In the bulk polypropylene plant, propylene is both a raw material and a product. If the propylene entrained in the gas discharged from the tail gas system is not recovered, it is not only a raw material loss on the book, but also an environmental pressure at the discharge port. The conventional condensation scheme can recover most of the propylene, but the concentration of propylene in the uncondensed tail gas is not enough to let it go-this part is enriched by membrane separation and refluxed, and the recovery rate can be more than 95%. This paper makes this configuration clear according to the sequence of tail gas source, membrane section configuration and reflux path.
The bulk process uses liquid-phase propylene as the reaction medium, and the powder generated by the reaction is separated and deactivated, and the tail gas system discharges the propylene entrained in the reaction and flash process. This part of the gas is large in quantity and propylene content is considerable. Directly entering the torch not only burns the raw materials but also increases the emission burden. All condensation recovery is affected by inert components in the tail gas-after the proportion of non-condensable gases such as propane and nitrogen increases, the condensation temperature level requirement is getting lower and lower, and the energy consumption curve is not cost-effective. The account of exhaust gas treatment should be calculated at the intersection of how much is recovered and how much energy is consumed.
The rubber separation membrane has high solubility for propylene, and the permeation rate of propylene is much faster than that of small molecules such as nitrogen and methane: the tail gas enters the membrane unit after compression and pressurization, and propylene preferentially permeates to form enriched gas. The permeation side is maintained at negative pressure by a vacuum pump to continuously extract the enriched gas; After the concentration of non-condensable gas on the interception side is increased, it is sent to the torch or used as fuel gas, and part of the condensate before the membrane. The enrichment multiple of the membrane section is high, which is equivalent to screening the propylene that cannot be collected by condensation again. The division of labor between the two stages is isomorphic to the condensation and membrane addition in oil and gas recovery.
The enriched propylene gas extracted from the permeate side has two routes: the one with appropriate concentration is directly returned to the compressor inlet or the reaction system for circulation, and the branch with higher concentration is condensed and liquefied and recycled. The return path must be fixed at the design stage-where the return point is connected, how the control valve is matched, and how it is connected with the pressure of the reaction system, these determine whether the membrane section can run smoothly. Flow rate and concentration monitoring shall be set on the return pipeline, and the data shall be controlled in the central control. In case of fluctuation, the working conditions before the membrane shall be checked first and then the return ratio shall be adjusted.
How to determine the scale of the device has a set of ready-made algorithms: take the long-term flow record of the tail gas system as distribution, push back the membrane area according to the recovery target concentration, and then make matching calculation with the temperature level capability of the condensation section. only after the two sets of data are collected can the promised value of the recovery rate be written in the scheme. Empirically, it is relatively safe to set aside about 20% allowance for the membrane area according to the peak working condition. If the allowance is too small, it will compact on the emission index. If it is too large, the vacuum pump will operate inefficiently for a long time and the power consumption will be wasted.
| Segment | Configuration considerations | Purpose |
|---|---|---|
| pre-film condensation | High temperature propylene | Reduce the load of membrane section |
| membrane unit | Rubber separation layer | propylene enrichment |
| Vacuum system | Maintain negative pressure on the osmotic side | Pull-off partial pressure difference |
| Enriched gas | Return to compressor inlet | Recycling |
| entrapped gas | To flare or fuel gas | Standard disposal |
| Monitoring | Concentration and flow online | Early detection of fluctuations |
After combined with condensation, the propylene recovery rate in uncondensed tail gas can be more than 95%, the specific level depends on the tail gas composition, membrane area and operating pressure, and is calculated according to the measured data of the device.
The inert components in the tail gas do not condense, and as their proportion increases, the condensation temperature level requirements become lower and lower, and energy consumption and equipment investment rise rapidly. The membrane section enriches low-concentration propylene and then treats it, just to make up for the concentration range where condensation is not cost-effective.
A filter and a demisting section are arranged in front of the membrane, and powder and droplets cannot reach the membrane surface. The filter material is replaced according to the pressure difference, and the pressure in front of the membrane is fixed in the daily trend, so that the pollution risk can be pressed.
This article by LuoyangWohong PetrochemicalEquipment Co., Ltd. arranges that the company provides design, manufacturing and operation and maintenance services for condensation recovery, membrane separation recovery and complete solvent recovery devices for chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please contact and exchange.
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