A case study of activated carbon fiber adsorption - steam desorption for toluene recovery in the Pharmaceutical industry

In the waste gas treatment of raw materials in the pharmaceutical industry, activated carbon fiber (ACF) has significant advantages over traditional activated carbon, as follows: Core advantages and technical characteristics:

Enhancement of adsorption rate and capacity
The specific surface area of activated carbon fiber can reach 1300-1500m²/g (BET method), far exceeding granular activated carbon (about 800-1000m²/g), and the microhole distribution is more dense, which can quickly capture low-concentration volatile organic compounds (VOCs) in waste gas. For example, the adsorption rate of medical solvents such as acetone and ethanol is increased by 2-3 times.

Precise selective adsorption
By regulating the surface functional groups (such as carboxyl, phenolic groups), the specific components (such as antibiotic intermediates, residual solvents) in the medical waste gas are preferentially adsorbed.

Physical-chemical stability
Resistant to strong acids, strong alkalis and high temperatures (can withstand up to 300℃), adapted to the complex waste gas environment in raw material production (such as mixed waste gases containing acidic and alkaline vapors)

Regeneration convenience
Activated carbon fiber can be regenerated in situ by steam desorption, with a solvent recovery rate of ≥95%, and the adsorption performance decay is less than 5% after regeneration, while the performance of granular activated carbon usually decreases by 10-20% after regeneration.

1. Process Description

The process treatment unit adopts an adsorption recovery system with two carbon fiber adsorbers as the main body, and all the adsorption-desorption-drying regeneration processes are completed inside the adsorber. The process flow of the activated carbon fiber organic waste gas adsorption recovery device is shown in the figure below.

The organic waste gas recovery device of active carbon fiber is composed of2adsorbers to form a pipeline system together. When adsorberA adsorbs, adsorber B desorbs and regenerates. The adsorption, desorption and regeneration of each adsorber are performed in turn. Organic waste gas generally enters the adsorber from the bottom, where organic matter is adsorbed by the active carbon fiber felt,and the purified tail gas is discharged from the top of the adsorber. The desorption steam enters from the top of the adsorber, passes through the active carbon fiber felt, and desorbs the organic matter adsorbed and brings it into the condenser, where the organic matter and water vapor are condensed and flow into the layering tank for separation by gravity settling. The organic phase separated out is transferred to an organic solvent storage tank, and the water phase separated out is discharged as sewage to the factory's chemical sewage system for centralized treatment and then discharged. The entire process is automatically controlled by the DCS program, alternately performing the adsorption, desorption and drying process.

2. System operating parameters and safety assurance

2.1 System operating parameters

(1)Exhaust gas composition and treatment requirements: This exhaust gas contains toluene, trace amounts of penicillin sulfoxide, ammonia, of which the emission mass concentration of toluene is about10000 mg/m3, the exhaust gas flow rate is5000 m3/h, and the exhaust gas temperature is 50 ℃.

(2)Determination of the gas volume to be processed: Based on the properties of toluene, the flow rate and concentration of the waste gas, and considering a 10% operational margin for the adsorption system, the actual waste gas treatment capacity entering the adsorption device is determinedQ = 5500m3/h.

(3)System resistance: The system resistance includes the resistance of the pipeline system and the adsorber itself. According to the calculation and practical experience, the resistance of the whole system is determined as5000Pa.

（4）Empty tower gas velocity: According to the requirements of HJ 2026-2013 "Technical Specification for Engineering of Adsorption Method for Industrial Organic Waste Gas Treatment", "the gas velocity of the adsorption layer (activated carbon fiber) of the fixed-bed adsorption device should be less than 0.15 m/s".
Based on this design, the gas velocity of the empty tower is determined to be 0.15m/s.

(5)Adsorption temperature:35 ℃.

(6)Desorption temperature:110 ℃.

2.2 Safety of the adsorption device

According to the technical regulations such as HJ 2000-2010 Technical Guide for Engineering of Atmospheric Pollution Control and HJ 2026-2013 Technical Specification for Industrial Organic Waste Gas Treatment by Adsorption, in order to operate the adsorption system effectively and safely, the following issues should be paid attention to:

(1)Consider the explosion limit of toluene-containing waste gas.The explosion limit of toluene is 1.2% -7.0%. Therefore, the corresponding toluene mass concentration should be below 25% of the lower explosive limit. The lower limit concentration calculated by volume fraction is 49360mg.

(2)Ensure the sealing of the system in place. Adsorption, desorption, and recovery. Since the entire system is always in frequent switching, special structural seals are designed to ensure that the system has good sealing, orpositive pressure ventilation is adjusted to negative pressure suction, to minimize the gas leakage of the entire system and ensure the safe operation of the equipment.

(3)Automation of the control system. The adsorption-desorption-regeneration process of this system is designed to be automatically controlled by the DCS system. When operation faults occur, the program automaticallylee automatically alarms and.

(4)Control of system temperature. The adsorption process is an exothermic process, and the bed temperature will continue to rise during continuous adsorption operation, resulting in a decrease in adsorption efficiency and also posing a risk to the safe operation of the system. The system usesbed temperature alarm device, when the temperature exceeds the set value, the system will automatically alarm and automatically switch to a safe position; at the same time, the cooling device is started to ensure the normal and safe operation of the system. Specific design regulations:

①High temperature alarm: When the adsorber is performing the adsorption operation, its maximumtemperature must not exceed 60 ℃（the boiling point of toluene is about 110℃），if the temperature ≥ 60 ℃, an alarm should be given.

②High temperature alarm interlock control: When the toluene temperature at the outlet of the cooler exceeds 60 ℃, an alarm is triggered, and at the same time, the signal is sent to the DCS, which issues a command to close the steam valve.

3. Operation of the device

The active carbon fiber toluene adsorption and recovery device has been officially installed and has been in operation for one year, with stable operation and high toluene recovery rate. The recovered toluene can be directly recycled back into the production system.

The recovery rate of toluene in the tail gas by the activated carbon fiber device is very high,stabilizing at 97% in most cases, but the emission concentration of toluene in the tail gas does not meet the second-level standard in Table 2 of GB 16297-1996 "Integrated Emission Standards for Air Pollutants", so it is necessary to further take control measures to make it meet the standard emission, and end adsorption can be considered.

4. Economic analysis

(1)The value of toluene recovered annually: According to the operation analysis, the amount of tail gas emitted by this pharmaceutical intermediate enterprise is 10000 m3/h, and the average mass concentration of toluene in the tail gas is

From10000 mg/m3 to30 mg/m3, if calculated this way,390 t of toluene can be recovered per year . The price of toluene is assumed to be 6000 yuan/t (the average price in recent years),the value of toluene recovered per year is234 ten thousand yuan.

(2)Equipment investment: less than 2 million yuan.

(3)Annual operating costs: electricity cost is about 115,200 yuan; steam cost is about 155,000 yuan; cooling water, instrument operation cost is about 70,000 yuan; equipment depreciation, capital interest, maintenance cost is about 2,000,000 yuan; the total is 5,400,000 yuan. 

(4)Net benefit per year: 234 - 54 = 180 ten thousand yuan. The investment cost can be recovered within one year.

Suggestions: When selecting activated carbon fiber for pharmaceutical enterprises, it is necessary to customize the surface modification scheme according to the composition of the waste gas (such as increasing alkaline functional groups for nitrogen-containing waste gas), and regularly monitor the iodine value of the fiber (recommended ≥1000mg/g) to ensure adsorption efficiency