger mounted in an outlet gas line of the reformer. The heat is further transferred to a purge gas for synthesis in a heat exchanger mounted in a flue gas line of the reformer.

Subsequently, the purge gas is passed into and expanded in a turbine for recovery of heat energy.

According to the above method, it may be possible to raise the preheating temperature.

However, when the methanol-synthetic process is viewed as a whole, it is general that the reforming system and the synthetic circulation system are physically separated through a process of starting synthetic gas compression. In addition, the operation loads of the respective systems are not always linearly proportional to each other, so that the operations of the systems and the design and installation of the systems become complicated. In fact, the recovery of waste heat of the reforming system is satisfactorily feasible by other method (e.g. using a waste heat boiler or a preheater for feed to a boiler), and no heat of high energy sufficient to use preheating of the purge gas is involved in the system.

It is an object of the invention to provide a method for the heat recovery from a reaction system for methanol synthesis which overcomes the drawbacks of the prior art.

It is another object of the invention to provide a method for the heat recovery from the reaction system for methanol synthesis in which heat is efficiently recovered from a purge gas discharged from the reaction system.

According to the present invention, there is provided a method for recovering heat from a reaction system for methanol synthesis, which comprises dividing an outlet gas from a methanol synthetic reactor into two groups, subjecting one group to heat exchange with a starting synthetic gas and the other group to heat exchange with part of a purge gas which is separated in a separator located downstream of the reactor, combining the two groups together for passage into the separator, and passing the heated purge gas into an expansion unit to recover the heat of the purge gas.

The invention will be further described by way of example with reference to the accompanying single Fig. of drawings, which is a flow chart illustrating an embodiment of the invention.

A synthetic circulation system, which has a synthetic reactor using a catalyst chiefly composed of copper, is operated under a pressure of 50 kg/cm2 to 150 kg/cm2. The outlet gas from the reactor is subjected to heat recovery and cooled, whereby methanol in the outlet gas is condensed and separated. The resultant non-condensed gas is combined with a starting synthetic gas and recycled to the reactor. Part of the circulation gas is withdrawn to outside, as a purge gas, in order to prevent accumulation of inert gases such as CH4, N2, Ar and the like, in the circulation system. The other circulation gas is heat-exchanged with the outlet gas in a synthetic gas preheater located at the outlet of the synthetic reactor and heated to a temperature of 180 to 300"C.

In the methanol synthesis reaction system described above, at least one preheater is arranged parallel to the synthetic gas preheater. The at least one preheater is used such that the purge gas is heat-exchanged with part of the outlet gas from the synthetic reactor and heated to a temperature of from 150 to 290"C. The heated purge gas is introduced into an expansion unit in which the energy power is recovered. The purge gas whose pressure is reduced in the expansion unit is re-utilized as a fuel.

In the practice of the invention, the purge gas, which has been preheated by heat exchange with the outlet gas from the synthetic, reactor, reaches a temperature not lower than 180"C and 300"C at most. The power recovered in the expansion unit becomes much greater than the power attained by a method which is described in the afore-indicated Japanese Laid-open Patent Application No. 56-40624.

According to the invention, the preheating of the purge gas is carried out in the same synthetic system, and thus the systems can be operated and installed in the same piace without difficulty. Thus, the method of the invention is free of the drawbacks involved in the method of the Japanese Patent Publication No. 59-3971.

Since the outlet gas from the synthetic reactor, which is a heating fluid in the preheater for the purge gas, has substantially the same pressure as the purge gas to be heated in the method of the invention, the designed pressure of tubes and heat transmission tubes which are used to make up of the purge gas preheater is conveniently determined because of the very small difference in pressure between the heating fluid and the fluid to be heated. This is very advantageous from the manufacture and maintenance of the systems.

Reference is now made to the accompanying drawing, in which a flowchart of one embodiment of the invention is shown.

A starting synthetic gas 1 is compressed in a synthetic gas compressor 101 to a pressure of rom 50 to 150 kg/cm2 necessary for synthetic reaction. The pressure of a non-condensed gas 4 from a separator 106 on a synthetic reactor outlet gas line is elevated in a circulator 107. The pressure-elevated gas 4 and the compressed synthetic gas 1 are combined together and preheated in a synthetic gas preheater 103, followed by feeding to a synthetic reactor 102.

An outlet gas 2 from the synthetic reactor 102 is divided into two groups. One group is fed to the synthetic gas preheater 103 and the other group is fed to a purge gas preheater 104 arranged parallel to the preheater 103. The gases discharged from the preheaters 103, 104 are combined again, and are cooled and condensed in a condenser 105 and passed into the separator 106.

A liquid 3 from the separator 106 is fed to outside, as condensed methanol, and part of the non-condensed gas 4 from the separator 106 is withdrawn as a purge gas 5 while the residue is fed to the circulator 107.

The purge gas 5 is passed into the purge gas preheater 104, in which it is preheated by means of part of the outlet gas from the reactor 102. The preheated purge gas 6 is reduced in pressure in an expansion unit 108 while recovering the heat energy thereof. The reduced purge gas 7 is used as a fuel.

The difference between the method of the invention and a prior art method is described by way of example.

Example (1) Purge Gas Composition: H2 58.74 vol% CO 5.59 CO2 8.96 vol% CH4 19.47 vol% N2 7.22 viol% CH3OH 0.02 vol% Total 100.00 vol% Flow Rate: 110 kmole/hr Pressure: 75.0 kg/cm2 (2) Expansion Unit Exhaustion pressure: 3.0 kg/cm2 Adiabatic efficiency: 65.0% The apparatus of the sole figure was used and the purge gas of the above composition was preheated at 240"C according to the method of the invention. The recovered power was found to be 1900 KW.

In constrast, when the purge gas was preheated at 160"C by the use of a discharge gas from the synthetic gas compressor as described in the Japanese Laid-open Patent Application No. 5640624, the recovered power was 1500 KW.

Claims (3)

1. A method for recovering heat from a reaction system for methanol synthesis, which comprises subjecting part of an outlet gas from a methanol synthesis reactor to heat exchange with a starting synthetic gas and another part of the outlet gas to heat exchange with part of a purge gas which is separated in a separator located downstream of the reactor, combining the two parts of the outlet gas for passage into the separator, and passing the heated purge gas into an expansion unit to recover heat from the purge gas.

2. A method according to claim 1, wherein the purge gas is separated from the noncondensed gas which has been separated in said separator, and the non-condensed gas is elevated in pressure and combined with the starting synthetic gas.

3. A method of recovering heat from a methanol synthesis process, substantially as hereinbefore described with reference to the accompanying drawing.