Propylene Hydroformylation

Transcription

1 Propylene Hydroformylation Objective: Building Fedbatch reactors with Pressure Control In this example, we simulate a multiphase batch reactor for Propylene Hydroformylation and also explain the features in REX for reactor pressure control. You may download the rex files to view the model. Fictitious values of reaction parameters and Henry phase distribution constants are used here. Features Illustrated Defining Compound Fragments Mixed mass and molar units in rate expression Virtual Experiments - Batch Reactor simulation with known kinetic parameters Calculation of Conversion and Selectivity auxiliary variables Options for Pressure specification in Fed-batch reactor Reaction Model Hydroformylation involves the addition of a formyl group and hydrogen atom to a C-C double bond, typically accomplished by reaction between an olefin and carbon monoxide-hydrogen mixture (synthesis gas). Only the terminal double bonds are active in hydroformylation. The reaction generates a linear aldehyde and its branched isomer as shown below: A large quantity of oxo-aldehydes obtained industrially via hydroformylation are converted to higher alcohols, used mainly for plasticizers. Over 75% of industrial units for oxo-aldehydes produce butyraldehydes using propylene as feedstock. The following hydroformylation and hydrogenation reactions are considered: R-nBA: Propylene + CO + H 2 n-butyraldehyde R-iBA: Propylene + CO + H 2 i-butyraldehyde R-Propane: Propylene + H 2 Propane The reactions are typically catalyzed by a rhodium-based complex that is soluble in a liquid phase. Rate equations provided for this system generally use mixed units - they use mass of catalyst in the rate expression while using moles for all other species. In this situation, there are two ways to model

2 the system, as described in this example. Here we will consider the first option, so the catalyst will be modeled as a compound. By selecting Molarity for Concentrations in the Units Configuration node, the rate will depend on the molar concentrations in the reacting phase. REX will show units of moles for catalyst, although the data will be entered in mass units. We assume that all reactions have same order as the molecularity plus the dependency on catalyst mass concentration. We set Rate Basis as Volume in the Chemistry Units Configuration node, so the rate will specified per unit volume (gmol/lit-h). For example, rate of n-butyraldehyde (nba) formation is expressed as: where the [ ] enclosure for compounds correspond to molarities for all species except catalyst, for which we use mass per unit volume. The Mass Balance for n-butyraldehyde is represented by: By definition, V Liq [Catalyst] is the total catalyst mass in the reactor W Cat. Then we can convert from Volume-based reaction rate to Catalyst Mass: We will work in REX using the volume based formula with direct inclusion of [Catalyst] concentration. Setting up the REX Project You may follow this example by importing the Propylene-Hydroformylation-1.rex file. As mentioned in the previous section, we select Molarity as the units for Concentrations in the Units Configuration node. For Rate Basis, Volume is used:

3 In the Chemistry Compounds node we have entered the reactants and products, plus the liquid solvent and the catalyst that will be considered as a compound. (Note that the catalyst is marked as Fragment. Fragment species are not considered when calculating physical properties, such as the densities, volumes or mole fractions for VLE calculations in a multiphase system. Typically, we use the fragment designation to monitor functional groups. Since catalyst quantity is quite small, we can exclude the catalyst from the VLE calculations by the fragment specification. In addition, because catalyst is in mass units, we avoid any distortion to the mole fraction calculations that are used in the VLE calculations.) The reactions are entered in Chemistry Reactions node. Only forward directions are included in the Kinetics node, and Mass Action kinetics is used. In the Kinetics Parameters node, we enter the values for pre-exponentials, activation energy and orders. All reactants have order same as their stoichiometric coefficients, and additionally have order one with respect to the catalyst as shown below:

4 Then, we go to the Estimation node. Since we are only doing reactor simulation, the kinetics are fixed and there is no need to open the bounds for parameters in the Estimation Parameters node. In the Reactor node, Multiphase Batch reactor is defined: Volume and Temperature are set as Interpolated from data. Pressure is defined as Free, so it is calculated in accordance with the changes in the reactor composition. The reaction phase is selected to be the liquid. In the Reactor Phase Distribution node, we set up the distribution of the compounds. Henry constants are entered for the compounds that are present in both liquid and gas phases:

5 Fedbatch flows and Outflows are also enabled in the Reactor node. In the Reactor Flows node, we define the inflows and outflows in the Flows tab: By selecting Reaction Mixture as Flow Contents for the gas phase Outlet flow, we specify the compound concentration of the outflow to be the same as the reactor gas phase concentration. The composition of the Inlet stream is defined in the Composition tab, by entering the mole fractions:

6 In the Experiments node, we enter the virtual experiments as sets that we will simulate. For simplicity, all sets have initial reactor molar composition same as inlet stream: C3/CO/H 2 = 2/1/1. In the Yield Calculations node, we define some auxiliary variables as described below. Conversion for reactants Propylene, CO and H 2 and Yield of n-butyraldehyde with respect to Propylene are defined in the Yield Calculations node:

7 As we are only running simulations, there is no need to select any compound for reconciliation in the Weights node. In the Run Solution Options node, we choose Kinetics Parameters = Simulate Only; thus kinetics parameters will not be calculated but kept fixed to values loaded in Parameters node. Next, we compare different options for pressure modelling. Pressure Free First, we simulate the batch reactor considering Pressure=Free, as in the Propylene- Hydroformylation-1.rex file. In the Experiments node, we have Set1 included; details can be seen in Experiments Measurements Set1 node. The initial charge of Propylene, CO and H 2 is specified so that for the batch Volume of 100 lt of gas, 100 lt of liquid and Temperature=100 C, the initial pressure is 8 atm. For the catalyst, we enter the mass in gr (even though the header indicates moles) As for the values of Pressure entered in the set, they are not enforced because Pressure is specified as free in the reactor node: actual pressure in the reactor will be calculated and will not necessarily be the same as the values entered in the sets. Finally, for Set1 we are considering an Inlet flow of 50 gmol/h during the whole batch operation, and zero flow rate of the Outlet. After running the simulation, we inspect the profiles along reaction time in the Model Data Comparison node. First, we see that Pressure increases with time:

