How can the membrane recovery of petrochemical plant tail gas be combined with compression condensation and absorption processes? Multi-process collaborative configuration

The tail gas from petrochemical plants is not the same as the oil and gas from gas stations and oil depots: it has complex components - light hydrocarbons, aromatic hydrocarbons, and oxygen-containing organic compounds may coexist; its gas volume fluctuates greatly - plant load, startup and shutdown, and maintenance periods are all changing; there are many emission points - storage tanks, loading and unloading, wastewater treatment, and recovery before flares each have their own operating conditions. It is not cost-effective in terms of investment and energy consumption to use a single process to handle all operating conditions. Multi-process collaboration is to place compression condensation, absorption, and membrane separation in their respective advantageous positions. This article explains the idea and key points of collaborative configuration based on the characteristics of petrochemical tail gas.

What does each of the three processes manage

Compression condensation is the main force in recovery: it pressurizes and cools the tail gas, and the hydrocarbons are liquefied according to their boiling points. Under high-concentration and high-volume conditions, the recovery rate is high. The shortcoming is that in the low-concentration range, the temperature needs to be lowered, leading to a steep increase in power consumption. Absorption is suitable for continuous tail gas with high flow and low concentration: lean oil is sprayed to absorb hydrocarbons, and rich liquid is regenerated and recycled. The operation is stable, but the quality of the recovered oil products is generally average. Membrane separation excels in enrichment control: low-concentration tail gas is enriched by membranes, and its concentration is increased. It is then sent back to the condensation or absorption inlet for further treatment, allowing the upstream process to operate in a high-efficiency range. These three processes are not substitutes, but rather have different roles to play.

How to arrange collaborative processes

The typical collaborative process is divided into three stages: compression and condensation as the initial stage, where the bulk of the gas is collected; the collection and pressurization of plant tail gas, followed by condensation and liquefaction, with the condensate sent to the recovery tank; absorption, where the remaining low-concentration tail gas is collected, and the high-flow, low-concentration tail gas after condensation enters the absorption tower to capture some of the remaining hydrocarbons; and membrane purification, where the exhaust gas from the top of the absorption tower and the low-concentration breathing gas from the tank area converge and enter the membrane unit, with the enriched gas returned to the inlet of the condensation section and the retained side discharged after meeting emission standards. The connection between these three stages relies on buffering and concentration monitoring: online monitoring of inlet and outlet concentrations at each stage, with the direction of the enriched gas switching according to its concentration, ensuring a closed-loop operation of the process chain.

How to determine the configuration aperture

Collaborative configuration is determined by data, not by experience: full analysis of exhaust gas components and gas concentration curves at each emission point are the inputs for configuration. The recovery target determines the depth of the process chain - components with high recovery value are prioritized for configuration in the recovery section, and emission compliance is the bottom-line constraint. Equipment specifications are verified based on adverse operating conditions, not on average values - exhaust gas conditions during startup, shutdown, and maintenance periods are often more severe. In terms of control strategy, each level is switched based on concentration and gas volume to avoid inefficient operation. Investment allocation follows this principle: allocate sufficient resources to the main section, allocate precise resources to the control section, and allocate enough resources to the auxiliary section.

There is also a set of practices for verifying collaborative schemes: first, perform material balance calculations based on actual measured data, ensuring that the input and output quantities and concentrations of each process stage are closed off segment by segment; then, gradually put each stage into operation during the commissioning phase - stabilize the condensation stage first, then the absorption stage, and finally connect the membrane stage at the end. Check the overall indicators each time a stage is put into operation. The commissioning method of putting all stages into operation at once can make it difficult to identify the problem at any specific stage, and gradually putting each stage into operation may take a few more days, but it results in faster problem localization and more accurate parameter tuning.

Multiple process configuration references

Operational collaboration management

1. Online monitoring of import and export concentration at all levels, with data integrated into central control;
2. Check the one-way valve of the enriched gas return pipeline regularly to prevent backflow;
3. The quality of the absorbed lean oil is tested weekly, and the regeneration system operates smoothly;
4. The condensation temperature level is adjusted seasonally according to the composition, and electricity consumption is calculated monthly;
5. Include the membrane pre-treatment pressure difference and pretreatment status in daily inspections;
6. Annual drill of tail gas routing plan during startup and shutdown periods.

Frequently Asked Questions

Do I still need to add a membrane to the existing condensation device?

Looking at the emission data: if the concentration in the uncondensed section after condensation is still high, adding a membrane control section can improve the overall recovery rate, and the investment is less than that for overall expansion; if the capacity of the front section is insufficient, expand the front section first, as the membrane section is the final stage for closing off.

How to choose absorbent oil?

Select absorbents based on exhaust gas components: For light hydrocarbons, diesel-based absorbents are preferred, while for oxygen-containing organics, attention should be paid to miscibility and regeneration difficulty. When selecting the type, provide the component analysis to the manufacturer for calculation, and compare lean oil loss with regeneration energy consumption.

How to prevent interference in three-level linkage?

Buffer tanks and concentration switching logic are set up between each level, and the enriched gas is only returned when the concentration at the inlet of the condensation section is permissible; control is independent at each level and interconnected along the chain, ensuring disturbances are leveled out in the buffer section.

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.