Design Criteria for Multiple Effect Evaporators in Alumina Refinery

Hi Friends,

In earlier post, we have already discussed that the net evaporation requirement in efficient Alumina refinery ranges from 2.10 to 2.50 tonnes per tonne of alumina production. This figure is used as a reference guideline to compare the same with any operative Alumina refinery to have an idea about dilution control as well as volume management in Alumina refinery.


In present post, we will cover the design and selection criteria along with associated operational issues for successful installation and operation of evaporators in Alumina refinery. 

Evaporation is an important unit operation of Alumina refinery for volume management of plant by evaporation of excessive water from the plant liquor. Spent liquor is used as the feed liquor with saturated steam as the heating media for evaporation of water resulting in generation of thick product liquor for recycle to digestion circuit of Bayer process. The equipment used for the purpose is known as Evaporator. Design and selection of evaporator is carried out based on nature of liquor to be handled aiming to meet following objectives-

1. High steam economy,
2. High temperature of product liquor,
3. Lower downtime for cleaning and
4. Lower capital requirement.

The design of evaporator for Alumina refinery is purely based on its suitability to handle Sodium aluminate liquor which is viscous in nature and pron to easy scale formation but does not crystallize on the tube surface of heaters. Backward feeding multiple effect evaporators (5 effects to 7 effects) are always the preferred choice aiming to achieve the steam economy between 3.5 to 4.5 tonnes evaporation on continuous basis per tonne of steam in Alumina refinery. Evaporator mainly consists of the heating chamber (Heater) and liquor separating chambers (Separators). Generally, saturated steam is used in the fist body and vacuum is applied in the last body of evaporators, thus evaporation takes place at positive pressure, atmospheric pressure and negative pressure (vacuum) depending on the pressure (or vacuum) profile in different bodies of evaporators. Pumps are used for forced circulation of liquor to improve the heat transfer coefficient. During circulation, liquor velocity varies from 1.5 to 3.5 m/sec. In Alumina refinery, falling film type evaporators have edge over rising film type evaporators. In falling film evaporators, the feed liquor enters from the top of heat exchanger, drops under gravity forming film along the inner walls of heater tubes and evaoprates to concentrate the liquor during travel downward. The vapour-liquor mixture flows to the bottom of heater and enters to the separator discharging liquor from the bottom of separator and releasing regenerated vapour (secondary steam)  from the top of the separator. The velocity of regenerative vapour at the outlet of separator varies from 30 to 50 m/sec at normal pressure where as vapour velocity ranges from 120 to 150 m/sec on vacuum side depending on vacuum level. The live steam condensate collected at the outlet of steam trap of 1st effect is returned to Co-generation plant to be used as feed water for boilers. The regenerative condensate collected from all other effects are send to regenerative condensate storage tank for hydrate washing and other purposes in process circuit. Typical structure of single effect falling film type evaporator is shown in following schematic diagram. I put on record my sincere thanks to my friend who has developed the structure of falling film type evaporator which is being presented here-

The inlet and outlet ports for various process streams have been clearly shown in this schematic view which are taken care of by all equipment suppliers. However, the mounting arrangement of heater and separator changes with different designers and fabricators of evaporators. The film formation on surface of tube increases the heat transfer coefficient resulting in high steam economy in evaporator. Presence of organic impurities in plant liquor causes generation of foam which may hinder the operation of evaporator because faulty indication of liquor level in evaporator bodies. Also, carryover of suspended hydrate with spent liquor may cause dirt formation on heater tubes ultimately reducing the heat transfer coefficient and performance of evaporator. Thus, due care is taken to control both these parameters within the desired limit.

Hope, the topic has been covered adequately. Please put your views / suggestions / remarks / comments, if any.

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Thanks and regards.

Kunwar Rajendra