Patent Application
20080209942
The present invention relates to a process for recovering low carbon olefins from a product gas obtained from an olefin conversion device. More specifically, the present invention concerns a process for recovering ethylene and propylene from the product gas from the methanol to olefin device and/or dimethyl ether to olefin device.
It is learned that the demand for low carbon olefins is increasing, but the starting material resources, e.g. naphtha, light diesel oil and so on, for the production of ethylene is confronted with serious shortage. The methanol to olefin (hereinafter referred to as MTO) or dimethyl ether to olefin (hereinafter referred to as DTO) technique, therefore, has drawn much attention. MTO or DTO technique is a technology for producing low carbon olefins, such as ethylene, propylene and the like, by using coal-based or natural gas-based synthesized methanol or dimethyl ether as the starting materials and by means of a fluidized bed reaction form similar to a catalytic cracking device. Said technology can be used for the production of low carbon olefins with a high selectivity, and the propylene/ethylene ratio may be optionally adjusted within a relatively broad scope.
Generally, the product gas obtained by the catalytic reaction of methanol via a MTO reaction device contains hydrogen, methane, ethylene, ethane, propylene, propane, and high carbon olefins, such as butylene or pentene, as well as water, carbon monoxide and carbon dioxide. The reaction products of the cracking of DTO are similar to those of MTO, but the conditions for DTO production are readily controllable. In order to obtain ethylene products in a polymerizable grade, the separation and recovering techniques of MTO product gases are very essential. Generally, the technique for recovering ethylene comprises, before recovering olefins, subjecting the MTO product gas to the conventional gas-liquid separation step to separate and remove water, carbon dioxide and heavy components of C5 and higher; cooling and introducing C4 & light components into the deethanizing column; discharging ethane and light components from the top of the deethanizing column; and then feeding them into the demethanizing column after removing ethyne via a hydrogenation device; removing methane and hydrogen from the top of the demethanizing column, and introducing the materials at the column bottom substantially containing ethylene and ethane into the C2 component separation column; and obtaining ethylene products at the top of the C2 component separation column. U.S. Pat. No. 5,811,621 has detailedly described the process. In said technology for recovering ethylene, many compression and pressure-increasing processes are required. Generally, the pressure of C4 & light components is required to be increased to 20-30 atm.A before feeding C4& light components into the deethanizing column. In addition, another pressure-increasing step is further needed to increase the pressure to higher than 30 atm.A after removing ethyne via the hydrogenation reaction device and before feeding C4& light components into the demethanizing column. In order to increase the recovery rate of ethylene, the materials at the top of the demethanizing column are further compressed and pressure-increased, and the compressed gas provides heat for the reboiler of the demethanizing column, and then are partially condensed. The uncondensed gas contains ethylene in an amount of 3-4% of the feedstock, and is discharged. It can be seen that three processing compressors and a propylene refrigeration compressor are required in said ethylene recovering technology, which increases the equipment investment. Meanwhile, said uncondensed gas is discharged in said technology so as to lose a small amount of ethylene. Accordingly, the ethylene recovery rate is not satisfactory, and generally about 95-97%. Accordingly, it highly needs a process for recovering low carbon olefins from a product gas from the methanol to olefin device and/or dimethyl ether to olefin device. Said process not only lowers energy consumption, but also may be used for recovering low carbon olefins in a high yield, so as to obtain ethylene and propylene products in the polymerizable grade.
The present invention provides a process for recovering low carbon olefins from MTO and/or DTO product gases, comprising the operation units of product gas pressure increasing, separation of C2 & light components and C3 & heavy components, demethanization of C2 & light components, rectification of ethylene and rectification of propylene, wherein said process is characterized in setting up an ethylene recovering device in the demethanizing unit of C2 & light components so as to recover ethylene in the uncondensed gas, and using the liquid phase C2 components in the separation unit of C2 & light components and C3 & heavy components and/or the ethylene rectification unit as a coolant in said ethylene recovering device.
