Presentation on theme: "Membrane Networks in Natural Gas Separation"— Presentation transcript:
1 Membrane Networks in Natural Gas Separation Nina Wright, Ernest West, Debora Faria and Miguel Bagajewicz
2 PurposeThe purpose of this presentation is to show the importance of membranes in natural gas purification and their potential to be highly cost effective.
3 Topics Modern Natural Gas Processes Membranes Membrane Networks SuperstructuresSuperstructure ModelModeling ResultsQuestions
4 Natural Gas Processing System-Today Major Components of Natural Gas Processing today include absortion, adsorption, molecular sieves and cryogenic processesAbsorption is used in:Glycol DehydrationAmine TreatingCryogenics is used in:NGL recoveryNatural Gas Process of today includes many stages, which I will discuss in subsequent slides.The absorbed NGLs in the rich solvent from the bottom of the NGL absorber column are fractionated in the solvent regenerator column which separates NGLs overhead and lean solvent produced at the bottom. After heat recuperation, the lean solvent is presaturated with absorber overhead gases. The chilled solvent flows in the top of the absorber column. The separated gas from the presaturator separator form the pipeline sales gas
5 Natural Gas with Membrane Separation Membranes could be used for:Wellhead dehydrationNitrogen RejectionAcid Gas RemovalDehydratorNGL fractionationMolecular sieves
6 Glycol DehydrationTriethylene glycol liquid absorbs water vapor from natural gasGlycol water solution is then heated to remove waterGlycol solution is then reused for further dehydration useRemoving water levels to below 5lbs/MMscfThe Environmental Protection Agency (EPA) estimated that more than 38,000 glycol dehydration units are operating in the U.S., collectively emitting about 18.6 billion cubic feet of CH4 per year into the atmosphereWet gas enters a contacting tower at the bottom. Dry glycol flows down the tower from the top, from tray to tray, or through packing material. A bubble cap configuration maximizes gas/glycol contact, removing water to levels below 5 lbs/MMscf. Systems can be designed to achieve levels down to 1lb/MMscf. The dehydrated gas leaves the tower at the top and returns to the pipeline or goes to other processing units. The water rich glycol leaves the tower at the bottom, and goes to the reconcentration system. In the reconcentration system, the wet glycol is filtered of impurities and heated to 400°F. Water escapes as steam, and the purified glycol returns to the tower where it contacts wet gas again.
7 Sulfur Removal-Sweetening Sour gas - H2S content exceeds 5.7 milligrams per cubic meter of natural gasSour gas is passed through contact tower containing amine solutionAmine solution absorbs H2SAmine is recycled for further absorptionCurrently Amine treating is used in 95% of U.S. gas sweetening operations
8 Methane Losses from Acid Gas Removal There are 291 acid gas removal (AGR) units in gas processing plantsEmit 646 MMcf annually6 McF/day emitted by average AGR unitMost AGR units use diethanol amine (DEA) or Selexol process
9 Cryogenic Method for NGL removal Expansion turbine is used to drop the temperature of the natural gasEthane and lighter hydrocarbons condense, methane remains in gaseous formAllows for 90%-95% recovery of ethane from the original gas streamIn cryogenic processing, almost the entire volume of natural gas must be refrigerated, usually compressed, and then heated again. Accordingly, cryogenic processing is expensive to install and operate.Cryogenic technology is believed only capable of cost effectively purifying reserves, which exceed 50,000,000 standard cubic feet of gas per day.
11 Largest membrane based natural gas plant Natural Gas Processing Plant Qadirpur, PakistanIn 1999: in the world (Dortmundt, UOP, 1999).Design: 265 MMSCFD natural gas at 59 bar.CO2 content is reduced from 6.5 % to less than 2 % using a cellulose acetate membrane.Also designed for gas dehydration.Plant processes all available gas.Plans for expansion to 400 MMSCFD.
12 Natural Gas Processing with Membranes AdvantagesLess Environmental ImpactLess Maintenance CostLess Workspace AreaAbility to treat gas at wellheadDisadvantagesReduced flux capacityHigh capitol costThermal instabilityMore research is needed for widespread industrial sized usageWhy should we process natural gas with membranes?
