Comparison of MVR, multi-effect evaporation, and TVR ( thermal vapor recompression ) technologies

MVR technology compresses the secondary steam that would otherwise require condensation to a higher pressure using a compressor, thereby increasing its internal energy. This enables continuous recirculation and reuse of this energy, replacing fresh steam. By consuming only a minimal amount of electrical energy, it allows the substantial internal energy of low-pressure steam to be recovered and reused.

MVR principle

1. Electrical energy enables the secondary steam generated during evaporation to be compressed and fully utilized, resulting in virtually no steam losses within the system.

2. The condensate and concentrate are discharged to exchange heat with the feed solution.

3. The non-condensable gas is heat-exchanged with the original liquid.

4. The compressor motor employs variable-frequency speed control.

To reduce or eliminate the need for live steam supply, recover waste heat steam, decrease cooling water consumption, and enhance system efficiency.

Comparison of MVR, Multi-Effect Evaporation, and TVR (Thermal Vapor Recompression) Technologies

A Comparison of Three Technologies Used in Evaporation and Distillation Apparatuses

Evaporation, distillation, evaporative crystallization, and evaporation‑drying equipment all consume substantial amounts of energy.

Energy consumption accounts for a significant share of the operating costs of these devices; therefore, reducing and optimizing specific energy consumption is critical to lowering overall operating expenses.

At present, there are three main technologies for minimizing specific energy consumption, which can be applied individually or in combination:

■ Multiple Effect Evaporator (MEE)

■ Thermal Vapour Recompressor (TVR) Technology

■ Mechanical Vapor Recompressor (MVR) Technology

1. Multi-effect technology

In a multi‑effect evaporation system, the steam generated in the first effect by heating with fresh steam does not enter a condenser; instead, it is reused as the heating medium for the second effect. This approach can reduce fresh steam consumption by approximately 50%. By extending this principle to additional effects, further reductions in fresh steam usage are achievable. The total temperature difference between the highest heating temperature of the first effect and the lowest boiling temperature of the last effect is distributed across the individual effects. Consequently, the temperature difference per effect diminishes as the number of effects increases. To maintain a specified evaporation rate, the heating surface area must therefore be enlarged. Preliminary estimates indicate that the total heating surface area required for all effects grows proportionally with the number of effects; as a result, while steam savings gradually decline, capital investment rises substantially.

A Product B Residual Steam C Concentrate D Live Steam

E Power‑steam condensate F Secondary‑steam condensate V Heat loss

Heat flow diagram of the second-effect direct-heating evaporator

2. Thermal Vapor Recompression Technology

In thermal vapor recompression, based on the heat pump principle, steam from the boiling chamber is compressed to a higher pressure in the heating chamber, thereby adding energy to the steam. Because the saturation temperature corresponding to this elevated heating‑chamber pressure is higher, the steam can be reused for heating. Steam‑jet compressors are employed for this purpose. Operating on the jet‑pump principle, they have no moving parts, feature a simple yet effective design, and ensure maximum operational reliability. Using a single thermal vapor compressor delivers the same steam‑saving and energy‑efficiency benefits as adding an additional effect to the evaporator. The operation of a thermal vapor compressor requires a certain amount of fresh steam, known as motive steam. This motive steam must either be conveyed to the next effect or discharged to the condenser as residual steam. The residual energy contained in the residual steam is roughly equivalent to the energy supplied by the motive steam.

A product B secondary steam B1 residual steam C concentrated liquid

D Power steam E Power steam condensate V Heat loss

Heat flow diagram of a thermal vapor recompression evaporator

3. Mechanical Vapor Recompression Technology

In mechanical vapor recompression, a mechanically driven compressor compresses the vapor generated in the evaporator to a higher pressure. Consequently, the recompressor also functions as a heat pump, thereby increasing the energy content of the vapor. Unlike compression heat pumps that use a circulating process fluid—i.e., a closed system operating on a refrigeration cycle—the vapor recompressor operates as an open system and can thus be regarded as a special type of compression heat pump. After the vapor is compressed and the resulting heated vapor condenses, the condensate exits the cycle. The heating vapor (the hot side) and the secondary vapor (the cold side) are separated by the evaporator’s heat-transfer surface. A comparison between open‑system and closed‑system compression heat pumps reveals that, in the open system, the evaporator surface essentially assumes the role of the expansion valve in the closed system. By consuming only a relatively small amount of energy—namely, the mechanical energy of the compressor impeller in a compression heat pump—energy is transferred to the process heating medium and sustained within a continuous cycle. Under these conditions, no primary steam is required as the heating medium.

A product B secondary steam B1 residual steam C concentrated liquid

D Electrical energy E Power steam condensate V Heat loss

Heat flow diagram of a mechanical vapor recompression evaporator

In a multi‑effect thermal vapor recompression system, the condensation heat to be rejected remains substantial. In a multi‑effect configuration, with n effects, the condensation heat amounts to approximately 1/n of the primary energy input. Moreover, the steam‑jet compressor can only compress a fraction of the secondary steam, and the remaining energy of the motive steam must be discharged as waste heat to the cooling water. However, by employing the principle of an open‑cycle compression heat pump, the heat rejected through the condenser can be significantly reduced or even eliminated. To achieve overall thermal equilibrium, a small amount of residual energy or condensation of leftover steam may still be required, thereby enabling a constant pressure ratio and stable operating conditions. Reasons for adopting mechanical vapor recompression include:

■ Low specific energy consumption

■ The low temperature difference ensures gentle evaporation of the product.

■ Due to the use of a single-effect evaporator, the product residence time is short.

■ Simple process and highly practical

■ Excellent part-load operating characteristics

■ Low operating costs