J 05 Pre-HeatTrain

Pre-Heat Train 1

Pre-Heat Train

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© 2004 AspenTech, Inc. - All Rights Reserved1.3.5 Pre-Heat Train.pdf

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WorkshopIn this module you will continue to develop your ability to create HYSYS simulations. Here, we will continue with the HYSYS case that was started in the previous module.

Typically, after some preheating the first unit that a crude oil will pass through is the desalter. This unit is responsible for removing salt, grit, and other impurities from the oil. This unit is normally placed along the Pre-Heat Train, needed to recover energy from products and to raise the temperature of the oil stream before it enters the Atmospheric Crude Column. In this module, you will learn how to build a Heat Exchanger in HYSYS.

Logical operations and the Databook are also introduced in this module.

Learning ObjectivesOnce you have completed this section, you will be able to:

• Install and converge Heat Exchangers.• Understand Logical Operations (Balances and Adjusts).• Use the Case Study tool to perform case studies on your

simulation.

PrerequisitesBefore beginning this section you need to complete the Oil Characterization module.

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Building the SimulationFor this module you will continue building on the case you worked on in the previous module by adding a Heater Exchanger, a Desalter, two simple Heaters and a Pre-Flash Separator, to complete a simple pre-heat train.

1. Open the case you saved at the end of the previous module (Oil Characterization).

Only the composition is known for stream Raw Crude. Add the following data to it:

Adding a StreamYou need to add a Water stream. Before adding it, you need to add Water to the Fluid Package. Afterwards, create a material stream called Water, at 15°C and a mass flow rate of 21600 kg/h (100% water). If you are using field units it is 59°F and 47600 lb/hr.

Adding a MixerA mixer is added to combine the raw crude with water to represent the oil sample entering the refinery. Downstream the free water is knocked out with the three - phase separator.

Add a Mixer with the following values:

Temperature 15°C (60°F)

Pressure 1000 kPa (145 psia)

mass Flow 6e+05 kg/h (1.3e+06 lb/hr)

In this cell... Enter...

Connections

Name Mixer

Inlets Raw Crude

Water

Outlets To Heater

To add components to the simulation, you must enter the Basis Environment by clicking the Basis Environment icon.

Enter Basis Environment icon

Mixer icon

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Adding a HeaterThe Pre-Heat Train consists of various heat exchangers. A heater will be added to account for them and raise the temperature to 65°C (149°F).

Add a Heater, we will use a Shell and Tube Heat Exchanger later.

1. Complete the first view with the following names.

2. Enter a Delta P of 50 kPa (7.25 psi) in the Parameters view.

3. In the worksheet specify a temperature of 65°C (150°F) for stream To Heat Exchanger.

Parameters

Pressure Assignment Equalize All


Connections

Name Simple Heater 1

Inlet To Heater

Energy H1 Q

Outlet To Heat Exchanger

Fluid Package Refinery

Figure 1


Heater icon

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Adding a StreamBefore adding the Heat Exchanger, we need to define the Pumparound stream, which will exchange heat with the Raw Crude.

1. Add a Material Stream.

2. Change the stream name to Hot Pumparound.

At this stage, we don’t know the pumparound composition. As a starting point, we are going to use the same composition as the Raw Crude. Of course, later, when we build the Crude Tower, we will change it to the actual stream.

3. Click the Define from Other Stream button on the bottom of the Hot Pump Around Stream view.

4. Highlight Raw Crude in the Available streams list.

5. Uncheck all the Copy Stream Conditions boxes except for Composition. You will be specifying your own conditions in the next step.

6. Click the OK button to return to the previous menu and enter the following values in the Hot Pump Around stream: Temperature 180 C, Pressure 200 kPa and Std Ideal Liq Vol Flow of 175 m3/h. (356°F, 30 psia and 2.6e+04 barrel/day if you are using field units).

Figure 2

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Adding the Heat ExchangerA Heater Exchanger is added to continue heating the crude stream. A shell and tube heat exchanger will be used to preheat the crude with the Pumparound, which is at 180°C (356°F).

The Heat Exchanger performs two-sided energy and material balance calculations. The Heat Exchanger is capable of solving for temperatures, pressures, heat flows (including heat loss and heat leak), material stream flows and UA.

1. Double-click on the Heat Exchanger icon in the Object Palette.

2. On the Connections page, enter the following information.


Connections

Name Heat Exchanger

Tube Side Inlet To Heat Exchanger

Tube Side Outlet To Desalter

Shell Side Inlet Hot Pumparound

Shell Side Outlet Cold Pumparound

Figure 3

Heat Exchanger icon

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3. Switch to the Parameters page. Complete the page as shown in the following figure. The pressure drops for the Tube and Shell sides, in field units, will be 35 kPa (5 psi) and 5 kPa (0.7 psi), respectively.

