How to reduce the loss of toluene in exhaust gases?

Secondary condensation means adding another condenser after the receiving tank.

I agree with this plan; it is best to use chilled brine as the cooling medium for the condenser on the collector.

Using secondary condensation with chilled water is a reasonable solution. Last edited in this post by *aohe8888 on 2009-1-6 00:02.]

:If no toluene is detected after the second cooling step in the handshake process, what could be the problem?

The exhaust flow rate is too high, resulting in poor heat exchange efficiency.

The effect of using two-stage condensation is very poor, and it is not recommended. We have encountered similar situations in production; when two-stage cooling was employed with chilled water at 5°C, there was almost no effect – very little organic solvent was condensed, and the exhaust gas still had a strong odor of organic solvents. Why is that? Because distillation is carried out under vacuum conditions, the gas coming out after cooling contains not only toluene but also many non-condensable gases. These non-condensable gases act as a carrier gas in this process. You are familiar with carrier gas distillation or the gas stripping of organic compounds, right? Due to the presence of these non-condensable gases, the vapor pressure of the organic compounds in the gas is altered and reduced, which is equivalent to a decrease in the partial pressure of the organic solvents. For the organic solvents to condense, their partial pressure must reach the saturation vapor pressure; as a result, the organic solvents in the gas become more difficult to condense because of these non-condensable gases, leading to a significant reduction in their condensation efficiency. For the treatment of the small amount of toluene present in your exhaust gases, the best and simplest method is still activated carbon adsorption. Due to the small quantity involved, the economic viability of recovering the toluene after adsorption is low; therefore, if permitted by the local environmental protection authorities, it is possible to simply dispose of the activated carbon along with the exhaust gases in a boiler as fuel. Otherwise, steam heating must be used to release the toluene trapped in the activated carbon. After the vapor is condensed, the resulting mixture separates into a toluene phase on top and an aqueous phase containing a small amount of toluene at the bottom; this aqueous phase can either be returned to the system or sent for wastewater treatment ； Another option to consider is to use a high-boiling-point fuel oil instead of activated carbon in the absorption tower; this fuel oil is used to absorb toluene from the cooled gas through a downstream absorption tower. As a result, the organic compounds in the exhaust gases end up in the absorption liquid, and the toluene that has been absorbed does not enter the vacuum pump system. The fuel oil that has absorbed toluene is then sent to the boiler to be burned as fuel.

:Handshake, thank you for your analysis and corrections. I’m planning to build a simple activated carbon adsorption unit. Do you have any suggestions?

Hello, I’ve read your suggestions regarding the removal of toluene from exhaust gases. It’s true that using activated carbon isn’t an effective method; I think using fuel oil for absorption is indeed a good approach! So, this would require a packed tower for spraying, right?

Secondary condensation is achieved using chilled brine, that is, an additional condensing coil is added inside the receiving tank to further reduce the temperature. The tank should have a large volume, ideally around 2000 liters. The internal coil is in the shape of a cone with two pyramid-like tips connected to each other; the pipe through which the exhaust gas enters the tank extends about 40-60 centimeters into the buffer tank, which improves the condensation effect. This allows for the recovery of a large amount of material while achieving the best results.

Everyone is considering adding a secondary condenser after the primary condensation, but due to the fast flow rate of the gas, this approach will inevitably yield poor results. A condensation unit can be installed on the vacuum exhaust pipeline, using low-temperature chilled water as the cooling medium (it is essential to consider slowing down the exhaust gas flow rate in order to enhance heat exchange). After testing through practical application, we adopted this method, and the recovery effect was significant.