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Based on: Industry Standards and Best Practices

Languages: English

Sample Transcript

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.

Black Liquor Evaporators - Equipment

Training Time: 36 minutes

The Black Liquor Evaporators – Equipment course describes several different evaporator designs that have been used to concentrate black liquor. These include rising film evaporators, falling film evaporators, falling film concentrators, and forced circulation concentrators. It also contains discussions on scaling and boil outs, the role of the surface condenser, overall driving force and capacity, and the removal of non-condensable gases from a multiple effect evaporator set.

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Different flow patterns can develop within a rising film evaporator, the most desirable flow pattern is called "annular flow".

In one design, one effect of the concentrator is online while the other is washed.

In one design, one effect of the concentrator is online while the other is washed.

NCGs are cascade from one effect to the next and then removed from the system through the surface condenser.

NCGs are cascade from one effect to the next and then removed from the system through the surface condenser.

Learning Objectives

  • 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
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Black Liquor Evaporators - Equipment FAQs

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.

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