14 Membrane DefinitionA membrane-a physical barrier from semi-permeable material that allows some component to pass through while others are held back. Usually, the driving force is pressure differenceFeedRetentatePermeateMembrane module
15 Permeability of a Non-Porous Membrane The rate of gas permeation through a membrane is determined permeability.Permeability = solubility (k) x diffusivity (D)This comes from the fact that the gas needs to dissolve first in the matrix and then diffuse.Solubility indicates how much gas can be taken up by the membrane and diffusivity measures of the mobility of the molecules in the membrane
16 Membrane-DiffusionMass transport through membranes described by Fick’s Lawis the diffusivity of the solute in the membraneaccounts for the solubility of the soluteis the permeability,
17 Membrane Characteristics SelectivityIdeal Membranes incorporate high permeability with high selectivityThis is accomplished usually with a highly selective membrane and making it as thin as possible to increase the permeability
18 Membrane ModulesMembrane modules divided into Flat sheets and Tubular formTwo membranes commonly used are spiral bound and hollow tubeSpiral bound are an example of Flat sheet modules- essentially a flat sheet module rolled into a spiral formSpiral shape accommodates more surface area then a regular flat sheetInsert caption
19 Hollow Fiber Membranes Tubular membranes are divided intoTubular >5mmCapillary 0.5 > 5mmHollow fiber < 0.5 mmLow operation costMore Area per unit vs. Spiral woundSusceptible to foulingWINNER!!Cross Section of Hollow Fiber
20 Cellulose AcetateCellulose acetate membrane-most commonly used in gas separation industry (especially CO2 removal)Drawbacks-sensitivity to water and plasticizing gasesLimited temperature and pressure resistanceAdvantages-increased selectivity towards CO2 over methane (CO2 with a selectivity of 10-20)How does a substance travel through the membrane?this depends on the solubility of the gas component, and the diffusivity of the gas component.
21 New membrane technology Recently, (Oct. 2007) information published about “thermally rearranged” plastic membrane from University of Texas shows great promise:Membrane pores modeled on ion channels in cellsSelectivity 4 times higher than Cellulose AcetateCO2 100 times more permeableHighly thermally stable, can operate over 600°F.500 times less space taken up by process compared to current conventional CO2 removal.
23 Hollow Fiber Modeling Assumptions: Uniform properties at each element Ideal gas behaviorSteady-state isothermal operationConstant ambient pressure at the shell sideConstant permeabilities (independent of composition)Negligible axial diffusion and pressure dropNegligible deformation of the fibers under pressure
24 Hollow Fiber Membrane Model Molar Component Material BalanceTube side Shell side
25 Hollow Fiber Membrane Model Active Area of membrane elementis the number of membrane tubesis the logarithmic mean diameteris the rate of transport of pure componentper unit area of the membraneis the permeability of component jis the thickness of the membrane
26 Hollow Fiber Model Mole fractions (source on nonlinearities) Tube side mole fraction for component jShell side mole fraction for component j
27 Modeling of Membrane Networks using Superstructures
28 Superstructure(a)(b)(c)(d)A superstructure is a flowsheet that contains all possible flowsheets simultaneously. Optimization determines the oprtimal configurationMathematical programming is needed.
31 Membrane network design (Qi, Henson, 2000) Generalized Superstructure usedModelSpiral-Wound moduleMulti-component gas mixturesMINLPCase study: CO2 and H2S separation from natural gas.Simultaneous optimization of flow in terms of total annual process costs.Using optimisation for membranes seems to be a growing field. From I found 7 articles when I searched for the words optimisation + membranes (in title, keywords and abstract). From there were 23 and from there were 695 and from there were 714.Some of the interesting new articles I have found uses optimisation with a superstructure approach.A superstructure is used to represent all the possible process configurations that is possible. For the membrane problem this means that all different membrane units, possible compressors, recycle streams, mixing and splitting of streams have been represented. When this is optimised, usually in terms of some capital and operating costs, the solution will give the best operating structure for the given problem. All the alternatives are then “tested” and the most economical one is found.This example is a mixed integer nonlinear programming solution to a CO2/methane separation using spiral wound modules. The optimisation of the flowsheet was done in terms of total annual costs.Ref: Qi., R., Henson, M.A., 1998
