1.1.10 oil stabilization with optimization_3

Oil Stabilization with Optimization 1

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Oil Stabilization with Optimization

2001 Hyprotech Ltd. - All Rights Reserved.1.1.10 Oil Stabilization with Optimization_3.pdf

2 Oil Stabilization with Optimization

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WorkshopIn this example, a poor-boy stabilization scheme is used to separate an oil and gas mixture into a stabilized oil and a saleable gas. This approach is used in many gas plants around Alberta where liquid production is small and does not warrant a full distillation column. A simple three-stage separation with heating between each stage is used and the object of the exercise is to select the let-down pressure and temperatures such that the products revenue less the utilities cost is maximized. A special tool in HYSYS, the Optimizer will be used to find the optimum operating conditions.

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

Use the Optimizer tool in HYSYS to optimize flowsheets Use the Spreadsheet to perform calculations

Prerequisites Adding Streams and Operations Using the Spreadsheet

Fast Track to page 9.

Process Overview


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Building the Simulation

Defining the Simulation BasisFor this case you will be using the Peng Robinson EOS with the following components: C1, C2, C3, i-C4, n-C4, i-C5, n-C5, C6, C7, C8, and C9.

Modifying the Unit SetFor this case, the units for Molar Flow are in m3/d_gas, instead of the default Molar Flow units, kgmole/h, and the units for Liquid Volume Flow are m3/d, not the default, m3/h.

1. From the Tools menu select Preferences, and go to the Variables tab, Units page.

2. Select the SI unit set as the default.

You cannot edit the default set, but you can make a copy of it by pressing the Clone Unit Set button.

3. Rename the cloned unit set to Optimizer.

4. Move the cursor to the Flow cell.

5. Select m3/d_gas from the drop down menu in the Edit Bar.

6. Move the cursor to the Liq. Vol. Flow cell.

7. Select m3/d from the drop down menu in the Edit Bar.

If you are working in Field units, choose MMSCFD for the Molar Flow and bbl/d for the Liquid Volume Flow.


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Starting the Simulation1. Add a Material stream with the following values:

The simulation contains three separators for separation of vapour and liquid. The first separator pressure will be fixed at the Inlet gas pressure. The pressure of the downstream separators is set by specifying the pressure of their outlet vapour streams. By default, the delta P on the Separator is 0 so you will have to delete this default value on the Parameters tab of Stage 2 and Stage 3 before you can specify the outlet stream pressures.

Each stage of separation has a preheater.

In this cell... Enter...

Name Feed

Temperature 10C (50F)

Pressure 4125 kPa (600 psia)

Molar Flow 28 200 m3/d_gas (1 MMSCFD)

Component Mole Fraction

C1 0.316

C2 0.158

C3 0.105

i-C4 0.105

n-C4 0.105

i-C5 0.053

n-C5 0.053

C6 0.027

C7 0.026

C8 0.026

C9 0.026


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2. Add 3 Heaters to the simulation with the following information:


Connections

Name Heater1

Inlet Feed

Outlet HotFeed1

Energy Steam 1

Parameters

Pressure Drop 0 kPa

Duty 4.25e+05 kJ/h (4.0e+05 Btu/hr)


Connections

Name Heater2

Inlet Stage1 Liq

Outlet HotFeed2

Energy Steam 2

Parameters

Pressure Drop 0 kPa



Connections

Name Heater3

Inlet Stage2 Liq

Outlet HotFeed3

Energy Steam 3

Connections

Pressure Drop 0 kPa



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3. Add 3 Separators with the following information:


Connections

Name Stage1

Feed HotFeed1

Liquid Outlet Stage1 Liq

Vapour Outlet Stage1 Vap

Parameters

Pressure Drop 0 kPa (default)


Connections

Name Stage2

Feed HotFeed2

Liquid Outlet Stage2 Liq


Parameters

Pressure Drop (delete default value)


Connections

Name Stage3

Feed HotFeed3

Liquid Outlet Liquid Product


Parameters

Pressure Drop (delete default value)


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4. Add 2 Compressors with the values shown in the following tables:

5. Add a Mixer with the information provided below:


Connections

Name Comp1

Inlet Stage2 Vap

Outlet Comp1 Out

Energy Comp1-hp

Parameters

Adiabatic Efficiency 75% (default)


Connections

Name Comp2

Inlet Stage3 Vap

Outlet Comp2 Out

Energy Comp2-hp

Parameters

Adiabatic Efficiency 75% (default)


Connections

Name Gas Mixer

Inlets Stage1 Vap

Comp1 Out

Comp2 Out

Outlet Gas Product

Parameters

Pressure Assignment Set Outlet to Lowest Inlet (default)


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6. Make the necessary stream specifications

Checking the Liquid RVPThe RVP of the Liquid Product stream should be about 96.5 kPa (14 psia) to satisfy the pipeline criterion.

The RVP for a stream is located in the Cold Properties Utility.

The Utility can be added by selecting Utilities from the Tools menu or from the Utility page of the Attachments tab of the Liquid Product stream property view.


Stage2 Vap, Pressure 2050 kPa (300 psia)

Stage3 Vap, Pressure 350 kPa (50 psia)

Comp1 Out, Pressure 4125 kPa (600 psia)

Comp2 Out, Pressure 4125 kPa (600 psia)

Open the case Optional10.hsc

Complete the following questions and then continue the Module.

