ConvergenceConvergenceblocks are defined to converge the tear streams,designspecifications,oroptimizationproblems.Convergenceblocksdeterminehowguesses for a tear stream or design specification manipulated variable areupdated from iteration toiteration.A sequence is determined,which includes all of the unit operation andconvergenceblocksIf you do not specify the tear streams, convergence blocks, or sequence,Aspen Plus determines them automatically. Every design specification and tearstream has an associated convergence block. The Aspen Plus generatedconvergence block names begin with the character"s."User-defined convergenceblocks should not begin with the character"s."Aspen Plus automaticallydetermines anyadditional specifications needed toexecute theflowsheet.By default, Aspen Plus also checks the user-specifiedsequences to ensure that all loops are torn.Convergence specifications you can make are:If you want to specifyUse this Convergence formFor more details seethissectioninthischapterConvergenceparametersand/ormethods forConvOptionsConvergenceOptionsconvergenceblocksTearSomeor all of the tear streams needed forSpecifying Tear Streamssystem-generated convergence blocksSome orall of the convergence blocks neededConvergenceSpecifying User-Defined Convergence BlocksConv OrderConvergenceorderfor someorallof theuser-Specifying Convergence Orderdefinedconvergence blocksSequenceSequencefor allor part of a flowsheetSpecifying the Calculation SequenceConvergence OptionsUsethe Convergence ConvOptions sheets to specify thefollowingfor convergenceblocks:Tear Convergence toleranceConvergencemethodsfortearstreams,design specifications,andoptimization problems used in convergence blocks generated by Aspen PlusParametersthataffectsequencingConvergence parameters for each method.The specifiedparameters are usedas defaults for convergence blocks you define and convergence blocksgenerated by Aspen Plus.17-2Aspen Plus User GuideVersion10.1-0
17-2 Aspen Plus User Guide Version 10.1-0 Convergence • Convergence blocks are defined to converge the tear streams, design specifications, or optimization problems. Convergence blocks determine how guesses for a tear stream or design specification manipulated variable are updated from iteration to iteration. • A sequence is determined, which includes all of the unit operation and convergence blocks. If you do not specify the tear streams, convergence blocks, or sequence, Aspen Plus determines them automatically. Every design specification and tear stream has an associated convergence block. The Aspen Plus generated convergence block names begin with the character “$.” User-defined convergence blocks should not begin with the character “$.” Aspen Plus automatically determines any additional specifications needed to execute the flowsheet. By default, Aspen Plus also checks the user-specified sequences to ensure that all loops are torn. Convergence specifications you can make are: If you want to specify Use this Convergence form For more details see this section in this chapter Convergence parameters and/or methods for convergence blocks Conv Options Convergence Options Some or all of the tear streams needed for system-generated convergence blocks Tear Specifying Tear Streams Some or all of the convergence blocks needed Convergence Specifying User-Defined Convergence Blocks Convergence order for some or all of the userdefined convergence blocks Conv Order Specifying Convergence Order Sequence for all or part of a flowsheet Sequence Specifying the Calculation Sequence Convergence Options Use the Convergence ConvOptions sheets to specify the following for convergence blocks: • Tear Convergence tolerance • Convergence methods for tear streams, design specifications, and optimization problems used in convergence blocks generated by Aspen Plus • Parameters that affect sequencing • Convergence parameters for each method. The specified parameters are used as defaults for convergence blocks you define and convergence blocks generated by Aspen Plus
Chapter17Specifying Tear Convergence ParametersA tear stream is converged when the following is true for all tear convergencevariables:Xealculed -Xasswmed tol-tol$XasstumedFor streams, the default convergencevariables aretotal moleflowall componentmole flows,pressure, and enthalpy.When the Trace Option is Cutoff (specified ontheConvergenceConvOptionsDefaultsTearConvergencesheet),AspenPlusbypassesthisconvergencetestforcomponentsthathaveamolefractionlessthan the Trace threshold. The default Trace threshold is Tolerance/1oo.Thealternative trace option, Trace-option = Gradual, adds a 1o0*Trace thresholdterm to the denominator.This setting gradually relaxes the convergence test fortrace components.To specifytear convergence parameters for convergence blocks:1.From the Data menu,point to Convergence, then Conv Options.2.Click the Tear