How to do the intake air pretreatment of the membrane separation section? Configuration points of defogging, cooling and filtration

In the solvent recovery system, the function of the membrane separation section is to concentrate the residual low-concentration vapor after condensation, so that the external displacement is smaller. Many devices have good results at the initial stage of operation, and the concentration capacity decreases after a period of operation. It is often not the quality problem of the membrane module itself, but the intake air pretreatment is not in place. Membrane modules are much more sensitive to intake conditions than heat exchangers, and liquid water, droplet mist, particulate matter and excessive temperatures all accelerate performance degradation.

What are membrane components afraid?

First is liquid water. When the membrane material works under high humidity or even liquid state, the separation performance will decrease, and the long-term operation may cause irreversible damage. Secondly, droplets and mist. The droplets entrained in the air flow will cover the surface of the membrane, forming local resistance and deteriorating the concentration effect. Three is particulate matter and high boiling point components. Dust, polymer particles, high boiling point impurities will be deposited on the surface of the film, the formation of difficult to remove the dirt layer. Fourth, the intake air temperature exceeds the applicable range of the membrane, the temperature is too high will affect the stability of the membrane material, too low is prone to condensate.

The four links of the pretreatment section

1. Defogging: A defogging structure is set up after the condensation section to stop the droplets, and the components should be easy to disassemble, wash and replace.
2. Cooling and temperature control: stabilize the intake air temperature within the applicable range of the membrane material, and the fluctuation range is as small as possible.
3. Filtration: Select the filtration accuracy according to the cleanliness of the air source, focusing on intercepting particulate matter and high boiling point sediments.
4. Drainage and heat preservation: drainage is set at the low point of the pipeline, and heat preservation is done when necessary to avoid secondary entrainment of condensate into the membrane section.

Why can't the order be reversed?

Pre-processing configuration comparison

Configuration Method	FAQ	Recommended practices
Filter before defogging	The filter element is blocked by droplets and replaced frequently	Change to defog first, then filter as needed
First cool down and then defog	Decreased efficiency of defogging element soaked in condensate	In front of defogging and immediately after cooling
Omit drainage point	The liquid in the pipeline is secondary entrained by the air flow.	Low point set drain and check regularly
Temperature control only by experience	Large temperature fluctuation, shortened membrane life	Set and record trend according to pneumatic conditions

A common mistake is to put filtering before defogging, or to merge the two sections to save space. Filter first and then defog, the filter element is quickly blocked by droplets, and replacement is frequent; first defogging and then filtering, the filter element burden is much smaller, and the replacement cycle is longer. The temperature should be reduced after defogging and before entering the film. If the temperature is reduced before defogging, the precipitated condensate will soak the defogging element in the liquid phase, and the defogging efficiency will obviously decrease. This sequence seems to be a detail, which actually directly affects the service life of the membrane module.

Operation and Maintenance Arrangements

The replacement cycle of consumables in the pretreatment section shall be determined according to the actual pressure difference and observation, and not only according to a fixed time. The performance of the membrane module is judged by data such as air intake, concentration ratio and outlet concentration. When there is a trend decline, first check whether the pretreatment section fails, and then consider whether the membrane module itself needs to be processed or replaced. It is recommended to keep the cleaning time of the defogging element, the replacement time of the filter element and the corresponding pressure difference in the operation record, so that the subsequent judgment can be based on.

Pre-configuration checklist

Confirm the humidity of the air source, dust content, high boiling point component content, exhaust temperature range and the actual outlet conditions of the condensing section, and then determine the defogging form, filtration accuracy and temperature control method. With all this data, the configuration of the preprocessing section will not be estimated only by experience.