Simulation of catalytic processes J.L. Valverde, C. Saez. University of Castilla-La Mancha. Spain. E-mail: [email protected]1. INTRODUCTION Computing packages that model the process units of chemical plants are introduced and utilized to model highly integrated flowsheets commonly designed to achieve more profitable operations. These packages are referred to as process simulators, most of which are used to simulate potential processes in the steady state- that is, to determine the unknown temperatures, pressures, and flow rates at steady state. Process simulators are also used by the design teams to calculate heat duties, power requirements, phase and chemical equilibria, and the performance of multistaged towers and reactors, among other calculations. Process flowsheets are the language of chemical processes. Analysis, or simulation, is the tool chemical engineers use to interpret process flowsheets to locate malfunctions, and to predict the performance of processes. The heart of analysis is the mathematical model, a collection of equations that relate the process variables, such as stream temperatures, pressure, flow rate, and composition, to surface area, valve settings and so on. HYSYS is an interactive, object-oriented program, which differs from many of the alternative commercial simulators (ASPEN PLUS, PRO/II and CHEMCAD) in two main respects. First, it has the facility for interactively interpreting commands, as they are entered one at a time, whereas most of the other flowsheet simulators require that a Run button be pressed after new entries are completed. Second, although HYSYS, like many other simulators, uses subroutines or procedures to model the process units, it has the unique feature that information propagates in both forward and reverse directions. This bidirectionality often makes iterative calculations unnecessary. These two features make the program fast responding and relatively easy to use. 2. BASICS OF THE STEADY STATE SIMULATION USING HYSYS Example 1 We are going to consider a simple example as the separation of ammonia and water. In this example, a mixture of ammonia and water in the vapour phase, saturated at 250 psia an containing 80 wt% ammonia, is passed through a condenser at a flow rate of 10000 lb/h, where heat is removed at the rate of 5.8 10 6 Btu/h. Its effluent is expanded to a pressure of 100 psia and fed into a flash vessel, as shown in Figure 1. Neglecting the heat loss from the equipment to the surrounding and the pressure drop in the condenser, it is desired to determine the composition of the liquid stream leaving the separator.
Transcript
Simulation of catalytic processes
J.L. Valverde, C. Saez. University of Castilla-La Mancha. Spain. E-mail: [email protected]
1. INTRODUCTION Computing packages that model the process units of chemical plants are introduced and utilized to model highly integrated flowsheets commonly designed to achieve more profitable operations. These packages are referred to as process simulators, most of which are used to simulate potential processes in the steady state- that is, to determine the unknown temperatures, pressures, and flow rates at steady state. Process simulators are also used by the design teams to calculate heat duties, power requirements, phase and chemical equilibria, and the performance of multistaged towers and reactors, among other calculations. Process flowsheets are the language of chemical processes. Analysis, or simulation, is the tool chemical engineers use to interpret process flowsheets to locate malfunctions, and to predict the performance of processes. The heart of analysis is the mathematical model, a collection of equations that relate the process variables, such as stream temperatures, pressure, flow rate, and composition, to surface area, valve settings and so on. HYSYS is an interactive, object-oriented program, which differs from many of the alternative commercial simulators (ASPEN PLUS, PRO/II and CHEMCAD) in two main respects. First, it has the facility for interactively interpreting commands, as they are entered one at a time, whereas most of the other flowsheet simulators require that a Run button be pressed after new entries are completed. Second, although HYSYS, like many other simulators, uses subroutines or procedures to model the process units, it has the unique feature that information propagates in both forward and reverse directions. This bidirectionality often makes iterative calculations unnecessary. These two features make the program fast responding and relatively easy to use. 2. BASICS OF THE STEADY STATE SIMULATION USING HYSYS Example 1 We are going to consider a simple example as the separation of ammonia and water. In this example, a mixture of ammonia and water in the vapour phase, saturated at 250 psia an containing 80 wt% ammonia, is passed through a condenser at a flow rate of 10000 lb/h, where heat is removed at the rate of 5.8 106 Btu/h. Its effluent is expanded to a pressure of 100 psia and fed into a flash vessel, as shown in Figure 1. Neglecting the heat loss from the equipment to the surrounding and the pressure drop in the condenser, it is desired to determine the composition of the liquid stream leaving the separator.