Convergence sheet.Specify tolerance and other convergenceparameters,such as TraceThreshold3.Sand TraceOption.The following parameters are available on the Tear Convergence sheet:FieldDefaultToTolerance0.001Specify Tear convergence toleranceA tear stream is converged when thefollowing is truefor all stream variables:XcalculatedXassumed≤tol-lolsXassumedTraceTolerance/100Specify thetracecomponent thresholdThresholdAspenPlus bypassesthisconverencetestforcomponentsthathaveamolefraction less than the Trace threshold.CutoffTraceSelect the Convergence test optionfor trace components.Traceoption=OptionGradual adds 100'TraceThreshold term to the denominator.This settinggradually relaxes the convergence test for trace components.ComponentAllcomponentsIdentifytheComponentgroupIDforcomponentstobeconverged intearGroupstreamsComponentqroupsaredefinedontheComponentsComp-Groupform(seeChapter6).Useacomponentgroupwhenyouknowthatsomecomponentshavezeroorconstantflowrates.AComponentGroupmaycauseconvergenceproblems ifthe unconverged components have significant flowComponent group specifications are intended primarily for use with the matrixconvergencemethods (Broyden,Newton,andSQP)toreducethematrixsizeand the number of numerical derivative perturbations.Continued17-3AspenPlusUserGuideVersion10.1-0
Aspen Plus User Guide 17-3 Version 10.1-0 Chapter 17 Specifying Tear Convergence Parameters A tear stream is converged when the following is true for all tear convergence variables: − ≤ − tol ≤ X X X tol calculated assumed assumed For streams, the default convergence variables are total mole flow,all component mole flows, pressure, and enthalpy. When the Trace Option is Cutoff (specified on the Convergence ConvOptions Defaults TearConvergence sheet), Aspen Plus bypasses this convergence test for components that have a mole fraction less than the Trace threshold. The default Trace threshold is Tolerance/100. The alternative trace option, Trace-option = Gradual, adds a 100*Trace threshold term to the denominator. This setting gradually relaxes the convergence test for trace components. To specify tear convergence parameters for convergence blocks: 1. From the Data menu, point to Convergence, then Conv Options. 2. Click the Tear Convergence sheet. 3. Specify tolerance and other convergence parameters, such as Trace Threshold and Trace Option. The following parameters are available on the Tear Convergence sheet: Field Default To Tolerance 0.001 Specify Tear convergence tolerance A tear stream is converged when the following is true for all stream variables: − ≤ − tol ≤ X X X tol calculated assumed assumed Trace Threshold Tolerance/100 Specify the trace component threshold Aspen Plus bypasses this convergence test for components that have a mole fraction less than the Trace threshold. Trace Option Cutoff Select the Convergence test option for trace components. Trace option = Gradual adds 100*TraceThreshold term to the denominator. This setting gradually relaxes the convergence test for trace components. Component Group All components Identify the Component group ID for components to be converged in tear streams Component groups are defined on the Components Comp-Group form (see Chapter 6). Use a component group when you know that some components have zero or constant flow rates. A Component Group may cause convergence problems if the unconverged components have significant flow. Component group specifications are intended primarily for use with the matrix convergence methods (Broyden, Newton, and SQP) to reduce the matrix size and the number of numerical derivative perturbations. Continued
ConvergenceFieldDefaultToStatePressure&EnthalpySelect the State variables to be converged.YoucanselectaStateoptionotherthanthedefault(Pressureandenthalpy)whenpressure isknowntobeconstant or enthalpyisnotcalculated(mass-balance-only simulations).State specifications are intended primarily for use with the matrixconvergence methods (Broyden, Newton,and SQP) to reduce the matrix sizeand thenumberof numerical derivativeperturbationsRestorecheckedRestore teartolast guessedvalue when thereisa convergenceerror.Tears onErrorFlash TearcheckedFlash tear streams after being updated by the convergence block.StreamsCheckFlashTearStreams if youaccess thetemperature,densityor entropyofthetearstreamthrough in-lineFortranorifyou needtoseeoruseintermediate or partial convergence results. Do not check Flash Tear Streamsif you want to save calculation time orif you do not need intermediateconvergence results.Flashing tear streams is independent of theconvergence method, with one exception. If Chemistry is associated with thetear stream,then the default is not toflashfortearstreams, irrespectiveofyour selection.DiagnosticsDisplay Maximum Error /Specity whether tables ofall variables or only thevariablewith themaximumToleranceerror should be generated.Specifying Default MethodsYou can specify the numerical methods to be used by the system-generatedconvergence blocks. See Convergence Methods, this chapter,for information onthenumerical methods.To specifythe numerical methods to beused bythe system-generatedconvergenceblocks:1.From the Data menu, point to Convergence, the Conv Options.2.Select the Default Methods sheet.3.You can specify the numerical methods to be used by the convergence blocks17-4Aspen Plus UserGuideVersion 10.1-0
