Reactions With HYSYS

Reactions With HYSYS 1

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Reactions With HYSYS

© 1999 AEA Technology plc - All Rights Reserved

ADV 2_2.pdf

2 Reactions With HYSYS

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WorkshopIn this module, you will simulate a Synthesis Gas Production facility. This will introduce you to the powerful reaction modelling capability of HYSYS.

The production of synthesis gas is an important step in the production of ammonia. Synthesis gas is comprised of hydrogen and nitrogen at a molar ratio of 3:1. The main role of the synthesis gas plant is to convert natural gas, primarily methane, into hydrogen.

In most synthesis gas plants, four reactors are used. However, in our simulation five reactors will be used to model this process. This is because the combustor, a single vessel, will be modelled as two reactors in series, with two different reaction types. The first reactor is a Conversion reactor and the second is an Equilibrium reactor.

Learning ObjectivesAfter completing this module, you will be able to:

• Simulate reactors and reactions in HYSYS• Use Set and Adjust Operations to modify a HYSYS simulation

PrerequisitesBefore beginning this module, you need to know how to:

• Navigate the PFD• Add Streams in the PFD or the Workbook• Add and connect Unit Operations


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Reactions and ReactorsThere are five different types of reactors that can be simulated with HYSYS, and by using combinations of these five reactors, virtually any reactor can be modelled with HYSYS. The five reactor types are:

• Conversion - given the stoichiometry of all the reactions occurring and the conversion of the base component, calculates the composition of the outlet stream.

• Equilibrium - determines the composition of the outlet stream given the stoichiometry of all reactions occurring and the value of equilibrium constant (or the temperature dependant parameters that govern the equilibrium constant) for each reaction.

• Gibbs - evaluates the equilibrium composition of the outlet stream by minimizing the total Gibbs free energy of the effluent mixture.

• CSTR - assumes that the reactor contents are completely mixed in computing the outlet stream conditions, given the stoichiometry for all the reactions that are occurring and the kinetic rate constant (or the temperature dependence parameters for determining the kinetic constant) for each reaction.

• PFR - assumes that the reaction stream passes through the reactor in plug flow in computing the outlet stream composition, given the stoichiometry of all the reactions occurring and a kinetic rate constant for each reaction.

Note that the required input is different depending on the type of reactor that is chosen. The last two types (CSTR and PFR) must have kinetic rate constants (or the formula to determine the kinetic rate constant) as inputs, as well as the stoichiometry of the reactions. All of the reactor types, except for the Gibbs type, must have the reaction stoichiometry as inputs.

The Tank, Separator, and Three Phase Separator Unit Operations can also process reactions if a reaction set is attached.

The process for entering the reaction stoichiometry is discussed in this module, as is the process for adding reactor Unit Operations to a HYSYS simulation.

Note that Kinetic, Kinetic (Rev Eqb), and Langmuir-Hinshelwood reactions can be modelled in the CSTR, PFR and Separator.

Process Overview


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Building the SimulationThe first step in simulating a synthesis gas plant is choosing an appropriate fluid package. We will be using the Peng Robinson (PR) EOS in this simulation.

Add the following components to the simulation: CH4, H2O, CO, CO2, H2, N2, and O2.

Adding the ReactionsReactions in HYSYS are added in a manner very similar to the method used to add components to the simulation:

1. Under the Rxns tab in the Fluid Package view, press the Add Set button to add the Global Reaction Set to the simulation. Press the Simulation Basis Mgr button.

2. Press the Add Comps button, and ensure that the FPkg Pool radio button is selected. Then press the Add this Group of Components button. This moves the entire component list over to the other side of the page.

3. Because nitrogen is an inert component for this simulation, you may remove it from the Selected Reaction Components group. Simply highlight it and press the Remove Comps button. When completed, the page should look like this:

4. Close this page.


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5. Press the Add Rxn button, and choose Conversion as the type from the displayed list. Enter the necessary information as shown:

6. Move to the Basis tab and enter the information as shown:

7. Repeat Steps 5 and 6 for two more Conversion reactions. Use the following data.

8. Add an Equilibrium reaction by selecting the reaction type as Equilibrium rather than Conversion. Under the Library tab, highlight the reaction with the form CO + H2O --> CO2 + H2. Press the Add Library Rxn button. This adds the reaction and all of the reaction’s data to the simulation.

When entering the values for the Stoichiometeric Coefficients, it is important to remember that "Products are positive and Reactants are negative."