Figure 1. Process simulated by using HYSYS. Next, it will be shown in an abridged way the procedure followed to simulate this process: Selection of components and properties package (Soave-Redlich-Kwong)
The selection of the Fluid package is a critical point to be considered. It can be used Equations of State (EOS) or Models based on activity coefficients for representing liquid behaviour (combined with equation of state for representing gas/vapour behaviour). Whenever the liquid phase presents a behaviour ideal or slightly non-ideal, EOS could be a good alternative. Among others, HYSYS allows to use the following EOS models:
• Peng Robinson: ideal (or close to ideal) L-V equilibria (especially for hydrocarbons).
• SRK: ideal (or close to ideal) L-V equilibria • Sour Peng Robinson and Sour SRK: ideal (or close to ideal) L-V equilibria
including polar species. Ions in solution should not be considered. If liquid phase presents a non-ideal behaviour, other models should be considered. UNIQUAC is valid for practically all the systems. NRTL is recommended for L-L equilibria. Ideal behaviour of the gas phase should be considered whenever the pressure of the system is lower than 5 atm.
Entering specifications for solution of the material and energy balances
Viewing results
3. REACTIONS IN HYSYS In HYSYS, a default reaction set, the Global Rxn Set, is present in every simulation. All compatible reactions that are added to the case are automatically included in this set. A reaction can be attached to a different set, but it also remains in the Global Rxn Set unless you remove it. To create a reaction, click the Add Rxn Button from the reaction Manager. The following table describes the five types of Reactions that can be modelled in HYSYS:
Reaction type Requirements Conversion Requires the stoichiometry of all the reactions and the conversion
of a base component in the reaction. Equilibrium Requires the stoichiometry of all the reactions. The term Ln (K)
may be calculated using one of several different methods, as explained later. The reaction order for each component is determined from the stoichiometric coefficients.
Heterogeneous catalytic
Requires the kinetics terms of the kinetic reaction as well as the activation energy, frequency factor, and component exponent terms of the adsorption kinetics.
Kinetic Requires the stoichiometry of all the reactions, as well as the activation energy and frequency factor in the Arrhenius equation for forward and reverse (optional) reactions. The forward and reverse orders of reaction for each component can be specified.
Simple Rate Requires the stoichiometry of all the reactions, as well as the activation energy and frequency factor in the Arrhenius equation for the forward reaction. The equilibrium expression constants are required for the reverse reaction.
The following procedure will demonstrate the minimum steps required for:
- The addition of components to the Reaction Manager. - The creation of a Reaction. - The addition of the reaction to a reaction set. - The attachment of the reaction set to a fluid package.
Example 2 The water-gas shift reaction will be considered. For this example, it is assumed that a New Case is created:
1. Within the component list, the following set of components are selected: CO, H2O, CO2 and H2.
2. Within the fluid package, the Peng Robinson property package is selected.
3. Go the Reactions tab of the Simulation Basis Manager. The selected components are present in the Rxn Component Group.
4. To install a reaction, click the Add Rxn button.
5. From the Reactions property view, highlight the Equilibrium reaction type and click the Add Reaction button. The Equilibrium Reaction property view appears.
6. On the Stoichiometry tab, select the first row of the Component column in the Stoichiometry Info Table.
7. Select CO from the drop-down list. The Mole Weight column automatically provides the molar weight of CO.
8. In the Stoich Coeff field, enter –1 (i.e., 1 mol of CO is consumed). 9. Now define the rest of the Stoichiometry tab as shown in the figure below
and click the balance button.
10. Go to the Basis tab and set Vapour Phase as Phase.
11. Close the property view. Attach the Reaction Set to the Fluid Package.
This example will be used later. PROBLEM Students have to enter a Conversion Reaction taken as a reference the water-gas shift reaction and define a degree of CO conversion equal to 40% . 4. REACTORS IN HYSYS Six types of reactors are considered in HYSYS:
The last four reactors are referred to as General Reactors. With the exception of the PFR, all the reactor operations share the same basic property view.