17-4 Aspen Plus User Guide Version 10.1-0 Convergence Field Default To State Pressure & Enthalpy Select the State variables to be converged. You can select a State option other than the default (Pressure and enthalpy) when pressure is known to be constant or enthalpy is not calculated (mass-balance-only simulations). State specifications are intended primarily for use with the matrix convergence methods (Broyden, Newton, and SQP) to reduce the matrix size and the number of numerical derivative perturbations. Restore Tears on Error checked Restore tear to last guessed value when there is a convergence error. Flash Tear Streams checked Flash tear streams after being updated by the convergence block. Check Flash Tear Streams if you access the temperature, density or entropy of the tear stream through in-line Fortran or if you need to see or use intermediate or partial convergence results. Do not check Flash Tear Streams if you want to save calculation time or if you do not need intermediate convergence results. Flashing tear streams is independent of the convergence method, with one exception. If Chemistry is associated with the tear stream, then the default is not to flash for tear streams, irrespective of your selection. Diagnostics Display Maximum Error / Tolerance Specify whether tables of all variables or only the variable with the maximum error should be generated. Specifying Default Methods You can specify the numerical methods to be used by the system-generated convergence blocks. See Convergence Methods, this chapter, for information on the numerical methods. To specify the numerical methods to be used by the system-generated convergence blocks: 1. From the Data menu, point to Convergence, the Conv Options. 2. Select the Default Methods sheet. 3. You can specify the numerical methods to be used by the convergence blocks
Chapter 17The followingparameters are available on theDefault Methods sheet:FieldDefaultTo specify the convergence method for system-generatedTearsWegsteinTearconvergenceblocksThe other methods available are Direct, Broyden,and NewtonSingle DesignSecantSingle design-spec convergence blocksSpecThe other methods available are Broyden and NewtonBroydenMultiple DesignMultiple design-spec convergence blocksSpecsThe other method available is Newton.Tears & DesignBroydenCombinedtears anddesign-specsconvergenceblocksSpecsThe other method available is Newton.SQPOptimizationOptimization convergence blocksSpecifying Sequencing ParametersYou canspecifyparameterstocontrol tear stream selectionand automaticsequencingTo specify the tearing and sequencing parameters:1.From the Data menu, point to Convergence, then Conv Options.2. Select the Sequencing sheet.3.You can specifythetearingand sequencingparameters.The following parameters are available on the Sequencing sheet:FieldDefaultTo specifyInsideDesign Spec NestingWhether design specifications should be nested inside tear stream loops,outsidetearstreamloops,orconvergedsimultaneouslywithtearstreamsDesign Spec Nesting doesnotapply toconvergenceblocksspecified intheConvergenceOrderform.When the tear of an outerloop is recalculated in an inner loop,the actualsequencegeneratedmay not strictlyfollow theloop-orderpreferencespecified in the Design Spec Nesting and User Nesting fields.User NestingOutsideUser Nesting lets you specify a preference for whether convergenceblocksspecifiedontheConvOrderformshouldbenestedinsideoroutside other convergence blocks (user-defined or system-generated)The User Nestingfield has precedenceoverthe DesignSpec Nestingfield.When the tear of an outer loop is recalculated in an inner loop, the actualsequence generated may not strictly follow the loop-order preferencespecifiedintheDesignSpecNestingandUserNestingfieldsContinued17-5AspenPlusUserGuideVersion 10.1-0
