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SCDS Column (SCDS)

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SCDS Column (SCDS) SCDS is a rigorous multi-stage vapor-liquid equilibrium module which simulates any single column calculation including distillation columns, absorbers, reboiled absorbers and strippers. Side products and side heaters/coolers can also be modeled rigorously by SCDS. Murphree tray efficiency can also be input and simulated by SCDS. SCDS handles columns with unlimited stages, five feed streams, and four side products. There is no limit to the number of SCDS UnitOps in a flowsheet. SCDS offers a variety of specifications, such as total mole flow rate, heat duty, reflux ratio, boil-up ratio, temperature, mole fraction, recovery fraction, component flow rate, and flow ratio of two components in products. This module can simulate rigorous two-phase or three-phase distillation systems. If the calculation is threephase, the user has the option to decant one of the liquid phases in the condenser, while refluxing the other. SCDS is mainly designed to simulate non-ideal K-value chemical systems. It uses a Newton-Raphson convergence method and calculates the derivatives of each equation rigorously, including the DK/DX (derivative of K-value with respect to composition) term which is significant in chemical system simulation. The run time of SCDS is usually longer than the other module TOWR especially when the number of components involved is greater than 10.

SCDS Topology The column stages are numbered from the top of the column down. As usual, the feed streams are entered as positive stream numbers while the output streams are entered as negative stream numbers. The feed streams must be entered from top to bottom. A side heater/cooler is simulated by specifying a feed stream to the desired stage with zero flow rate but having the desired enthalpy rate. Positive enthalpy indicates a heater; negative enthalpy indicates a cooler. The first output stream will always be the distillate and the second output stream will always be the product from the bottom of the column. If the condenser type is 2 or 3, the decanted stream is always the third outlet stream. All additional output streams are treated as side products and should be entered from top to bottom. If the column has a partial condenser and has both vapor and liquid products from the condenser, the vapor from the condenser will be defined as distillate and the liquid product will be defined as the first side product. If the column has a total condenser, there will be no vapor from the condenser and the liquid product from the condenser is defined as a distillate. Example: SCDS 1 4 -2 -5 -3 -7 -9 Suppose the column has a partial condenser (condenser type 1) and a liquid product from the condenser. The topology stated above provides the following information: The column has two feed streams, 1 and 4. Stream 1 enters the column in a higher location. Stream 2 is the vapor product (distillate) from the condenser and stream 5 is the liquid product from the bottom of the column. Stream 3 is the liquid product from the condenser (treated as the first side product stream) and streams 7 and 9 are two more side products located somewhere down the column.

Stage Flow Rates In CHEMCAD, the top stage is stage 1 and the bottom stage is stage N. If a condenser and/or reboiler are present, they are stage 1 and N, respectively. The vapor or liquid flow rate of a stage is defined as the mole flow rate leaving the stage, as illustrated below:

mk:@MSITStore:C:\Program%20Files%20(x86)\Chemstations\CHEMCAD\cc6.chm:... 20/07/2010

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where: Vm = Vapor mole flow rate of stage m Lm = Liquid mole flow rate of stage m SV = Vapor side stream mole flow rate SL = Liquid side stream mole flow rate Vm+1 = Vapor mole flow rate of stage m+1 Lm-1 = Liquid mole flow rate of stage m-1 The reflux from the condenser is always considered as L1.

Methods SCDS uses the simultaneous correction method for rigorous fractionation simulation. Although the algorithm is proprietary, you can find a discussion of the general method in Kister, H.Z.; Distillation Design, McGrawHill (1992).