Since DTO flow scheme is substantially the same as MTO flow scheme, only MTO reaction device is detailedly illustrated as follows.
Generally, 80% or more (based on carbon element) methanol is industrially converted to ethylene and propylene after the catalytic reaction of methanol in MTO reaction device. Meanwhile, a small amount of methane, carbon dioxide, oxide and hydrocarbons of C4 or higher are also produced, in addition to an extremely small amount of hydrogen and ethyne. In most cases, most oxides are removed from the gas from the MTO reactor after rapid cooling and washing, and the pressure thereof is less than 0.2 MpaG. Such materials are the MTO product gas as called in the present invention. The process for recovering low carbon olefins of the present invention can be used to recover low carbon olefins, i.e. mainly ethylene and propylene, from such MTO product gas, and both ethylene and propylene obtained thereby can reach the polymerization grade standard.
Preferably, the process for recovering low carbon olefins of the present invention comprises the steps of:
Specifically, the present invention provides a process for recovering low carbon olefins from MTO product gases, mainly comprising a pressure-increasing and acidic gas removing system, a C2 & light component and C3 & heavy component separation system, an ethylene rectification system and a propylene rectification system. Further explanations are as follows by reference to
(1) Pressure-increasing and acidic gas removing system. In said system, the pressure of the product gas 1 from the MTO device is increased via a pressure-increasing apparatus, e.g. processing gas compressor 2, to 2.0-4.0 MpaG, preferably 2.0-3.0 MpaG, and an oxide removing system 5 is set up at a suitable site (e.g. between sections 3 and 4 of the compressor). Unreacted methanol in the feedstock and dimethyl ether produced in the reaction are removed via washing and stripping, and simultaneously an acidic gas removing system 6 is set up to remove CO2 produced in the reaction via alkaline washing.
(2) C2 & light component and C3 & heavy component separation system. In said system, the gas obtained from said pressure-increasing and acidic gas removing system is cooled and fed into a desiccator to remove water. An adsorbing system, if necessary, may also be set up to remove some other impurities, such as oxides. Then, the gas is fed into the deethanizing column 10 so as to separate C2 & light components from C3 & heavy components, wherein the operation temperature of said deethanizing column 10 is generally from 0 to −25° C. The materials at the top of the deethanizing column 10 are introduced into the C2 hydrogenating system 12 to remove a trace amount of ethyne from the materials, so as to satisfy the specification requirements of the ethylene products. The C2 hydrogenating system 12 is usually a fixed-bed reactor, and G58C from Sud Chemie or other common hydrogenation catalysts in the field may be used. Additionally, a small amount of hydrogen may also be supplemented if the amount of hydrogen in the MTO product gas cannot satisfy the hydrogenation requirements.
(3) Demethanizing system. The hydrogenated product gas is cooled and fed into the demethanizing column 13, wherein operation temperature of the demethanizing column 13 is generally from −30 to −37° C. Said demethanizing column is used to make the ethylene product satisfy the quality requirements, but it is more important that it is used to recover ethylene from the product gas. The materials at the top of the demethanizing column 13 are cooled and fed into the reflux tank 17. Due to the presence of methane and hydrogen, the materials at the top of the demethanizing column 13 are not completely condensed. There is accordingly uncondensed tail gas in the reflux tank 17, and said tail gas contains a small amount of ethylene. If said tail gas is discharged or burned as fuel gas, there will be much economic loss. Therefore, in accordance with the present invention, an ethylene recovering device is set up on the reflux tank of the demethanizing column 13.
Said ethylene recovering device comprises at least a condenser 19, which is generally a flat type one.
