At the end of this module, you will be able to:
- List the objectives of black liquor evaporators
- Differentiate between the designs and operation of several common types of black liquor evaporators and concentrators
- Define "scaling" and recognize the need of a "boil-out"
- Describe the role of the surface condenser
- Describe overall driving force and capacity
- State the importance of non-condensable gas (NCG) removal
The following key questions are answered in this module:
What is the primary purpose of black liquor evaporators?
The primary purpose of the black liquor evaporators is to evaporate water vapor and increase the concentration of the solids in the liquor.
Falling film evaporators have some advantages over rising film evaporators. What is their major disadvantage?
Falling film evaporators require a recirculation pump which increases the operating costs of this type of evaporator.
What is the difference between black liquor concentrators and evaporators?
Black liquor concentrators have design features to deal with the viscosity and salts which precipitate out of the liquor at higher concentrations.
What is the role of a surface condenser in a mulitple effect evaporator set?
The surface condenser condenses vapor from the lowest pressure effect, creating a vacuum which drives vapor flow through the set.
How do non-condensible gases affect evaporator performance?
NCGs on the vapor side of an evaporator set interfere with the movement and condensation of water vapor, causing a drop in evaporator capacity.
Below is a transcript of the video sample provided for this module:
There are several different evaporator designs used to concentrate black liquor, and they all contain two major parts. The first is the heating element, usually a vertically-arranged heat exchanger where the vapor or steam condenses and transfers its heat to the black liquor. The second part is the vapor body, also referred to as the vapor head or dome, which provides space for the concentrated liquor and water vapor to separate. The heating element is designed to provide as much surface area as possible for the vapor to condense and transfer heat to the liquor. The more heating surface that is available, the more the solution can be heated. Sets of tubes or parallel plates are commonly used. Black liquor and vapor or steam are both continuously fed into the evaporator, and concentrated liquor, water vapor, and condensate are all continuously discharged.
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