The Exchanger Design models are defined as follows:

• Weighted. The heating curves are broken into intervals, which then exchange energy individually. An LMTD and UA are calculated for each interval in the heat curve and summed to calculate the overall exchanger UA. The Weighted method is available only for Counter-Current exchangers.

• Endpoint. A single LMTD and UA are calculated from the inlet and outlet conditions. For simple problems where there is no phase change and Cp is relatively constant, this option may be sufficient.

4. Choose Exchanger Design (Weighted) in the Heat Exchanger Model.

5. Go to the Specs page.

In order to solve the Heat Exchanger, unknown parameters (flows, temperatures) are manipulated by the solver. Each parameter specification will reduce the degrees of freedom by one. The number of constraints (specifications) must equal the number of unknown variables. When this is the case, the degrees of freedom will be equal to zero, and a solution will be calculated.

Figure 4

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Two specifications are needed for this exchanger:

• Heat Balance = 0. This is a Duty Error specification and is needed to ensure that the heat equation balances. This is a default specification that is always added by HYSYS so you do not need to supply it.

• Min Approach = 30°C (54°F). This is the minimum temperature difference between the hot and the cold stream.

You can have multiple Estimate specifications. The Heat Exchanger will only use the Active specifications for convergence.

6. You will first need to deactivate the UA specification. To do this, remove the X from the Active checkbox for the UA specification so that the view is the same as below.

7. To add a specification, click the Add button, the Exchanger Specification view displays.

8. Provide the following information using the drop-down list to specify the Type and Pass:

Figure 5


Name Temp Approach

Type Min Approach

Pass Overall

Spec Value 30°C (54°F)

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When you change the type of specification, the view will change accordingly. Once all the information has been provided, the view will be as below. Close the Temp Approach window.

The completed heat exchanger Specs page should look like this:

Figure 6

Figure 7

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Adding the DesalterThe Three-Phase Separator divides the vessel feed into vapour, light liquid and heavy liquid (Aqueous) phases.

1. Add a Three-Phase Separator with the following information.


Connections

Name Desalter

Inlet To Desalter

Vapour Ovhd Vapour

Light Liquid Crude

Heavy Liquid Desalter Water

Figure 8

What is the Molar Flow rate of Crude?__________________________________

What is the Molar Flow rate of Desalter Water?__________________________

Three-Phase icon

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Finishing the SimulationYou need to add the last three units before the crude enters the Atmospheric Column: two heaters and a Preflash separator to reduce the light components in the feed. Since you already know how to use shell and tube heat exchangers, we will simulate these heaters in a simplified mode.

Adding the First HeaterAdd a Heater with the following values:

The Delta P represents the result of the pressure drop along the various heaters.

Adding the Pre FlashClick on the Separator icon from the Object Palette.


Connections

Name Simple Heater 2

Inlet Crude

Outlet To Preflash

Energy H2 Q

Parameters

Delta P 375 kPa (54 psi)

Worksheet

To Preflash Temperature 175°C (347°F)


Connections

Name Pre-Flash

Inlets To Preflash

Vapour Outlet Light Prod

Liquid Outlet Bttm Liq

Separator icon

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Adding the Last Heater

Add a Heater with the following values:

Adding the BalanceThe Balance operation provides a general-purpose heat and material balance facility. There are different Balances available in HYSYS.

• Mole. An overall balance is performed where only the molar flow of each component is conserved. Outlet streams will have the same molar flow rate and composition as the inlet stream, but will contain no vapour fraction, temperature, or pressure values.

• Mass. An overall balance is performed where only the mass flow is conserved. The outlet stream will contain no composition, vapour fraction, temperature, or pressure.

• Heat. An overall balance is performed where only the heat flow is conserved.

• Mole and Heat. An overall balance is performed where the heat and molar flow is conserved.


Connections

Name Furnace

Inlet Bttm Liq

Outlet ATM Feed

Energy H3 Q

Parameters

Delta P 250 kPa (36 psi)

Worksheet

ATM Feed Temperature 400°C (752°F)

Save your case!

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Look at the Vapour Fraction for stream To Desalter. The feed to the Desalter has to be liquid, in this case it is, but it could change with the pressure. The stream parameters are calculated so there is no way you can force the stream to calculate a bubble point. Adding a Mole Balance allows you to create a second steam with the same molar flowrate and composition as To Desalter, but no vapour fraction, temperature or pressure.

1. Double-click on the Balance icon in the Object Palette.

2. Add the following information

3. Specify a Pressure of 915 kPa (133 psia) for the stream Bubble Temperature.

4. Set the Vapour Fraction to calculate the bubble point temperature.

Adding the AdjustThe Adjust operation is a Logical Operation: a mathematical operation rather than a physical operation. It will vary the value of one stream variable (the independent variable) to meet a required value or specification (the depend variable) in other stream of operation.

We are going to use this tool to optimize the heating-train, by specifying the Flowrate of Light Prod in the Pre-Flash, which depends on the temperature of the feed stream.

