Comparison of MVR with Multi Effect and TVR Technologies


Comparison of Three Technologies Used for Evaporative Distillation and Other Devices

Evaporation, distillation, crystallization, and drying devices are all high energy consuming.

Energy consumption accounts for a large proportion of the operating costs of these devices, so reducing and optimizing unit energy consumption is crucial for reducing the overall operating costs.

There are currently three main technologies to achieve the minimization of specific energy consumption, which can be applied separately or in combination:

Multiple Effect Evolutioner (MEE)

Thermal Vapor Recompression (TVR) technology

Mechanical Vapor Recompression (MVR) technology

1. Multi effect technology

In a multi effect evaporation device, the steam generated by heating the first effect with fresh steam does not enter the condenser, but is reused as the heating medium for the second effect. This can effectively reduce the consumption of new steam by about 50%. Reusing this principle can further reduce the consumption of fresh steam. The total temperature difference is formed between the highest heating temperature of the first effect and the lowest boiling point temperature of the last effect, distributed throughout each effect. As a result, the temperature difference per effect decreases with the increase of the number of effects. So in order to achieve the specified evaporation rate, it is necessary to increase the heating area. Preliminary estimates indicate that the effective heating area used increases proportionally with the efficiency, resulting in a gradual decrease in steam savings and a significant increase in investment costs.

 

A product B residual steam C concentrated liquid D power steam

E power steam condensate F secondary steam condensate V heat loss

Heat flow diagram of dual effect direct heating evaporator

 

2. Thermal steam recompression technology

When the thermal steam is compressed again, according to the principle of a heat pump, the steam from the boiling chamber is compressed to a higher pressure in the heating chamber; That is, energy is added to steam. Due to the higher saturated steam temperature corresponding to the heating chamber pressure, the steam can be reused for heating. For this purpose, a steam jet compressor is used. They operate based on the principle of jet pumps, have no moving parts, are designed simply and effectively, and can ensure the highest working reliability. The use of a thermal steam compressor has the same steam/energy saving effect as adding a one effect evaporator. The operation of a thermal steam compressor requires a certain amount of fresh steam, also known as power steam. These power steam must be transported to the next effect or sent to the condenser as residual steam. The remaining energy contained in the residual steam is approximately equivalent to the energy provided by the power steam.

 


A product B secondary steam B1 residual steam C concentrate

D power steam E power steam condensate V heat loss

Heat flow diagram of a hot steam re compression heating evaporator

 

3. Mechanical steam recompression technology

When mechanical steam is compressed again, the steam evaporated from the evaporator is compressed to a higher pressure through a mechanically driven compressor. Therefore, the compressor also works as a heat pump, adding energy to the steam. Unlike compression heat pumps that use circulating process fluids (i.e. closed systems, refrigeration cycles), steam compressors can be considered as special compression heat pumps because they operate as open systems. After steam compression and subsequent heating, the condensate leaves the circulation. The heated steam (hot side) and the secondary steam (cold side) are separated by the heat exchange surface of the evaporator. The comparison between open compression heat pumps and closed compression heat pumps shows that the evaporator surface in open systems basically replaces the function of the process fluid expansion valve in closed systems. By using relatively less energy, i.e. the mechanical energy of the compressor impeller in the case of a compressed heat pump, the energy is added to the process heating medium and enters a continuous cycle. In this case, it is not necessary to use steam as the heating medium once.

 

A product B secondary steam B1 residual steam C concentrate

D electric energy E power steam condensate V heat loss

Heat flow diagram of mechanical steam re compression heating evaporator

 

In a multi effect steam re compression system, the amount of condensation heat to be released is still very high. In a multi effect device, if there is an n effect, the condensation heat is about 1/n of the primary energy input. Moreover, the steam jet compressor can only compress a portion of the secondary steam, and the energy of the power steam must be released as waste heat to the cooling water. However, the use of the open compression heat pump principle can significantly reduce or even eliminate the heat released through the condenser. To achieve the final thermal equilibrium, a small amount of residual energy or condensation of residual steam may be required, allowing for a constant pressure ratio and stable operating conditions. The reason for using mechanical steam recompression is:

Low energy consumption per unit

Due to the low temperature difference, the evaporation of the product is mild

Due to the commonly used single effect, the product has a short residence time

Simple process and strong practicality

Excellent partial load operation characteristics

Low operating costs

 

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