Mechanical Vapor Compression
Introduction
Mechanical Vapor Compression (MVC) desalination is an evaporative, compressor-driven process where vapor generated from saline feedwater is compressed to raise its temperature and pressure. This compressed vapor then condenses, releasing latent heat to evaporate the feedwater, thus producing distillate without external heating or cooling. The system primarily consists of an evaporator/condenser and a compressor, with optional heat exchangers for enhanced heat integration, and may utilize external steam to initiate the process.
Implementation
Figure 1 illustrates the process flow diagram for a Mechanical Vapor Compression (MVC) desalination system. In this configuration, feedwater is pumped and divided between the cold inlets of a distillate heat exchanger (HEX) and a brine HEX. The preheated feed exiting these heat exchangers combines and enters the evaporator, where it absorbs heat from the condensing steam and vaporizes, leaving behind concentrated brine. The generated vapor is then directed to a compressor, which elevates its temperature and pressure. This compressed vapor is recycled back to the evaporator, where it condenses and provides the necessary heat for evaporating the preheated feed. The hot concentrated brine and distillate pass through heat exchangers to preheat the incoming feedwater before exiting the system. The flowsheet relies on the following key assumptions:
supports steady-state only
property package(s) supporting liquid and vapors is provided
Figure 1. MVC flowsheet
- Documentation for each of the WaterTAP unit models can be found below.
- Documentation for each of the IDAES unit models can be found below.
- Documentation for each of the property models can be found below.
- Documentation for the costing relationships can be found below.
The objective function is to minimize the levelized cost of water, which can be represented by the following equation where \(Q\) represents volumetric flow, \(f_{crf}\) represents capital recovery factor \(C_{cap,tot}\) represents total capital cost, \(C_{op,tot}\) represents total operating cost, and \(f_{util}\) represents the utilization factor:
\[LCOW_{Q} = \frac{f_{crf} C_{cap,tot} + C_{op,tot}}{f_{util} Q}\]
Degrees of Freedom
- The following variables are specified for the MVC flowsheet based on the default settings:
feed water conditions (mass flow, mass fractions, temperature, and pressure)
feed pump efficiency and pressure change (ΔP)
distillate HEX heat transfer coefficient, cold-side ΔP, and hot-side ΔP
brine HEX heat transfer coefficient, cold-side ΔP, and hot-side ΔP
evaporator overall heat transfer coefficient
compressor efficiency
brine pump efficiency and ΔP
distillate pump efficiency and ΔP
translator block outlet TDS concentration
Flowsheet Specifications
Description |
Value |
Units |
|---|---|---|
Feed Water |
||
Water mass flow |
40 |
\(\text{kg/s}\) |
TDS mass fraction |
0.1 |
\(\text{dimensionless}\) |
Temperature |
298.15 |
\(\text{K}\) |
Pressure |
101325 |
\(\text{Pa}\) |
Feed Pump |
||
Pump efficiency |
0.8 |
\(\text{dimensionless}\) |
Pressure change |
7000 |
\(\text{Pa}\) |
Separator |
||
Total flow split fraction to distillate HEX |
0.5 |
\(\text{dimensionless}\) |
Distillate HEX |
||
Overall heat transfer coefficient |
2000 |
\(W/\left(m^2K\right)\) |
Cold-side pressure change |
7000 |
\(\text{Pa}\) |
Hot-side pressure change |
7000 |
\(\text{Pa}\) |
Brine HEX |
||
Overall heat transfer coefficient |
2000 |
\(W/\left(m^2K\right)\) |
Cold-side pressure change |
7000 |
\(\text{Pa}\) |
Hot-side pressure change |
7000 |
\(\text{Pa}\) |
Evaporator |
||
Overall heat transfer coefficient |
3000 |
\(W/\left(m^2K\right)\) |
Compressor |
||
Compressor efficiency |
0.8 |
\(\text{dimensionless}\) |
Brine Pump |
||
Pump efficiency |
0.8 |
\(\text{dimensionless}\) |
Pressure change |
40000 |
\(\text{Pa}\) |
Distillate Pump |
||
Pump efficiency |
0.8 |
\(\text{dimensionless}\) |
Pressure change |
40000 |
\(\text{Pa}\) |
Translator Block |
||
Outlet TDS mass flow |
1e-5 |
\(\text{kg/s}\) |