Three-effect evaporation and MVR evaporation of sodium sulfate

MVR is good – it saves energy and has low operating costs. However, it is not recommended for use when the evaporation volume is too small; for evaporation and crystallization processes, MVR remains the suitable choice

MVR, but with much higher investment costs

Last year, I was involved in tuning up a set of MVR evaporation crystallization equipment used for treating sodium sulfate; the salt production results were quite good. It was used for treating the wastewater generated from the soap-making process at Shandong Zouping Samsung Oil, after fatty acids had been extracted, with a treatment capacity of around four tons per day. But there are also issues. Indeed, the steam consumption of MVR is low, yet the power required by the steam compressor and circulation pump is not insignificant. If your company has an ample supply of heat, three-effect evaporation could be considered; overall, however, MVR is more cost-effective. Additionally, the crystal salt produced needs to be processed as well, so it’s best to think this through before making any plans

The discharge ratio is set at 1:3 between solids and liquids. Can it be fed directly into a centrifuge for crystal separation? Is it necessary to use a hydrocyclone?

Oh, I see. We use MVR, film evaporation, followed by a crystallizer of the OSLO type; then the fluid is forced to circulate back into the film evaporator. The secondary steam coming from the top of the crystallizer goes into steam compressor 94, where its temperature is raised to around 102 degrees, in order to heat the film evaporator. After that, the heated fluid is sent to a preheater to preheat the feed material. Efforts are made to use the heat source in a cost-effective manner. As for discharging the material, it is done in a batch process, with a cycle time of two hours; the processing capacity is around 8 cubic meters. The material is fed into a slurry thickener with a capacity of 2 cubic meters, and the solid-liquid ratio is maintained at 25%. The secondary steam is converted into condensed water, making this processing step simple. As for the centrifuge, we initially used a horizontal type with filters, but since the material being processed was oil waste containing a lot of organic matter, it clogged the filters. Later, we switched to a horizontal screw-type centrifuge, which gives much better discharge results

It is better to add a swirl chamber, as this allows for a higher solid content in the output material, resulting in more stable operation of the centrifuge. As for the recommended solution, MVR is the best choice; although its investment cost is twice that of a three-effect system, its operating costs are low, and it is possible to recoup the initial investment within 1–2 years.

Using falling film technology can help reduce operating costs in your case, but for materials containing a high amount of organic matter, isn’t it necessary to treat them first? As the concentration increases, those organic substances get increasingly concentrated, which will raise the boiling point significantly, thereby affecting the evaporation efficiency. Isn’t any preliminary treatment required?

Does salt deposition on the wall affect heat transfer during the concentration process? Currently, the efficiency of MVR concentration is very low

Specifically, which salt is it? Sodium sulfate does not remain on the walls of the heat exchange tubes; could it be calcium and magnesium scaling, or some other salt? It’s also possible that the composition of your materials has changed

Sir, for your project, what concentrations of solution are achieved after evaporation at the first and second stages? We are also working on a system for evaporating Na2SO4 to dryness