Returning acid washing waste liquid to the stock tank will indeed alter the foaming characteristics of the system and is highly likely to exacerbate carryover during evaporation; the overall effect is the cumulative result of multiple factors, rather than being caused by a single change in pH. The acid washing process strips away silicate scale, metal oxides and small amounts of organic colloids deposited on heat exchange surfaces and chamber walls. The washwater contains large quantities of dissolved metal ions, as well as the stripped-off fine colloidal particles, residual acid and acid washing additives, such as corrosion inhibitors and surfactants. Once these substances enter the feed tank, they continuously alter the surface tension and foam film strength of the feed solution, making it easier for a stable foam layer to form under vacuum evaporation conditions.

Firstly, there is the impact of metal ions: iron and aluminium ions leached during acid washing enter the feed solution and, under the solution’s original pH conditions, hydrolyse to form fine hydroxide colloids. These colloids act as foam stabilisers, adhering to the liquid film on the bubbles to increase its toughness; once bubbles form inside the vacuum evaporation chamber, they do not burst easily, and the foam layer continues to thicken. causing the apparent liquid level to be falsely elevated. The automatic level control system detects an erroneous signal and continues feeding the feedstock, making it even easier for the foam to pass beyond the gas-liquid separation section and be carried along by the vacuum airflow—a phenomenon known as ‘evaporation with carryover’. Compared to the native foam formed in the feedstock itself, the foam formed by these colloids ruptures much more slowly, and the effectiveness of conventional defoamers is significantly reduced.

Secondly, there is the effect of residual acid-washing agents. Acid-washing formulations often contain small amounts of wetting and penetrating additives; even after the main acid has been neutralised, trace amounts of surfactant components remaining in the feed solution reduce its surface tension, making it easier for bubbles to form. Many corrosion inhibitors consist of nitrogen-containing organic compounds, which possess a certain degree of foam-stabilising capacity. As these residues accumulate continuously within the raw liquid system—particularly following repeated recycling of washed waste liquid—the concentration of foam-stabilising substances gradually increases, causing the carryover problem to become progressively more pronounced with each cycle. If the acid wash is not thoroughly rinsed, residual acid entering the stock solution tank will also alter its pH. The silicate colloids originally present in the stock solution undergo a phase transition when the pH changes, forming ultrafine suspended particles which further enhance foam stability.

Another point that is easily overlooked on site is that the scale debris stripped away by the acid wash—micrometre-sized solid particles suspended in the stock solution—act as a scaffold for the bubbles. Whilst pure liquid-phase foam tends to collapse easily, when solid microparticles become embedded in the bubble walls, the bubbles’ tear resistance increases significantly. Under negative pressure suction, the foam is less likely to break apart, and fine droplets, together with the solid particles, are drawn directly into the condensation system, causing contamination of the condensate. In severe cases, this can lead to blockages in the vacuum piping and the activated carbon adsorption bed.

Changes in the system can also produce a lag effect; adding a small amount of acid washing waste liquid in a single batch may not immediately result in observable changes to the foam. However, after multiple washing cycles, colloids and metal ions continue to accumulate, and foam problems may suddenly erupt. Many sites misinterpret this as fluctuations in the quality of the feed solution, overlooking the root cause—the reuse of acid washing waste liquid. Once foaming intensifies, the load on the gas-liquid separator exceeds its design capacity. Material carried over by the overflow deposits on the condenser side, further reducing heat transfer efficiency and exacerbating vacuum fluctuations, thereby creating a chain reaction of faults linked to the interference from non-condensable gases in the vacuum system, as discussed previously.

It is also important to distinguish between different operating conditions. If the acid washing waste liquid has been thoroughly neutralised, filtered to remove suspended solids, and the proportion returned to the tank is controlled—with small amounts added in each batch—changes in foaming characteristics will be relatively minor. However, whenever the waste liquid carries a large amount of fine scale and dissolved metal ions and is returned directly to the stock tank, the risk of carryover increases significantly. The prudent approach is generally to collect and treat the acid washing waste liquid separately, rather than returning it directly to the stock tank. If reuse is being considered, a sedimentation and filtration unit should be installed to remove suspended solids, metal ion concentrations should be monitored online, and the capacity of the defoaming system should be verified accordingly.