Reactor

Reactor Type / Phase Behavior

In this grid you may select the reactor type and the phases. The default option is Single Liquid Batch reactor
After selecting the option, you must press the Apply button to proceed with the changes.

The reactor types are:

Batch			n-CSTR				PFR

Note: If Batch reactor type is selected, the Flows Node appears as a child node of the reactor node. However, the grids of the Flows node will be locked unless you select the option Yes in Fedbatch Flows and/or Outflows. REX provides not only the standard batch reactor, but also several forms of the semibatch reactors.

According to the phases selected in this grid, we have two main reactor categories:

• Single Liquid or Gas Reactor, where all the compounds are considered to be in the same liquid or gas phase.


• MultiPhase: When two or more phases are selected, compounds can be present in more than one phase. Here, it is necessary to add additional information in the Reactor → Phase Distribution node. Gas, liquid and solid species are allowed for Batch and CSTR reactors. For PFR reactors, multiphase operation is not supported, but a membrane reactor can be modeled by activating the special Gas-Gas option. The Gas-Gas membrane feature is not supported for batch reactors.
  In general multiphase reactors, all initial moles entered are distributed between the phases in accordance with the phase distribution equations. Phase equilibrium is applied at all times, so diffusion processes are not considered. The only exception is in gas-gas membrane diffusion reactors where initial moles entered for a compound which is present in both gas phases are allocated completely to the reaction phase. As the flow moves through the reactor, membrane diffusion will distribute the compounds. Therefore, you must have a compound allocated exclusively to the non-reaction phase (in Reactor → Phase Distribution) so that initial compositions can be calculated before diffusion starts to the non-reaction phase. Usually, this compound flow (also called the sweep gas) helps to reduce the partial pressure of the compound diffusing from the reaction phase. If there is no sweep gas, you must create an artificial compound with a very small molar flow so that initial compositions in the non-reaction phase can be calculated.
  
  For gas-gas membrane reactors, you must also provide the Specific Membrane Area. For solid catalyst, this is typically in m2/gram of catalyst and can change along the length of the reactor. It should be entered in Experiments → Measurements → Sets for each CSTR reactor or is interpolated from provided data for a PFR reactor.

Since this node defines important criteria for the information structure in the sub-nodes, when a property value is applied, the tree is refreshed automatically.


Phases

In this grid, the phases of the system are listed, according to the selection in the Reactor Type / Phase Behavior grid.
The reaction phase must be specified, which can be any of the phases except a solid phase.
You may rename the phases by editing the Phases Column. The phase names must follow a Special Syntax.

For a CSTR reactor with Intermediate Feed and the Use Energy Balance option enabled for Temperature in the Reactor Properties grid, you must also provide here the phase of the intermediate feed.


The first time you define a multiphase reactor, REX initializes all the compounds to be present in the reaction phase, as you will see in the Reactor → Phase Distribution node. Following this, if you change the reaction phase, REX does not make any changes. However, you should verify that the following phase-sensitive nodes have the correct information:

• Reactor → Phase Distribution
  
• Experiments → Measurements
  
• Measurements → SetName Total Moles → Phase: PhaseName
  
• Weights
  
• Weights → Total Moles → Phase: PhaseName
  
• Initialization Values → SetName Total Moles → Phase: PhaseName

Note: You may run Checkmodel to verify that the information about the phases have been entered correctly


Reactor Properties

This grid displays the reactor properties, which vary depending on the reactor type, namely Batch, CSTR or PFR and also on the number and type of the phases. The contents of the grid for each reactor are explained below.

Batch

If the reactor type selected in the Reactor grid is Batch, the following properties are listed:

• Volume: Interpolate from Data, Use Density.
  If it is Interpolate from Data, the reactor volume at any time-point is set by a linear interpolation of the specified volumes between two consecutive data points containing the time-point.
  If Use Density is selected, volume is calculated based on the Molecular Weights and Density Values loaded in Chemistry → Compounds → Properties node. When Density calculations are enabled for a multiphase batch reactor that has a Gas Phase, the Total Volume experimental data loaded in Measurements → Sets → Total Moles is interpolated along the operation time, the Liquid/Solid phase volume evolution with time is calculated using density parameters, while Gas Phase volume is obtained by:
  
  Volume(Gas) = Volume(Total) - [Volume(Liquid) + Volume(Solid) ]
  
  For a batch system without a Gas phase, the experimental values for Total Volume are not considered in the model, while its calculated value is just the addition of all phase volumes.
You may explore this example to see how to model a reactor with the density option enabled.


