The activated carbon adsorption unit is installed in the exhaust gas pipeline upstream of the vacuum pump to capture alcohols, volatile organic compounds, and small amounts of mist. If the carbon bed reaches adsorption saturation and is not replaced in a timely manner, it will indeed lead to a deterioration in the quality of the condensate. The contamination pathways involve multiple levels and are not limited to a single desorption process; this is fully explained in conjunction with the unique characteristics of low-temperature evaporation under negative pressure conditions.

As the system operates under continuous negative pressure, the pores within the activated carbon are gradually filled with organic compounds, causing it to lose its adsorption capacity. At this point, unretained volatile organic compounds in the exhaust gas pass directly through the carbon bed, travel with the gas phase to the condenser, condense on the low-temperature heat exchange surface, and mix into the condensate. The most obvious manifestation of this is a continuous increase in the COD of the condensate. As the negative-pressure gas flow passes through the saturated carbon bed, it also creates turbulence. Loose, fine carbon powder from the surface of the carbon material is carried forward by the gas flow. This carbon powder enters the condensation system along with the vapor and becomes suspended in the condensate, causing the effluent to become turbid and resulting in the presence of black suspended solids.

Operating conditions in low-temperature evaporation systems are subject to fluctuations. Unit start-ups and shutdowns, vacuum oscillations, and changes in evaporation load can cause slight fluctuations in the vapor temperature within the piping. The adsorption equilibrium of organic substances captured by activated carbon changes with temperature and negative pressure. When the temperature rises or the vacuum draw changes abruptly, some of the organic matter adsorbed within the carbon pores desorbs, re-enters the gas phase, enters the vacuum piping, and migrates toward the condenser, continuously contaminating the condensed water. This desorption occurs intermittently; water quality abnormalities often manifest as fluctuating performance, making it difficult to directly pinpoint issues with the activated carbon.

Trace droplets and water vapor carried in the exhaust gas continuously saturate the activated carbon over time, making the saturated carbon bed prone to forming an anaerobic environment. The adsorbed organic pollutants undergo slow microbial decomposition within the carbon bed, producing small-molecule organic acids and aldehydes. These small-molecule substances are more likely to volatilize, penetrate the carbon bed, and enter the condensation system. This not only elevates the organic matter levels in the condensate but also causes the condensate to develop an unpleasant odor. At the same time, the acidic substances produced by degradation will slowly migrate with the airflow, and over the long term, this will accelerate the degradation of the vacuum piping and vacuum pump oil.

The risk of reverse flow is often overlooked at many sites. During the vacuum break phase when the unit is shut down, outside air flows back into the adsorption tank from the outlet, causing the gas flow to pass through the carbon bed in the opposite direction. Pollutants adsorbed on the surface of the carbon bed, along with loose carbon dust, are carried by this reverse airflow back toward the evaporator chamber and condenser, where they adhere to the heat exchange surfaces and the inner walls of the demister. During the next evaporation cycle, these substances volatilize again upon heating and continue to dissolve into the condensate.

In addition, the saturated activated carbon’s ability to intercept mist droplets decreases significantly. Under normal conditions, the activated carbon bed provides a certain degree of gas-liquid separation, capturing mother liquor droplets carried by the exhaust gas; however, once adsorption reaches saturation, the carbon pores become blocked by organic matter, causing the pores to lose their hydrophobic properties. As a result, mist droplets penetrate the adsorption unit directly, and mother liquor droplets containing salts and colloids enter the condensation system, further exacerbating condensate contamination.

In many operating conditions, this failure exhibits a time lag: when the activated carbon layer first begins to be penetrated, the change in water quality is minimal and difficult to detect via online monitoring; after continuous operation for a period, pollutants accumulate, and condensate parameters suddenly exceed limits. In addition to regularly replacing the activated carbon, routine management can also involve monitoring changes in the condensate’s COD as a basis for assessment. Installing a high-efficiency gas-liquid separator upstream of the adsorption tank reduces the entry of water vapor and mist droplets into the carbon bed, thereby delaying saturation. Properly arranging check valves in the piping system helps minimize the reverse flow of contaminants carried by air during shutdown periods.