In order to achieve better separation effects, one preferred embodiment involves that said ethylene recovering device further comprises a rectification column section 18 directly linking with the demethanizing column reflux tank 17. The uncondensed tail gas in the reflux tank 17 is fed into the. rectification column section 18. Various internal column components may be used inside the rectification column section 18, e.g. fillings, float valves and the like. Generally, the operation temperature of said rectification column section 18 ranges from −37 to −98° C. Said plate type condenser 19 is set up at the top of the rectification column section 18. The ethylene recovering device consisting of the rectification column section 18 and the condenser 19 is used to further recover ethylene in the uncondensed gas by using the fractionation principle, so as to increase the ethylene recovery rate.
In another embodiment of the process of the present invention, the condenser 15 at the top of the demethanizing column 13, the reflux tank 17, the rectification column section 18 and the plate type condenser 19 at the top of the rectification column section 18 are combined together and set up at the top of the demethanizing column 13 (see
Coolant at a temperature of less than −40° C. is usually needed for the condenser 19 in the ethylene recovering device in any form above. In order to avoid designing any new refrigeration system, e.g. ethylene refrigeration system, and lower energy consumption, C2 components produced by the system per se are sufficiently utilized in the present invention to provide the refrigeration as required. The coolant in the condenser 19 is one selected from the group consisting of
After the pressure of the coolant is reduced via the throttle valve 14, it is fed into the condenser 19 of the ethylene recovering device. In said condenser 19, said coolant is vaporized so as to provide the refrigeration for the ethylene recovering device, and then refluxed back to suitable site of the compressor 2 (determined by pressure) after the refrigeration is further recovered by the system.
(4) Ethylene rectification system. The materials at the bottom of the demethanizing column 13 are fed into the ethylene rectification system 22. At the top of the ethylene rectification system 22 is obtained the ethylene product in a polymerizable grade, which has an ethylene content of higher than 99.9 mol %. Ethane obtained at the bottom of the ethylene rectification column 22 can be delivered out as a product or incorporated into the fuel system. Since there are only C2 components in the materials fed into the ethylene rectification column 22, open-type heat pump technique may be used in the ethylene rectification column, if required by the consumer, to reduce the energy consumption.
(5) Propylene rectification system. The materials at the bottom of the deethanizing column 10 are fed into the depropanizing column 23. C3 components are separated out at the top of the depropanizing column 23, and C4 components and higher are separated out at the bottom of the depropanizing column 23. C3 components at the top of the ethanizing column 23 are fed into the propylene rectification column 25. At the top of the propylene rectification column 25 is obtained the propylene in a polymerizable grade, which has a propylene content of higher than 99.5 mol %. Propane obtained at the bottom of the propylene rectification column 25 can be delivered out as a product or incorporated into the fuel system.
In a specific embodiment, if the common engineering system is deficit in steam, the system of the combination of the rectification column with the open-type heat pump technique may be used as the propylene rectification system to reduce the energy consumption of the device. Meanwhile, the combination with propylene refrigeration compressor may also be used to reduce the compressor units. Specifically, the gas at the top of the propylene rectification column 25 is firstly fed into the heat pump compressor 27 so as to increase the pressure to a sufficient extent and to condense propylene in the reboiler 28. A part of condensed propylene is taken out, and the remaining is refluxed back to the top of the column. Therefore, the propylene rectification column needs no steam to provide heat for the reboiler.
Under the circumstance of the presence of sufficient steam, particularly waste steam, a common flow scheme may be used in the propylene rectification system.
The process for recovering low carbon olefins of the present invention should also comprise a conventional closed circuit refrigeration system using propylene as a coolant to provide the refrigeration of various temperature ranges for the separation parts.
It is noteworthy that the process for recovering low carbon olefins from the MTO product gas of the present invention have notably advantages over the prior art:
The present invention is detailedly disclosed by the following examples, but it is not limited by said examples.
According to the scheme as shown in
The scheme simulation for the scheme of
The computation results showed that the recovery rate of ethylene may achieve 99.5% by using the process of the present invention to separate the MTO product gas.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2005-1015587.3 | Sep 2005 | CN | national |
