Flowsheet Tools

Sensitivity Analysis

You can use the Sensitivity Analysis Utility in order to study the influence of up to 2 variables into other dependent flowsheet variables. The changes in variables are defined by a value range and a number of equally spaced points within this range. For example, you can analyze the influence of temperature and pressure in the enthalpy of a mixture, from 200 to 400 K and from 100 to 1000 kPa, nine points for temperature and 5 points for pressure, totaling 45 points on which the enthalpy will be calculated at different temperatures and pressures. This also means that the flowsheet will be recalculated 45 times (!), so be careful with the number of points you choose as the calculation time can be prohibitive.

The sensitivity analysis utility is based on case studies. In a single simulation one can define a number of cases, each one with its own variables, ranges and results. These cases will be saved together with the simulation, and cannot be exported to other ones. The results are shown in a table, so the data can be copied and pasted into another specialized data analysis software or sent directly to the data regression plugin.

Optimization

The Optimizer in DWSIM handles single and multivariate optimization problems with or without bound constraints. The objective function can be either a variable in the flowsheet or an expression as a function of as many variables as you need.

The interface is very similar to Sensitivity Analysis's one. One can define a number of cases, each one with its own variables, ranges and results. These cases will be saved together with the current simulation, and cannot be exported to other simulations.

There are some options to choose from in DWSIM's Multivariate Optimizer. It is possible to select the type of the optimization (minimization or maximization of the objective function), choose if the indendent variables will have lower and/or upper bounds and if the objective function will be a flowsheet variable or an expression based on flowsheet variables. One can also define a maximum number for the iterations and a tolerance for the variation of the calculated value for the objective function - if the variation is less than the defined value, the flowsheet is considered optimized and the process stops. There is also an option to choose if the flowsheet will be returned to its original state after optimization, so the results will be shown only in the current window, and the flowsheet initial configuration will remain intact.

In order to define variables to be used in the optimization process, a variable can be added by clicking/touching on the "+" button. With the variable added to the list, one chooses an object, then the desired property and the type of variable (IND for independent, AUX for auxiliary or DEP for dependent variables). If necessary, one can define a lower and/or upper limit for the IND variables, according to the current unit system. The variable name is the one which will be used in the expression.

DWSIM only considers bounds for independent variables. Also, if the objective function is a DEP variable, and you defined multiple DEP variables, only the first will be used. AUX variables are used in the context of an expression only. To remove a variable, click/touch the "-" button.

With all the variables defined and the case configured, the optimization can be carried out by clicking on the appropriate button - the button will become disabled. After some time, if the optimization converges, the button will become active again, indicating that the the optimization process is over.

Chemical Reactions

DWSIM classifies chemical reactions in three different types: Conversion, where the conversion of a reagent can be specified as a function of temperature; Equilibrium, where the reaction is characterized by an equilibrium constant K, and Kinetic, where the reaction is led by a velocity expression which is a function of concentration of reagents and/or products.

When the reactions and their respective reaction sets are correctly defined, they will be available for selection in the property window of a reactor in the simulation. The reactor will then look for active reactions inside the selected set.

Characterization of Petroleum Fractions

DWSIM provides two tools for characterization of petroleum fractions. One of them characterizes C7+ fractions from bulk properties. The other characterizes the oil from an ASTM or TBP distillation curve.

In both tools, the characterization is done through the creation of various components with different boiling points (pseudocomponents) which together represent the assay as a whole.

Characterization from bulk properties

The method itself requires a minimum of information to generate the pseudocomponents, though the more data the user provides, the better will be the results. It is recommended that the user provides the specific gravity of the C7+ fraction at least. Viscosity data is also very important.

Characterization from distillation curves

This tool gets data from an ASTM or TBP distillation curve to generate pseudocomponents. It is also possible to include viscosity, molecular weight and specific gravity curves to enhance the characterization.

After the pseudocomponents are created, a material stream with a defined composition is also created, which represents the characterized petroleum fraction.

The pseudocomponents are available for use only in the simulation in which they were generated, even if there is more than one opened simulation in DWSIM. Nevertheless, the user can export these components to a file and import them into another simulation.