Aspen Plus User Guide 17-5 Version 10.1-0 Chapter 17 The following parameters are available on the Default Methods sheet: Field Default To specify the convergence method for system-generated Tears Wegstein Tear convergence blocks The other methods available are Direct, Broyden, and Newton. Single Design Spec Secant Single design-spec convergence blocks The other methods available are Broyden and Newton. Multiple Design Specs Broyden Multiple design-spec convergence blocks The other method available is Newton. Tears & Design Specs Broyden Combined tears and design-specs convergence blocks The other method available is Newton. Optimization SQP Optimization convergence blocks Specifying Sequencing Parameters You can specify parameters to control tear stream selection and automatic sequencing. To specify the tearing and sequencing parameters: 1. From the Data menu, point to Convergence, then Conv Options. 2. Select the Sequencing sheet. 3. You can specify the tearing and sequencing parameters. The following parameters are available on the Sequencing sheet: Field Default To specify Design Spec Nesting Inside Whether design specifications should be nested inside tear stream loops, outside tear stream loops, or converged simultaneously with tear streams Design Spec Nesting does not apply to convergence blocks specified in the Convergence Order form. When the tear of an outer loop is recalculated in an inner loop, the actual sequence generated may not strictly follow the loop-order preference specified in the Design Spec Nesting and User Nesting fields. User Nesting Outside User Nesting lets you specify a preference for whether convergence blocks specified on the Conv Order form should be nested inside or outside other convergence blocks (user-defined or system-generated) The User Nesting field has precedence over the Design Spec Nesting field. When the tear of an outer loop is recalculated in an inner loop, the actual sequence generated may not strictly follow the loop-order preference specified in the Design Spec Nesting and User Nesting fields. Continued
ConvergenceDefaultFieldTo specify1Variable WeightTear variable weighting factor for tearing algorithmIf VariableWeight is a largenumber,thetearingalgorithmminimizesthenumber of ton variables.1Loop WeightLoop weighting factor for tearing algorithmIf Loop Weight isalargenumber,thetearing algorithm minimizesthenumber of loops torn.Tear Fortran Writenot checkedWhetherFortranblockvariablescanbetomwhenFortranblocksappearVariablesinfeedback loopsSee Chapter19,Fortran,in the section,Converging Loops Introduced byFortran Variables formoreinformaltion andan exampleCheck SequencecheckedWhether Aspen Plus checks a user-specifiedsequence to ensure that allloops are tornSpecifying Convergence Method ParametersYou can specify additional parameters foreach numerical method.Select theappropriate tab for the convergence method. See Convergence Methods, thischapter, for information on the numerical methods.To specifytheadditional parameters:1. From the Data menu, point to Convergence, then Conv Options.2.Intheleftpaneof theDataBrowserwindow,selecttheMethodsform.3.Selecttheappropriatesheetfortheconvergencemethod.4.Specify theparameters for that method.Specifying Tear StreamsUse theTear Specifications sheet to identify tear streams to be converged bysystem-generated convergence blocks.If you specify an incomplete tear set for yourflowsheet, Aspen Plus automatically chooses the remaining set of streams. If youspecifyaredundanttear set (toomanytearstreams),AspenPlusmayignoresometears orfindan inefficient sequence.Tospecifyatearstream:1.From the Data menu, pointto Convergence, then Tear.17-6AspenPlusUserGuideVersion10.1-0
17-6 Aspen Plus User Guide Version 10.1-0 Convergence Field Default To specify Variable Weight 1 Tear variable weighting factor for tearing algorithm If Variable Weight is a large number, the tearing algorithm minimizes the number of torn variables. Loop Weight 1 Loop weighting factor for tearing algorithm If Loop Weight is a large number, the tearing algorithm minimizes the number of loops torn. Tear Fortran Write Variables not checked Whether Fortran block variables can be torn when Fortran blocks appear in feedback loops See Chapter 19, Fortran, in the section, Converging Loops Introduced by Fortran Variables for more information and an example Check Sequence checked Whether Aspen Plus checks a user-specified sequence to ensure that all loops are torn Specifying Convergence Method Parameters You can specify additional parameters for each numerical method. Select the appropriate tab for the convergence method. See Convergence Methods, this chapter, for information on the numerical methods. To specify the additional parameters: 1. From the Data menu, point to Convergence, then Conv Options. 2. In the left pane of the Data Browser window, select the Methods form. 3. Select the appropriate sheet for the convergence method. 4. Specify the parameters for that method. Specifying Tear Streams Use the Tear Specifications sheet to identify tear streams to be converged by system-generated convergence blocks. If you specify an incomplete tear set for your flowsheet, Aspen Plus automatically chooses the remaining set of streams. If you specify a redundant tear set (too many tear streams), Aspen Plus may ignore some tears or find an inefficient sequence. To specify a tear stream: 1. From the Data menu, point to Convergence, then Tear