32 Membrane network design (Kookos, 2002) ModelHollow-fibre membrane systemMulti-component gas mixturesLimit study to two membranes networkNLPCase study: O2 and N2 separation from airFind the optimal membrane permeability, selectivity and the optimum structure.Configuration and membrane properties are optimized together.Ref: Kookos, I.K, 2002
33 Our Mathematical Model Tube side Shell sideFeedRetentateKK+1K+2Permeate
38 Membrane Optimization One Membrane Optimization while Minimizing Area, and Flow of CO2 Tube sideLength(m)50CO2 Material Balance Tube (Mol/s)0.07CH4 Material Balance Tube(Mol/s)6.73CO2 Material Balance Shell (Mol/s)1.95CH4 Material Balance Shell0.15Area(m2 )2.09Mole fraction CO2 (tube side)0.01Mole fraction CH4 (tube side)0.99Mole fraction CO2 (shell side)0.93Mole fraction CH4 (shell side)Feasibility8The Feasibility of the Membrane is discovered to be an eight.Eight is considered feasible
39 Complex Membrane Model Two membranesFour componentsReasonable Values for Objective FunctionDiscretization of Continuous ModelAddition of Non-Linear Solver
40 Objective Function Capital Costs $1000 per kWh per compressor $200 per m2 of membrane area for housingStainless Steel compressorsWorking capital = 10% of FCI27% of TCI paid each year
41 Objective Function Operating Costs Power costs = fuel cost*working days per year*compressor power / heat content of fuel / efficiency$30/yr/m2 cost of membrane replacementLabor cost is assumed to be 5% of FCI/yearProduct losses = fuel cost * working days per year * rate of loss of product
42 Additional Information Efficiency of compressors is 70%Stainless steel compressorsFeed available at 3.5MPaPermeate pressure at .105MPa (~1bar)10mol/s total feed19% CO2, 73% CH4, 7%C2+, 1%H2SMembrane Area available in 40m2 incrementsOperates 300 days/year
43 Mathematical ModelModel uses a combination of mixed integer linear programming (MILP) and mixed integer non-linear programming (MINLP).The MILP solves a initial linear problem to obtain initial guesses and provides a lower boundThe MINLP then solves the non-linear problem to obtain a feasible solution
45 Difficulty in Finding Optimum Model is highly non-linear, so obtaining solutions is difficultMost simple model involves no recycle, model preferentially chooses parallel configurationMILP solution generates a lower bound that is too low and does not provide good enough initial guesses on its ownWhen forcing the model to recycle, much lower objective function occurs
46 Increasing Maxcycles and Including Infeasder Added infeasder option and increased maxcycles from 20 to 100.Infeasder unable to be used because there were too many non-linear sub-problems in the modelIncreasing maxcycles occasionally causes the model to obtain a feasible solution if model cannot converge with 20 cyclesDoes not improve model enough
47 Addition of Loop Function Model cannot obtain optimum solution without initial guesses or boundsLoop generates initial guess for recycle flows and feed flowsSuccessfully is able to obtain much lower minimums than original model run on high max cycles
48 Membrane System Design from Qi, and Henson -Two stage separation process -2% CO2 composition in final retentate -Spiral wound membranes vs. hollow fiber membranes -Four component system
49 Two Membrane Network Two Membrane Network Partial Recycle of the Permeate of the Second MembraneFraction of the Retentate from Membrane 1 to Membrane 2Comparable compressor resultsComparable Cost of Processing
50 Two Membrane Network P=5MPa Total Area = 80m2Cost of treating = $8.14/km3P (tube side) =5 MpaComplete Recycle of Permeate of Membrane 1
51 ConclusionsSuccessfully modeled two component, counter current flow membraneSeveral computational problems were highlightedParallel configurationsLess Total Membrane AreaTwo Membrane Networks modeled from Superstructure
53 ReferencesQi., R., Henson, M.A., ‘Optimization-based design of spiral-wound membranes systems for CO2/CH4 separations’, Separation and Purification Technology, 13, , 1998 Qi., R., Henson, M.A., ‘Membrane system design for multicomponent gas mixtures via mixed-integer nonlinear programming’, Computers and Chemical Engineering, 24, , 2000 Kookos, I.K., ‘A targeting approach to the synthesis of membrane network for gas separations’, j. Membrane Science, 208, , 2002 Mark C. Porter, "Handbook of Industrial Membrane Technology", Westwood, New Jersey, Noyes Publications, Seader, J. D., and Henley, E. J. "Separation Process Principles". New York: John Wiley & Sons, Inc., Natural Gas Supply Association, John A. Howell, The Watt Committee on Energy, "The Membrane Alternative, London, Elsevier Science, 1990.Polymeric Gas Separation Membranes by Donald R. Paul, Yuri P. Yampolskii, 1996.