Over time, thermal cycling causes micro-gaps to form in the manhole gaskets, resulting in continuous gas seepage. This is not a leak that develops later in the process; rather, its impact is minimal during the dilute feed stage. As the system enters the final stage of concentration and operating conditions change, the negative effects of gas seepage become concentrated and pronounced, giving the appearance on-site that the leak only occurred during the late concentration phase.

First, the physical properties of the feed solution undergo significant changes. In the early stages, the feed solution has low salt content and low viscosity, resulting in a relatively thin foam layer generated by boiling, where bubbles tend to burst spontaneously. In the late stages of concentration, however, the ion concentration rises sharply. Combined with residual complexing agents and trace organic impurities in the system, the foam becomes significantly more resilient—dense and less prone to collapse. When external air infiltrates the chamber, it forms a large number of microscopic bubbles that continuously integrate into the boiling liquid layer, steadily increasing the height of the foam layer. Even a minor air leak can cause the foam to rise rapidly, overflow the demister screen, and result in clearly visible product carryover.

Second, there is a massive difference in the gas-phase load and vacuum tolerance within the chamber before and after the leak. During the feed stage, the solution has a high water content; continuous evaporation produces a large amount of water vapor, which occupies the vast majority of the gas phase space. The small amount of infiltrating air is rapidly entrained and drawn away by the steam, causing only limited disturbance to the overall vacuum. Toward the end of the concentration process, the water content decreases significantly, steam production continues to decline, and the partial pressure of steam inside the chamber drops; for the same volume of air infiltration, the proportion of non-condensable air increases markedly. While vacuum pumps are highly efficient at removing water vapor, the load of extracting air is much greater, directly causing continuous vacuum drift and making it difficult to maintain a stable negative pressure, leading to a concentration of various chain-reaction abnormalities.

Furthermore, changes in liquid level alter the flow pattern. As the batch concentration process progresses to its later stages, the total volume of feed liquid inside the chamber decreases, and the available space in the gas phase increases. Infiltrating air is not rapidly absorbed by the liquid surface, making it prone to forming turbulence and vortices in the upper part of the chamber. These continuously impact the foam layer and the demister mesh, shattering liquid droplets and forming large amounts of ultrafine salt-containing aerosols, which drastically exacerbates the problem of mist entrainment. When the feed liquid level is high, the gas phase space is narrow, and the gas flow is relatively stable, so the agitation effect of trace air infiltration is hardly noticeable.

At the same time, saturated mother liquor is more sensitive to vacuum fluctuations. In the late stages of concentration, as the solution approaches salt crystal saturation, its boiling point becomes significantly more sensitive to changes in negative pressure. Air ingress causes a slight drop in vacuum and a rise in boiling point, leading to a sudden intensification of boiling within the chamber and increased turbulence at the liquid surface; whereas during the dilute feed stage, when saturation is low, minor vacuum fluctuations do not result in drastic changes in the boiling state.

It is also worth noting the dynamic changes in the sealing conditions themselves. During the heating phase, the gasket expands due to heat, causing minute gaps to close to some extent; during cooling, the gasket contracts, causing the gaps to open slightly. In the late stages of concentration, the system is often maintained at a stable temperature for extended periods, lacking the sealing effect provided by gasket expansion caused by significant temperature increases, so the micro-leakage pathways remain relatively fixed. The aging and hardening effects of the gasket, accumulated from daily thermal cycles, will also gradually become more pronounced during prolonged operation at high concentrations.

Common Misconceptions on Site

Many operators assume that there were no leaks in the early stages and that the gaskets failed only during the late concentration phase. In reality, gas leakage has existed from the very beginning; however, the dilute liquid conditions provided a buffering effect that masked the defect. As the high-concentration mother liquor exhibits deteriorated foaming characteristics, reduced steam production, and weakened vacuum stability, the disturbances caused by trace gas leakage can no longer be offset, leading to concentrated exposure of issues such as carryover and unstable vacuum.

Derived Chain Reactions

Gas seepage in the later stages triggers vacuum oscillations, further impairing the gas-liquid separation efficiency of the demister and causing the conductivity of the condensate to rise rapidly. The persistent presence of non-condensable gases within the chamber increases heat transfer resistance, leading to a simultaneous rise in unit energy consumption. It also causes the vacuum pump load to remain consistently high, increases the concentration of non-condensable gases in the tail gas, and accelerates the failure of the upstream activated carbon adsorption layer.