The Adjusted Variable must always be a user specified value.


Connections

Name Bubble Point

Inlet Streams To Desalter

Outlet Streams Bubble Temperature

Parameters

Balance Type Mole

Up to what Temperature can the Feed be heated in the Pre-heat Train so that it remains liquid?____________________________________________________

Balance icon

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1. Double-click on the Adjust icon in the Object Palette; the Adjust property view displays.

2. Click the Select Var… button in the Adjusted Variable group to open the Variable Navigator.

3. From the Object List select To Preflash. From the Variable List, which is now visible, select Temperature.

Always work left to right in the Variable Navigator. Don't forget you can use the Object Filter on the right-hand side when the Object list is large.

Figure 9

Figure 10

Adjust icon

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4. Click the OK button to accept the variable and return to the Adjust property view.

5. Click the Select Var button in the Target Variable group.

6. Select Light Prod Molar Flow as the target Variable.

7. Enter a value of 200 kgmole/h (440 lbmole/hr) in the Specified Target Value box.

8. The completed Connections tab is shown below.

Figure 11

Figure 12

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9. Switch to the Parameters tab, and leave the parameters at the default values. Note the Tolerance and step Size values. When considering step sizes, use larger rather than smaller sizes. The Secant method works best once the solution has been bracketed and by using a larger step size, you are more likely to bracket the solution quickly.

When adjusting certain variables, it is usually a good idea to provide a minimum or maximum that corresponds to a physical boundary, such as zero for pressure or flow.

10. Click the Start button to begin calculations.

11. To view the progress of the Adjust go to the Monitor page.

Note how the Secant method works.

Figure 13

What is the To Preflash Temperature to achieve the Light Prod Flow specification?________________________________________________________

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Using the Case StudyThe Case Study tool allows you to monitor the steady state response of key process variables to changes in your process. You select independent variables to change and dependent variables to monitor.

HYSYS varies the independent variables one at a time, and with each change, the dependent variables are calculated.

Instead of using the Adjust to find a feed temperature to achieve a required flowrate you can use the Case Study to examine a range of temperatures and flow.

Before installing the Case Study, the Adjust has to be turned off so that it does not conflict with the Case Study.

Any Unit Operation can be temporarily removed from the calculations by selecting the Ignore check box.

1. Open the Adjust property view.

2. Check the Ignored checkbox.

3. Close the Adjust property view.

4. From the Tools menu select Databook, or use the CTRL D hot key, to open the Databook.

Both the independent and the dependent variables are added to the Databook from the Variables tab.

Figure 14

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5. On the Variables tab, click the Insert button to open the Variable Navigator.

6. Select the Object as To Preflash and choose Temperature as the first variable.

7. Click the Add button to add the variable. This will leave you in the Variable Navigator to add the second variable.

8. Select Light Prod - Molar Flow and click the Add button.

9. Close the view to return to the Databook.

10. In the Databook, switch to the Case Studies tab.

11. Click the Add button to add a new Case Study.

12. Select To Preflash Temperature as the Independent Variable and Light Prod - Molar Flow as the Dependent Variable.

Only user supplied variables can be selected as Independent Variables.

13. Click the View button to setup the Case Study.

Figure 15

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14. Enter values for Low Bound, High Bound, and Step Size of 150°C (300°F), 250°C (482°F) and 10°C (18°F) respectively.

15. Click the Start button to begin de calculations.

Once you have finished the Case Study, activate the Adjust operation by unchecking the Ignored box.

Figure 16

What is the influence of temperature in Light Prod flowrate?______________

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Exploring with the Simulation

Exercise 1

When you added the Balance, a new stream was created, and then you introduced a value of 915 kPa for the pressure, because it was that of To Desalter stream. But the pressure in the To Desalter stream could change, and consequently the bubble point. There is a tool in HYSYS, which allows you to set the pressure of Bubble Temperature as a function of To Desalter pressure. It is a Logical Unit Operation called Set.

The Set is a steady-state operation used to set the value of a specific Process Variable (PV) in relation to another PV. The relationship is between the same PV in two like objects; for instance, the temperature of two streams, or the UA of two exchangers.

In this case we want it to be the same, but in other cases, it could be useful to use a linear dependency.

Try to use this option to set the Bubble Temperature pressure.

Exercise 2

You are very concerned about the behaviour of the Pre-flash. You want to achieve a good separation of the light components. Although you adjusted the temperature to the required flowrate, usually this value of the molar flow is not known, and the specification is a desired molar fraction of light products in the stream entering the Atmospheric Column.

Use the Adjust to achieve a molar fraction of light products (C1, C2, C3, i-C4, n-C4, i-C5, n-C5 and Water) in the Bttm Liq of 0.12.

What is the required temperature?_____________________________________

Set icon

Use a spreadsheet to create this new variable.

Save your case!

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J 05 Pre-HeatTrain

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