Yes, and this is a very common potential hazard in low-temperature, negative-pressure evaporation systems. As the light organic phase accumulates in the upper layer, it continuously desorbs and vapourises, returning via the vapour line at the top of the tank**. This is not a one-off flash, but rather a steady-state, partial-pressure-driven continuous volatilisation, which is significantly exacerbated by fluctuations in operating conditions.

This differs slightly from the adiabatic flash mechanism seen in feed-depressurisation systems and can be understood in two parts:

The condenser tank itself is pressure-communicating with the vacuum side, meaning the interior of the tank is under the negative pressure of the evaporation system. When light components float to the upper layer to form a distinct organic phase—no longer diluted by the aqueous phase—their activity approaches 1; at the same temperature, the saturated vapour pressure of the organic components is significantly higher than when they are dissolved in water. The vapour phase within the tank will gradually become saturated with this organic vapour. As long as the vapour phase is not fully captured by the condenser, this organic vapour will flow along the equilibrium line at the top of the tank towards the vacuum side, entering the main vacuum line, the vacuum pump or the pre-cooling trap.

True **violent reverse flash (massive instantaneous vapourisation)** is generally triggered under disturbed conditions: excessively high condensate temperature, fluctuations in cooling water temperature, a sudden increase in vacuum, a rapid drop in liquid level, brief pressure fluctuations within the tank during drainage, or a very thick organic layer leading to localised heat accumulation, where the liquid phase temperature exceeds the bubble point of the organic compound at the current negative pressure. In such cases, noticeable flash bubbling occurs, causing the vapour load to surge instantaneously. Under stable operating conditions, the phenomenon is more commonly characterised by **continuous desorption and volatilisation**. Many operators mistakenly assume this is merely a small amount of non-condensable gas; however, long-term accumulation can lead to vacuum pump oil contamination, seal swelling, excessive loads on downstream cold traps, and a rebound in COD levels in the condensate on the evaporation side.

There are several structural details where pitfalls are common:

In many low-temperature evaporation systems, the vapour-liquid equilibrium port of the condenser tank is located directly at the top of the tank, precisely above the organic enrichment layer. This effectively draws vapour directly from the organic phase, making this configuration highly prone to vapour backflow. Whilst the risk is significantly lower with a bottom-inserted immersion tube and liquid seal configuration, the organic layer gradually rises; once it submerges the tube inlet, the liquid seal fails and the risk increases immediately. Furthermore, in batch discharge mode, the liquid level in the tank drops instantly upon discharge, increasing the vapour space and causing a sudden surge in vacuum flow rate, which can carry away large quantities of surface organic vapour.

The corresponding engineering control approach, which is also commonly used on site, is as follows:

Prioritise **phase separation**; fit the condenser tank with a side-line outlet for the oil phase to discharge the upper layer of light organic vapour at regular intervals or continuously, preventing it from accumulating in the tank over time; install a secondary trap or mist eliminator on the tank top vent line to specifically capture this rising organic vapour; operate the condenser tank at as low a liquid level as possible to minimise the thickness of the organic phase; control the supercooling of the condensate to lower the temperature of the condensation tank further, thereby reducing the saturated vapour pressure of the organic phase; avoid a simple direct connection between the condensation tank and the vacuum line; instead, install a liquid seal pipe or a barometric leg to prevent direct gas-phase exchange.

I would like to add a common phenomenon observed in this scenario: sometimes the main condenser appears to be cooling adequately and the effluent COD is very low, yet the oil on the vacuum pump side discolours rapidly and VOC levels consistently exceed limits. The root cause is often the long-term desorption and return of vapour from the upper organic phase in the condensate receiving tank, as the main condenser is unable to capture this secondary vapour.