General Tab Condenser type Select a condenser type from the drop-down list. For option 0, two-phase total condenser, the distillate will be liquid. For option 1, two-phase partial condenser, the distillate will be only the vapor from the condenser. All of the liquid is refluxed in the default condition. If the user has a partial condenser and wishes part of the liquid coming from the condenser to be withdrawn as a top product (with the vapor), he/she must specify a side product stream coming from stage 1 (the condenser). This stream must be identified in the flowsheet and its presence creates another degree of freedom in the problem which must be matched by a side product specification (below). For option 2, total condenser with decant, a three-phase total condenser with decant, two liquid phases form in the condenser. The user may specify what portion of condensate is refluxed, what portion is decanted, and what portion is withdrawn as product. The distillate, however, is always made up of some portion of the


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light liquid phase. Normally, of course, one of the phases, is completely decanted, but this is not required by CHEMCAD. This option requires that you set up a side stream from the column. This must be done in the flowsheet graphics or Topology. This side stream is for the decanted liquid. Its presence does not create an extra degree of freedom and, therefore, does not require a side product specification. When using 3phase/decanter options 2 (above) or 3 (below) you should NOT define the decant stage or specification. SCDS assumes that the first side draw defined by graphics or topology will be the condensate decanter. The portion of each liquid phase which goes to the decanter is specified on page 1 of the SCDS menu in the decanter split fraction fields (upper layer Alpha, lower layer Beta). For option 3, partial condenser with decant, a three-phase partial condenser with decant, one vapor, and two liquid phases form in the condenser. The vapor is the distillate. The two liquid phases are either decanted or refluxed. The user may specify how much of each liquid phase is decanted and how much is refluxed using the decanter split fields below. This option requires that the user set up a side stream from the column. This must be done using flowsheet graphics or Topology. This side stream is for the decanted liquids. Its presence does not create an extra degree of freedom and, therefore, does not require that a side product specification be made. Subcooled delta T Subcooling may be specified for a total condenser. The number entered is the number of degrees below the bubble point of the liquid distillate. It is NOT the condenser temperature. For instance, to specify a total condenser which operates at a temperature 20 F below the bubble point of the liquid distillate, set the condenser type to 0 and the condenser degrees of subcooling to 20. Top pressure The pressure at condenser or top of the column. If not entered, the first feed stream pressure will be used. Cond press drop Enter the condenser pressure drop as a positive number. Colm press drop Enter the pressure drop through the column and reboiler as a positive number. Pressure on each tray will be calculated by linearization of pressure between top of the column (excluding condenser) and bottom of the column (including reboiler if it exists). Reflux pump press. Determines the pressure of distillate and reflux. Bottom pump press. Determines the pressure of the bottoms stream. No. of stages For Regular VLE model, this is the number of stages including condenser and reboiler. Stages are numbered from the top of the column own. If a condenser is present, it is treated as stage 1. If a reboiler is present, it is treated as stage N. Theoretically, there is no limit to the number of stages. The actual number of stages allowed depends on computer memory and number of components in the system. The minimum number of stages is 2. For Tray Mass Transfer models, this is the number of theoretical (not ideal) trays to the column. Tray 1 and N are still the condenser and reboiler, if present. For Packed Column Mass Transfer models, this is the number of segments used as calculation segments in the numerical methods of the mass transfer model. In regular VLE, CHEMCAD performs a VLE calculation around each stage. For the packed column, CHEMCAD performs VLE and mass transfer calculations around each segment. Using too few segments is analogous to using too large a step size in numerical integration; the result will be a choppy fit to a smooth curve. Using too many segments will provide the correct answer, but will take longer to calculate. As a general rule, use 25-100 segments, depending on how non-ideal the system will behave. Feed stages The feed stages are listed in order from top to bottom. Side heaters/coolers are entered as enthalpy streams (streams have enthalpy and no component flow rate), and the locations must be entered as feed stages. A feed stream entering a condenser or a reboiler is permitted. If you require multiple feed streams to enter the same stage, the Mixer UnitOp should be used before the column. Simulation model Choose regular VLE model to use the normal SCDS model for a column of N stages. This model assumes equilibrium mixing on each stage. The mass transfer options use Maxwell-Stefan diffusivities and empirical


