Increasing energy cost and pressures on improving process efficiency are forcing process engineers to minimize wasteful losses. Efforts are continually being made to minimize all such losses. In many industrial processes low pressure spent steam is let off into atmosphere and goes off as waste heat. Thermal separation processes such as evaporation and distillation are energy intensive. The need for reducing energy costs led to multieffect plants, then to thermal vapor compression and finally to use of mechanical vapor compression systems. Under steady state conditions, sum of all energy and enthalpy inputs must equal the sum of all energy and enthalpy outputs. It, therefore, becomes important to ensure that energy imparted to the vapors is recovered back/reused. The following options are generally adopted in the industry for recovery of
energy:
In a multi effect evaporation plant, the vapors produced in the first effect are utilized as the heating medium of the second effect and so on. This effectively reduces steam consumption in proportion to the number of effects. Ideally unit mass of vapor on condensation can evaporate unit mass of liquid. The vapors generated at the first effect are condensed in the second stage to further evaporate the liquid from the second stage and so on. A temperature gradient of about 7-10°C is maintained between stages for maximum efficiency. So a triple effect evaporator would consume only 35-36% of the energy in comparison to a single effect system.
Vapor Recompression:
In vapor recompression arrangement the heat of condensation of the evaporated vapor is recovered in single effect only by raising the pressure and temperature of the generated vapor and then their condensation in the same evaporator. The vapor compression can be done by Thermal Vapor Compression or Mechanical Vapor Compression Process.
Thermal Vapor Compression: In thermal vapor recompression steam jet ejectors are used to raise the pressure and temperature of the generated vapors. The motive steam mixes with the vapor and to maintain the steady flow heat balance some of the vapor steam mixture has to be taken to second effect for full recovery of latent heat of vapor and, therefore, excess vapor is to be conveyed to next effect for recovery.
Initially, heating steam is used to initialize evaporation. The vapors evaporated are compressed to higher pressure and temperature by steam jet ejector, condensed back for heat recovery and the residual vaporsare taken to second stage for condensation / heat recovery. The amount of surplus energy contained in the residual vapor corresponds to the amount of energy supplied for steam jet ejector operation. This is taken as additional heat input / work done for recovery of large heat content of the evaporated vapors.
Mechanical Vapor Compression: In mechanical vapor compression, positive displacement compressors or multi stage centrifugal compressors are generally used to raise the pressure and temperature of the generated vapors. Since mechanical compressors do not require any motive steam, all vapors can be compressed to elevated pressure and temperature eliminating the need for subsequent recovery system. The energy supplied to the compressor constitutes the additional energy input to vapors. After compression of vapor and subsequent condensation of the same, hot condensate leaves the system. A typical mechanical vapor recompression cycle would be as illustrated in figure below:
For mechanical vapor compressors, the specific energy input depends upon the compression ratio (ratio of input pressure to discharge pressure). Compression ratio, therefore, must be maintained to the lowest required.
The compression ratio is influenced by:
1. The boiling point elevation of the liquid to be evaporated. Higher the boiling point rise higher is the compression ratio required.
2. Minimum differential temperature gradient required for effective heat transfer. Indirect condensers require a minimum temperature gradient across the fluids exchanging heat. The condensers should be designed for least ∆T operation.
3. Total system pressure drop in the piping and valves. Adequate size of piping and valve selection should be done for minimum pressure drop during transfer of fluid through them.
The working cycle of Everest mechanical compressor for steam, as fluid handled, is explained under.
energy:
a) Multi-effect Evaporation
b) Vapor Recompression
- Thermal vapor recompression
- Mechanical Vapor recompression
In a multi effect evaporation plant, the vapors produced in the first effect are utilized as the heating medium of the second effect and so on. This effectively reduces steam consumption in proportion to the number of effects. Ideally unit mass of vapor on condensation can evaporate unit mass of liquid. The vapors generated at the first effect are condensed in the second stage to further evaporate the liquid from the second stage and so on. A temperature gradient of about 7-10°C is maintained between stages for maximum efficiency. So a triple effect evaporator would consume only 35-36% of the energy in comparison to a single effect system.
Vapor Recompression:
In vapor recompression arrangement the heat of condensation of the evaporated vapor is recovered in single effect only by raising the pressure and temperature of the generated vapor and then their condensation in the same evaporator. The vapor compression can be done by Thermal Vapor Compression or Mechanical Vapor Compression Process.
Thermal Vapor Compression: In thermal vapor recompression steam jet ejectors are used to raise the pressure and temperature of the generated vapors. The motive steam mixes with the vapor and to maintain the steady flow heat balance some of the vapor steam mixture has to be taken to second effect for full recovery of latent heat of vapor and, therefore, excess vapor is to be conveyed to next effect for recovery.
Initially, heating steam is used to initialize evaporation. The vapors evaporated are compressed to higher pressure and temperature by steam jet ejector, condensed back for heat recovery and the residual vaporsare taken to second stage for condensation / heat recovery. The amount of surplus energy contained in the residual vapor corresponds to the amount of energy supplied for steam jet ejector operation. This is taken as additional heat input / work done for recovery of large heat content of the evaporated vapors.
Mechanical Vapor Compression: In mechanical vapor compression, positive displacement compressors or multi stage centrifugal compressors are generally used to raise the pressure and temperature of the generated vapors. Since mechanical compressors do not require any motive steam, all vapors can be compressed to elevated pressure and temperature eliminating the need for subsequent recovery system. The energy supplied to the compressor constitutes the additional energy input to vapors. After compression of vapor and subsequent condensation of the same, hot condensate leaves the system. A typical mechanical vapor recompression cycle would be as illustrated in figure below:
The compression ratio is influenced by:
1. The boiling point elevation of the liquid to be evaporated. Higher the boiling point rise higher is the compression ratio required.
2. Minimum differential temperature gradient required for effective heat transfer. Indirect condensers require a minimum temperature gradient across the fluids exchanging heat. The condensers should be designed for least ∆T operation.
3. Total system pressure drop in the piping and valves. Adequate size of piping and valve selection should be done for minimum pressure drop during transfer of fluid through them.
The working cycle of Everest mechanical compressor for steam, as fluid handled, is explained under.
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