In many cases of water scarcity or due to regulations it becomes imperative that waste water is recycled at least partially. Waste water recycle means treating waste water in such a way that it can be used back for process or boiler feed as the case may be. This kind of recycle is done in mainly two ways i.e. Effluent Treatment & Reverse Osmosis. One disadvantage of Reverse Osmosis process is disposal of reject stream or concentrate. The volumes are considerable in large size plants and this High TDS water cannot be discharged to water bodies in many places due to detrimental Environmental issues.
In order to reduce discharge volumes Waste concentration systems are employed. Following are the options of Waste Concentration Systems:
1. Reject RO: The Recovery is limited to maximum of 50 -60% as solubility saturation point of some or other salt (mostly SO4 or SiO2) is exceeded.
2. Thermal Concentration: Thermal methods mainly comprise of
a. Mechanical Vapour Compression
b. Thermal Vapour Compression.
Al the three systems have been discussed in detail below.
A. Reject RO System:
The system consists of Reject Holding Tank, Feed Pump, Cartridge Filter, High Pressure Pump, RO Block & dosing system as required.
A schematic is given below for better understanding
B. Mechanical Vapour Compression:
Increasing energy costs have justified the increased use of mechanical recompression evaporators. The principle is simple. Vapour 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.
Mechanical recompression is most practical for low ΔT’s (larger heat-transfer areas) and low boiling-point elevations.
Evaporation plants heated by mechanical vapour recompressors require particularly low amounts of energy. Whereas steam jet compressors only compress part of the vapour, mechanical vapour recompressors recycle all of the vapour leaving the evaporator. The vapour is recompressed to the pressure of the corresponding heating steam temperature of the evaporator, using a mere fraction of electrical energy relative to the enthalpy recovered in the vapour. The operating
principle is similar to that of a heat pump. The energy of the vapour condensate is frequently utilized for the preheating of the product feed. The amounts of heat to be dissipated are considerably reduced, with the evaporator itself re-utilizing the energy normally dissipated through the condenser cooling water. Depending on the operating conditions of the plant, a small quantity of additional steam, or the condensation of a small quantity of excess vapour may be required to maintain
the overall evaporator heat balance and to ensure stable operating conditions.
Due to their simplicity and maintenance friendly design, single stage centrifugal fans are used in evaporation plants. units are supplied as high pressure fans or turbo-compressors.
They operate at high flow velocities and are therefore suited for large and very large flow rates at vapour compression ratios of 1:1.2 to 1:2. Rational speeds typically are 3,000 up to 18,000 rpm. For high pressure increases, multiple-stage compressors can be used.
B. Thermal Vapour Compression:
To reduce energy consumption, water vapour from an evaporator is entrained and compressed with high pressure steam in a thermocompressor so it can be condensed in the evaporator heat exchanger. The resultant pressure is intermediate to that of the motive steam and the water vapour. A thermocompressor is similar to a steam-jet air ejector used to maintain vacuum in an evaporator. Only a portion of the vapour from an evaporator can be compressed in a thermocompressor with the remainder condensed in the next-effect heat exchanger or a condenser. A thermocompressor is normally used on a single-effect
evaporator or on the first effect of a double- or triple-effect evaporator to reduce energy consumption. As with mechanical recompression, thermal recompression is more applicable to low boiling-point rise liquids and low to moderate ΔT’s in the heat exchanger to minimize the compression ratio.
During thermal vapour recompression, vapour from a boiling chamber is recompressed to the higher pressure of a heating chamber in accordance with the heat pump principle. The saturated steam temperature corresponding to the heating chamber pressure is higher so that the vapour can be reused
for heating. To this end, steam jet vapour recompressors are used. They operate according to the steam jet pump principle. They have no moving parts and are therefore not subject to wear and tear. This ensures maximum operational reliability. A certain steam quantity, the so-called motive steam, is required
for operation of a thermal vapour recompressor. This motive steam portion is transferred as excess vapour to the next effect or to the condenser. The energy of the excess vapours approximates the energy of the motive steam quantity used. The use of a thermal vapour recompressor gives the same steam/energy saving as an additional evaporation effect.
