Aspen Plus Reactor Simulation
Martha Weitz
This section is a tutorial to walk you through Problem 11-3 for the 1st edition of Essentials of Chemical Reaction Engineering. You can download the ASPEN backup file here that completes this problem.
Normal butane, C4H10, is to be isomerized to isobutane in a plug-flow reactor. This elementary reversible reaction is to be carried out adiabatically in the liquid phase under high pressure using a liquid catalyst which gives a specific reaction rate of 31.1 h-1 at 360 K. The feed enters at 330 K.
Three components are considered in the Aspen model: C4H10 (n-butane), IC4H10 (isobutane) and IPENTANE (2-methyl-butane). The liquid catalyst is not included because its flowrate is not known and the specific reaction rate has been given for the reaction condition. These three components are called directly from built-in Aspen pure component databanks.PropertiesDifferent property models can yield different predictions for various thermophysical properties used in mass and energy balance calculations. PENG-ROB, Aspen Peng-Robinson equation-of-state property model, is chosen to describe the thermophysical properties of this hydrocarbon liquid mixture. One of several equations-of-state well-known to be suitable for hydrocarbon systems, Peng-Robinson equation-of-state should provide reasonable calculations for heats of reaction and heat capacities.
To describe the n-butane isomerization reaction, an Aspen reaction model of POWERLAW type is created: ISOMER. The ISOMER reaction is rate-controlled. The reaction stoichiometry is shown below:Both forward and reverse reactions are 1st order with respect to reactants.
Problem (a)Aspen RPLUG reactor model is used with reactor type Adiabatic Reactor and reaction model ISOMER. Aspen requires input of reactor dimensions in lieu of reactor volume. To start the simulation, initial values of 0.1 meter in diameter and 1000 meter in length are assumed.
It is found that the conversion of C4H10 reaches a maximum of about 72%. 70% of C4H10 conversion is achieved with reactor length of 410 meter, or 603 second of residence time. That gives reactor volume of 3.22 m3.
A DESIGN-SPEC block is set up to find the reactor volume required for 70% C4H10 conversion. The study shows a reactor liquid volume of 20.6 m3 is required to achieve 70% C4H10 conversion. That corresponds to a residence time of to 3678 seconds, more than 6 times that of the RPLUG residence time.
Given the models, various reactor analyses can be performed. For example, a PFR reactor liquid volume of 1.41 m3 is required to achieved 40% C4H10 conversion while a CSTR reactor requires only liquid volume of 1.30 m3.
This simulation uses AspenPlus to model the plug flow reactordesign created in the Matlab program plugr1, which simulates aplug flow reactor. Although a detaileddescription of building an Aspen model may be found elsewhere,this section briefly covers building a model of a reactor inAspen.
After the flowsheet is complete, it is time to specify the model.On the "Setup.Main" page, we let Aspen know that we would like toview the products as both mole flows and mole fractions. The"Components" window for this setup should look like this.
For the property set, choose ideal since the Matlab model of thisreactor is based on ideal thermodynamics. When specifying the feedstream to the reactor, fill in your window such that it resemblesthis one. Keep in mind that these are the same conditions that may befound in the feed stream of the Matlab plug flow model.
The next page of importance to appear is the "Rplug.Main" page.Note the section on this page entitled "Reactions." Since we aregoing to specify the reactions and the kinetics of the reactions thatare going to occur in the reactor, we need to create a database onthese reactions before we complete the "Rplug.Main" page. The firstthing we have to do, though, is determine the kinetics of thesystem.
Using kinetics, the following two rate equations may be written.Keep in mind that the first equation applies to the forward-onlymethane reaction, while the second equation incorporates both theforward and reverse reactions of the water-gas shift reaction.
Once this has been determined, we need to make sure that thecorrect units are being employed. When using powerlaw equations inAspen, the units used for the equation as a whole are based on theunits used in the concentration variables. One of the choices Aspenhas is molarity, which allows us to use kgmol/m3. To getk1 in the right units, we must do the following.
Now that we have determined the reaction rate equation for thefirst reaction, we need to do the same for the second and thirdreactions. The second reaction is the forward reaction of thewater-gas shift.
The next thing to do is enter the reaction data. Double click onthe "Reactions" folder in the input section of the Flow Sheet window.The menu now present should have "Chemistry" and "Reactions." Clickon "Reactions" to bring up a window where you can enter informationabout the reactions. Click on the "New" button in that window.
