Mechanical Recompression Evaporator
Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapor from an evaporator is compressed (with a positive-displacement, centrifugal or axial-flow compressor) to a higher pressure so that it can be condensed in the evaporator heat exchanger. Various combinations are possible, including single-effect recompression, multiple-effect recompression, multiple-stage recompression, and single-effect recompression combined with a multiple-effect evaporator.
A simplified flowsheet of a single-effect recompression evaporator illustrates why mechanical recompression is energy efficient. All of the pertinent pressures and temperatures are given in Figure 8 for this example.
Based upon a 75% isentropic (adiabatic, reversible) compressor efficiency and a combined electric drive motor and gear reducer efficiency of 92%, the energy required to compress a single pound of vapor from 14.1 to 22.8 psia is only 49.3 BTU. To produce the equivalent steam from one pound of 234oF evaporator condensate requires 999 BTU. Therefore, the energy savings for a recompression evaporator are highly competitive with those of multiple-effect evaporators and depend upon the compression pressure ratio required and the relative cost of electric power and steam.
The compression ratio required is comprised of three components which are:
- The boiling-point rise, i.e., the temperature of the boiling liquor minus the temperature of boiling water at the same pressure.
- The Delta-T required for heat transfer.
- The pressure drop in the vapor pipe to and from the compressor.
Mechanical recompression is most practical for low Delta-T's (larger heat-transfer areas) and low boiling-point elevations.
In Figure 7, a simplified flowsheet is shown for a single-effect recompression soda ash evaporator which has replaced the traditional triple-effect evaporators used for this application. The vapor body shown has the alternate Swenson vertical-inlet baffle design, which has proven to be effective in minimizing short circuiting. Vapor from the body is compressed with a single-stage centrifugal compressor and condensed in the vertical heat exchanger. Condensate is sprayed into the vapor discharged from the compressor to reduce super-heat. Some make-up steam is required to supplement the mechanical energy from the compressor. For some applications, make-up steam is not required.
Most submerged-inlet evaporators short circuit. That is, some of the heated liquor which enters the vapor body short circuits to the outlet instead of rising to the boiling surface. The boiling temperature of the liquor is increased above the equilibrium value (denoted as degrees of short circuiting), which decreases the Delta-T available for heat transfer. It is particularly important to minimize short circuiting in recompression evaporators because short circuiting increases the compression ratio required, thereby increasing power consumption.
The following table compares the energy required for this system versus that required for a triple-effect evaporator:
UTILITIES REQUIRED PER ONE TON OF SODIUM CARBONATE MONOHYDRATE
|Tripple Effect||Recompression Evaporator|
|Condenser H2O, gpm||75.3||0|
|Steam & Power cost||$9.93||$6.80|
(Based upon $6 per 1,000 lb steam & $0.05 per kwh power)
Mechanical recompression is not limited to single-effect evaporation. It is sometimes economical to compress vapor from the last effect of a double- or triple-effect evaporator so that the vapor can be condensed in the first-effect heat exchanger.
A multiple-stage, LTV falling-film, mechanical recompression evaporator is shown in Figure 4.
The vapor recompression evaporator (shown in Figure 8) uses high pressure steam (880 psia, 900oF) fed through two turbines in parallel; one turbine drives a generator to produce electricity for the plant and the other turbine drives a multi-stage, axial-flow compressor for the single-effect recompression evaporator. The 44 psia steam discharged from both turbines is condensed in the first effect of the quadruple-effect salt evaporator and a small quantity is condensed as make-up steam in the recompression evaporator heat exchangers.