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correlation to calculate the matrix of overall mass transfer coefficients. Check here for reactive distillation A check in this box indicates that the distillation is reactive. After checking this box and clicking OK, you will see data input screens for reactive distillation. The input screens for reactive distillation are described below. Upper Layer (Alpha) Upper layer is the decant split fraction for the light liquid phase in the condenser. It is only used when the condenser type is 2 or 3, i.e., when two liquid phases exist in the condenser. The default is 0. When upper layer = 0, all the light phase is refluxed, and none is sent to the decanter. When upper layer = 1, all the light phase is sent to the decanter and none is refluxed. Any number between 0 and 1 is permitted. Even if a distillate or side product of light liquid has been taken off the condenser, the fraction is applied to the total amount of light liquid product condensed by the condenser. This parameter is referred to as Alpha in previous versions of CHEMCAD. Lower Layer (Beta) Lower Layer is the decant split fraction for the heavy liquid phase in the condenser. It is only used when the condenser type is 2 or 3, i.e., when two-liquid phases exist in the condenser. The default is 0. When Lower Layer = 0, all the heavy phase is refluxed and none is decanted. When Lower Layer = 1, all the heavy phase is sent to the decanter and none is refluxed. Beta may be any number between 0 and 1. Beta is applied to the total amount of heavy liquid condensed by the condenser. This parameter is referred to as Beta in previous versions of CHEMCAD. A check mark here indicates that the distillation is reactive. If this field is checked, data input screens for reactive distillation will pop up when you click OK. Three-phase flag The default mode (0) is for two-phase distillation. The user can convert to three-phase distillation by either putting a one in the field to allow for LLV in this unit (local) or by using the LLV K-value option (global) in the K-value menu. Three-phase stage from When a three-phase simulation has been specified, the default is to use rigorous, three-phase calculations on all stages. If these phases exist, they will be shown; if not, they are not shown and nothing is lost except computation time. This method of looking for three phases on all stages is the safest approach. It is also the slowest. To speed up the calculation, the user can tell the program to look only on certain stages for two liquid phases. In this field, the starting stage is identified. Three-phase stage to This is the ending stage for the above specification. Ambient Heat Transfer If U, A, and ambient T are specified, it is possible to calculate ambient heat transfer to or from the environment. This will affect the temperature and/or heat duties for the column. The column profile may also be affected. Heat transfer area per stage Enter the area per stage which is available for heat transfer. The units are indicated on the screen. Heat transfer coefficient (U) Enter the overall heat transfer coefficient in this field. The units will be indicated on the screen. Ambient temperature Enter the ambient temperature to be used by the heat transfer calculations. Units are indicated on the screen.

Specifications Tab Condenser mode Select a condenser specification mode from the drop-down list. Mode 0 indicates that there is no condenser; any other mode indicates that the condenser exists and specifications are provided. Specification Enter the condenser specification according to the condenser mode that you have selected. Condenser heat duty must be a negative real number. If mode is 0, this field does not appear. Component For condenser mode 5, 6, 7, 11, or 12, select a component from the drop-down list if the specification of a


