How to recycle the solvent exhaust gas from the pharmaceutical drying process? Collection configuration of exhaust gas from drying ovens and vacuum drying

In pharmaceutical production, the drying process is the concentrated stage of solvent evaporation: after crystallization, the wet material is placed in an oven or vacuum dryer, and the solvent escapes in large quantities with the hot airflow or vacuum system. The exhaust gas has high temperature and high concentration, fluctuating with each batch. If this exhaust gas is directly vented, both solvent loss and emission burden will be imposed on the enterprise; if collected and condensed for recovery, the recovery rate can be considerable under most working conditions, making it a clearly beneficial link in the recycling transformation of pharmaceutical enterprises.

The difficulty in recycling dry exhaust gas lies in the operating conditions: high temperature means that the inlet gas cannot be directly sent to the condensation section and needs to be cooled first; batch fluctuations mean that the load changes greatly, and the device must be able to keep up with these fluctuations. The collection and configuration plan should be tailored to these two factors.

How to collect exhaust from an oven

The drying oven primarily operates through hot air circulation, with the solvent concentration at the exhaust port varying according to the drying stages - higher during the constant rate phase and lower during the deceleration phase. Collection is managed by connecting pipes from each drying oven, with the exhaust gas first undergoing heat recovery or pre-cooling before entering the main condensation recovery section. When multiple drying ovens are operated during off-peak hours, the total pipe load is calculated based on the concurrent rate, rather than simply adding peak values together. The insulation and slope of the air ducts are optimized to prevent condensate from accumulating in the pipes during the cooling phase, and drainage outlets are installed at low points in the piping for regular condensate drainage.

The majority of windows for collection and modification are scheduled during the production downtime for maintenance: the locations of openings and the routing of pipelines are confirmed in advance with the user, and prefabrication and processing are completed before the windows. On-site work only involves installation and purging, ensuring the project timeline is manageable.

How to handle vacuum drying

The vacuum-dried solvent is carried away with the exhaust from the vacuum pump, with high concentration and temperature. The treatment path is to condense first after the pump: primary condensation to recover most of the solvent, and non-condensable gas is then subject to adsorption or secondary condensation for quality control. The exhaust buffer tank of the vacuum pump also serves as a pre-separator. The selection of the working fluid for the pump should consider solvent miscibility to avoid performance degradation due to contamination of the working fluid. The hot air exhaust from the oven and the exhaust from the vacuum pump are collected and metered separately, ensuring clear accounting of recovery and loss.

Condensate is a mixed solvent, which is tested and graded by batch: batches dominated by a single solvent are directly reused, while those with mixed components are blended according to their purposes or sent for refining. The disposal destination is determined during the planning stage. The storage time of condensate tanks is also managed - the temperature of condensate in the solvent wet material drying process is high, and the quality and safety risks of long-term storage are significant. The principle of quick collection and quick transfer can be followed by copying the practices of the main condensation line.

Collection configuration reference

How to implement the configuration

1. Each drying oven is connected to a separate pipe for collection, and the total pipe load is calculated based on the contemporaneous rate;
2. The exhaust gas is precooled and then enters the condensation section, with the pipeline slope and condensate drainage in place;
3. The subsequent stage of the vacuum pump is condensation, with non-condensable gas being monitored through series adsorption;
4. The two exhaust branches are metered separately, with separate accounts for recovery and loss;
5. The renovation of windows should be scheduled in advance, with prefabrication taking precedence and only installation without processing on site;
6. The condensate is tested batch by batch, and then reused or sent out for disposal according to its grade, with the final destination documented.

Frequently Asked Questions

Can't we directly send the high-temperature dry exhaust gas into the condensation section?

No, the intake temperature exceeds the design operating conditions of the condensing unit, affecting both refrigeration efficiency and equipment safety. It is standard practice to cool down first and then recover: the pre-cooling stage brings the temperature to an appropriate range, and the cooling process itself can recover some of the condensate, achieving two goals with one action.

With significant batch-to-batch fluctuations, how can the equipment keep up?

The main body of the device is selected based on the median load of multiple batches, with the peak segment being undertaken by the buffer and gate-keeping segment. In terms of control, it is adjusted in conjunction with the inlet gas concentration, and when the load is low, it operates at a reduced speed to save energy consumption. The actual concentration measurement data before commissioning serves as the input for model selection, which is more reliable than estimation.

Is the collection and renovation of the existing drying room a big undertaking, both in terms of effort and impact?

Most of the modifications are minor: the exhaust port of the drying oven is connected to a pipe for leading out, and the pre-cooling section and recovery device are arranged outdoors. The modifications are mainly focused on the air duct and connections, and the production impact can be absorbed through the downtime for maintenance. During the planning phase, the opening and pipeline routing should be confirmed with the user, ensuring that the construction period is manageable.

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.