At the end of this module, you will be able to:
- Describe the purpose of evaporators
- List the objectives of black liquor evaporators
- Describe the key principles of steam theory, including the relationship between temperature and pressure, and flashing
- Describe the effect of concentration on boiling point rise
- Describe why evaporator body sizes are not all the same
The following key questions are answered in this module:
What is boiling?
Boiling is the conversion a liquid to a gas by the addition of heat.
What happens to the solids in a water solution when the solution is boiled?
As water boils into steam, the solids in the solution do not boil away. As the amount of liquid water in the solution becomes smaller, the concentration of solids in the solution increases.
How can the boiling temperature of water be changed?
The boiling temperature of water is dependent on its pressure. As the pressure above a body of water decreases, so does the boiling temperature.
What is latent heat?
Latent heat is the heat which is absorbed or released during a phase change, such as when water is boiled to create steam.
Why are the final effects in multiple effect evaporators sometimes larger than the initial effects?
The pressures in the later effects of a multiple effect evaporators are lower than the initial effects. Gases expand as the pressure drops, so the later stages are made larger in an attempt to keep gas velocities down.
Below is a transcript of the video sample provided for this module:
At a kraft pulp mill, evaporators are used to remove water from the used pulping chemical solution in order to concentrate it. This is the first step in the process that is used to recover the chemicals so that they can be reused in the pulping process. Black liquor evaporators have several objectives. One, efficiently heat and remove water from the black liquor so that it can be fired into the recovery boiler. Two, avoid scale formation inside the evaporators. Three, produce condensate or warm water that is clean enough for reuse in the pulp mill and recausticizing area in order to reduce fresh water usage. And four, safely remove any volatile components and NCGs or non-condensable gases that are produced.
Use the additional resources and links below to learn more about this topic: