Design of Gas Permeation
Membrane Systems
Ramagopal V S Uppaluri
Supervisor: Robin Smith
6-1
XVIII PIRC Annual Research Meeting 2001
Abstract:
This presentation addresses the design of gas permeation membrane networks.
Three salient features are investigated, namely pressure ratio optimisation,
membrane screening and selection and novel membrane design.
Pressure ratio optimisation refers to the optimisation of permeate and retentate
pressures, often specified as parameters of the synthesis problem. The optimisation
procedure involves development of pressure ratio selection criteria coupled with
process design calculations. Membrane screening and selection procedures can
address membranes of diversified properties and manufacturing costs. Many salient
features of industrial gas processing can be considered such as debottlenecking,
high purity applications and lean feed processing. Novel membrane design
procedures indicate the development of optimal membrane properties.
Modifications proposed for novel membranes can be screened using experimental
data to provide membrane properties. Input from polymer developments can
provide more streamlined search procedures.
Illustrative examples are provided for the developments. Pressure optimisation
studies are performed for air separation, membrane screening procedures for natural
gas sweetening process, and novel membrane design for hydrogen recovery from
multi-component refinery gas streams.
Outline
1 Introduction
2 Process Design Framework
2.1 Modelling framework
2.2 Optimisation framework
3 Pressure Ratio Optimisation
4Membrane Screening
5Novel membrane Design
6Conclusions & Future work
6-2
Design of Gas Permeation Membrane Systems
1. Introduction
6-3
Design of Gas Permeation Membrane Systems
Design philosphy#
Process
Synthesis
Network design
#
PIRC Meeting 2000
6-4
• Automated design procedure
Variations in network structure
Simple networks to start with
• Network design for established applications
Hydrogen recovery
Air separation
• Optimisation features
Robust procedure
Multiple solutions
Solutions close to global optimal
• Optimisation considers
Structural optimisation
Area allotment
Feed rate optimisation
Flow patterns
• Optimisation constraints
Permeate/retentate purity
Permeate/retentate flow rate
Recovery
Design of Gas Permeation Membrane Systems
Existing design philosophies for gas permeation membrane systems are limited in
their capability to design a membrane network for required product specifications.
A new synthesis model has been formulated to allow robustness in design
calculations used in process synthesis. Quality solutions close to the global optimal
domain can be achievable.
Existing philosophies for design of gas permeation membrane systems only
consider a membrane network with assumed retentate and permeate pressures and
cannot exploit many oppurtunities that remain unaddressed. Membrane
applications for high purity applications, debottlenecking, lean feed processing are
of utmost importance for stabilising membrane technology in the chemical industry.
These issues need to be addressed with a systematic methdology.
Membrane systems: Research activity
Network
development
15%
Module development
Modelling issues
35%
50%
Membrane modification
Polymer chemistry
6-5
Contributions
from process
synthesis can be
beneficial for
chemical industry
Design of Gas Permeation Membrane Systems
Network development issues include membrane network upgrading from
conventional polymers, parametric analysis of system parameters etc. Membrane
modification involves complex polymer formulations coupled with network design
insights. Both these areas can be substantiated with ample oppurtunities evolving
from process synthesis techniques. Contributions henceforth, could evolve
extending the design philosphy.
Network development:
Degrees of freedom:
PH
PL
Retentate
pressure
(> 1 atm)
Permeate
pressure
(</=/> 1 atm)
Membrane
properties*
• Permeabilities (PM)
• Selectivities (S)
• Annualised cost(CM)
Network cost = f(PH, PL,PM, S, CM)
PM: Component flux per unit pressure
S: Ratio of permeaibilities
6-6
Design of Gas Permeation Membrane Systems
Major degrees of freedom for a membrane network include a) pressure ratios across
the membrane system and b) membrane properties. Pressure ratio can be varied as
mentioned in this work to be vacuum, atmospheric or higher pressures for permeate
and high pressures on the retentate side. Membrane properties that need to be
screened include permeabilities, selectivities and annualised cost of membranes,
coupled to offer economic feasibility.
Network development: Summary
Network cost = f(PH, PL,PM, S, CM)
6-7
Network cost = f(PH, PL)
Membrane properties frozen
Network cost = f(PM, S, CM)
Pressures frozen
Pressure ratio
optimisation
Membrane screening and selection
procedures
Design of Gas Permeation Membrane Systems
Membrane screening: Overview
Different membranes
•
•
•
•
High permeability low selectivity
Low permeability high selectivity
Moderate permeability, selectivity
High membrane cost coupled with
improvement in properties
Network design
Industrial concerns
•
•
•
•
Network cost reduction
Lean feed processing
Debottlenecking
High purity applications
Effective screening
• Best membrane
• Performance limits of all
membranes
• Process economics
6-8
Design of Gas Permeation Membrane Systems
Rapid development in membrane formulations coupled with experimental validity
of membrane properties offer a large number of membranes with diverse properties
such as high permeability and low selectivity, low permeability and high selectivity
etc. Utilisation of network design procedures systematically for such membranes
would evolve searching for the most economic membrane and quantify perfomance
limits of the membrane process in comparision with alternative separation
processes.
Novel membrane development: Summary
Targets
• Increasing S at
constant PM
• Increasing PM at
constant S
• Increasing PM with
appropriate increase
in S
Advantages
• Network cost
reduction
• High purity
applications
• Lean feed
processing
Limitations
• High manufacturing
costs
• Difficulty in estimating
performance
• Difficulty in crossing
PM - S upper bound
(see later)
Novel membranes: Disadvantages
• Membranes developed without considering economic feasibility
• Limited applications in the industrial regime
• Applications finally culminate in small scale applications
Palladium membranes: Costly
Surface selective flow (SSF) nanoporous membrane: Limited to membrane PSA systems
6-9
Design of Gas Permeation Membrane Systems
Novel membrane development aims to target developing membranes with enhanced
properties such as high selectivity at existing permeability or high permeability at
existing selectivity or moderately high permeability and selectivity. Stumbling
blocks to such developments are of course, complexity of bringing these properties
out at the polymer chemistry level itself. However, design insights from process
design experts can provide greater understanding for various membrane
modifications. Existing trends in developing novel membranes are confined to
niche applications or to develop membranes to be accomodated in hybrid membrane
systems.
Polymer chemistry & process development: Overview
Membrane preparation &
characterisation
Economic evaluation
• Specify PM, S, manufacturing cost
• Specify performance limits
Polymer chemist
• Single stage systems
• Non-automated network design
Lost
opportunties!
Membrane process
design specialist
Chemistry modification
• Modify performance
• Modify S
• Modify PM
• Modify PM,S
• Specify maximum
manufacturing cost
In situ experience:
• Membrane performance as defined
• Bottlenecks in purity, throughput
• Better to use other separation
technology?
• Limited information to chemist.
• Chemist may not explore development of the best membranes
• Other separation technology may completely replace membranes
6 - 10
Design of Gas Permeation Membrane Systems
Existing links between polymer chemistry development and subsequent studies on
economic viability of developed polymers are very weak. A set of guidelines is not
available for polymer chemists to propose membranes with properties that are
suitable for industrial scenarios. Process design procedures need to address such
guidelines.
Summary of present objectives:
Membrane
screening
Pressure ratio
optimisation
Robust process
design
framework
Novel membrane
development
6 - 11
Design of Gas Permeation Membrane Systems
2. Process Design Framework
6 - 12
Design of Gas Permeation Membrane Systems
2.1 Modelling framework
state 1 (HP)
SP1,1
SP1,ffeed
c=1
f =1
c=3
c=2
MI1,1
MI1,2
SP1,prprod
MI1,prprod pr =1
MI1,3
MISC2,1,2
MISC2,1,1
SP1,3
SP1,2
MISC2,1,3
SP2,prprod
c=3
c=2
c=1
MI2,prprod
SPSCP2,2
pr =1
SPSCP2,3
state 2 (LP)
COM1,f
COM2,1,c
VAC2,c
• Accomodates co-current, counter-current and cross-flow patterns
• Accomodates generic vacuum pump representation (if necessary)
• Optional feed compression (based on feed condition)
• Pressure drop along the membrane neglected
6 - 13
Design of Gas Permeation Membrane Systems
The modelling framework is a superstructure of membrane compartments, vacuum
pumps, feed compressor (optional), permeate recycle compressor, permeate product
compressor (optional). The stream network allows realisation of industrial
membrane permeators. Vacuum pump allocation follows from the methodology,
where a vacuum pump is allowed for each membrane unit whose permeate stream
undergoes partial permeate recycle. A vacuum pump is alloted to all those permeate
streams which do not undergo any permeate recycle.
Networks developed from modelling framework:
Conventional networks:
A) One stage system
D) Two stage stripping cascade
C) Recycle stripper
B) Two stages in series
E) Enriching cascade with extra stage
F) Two stage enriching cascade
Novel networks:
A)Feed contacting
6 - 14
B) Partial retentate processing
Design of Gas Permeation Membrane Systems
C) Partial product(s) processing
2.2 Optimisation framework
Problem data
and
specifications:
Superstructure
network model
Biases
Stochastic
optimisation
• Fixed network
optimisation
• Fixed flow
patterns
New representation
• Non-linear modelling framework
• Easy to get trapped into local
solutions
Structural optimisation
Designs
Analysis
6 - 15
• Solutions near to global optimal
• Multitude of solutions
• Conventional networks
• Novel networks
Design of Gas Permeation Membrane Systems
Synthesis methodology involves stochastic optimisation. Stochastic optimisation is
highly robust and provides solutions close to the global optimal solution domain.
Further, any analysis can be subjected to optimised network structures by allowing
biases in the network design problem. Conventional networks can be developed
from novel networks with similar biases for industrial network simplicity.
Process synthesis : Implementation framework
Problem specification:
• Permeabilities • HP , LP values
• Selectivities
Perturbation Moves:
• Network changes
• Operational changes
6 - 16
• Cost parameters
• Product purities
Simulated annealing
decisions:
Accept/Reject/Terminate
Design of Gas Permeation Membrane Systems
Optimal
solution
Optimisation variables
Moves allow transition from intial superstructure to final optimal
structure through different changes
Moves
6 - 17
Structure moves
Area move
• Add unit
• Remove unit
• Modify area
Stream moves
• Add stream
• Remove stream
• Modify stream
Design of Gas Permeation Membrane Systems
The membrane network superstructure optimisation procedure involves both
structural and operational changes. Structural changes involve adding and removing
membrane compartment units. Operational changes involve adding, removing and
modifying streams and modifying membrane area. Optional changes involve feed
rate optimisation moves for optimising feed rate (if necessary) for particular product
specifications.
3. Pressure Ratio Optimisation
6 - 18
Design of Gas Permeation Membrane Systems
Optimal pressure ratio:
Optimal pressure ratio is the pressure ratio that provides minumum network
cost for required product specifications
• Optimal pressure ratio a strong function of membrane properties
Membrane thickness
Membrane permeabilities (PM) and selectivities (S)
Cost of membrane
• Optimal pressure ratio a strong function of product specifications
Higher product purities force higher pressure ratios
Lower product purities require lower pressure ratios
• Optimal pressure ratio is a strong function of permeate pressure
High permeate pressure would force high retentate pressures
Vacuum applications on the permeate side may be advantageous to reduce compression cost
• Optimal pressure ratio affects overall process economics
Membrane properties play a major role in determining
optimal pressure ratio
6 - 19
Design of Gas Permeation Membrane Systems
Limitations of existing procedures:
• Optimisation oriented
Fixed network structure and subsequent analysis for pressure optimisation
Solving non-linear framework imposes difficulty in robustness
Solutions close to global optimal domain cannot be reached easily
• Operation at locally optimal pressure ratio
• Conceptual oriented
Non-generalised concepts for retenate and permeate pressure selection
Rules of thumb may lead to operation at inferior pressure ratios
Lack of concepts for networks with and without feed compressor
Contributions from present work:
Conceptual approach coupled with robust process
synthesis
• Pressure ratio selection close to global optimum
• Extensions to existing research directions
Statistical design of pressure ratio optimisation experiments
Knowledge based insights
6 - 20
Design of Gas Permeation Membrane Systems
Contributions from the present work include the development of conceptual criteria
coupled with a process synthesis framework. Such methodology could evolve
optimal pressure ratio evaluation close to the global value. Enhanced developments
include statistical design of experiments in case other parameters such as membrane
thickness need to be addressed. Knowledge based insights can be provided for
membranes whose cost information is not available. Case studies can also be
performed for membranes with different product specification constraints to study
the impact of system parameters on pressure ratio optimisation.
Pressure ratio selection: Methodology
Feasible
pressure ratio
Conceptual
selection criteria
+
Modify pressure
ratio
Process
synthesis
framework
Analysis
6 - 21
Optimal
pressure ratio
Design of Gas Permeation Membrane Systems
Membrane system pressure ratio feasibility is directly related to membrane area
bounds. An infinite area is required for a negligible pressure ratio across the
membrane system, but feasible area bounds would confine the study of membrane
system pressure optimisation to feasible pressure ratios. Modification of pressure
ratio needs trial and error based procedures to evolve trends of network cost with
respect to permeate and retenate pressures. Later modification of pressure ratio can
be set to definite values basing on the trends obtained from graphical analysis.
Conceptual selection criteria:
Networks belong to ...........
A) Networks with feed compressors
8
•Air separation
8
B) Networks without feed compressors
•Hydrogen recovery
8
8 Desired product streams
6 - 22
Design of Gas Permeation Membrane Systems
Conventional membrane networks involve networks with and without feed
compressors. Other modifications are possible for these networks, such as addition
of energy saving equipment for retentate product streams and networks with
permeate product compressors for permetae product streams.
A) Networks with feed compressors: Conceptual selection criteria
#Optimal permeate pressure is a strong
function of vacuum costing equations
Q Vacuum costing equations require more concern
Q Pressure above 1 atm. Can be controlled using
valves (hence, no costing equations desired)
PP1
PP2
PP3
FPopt
Feed pressure
#Optimal feed pressure = f (permeate
pressure)
#Other costing equations in network
optimisation framework and flow patterns
can be considered
#Best pressure ratio corresponds to (FPopt,
PP3)
#General rules of thumb discarded
Q Area contribution = at least 50 % of network cost
Q Compressor contribution = at least 30 % of
network cost
Complete mathematical treatment of pressure optimisation problem!
6 - 23
Design of Gas Permeation Membrane Systems
For networks with feed compressors, conceptual criteria follow from the fact that
both permeate and retentate pressures need to be searched before finding optimal
pressure ratio. The optimal pressure ratio at a particular permeate pressure need not
be the globally optimal pressure ratio. Further, permeate pressure should be studied
ranging from sub-atmospheric to atmospheric. Application of vacuum on permeate
side requires a vacuum pump operating with additional fixed costs and a sharp
decline in membrane area cost. Hence, optimisation of permeate pressure
considering vacuum is important for systems with feed compressors.
B) Networks without feed compressors: Conceptual selection criteria
#No feed compressor
#High permeate pressure involves
low purity, low recovery, low
recycle compressor costing, high
area cost
#Low permeate pressure involves
high purity/recovery and low area
cost
PPopt
Permeate pressure
Key information for pressure ratio selection:
Cost of membrane
Vacuum application
Lower bound for vacuum application to be defined
Permeability and selectivity data to be consistent with industrial
performance
Deteriotation of membrane forces operation at locally optimal pressure ratio or nonoptimal pressure ratio.
6 - 24
Design of Gas Permeation Membrane Systems
Networks without feed compressors are common in applications for gas processing.
Major networks for hydrogen recovery from purge streams involve networks
without feed compressors, as the feed is already is at high pressure. For such
systems, permeate pressure needs to be optimised for the particular membrane cost.
Again the search is only confined to a narrow range where optimal pressure is
located.
Summary: Pressure Ratio Selection
Systematic method developed for selection of optimal pressure
ratio
Accounts for
• All system costs
• Network structure
• Vacuum application
• Selection of membrane
6 - 25
Design of Gas Permeation Membrane Systems
CASE STUDY: Enriched N2
Product specifications#
Feed
O2
0.01045 kmol/s
N2
0.03955 kmol/s
#
N2 purity
(mole fraction)
0.95
N2 recovery
0.80
Retentate stream
Search for
• Pressure ratio range for optimality
• Variation of optimal network cost with permeate pressure
• Utilisation of vacuum economic ?
• Best pressure ratio combination (retentate, permeate)
• Network configuration for optimal pressure ratio
6 - 26
Design of Gas Permeation Membrane Systems
Problem specifications:
P  kmol 
δ ~  m2 . s. bar 
Membrane*
O2
3.01438 X 10-6
N2
4.01918 X 10-7
δ = 10-6 m
~
cm 3 (STP)
P (O2) cm 2 .(cmHg). cm X10
10
Membrane
cost
(Annualised)
100 $/m2
Area bounds
[50, 7000]
m2
* Singh A., Koros W.J., 1996. Significance of entropic selectivity for advanced gas separation
membranes, Ind. Chem. Eng. Res., 35, 1231-34
6 - 27
Design of Gas Permeation Membrane Systems
Membrane specifications considered for pressure optimisation case studies are
taken from Singh & Koros (1996). The membrane selected is available on the
upper bound of P - S line as shown in the figure. Further, out of all those
membranes existing on the P - S upper bound, the membrane selected offers the
best combination of both permeability and selectivity. Membrane thickness is taken
to be about 1 micro m. Feasible area bounds were provided as shown in the slide.
Membrane annualised cost is taken as $ 100 per unit square meter of area.
Vacuum pump 1 & recycle compressor costing 2:
Vacuum pump
Compressor, inlet P = 1.0 bar
Compressor, inlet P = 2.0 bar
25000
20000
Power =
15000
Power parameter xflow
efficiency
Op.cost ($ / yr) = Power x 294.52
10000
a
Fixed cost ($ / yr) =
Power x Cost constant
Plant life
5000
Efficiency
0
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
5
10
15
20
25
30
35
40
2.
Vacuum
0.60
3250.0 0.50
Compressor
0.75
3791.5 0.82
45
Compressor outlet pressure (bar)
1
Plant life = 3 yrs.
According to Bhide & Stern, J. Mem. Sci. 62, 13 - 35
Douglas J., 1989. Conceptual Design of Chemical processes, Mc Graw Hill , NewYork
6 - 28
a
0.9
Vacuum pressure (bar)
0
Cost
constant
Design of Gas Permeation Membrane Systems
Feed compressor costing2:
1e+7
9e+6
Fixed cost
Operating cost
8e+6
Op. cost ($ / yr.) = Cost parameterop x flow
Fixed cost ($ / yr.) = Cost parameterfixed
7e+6
x flow 0.82
6e+6
5e+6
[Area cost
+ recycle compressor cost
Objective = ∑
+ vacuum pump cost
4e+6
+ feed compressor cost]
3e+6
2e+6
1e+6
0
5
10
15
20
25
30
35
40
45
Retentate pressure (bar)
2
Douglas J., 1989. Conceptual Design of Chemical processes, Mc Graw Hill, NewYork
6 - 29
Design of Gas Permeation Membrane Systems
Results:
Optimal pressure ratio
Impact of vacuum
160
122
Permeate pressure
0.1 bar
0.2 bar
0.5 bar
0.8 bar
1.0 bar
1.2 bar
2.0 bar
150
140
35
Network cost
Optimal retentate pressure
120
30
118
25
116
130
20
114
120
15
112
110
0
5
10
15
20
25
30
35
40
45
110
10
0.0
Retentate pressure (bar)
6 - 30
0.5
1.0
1.5
2.0
2.5
Permeate pressure (bar)
Design of Gas Permeation Membrane Systems
The graphs provide a summary of results obtained after the search for the optimum
pressure ratio. Each point in the graphs refers to network costs close to the global
solution and are best solutions for network cost. Further, simulations were
performed to consider a wide range of permeate pressures ranging from 0.1 bar to
2.0 bar. Emphasis is to analyse optimal pressure ratio combination from such
studies.
Optimal pressure ratio and network configuration:
16 bar
16 bar
16 bar
1096.7
209.4
1092.7
0.5 bar
0.5 bar
0.5 bar
0.20
Network cost = $ 1,111,300
Conclusions:
• Optimal retenate pressure range varied from 12 bar to 24 bar from
permeate pressures of 0.1 bar to 2.0 bar
• Network cost optimal at 0.5 bar vacuum. This shows the significance of
vacuum for present case.
• Quality of solution only improved to a maximum of 8.3 % with permeate
pressure optimisation
• Optimal combination of permeate/retentate pressures is around [0.5 bar,
16 bar]
6 - 31
Design of Gas Permeation Membrane Systems
Vacuum pump
Recycle compressor
Feed compressor
4. Membrane Screening
6 - 32
Design of Gas Permeation Membrane Systems
Established screening procedure:
Selection of membrane
Quantification of membrane
properties
• Specify PM, S, manufacturing cost
• Specify minumum membrane
thickness
Polymer chemist
Membrane process
design specialist
Industrially
feasible
membrane(s)
Knowledge based systems:
• Process economics of
separation systems
• Select a particular membrane
• Assume a simple membrane
network (usually single stage)
• Perform economic evaluation
• Repeat for all available membranes
• Choose the best memrbane
Insitu experience:
Variation with product specifications
Variation with throughput
Variation with feed composition
• Deterioating membrane
performance
• Bottlenecks in purity, throuput
• Advances in other separation
technology offer better economics
Economic evaluation is used in the heart of the procedure
6 - 33
Design of Gas Permeation Membrane Systems
Established screening procedures are summarised as follows. Polymer researchers
specify membrane properties such as permeabilities, selecitivities, membrane
thickness and annualised membrane cost. Selection procedures for a particular
membrane use economic evaluation procedures at the core of selection. Input from
industrial membrane performance would allow more flexibility for selection.
Further, knowledge based insights also provide key information for membrane
process industrial applications.
Limitations of Previous Work:
• Confined to limited network parameters
• Non-automated procedure
• Non-systematic procedure
• Poor solution space due to inadequate computation
• Membrane network cost may or may not be global optimal solution
• Targeting optimal pressure ratio, area bounds difficult
• Targeting for higher throughput, purity not considered
Basic disadvantages:
• Lack of robustness (convergence not guarenteed)
• Poor quality of solution (NLP or MINLP procedures)
• Difficult to incorporate knowledge based systems into screening
Established screening procedures could not provide
enough insights for industrial concerns
6 - 34
Design of Gas Permeation Membrane Systems
New Screening Methodology:
Step 1: Preliminary screening framework
• Eliminate economically infeasible membranes
• Streamline polymer chemistry research activity
Step 2: Quantify membrane performance:
• To systematically exploit many oppurtunities
• To evaluate the best membrane economically feasible
• To search for alternate membranes in case best membrane deteriorates in
performance through time
Step 3: Knowledge based insights
• To quantify membrane network process economics
• Useful for the design of competing separation processes
as well
Alternatively, methodology can be terminated at any level
due to practical limitations and problem uncertainties
6 - 35
Design of Gas Permeation Membrane Systems
Preliminary screening framework:
Network
development
Consider all membranes
[Pi,M,Si,M,CM,δΜ]
Network
upgrading
Consider a conventional
membrane
[Pi,M*,Si,M*,CM*,δM*]
Knowledge based
separation process
economics
Robust process design
framework
i: Component
M: Membrane
Select all economically feasible
membranes
6 - 36
δ: thickness
Design of Gas Permeation Membrane Systems
Network development refers to those applications where a membrane network does
not exist and a membrane network would be economically effective. Membrane
network upgrading refers to replacing a conventional membane network with a new
membrane network where the membrane offers better properties (permeabilities,
selectivities or cost or a combination of all three). Preliminary analysis considers
screening procedures and reduces computational time.
Quantify membrane performance:
Consider all economically feasible
membranes
[Pi,M,Si,M,CM,δΜ]
Consider a conventional
membrane
[Pi,M*,Si,M*,CM*,δM*]
Knowledge based
separation process
economics
Robust process design
framework
Threshold
throughput
FTM
Threshold retentate
/permeate purity
XP,i,MT/ XR,i,MT
Least feed
composition
XF,i,ML
Lowest network cost
reduction
NCML
Determination of parameters quantify mutual performance
and provide best membrane economically feasible
6 - 37
Design of Gas Permeation Membrane Systems
Typical industrial gas processing involves many complexities. Firstly, network cost
reduction needs to be considered where network costs can be saved by using a
different membrane with advanced properties. Initial confidence could be obtained
from preliminary screening procedures. Secondly, network cost reduction can be
addressed by studying purity and throughput specifications. Process economics of
high purity applications can provide a significant contribution. Similarly, high
throughput applications can be addressed for systems where feed flow rate
flexibility needs to be considered. An important feature of membrane performance
is to examine lower feed concentration. This would result in a preliminary
screening framework to allow industrial designers to study the economic impact on
feed concentration and subsequent advantages arising from advanced membrane
properties.
Membrane performance needs to be addressed at this stage to identify the best few
membranes to be economic and allow stability for a number of applications.
Knowledge based insights:
Consider best membrane
economically feasible
[Pi,M,Si,M,CM,δΜ]
Consider a conventional
membrane
[Pi,M*,Si,M*,CM*,δM*]
Knowledge based
separation processs
economics
Robust process design
framework
Network cost
variation with feed
flow rate
Network cost
Network cost
Network cost
variation with
variation with
variation with
feed composition product(s) flow rate product(s) purity
Knowledge based process economics useful for assessment
and quality of all types of separation process
6 - 38
Design of Gas Permeation Membrane Systems
Knowledge based information could be useful for an overall study of different
separation processes and their competiveness for different product and feed
specifications. Such issues need to be addressed with the membrane that would
offer best performance in the overall screening procedures.
Summary: Membrane Screening
Accounts for
• Membranes of diversified properties
Permeabilities
Selectivities
Manufacturing cost
• Rich processing oppurtunities
High throughput
High purity
Lean feed processing
• Methdology subjected to limitations due to industrial uncertainities
Poor membrane performance at different specifications
• High throughput
• Higth purity
• Lean feed processing
Limited membrane performance specification
• As provided by polymer chemist
6 - 39
Design of Gas Permeation Membrane Systems
CASE STUDY: Natural gas sweetening
Purity
Ret. pres. 14.0 bar
CO2 0.015
0.3
Per. Pres. 1.0 bar
CH4
0.7
Kmol/s mol fr
0.035
Membrane cost
Network parameters
Feed
No. of
stages
Area
bounds
Membranes Cost
100
M1, M2
190
M3, M5, M8
280
M4, M7, M10
M5, M8, M11 370
Max. 3
[30 4000]
[10 4000]
Product specs
M1,2
M3-11
Objective
Stream Purity Recovery
CH4
Ret
0.98
0.90
CO2
Per
0.95
0.70
Areacost + Recycle
compressor cost
Min ∑
+Permeate compression
cost
Select and quantify membrane performance for enhanced
gas separation
6 - 40
Design of Gas Permeation Membrane Systems
Membrane specifications
Matrix polymers*:
S.No. Polymer
1
6FDA-IPDA
2
6FDA-6FpDA
2.3445
5.8613
3
30 % CMS 800 - 2
70 % 6FDA - 6FDA
2.00959
2.41151
60 % CMS 800 - 2
40 % 6FDA - 6FDA
1.67466
1.3221
90 % CMS 800 - 2
10 % 6FDA - 6FDA
1.33973
0.7881
30 % CMS 550 - 8
70 % 6FDA - 6FDA
5.0234
9.5392
4
5
6
7
8
9
10
60 % CMS 550 - 8
40 % 6FDA - 6FDA
90 % CMS 550 - 8
10 % 6FDA - 6FDA
7.70343 11.79096
10.383 13.3114
30 % CMS 550 - 2
70 % 6FDA - 6FDA
9.37809
20.5359
60 % CMS 550 - 2
40 % 6FDA - 6FDA
16.4116
23.4452
31.971
41.132
90 % CMS 550 - 2
10 % 6FDA - 6FDA
CO2 Permeability (Barrers)
P+ X 105 P+ X 107
CH4
CO2
1.0048
2.2329
11
+
kmol/(m2.s.bar)
* Mahajan R., Koros W.J., 2000. Pushing the limits on possibilities for large scale gas
separation: which strategies? J. Mem. Sci., 175, 181-196
6 - 41
Design of Gas Permeation Membrane Systems
Matrix polymers were reported by Mahajan & Koros (2000) as the best options to
achieve a leap in permeabilities and selectivities. For natural gas sweetening
processes, three types of matrix polymers were reported by the authors. However,
the authors have not provided economic viability of such membranes. The costing
of matrix polymers is considered as a linear function of the costing of conventional
polymer and inorganic membranes, based on the composition of inorganic
membrane and polymer in the matrix. Two conventional polymers near to the P - S
upper bound line and nine different matrix polymers were selected for screening
studies. The matrix polymers selected were three different polymers of different
polymer composition for each inorganic membrane as reported in the table.
Procedure for membrane screening and selection:
• Perform preliminary screening initially to eliminate membranes
economically infeasible
A cost of about $1,000,000 is assumed as the basis for screening
• Comparative membrane performance
Study network cost vs feed flow rate to membrane network Debottlenecking factor plot
Study network cost vs feed concentration - Lean feed factor plot
Study network cost vs retentate product purity - Purity factor plot
Use $ 1,000,000 as the permissible maximum cost for networks
• Knowledge based insights
For best membrane economically feasible, perform calculations for
various system parameters to provide additional data.
6 - 42
Design of Gas Permeation Membrane Systems
Results: Preliminary screening
Conclusions
200
180
• 6FDA-IFDA polymer performance
better than 6FDA- 6IPDA polymer
• CMS800-2 matrix polymers could
not perform well
• Both CMS550-8 and CMS550-2
matrix polymers provided best
performance
• Membranes 2, 8, 11 screened as
best membranes among
conventional and matrix polymers
160
140
120
100
80
60
40
20
0
1
2
3
4
5
6
7
8
9
10
11
Membrane number
Preliminary screening eliminated many membranes
6 - 43
Design of Gas Permeation Membrane Systems
It is interesting to note that, even though the costing framework for matrix polymers
involves calculation of module cost basing on the fractional contribution of
advanced polymer composition into matrices, only few formulations were
beneficial. The first set of matrix polymers made from CMS800-2 clearly illustrate
that matrix polymers of low composition are preferred. Such polymers can be used
only for niche applications. CMS550-2 and CMS550-8 matrix polymers on the
other hand offer better permeabilities and selectivites without any impact of module
cost. Hence, future polymer development directions need to address such
formulations.
Comparative membrane performance:
Feed purity~
Throughput enhancement*
120
140
Membrane 2
Membrane 8
Membrane 11
110
100
Membrane 2
Membrane 8
Membrane 11
120
90
100
80
80
70
60
60
50
40
40
20
30
0.04
0.06
0.08
0.10
0.12
Feed flow rate (kmol/s)
0.2
0.3
0.5
0.6
0.7
Feed composition (Mole
fraction CH4)
*Based on 90 % and 75 % recoveries of feed rate
~
Feed composition altered by increasing CO2 rate
6 - 44
0.4
Design of Gas Permeation Membrane Systems
0.8
Comparative membrane performance:
Product purity
Conclusions
120
110
100
Membrane 2
Membrane 8
Membrane 11
• Bottleneck factor
Membrane 11 best for bottleneck
Feed throuhput can be increased by 120 %
( 0.11 kmol/s)
90
• Lean feed concentration
M11 best membrane for low feed targeting
Feed concentration can be decreased up to
30 % CH4
80
70
60
• Purity factor for methane
Very high purity can be targeted using
membrane 11
CH4 purity in product can be around 0.9993
mole fraction
50
40
30
0.975
0.980
0.985
0.990
0.995
1.000
Retentate purity (mole fraction CH4)
Performance of membranes 8 and 11 show
similar trends
6 - 45
Design of Gas Permeation Membrane Systems
Knowledge based insights:
Throughput enhancement
Feed purity
260
240
240
220
220
200
200
180
180
160
160
140
140
120
120
100
0.10
0.15
0.20
0.25
0.30
Feed flow rate (kmol/s)
0.35
0.10
0.15
0.20
0.25
0.30
Feed composition (Mole
fraction CH4)
Knowledge based insights restricted to CMS550-2
membrane only (best membrane)
6 - 46
Design of Gas Permeation Membrane Systems
Knowledge based insights refer to calculations for membrane networks with
variation in purity and throughput within a particular costing value. For this
example, network costing is allowed to a maximum of $ 2,500,000 as the total
annualised cost (TAC) for the present case. For such a value, variations in both
feed throughput and feed compositions were studied to provide data regarding
membrane process economics. Its interesting to note that, feed throughput can be
increased to about 0.30 kmol/s and feed composition can be varied to a value of
about 0.15 mole fraction with in the specified maximum network cost.
Conclusions:
Preliminary screening:
• 6 FDA-IPDA best among conventional polymers
• CMS800-2 matrix polymers cannot provide economic sustanance
Mutual membrane performance:
• CMS550-2 provides best debottleneck
• CMS550-2 can be subjected to a lean feed of about 30 % CH4
• CMS550-2 can provide purities upto 99.993 % methane in product
• Performance of CMS550-2 and CMS550-8 is mutually competitive
Knowledge based insights:
• High throughput can be addressed
• Lean feed can be subjected for processing
• No insights provided for high purity (due to high purity addressed in
mutual membrane performance itself)
6 - 47
Design of Gas Permeation Membrane Systems
5. Novel Membrane Design
6 - 48
Design of Gas Permeation Membrane Systems
Existing design method:
• Predict novel membrane
permabilities and
selectivities
• Predict threshold module
manufacturing cost
Economic evaluation
• Input to polymer chemist
• Input to quantify bounds for
membrane performance
• Input for manufacturing cost
optimisation
Economically feasibile
novel membrane
Economic evaluation procedures have been proposed in the
literature+
+
Bhide B.D., Stern S.A, 1991. A new evaluation of membrane processes for the oxygen
enrichment of air: 2. Effects of economic parameters and membrane properties. J. Membr.
Sci., 62, 37
+
Porter K.E., Hinchcliffe A.B., Tighe B.T., 1997. Developing a new membrane for the
separation of hydrogen and carbon monoxide using the targeting approach. Ind. Eng.
Chem. Res. , 36, 830 - 37
6 - 49
Design of Gas Permeation Membrane Systems
Bhide & Stern (1991) provided selectivity information for oxygen enrichment from
air. They reported that oxygen selectivity needs to be increased to a particular value
at a particular value of permeability. However, Bhide & Stern failed to provide a
systematic methodology with conceptual insights.
Porter et al (1997) reported conceptual information quoting a new design parameter
called the CP factor defined as permeability of the fast gas component divided by
network cost. Porter et al based their novel membrane design insights based on the
CP factor at various selectivities and applied the same to a costing framework for
different fixed membrane network systems. Finally optimal CP values, selectivity
values were obtained from their analysis to develop a polymer. Major draw backs
in such procedures involve developing a number of costing framework equations
for various membrane properties and matching the CP and selectivity values for
different frameworks to evolve novel membrane properties.
Limitations of economic evaluation:
Bhide et al
S.No.
Porter et al
1
Graphical procedure
(Two components only)
2
Fixed network evaluation
3
Limited NLPs to be solved
Semi-graphical procedure with
conceptual approach
Every evaluation needs to
develop complex cost functions
Number of NLPs to be solved
4
Pressure ratio reduction
not stressed
Pressure ratio reduction not
considered
5
Membrane developed met the
Previous membrane modification
proposed
modifications which were
experience not considered
not considered
Summary:
• Solution may result in unnecessary modifications (due to local solution)
• Procedures do not consider previous modification experience
• Multi-component problems not considered
6 - 50
Design of Gas Permeation Membrane Systems
Targeting methodology adopted in this work:
• Predict novel membrane
permabilities and
selectivities
• Predict threshold module
manufacturing cost
Process synthesis
(instead of economic
evaluation)
• Input to polymer chemist
• Input to quantify bounds for
membrane performance
• Input for manufacturing cost
optimisation
Economically feasibile
novel membrane
• Previous experience in polymer modification considered
• Multi-component system modification criteria can be addressed
• Contributions of polymer chemist can enhance search process
• Alternatively, modification can be performed without considering
existing membrane selectivity
6 - 51
Design of Gas Permeation Membrane Systems
This work considers targeting novel membrane properties with a systematic
conceptual methodology general enough to apply to problems of different types.
Extensions to the proposed methodology can be performed at any level basing on
the intentions of polymer chemist to offer flexibility to the procedure.
Modification methodology:
Existing membrane
specification
[PM, S, C]
Novel [P,S] available or
achievable
• Modify P based on area
requirement
• Modify S based on existing
selectivity and suitable
correlations
[NewPM, NewS, C]
[PM*, S*, NewC]
Process synthesis
Process synthesis
[OptPM, OptS, C]
Optimal membrane properties
[PM*, S*, OptC]
Threshold module cost
Methodology can effectively incorporate contemporary
membrane development information
6 - 52
Design of Gas Permeation Membrane Systems
Modification methdology is based on two search directions feasible. The first
search direction involves the concept that of a specified annualised membrane cost.
Novel membrane properties need to be proposed that are optimal with respect to the
conventional separation (membrane) process cost. The other search direction
attempts to identify the threshold annualised membrane cost (and thereby threshold
manufacturing cost) for specified membrane properties.
The evolution of optimal module properties and cost are based on trial and error
simulations using step by step modification of membrane properties. Since there is
no clear cut definition of network cost versus selectivity modification, a trial and
error based procedure is the only way to develop such criteria.
Development of novel polymers:
Step 1: Multicomponent modification criteria
• Consider various correlations
• Assume component selectivities using subsequent
correlations
Step 2: Screening of component selectivities
• Consider existing novel membrane selectivity data
• Superimpose on assumed selectivities
• Screen correlations suitable for study
Step 3: Optimal selectivity search
• Fix base component permeability
• Search for optimal selectivity values
Search for different correlations
• Repeat for different base component permeabilities
6 - 53
Design of Gas Permeation Membrane Systems
The development procedure assists contemporary experimental development. The
first step is to propose a multicomponent modification strategy. Various correlations
can be proposed for different component selectivity modifications. Basing on
correlations, component selectivities need to be evaluated. These correlations can
be linear, non-linear, logarithmic or more complex based on the input from
selectivity trends and the intentions of the polymer chemist. The second step is to
screen and eliminate all those correlations whose properties don't match those
proposed. This provides more flexibility for the search domain to be confined to
those selectivity values that are achieved or achievable. All major correlations can
be considered after this step. The last step involves a trial and error procedure to
evaluate optimal membrane properties (permeabilities and selectivities), based on
screened correlations.
Modification criteria: Summary
Modification of Permeability (PM)
• Order of PM maintained to be near the existing developments
• Novelty addresses modification of S at at least existing PM value for fast permeating
component for low pressure permeating membranes
• Area bounds match the values of P for high pressure rejecting membranes
Modification of Selectivity (S)
• For binary systems
Use existing novel polymer selectivity data available
Use selectivities as desired if no novel polymer exists
• Multi-component systems
Use existing multi-component selectivity for all components
Prepare graphical plots
Apply different correlations
Search for the best correlation (using contemporary membrane properties)
6 - 54
Design of Gas Permeation Membrane Systems
Summary: Novel Membrane Development
Methodology accounts for
# Two types of search directions
Q Optimal membrane properties at specified annualised (manufacturing) cost
Q Optimal annualised (manufacturing) cost at specified membrane properties
# Quicker search for optimal values
Q Membrane properties
Q Membrane cost
# High degree of practicality
Q Multi-component modification criteria based on experimental data
Q Polymer chemist to provide additional information
6 - 55
Design of Gas Permeation Membrane Systems
CASE STUDY: Hydrogen recovery2
Feed, membrane specifications
Feed rate
kmol/s
Permeability
kmol/(m2.s.bar)
Network parameters
Number of stages
Area bounds m2
3
[10, 2500]
-6
H2
0.01566
9.713018 x 10
CH4
0.01134
1.239247 x 10-7
C2H6
C3H8
0.008526
2.14355 x 10-8
0.0088392*
1.54906 x 10-8@
* C3, C4+ taken as C3 @ Based on S C3/C1 = 8
Pressures
12
bar
1
bar
Problem summary
• Membrane cost (annualised)= $ 100/m2
• Compressor costing according to Douglas (1989)
• Product specifications
Case1: 95 % pure H2 with 95 % recovery at 12 bar#
Case 2: 99 % pure H2 with 95 % recovery at 12 bar
Determine novel polymer properties that can provide the same
network cost as a conventional polymer
2
Kaldis S.P., Kapantaidikis G.C., Sakellaropoulos G.P., 2000. Simulation of multicomponent gas separation
in a hollow fiber membrane by orthogonal collocation - hydrogen recovery from refinery gases. J. Mem. Sci.
173(2), 61 - 71
6 - 56
Design of Gas Permeation Membrane Systems
Results from optimisation: Conventional polymer
Case 1:
Remarks
1074.6
1509
1684
Optimal process cost = $ 674,000
Case 2:
0.85
329.4
2463.5
2038.6
• Area cost
contribution is high
• Contribution of
permeate recycle
cost is high to
optimal process cost
• Search directed for
high pressure
rejecting membrane
Low area costs
High permeate recycle
compressor cost
No contribution of
permeate recycle
product compressor
cost
Optimal process cost = $905,000
Membrane needs to be developed that can reject hydrogen on the
retentate side to avoid product compression cost
6 - 57
Design of Gas Permeation Membrane Systems
Procedure for targeting membrane
properties
Base case: PDMS membrane
Step 1: Multicomponent modification
criteria
• Consider various correlations
• Assume component selectivities using
subsequent correlations
Step 2: Screening of component
selectivities
• Consider existing novel membrane
selectivity data
• Superimpose on assumed
selectivities
• Screen correlations suitable for study
Boiling point
S#
Step 3: Optimal selectivity search
• Fix H2 Permeability
• Search for optimal selectivity values
Search for different correlations
• Repeat for different H2
Permeabilities
6 - 58
#
Assuming PH2 = 550 Barrers
Design of Gas Permeation Membrane Systems
C1/H2
2.43
C2/H2
6.80
C3/H2
10.5
Existing
Step 1: Multicomponent criteria
Modification 1
Modification 2
Modification 3
Modification 4
Assume selectivities:
C1
C2
22
C3
C1
C2
C1
C3
100
100
10
10
C2
C3
20
18
16
14
12
10
8
6
4
2
0
10
15
20
25
30
35
40
45
50
Molecular weight
Linear correlation
6 - 59
1
1
10
15
20
25
30
35
40
45
50
10
15
20
25
30
35
40
45
50
Molecular weight
Molecular weight
Logarithmic - linear
correlation
Logarithmic segmented correlation
Design of Gas Permeation Membrane Systems
Various correlations considered for this case study include linear, logarithmic and
logarithmic - segmented correlations. Subsequent values of C2 and C3 selectivities
are calculated based on these correlations. The graphs illustrate an overview of
those selectivity values for all the correlations.
Step 2: Screening of correlations
MFI - Zeolite membrane exhibits
PDMS type behaviour (solution diffusion mechanism)*
100
25
Linear correlation
Logarithmic - linear correlation
Logarithmic - segmented correlation
o
MFI - Zeolite 40 C
Modification 4
o
25 C
o
105 C
20
o
40 C ( Correlated)
15
10
5
0
10
0
10
20
30
40
50
10
15
Molecular weight
20
25
30
35
40
45
50
Molecular weight
Logarithmic correlations agree with experimental data
* Dong J., Lin Y.S., Liu W, 2000. Multicomponent hydrogen/hydrocarbon separation by
MFI-type zeolite membranes. AIChE Journal, 46(10) 1966.
6 - 60
Design of Gas Permeation Membrane Systems
Dong, Lin and Liu (2000) provided multicomponent selectivity values for MFI Zeolite membranes that exhibits behaviour similar to PDMS membranes (high
pressure hydrogen rejecting membrane). The authors reported permeance values at
25 C and 105 C. Permeance values were calculated at 40 C based on linear
interpolation of P vs 1/T data for each component. Selectivity data were then
calculated at 40 C.
Its interesting to note that both logarithmic correlations ( linear and segmented)
matched with experimental trends. Logarithmic - linear correlations matched more
closely than logarithmic - segmented correlations. Hence, logarithmic correlations
were considered to be the best.
Procedure for optimal selectivity search:
Selectivity values
(based on logarithimic
correlations)
Process synthesis
framework
Modify selectivities
Analysis
Optimal membrane
selectivities
6 - 61
Design of Gas Permeation Membrane Systems
Step 3: Optimal selectivity search results for Case 1
0.95
Component
H2
C1
C2
C3
-5
P 1.8421242 x 10
(kmol/(m .s.bar)
S
S
9.72
S
42.84
10.0
10.0
24.0
63.6
0.20
Optimal cost = $587,700
0.94
Component
H2
C1
C2
C3
6 - 62
-6
P 1.8421242 x 10
(kmol/(m .s.bar)
109.6
330.7
135.8
11.0
S
S
27.132
S
48.477
Optimal process cost = $564,100
Design of Gas Permeation Membrane Systems
Optimal membrane properties indicate different sets of optimal values at different
base component (hydrogen) permeabilities.
Step 3: Optimal selectivity search results for Case 2
0.934
Component
H2
C1
C2
C3
-5
P 1.8421242 x 10
(kmol/(m .s.bar)
S
S
S
11.0
72.1
34.8
11.5
28.365
50.681
Optimal cost = $855,400
Component
H2
1.842124 x 10-6
P (kmol/(m
2
.s.bar)
C1
S
S
33.298
S
59.495
C2
C3
6 - 63
99
914.5
83.1
13.5
Optimal process cost = $850,800
Design of Gas Permeation Membrane Systems
Conclusions:
• Search restricted to high pressure side hydrogen rejecting
membrane
• MFI Zeolite membrane data used to provide experimental trends.
• Logarithmic correlations work out to be best for selectivity
modification
• Process synthesis provides guidelines for optimal selectivity
modification
At least 4 - 5 times C1/H2 selectivity as that offered by conventional PDMS type
membrane with same membrane annualised cost
Selectivity specifications change w.r.t product purity
Direct polymer membrane research to approach
economic performance limits
6 - 64
Design of Gas Permeation Membrane Systems
6. Conclusions & Future Work
6 - 65
Design of Gas Permeation Membrane Systems
Pressure ratio optimisation:
Conclusions
• Solutions close to global optimal
pressure ratio
• Robust framework for
Membrane screening
• Preliminary screening
Allows discarding membranes of no
industrial feasibility
Evaluation of optimal retentate
pressure w.r.t permeate pressure
Evaluation of optimal network cost
w.r.t permeate pressure
• Combination of Permeabilities,
Selectivities, Module cost
Selection of a set of membranes
• Mutual performance
Threshold purity
Threshold feed rate
Least feed concentration
Network cost reduction
Novel membrane development
• Multi-component modification criteria
introduced
Screening of various property modifications with
experimental validation
• Knowledge based process
economics
• Two search directions feasible
Threshold module cost at fixed properties
Optimal membrane properties at desired cost
• Robust network design with quality
solutions
Quality solutions provide achievable modifications
Faster search
6 - 66
Design of Gas Permeation Membrane Systems
Economic growth of membrane
process
Economic growth of separation
processes competing with membrane
processes
Future work:
• Synthesis of hybrid - gas permeation membrane systems
Membrane - PSA
Membrane - absorption
• Synthesis of reverse osmosis systems
Desalination
Waste water treatment
• Synthesis of ultrafiltration systems
Waste water processing
• Synthesis of pervaporation systems
Contaminant removal
Organic recovery from aqueous solutions
• Synthesis of hybrid pervaporation systems
Distillation-membrane hybrids
Reactor-membrane hybrids
6 - 67
Design of Gas Permeation Membrane Systems