Chapter 1: Basic Concepts
The basic educational goals of this WebBook (and the associated
Separation Processes course at UMBC) can described through
several illustrative separation
processes.
Example 1: Production of decaffeinated instant coffee.
The process for making decaffeinating instant coffee consists of the following steps:
- Caffeine removal from (steamed) green beans.
- Roasting and grinding of green beans.
- Coffee extraction.
- Multieffect evaporation to remove bulk of water from extract.
- Spray (or freeze) drying of remainder of water.
Caffeine removal in above process can be accomplished by several methods:
- Direct method: contacting with methylene chloride.
- Indirect method: contact coffee beans with hot water, caffeine removed from hot water
by contacting with methylene chloride.
- Swiss water method: contact coffee beans with hot water, caffeine removed from hot water
by adsorption onto activated carbon.
- Contact green beans with supercritical carbon dioxide.
Question to consider which illustrate goals (answers
to these questions will be considered later):
- Cost of separation equipment for a chemical process is generally greater than 50% of
capital cost, and for production of decaffeinated instant coffee it is greater than 90%.
- Why use countercurrent flow?
- Why concentrate extract in several steps?
- Why is aroma retention a consideration in multieffect evaporation but not spray drying?
- What considerations lead to the selection of solvents in decaffeination steps?
This brings us to the goals of this WebBook (and the Separation Processes
course at UMBC), which are to:
- Develop skills in qualitative reasoning and conceptual design.
- Develop skills in quantitative design (as illustrated in
the next separation processes example).
Example 2: Sweetening of sour gas
Natural gas often contains small amounts of hydrogen sulfide, the removal of which is
termed "sweetening."
The most common process is countercurrent absorption into a mixture of water and
ethanolamine.
Quantitative design considerations:
- What are the diameter and height for absorption column?
- What is inside the absorption
column (is it empty, full of a "packing", or does it have
"plates")?
- For a given flow rate of
natural gas, what should the flow rate of absorbing liquid be?
- What is the operating pressure?
- Mechanical-type design questions (e.g., how thick should the walls of absorption
column be) will not be considered.
General features of separation processes:
All separation processes require a "separating agent" to
function, due to the fact that mixing
is a spontaneous process. Separation processes are the opposite of mixing process and are
therefore never spontaneous. The separating agent makes the separation process run in the
desired direction.
Separation processes can be classified by various methods, e.g.,
- What is the separating agent (energy, mass etc.)?
- Is it a mechanical process (heterogenous feed) or a diffusional process
(homogeneous feed) where there is diffusional transfer from a "feed" phase to a "product"
phase?
- Diffusional processes can be further subdivided into equilibration processes, which
operate through equilibrium between phases, or rate-governed processes, which involve
differential rates of mass transfer to achieve a separation.
Additional Information:
