At the end of this module, you will be able to:
- List in order the key steps in the kraft chemical pulping process
- Describe the role of black liquor evaporators at a kraft pulp mill
- Identify the flows of steam, vapor, and black liquor in a multiple effect evaporator system
- Define "steam economy"
The following key questions are answered in this module:
Where is black liquor separated from cooked pulp?
Black liquor is separated from cooked pulp in the brown stock washers.
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.
What is a mulitple effect evaporator?
A multiple effect evaporator is an evaporator made up of multiple evaporator bodies, or stages of evaporation.
What is the largest advantage of a multiple effect evaporator?
Multiple effect evaporators can evaporate more than one pound of water for every pound of steam used.
What is steam economy in a multiple effect evaporator?
Steam economy refers to the pounds of water evaporated per pound of fresh steam used in a multiple effect evaporator set.
Below is a transcript of the video sample provided for this module:
At kraft pulp mills, black liquor evaporators are typically installed in multiple effect systems. These systems usually contain from five to seven evaporator bodies that are set up in a series. The term multiple effect comes from the effective use of steam energy multiple times to evaporate water from the black liquor. Each effect is made up of one or two evaporator bodies which operate at a different pressure and temperature from the other effects. In a typical system fresh steam from the recovery boiler is used to heat the black liquor in the first effect only, which operates at the highest temperature and pressure. The vapor generated from boiling the black liquor in the first effect is then used to heat the liquor in the second effect. This process of using vapor from one effect to heat the liquor in the next effect continues through the entire set with each effect operating at a slightly lower temperature and pressure than the previous effect.
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