Vapor-compression evaporation

Vapor-compression evaporation

Vapor-compression evaporation is the evaporation method by which a blower, compressor or jet ejector is used to compress, and thus, increase the pressure of the vapor produced. Since the pressure increase of the vapour also generates an increase in the condensation temperature, the same vapour can serve as the heating medium for its "mother" liquid or solution being concentrated, from which the vapor was generated to begin with. If no compression was provided, the vapor would be at the same temperature as the boiling liquid/solution, and no heat transfer could take place.

If compression is performed by a mechanically-driven compressor or blower, this evaporation process is usually referred to ad MVR (Mechanical Vapour Recompression. In case of compression performed by high pressure motive steam ejectors, the process is usually called Thermocompression or Steam Compression.

MVR Process

In this case the energy input to the system lies in the pumping energy of the compressor. The theoretical energy consumption will be equal to E=Q*(H2-H1), where

  • E is the total theoretical pumping energy
  • Q is the mass of vapours passing through the compressor
  • H1, H2 are the total heat content of unit mass of vapours, respectively upstream and downstream the compressor.

In SI units, these are respectively measured in kJ, kg and kJ/kg. This theoretical value shall be increased by the efficiency, usually in the order of 30 to 60 %. In a large unit, the compression energy is between 35 and 45 kW per metric ton of compressed vapours.

Equipment for MVR Evaporators

The compressor is necessarily the core of the unit. Compressors used for this application are usually of the centrifugal type, or positive displacement units such as the Roots blowers, similar to the (much smaller) Roots type supercharger. Very large units (evaporation capacity 100 metric tons per hour or more) use sometimes Axial-flow compressors. The compression work will deliver the steam superheated if compared to the theoretical pressure/temperature equilibrium. For this reason, the vast majority of MVR units feature a desuperheater between the compressor and the main heat exchanger.

Thermocompression

The energy input is here given by the energy of a quantity of steam (motive steam), at a pressure higher than those of both the inlet and the outlet vapours. The quantity of compressed vapours is therefore higher that the inlet : Qd = Qs + Qm
Where Qd is the steam quantity at ejector delivery, Qs at ejector suction and Qm is the motive steam quantity. For this reason, a thermocompression evaporator often features a vapour condenser, due to the possible excess of steam necessary for the compression if compared with the steam required to evaporate the solution. The quantity Qm of motive steam per unit suction quantity is a function of both the motive ratio of motive steam pressure vs. suction pressure and the compression ratio of delivery pressure vs. suction pressure. In principle, the higher the compression ratio and the lower the motive ratio the higher will be the specific motive steam consumption, i. e. the less efficient the energy balance.

Thermocompression Equipment

The heart of any thermocompression evaporator is clearly the steam ejector, exhaustively described in the relevant page. The size of the other pieces of equipment, such as the main heat exchanger, the vapor head, etc (see evaporator for details), is governed by the evaporation process.

Comparison

These two compression-type evaporators have different field of application, although overlapping sometimes.

  • An MVR unit will be preferable for a large unit, thanks to the reduced energy consumption. What was the largest single body MVR evaporator built (1968, by Whiting Co., later Swenson Evaporator Co., Harvey, Ill. in Cirò Marina, Italy) was a salt crystallizer, evaporating approximately 400 metric tons per hour of water, featuring an axial-flow compressor (Brown Boveri, later ABB). This unit was transformed around 1990 to become the first effect of a Multiple effect evaporator. MVR evaporators with 10 tons and up evaporating capacity are common.
  • The compression ratio in a MVR unit does not usually exceed 1.8. This means that, if evaporating at atmospheric pressure (0.101 MPa and 100 °C), the condensation pressure at the heat exchanger will be 116.9 °C. Deducting the boiling point elevation boiling point rise (8 K for a saturated salt solution), this leaves less than 8 K delta T at the heat exchanger, resulting in a very large heating surface. Axial-flow and Roots compressor may reach slightly higher compression ratios.
  • Thermocompression evaporators may reach higher compression ratios - at a cost. A compression ratio of 2 is possible (and something more) but unless the motive steam is at a reasonably high pressure (say, 16 bar g - 250 psig - or more), the motive steam consumption will be in the range of 2 kg per kg of suction vapours. A higher compression ratio means a smaller heat exchanger, and a reduced investment cost. Moreover, a compressor is an expensive machine, while an ejector is much simpler and cheap.

As a conclusion, MVR machines are used in large, energy-efficient units, while thermocompression units tend to limit their use to small units, where energy consumption is not a big issue.

The efficiency and feasibility of this process lie on the efficiency of the compressing device (e.g., blower, compressor or steam ejector) and the heat transfer coefficient attained in the heat exchanger contacting the condensing vapor and the boiling "mother" solution/liquid. Theoretically, if the resulting condensate is subcooled, this process could allow full recovery of the latent heat of vaporization, which would otherwise be lost if the vapor, rather than the condensate, was the final product; therefore, this method of evaporation is very energy efficient. The evaporation process may be solely driven by the mechanical work provided by the compressing device.

Some uses

A vapor-compression evaporator, like most evaporators can make reasonably clean water from any water source. In a salt crystallizer, for example, a typical analysis of the resulting condensate shows a typical content of residual salt not higher than 50 ppm or, in a different concept, not higher than 10 μS/cm. This results in a drinkable water, if the other sanitary requirements are fulfilled. While this cannot compete in the marketplace with reverse osmosis or demineralisation, but vapor compression chiefly differs from these thanks to its ability to make clean water from saturated or even crystallizing brines with total dissolved solids (TDS) up to 650,000 mg/L. The other two technologies can make clean water from sources no higher in TDS than approximately 35,000 mg/L.

For economic reasons evaporators are seldom operated on low-TDS water sources. Those applications are filled by reverse osmosis. The already brackish water which enters a typical evaporator is concentrated further. The increased dissolved solids act to increase the boiling point well beyond that of pure water. Seawater with a TDS of approximately 30,000 mg/L— exhibits a boiling point elevation of less than 1 degree F. But saturated sodium chloride at 360,000 mg/L has a boiling point elevation of about 13 degrees F. This boiling point elevation represents a challenge for vapor-compression evaporation in that it increases the pressure ratio that the steam compressor must attain to effect vaporization. Since boiling point elevation determines the pressure ratio in the compressor, it is the main overall factor in operating costs.

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