Unit Operation and Stream Models
Streams
Material Stream
The Material Stream is used to represent matter which enters and leaves the limits of the simulation, passing through the unit operations.
Input Parameters
? Specification:
? Temperature and Pressure
? Pressure and Enthalpy
? Pressure and Entropy
? Pressure and Vapor Fraction
? Temperature and Vapor Fraction
The above selection dictates which variables should be defined in order to DWSIM calculate the others.
? Temperature: stream temperature;
? Pressure: stream pressure;
? Specific Enthalpy: stream's specific enthalpy;
? Specific Entropy: stream's specific entropy;
? Molar Fraction (Vapor Phase): the vapor phase mole fraction of the stream;
? Composition: the stream composition can be entered on Mole/Mass Fraction/Flow.
For the mole/mass fraction basis options, the user must enter the composition in such a way that the amounts sum up to 1. Regardless of the input basis option, new values are saved only when the "COMMIT CHANGES" button is pressed.
? Flow: one of the three types of flow (mass, molar or volumetric) must be given - the other two are calculated in the case of temperature and pressure are already defined.
The material stream composition can only be edited if the stream is not connected to any unit op upstream, that is, if it doesn't work as an output of any operation. If that is the case, the stream con able to edit any of its properties directly.
Calculation Method
When the four properties described above are defined, the material stream is calculated and its properties are shown in the same window. The calculation sequence for the material stream is the following:
1. A flash calculation is done to know the component distribution between phases;
2. Properties of each phase are calculated individually;
3. Finally, the mixture properties are calculated.
In the first step, a TP Flash is done by default, but it can be replaced by a PH flash if the user defines this option in the simulation configuration window. When in "read-only" mode, the stream properties are calculated according to parameters given by the upstream unit operation (in the majority of cases, a TP flash is done as well).
Output parameters
Component distribution between phases and phase properties: specific enthalpy, specific entropy, molecular weight, density, volumetric flow rate @ T and P, phase molar and mass fraction, compressibility factor Z, constant-pressure heat capacity (Cp), Cp/Cv, thermal conductivity, surface tension (liquid phase only) and kinematic and dynamic viscosity. Mixture properties: specific enthalpy and entropy, molecular weight, density and thermal conductivity. If the global setting ?Calculate bubble and dew points at stream conditions? is activated, these are also shown in the ?Mixture? properties section.
Energy Stream
The energy stream is used to represent energy entering and leaving the limits of the simulation, used by the unit operations, either to represent loss, demand or power generation. As we are dealing with steady-state simulations, one defines the energy stream in terms of power (energy by unit of time) and not energy itself.
Input Parameters
? Energy: Energy by unit of time (power) which is represented by the stream;
Output Parameters
There are no output parameters for this object.
Unit Operations
Mixer
The Mixer is used to mix up to six material streams into one, while executing all the mass and energy balances.
Input Parameters
? Downstream pressure: defines how the downstream pressure must be calculated from the pressure of the streams connected to the mixer inlets.
Calculation Method
The mixer does the mass balance in the equipment and determines the mass flow and the composition of the outlet stream. Pressure is calculated according to the parameter defined by the user. Temperature is calculated by doing a PH Flash in the outlet stream, with the enthalpy calculated from the inlet streams (energy balance).
Output Parameters
There are no output parameters for this object.
Splitter
The splitter is a mass balance unit operation - divides a material stream into two or three other streams.
Input Parameters
? Stream fractions: this property defines the mass flow fraction to be passed to each outlet stream in the splitter. Each fraction must have a value between 0 and 1 and the total sum must not be bigger than 1.
Output Parameters
There are no output parameters for this object.
Separator Vessel
The separator vessel (also known as flash drum) is used to separate liquid phases from vapor in a mixed material stream.
Input Parameters
? Override separation temperature and pressure: these properties define if the flash calculation will be done with the temperature and pressure of the inlet stream or the ones defined by the user.
Calculation Method
The separator vessel just divides the inlet stream phases into two or three distinct streams. If the user defines values for the separation temperature and/or pressure, a TP Flash is done in the new conditions before the distribution of phases through the outlet streams.
Output Parameters
There are no output parameters for this object.
Pipe Segment
The Pipe Segment unit operation can be used to simulate fluid flow process in a pipe. Two of the most used correlations for the calculation of pressure drop are available in DWSIM. Temperature can be rigorously calculated considering the influence of the environment. With the help of the Recycle Logical Operation, the user can build large water distribution systems, as an example.
