Ch E 452: Process Design, Analysis, and Simulation Flowsheet Separation Train

1
Ch E 452 Process Design,Analysis, and
SimulationFlowsheet Separation Train

• David A. Rockstraw, Ph.D., P.E.
• New Mexico State University
• Chemical Engineering

2
Separation Train Structure

• To determine the general structure of the
  separation train, determine the phase of reactor
  effluent.
• For vapor-liquid systems, there are three
  possible scenarios
• Assume
• phase splits are cheapest method of separation
• some type of distillation separation is possible

3
Separation Train Structure

• Reactor effluent is liquid
• assume that we only need a liquid separation
  system, which may include distillation columns,
  extraction units, azeotropic distillation, etc.

reactor system
liquid separation system
liquid
products
feeds
liquid recycle
4
Separation Train Structure

• Reactor effluent is 2-phase
• Use the reactor as a phase splitter (or put a
  flash drum right after the reactor to separate at
  a pressure different from the reaction).

vapor
reactor system
liquid
feeds
5
Separation Train Structure

• Reactor effluent is 2-phase
• Send liquid to a liquid separation system.

vapor
liquid separation system
reactor system
liquid
products
feeds
liquid recycle
6
Separation Train Structure

• Reactor effluent is 2-phase
• If operating above cooling-water temperature,
  cool reactor vapor to 100F and phase-split.

phase split
vapor
vapor
liquid
liquid separation system
reactor system
liquid
products
feeds
liquid recycle
7
Separation Train Structure

• Reactor effluent is 2-phase
• If the low-temperature flash liquid contains
  mostly reactants (an no product components formed
  as intermediates in a consecutive reaction),
  recycle.

phase split
vapor
liquid
vapor
liquid separation system
reactor system
liquid
products
feeds
liquid recycle
8
Separation Train Structure

• Reactor effluent is 2-phase
• If the low-temperature flash liquid contains
  mostly products, send to liquid separation system.

phase split
vapor
vapor
liquid
liquid separation system
reactor system
liquid
products
feeds
liquid recycle
9
Separation Train Structure

• Reactor effluent is gas
• Cool to cooling water temperature (100F) and
  phase split or completely condense.

vapor
reactor system
vapor
phase split
feeds
liquid
10
Separation Train Structure

• Reactor effluent is gas
• Cool to cooling water temperature (100F) and
  phase split or completely condense.

gas recovery system
purge
vapor
reactor system
vapor
phase split
liquid
feeds
liquid
liq sep system
products
liquid recycle
11
Separation Train Structure

• Reactor effluent is gas
• Send condensed liquid to liquid recovery system,
  vapor to gas recovery system.

gas recovery system
purge
vapor
reactor system
vapor
phase split
liquid
feeds
liquid
liq sep system
products
liquid recycle
12
Separation Train Structure

• Reactor effluent is gas
• If no phase split is obtained under cooling
  water,
• can reactor system be pressurized to induce a
  split without effecting product distribution?
• If not, consider use of high pressure and a
  refrigerated partial condenser.
• If not, consider sending reactor effluent
  directly to vapor recovery system.

13
Vapor Recovery System Decisions

• What is the best location?
• What is the cheapest type of vapor recovery
  system?
• Location choices
• Purge stream
• Gas-recycle stream
• Flash vapor stream
• none

purge
gas recycle
prevent loss of valuable or undesirable material
prevent recycle of certain components
vapor from phase split
14
Vapor Recovery Systems

• The best location for the VRS is
• the purge stream if significant amounts of
  valuable materials are being lost in the purge
• the gas-recycle stream if materials that are
  deleterious to the reactor operation are present
• The flash vapor if both items are important

purge
gas recycle
prevent recycle of certain components
prevent loss of valuable or undesirable material
vapor from phase split
15
Vapor Recovery Systems

• The types of VRSs available are
• Condensation (high pressure, low temperature,
  both)
• Absorption (into liquid-phase)
• Adsorption (onto solid adsorbent, i.e., carbon,
  zeolite, etc.)
• Membranes (pervaporation, ceramics)
• Reactors (chemical nature of vapor is changed)

16
Vapor Recovery Systems

• VRS Strategy
• Design VRS before LRS because each of the VRS
  processes typically generates a liquid stream
  that must be further purified
• Combine the VRS and LRS and feed the reactor
  effluent directly to the LRS when
• A partial condenser and flash drum are being
  considered to phase-split the reactor effluent
  and only a small amount of the liquid components
  are leaving with the flash vapor
• And the first LRS unit is chosen to be
  distillation
• Increases distillation column diameter, but
  increased cost may be less than costs associated
  with using a VRS
• No heuristic is available, so this must be
  considered as a process alternative

17
Liquid Recovery System Desicions

• How should light ends be removed if they
  contaminate the product?
• What should be the destination of light ends?
• Should components forming azeotropes with the
  reactants be recycled, or should azeotrope be
  broken?
• What separations can be made by distillation?
• What sequence of columns should be used?
• How should separations be accomplished if
  distillation is not feasible?

18
LRS - Light Ends

• Alternatives for light-ends disposal
• Drop pressure or increase temperature of a
  stream, and remove the light ends in a phase
  splitter (flash drum).
• Use a partial condenser on the product column.
• Use a stripper (pasteurization) section on the
  product column or a stripper (stabilizer column)
  before the product column.
• Destination
• Vent to flare
• low value
• environmental
• Send to fuel
• Flammable w/ fuel value
• Recycle to VRS
• Valuable
• Introduces another recycle

19
LRS Azeotropes with Reactants

• Recycle azeotropic mixture
• Requires all equipment in the recycle loop be
  oversized to handle incremental flow of the extra
  components
• Break the azeotrope
• Usually requires two columns, one operating at
  non-atmospheric pressure

20
Applicability of Distillation

• In general, distillation is the least expensive
  means of separating mixtures of liquids.
• If the relative volatilities of two components
  with neighboring boiling points is less than
  about 1.1, distillation becomes very expensive
  due to
• Large reflux ratios, corresponding to large vapor
  rates and Large column diameters.
• Large condensers/reboilers, corresponding to
  large utility demands.
• Group such close a neighboring boilers, and treat
  as a single component, and develop the best
  distillation strategy for this group.