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certain component in the distillate is necessary. Divided by For condenser mode 9, use this field along with the Component field to specify the molar flow ratio of two components in the distillate. Reboiler mode Select a reboiler specification mode from the drop-down list. Mode 0 indicates no reboiler (such as an absorber). Any other mode indicates that the reboiler exists and specifications are provided. Specification Enter the reboiler specification according to the reboiler mode that you have selected. Reboiler heat duty must be a positive real number. If the selected mode is 0, this field does not appear. Component For reboiler mode 5, 6, 7, 11, or 12, select a component from the drop-down list if the specification of a certain component in the bottom is necessary. Divided by For reboiler mode 9, use this field along with the Component field to specify the molar flow ratio of two components in the bottom. Side Product Specifications Stage The side product stage locations must be entered from stream stage locations top to bottom. The number of side product stage locations must be the same as the number of side product streams defined in the column topology. Example: TOWR 2 3 -4 -7 -9 -12 1st feed stage no. 5 2nd feed stage no. 15 1st side product stage no. 3 2nd side product stage no. 10 The column has two feed streams entering on stages 5 and 15 respectively. Stream 4 is the distillate from the top of the column and stream 7 is the bottom product. Steam 9 is the first side product from stage 3 and stream 12 is the second side product from stage 10. Side product mode For each side product stream, select a side product specification mode from the drop-down list. Specification For each side product stream, enter the appropriate side stream specification. Optional Tray Specification It is sometimes desirable to make a specification at a particular tray rather than at the condenser, reboiler, or side draw whose presence created the degree of freedom requiring the specification. It is still necessary to make the original spec at your condenser, reboiler, or side stream in the normal way. The tray specification then varies the value of the original specification to meet the tray specification. Adjust the specification for The program will vary the value of the condenser spec, the reboiler spec, or any of the side product specs. Use the drop-down list to select an option. until the This is the setpoint variable type. It may be on a tray or in the distillate. Tray If the measured variable is on a tray, enter the tray number here. Variable Use the drop-down list to identify which variable of the specified tray is to be set to the specified value. Phase Identify whether the above specified variable in the vapor phase leaving the stage or the liquid phase leaving the stage. Component


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If the specified tray specification is a composition variable, select the appropriate component. is equal to a specified value of Give the numerical value of the specified variable in global flowsheet engineering units.

Convergence Tab Estimates Initialization The distillation calculation algorithm enables you to supply initial estimates for the distillation calculation. The initialization options are as follows: 0 Use simple estimations

This option requires the minimum amount of input information (distillate flow rate, reflux rate, etc.) and starts its own profile estimation by the built-in profile generation algorithm. In most cases, option 0 has no trouble converging.

1 Reload column profile

Reloads the entire column profile from the previous simulated results and is most effective for case studies. Make sure you already have a base case from the previous run before you select option 1. The number of stages and number of components must remain the same as the base case. This option is used internally if the column is part of a recycle loop.

2 Temp. profile only

Estimate temperature profile.

3 Temp and Vapor Profile

Estimate temperature, and vapor flow rate.

4 Temp, Vapor, Liquid Profile

Estimate temperature, vapor and liquid flow. Note that the temperature and vapor and liquid mole flow rates must have the same units as you selected in the unit selection section.

5 Temp, Vapor, Liquid, Pressure

Estimate temperature, vapor flow, liquid flow, and pressure. Note that the pressures provided are fixed values, i.e., they fix the pressure profile. The T, V, and L values are initial estimates only. To fix a pressure profile in the column other than the linear profile normally created by the program, select this option.

6 Auto estimation

This is the default option. If you select it, you do not need to enter any estimates for the column.

If SCDS fails to converge because of extremely non-linear vapor or liquid temperature profiles in the column, you may want to estimate the initial profile as follows:  For options 2, 3, 4, and 5, complete the Profile Estimation dialog box, which will appear immediately following the SCDS menu when you click OK.  It is not necessary to give estimates for every stage on the menu. Give only those stages desired and the program will perform linear interpolation to determine initial estimates for the rest. The top and bottom stages must be provided. Dist. rate Estimated distillate mole flow rate. Recommended input. Do not enter an arbitrary number. If you have no idea about distillate rate, leave this specification blank. SCDS will use its own estimation. Reflux rate Estimated reflux mole flow rate. This is not the specified reflux ratio and should not be input if it is not approximately known. It is recommended input if known, especially for a very large reflux. T top Enter the estimated condenser or top stage temperature. Recommended input. Leave blank if unknown. T2 (ovhd) Enter the estimated temperature for stage 2. This is recommended for problems having a large temperature difference between T top and T2 such as columns with a subcooled condenser. Optional input. T (n-1) Enter the estimated temperature for stage N-1. This is recommended for problems having a large


