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Combustion Reactor Simulation with Aspen HYSYS

by Chemical Engineering Expert

 

Welcome to our adventure into the world of combustion reactors! Who hasn't been mesmerized by the sheen of raging flames and the energy they produce? In the world of technology and industry, combustion reactors are at the center of everything. Let's explore the basic principles, types, and important applications of combustion reactors that make them a marvel of modern technology.

Basic Principles of Combustion Reactors


A combustion reactor is a magical device that converts fuel and oxygen into heat, energy, and reaction products. It is the place where the "fire dance" happens. In this process, fuel (usually hydrocarbons) and oxygen from the air come together, producing invaluable heat and by-products.

Types of Combustion Reactors

1. Open Combustion Reactor: Experience Natural Energy

These reactors are the stage for open-air combustion. They can be found in the form of an open stove or perhaps around a campfire while camping. The thrill of a real fire!

2. Closed Combustion Reactor: The Power Behind the Control

Closed combustion reactors, on the other hand, are sophisticated devices that control the supply of air and fuel to produce the desired energy. They are at the heart of various industries, including power generation, manufacturing, and waste treatment.

Main Components of a Combustion Reactor

  • Fuel: This is the main star of the show. Fuels such as gasoline, natural gas, or coal become the main source of energy.
  • Oxygen: Air contains the oxygen we need for combustion reactions.
  • Combustion Chamber: This is where fuel and oxygen meet and dance. This is the main stage where the magic happens.
  • Combustion System: This system carefully regulates the meeting of fuel and air to achieve optimal combustion conditions.
  • Temperature Regulation: Temperature control is very important. It ensures that the process runs efficiently and safely.

Combustion Process: The Mesmerizing Dance of Flames

The combustion process is the main show in a combustion reactor. It is where fuel and oxygen produce the heat we want. The main results are heat, carbon dioxide (CO2), and water (H2O). However, some side actors play in this show, such as oxides of nitrogen (NOx) and oxides of sulfur (SOx).

Combustion Reactor Applications: Mastering Energy

  • Power Generation: Combustion reactors are the main engines in power plants. They convert heat energy into electrical energy that supplies our homes and businesses.
  • Transportation: The internal combustion engines in cars, planes, and ships are another example of the use of combustion reactors. They convert fuel into kinetic power that powers our vehicles.
  • Chemical Industry: Combustion reactors are also used in chemical industry processes to generate heat or trigger important chemical reactions.

Background to the problem

Natural gas, which is mostly methane, is distributed through underground pipelines. The pressure in these pipes varies depending on where the pipe is located: the closer to the pumping station, the higher the pressure. Industrial customers can expect to get natural gas at around 60 psig and are usually charged per cubic foot of natural gas used. Methane burns in the following reaction: 

Case study

Determine how much energy is available from a 5 ft3/hr (0.472 kg/hr) fuel stream consisting solely of methane at 60 psig. The air feed should be approximated with 80 mol% nitrogen and 20 mol% oxygen. There should be 10% excess oxygen in the air stream so that the fuel-air mixture is not too rich. Assume the exhaust temperature is 182°C. Report the air flow rate in mol/hour and ft3/hour (at 1 atm) in addition to the heat available in kW.

Mole Balance

Two moles of oxygen are required to burn every mole of methane. Oxygen is one-fifth of the moles in air. Therefore, ten moles of air are required for every mole of methane for a stoichiometric tric mixture. A 10% excess requires a 10% increase in the relative amount of air or 11 moles of air for each mole of methane.

 Aspen HYSYS Completion

Open Aspen HYSYS and create a new simulation

In the Component Lists section of the Navigation pane. Enter the components involved in this simulation. Add Oxygen, Nitrogen, Methane, Carbon Dioxide, and Water to the component list.

Define the fluid package used. In the Fluid Packages section select Add and select Peng-Robinson as Property Package.


Define the reaction used. Go to Reactions and click New to create a new reaction set. On the form for the newly created reaction set, click Add Reaction and select Hysys, Conversion.


Double-click Rxn-1 to open the Conversion Reaction: Rnx-1 window. Enter the following information. Note that the Reaction Heat value will be automatically calculated as -8.0e+05 kj/kg mole.

Attach the reaction to the fluid pack. On the Reaction Set 1 form, click the Add to FP button. Select Basis-1

At this point, you are ready to move to the simulation environment. To do so, click the Simulate button at the bottom left of the screen.

On the main flow sheet, create a material flow using the Model Palette. Select the icon for the material stream and place it on top of the flow chart


Double-click the stream to open the stream properties window. Change the stream name to METHANE, and enter Temperature 25°C, Pressure 515 kPa, and Mass Flow 0.472 kg/hour.


Open the Composition form and enter a Mole Fraction of 1 for METHANE. You will notice that after entering the stream composition, the status bar will turn green and say OK. This indicates that the flow is fully defined and finalized for all parameters.

Create a second material stream as the AIR stream required for combustion. Double-click on the new material stream and enter the following information. Bold blue letters indicate user-entered values. From the completed METHANE stream, we know that there is 0.02942 kg mol/hour of Methane. We want there to be 11 moles of air for every mole of methane, therefore we will enter a molar flow rate of 0.324 kg mol/hr for the air stream. Enter a Mole Fraction of 0.2 for Oxygen and a mole fraction of 0.8 for Nitrogen. The stream should then complete

The flow sheet will now look like the following.

We will now place a valve to reduce the pressure of the methane stream to ambient pressure. Select Control Valve from the Model Palette and place it on top of the flowsheet.

Double-click the valve to open the valve properties window. On the Connections, page select METHANE as the Inlet stream and create an Outlet named METHANE-LP.

Determine the outlet pressure of the valve. Go to the Worksheet tab and enter a Pressure of 101.3 kPa for the METHANE-LP Flow. The valve should be open.


Insert the reactor. Select the Reactor radio button and add the Conversion Reactor to the flow sheet.

In the Conversion Reactor properties window, select the AIR and METHANE-LP streams as Inlet streams. Create a Vapor Outlet stream named VAP-OUT and a Liquid Outlet named LIQ-OUT.

Go to the Reaction tab. Select Set-1 for the Reaction Set. The reactor should complete and its status should turn green and say OK.

The flow sheet should now look like the following

To check the results, go to the Conversion Reactor at Worksheet tab. You can see that the VAP-OUT stream leaves the reactor at a very high temperature. This is due to the high heat of the reaction. To calculate exactly how much energy is released from this reaction, simply take the heat of the reaction found on the Reactions tab and multiply it by the molar flow rate of methane. In this case, burning 5 ft3/hour of methane would release 6.5 kW.

Conclusion

5 ft3/hour of methane produces 6.5 kW of heat. To run a quality, lean mixture, there must be 280 ft3/hr of air (i.e. 20 mol-% oxygen) which is 0.324 kg mol/hr. Conversion reactor blocks are useful for fast simulations with well-understood reactions. Reactions with slow kinetics, or complex systems with series or parallel reactions are beyond the scope of this reactor model.

These simulations can also be made using the Gibbs reactor block. The Gibbs reactor is unique in that it can function without a defined reaction set. This reactor block will minimize the Gibbs free energy of the system reaction to calculate the product composition. This reactor block is useful when the exact reaction or kinetics are unknown, and the reaction reaches equilibrium very quickly. It may be a useful exercise to repeat this using a Gibbs reactor and compare the results.

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