Simulation of Catalytic Processes

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 6 lb/h, where heat is removed at the rate of 5.8 10 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 l iquid stream leaving the separator.

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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 i n every simulation. All















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.















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.































11. Close the property view. Attach the Reaction Set to the Fluid Package.















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 composition is identical identical to that that of the reactor contents. Given 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















Example 3

Consider again the water-gas shift reaction. Complete the following flowsheet constituted by a mixer and an equilibrium reactor:













































Example 4

Consider the reaction of aniline and hydrogen to form cyclohexylamine (CHA):

C 6 H 7 N + 3 H 2 ↔ C 6 H 13 N The assumed reactor conditions are 121 ºC y 44.82 bar. The volume occupied by the 3 3 liquid phase is 13.6 m (the reactor volume is 27.2 m ). The reaction kinetics are picked such that the aniline conversion is close to 70%. The activation energy is 11111



























































Example 5















Is the reaction exothermic or endothermic?





































































Simulation of Catalytic Processes

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