8 Pressure increases due to the continuous addition of moles through the Inlet stream, even though the reactions decrease the amount of moles. We can see the profile of several compounds for the same set by executing the Single Set action by right clicking on the Model Data Comparison node, or by pressing the icon in the toolbar. The profiles for the reactants inside the reactor is shown below. CO and H 2 are almost exactly superposed as their initial charge and inlet flows are the same, while their consumption only differs due to the propane formation reaction which is slow: The product profiles in the reactor are as follows:

9 Now we simulate Set2 and Set3 also, by adding them in the Experiments node and then running the model again. These sets have almost the same specification as Set1, differing only in the Outlet stream that is higher: 25 lt/h and 50 lt/h respectively: The following chart shows the pressure profiles, as outflow increases, reactor pressure drops: As Pressure in Set3 is the lowest, partial pressure also decreases for all compounds thus making the reactions slower. Therefore, n-butyraldehyde yield decreases as outflow increases:

10 In the following sections, we illustrate features for pressure control. Pressure Controlled Simulation Now, we will implement pressure control for the propylene hydroformylation reactor. You may import the Propylene-Hydroformylation-2.rex file to follow the implementation below. Pressure control can be enabled for any batch reactor with a gas phase and at least one inflow or outflow. The model will try to keep actual pressure as close as possible to the pressure setpoint by adjusting a chosen inlet or outlet stream. First, we enable the option for Controlled pressure in Reactor node: Next, we assign the flow which will be manipulated to control pressure in the Specification tab of the Reactor Flows node. In this example, we choose the Outlet stream as Float for Pressure Control to do that: By selecting this option, the values entered for the flow in Measurements Set are not used anymore during the simulation. The flow will be calculated to match the pressure values (which are now

11 pressure setpoints) entered in Measurements Set. Proper bounds must be defined for the flow in the Run Solution Options Bounds node. We start by constraining the outflow stream to be between 0 and 75 lt/h: In order to control Pressure, we must select it as a reconciled measurement in Weights node and define weights for it. Higher weights will reduce the deviation between the actual reactor pressure and the setpoint. In this example, we autogenerate hybrid weights:

12 The flow selected as Float for pressure control will be calculated along time in order to minimize the pressure deviation from setpoint. The Pressure setpoint values are interpolated from the pressure values entered in Experiments Sets node, which for Set1 indicate a constant setpoint of 8 atm. After running the project, we inspect the resulting Pressure and Outflow profiles: We see that the pressure profile does not follow the setpoint well. The flow profile chart on the right gives us a clue to the reason. The outlet flow is at the upper bound and the only way to reduce the pressure would be to increase the upper bound. (A look at the Results Marginal Values node shows that this is the only constraint that has a marginal value, indicating that relieving this bound could improve the pressure control) We now increase the upper bound for the outflow to 150 lt/h, as it is done in Set2. After including that set and running the model, we have the following profiles: We now have almost perfect pressure control after increasing the upper bound for the Outlet flow.

13 We can also simulate a varying pressure setpoint with reaction time. For example, we can evaluate an experiment where the pressure is to increase from 8 atm to 10 atm in the first 2 hours followed by a constant profile. To do this, extra data points must be added for the set as displayed below for Set3: Resulting values for Pressure and the outlet flow are shown below: One issue that occurs in these systems is that at time=0, the pressure setpoint may be different from the pressure calculated by the equation of state based on the initial conditions in the reactor. When controlling pressure for these cases, in general it take some time to achieve the desired pressure. For example, consider Set4 that is exactly same as Set2 except that pressure setpoint is 15 atm:

14 Pressure needs to increase at the beginning of the operation, thus the Outlet flow is zero in that region. Pressure builds up with the inlet flow until it reaches the setpoint, following which the outlet flow increases to keep the pressure from going above the setpoint. Pressure Interpolated from Data In the section above, we learned how to control the reactor pressure. REX also allows you precisely fix the pressure values through the option Interpolated from Data. For that option, the mathematical model forces the pressure to be exactly same as setpoint. However, using this option carries a risk. If you enter a tight flow bound (like the 75 lit/h outflow bound shown earlier) and still enforce the pressure, the solver will declare the model infeasible and fail. In general, this approach to controlling the pressure is not recommended. When the Interpolated from Data option is chosen, the setpoint for pressure will be the linearly interpolated pressure values entered in the Measurements Set node. You can import Propylene-Hydroformylation-3.rex file, that has the same sets loaded as in the controlled pressure example. The only change is that pressure is selected as Interpolated from Data. You can try running one set at a time. Only Set2 and Set3 will have a feasible solution as their conditions allow for exactly matching the pressure to the setpoint. Set1 and Set4 do not give a feasible solution, as the pressure can not be forced to be at the setpoint. In general, setting Pressure as Interpolated is not recommended for the reasons described above. Another reason is that the initial charge must exactly correspond to the pressure setpoint at the start of the batch: small errors due to rounding values loaded into the Measurements Sets could cause infeasibilities. Setting pressure to Controlled is a better option for reliability.

15 Further Studies We have shown the different options for modeling pressure control in the batch reactor by simulating several experiments in Estimation mode. The same idea applies when estimating parameters as well as in Optimization mode. You may switch to Optimization mode and follow these guidelines to run optimization cases. The pressure control ideas shown here are not only for multiphase reactions, but also apply to single gas phase batch reactor.