Reactor

Authors: Vincent Kenny ^[2015] and Stephen Lenzini ^[2015]

Steward: Jian Gong and Fengqi You

Introduction

Process simulation is extremely beneficial to engineers, allowing them to further understand processes, identify process advantages and limitations, and provide quantitative process outputs and properties. Modeling reactors and their corresponding reactions is difficult by nature but can be rewarding if done correctly. This page provides essential information on the topic of reactor simulation using the computer program Aspen HYSYS.

Aspen HYSYS Reactor Simulation Basics

The HYSYS program allows the user to define reactions primarily based on desired model outputs and available information. After defining process components, the user can choose a reaction type as listed in the section below. HYSYS includes a number of different reactor models for the various reaction types, desired outputs, and specification limitations as well as standard PFR and CSTR models as described in this section.

Unlike other simulation programs, HYSYS attempts to calculate all variables at all times which can be helpful or frustrating at times. The user can utilize the "hold" function in order to cease this process.

Limitations

Because simulation requires reaction characteristics, parameters, and other information, it is important to conduct background research appropriate to the reaction of interest before beginning the actual simulation. If theoretical or empirical data do not exist for the reaction, it may be difficult or impossible to conduct a computer simulation (see Additional Options). It follows that an increasing number of reaction byproducts requires an increasing amount of reaction data. Users should not be concerned if they cannot quantitatively specify or simulate all reaction by-products or outputs; in the end, a simulation as a process approximation and is inherently unable to model the process completely.

Of course, the phase of the reaction must be known; unfortunately, however, HYSYS does not support solid phase modeling and thus a different approach must be chosen.

HYSYS Reactors

In general, the user will attach a defined reaction set, generated as described above, to each reactor. This allows precise coordination of reactions that may be occurring in different units in the entire process.

Plug Flow Reactor (PFR)

Plug Flow Reactors are tubular reactors that assume perfect radial mixing and approximate zero axial dispersion. A PFR is used primarily for gas phase reactions. HYSYS PFRs can utilize kinetic, equilibrium, and heterogenous catalysis reaction types. The user must define an inlet streams and an outlet stream; like most HYSYS units, multiple streams can enter the same unit. An optional energy stream can be specified to either provide or remove heat; if no stream is specified, HYSYS assumes that the unit operates adiabatically. However, depending on user specifications, HYSYS can determine required energy for the specified operation.

The user can view parameters and variables resulting from the integration of physical specifications and the specified chemical reaction after the operation converges.

Sub-volume Integration

The HYSYS PFR divides the unit into sub-volumes based on total length where calculations occur throughout the PFR. The default number of sub-volumes is 20, but this number can be specified by the user. Of course, a higher number of sub-volumes will generally lead to a more accurate simulation but may be unnecessary given the operation or limitations due to time and computation capacity. Properties of unresolved sub-volumes can be user-specified using the "Minimum Step Fraction" and "Minimum Step Length" inputs.

After convergence, the user can view various graphs of process variable changes throughout each sub-volume.

Unique Parameters

The user must define physical properties of the reactor such as volume, cross sectional area, and length. Additionally, the program provides the option to define a number of tubes included in the unit. The specified length will encompass all tubes.

Pressure drop through the PFR can be defined by the user, approximated as zero, or left for HYSYS to calculate. User-defined pressure drops can be useful in accordance with the process as a whole but a zero approximation can serve for initial simulations if a pressure drop is not know. HYSYS will calculate using the Ergun equation.

A void fraction must be specified with unity referring to zero reactor packing. If a value other than unity is specified, the user must input data for the packing. Note that this data has a purely physical relationship with the system such as mean particle size, solid density, solid heat capacity, etc. and does not relate the the chemical reaction occurring in the unit. If the PFR is using a reaction set with a Heterogeneous Catalytic Reaction, that information will be specified in the reaction itself.

Continuous Stirred Tank Reactor (CSTR)

The Continuous Stirred Tank Reactor assumes perfect mixing throughout the volume of the reactor and the outlet conditions are approximated as equal to conditions inside the reactor. The "General Reactors" (Equilibrium Reactor, Conversion Reactor, Gibbs Reactor) each is a CSTR with a specialization in its corresponding HYSYS Reaction Type. The kinetics, equilibrium, and heterogeneous catalysis reaction types can be used with the generic CSTR.

