MED with Thermal Vapor Compression

MED with thermal vapor compression (MED-TVC), or simply TVC, is the most rapidly growing distilla­tion process. The process combines the basic MED system with a steam jet ejector, which enables the plant to utilize enthalpy and kinetic energy from motive steam and is shown in Fig. 7. Like the MED system, each kilogram of steam entering the first effect can yield a distillate production of 0.85Nkg, where

Подпись: Ejector Cooling water out FIGURE 7 Multiple-effect distillation system with thermal vapor compression.

N is the number of effects. The important point to note is that each kilogram of steam entering the first effect is a combination of live steam and recycled low – pressure process vapor. The net result is that when utilizing medium-pressure steam, it is possible to achieve a GOR of over 8 with only four effects.

The addition of a thermocompression device adds approximately 1% to the CAPEX of a basic MED unit, yet the efficiency can be doubled. In cases where the steam is produced from waste gases, or where there is little commercial value difference between medium – and low-pressure steam, the use of TVC has grown tremendously. Using medium-pressure steam, it has been possible to achieve GORs of over 15. Such high GOR values also mean that less heat must be rejected from the system via cooling water, which can be an important consideration in many projects (for environmental or permit reasons).

The most common method for large-scale desalting of seawater had been the MSF process, which typically was combined with power production and utilized turbine extraction steam at 5 bar. Higher heat transfer coefficients, lower pumping pressures (and therefore lighter mechanical design requirements), and greater design flexibility mean that TVC designs are producing the same amount of water from this 5 bar steam but at lower CAPEX and OPEX than MSF. For distillation units in the range of 1000-25,000 ton/day capacity, the TVC process is beginning to dominate.

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