How to recycle the non-condensable gas at the top of the pharmaceutical distillation tower? Collection and control configuration of distillation tail gas
Release time: 2026-10-01
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The distillation process in pharmaceutical production serves as a pivotal hub for solvent recovery. The overhead distillate is either condensed and refluxed or collected, yet a portion of the gas phase remains uncondensable—this constitutes the overhead non-condensable gas. This tail gas is characterized by its high concentration, low volume, and composition identical to the solvent being distilled. High concentration implies significant recovery value, low volume indicates a manageable processing load, and the cost of exhaustive treatment is not high in terms of configuration. These factors collectively make the treatment of this tail gas a profitable aspect of pharmaceutical companies' waste gas management.
The presence of non-condensable gas is related to the light components in the system: low-boiling-point impurities, trace moisture, and non-condensable gases introduced into the system. These components fail to meet the condensation conditions in the overhead condenser and are discharged with the gas phase. If non-condensable gas is not discharged, the heat transfer of the condenser will be deteriorated by the gas phase cushion, leading to an increase in tower pressure and unstable operation of distillation. Therefore, there must be an outlet for non-condensable gas, and if the outlet is well designed, recovery can be achieved as a by-product.
How to collect
The non-condensable gas is drawn out from the gas phase outlet of the condenser at the top of the tower, with short piping and proper insulation to avoid condensation and liquid accumulation en route. When multiple distillation towers converge their non-condensable gas, valves are interlocked according to the operating periods of the towers. The branches of the towers that are shut down are closed to avoid gas dilution and pressure crossover. The diameter of the converging pipe is calculated based on the gas volume during the same period. Although the total amount of non-condensable gas is small, the pressure at the top of the tower is sensitive to back pressure, so the resistance of the converging pipe should be kept low. A large pressure difference will affect the operating pressure of the tower.
For towers with intermittent production, the amount of non-condensable gas fluctuates from batch to batch. A buffer tank is installed on the collection pipeline to stabilize the pressure, which stabilizes the load on the condensation section and makes defrosting and liquid level management easier.
How to match recycling with quality control
The treatment of non-condensable gas is carried out in two stages: the first stage is condensation, with the temperature level set according to the dew point of the solvent. Under most operating conditions, the majority of the solvent in the non-condensable gas can be condensed, and the condensate is returned to the recovery system. The second stage is the final check, where residual low-concentration tail gas is treated through tandem adsorption or membrane separation, and the outlet is controlled according to emission targets. No bypass is set between the two stages for direct discharge - if the concentration of non-condensable gas is high, the leakage amount through the bypass can be considerable when converted into annual solvent loss, making both the economic efficiency and compliance of the final check stage valid.
The condensate reflux method in the condensation section is determined based on the system: direct reflux and extraction system with the same components as the overhead distillate, separation and disposal of impurities containing light components, to avoid the accumulation of impurities in the system. The adsorption material in the control section is selected based on the solvent components, and the regenerated desorption gas returns to the condensation section, forming a closed loop, with no secondary emission of desorption tail gas.
Configuration reference
| link | characteristic | Configuration arrangement |
|---|---|---|
| Lead out from the top of the tower | High concentration and low air volume | Short circuit insulation to prevent fluid accumulation |
| Multi-tower manifold | Tower pressure is sensitive to back pressure | Valve linkage resistance pressure reduction |
| Buffering and voltage stabilization | Batch fluctuation | Stable load of buffer tank |
| Primary condensation | Large head solvent coagulation | The temperature level is set according to the dew point |
| Secondary check | Low-concentration residue | Back-up solution for adsorption or membrane separation |
| Desorption gas | Regenerative emissions | Return to the closed-loop condensation section |
How to implement operation management
- The non-condensable gas branch is equipped with valve linkage for each tower, and the branch is closed when the tower is shut down;
- The calculation of manifold resistance is accurate, and the back pressure at the top of each tower is monitored;
- The condensation temperature level is set according to the dew point, and the condensate reflux is divided according to the composition;
- Ensure that all stages are connected and covered, without bypassing or direct discharge;
- The desorbed gas is returned to the condensation section, and the closed loop is not discharged twice;
- Record the non-condensable gas volume and condensate volume batch by batch, and maintain a closed-loop recovery accounting system.
Frequently Asked Questions
Is the amount of non-condensable gas so small that it's not worth setting up a separate recovery system for?
It's worth keeping track of the costs: high concentration means a large solvent content per unit of gas volume, which translates to considerable annual losses. The main approach is to configure with condensation and small-scale checks, which requires little investment, and the accounting results of most pharmaceutical companies are valid.
How to coordinate the pressure at the top of the tower affected by back pressure?
The calculation of manifold diameter and resistance should be fully considered during the planning stage. Valves should be installed on each tower branch for linkage, and the back pressure of the operating tower should be controlled within a set range. If tower pressure fluctuations are found to be related to the manifold during operation, the status of the branch valves and the resistance of the manifold should be checked first.
What about the light component impurities in the non-condensable gas?
Condensate is separated by component: The reflux extraction system with the same components as the overhead distillate is used, and the impurity-enriched part is separated and disposed of to avoid backflow into the tower and subsequent accumulation, which could affect product purity and tower operation.
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.
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