The CSTR requires at least one input stream and can consider both vapor and liquid phases internally with corresponding liquid and vapor outlets. Primarily, CSTRs are used for liquid reactions, and thus the vapor outlet can be omitted, but the option exists nonetheless. The user can also omit the liquid outlet but this is not recommended and a PFR should be considered instead.

As with the PFR, an optional energy input can be specified or omitted to assume an adiabatic operation. Additionally, HYSYS can calculate the energy input given that the user specifies the appropriate variables.

A pressure drop is unlikely to be substantial with a CSTR but can be entered by the user; the default pressure drop is assumed to be zero, or HYSYS can calculate a pressure drop based on outlet stream information. The volume should be calculated or estimated and user-defined on the unit information page.

Unique to the CSTR, an option exists to model as a flash drum separator if a solution cannot be found. This option may enhance or limit the desired outputs depending on the situation.

Like the PFR, after convergence, the CSTR unit page can show parameter and variable outputs based on integration of physical specifications and the chemical reaction.

Equilibrium Reactor

The Equilibrium Reactor is a CSTR with a specialization in equilibrium reactions and thus can only function with reaction sets including that reaction type. The outlets from this reactor will be in a state of chemical and physical equilibrium according to parameters defined in the reaction set; as a consequence, the operation is extremely sensitive to input parameters and variables. The user might find it easier to use the Gibbs Reactor in situations where sensitivity affects the program's ability to reach convergence.

Conversion Reactor

The Conversion Reactor is a CSTR with a specialization in conversion reactions and thus can only function with reaction sets including that reaction type. Unique to this reactor, conversion X becomes a process variable defined as

$X=(N_{A,in}-N_{A,out})/N_{A,in}$

where A is the base component of the reaction. Like other variables, this can be solved for by HYSYS or specified by the user depending on the degrees of freedom analysis.

Due to the simplicity of this reaction type, this reactor can integrate multiple different reactions in the same unit. The product of one reaction can serve as the reactant of another, and so on. This can be extremely useful for complex reactor designs.

Gibbs Reactor

The Gibbs Reactor is an extremely unique CSTR in that it does not require a reaction set to be attached in order to function. The Gibbs Reactor will simply produce an outlet in which the Gibbs free energy of the mixture is minimized. Of course, a reactor set can be attached to this reactor, but it is important to note that any parameters specified in the set will not be included in the simulation as the minimization of free energy will be the dominant simulation method. An exception to this rule is the stoichiometry of the reaction set. Without a set, stoichiometry will not be considered in the simulation; however, with an attached reaction set with stoichiometric parameters, the simulation will account for them and the outlet conditions can change. However, if a set is attached, only the components specified in the set will reach an equilibrium point; other components will be neglected.

The Gibbs Reactor can be very useful if the user does not possess any data pertinent to the reaction or desires only a simulation of the equilibrium state. At the very least, the Gibbs Reactor can provide simulation estimates as a starting point for a more rigorous simulation through another reactor type.

Simulation

Degrees of Freedom

A degree of freedom analysis will assist in reaching a state of convergence for the reactor and downstream units. For a reactor,

$N_{dof}=N_{unknowns}+N_{reactions}-N_{balances}$

in which the number of degrees of freedom is expressed as the number of unknown variables plus the number of reactions occurring minus the number of material balances able to be performed on the system. In order for the simulation to converge, the user must specify as many variables as existing degrees of freedom. The simulation will then calculate unspecified variables.

Conclusion

Additional Options

In general, the AspenPlus simulation program can handle more rigorous process simulation. Several other types of simulation programs are available as well.

If reactor simulation through a program fails, an engineer can execute manual simulation for a unit by hand or through Microsoft Excel, Matlab, or another computational method. Because of the manual method, the results from this simulation should be more intuitively clear and can be added to the overall process on HYSYS absent an actual reactor and/or reaction.