Original notes of Dr. F. Iskanderani
• A system is any portion of a process that can be enclosed within a hypothetical box
(boundary). It may be the entire process, a
single unit, or a point where streams converge or combine.
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
• Boundary A encloses the entire process.
– inputs: Streams 1, 2, and 3 – products: 1, 2, and 3
– Balances on A would be considered overall balances – internal streams would not be included in balances
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
• B: an internal mixing point (2 inputs, 1 product)
• C: Unit 1 (1 input, 2 products)
• D: an internal splitting point (1 input, 2 products)
• E: Unit 2 (2 inputs, 1 product)
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
• B: an internal mixing point (2 inputs, 1 product)
• C: Unit 1 (1 input, 2 products)
• D: an internal splitting point (1 input, 2 products)
• E: Unit 2 (2 inputs, 1 product)
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
• B: an internal mixing point (2 inputs, 1 product)
• C: Unit 1 (1 input, 2 products)
• D: an internal splitting point (1 input, 2 products)
• E: Unit 2 (2 inputs, 1 product)
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
• B: an internal mixing point (2 inputs, 1 product)
• C: Unit 1 (1 input, 2 products)
• D: an internal splitting point (1 input, 2 products)
• E: Unit 2 (2 inputs, 1 product)
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
• The procedure for solving material balances on multi-unit processes is the same as for a single unit; though, it may be necessary to perform balances on several process subsystems to get enough equations to determine all unknown stream variables.
Balances on Multiple Unit Ops
Original notes of Dr. F. Iskanderani
Two-Unit Process Example
• Variables for Streams 1, 2, and 3 are
unknown
Original notes of Dr. F. Iskanderani
Two-Unit Process Example
• Variables for Streams 1, 2, and 3 are
unknown
Original notes of Dr. F. Iskanderani
• Variables for Streams 1, 2, and 3 are unknown
• Label unknown stream variables Two-Unit Process Example
mixer
Original notes of Dr. F. Iskanderani
• Degree-of-freedom analysis
– overall system: 2 unknowns – 2 balances = 0 (find , x
3) – mixer: 4 unknowns – 2 balances = 2
– Unit 1: 2 unknowns – 2 balances = 0 (find , x
1) – mixer: 2 unknowns – 2 balances = 0 (find , x
2)
Two-Unit Process Example
mixer Unit 1
Original notes of Dr. F. Iskanderani
• Degree-of-freedom analysis
– overall system: 2 unknowns – 2 balances = 0 (find , x
3) – mixer: 4 unknowns – 2 balances = 2
– Unit 1: 2 unknowns – 2 balances = 0 (find , x
1) – mixer: 2 unknowns – 2 balances = 0 (find , x
2)
Two-Unit Process Example
mixer Unit 1
Original notes of Dr. F. Iskanderani
Extraction-Distillation Process
Original notes of Dr. F. Iskanderani
Simultaneously solve total mass and acetone balances to determine m1 and m3.
Solve MIBK* balance to determine xM1.
Extraction-Distillation Process
*MIBK: Methyl Iso Butyl Ketone
Original notes of Dr. F. Iskanderani
Solve acetone, MIBK, and water balances to determine mA4, mM4, and mW4.
Extraction-Distillation Process
Original notes of Dr. F. Iskanderani
For either (just 1) extractor unit, solve acetone, MIBK, and water balances to determine mA2, mM2, and mW2.
Extraction-Distillation Process
Original notes of Dr. F. Iskanderani
ndf = 4 unknowns (mA6, mM6, mW6, and m5) – 3 balances = 1 underspecified
Extraction-Distillation Process
Original notes of Dr. F. Iskanderani
ndf = 4 unknowns (mA6, mM6, mW6, and m5) – 3 balances = 1 underspecified
Extraction-Distillation Process
