Content area
Full Text
THE DESIGNS of six evaporator suppliers were evaluated: Ahlstrom, Goslin-Birmingham, HPD, Lurgi, RCC and Tampella.
The following energy factors were reviewed:
Steam economy;
Electrical power consumption.
The following design parameters were compared:
Heat transfer coefficient;
Specific loading;
Heat flux.
BACKGROUND
In the last decade, the falling-film evaporator dominated the market. In the previous decade, the rising-film or LTV (long tube vertical) evaporator type was the most popular. A final product solids of about 64% was the target some years ago. Today, evaporator trains are being designed for 75% solids and higher. Higher solids firing in recovery boilers will significantly reduce sulphur dioxide and total reduced sulphur (TRS) emissions as well as increase thermal efficiency. However, nitrous oxide (NOx) and particulate emissions may be higher.
The main challenge in obtaining high solids liquor is to avoid rapid fouling of the heat transfer surfaces. Tampella and Goslin-Birmingham have achieved 80% solids by controlling crystallization in the liquor such that the crystals grow within the bulk of the liquor and not on heat transfer surfaces [1,2,3].
Another of the potential difficulties with producing high solids is that liquor viscosity can approach the limits of centrifugal pumps. Two design approaches have been used by various suppliers. One method involves heating the liquor to a higher temperature to reduce its viscosity. Storage of such liquor can only be done in a pressurized state. As an alternative, Ahlstrom has introduced a heat treatment process which reduces the liquor viscosity and allows the high solids liquor to be stored at atmospheric pressure [4].
DESIGN SPECIFICATION
The consulting engineer or purchaser usually specifies the duty of the black liquor evaporators. Some evaporators include a foul condensate stripper which is usually integrated. This specification typically includes the following:
Feed liquor flow;
Feed liquor solids concentration;
Feed liquor temperature;
Product solids concentration;
Evaporation rate (stream);
Boiling point rise at various liquor concentrations (if available);
Digester foul condensate flow;
Number of effects;
Surface condenser cooling water temperatures;
Steam pressure and temperature.
The stream (design) evaporation rate usually includes a 10% margin to allow for processing liquor spills and dumped (high conductivity) process condensate.
The number of effects is usually determined by the consulting engineer, taking into account the mill steam and power balance, equipment capital...