Name ReactionBase Component

Conversion (%)

Rxn-2 CH4 + 2H2O --> CO2 + 4H2 Methane 65

Rxn-3 CH4 + 2O2 --> CO2 + 2H2O Methane 100


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Adding the Reaction SetsOnce all four reactions are entered and defined, you can create reaction sets for each type of reactor.

1. Still on the Reactions page, press the Add Set button. Call the first set Reformer Rxn Set, and add Rxn-1 and Rxn-2. Reactions are added by highlighting the <empty> field in the Active List group, and selecting the desired reaction from the drop down list. The view should look like this after you are finished:

2. Create two more reaction sets with the following information:

Attaching Reaction Sets to the Fluid Package

After the three reaction sets have been created, they must be added to the current fluid package in order for HYSYS to use them.

1. Highlight the desired Reaction Set and press Add to FP.

2. Select the only available Fluid Package and press the Add Set to Fluid Package button.

3. Repeat Steps 1 and 2 to add all three reaction sets (Reformer, Combustor, and Shift).

4. If desired, you can save the Fluid Package with the attached reaction sets. This will allow you to reopen this FP in any number of HYSYS simulations.

Only reactions of the same type can be included in a reaction set. For example, Equilibrium and Conversion reactions can not be grouped into the same reaction set.

Reaction Set Name Active Reactions

Combustor Rxn Set Rxn-1, Rxn-2, Rxn-3

Shift Rxn Set Rxn-4


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Once all three reaction sets are added to the Fluid Package, you can enter the Simulation Environment and begin construction of the simulation.

Installing the Material Streams

Create four new material streams with the following information:

Adding the Conversion ReactorsThe first reactor in the synthesis gas plant is the Reformer. This reactor will be modelled as a Conversion Reactor.

1. From the Object Palette, click General Reactors. Another palette appears with three reactor types: Gibbs, Equilibrium and Conversion. Select the Conversion Reactor, and enter it into the PFD.

2. Name this reactor Reformer and attach Natural Gas and Reformer Steam as feeds. Name the vapour outlet Combustor Feed and the energy stream as Reformer Q. Even though the liquid product from this reactor will be zero, we still must name the stream. Name the liquid product stream as Reformer LP.

3. On the Parameters page, choose the duty as Heating.

NameNatural Gas Reformer Steam Air Combustor

Steam

Temp., oC (oF)

370 (700) 250 (475) 16 (60) 250 (475)

Pressure, kPa (psia)

3500 (500) <empty> <empty> <empty>

Molar Flow,kgmole/hr(lbmole/hr)

90 (200) 240 (520) 90 (200) 140 (300)

Molar Composition

100% - CH4 100% - H2O 79% - N2

21% - O2

100% - H2O

Conversion Reactor Button


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4. On the Details page of theReactions tab, select Reformer Rxn Set as the reaction set. This will automatically connect the proper reactions to this reactor. On the Results page, change the Spec % Cnv value for Rxn-1 to be 40% and for Rxn-2 to be 30 %. The screen should look like this:

5. On the Worksheet tab, enter a temperature of 930 oC (1700 oF) for the outlet stream Combustor Feed.

The second reactor in a synthesis gas plant is the Combustor. The Combustor will be modelled as a Conversion reactor and an Equilibrium reactor in series. This is because Conversion reactions and Equilibrium reactions cannot occur in reactors of the opposite type, i.e. conversion reactions cannot be associated with equilibrium reactors, and vice versa.


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6. Add another Conversion Reactor with the following data:

Adding the Set OperationsRecall that we did not enter any pressures, except for the natural gas, when we added the material streams to the PFD. This is so that we could now add Set Operators to the PFD to set the pressures of the remaining streams.

1. Select the Set Operation button from the Object Palette.

2. Enter Reformer Steam Pressure as the Target Variable, and Natural Gas as the Source Variable. This process links the Target Variable to the Source Variable, so that if the Natural Gas Pressure were to change, the Reformer Steam Pressure would vary to match it. The completed view is shown here:

In This Cell... Enter...

Name Combustor

Feeds Combustor Feed, Air,

Combustor Steam

Vapour Product Mid Combust

Liquid Product Combustor LP

Reaction Set Combustor Rxn Set

Rxn-1 Conversion 35% (Default Value)



Set Operation Button


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3. Repeat Steps 1 and 2 with Combustor Steam Pressure, and Air Pressure as Target Variables, and Natural Gas as the Source Variable in both cases.

