How does the vacuum degree on the permeate side affect the membrane recovery rate? Configuration and adjustment of driving force

After the membrane-based oil and gas recovery device has been operating for a period of time, the recovery rate tends to decrease. Eighty percent of the problem lies not in the membrane itself, but in the driving force - the partial pressure difference on both sides of the membrane has decreased. The partial pressure difference is determined by both ends: the feed side is boosted by the compressor, while the permeate side is pumped down by the vacuum pump. If either end fails, the amount of oil and gas passing through the membrane will decrease. This article clarifies several things in order of operation: where the driving force comes from, how to adjust the vacuum degree, and how to troubleshoot abnormalities. The personnel on duty can refer to it directly.

First, let's establish a benchmark for judgment: the membrane layer itself is a slowly changing component, and the recovery rate curve during the initial operation period can serve as a baseline. Any subsequent fluctuations can be compared to this baseline. Periods where the rate drops rapidly can almost be traced back to driving forces or pretreatment events. By drawing this baseline, future operation and maintenance will have a reference point.

Where does the driving force come from

The driving force behind gas membrane permeation is the partial pressure difference between components: the higher the partial pressure of oil and gas on the feed side and the lower the partial pressure of oil and gas on the permeate side, the greater the driving force for membrane permeation. In engineering, a combination of pressure boosting and vacuum pumping is used to create a pressure difference between the two sides - a compressor raises the pressure of the mixed gas before the membrane, increasing the partial pressure on the feed side; a vacuum pump maintains the permeate side at negative pressure, allowing the permeated oil and gas to be promptly pumped away and not accumulate on the surface area behind the membrane. If one component is missing, the flux drops significantly immediately, which is why the vacuum pump and compressor of a membrane device should be configured and maintained as a pair.

How does vacuum degree affect recovery rate

The lower the permeate side pressure, the greater the partial pressure difference across the membrane, the faster the oil and gas permeation rate, and the higher the enriched gas concentration. When the vacuum degree operates close to the design value, the recovery rate of the device is stable; as the vacuum degree decreases, the permeation rate decreases accordingly, the concentration on the emission side first increases, and the bookkeeping of the recovery amount subsequently becomes unfavorable. Conversely, blindly pumping the vacuum to the extreme also has costs: the load and power consumption of the vacuum pump increase, the temperature rise of the pump body accelerates, and the wear of the seals becomes faster. The goal of operation is to stabilize the vacuum degree within the design range, rather than pumping it to the limit.

How to coordinate boosting and vacuum

The idea of configuration is to allocate the investment at both ends to the most critical areas: after increasing the pressure on the raw material side to the design pressure before the membrane, the vacuum degree only needs to match the corresponding range to achieve the target flux. A common problem is the disconnection between the two ends - the compressor outlet pressure is sufficient, but the resistance in the pipeline before the membrane is high and the temperature is high, resulting in insufficient partial pressure actually reaching the membrane surface; or the vacuum pump has sufficient pumping capacity, but the seal is aging and the pipeline is leaking, preventing the vacuum degree from increasing. Checking the design parameters against the actual readings and identifying the gap between the two ends is much more cost-effective than simply replacing the membrane.

Configuration reference

How to adjust and conduct routine inspections in daily life

1. Record the three items: membrane front pressure, permeate side vacuum degree, and permeate side flow rate daily, and draw trend lines to observe the trends;
2. If the vacuum degree deviates from the design range, first check for seal and pipeline leaks, and then check the oil level and temperature rise of the pump;
3. If the pressure before the membrane is insufficient, first check the pressure difference of the filter. Blockage of the filter material is a common cause of pressure drop;
4. Make small adjustments to avoid sudden pressure changes that could impact the separation layer;
5. Compare quarterly performance calibration with daily trends, and first investigate the driving force behind recovery rate fluctuations before evaluating the membrane layer.

Frequently Asked Questions

Is it better to have a higher vacuum level?

No. When the vacuum degree exceeds the design range, both power consumption and pump body wear increase significantly, while the increase in recovery rate is very limited. Maintaining the vacuum degree within the design range is more cost-effective than pursuing extremes, and it also extends the lifespan of the pump.

Why do we first check the driving force when the recovery rate decreases?

The change in membrane performance is a slow process, while abnormal driving force can occur overnight. First, check the readings of pressure and vacuum degree. Most fluctuations can be traced back to pre-treatment, sealing, and pumping.

How to switch between two vacuum pumps, one in operation and the other as standby?

Regularly rotate the pumps based on their operating hours, and observe the time it takes for the vacuum level to rebound and the temperature rise of the pump body after switching. Long-term storage of standby pumps can easily lead to issues, while rotating operations can keep both pumps in good condition.

This article is compiled by Luoyang Wohong Petrochemical Equipment Co., Ltd., which provides design, manufacturing, and operation and maintenance services for condensation recovery, membrane separation recovery, and complete solvent recovery equipment to chemical and pharmaceutical enterprises. If you need process evaluation or scheme configuration, please feel free to contact us for discussion.