The pipe segment is divided in sections, which can be straight tubes, valves, curves, etc. Each section is subdivided in small sections for calculation purposes, as defined by the user.
Input Parameters
? Hydraulic profile: clicking on the ellipsis button opens the pipe hydraulic profile editor . In the hydraulic profile editor, the user adds sections, define their type and in how many increments it will be divided during the calculations, the pipe material, length, elevation and internal and external diameters. Each change can be saved by clicking in the "Apply" button.
? Pressure drop correlation: select the model to be used for the pressure drop calculation in the pipe segment.
? Thermal profile: clicking on the ellipsis button opens the pipe thermal profile editor . In the thermal profile editor it is possible to define how the temperature profile in the pipe should be calculated. The configurations in this window are valid for the entire pipe segment. Changes are saved automatically.
Calculation Method
The pipe segment is calculated based on incremental mass and energy balances. The complete algorithm consists in three nested loops. The external loop iterates on the sections (increments), the middle loop iterates on the temperature and the internal loop calculates the pressure. The pressure and temperature are calculated as follows:
1. The inlet temperature and pressure are used to estimate the increment outlet pressure and temperature.
2. Fluid properties are calculated based in a arithmetic mean of inlet and outlet conditions.
3. The calculated properties and the inlet pressure are used to calculate the pressure drop. With it, the outlet pressure is calculated.
4. The calculated and estimated pressure are compared, and if their difference exceeds the tolerance, a new outlet pressure is estimated, and the steps 2 and 3 are repeated.
5. Once the internal loop has converged, the outlet temperature is calculated. If the global heat transfer coefficient (U) was given, the outlet temperature is calculated from the following equation:
`Q=UA\Delta T_{ml}`
where: Q = heat transferred, A = heat transfer area (external surface) and `\Delta T_{ml}` = logarithmic mean temperature difference.
6. The calculated temperature is compared to the estimated one, and if their difference exceeds the specified tolerance, a new temperature is estimated and new properties are calculated (return to step 2).
7. When both pressure and temperature converges, the results are passed to the next increment, where calculation restarts.
Output Parameters
? Delta-T: temperature variation in the pipe segment.
? Delta-P: pressure variation in the pipe segment.
? Heat exchanged: amount of heat exchanged with the environment, or lost by friction in the pipe walls.
? Results (table): results are show section by section in a table.
? Results (graph): a graph shows the temperature, pressure, liquid holdup, velocity and heat exchanged profiles.
Valve
The Valve works like a fixed pressure drop for the process, where the outlet material stream properties are calculated beginning from the principle that the expansion is an isenthalpic process.
Input Parameters
? Pressure drop: pressure difference between the outlet and inlet streams.
Calculation Method
The outlet stream pressure is calculated from the inlet pressure and the pressure drop. The outlet stream temperature is found by doing a PH Flash. This way, in the majority of cases, the outlet temperature will be less than or equal to the inlet one.
Output Parameters
? Delta-T: temperature drop observed in the valve expansion process.
Pump
The pump is used to provide energy to a liquid stream in the form of pressure. The process is isenthalpic, and the non-idealities are considered according to the pump efficiency, which is defined by the user .
Input Parameters
? Delta-P: pressure rise in the pump.
? Efficiency: pump adiabatic efficiency;
? Ignore vapor in the inlet stream: defines if the calculator should ignore any vapor in the inlet stream;
? Use the provided Delta-P: defines if the pressure of the outlet stream will be calculated by the user-defined Delta-P or the energy stream connected to the pump.
Calculation Method
The calculation method for the pump is different for the two cases (when the provided delta-p or the potency of the energy stream is used). In the first method, we have the following sequence:
? Outlet stream enthalpy:
`H_{2}=H_{1}+\frac{\Delta P}{\rho},`
? Pump discharge pressure:
`P_{2}=P_{1}+\Delta P`
? Pump required power:
`Pot=\frac{W(H_{2}-H_{1})}{\eta},`
where:
`Pot` pump power
`W` mass flow
`H_{2}` outlet stream specific enthalpy
`H_{1}` inlet stream specific enthalpy
`\eta` pump efficiency
? Outlet temperature: PH Flash (with P2 and H2).