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temperature difference between T bottom and T (N-1). Optional input. T bottom Enter the estimated reboiler or the last stage temperature. Recommended input. Leave blank if unknown. Side product estimations Enter stages and estimated flow rates for side products. Convergence Iterations Enter the maximum number of iterations for convergence. The default value is 20. Tolerance You can specify a tolerance if desired. Note that SCDS does not use relative error for tolerance checks. When an SCDS calculates, the calculated tolerance is displayed under calculated results. If you are modeling at ppm concentrations you may have to tighten the convergence. Note that an extremely small tolerance may cause some round-off errors. SCDS calculates the convergence tolerance by the following equations: Factor=(v2+L1+Feed/3)/2 Tolerance= 1.0 E-9 *((2*nc +1)*nstage)*[Factor*4]^2 Where: V2 = Vapor flow at stage 2 L1 = Liquid flow at stage 1 Feed = Total feed moles nc = No. of components nstage = No. of stages Damping factor Enter a value between 0 - 1 if SCDS does not converge using ordinary methods. This triggers a lower starting efficiency which may stabilize difficult columns. Trace flag Advanced users can use this field to increase the level of detail in the Trace Messages dialog box. Run optimization for infeasible design specs. This box, which is checked by default, allows CHEMCAD to adjust the column specifications for convergence. Stage efficiency Tray efficiency profile This option allows you to specify Murphree tray efficiencies by tray or by component and tray. If an incomplete profile is provided, the program interpolates to get the missing values. If you select an option other than none, the efficiency profile will override top stage and last stage efficiencies (above). If this option is selected, specification input screens appear upon exiting the SCDS input screen. Top stage Enter a Murphree tray efficiency for the top of the column. If you specify a bottom stage efficiency, a straight line will be interpolated across the column. If you specify only the top stage efficiency, it will be used for all stages. If you use a tray efficiency profile, it will override this setting. Last stage Enter a Murphree tray efficiency for the bottom of the column. If you specify a top stage efficiency, a straight line will be interpolated across the column. If you specify only the bottom stage efficiency, it will be used for all stages. If you use a tray efficiency profile, it will override this setting. Miscellaneous Pressure profile Pressure profile can be either user-specified or predicted using the column sizing tools.  The default mode is Specify pressure drop. For this mode, the pressure drops entered on the General tab will be used to determine pressure through the column.  If you select Specify pressure profile, you can specify pressure versus stage. Pressure throughout the column will be interpolated from your specifications. The pressure profile dialog will appear when you click OK.


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 If you select Use tray column rating, CHEMCAD will use generalized tray sizing methods to estimate pressure drop through a column. When you click OK, screens for tray column geometry will be displayed.  If you select Use packed column rating, CHEMCAD will use generalized packed column sizing methods to estimate pressure drop through a column. When you click OK, screens for packed column geometry will be displayed. Thermosyphon vapor fraction If your reboiler is a thermosyphon reboiler, you may specify the outlet vapor fraction of the reboiler. If you do so, CHEMCAD will calculate the flow rate and heating curve of the reboiler. This calculation does not affect the distillation calculation, only the reporting of the reboiler information. Calculated results Condenser duty Displayed is the value CHEMCAD calculates for the condenser heat duty, based on user specifications. Reboiler duty Displayed is the value CHEMCAD calculates for the reboiler heat duty, based on user specifications. Reflux mole This field displays the liquid leaving the ‘theoretical plate’ on stage 1. Note that if you do not have a condenser this is a non-zero number, as you have liquid leaving the top stage. For a column with condenser, this is the Reflux mole rate. Reflux mass Displayed is the liquid leaving the ‘theoretical plate’ on stage 1. Note that if you do not have a condenser this is a non-zero number, as you have liquid leaving the top stage. For a column with condenser, this is the Reflux mass rate. Reflux ratio Displayed is the reflux ratio calculated by CHEMCAD, based on user specifications. Note that if you do not have a condenser this is a non-zero number, as you have liquid leaving the top stage. For a column with condenser, this is the Reflux mole rate. Tolerance Displayed is the calculated tolerance from the previous calculation of this SCDS. This is useful if you intend to specify your own tolerance for the SCDS, as above.

