Should a horizontal or vertical evaporator be used for evaporating high-salt wastewater?
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This post was last edited by tudou0710 on 2016-10-10 at 10:17. What we are designing is a zero-discharge project for high-salt wastewater from coal chemical plants; after pretreatment and concentration, the wastewater is fed into an evaporation crystallization system. The salt content in the wastewater entering the evaporator is 10%, with NaCl accounting for 99% of that amount, Na2SO4 making up less than 1%, along with very small amounts of sodium nitrate and sodium carbonate. We have been using vertical evaporators in the past; now some manufacturers are suggesting vertical evaporators to us, as they can reduce the height of the factory building, facilitate monitoring and maintenance, and result in better water quality of the condensed liquid. However, our research shows that vertical evaporators are still widely used. So the first question I would like to ask is, is it better to choose a horizontal or a vertical evaporator? Can both falling film evaporation and forced circulation evaporation use vertical evaporators? Could you please explain in detail? Thank you! Additionally, due to the lack of a steam source on site, we have chosen MVR evaporation. Since we have no experience with MVR evaporation, I have a few questions: (1) Where should the start-up steam and make-up steam be supplied? Is it the inlet, outlet, or heat exchanger of the centrifugal compressor? What’s the difference? (2) Selection between imported compressors and domestic compressors? What is the quality of domestic compressors? Is the response time for maintaining and replacing parts for imported compressors very long?
1. Compressor inlet. 2. A domestic MVR will suffice; the average annual maintenance cost is around 10,000.
Are there any requirements for the steam supplied at the compressor inlet? Can low-pressure steam be used?
For evaporators: if the vaporization rate is greater than 30%, use a horizontal kettle-type evaporator. Vertical types generally have a low gasification rate, but they can operate in multiple stages. The advantages of a horizontal design lie not only in its height but also in the ease of maintenance. BKU format, with core extraction possible. Vertical versions are generally not feasible, unless it’s a BEU vertical type.
1. In both natural circulation and falling film evaporators, vertical equipment is used, as a height difference and density difference are required; this results in a high circulation rate and efficient heat transfer. At the same time, the rising vapor film does not easily tear the liquid film, which facilitates the process of evaporation. At the same time, a certain space for gas-liquid separation is required. 2. Horizontal evaporators: Steam has an impact on heat transfer across the tube walls, which can disrupt the liquid film; unless forced circulation together with special tube types (porous surface tubes or grooved tubes) is used, the energy consumption and economic efficiency are very poor. 3. The horizontal dry and wet evaporators used in refrigeration units are different from those used for evaporating high-salt wastewater; since this type of wastewater flows outside the tubes, it will cause blockages, and cleaning it is quite difficult. When using horizontal pipes within the system, evaporation in such pipes will inevitably encounter the aforementioned problems. 4. Vertical equipment must be used; conventional options are sufficient, and there is no need for anything new.
I’ve never seen the horizontal evaporator you mentioned; our factory also uses MVR systems, and both falling-film evaporators and forced-circulation evaporators are of the vertical type. I’m not sure if it’s due to the high salt content (over 100,000 units), but the falling-film evaporator gets clogged frequently, and the steam compressor experiences frequent surging. Still looking for the reason.
1. Falling-film evaporators are definitely vertical. The key design aspects are the liquid circulation system, including the circulation volume, membrane distributor design, and installation height. 2. For forced circulation, the design is based on the crystallizer; in addition to considering heat transfer, scaling, and installation height, factors such as crystal growth control, circulation flow rate, and the selection of circulation pumps also need to be taken into account. Vertical single-pass single-heater designs are commonly used both domestically and internationally. 3. For MVR fans, domestic single-stage models can be used, or two imported units connected in series is also an option; imported units offer higher reliability, but price and delivery time need to be taken into consideration.
Of course, MVR products imported are of high quality, but their prices are high and the delivery times are long. Domestic versions are said to be good these days as well, but I’m not sure; I haven’t used any domestic ones yet!
Could a blockage in the falling film evaporator be caused by an excessively high TDS level in the evaporated liquid? Frequent compressor surging may be due to an insufficient steam supply, failing to meet the compressor’s rated steam requirement?
The cause of the blockage in the falling film evaporator is still under investigation; it’s certain that the TDS level is high. After the first jet cleaning, the evaporation rate was quite good, but it gradually declined again. I took it apart a couple of days ago and found that more than half of it was blocked once again. No good solution has been found yet. Surge: By increasing the steam volume, surge was indeed alleviated to some extent. (Doubled compared to before).
I have been working in the MVR industry for 5 years, and I have had a lot of contact with design institutes in the past. Just by looking at the way the question was phrased, it’s clear that it follows the style of design institutes. 1. Vertical evaporators are generally used more often, as when evaporating high-salt wastewater, the salt particles can settle due to their own gravity. This advantage is especially noticeable in cases of emergency shutdowns or power outages, whereas horizontal evaporators would get clogged. Vertical heat exchangers used for falling film evaporation differ significantly from those with forced circulation, and their working principles are also different. In a falling film evaporator, the liquid feed enters through the feed pipe and then overflows from the gaps between the teeth of the dispersing disk; it falls through the gap between the dispersing disk and the retaining ring onto the upper dispersing disk, where it undergoes initial distribution. From there, it flows across the sieve plates on the upper, middle, and lower dispersing disks before overflowing onto the plate bridges of the tube sheet ； The forced circulation heat exchanger is of the traditional type. 2. The steam for starting up is supplied at the plate exchanger where the material is preheated, while the supplementary steam is provided at the outlet of the compressor. 3. The advantage of using a compressor at the inlet side is its suitability for transporting gases such as hydrogen (which are hazardous), whereas the compressors used in MVR systems are domestically produced and already well-developed, with many application examples available. Imported compressors are expensive, have long production cycles, and come with complicated after-sales service. Domestic options are now trustworthy; feel free to contact me if you need anything. QQ: 691951325. On-site inspections are also possible