In the second case (calculated outlet pressure), we have the following sequence:
? Outlet stream enthalpy:
`H_{2}=H_{1}+\frac{Pot\,\eta}{W},`
? \Delta P:
`\Delta P=\rho(H_{2}-H_{1}),`
? Discharge pressure:
`P_{2}=P_{1}+\Delta P`
? Outlet temperature: PH Flash.
Outlet Parameters
? Delta-T: temperature variation in the pumping process.
? Power required: power required by the pump.
Compressor
The compressor is used to provide energy to a vapor stream in the form of pressure. The ideal process is isentropic (constant entropy) and the non-idealities are considered according to the compressor efficiency, which is defined by the user.
Input Parameters
? Delta-P: pressure rise in the compressor.
? Efficiency: compressor adiabatic efficiency;
? Ignore liquid in the inlet stream: defines if the calculator should ignore any liquid in the inlet stream;
? Use the provided Delta-P: defines if the pressure of the outlet stream will be calculated by the user-defined Delta-P or the energy stream connected to the compressor.
Calculation Method
The compressor calculation is different for the two cases (when the provided delta-p or the potency of the energy stream is used). In the first method, we have the following sequence:
? Outlet pressure calculation:
`P_{2}=P_{1}+\Delta P`
? Outlet enthalpy: A PS Flash (Pressure-Entropy) is done to obtain the ideal process enthalpy change. The outlet real enthalpy is then calculated by:
`H_{2}=H_{1}+\frac{\Delta H_{id}}{\eta\,W},`
? Power required by the compressor:
`Pot=\frac{W(H_{2_{id}}-H_{1})}{\eta},`
? Outlet temperature: PH Flash with `P_{2}` and `H_{2}`.
In the second case (calculated outlet pressure), we have the following sequence:
? Discharge pressure:
`P_{2}=P_{1}[1+\frac{Pot}{\eta W}\frac{k-1}{k}\frac{MM}{8.314T_{1}}]^{[k/(k-1)]},`
where:
`P_{2}` outlet stream pressure
`P_{1}` inlet stream pressure
`Pot` compressor power
`W` mass flow
`\eta` compressor adiabatic efficiency
`k` adiabatic coefficient `(Cp_{gi}/Cv_{gi})`
`MM` gas molecular weight
`T_{1}` inlet stream temperature
? Outlet enthalpy: A PS Flash (Pressure-Entropy) is done to obtain the ideal process enthalpy change. The outlet real enthalpy is then calculated by:
`H_{2}=H_{1}+\frac{\Delta H_{id}}{\eta\,W},`
? Outlet temperature: PH Flash with `P_{2}` and `H_{2}`.
Output Parameters
? Delta-T: temperature change in the compression process.
? Power required: power required by the compressor.
Expander
The expander is used to extract energy from a high-pressure vapor stream. The ideal process is isentropic (constant entropy) and the non-idealities are considered according to the expander efficiency, which is defined by the user.
Input Parameters
? Delta-P: pressure drop in the expander.
? Efficiency: expander adiabatic efficiency;
? Ignore liquid in the inlet stream: defines if the calculator should ignore any liquid in the inlet stream;
Calculation Method
? Discharge pressure calculation:
`P_{2}=P_{1}-\Delta P`
? Outlet enthalpy: A PS Flash (Pressure-Entropy) is done to obtain the ideal process enthalpy change. The outlet real enthalpy is then calculated by:
`H_{2}=H_{1}+\frac{\Delta H_{id}}{\eta\,W},`
? Power generated by the expander:
`Pot=\frac{W(H_{2_{id}}-H_{1})}{\eta},`
? Outlet temperature: PH Flash with `P_{2}` and `H_{2}`.
Output Parameters
? Delta-T: temperature change in the compression process.
? Power generated: power generated by the expander.
Heater
The heater simulates a stream heating process.
Input Parameters
? Pressure drop: pressure drop in the heater.
? Heat added: amount of heat added in the heater.
? Efficiency: heater efficiency.
? Use heat provided: defines if the heat added is determined by the value informed by the user or by the energy stream connected to the heater.
Calculation Method
The outlet stream temperature is calculated by doing a PH Flash, were the outlet stream enthalpy is calculated by a energy balance in the heater.
Output Parameters
? Delta-T: temperature rise observed in the heating process.
Cooler
The cooler simulates a stream cooling process.
