Comparison of MVR evaporators and vapor compressors-Hezhong Environment (Beijing) Co., Ltd. was established in 2003

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Comparison of MVR evaporators and vapor compressors

Published Time:

2024-09-13

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Abstract: The steam compressors selected for MVR evaporators include centrifugal steam compressors, Roots steam compressors, and single-stage screw steam compressors. The first two are commonly used compressors in recent decades, while the last one has only been used in MVR evaporators in recent years. Its biggest feature is the higher temperature rise during the evaporator operation. The MVR evaporator compresses all secondary steam through a steam compressor as a heating heat source to heat the evaporating liquid in the evaporator shell side. The MVR evaporator needs continuous feeding and discharging, and the material must be heated to the boiling point or above.

In practical applications, because the preheating temperature of the material before entering the evaporator is low, some have not reached the boiling point or above, resulting in the inability to start the compressor in time, and even more cases of compressor surge during operation due to insufficient secondary steam. Some evaporators are relatively small, and the using units do not have steam sources. Most of them are steam generators provided by manufacturers. Some steam generators have too small gas production, resulting in the preheating temperature not reaching the required temperature for a long time, and the steam compressor cannot be started on time. This is also common. The preheating effect of the liquid before entering the evaporator is directly related to whether the MVR evaporator can operate normally. Even if the preheating effect is good, due to the increase in boiling point, heat loss, and the needs of the compressor itself during production, a certain amount of additional steam must be added, otherwise, evaporation is difficult to proceed normally. To achieve the ideal evaporation state, among many influencing factors, the amount of spray water in the steam compressor that participates in heating is also important. This article elaborates on the MVR evaporator and the application of three compressors to provide a reference for similar applications.

Comparison of the application of different steam compressors in single-effect MVR evaporators
The steam compressors selected for MVR evaporators include centrifugal steam compressors, Roots steam compressors, and single-stage screw steam compressors. The first two are commonly used compressors in recent decades, while the last one has only been used in MVR evaporators in recent years. Its biggest feature is the higher temperature rise during the evaporator operation, and no additional steam needs to be added. This article elaborates on the heating temperature rise and application of the three compressors, taking the MVR01-2000 single-effect forced circulation evaporator for evaporating salt-containing wastewater as an example.

The outlet temperature rise of centrifugal steam compressors and Roots steam compressors is low, generally around 21℃, and additional steam needs to be added during operation, otherwise, it is difficult to meet the normal evaporation needs. The last one is a single-stage screw steam compressor. This compressor has a high temperature rise and does not need to add steam during use, but its power consumption and cooling water consumption are greater than the first two compressors with the same production capacity. Below, taking an evaporator with an evaporation capacity of 2000kg/h and a single-effect forced circulation type as an example, the two compressors are used for heating and evaporation for comparison.

Main technical parameters are as follows:
① Material medium: salt-containing industrial wastewater; ② Production capacity: 2000kg/h; ③ Inlet temperature: 95℃; ④ Inlet mass fraction: 8%; ⑤ Inlet viscosity: 10cp; ⑥ Outlet mass fraction: 45%; ⑦ Evaporation temperature: 85℃; ⑧ Boiling point temperature: 92℃; ⑨ Shell-side heating temperature: 105℃ (saturated steam temperature after secondary steam is compressed by the compressor and goes to the shell-side heating); ⑩ Compressor temperature rise: 20℃.

Comparison of centrifugal steam compressor and single-stage screw steam compressor
The main parameters of the centrifugal compressor with an inlet temperature of 85℃ and an outlet temperature of 105℃ are: power is 185kW, and cooling water consumption is 19T/h. In the MVR evaporator using a centrifugal compressor, because the amount of cooling water vaporized during the process of saturated steam compression overheating and then becoming saturated steam is very small, the amount of steam participating in heating is insufficient, and it is difficult to meet the evaporation needs. Therefore, a certain amount of saturated steam needs to be added during production to maintain normal production.

However, if a single-stage screw steam compressor is used, because the amount of cooling water vaporization participating in heating is relatively large, and its temperature rise is also relatively high, no raw steam needs to be added during production. Taking the above parameters as an example, the required temperature rise is calculated and then compared.

Heating steam consumption: D=36.3kg/h (1)

This is the case with feed above the boiling point. In many cases, even with preheating, some can only reach the boiling point temperature close to evaporation. The material temperature before entering the evaporator is very low, and preheating is required. Preheating close to, equal to, or higher than the boiling point temperature is beneficial to evaporation, especially for MVR evaporators. Low-temperature materials cannot start the steam compressor without preheating. The higher the preheating temperature, the smaller the heating steam consumption. The source of preheating heat is mostly primary saturated steam used in the final stage of preheating, in addition to waste heat recovery. A single-stage screw steam compressor needs to provide 2136.3kg/h of heating steam to meet and maintain normal evaporation.

A domestic company provides 130kg/h of saturated steam for cooling water vaporization participating in heating, with a power of 115kW, and its total saturated steam is: 2000+130=2130kg/h (2)

The heat of 105℃ saturated steam is: 2130×535.8=1141254kcal/h, which is close to the actual heat required to maintain evaporation. Therefore, under good insulation and heat insulation conditions, no additional raw steam needs to be added during production, which is also the case in practical applications. However, additional raw steam is still needed for feed below the boiling point.

Double-effect MVR form
Currently, MVR evaporators employing double-effect evaporation primarily utilize compressors to simultaneously compress the primary and secondary steam from the first and second effects, respectively, serving as the heating sources for each effect. The material typically flows concurrently, passing through the first-effect evaporation before entering the second; countercurrent feeding is also possible. This evaporation can involve a single evaporator type or a mixed type (the evaporation system contains two different evaporators), with the latter being more common. For instance, in the evaporation of industrial saline wastewater, falling-film evaporators and forced-circulation evaporators are often combined. Initially, with lower feed concentration and low viscosity, and without crystallization, falling-film evaporators are preferred due to their high evaporation rate and efficiency. Compressing the secondary steam from the final effect of double or multiple effects to heat the first effect is rarely applied and usually unsuccessful, mainly because the heating capacity is insufficient to maintain evaporation needs. Even though single-stage screw steam compressors have a wide temperature rise range, they present challenges such as high power consumption, large size, and excessive supplementary steam requirements, rendering double or multiple effects meaningless.

Precautions
Compressor selection depends on material properties. Industrial wastewater evaporation often involves crystallization and salt precipitation. Many salts have high boiling point elevations, making evaporation difficult or impossible. For example, the boiling point elevation of sodium hydroxide solutions increases significantly with concentration. A 50% NaOH solution can have a boiling point elevation as high as 40℃, making it difficult for centrifugal and screw steam compressors to meet evaporation needs. In such cases, MVR evaporators are unsuitable, highlighting the importance of careful evaporator selection.

Conclusion
Currently, centrifugal and Roots steam compressors are mostly used in MVR evaporators with good results, although they require supplementary live steam. Single-stage screw steam compressors are less common and more recent in MVR applications. Because the compressed secondary steam has a higher temperature rise, it doesn't need additional steam, thus gaining attention in recent years. However, issues like oil leaks still need to be addressed.

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