21
Column Sequencing

• Alternatives for a ternary mixture

22
Column Sequencing
As number of components increase, so do the
alternate sequences available to the designer.
23
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCDE BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

possible sequences for a 5-component separation
24
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

20 possible column designs
25
Column Sequencing Heuristics

• Remove corrosives as soon as possible.
• Remove reactive components or monomers as soon as
  possible.
• Remove products as distillates.
• Remove recycle streams as distillates,
  particularly if they are recycled to a packed bed
  reactor.

26
Column Sequencing Heuristics

• First
• Most plentiful (composition)
• Lightest (relative volatility)
• Last
• High-recovery separations
• Difficult separations (relative volatility)
• Favor equimolar splits (composition)
• Next separation should be cheapest
• Conflicts in heuristics will occur

27
Separation System/Process Interactions

• Consider these two process alternatives

28
Separation System/Process Interactions

• Selecting the best separation sequence cannnot
  always be isolated from the design of the
  remainder of the process.
• Thus, select the sequence that minimizes the
  number of columns in a recycle loop.

29
Complex Column Heuristics

• Ease of separation index (ESI)
• If ESI lt 1, A/B split is harder than B/C split
• If ESI gt 1, A/B split is easier than B/C split

30
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI lt 1.6 40-80 middle product nearly
equal amounts of overhead and bottom present
31
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI lt 1.6 50 middle product lt 5 is
bottoms
32
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI lt 1.6 50 middle product lt 5 is
overheads
33
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI lt 1.6 lt15 middle product equal amounts
overhead and bottoms
34
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI lt 1.6 otherwise, whichever of 1 or 2 that
removes the most plentiful component first
35
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI gt 1.6 gt50 bottoms product
36
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI gt 1.6 gt50 middle product 5-20 is
bottoms
37
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI gt 1.6 gt50 middle product lt 5 bottoms
38
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI gt 1.6 gt50 middle product and lt 5
overheads
39
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

ESI gt 1.6 otherwise, favor design 3
40
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

Alternatives, if less than half the feed is
middle product
41
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

Considered when a low, middle product purity is
acceptable
42
Column Sequencing

• column 1 column 2 column 3 column 4
• A/BCDE B/CDE C/DE D/E
• A/BCDE B/CDE CD/E C/D
• A/BCDE BC/DE B/C D/E
• A/BCDE BCD/E B/CD C/D
• A/BCDE BCD/E BC/D B/C
• AB/CDE A/B C/DE D/E
• AB/CDE A/B CD/E C/D
• ABC/DE D/E A/BC B/C
• ABC/DE D/E AB/C A/B
• ABCD/E A/BCD B/CD C/D
• ABCD/E A/BCD BC/D B/C
• ABCD/E AB/CD A/B C/D
• ABCD/E ABC/D A/BC B/C
• ABCD/E ABC/D AB/C A/B

Why havethese not beendiscussed?
43
Complex Column Heuristics

• Use the direct sequence if

B
A
A B C
C
44
Complex Column Heuristics

• Use the indirect sequence if

45
Complex Column Heuristics

• consider a sidestream
• when lt 30 of the feed is intermediate
• when xAF and/or xCF lt 0.1
• intermediate is recycled, high purity not
  required
• volatiles are not evenly distributed
• above the feed when the intermediate is more
  difficult to separate from the heavy than from
  the light. Otherwise, consider a sidestream
  below the feed.

A
A
B
A B C
A B C
B
sidestream stripper
sidestream rectifier
C
C
46
Complex Column Heuristics

• consider a Petlyuk Column
• for large or moderate xB, when
• volatilities balanced both splits difficult,
  i.e., aAB ? aBC ? 2
• Split A/B is difficult, and B/C is easy, i.e.,
  aAB lt aBC
• When A/B and B/C splits are of similar
  difficulty, and xA gt 0.5
• for low xB, when xA is close to (aAB 1)/(aAC 1)
• though a sidesection column may be better since
  it has similar vaor savings, but less trays
• Heuristics for use of the Petlyuk column are not
  always correct because performancedepends upon
  volatilities. If thevolatilities are evenly
  distributed,Petlyuk column should be considered.

A
A,B
A B C
Petlyuk Column
B
B,C
C
47
Other Types of Separations

• If distillation is too expensive, alternatives
  are commonly considered
• Extraction
• Extractive distillation
• Azeotropic distillation
• Reactive distillation
• Crystallization
• Adsorption
• reaction

48
Extraction

• Solvent S is added to B/C mixture.

B
S
C (B)
B C
C,S
C
49
Extractive Distillation

• A heavy component (S) is added to change the VLE.
  The heavy component is recycled in the system.

B
C
S
B C
Example B HNO3 C H2O S H2SO4
C,S
50
Azeotropic Distillation

• Azeotropes can be broken by
• With 3 columns by addition of a third component
• With 2 columns by changing operating pressure

BCS ternary hetrogeneous azeotrope
Example B ethanol C water S benzene
B,C Azeotrope
B C
C
B,C
B
51
Reactive Distillation

• Addition of an entrainer can often react with one
  of the components in the mixture that is
  difficult to separate. The reaction is then
  reversed in a second column.

B
S
Example B C xylenes aBC 1.03 S
organometallic aB/CS 30
B C
CS
C