Input Parameters
? Pressure drop: pressure drop in the cooler.
? Heat removed: amount of heat removed by the cooler.
? Efficiency: cooler efficiency.
Calculation Method
The outlet stream temperature is calculated by doing a PH Flash, were the outlet stream enthalpy is calculated by a energy balance in the cooler.
Output Parameters
? Delta-T: temperature drop observed in the cooling process.
Shortcut Column
The shortcut column is used to calculate the minimum reflux and distribution of products in a distillation column by the method of Fenske-Underwood-Gilliland [1]. The column should have a single feed stage, two products (top and bottom), condenser (partial or total) and reboiler. The results are the minimum reflux, thermal loads and temperature of the condenser and reboiler for a fixed reflux ratio, in addition to determining the optimum feed stage and the minimum number of stages.
Input Parameters
? Connections: feed, product, top, bottom and heat loads (condenser/reboiler).
? Type of condenser: partial or total.
? Reflux Ratio: ratio between the flow of liquid that returns from the condenser to the column and the one that leaves the condenser as the top product.
? Light Key: component used as a reference so that the lighter ones are present only in the top product.
? Heavy Key: component used as a reference so that the heavier ones are present only in the product of fund.
? Condenser pressure: pressure of the condenser.
? Reboiler pressure: pressure of the reboiler.
Output Parameters
? Minimum reflux: reflux ratio of minimum to ensure the separation specified.
? Minimum number of stages: the minimum number of training which ensures the separation specified.
? Optimal feed stage: the feed stage that minimizes the thermal load of the reboiler.
? Liquid / Vapor flows: internal flows in sections of rectification and stripping of the column.
? Thermal loads: thermal loads of condenser and reboiler.
Heat Exchanger
DWSIM has a model for the countercurrent, two-stream heat exchanger which supports phase change and multiple phases in a stream.
Input Parameters
The heat exchanger in DWSIM has five calculation modes:
1. Calculate hot fluid outlet temperature: you must provide the cold fluid outlet temperature and the exchange area to calculate the hot fluid temperature.
2. Calculate cold fluid outlet temperature: in this mode, DWSIM needs the hot fluid outlet temperature and the exchange area to calculate the cold fluid temperature.
3. Calculate both temperatures: in this mode, DWSIM needs the exchange area and the heat exchanged to calculate both temperatures.
4. Calculate area: in this mode you must provide the HTC and both temperatures to calculate the exchange area.
5. Rate a Shell and Tube exchanger: in this mode you must provide the exchanger geometry and DWSIM will calculate output temperatures, pressure drop on the shell and tubes, overall HTC, LMTD, and exchange area. This calculation mode uses a simplified version of Tinker's method [4] for Shell and Tube exchanger calculations.
You can provide the pressure drop for both fluids in the exchanger for modes 1 to 4 only.
Calculation Mode
The heat exchanger in DWSIM is calculated using the simple convection heat equation:
`Q=UA\Delta T_{ml},`
where: Q = heat transferred, A = heat transfer area (external surface) and `\Delta T_{ml} `= Logarithmic Mean Temperature Difference (LMTD). We also remember that:
`Q=m\Delta H,`
where: `Q` = heat transferred from/to the fluid and `\Delta H `= outlet-inlet enthalpy difference.
The calculation procedure depends on the mode selected:
1. Calculate hot fluid outlet temperature: HTC (Heat Transfer Coefficient), hot fluid outlet temperature, heat load and LMTD.
2. Calculate cold fluid outlet temperature: HTC, cold fluid outlet temperature, heat load and LMTD.
3. Calculate both temperatures: HTC, cold and hot fluid outlet temperatures and LMTD.
4. Calculate area: exchange area and LMTD.
5. Rate Shell and Tube exchanger: exchanger geometry information.
Results
The results given by the heat exchanger after calculation depends on the mode selected:
1. Calculate hot fluid outlet temperature: overall HTC, hot fluid outlet temperature, heat load and LMTD.
2. Calculate cold fluid outlet temperature: overall HTC, cold fluid outlet temperature, heat load and LMTD.
3. Calculate both temperatures: overall HTC, cold and hot fluid outlet temperatures and LMTD.
4. Calculate area: exchange area and LMTD.
5. Rate Shell and Tube exchanger: area, LMTD, LMTD correction factor (F), overall HTC, hot fluid outlet temperature, cold fluid outlet temperature, hot fluid pressure drop (shell/tubes only), cold fluid pressure drop (shell/tubes only).