PFR Reactor The PFR generally consists of a bank of cylindrical pipes or tubes. The flow field is modelled as plug flow, implying that the stream is radially isotropic (without mass or energy gradients). This also implies that axial mixing is negligible. CSTR Reactor The CSTR is a vessel in which kinetics, heterogeneous catalytic, and simple rate reactions can be performed. The conversion in the reactor depends on the rate expression of the reactions associated with the reaction type. The inlet stream is assumed to be perfectly mixed with the material already in the reactor, so that the outlet stream composition is identical to that of the reactor contents. Given the reactor volume, a consistent rate expression for each reaction and the reaction stoichiometry, the CSTR computes the conversion of each component entering the reactor. Gibbs Reactor The Gibbs reactor calculates the existing compositions such that the phase and chemical equilibria of the outlet streams are attained. However, the Gibbs reactor does not need to make use of a specified reaction stoichiometry to compute the outlet stream composition. The condition that the Gibbs free energy of the reacting system is at a minimum at equilibrium is used to calculate the product mixture composition. As with the Equilibrium reactor, neither pure components nor the reaction mixture are assumed to behave ideally. Equilibrium reactor The Equilibrium reactor is a vessel which models equilibrium reactions. The outlet streams of the reactor are in a state of chemical and physical equilibrium. The reaction set which is attached to the Equilibrium reactor can contain an unlimited number of equilibrium reactions, which are simultaneously or sequentially solved. Neither the components nor the mixing process need to be ideal, since HYSYS can compute the chemical activity of each component in the mixture based on mixture and pure component fugacities. Conversion reactor The conversion reactor is a vessel in which conversion reactions are performed. You can only attach reaction sets that contain conversion reactions. Each reaction in the set proceeds until the specified conversion is attained or until a limiting reactant is depleted. Yield Shift Reactor The Yield Shift reactor unit operation supports efficient modelling of reactors by using data tables to perform shift calculations. The operation can be used for complex reactors where no model is available, or where models that are too computationally expensive.
Example 3 Consider again the water-gas shift reaction. Complete the following flowsheet constituted by a mixer and an equilibrium reactor:
Is the reaction exothermic or endothermic? Evaluate the CO conversion and the heat flow in the reactor at the temperatures and water/CO molar ratios listed in the next table:
Example 4 Consider the reaction of aniline and hydrogen to form cyclohexylamine (CHA):
NHCHNHC 136276 3 ↔+ The assumed reactor conditions are 121 ºC y 44.82 bar. The volume occupied by the liquid phase is 13.6 m3 (the reactor volume is 27.2 m3). The reaction kinetics are picked such that the aniline conversion is close to 70%. The activation energy is 11111 cal/mol). The reaction rate is first order in the molar concentration (kmol/m3) of each of the reactants:
273 11111exp100.1)./( HAnCC
RTxmhkmolr ⎥⎦
⎤⎢⎣⎡−=
The pure reactants are fed into the reactor in two fresh feed streams. The physical property package used is UNIQUAC-Virial. Complete the following flowsheet.
Is the reaction exothermic or endothermic? Evaluate the reactor volume that would lead to an aniline conversion higher than 80%. Do the same by considering the reactor temperature and a reactor volume of 27.2 m3 (volume occupied by the liquid: 13.6 m3). What is the heat duty in both cases?
Example 5 The numerical case considered is the chlorination of propylene. There are two parallel gas phase reactions. The first forms allyl chloride and HCl:
HClClCHCHCHClHC +−=↔+ 22263 The second forms 1,2 dichloro propane:
32263 CHCHClClCHClHC −−↔+
The reaction takes place in a pipe, which is 2 inches in diameter and 22 feet long. A porous solid catalyst is placed inside the tube. The catalyst properties are as follows:
Reactions have a first-order dependence on the partial pressures of the reactants. The reaction rates are given in lb-mol/h.ft3, pressure in atm, activation energy in cal/mol and temperature in R:
[ ]( )[ ]( )232
235
1
)/(3811exp7.11)/(15111exp1006.2
ClC
ClC
PPRTrPPRTr
−=−×=
The reactions must be specified as heterogeneous catalyst when the choice of the reaction type is selected. The physical property package used is PRSV. Complete the following flowsheet.
Is the reaction exothermic or endothermic? What is the propylene conversion with this configuration? What is the temperature of stream out? Evaluate the propylene conversion if the reactor length is twice and the operation is kept adiabatic. What is the heat flow entering/coming from the reactor with this configuration? Evaluate the propylene conversion if the temperature of stream out is 200 ºC and the reactor is 22 feet long? What is the heat flow entering/coming from the reactor?
Literature used - Process Design Principles. Synthesis, analysis and evaluation. W.D. Seider, J.D.
Seader, D.R. Lewin, John Wiley & Sons, New York (1999). - Plantwide Dynamic Simulators in Chemical Processing and Control, W.L. Luyben,
Marcel Dekker, Inc., New York (2002). - ASPEN HYSYS 2006.5. Documentation. ASPEN technology, Inc. Burlington (2007).