Reactive Distillation Option Reactive distillation is simulated using SCDS as a platform or template for the addition of chemical reaction specifications. The reactions may be defined as kinetic and/or equilibrium and may occur in the liquid and/or vapor phase, simultaneously. Expressions for equilibrium and rates of reaction are the same as those described for the equilbrium reactor and kinetic reactor. To activate the reactive distillation screens, check the Check here for reactive distillation box, found on the General tab of the SCDS UnitOp specification dialog. The screens for reactive distillation are as follows.

General Data - Reactive Distillation Dialog Box No. of liquid reactions A combined total of 20 liquid and/or vapor reactions is permitted. No. of vapor reactions A combined total of 20 liquid and/or vapor reactions is permitted. Units selection for rate equations Molar flow unit Molar flow units are needed to properly define concentration terms in the reaction equations. Select a flow unit from the drop-down list. Activation energy Select activation energy from the drop-down list. Volume unit Volume units are needed to properly define concentration terms in the reaction equations. Select a volume unit from the drop-down list. Time unit Select a time unit from the drop-down list.


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For vapor phase reactions: Concentration flag For vapor phase reactions, you can define the concentration terms as moles/volume or partial pressure. Select an item from the drop-down list. Pressure units Select pressure units from the drop-down list: For equilibrium reactions: Temperature units For equilibrium reactions only, you must define the units of temperature in the equation expressing the dependence of Keq on temperature. This temperature unit is not used in the kinetic expression since the temperature unit is inferred from the activation energy unit. Select units from the drop-down list.

Reaction Type Dialog Box On this screen, simply choose the reaction type for each specified reaction. The default setting for all reactions is Kinetic.

Reaction Volume Dialog Box Enter the stage number and liquid and/or vapor volumes for each specified stage.

Kinetic Data Dialog Box This screen requests the reaction parameters and stoichiometry for each reaction. The screen will appear once for each specified kinetic reaction. In the Reaction phase field, specify liquid or vapor phase reaction for the reaction number shown in the top right corner. The entries in the remaining fields will define the rate of reaction. Recall that the total rate of reaction for a single component in a simultaneous reaction is given by the following expression:

Note: Most users will not have a need for the absorption terms contained in the second brackets. Leaving out such data, the rate expression reverts to the traditional stoichiometric reactant-concentration dependent form with Arrhenius temperature dependence. The entire equation may be generalized as:

where: ri =

Rate of formation for component i, mole/volume-time

i k j Nij

= = = =

Subscript for component i Subscript for reactant k Subscript for reaction j Stoichiometric coefficient for component I in reaction j

Aj

=

Frequency factor (Arrhenius parameter) in reaction j

Ej

=

Activation energy in reaction j

R T Ck

= = =

Universal gas constant Absolute temperature Concentration of reactant k, mole/volume or the partial pressure of reactant k

akj

=

Exponential factor for reactant k in reaction j

n nrx

= =

Number of reactants Number of reaction

ϕkj

=

Absorption frequency factor for component k (Adsorption factor)

Ekj

=

Absorption energy factor for component k (Adsorption E) in reaction j

βj

=

Π =

Power factor for absorption sites term for reaction j (Beta factor) Product symbol


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Summation symbol Adsorption exponential factor for reactant k in reaction j

Equilibrium Data Dialog Box This screen requests the reaction parameters and stoichiometry for each reaction. The screen will appear once for each specified equilibrium reaction. Reaction phase Specify liquid phase reaction or vapor phase reaction. Parameters (A) through (D) represent the coefficients for the following reaction equilibrium equation:

The definition of Keq depends upon the phase of the reaction. Component, Stoich. For each component involved in the reaction, use the drop-down list to enter it in the Component field; then enter the appropriate stoichiometric coefficient to the right of each component. Liquid Keq option Use this optional field to define Keq in terms of either composition or activity. Approach DT The temperature at which Keq is calculated is normally the stage temperature. You can use this field to adjust the temperature for the purpose of calculating Keq.

Disabled Reactions Dialog Box Use this screen to disable reactions on selected stages:


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