• Temperature: Interpolate from Data or Use Energy Balance. The Interpolate from Data option here works exactly the same way as the volume.
When Use Energy Balance is selected, the Temperature profile is calculated by energy balance, assuming adiabatic conditions. This example illustrates the use of this feature.

• Pressure: Controlled, Free or Interpolated from Data. The default option is Free, which is the only option available for single liquid phase. The other options try to adjust an inflow/outflow flowrate in order that the calculated pressure be close or equal to a experimental pressure. A detailed explanation is given below.
• Fedbatch Flows: No or Yes.
• Outflows: No or Yes.
• Volume from Flow Effects Only: No, Yes.
This row only becomes available for a multiphase batch reactor with density calculations enabled in the Volume row and with at least one Fedbatch or Outflow.
This special option is only intended to be used for a liquid-gas batch reactor. It allows you to calculate liquid volume using a relaxed density model: the initial liquid volume is set to its experimental value, while its variation along time is obtained by adding/subtracting the equivalent liquid volume of compounds entering/leaving the system through Fedbatch/Outflows. Another simplification is that compounds whose density parameters are not loaded are ignored in the volume calculation: this is useful in that you need not provide density parameters for all compounds.
Volume of gas phase is calculated by difference between total volume provided in experimental data and the calculated liquid volume.

Explanation of the Pressure options - Batch reactor


Free

The gas phase pressure is calculated (based on ideal gas law) and is not restricted between any bounds.
When selecting the pressure as Free, you may reconcile it if you choose. To reconcile presure, you must follow these steps:
1. Select pressure in Measurements node.
   
2. Enter the experimental pressure values in Experiments → Measurements → Total Moles.
   
3. Check the pressure checkbox in Weights node.
   
4. Fill the pressure weights, either manually or by autogenerating them.

Controlled

The pressure will be controlled by adjusting a given inflow/outflow. This flow will be adjusted to minimize the difference between the experimental and the calculated pressure profile, by using the least square approach.
In order to apply this option:
1. One flow must be defined as Float for Pressure Control in the Flows Node.
   
2. Pressure must be selected in Measurements node and the pressure data must be loaded in Experiments → Measurements → Total Moles.
   
3. The check box for pressure Weights node must be selected, and the pressure weights must have nonzero values in order to control the pressure. These weights are similar to a controller tuning factor.
4. The lower and upper bounds for the flow must be entered in the Solution Options → Bounds node. By default, the lower and upper bounds are equal to zero, thus meaning that no pressure control will be performed unless you change these bounds to reasonable values.
The deviation between the calculated and experimental pressure decrease as the weights are increased, provided that the bounds for the floating inflow/outflow are widen enough to do a good pressure control.

Interpolated from Data

When choosing this option, the reactor pressure at any time-point is set (fixed) by a linear interpolation of the specified pressure between two consecutive data points containing the time-point.
In order to apply this option, execute the steps 1, 2 and 4 that were described previously for the Controlled pressure option.
Note: As REX fixes the calculated and experimental pressure to have the same values along the batch time, convergence problems may appear. REX may be unable to achieve the specified pressure due to constraints such as tight bounds on the flows, improper specifications of compounds moles, and so on. For these cases, it would be better to try the Controlled option for pressure that results in more degrees of freedom, and therefore, a more relaxed mathematical problem.

CSTR

If the reactor type selected in the Reactor grid is CSTR, the following properties are listed:

• Flow: Fixed to Data, Float for Pressure Control (only for Single Gas or Multiphase with a Gas Phase defined), plus Use Density for Single Liquid or Multiphase system.
  If Fixed to Data is selected, the flow values are set equal to the value at each datapoint in the experimental data; in case of selecting Float for Pressure Control, the Gas Flow is calculated in order to match the Pressure to the experimental values.
When Use Density is selected, liquid/solid flows are calculated based on the compounds Molecular Weights and Density Values in the Chemistry → Compounds → Properties node.
• Pressure: Free, Fixed to Data (Last option only available for Single Gas or Multiphase system with a Gas Phase defined).
  When Free is selected, pressure will be calculated using ideal gas law; this option also allows to reconcile pressure by selecting it in Weights node. When Fixed to Data is chosen, pressure will be kept fixed to the experimental values, while Flow variable will be calculated.
  
