EVAPORATORS

Introduction

Brine treatment and waste water discharge is a challenge for many industries and needs to be done safely while meeting environmental regulations. But the ideal solution is ZLD (Zero liquid discharge), a complete closed loop cycle where discharge is eliminated.

The ZLD system removes dissolved solids from the waste water and returned distilled water to the process. Reverse osmosis (membrane filtration) may be used to concentrate a portion of the waste stream and return the clean permeate to the process. In this case, a much smaller volume (the reject) will require evaporation, thus enhancing performance and reducing power consumption. The water vapour from evaporator is condensed and returned to the process.

Following are the key benefits of Zero liquid discharge systems :

  1. Recovers valuable ingredients from effluent waste water.
  2. Reduces process water disposal cost.
  3. Meets environmental permit targets
  4. Reliable and robust solutions allows focus on main production business.

Following are the technologies used to further concentrate the brine or the reject for RO plants.

  1. Forced circulation evaporator
  2. Falling film evaporator
  3. Hibrid thermal and mechanical vapour recompression

    4. Forced circulation evaporators :

    Forced circulation evaporators are used if boiling of the product on the heating surfaces is to be avoided due to the fouling characteristics of the product, or to avoid crystallization. The flow velocity in the tubes must be high, and high capacity pumps are required.

    1. Product
    2. Vapor
    3. Concentrate
    4. Heating System
    5. Condensate

    1. Heat Exchanger
    2. Flash Vessel (Separator)
    3. Circulation Pump
    4. Concentrate Pump

    The circulating product is heated when it flows through the heat exchanger(1) and then partially evaporated when the pressure is reduced in the flash vessel (separator)(2). The liquid product is typically heated only a few degrees for each pass through the heat exchanger. To maintain a good heat transfer within the heat exchanger, it is necessary to have a high recirculation rate.

    Falling film evaporators :

    Falling film evaporators can be operated with very low temperature differences between the heating media and the boiling liquid, and they also have very short product contact times, typically just a few seconds per pass. These are more economical in terms of capital and energy costs as compared to forced circulation evaporators. However, since laminar flow range is employed, they may be susceptible to scaling when operating at higher concentration of brine.

    1. Product
    2. Vapor
    3. Concentrate
    4. Heating System
    5. Condensate

    1. Head
    2. Calandria
    3. Calandria, Lower part
    4. Mixing Channel
    5. Vapor Separator

    In falling film evaporators the liquid (A) usually enters the evaporators at the head (1) of the evaporator. In the head of the product is evenly distributed into the heating tubes. A thin film enters the heating tube as it flows downwards at boiling temperature and is partially evaporated. In most cases steam (D) is used for heating the evaporator. The product and the vapor both flow downwards in parallel flow. This gravity-induced downward movement is increasingly augmented by the co-current vapor flow. The separation of the concentrated product (C) from its vapor (B) is achieved in the lower part of the heat exchanger (3) and the vapor separator (5).

    Thermal Vapour Recompression (TVR) :

    Thermal Vapour Recompression (TVR) : Wherever steam is available at a high pressure as compared with the pressure needed in the evaporator, the use of thermal vapour recompressor gives the same steam / energy saving as an additional evaporation effect. TVR plants can be single or multiple effects.

    During Thermal vapour recompression, a fraction of the process vapour from the boiling chamber is recycled and recompressed in a steam jet ejector with high pressure live steam, called motive steam, to a higher pressure and temperature as required for the heating of the evaporator.

    Consequently, the live steam condensates are mixed with the process condensates. Thanks to the recycle of process vapour, the amount of steam required for heating is lower and flow of process vapour going to the subsequent effect or condenser is therefore reduced, decreasing the cooling water demand.

    This technology costs slightly more than a single effect and has the efficiency of nearly a triple effect. The other benefit of steam jet ejector is that they have no moving parts and are therefore not subject to wear and tear, this ensures maximum operational reliability. TVR can also be installed on existing units to increase their evaporation capacities.

    TVR operating conditions should be close to their design conditions in order to get optimum yield. Should variability in capacity is required, several steam ejector for various capacities can be installed.

    The main feature of TVR are as follows :

    1. Usable when high pressure steam is available.
    2. Single or multiple effect
    3. Energy and cooling water savings.
    4. No moving parts in steam jet ejector.
    5. Operating close to design conditions.
    6. Suitable for low boiling elevation and non-incrusting products.

    Mechanical Vapour Recompression (MVR) :

    Evaporation or crystallisation plants using mechanical vapour recompression requires particularly low amounts of energy. Whereas in TVR, only part of the process vapour are recompressed, in MVR all the process vapours leaving the evaporator are recompressed to the pressure of the corresponding heating steam pressure / temperature used to heat the evaporator. The amounts of heat to be dissipated are considerably reduced, with the evaporator itself re-utilising the energy normally dissipated through the condenser cooling water. The energy required to drive the compressor is only the energy difference between the process vapour and the recompressed vapours, making such system energy efficient. 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 insure stable operating conditions. Any excess vapour is condensed or may be utilised in another concentrator. Low pressure steam will be required for preheating when starting up the process.

    Generally MVR plants are single effect, but if required can be multiple effects. The compressor can be either positive displacement type (lobes or screw) or centrifugal type with one or several stages of compression, or fans. Due to their simplicity and maintenance friendly design, centrifugal fans are often used.

    The feed process solution is often preheated with hot process condensates and/or can be by concentrated solution leaving the evaporator. The present cost of thermal energy makes MVR attractive for the evaporation of any solution whose boiling point rise is moderate.

    The main features of MVR are as follows :

    1. Most energy efficient evaporation technology (n-effect)
    2. Flexible and simple to use
    3. Generally single effect (reduced footprint)
    4. Almost no steam and cooling water consumption (boiler and cooling tower cost reduction)
    5. Suitable for moderate boiling elevation products
    6. Capital cost vs. operating cost to be studied.

    In Hybrid thermal and mechanical vapour compression, first effect can be mechanical vapour compression and polisher can be with thermal vapour compression.