Component Separator
The Component Separator is a mass balance unit operation. The components are separated between two streams, specified as fractions or absolute flow rates. The energy balance is then calculated after the separation.
Input Parameters
? Specified stream: sets the stream to which the separation specifications will be applied. ?0? corresponds to the Outlet stream 1 (overhead) and ?1? corresponds to the Outlet stream 2 (bottoms).
Results
? Energy imbalance: Difference between enthalpy of outlet and inlet streams. in some cases it can be interpreted as the energy necessary to do the separation.
Solids Separator
The solids separator is used to separate solids from a liquid phase in a mixed material stream.
Input Parameters
? Solids Separation Efficiency: defines the amount of solids in the liquid stream. 100% efficiency means no solids in the liquid stream.
? Liquids Separation Efficiency: defines the amount of liquid in the solids stream. 100% efficiency means no liquid in the solids stream.
Calculation Method
The solids separator does a mass balance and splits the inlet stream phases into two distinct streams.
Reactors
The reactors in DWSIM are specialized modules that solve a particular set of reactions in sequence or in parallel.
Input Parameters
All reactors share the same basic interface. Should be set the input and output streams, energy streams that represent the heat exchanged with the environment, the reaction set to be used, the mode of operation (isothermal or adiabatic) and the pressure drop through the reactor. For the PFR and CSTR, it is also necessary to inform the volume of the reaction medium.
Output Parameters
As results, are shown the conversions of the components involved in the reactions, the variation in temperature and the heat exchanged in the reactor. For the PFR are also shown the profiles of concentration of reactants and products ( ) along the longitudinal axis of the reactor (assuming the concentration does not vary radially).
The following points should be observed when using reactors in DWSIM:
? Only the active and compatible reactions in the selected reaction set are considered.
? The index of each reaction defines the solving method: equal indices represent reactions in parallel, while different indexes define sequential reactions.
Calculation Method
The Conversion Reactor is solved by simple energy and mass balances, calculating the reaction heat by considering the variation in the amount of the base component. The PFR and CSTR are solved by a numerical method for systems of ordinary differential equations (ODEs). Equilibrium and Gibbs Reactors are solved by using the procedure described by Michelsen.
Logical Operations
Recycle
The Recycle operation is composed by a block in the flowsheet which does convergence verifications in systems were downstream material connects somewhere upstream in the diagram. With this tool it is possible to build complex flowsheets, with many recycles, and solve them in an efficient way by using the acceleration methods presents in this logical operation.
There are two acceleration methods available: Wegstein and Dominant Eigenvalue. The Wegstein method must be used when there isn't a significant interaction between convergent variables, in the contrary the other method can be used. The successive substitution method is slow, but convergence is guaranteed.
The Wegstein method requires some parameters which can be edited by the user. The dominant eigenvalue does not require any additional parameter. The user can define convergence parameters for temperature, pressure and mass flow in the recycle, that is, the minimum acceptable values for the difference in these values between the inlet and outlet streams, which, rigorously, must be identical. The smaller these values are, the more time is used by the calculator in order to converge the recycle.
As a result, the actual error values are shown, together with the necessary convergence iteration steps.
Adjust
The Adjust is a logical operation which changes the value of a variable in a stream in order to attain a specification which can be a user-defined value or the value of other variable, in other stream. The adjust operation is very useful when there is a specification which cannot be accomplished directly, imposing the necessity of doing a trial and error calculation. If this is the case, the Adjust does everything automatically.
The user selects the controlled (specified) variable and the manipulated one. Then he defines the parameters:
? Adjust value (or offset): desired value for the variable or the value to be added or subtracted from the referenced variable.
? Maximum iterations: maximum number of iterations to be executed by the adjust;
In the Adjust Control Panel, the user controls the operation convergence. Two convergence methods are available.
References
[1] Henry Z. Kister, "Distillation Design", McGraw-Hill Professional (1992).
[2] McCabe, W.L. and Smith, J. and Harriott, P., "Unit Operations of Chemical Engineering", McGraw-Hill Education (2005).
[3] J. D. Seader and Ernest J. Henley, "Separation Process Principles", Wiley (2005).
[4] Tinker, T., "Shell side characteristics of shell and tube heat exchangers", Trans. ASME (1958), 80.