Adding the Shift ReactorsAs mentioned before, the Combustor is to be modelled as a Conversion reactor followed by an Equilibrium reactor. The Shift Reactors will also be modelled as Equilibrium Reactors. This means that a total of three equilibrium reactors must be added to the PFD.

1. Add an Equilibrium Reactor with the following information:

2. Enter another Equilibrium Reactor with the following information:

HYSYS knows to use the pressure value of Natural Gas as the source because a pressure value was selected as the Target Variable.


Name Combustor Shift

Feed Mid Combust

Vapour Product Shift1 Feed

Liquid Product Combustor Shift LP

Reaction Set Shift Rxn Set

The Equilibrium Reactor Button


Name Shifter 1

Feed Shift1 Feed

Vapour Product Shift2 Feed

Liquid Product Shifter 1 LP

Energy Stream Shift1 Q

Duty Cooling

Shift2 Feed Temperature 450 oC (850 oF)


Remember: Set temperature values on the Work Sheet page.


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3. Enter the third Equilibrium Reactor with the following information:


Name Shifter 2

Feed Shift2 Feed

Vapour Product Synthesis Gas

Liquid Product Shifter 2 LP

Energy Stream Shift2 Q

Duty Cooling

Synthesis Gas Temperature 400 oC (750 oF)


What is the mole fraction of Hydrogen in the Synthesis Gas stream? __________

What is the mole fraction of Nitrogen in the Synthesis Gas stream? __________

What is the ratio of H2 / N2 in the Synthesis Gas stream? __________

Save your case!


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Adding the Adjust OperationsIn order to control the temperature of the product stream leaving the Combustor (the second Conversion reactor), the flow rate of steam to this reactor is controlled. We desire to have an outlet temperature from the first shift reactor of 930oC (1700 oF). The steam flow can be adjusted manually until the desired temperature is achieved; however, this takes a lot of time and will not be automatically updated if something were to change. HYSYS contains an adjust function that allows the user to have HYSYS adjust one variable until the desired condition is met.

1. Select the Adjust Operation button from the Object Palette, and enter it into the PFD. Enter the information as shown:

2. On the Parameters tab, enter the information as shown below. The step size in field units will be 40 lbmole/h.

3. Move to the Monitor tab, and press the Start button. HYSYS will adjust the steam flow rate until the desired condition is met.

The Adjust Operation Button

You don’t have to be on the Monitor page to start the Adjust Operation, but it shows you the values that HYSYS is using in the calculations.


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We will use a second Adjust Operation to control the Air Flow rate. The Air Flow rate determines the ratio of H2 to N2 in the synthesis gas product. We want this value to be set at 3.05.

The process for this is not as straight forward as it was for the previous Adjust operation. This is because the molar ratio of H2 to N2 is not a selectable variable. Therefore, we must use the Spreadsheet to transfer the H2 / N2 ratio onto a "dummy stream."

1. Create a new Material Stream in the PFD, and call it Dummy Stream.

2. Add the Spreadsheet operation to the PFD. (The Spreadsheet is added in the same manner as other unit operations).

3. Import Synthesis Gas Comp Molar Flow [Hydrogen] and Synthesis Gas Comp Molar Flow [Nitrogen] into the Spreadsheet.

4. Add a ratio formula to an empty cell in the Spreadsheet, e.g. +A1/A2.

5. Export the value of the molar ratio into the Molar Flow value for Dummy Stream. The Connections page of the Spreadsheet should look like this when completed:

Spreadsheet Button

The Spreadsheet can be used to transfer variables of different types. Here we transferred a molar ratio into a molar flow value.

Make sure that the Exported Variable Cell shown on the Connections page actually contains the ratio formula.


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6. Add another Adjust operation. Select Air - Molar Flow as the Adjusted Variable, and Dummy Stream - Molar Flow as the Target Variable, with a Specified Target Value of 3.05.

7. On the Parameters page, choose a tolerance of 0.003 kgmole/hr (0.005 lbmole/hr), and a step size of 20 kgmole/hr (50 lbmole/hr). Change the number of iterations to 50.

8. Move to the Monitor page, and press the Start button. HYSYS will adjust the Air flow rate until the desired ratio is achieved.

What is the required Air Flow rate? __________

What is the molar flow rate of Synthesis Gas product? __________

Save your case!


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Reactions With HYSYS

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