In the low-pressure gas phase fluidized bed polymerization process for recovering polyethylene by one-step condensation and two-step condensation membrane separation, the exhaust gas recovery unit is mainly used for recovering the condensing conditions of the comonomer and the condensing agent, taking a certain butan number in the condensation operation as an example. In the existing process, the outlet pressure of the exhaust gas recovery compressor is to increase the compressor outlet pressure, and other process conditions are unchanged, and the degassing chamber exhaust gas in the process flow of the polyethylene device exhaust gas recovery process. First, a part of the liquid is recovered through the low pressure cooler and the low pressure condenser, and then the compressor is boosted by the tail gas recovery compressor and then cooled by a high pressure cooler to a high pressure condenser to condense to recover more liquid. The recovered comonomer and condensate liquid is pumped back to the reaction feed system. A portion of the non-condensable gas is returned to the product blow-out tank for conveying the resin product to the product degassing bin, and excess exhaust gas is sent to the torch. At present, several sets of polyethylene unit recovery unit processes use low-pressure and high-pressure two-step condensation, that is, a low-pressure condenser compressor and a high-pressure condenser. The compressor outlet pressure is generally the following to analyze the influence of the existing compressor outlet pressure. And the rationality of the two-step condensation. When the equipment cost is not considered, although the pressure is increased and the energy consumption is increased, the recovery of the monomer and the condensing agent is increased, the raw material consumption is reduced, and the net income is still proportional to the compressor outlet pressure. The higher the pressure, the net income. more. However, from a fixed investment, the higher the pressure, the better. When the compressor outlet pressure is required to be too high, more compression stages may be required, and the equipment cost will be higher. Therefore, when industrially applied, it is necessary to comprehensively compare various factors such as equipment costs and operating costs, and select the most appropriate operating pressure. The compressor outlet pressure rises, the ice machine power decreases, and the cooling water consumption increases. This is because the pressure rises, the dew point of the exhaust gas rises, and the liquid is already condensed in the high pressure cooler, so that the circulating water consumption is greatly increased, and the condenser heat load is lowered. The total power consumption of the compressor and ice machine also increases as the compressor outlet pressure increases. In short, increasing the compressor outlet pressure, the consumption of monomer and condensing agent can be reduced, but the energy consumption also increases. Without a low pressure condenser, the compressor power is increased, the circulating water consumption is increased, but the electromechanical consumption of ice is reduced. After the power consumption and circulating water consumption are converted into energy consumption according to the energy conversion coefficient, the total energy consumption is reduced. Therefore, no low-pressure condenser is provided, that is, a one-step condensation method can be used to save energy. Without a low-pressure condenser, the compressor power is increased, the circulating water consumption is increased, but the electromechanical consumption of ice is reduced, and the total energy consumption is reduced. Therefore, the low-pressure condenser is not provided, that is, the one-step condensation method can save energy and reduce material consumption. This part of the vent gas contains a large amount of nitrogen and hydrocarbons. If the nitrogen gas can be separated as part of the purge gas of the degassing tank, the amount of fresh nitrogen can be greatly reduced. At the same time, if the hydrocarbons in the vent gas can be further recovered, the consumption of the monomer can also be reduced. This can be achieved by using different membrane selectivity for hydrocarbons and nitrogen. The membrane separation system can effectively separate the nitrogen and hydrocarbons in the exhaust gas of the exhaust gas recovery unit, and use the purified nitrogen to return to the degassing chamber to reduce the exhaust gas emissions, and at the same time can improve the recovery rate of the monomer, which is a good method for reducing the monomer consumption. Among them, ethylene-butene pentane can be returned to the reaction system, and the nitrogen separated by the membrane separation and purification is returned to the degassing chamber to supplement the purge gas, which can greatly reduce the consumption of the raw material ethylene-butene pentane and nitrogen. Membrane separation process The degassing chamber exhaust gas is compressed by the compressor, cooled and condensed by the heat exchanger, and the condensate is returned to the reaction system. The gas is separated by the membrane for secondary separation, and the hydrocarbon-rich permeate stream is recycled back to the compressor inlet. The stock is a nitrogen-rich stream, in which the nitrogen concentration can be purified to above 9. According to the data provided by the manufacturer, when the concentration of nitrogen recovered is used, the comparison between the gas volume of the torch and the recovery rate of nitrogen hydrocarbons before and after membrane separation is shown in the table and the optimized recovery process. The above analysis can be improved from the following aspects. The existing exhaust gas tail gas recovery process uses one-step condensation to eliminate the low pressure condenser. Increasing the outlet pressure of the compressor can increase the amount of condensate and increase the monomer recovery rate, thereby reducing material consumption. Increasing the membrane separation system can reduce tail gas emissions, further recover monomer and nitrogen, and utilize the recovered nitrogen to reduce material consumption.
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Low pressure gas phase fluidized bed polymerization process
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