By selecting "New" we are telling Aspen that we want to define aset of reactions that is going to take place in one of the units inthe model. From the "Object Type" menu that just appeared, select"Powerlaw." Next, name your reaction set in the "Create" window thatjust appeared. This example will use the default name, "R-1." We canthen edit the reaction window for the first reaction to make it looklike.
Notice that the reactor diameter is different from the reactordiameter in the Matlab example. In the Matlab example, the porosity(phi) is 0.48. To account for the porosity, we use the followingrelation:
We are now ready to execute the system. To dothis, first "View" the Control Panel and execute with the "Run"button under the "Run" menu.. When Aspen has finished thecalculations, pull up the results under the Data menu: ResultsSummary: Streams to see:
To allow you to see how the results compare, the following graphcompares the mole fraction of water in both systems. The dotted blueline represents the Matlab data while the solid red line correspondsto the Aspen data.
Chemical Looping Combustion (CLC) is a technology that efficiently combines power generation and CO2 capture. In CLC, the fuel is oxidized by a metal oxide called an oxygen carrier (OC). CLC uses two reactors: a fuel reactor and an air reactor. The fuel reactor oxidizes the fuel and reduces the OC. The air reactor oxidizes the OC using air and then the OC is cycled back to the fuel reactor. It is typical for both the fuel and the air reactors to be fluidized beds (FBs). In this research, an Aspen Plus model was developed to simulate a CLC system. Aspen Plus has recently included a built-in FB unit operation module. To our knowledge, no literature has been reported using this FB module for simulating fluidized bed combustion or gasification. This FB unit process was investigated in Aspen Plus and a kinetic based model was used and compared the simulation results to experimental data and the commonly used Gibbs equilibrium model. The FB unit and the kinetic model well fit the experimental data for syngas and methane combustion within 2% of the molar composition of syngas combustion and within 4% for the methane combustion. An advantage of this model over other kinetic models in literature is that the core shrinking model kinetic rate equations have been converted into a power law form. This allows Aspen Plus to use a calculator instead of an external Fortran compiler. This greatly simplifies the modeling process. The reaction rate equations are given for all reactions. A sensitivity analysis of the reaction kinetics was conducted. All data, code, and simulation files are given.
The world has an ever-increasing need for clean energy to help reduce global warming driven by human generated greenhouse gas (GHG) emissions. Carbon dioxide (CO2) accounts for about 80% of anthropogenic GHG emissions and methane accounts for approximately 10% [1, 2]. Further, more than 40% of all anthropogenic CO2 emissions is produced by coal fired power plants [3].
Three main categories of carbon capture systems are: post-combustion capture, pre-combustion capture, and oxy-fuel combustion (also called oxy-combustion). Post-combustion capture removes the CO2 from combustion flue gases after combustion using air, but these separation units often involve a large parasitic energy expense for fossil fuel power plants [4]. The most common method of post-combustion CO2 separation uses monoethanolamine (MEA) scrubbing. The flue gas is cooled and enters an absorber where amine is used as a solvent to remove CO2 from the flue gas stream. The amine solvent is then regenerated in a stripper unit where the temperature is higher than the absorber unit. CO2 can then be recovered at lower pressure. MEA and other solvent scrubbers have many drawbacks, such as corrosion and energy intensive solvent regeneration [5,6,7]. Also the presence of other flue gas contaminants such as SOx and NOx negatively impact the solvent based scrubbers' performance. There are also CO2 scrubbers specific to syngas. Materials such as zeolites, alumina molecular sieves, and activated carbon are used to adsorb CO2 in the production of hydrogen from syngas and in natural gas "sweetening." But again, the regeneration of the adsorbent is often energy intensive [4]. Membranes can also be used to separate gases of different sizes, however, this has been difficult to scale and has high capital cost [7].
Chemical Looping Combustion (CLC) would be considered an oxy-combustion carbon capture technology in the previous three categories. It involves an intrinsic oxygen separation and is therefore a more efficient alternative for carbon capture [8,9,10,11,12]. CLC can operate with many different fuel types, including coal, biomass, and natural gas. Most exciting environmentally is that CLC and chemical looping gasification (CLG) can be carbon-negative [13, 14].
The CLC process usually uses a transition metal oxide as an oxygen carrier (OC) to transport oxygen from the air reactor (AR) to the fuel reactor (FR). The OC is circulated between the AR and the FR. The FR oxidizes the fuel and reduces the OC. The AR oxidizes the OC using air and then the OC is cycled back to the FR. This is illustrated in Fig. 1. In this way, the fuel does not need to be in contact with air, and the resulting combustion gases are free of nitrogen.
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