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Separation (Mass Transfer) Unit Ops
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Components are separated using a semipermeable membrane
1. Membrane Separation Components are separated using a semipermeable membrane
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Function of a membrane separator
Separate a stream based upon phase or properties Prevent two phases from mixing Control concentration
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Types of membranes Porous Membranes:
Separation based on size and diffusivity Nonporous Membrane: Separation based on diffusivity
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Components are separated using a semipermeable membrane.
The permeate passes through the membrane The retentate does not pass through the membrane Common uses include: hemodialysis, desalination of brackish water, and sterilization of food and drugs
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Example flow diagrams of multiple separators
In parallel In series
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Visualization of a spiral-wound membrane
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Useful calculations Separation factor: A measure of relative diffusibility through membrane Cut: The fraction of feed which permeates through the membrane
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2. Gas Absorption/Stripping
Components are separated by contacting different phases
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Components are separated by contacting different phases
Gas absorption occurs when components of a gas are removed through contact with a liquid Stripping occurs when components of a liquid are removed through contact with a gas An example of absorption: the removal of ammonia from air using water An example of stripping: the removal of VOCs (volatile organic compounds) in wastewater
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Absorption and stripping are done in packed columns
Absorption and stripping are done in packed columns. The columns are filled with ‘packing’ that promotes contact between the vapor and liquid streams and facilitates mass transfer
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Absorption/Stripping Graphical Procedure
Plot the equilibrium curve and operating lines Count the theoretical number of stages required Find total height of packing can be found using HETP correlation info and number of theoretical stages Algebraic Solution of Equilibrium Stage Problems: The Kremser Equation (accessed Feb 3, 2020).
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Absorption/Stripping Kremser Method
Provides convenient algebraic solution for several separations cases Convert both equilibrium and operating information into linear format Can solve for # stages using Kremser equation (accessed Feb 3, 2020). Algebraic Solution of Equilibrium Stage Problems: The Kremser Equation (accessed Feb 3, 2020).
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3. Distillation A mixture of two or more species enters a vertical column to be purified, and separated via boiling point.
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Distillation Columns The column is made of consecutive stacked trays
The feed enters the column at one or more designated tray(s) There are holes in the trays to allow for the vapor to bubble up through the tray
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Distillation Columns Continued
Volatility The boiling points of the components are exploited to determine the optimal temperature for the column. Phases Due to the temperature of the column, the components reach different phases. The vapor rises as the liquid falls down the column Interactions As the two components interact more with each other, they exchange energy as they travel in opposite directions within the column. This results in desired purities.
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Distillation Column Design
Reflux Ratio: The ratio of fluid put back into the column to distillate product removed. Reboiler Concentration Number of Stages Feed Stage
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