• Intermediate Feed: No(default) or Yes. By enabling this option, you can define intermediate feed between CSTR units.
  Please note that for the Gas - Gas Membrane reactor for which there is no phase equilibrium but diffusion of compounds through the membrane, some special considerations apply:
  • For Compounds that are present in the reaction phase: All moles of intermediate feed for such compounds are added to the reaction phase. That is done regardless of the compound being also present in the permeate (non-reaction) phase.
  • Compounds only present in the permeate (non-reaction) phase: All moles of intermediate feed for such compounds are added to the Permeate phase.
• Intermediate Separation: No, Overall Based and Phase Based.
  When Intermediate Separation is enabled, you can perform separations using a component splitter, between consecutive CSTR reactors in the CSTR series. By selecting Overall Based, you may specify the percentage of each compound removed at the end of the CSTR unit; if Phase Based is chosen, a fraction of one or more phases leaving a reactor can be removed. Graphical help for entering such data is provided by clicking on the Description tab in data entry sheets for the Intermediate Separation.
  Please note that when Overall Based is chosen for the Gas - Gas Membrane reactor, some special considerations apply:
  • For Compounds that are present in the reaction phase: The removal percentage refers only to the moles in the reaction phase, from where the compounds are removed. Moles present in the permeate phase are untouched and ignored for separation calculations.
  • Compounds only present in the permeate (non-reaction) phase cannot be removed.
• Temperature: Fixed to Data and Use Energy Balance.
  If Fixed to Data is selected, temperature values are fixed to the values at each CSTR datapoint in the experimental data. When Use Energy Balance is selected, the CSTR outlet temperature for each reactor is calculated by energy balance assuming adiabatic conditions. This example illustrates the use of this feature.
  

PFR

If the reactor type selected in the Reactor Type / Phase Behavior grid is PFR, the following properties are listed:

• Temperature: Interpolate from Data(Default), or Use Energy Balance.
  When Use Energy Balance is selected, the Temperature profile is calculated by energy balance, assuming adiabatic conditions. This example illustrates the use of this feature.
• Pressure: Free is the Default option, and the only available option for Single Liquid Phase. For Single Gas Phase or Gas - Gas membrane reactor, the option Interpolate from Data is also available. These options allow you to calculate the Flow along the PFR reactor so that the specified pressure values are exactly matched. More explanation is given below.
• Flow: The general option here applicable for all reactors is Interpolate from Data. For Single Gas or Gas - Gas membrane reactor, the Float for Pressure Control option is also available. For Single Liquid, the Use Density option becomes available, thus allowing to calculate the volumetric Flow based on density values of all compounds.Constant option is available for gas PFR reactor where recycle is enabled, so as to enforce same flow value inside the reactor.
• Recycle: This enables the recycle of selected species from the reactor output to be reprocessed. More details can be found in this Example.

Explanation of the Pressure options - PFR reactor

Free

The gas phase pressure is calculated based on ideal gas law and is not restricted between any bounds.
When pressure is selected as Free, you may reconcile it. To do so, you must follow these steps:
1. Select pressure in Measurements node.
   
2. Enter the experimental pressure values in Experiments → Measurements → Total Moles.
   
3. Include the pressure as reconciled in the Weights node.
   
4. Fill the pressure weights, either manually or by autogenerating them.
   

Interpolated from Data

When choosing this option, the reactor pressure at any point is set (fixed) by a linear interpolation of the specified pressure between two consecutive data points.
In order to apply this option:


1. Flow must be defined as Float for Pressure Control in the Reactor node.
   
2. Pressure must be selected in Measurements node and the pressure data must be entered in Experiments → Measurements → Total Moles.
   

Project Explorer

Compounds → Properties

Estimation

Phase Distribution

Flows

Recycle

Example Recycle

Example Using Energy Balance