What is the volumetric Liquid Product Flow? __________

What is the molar Gas Product Flow? __________

What is the current RVP of Liquid Product? __________

HYSYS treats Utilities as stream properties and so will recalculate the Utility every time the stream is recalculated.


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The OptimizerHYSYS contains a multi-variable Steady State Optimizer. Once your Flowsheet has been built and a converged solution has been obtained, you can use the Optimizer to find the operating conditions which minimize or maximize an Objective Function. The Optimizer owns its own Spreadsheet for defining the Objective Functions as well as any constraint expressions to be used. This allows you to construct Objective Functions which maximize profit, minimize utilities or minimize exchanger UA.

Primary Variables - these are flowsheet variables whose values are manipulated in order to minimize (or maximize) the objective function. You set the upper and lower bound for the primary variables, which are used to set the search range.

Objective Function - this is the function which is to be minimized or maximized. The function has to be defined within the Spreadsheet. This allows the user a great deal of flexibility in defining the function.

Constraint Functions - inequality and equality functions are defined in the Spreadsheet. In solving the Objective Function, the Optimizer must also meet any constraints that are defined by the user.

In our case, we want to maximize the total profit while achieving an RVP of Liquid Product less than 96.5 kPa.

The Revenues from the Plant are the Gas Product and the Liquid Product. The associated costs are the Steam Costs for each Heater plus the Compression Cost for each Compressor.

Profit = Revenue - Cost

Profit = Gas Product + Liquid Product - Steam Costs - Compression Cost

Which Variables can we change to affect the Steam Cost? _________

How is the Compression Cost measured? __________

Which Variables can we change to affect the Compression Cost (remember the Compressor outlet pressure is fixed)? __________

What should the Process (Adjusted) Variables be to maximize profit (there are five)? __________, __________, __________, __________, __________

Only user-specified process variables can be used as Primary Variables

Restrictions on the Optimizer

only available for Steady-State calculations

it cannot be used in Templates.


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To invoke the Optimizer, select Optimizer under Simulation in the Menu Bar, or press .

Variables Tab

When you invoke the Optimizer for the first time, you are placed on the Configuration tab. Select the Default model. Then move to the Variables tab. On the Variables tab you define the Process (Adjusted) Variables to be used in the Optimization.

1. Press the Add button to add the first variable, Steam1, Heat Flow.

2. Set the Low Bound at 0 and the High Bound at 1.0e6 kJ/h (1.0e6 Btu/hr).

3. The complete list of Process Variables are shown in the table below.

The Optimizer is not an operation block and it will not show up in the PFD.

Reasonable upper and lower bounds are important. Set values which can be achieved in your actual design, i.e., dont set a high bound for a Steam Heat Flow that is more than what is available at your plant.

ObjectVariable Description

UnitsLow Bound

High Bound

Steam 1 Heat Flow kJ/h (Btu/hr) 0.0 (0.0) 1.0e+6

(1.0e+6)


(1.0e+6)


(1.0e+6)

Stage2 Vap Pressure kPa (psia) 650 (95) 3500 (510)

Stage3 Vap Pressure kPa (psia) 70 (10) 1000 (145)


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Defining the SpreadsheetThe Optimizer has its own Spreadsheet for defining the Objective and Constraint functions. Primary Variables may be imported and functions defined within the Optimizer Spreadsheet, which possesses the same capabilities as the Main Flowsheet Spreadsheet.

1. Press the Spreadsheet button on the Optimizer view to open the Spreadsheet.

2. On the Parameters tab of the Spreadsheet view, increase the Number of Rows from 10 to 15.

3. Move to the Spreadsheet tab.

Importing and Exporting Variables

You may import virtually any variable in the simulation into the Spreadsheet and you can export a cells value to any specific field in your simulation.

Object Inspection - object inspect (secondary mouse button) the cell which you want to Import into, or Export from. From the Menu that appears, select Import Variable or Export Formula Result. Then, using the Variable Navigator, select the variable you wish to import or export.

Connections page tab - select the Add Import or Add Export button. Then, using the Variable Navigator, select the variable you wish to import or export.

Drag n Drop - using the secondary mouse button, click the variable value (from the WorkBook or Property View) you wish to import, and drag it to the desired location in the Spreadsheet. If you are exporting the variable, drag it from the Spreadsheet to the exported location.

Adding Formulas

Complex mathematical formulas can be created, using syntax that is similar to conventional Spreadsheets. Arithmetic, logarithmic and trigonometric functions can be performed in the Spreadsheet.

All normal functions must be preceded by a + or = symbol. Special Functions must be preceded by the @ symbol.

Some of the functions available are:

Addition (+): +A1+A2 Subtraction (-): +A1-A2

The Spreadsheet is an operation and thus the Spreadsheet cells get updated when Flowsheet variables change.


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Multiplication (*): +A1*A2 Division (/): +A1/A2 Power (^): +A1^3 Absolute Value (@ABS): @ABS(A1) Square Root (@SQRT): @SQRT(A1) Natural Log (@ln): @ln(A1) Exponential (@exp): @exp(A1)

The following variables need to be imported into the Spreadsheet.

The following constants should be added to the spreadsheet

Conversion factors you may need:

1 kw-h = 3412 Btu

1 kw-h = 3600 kJ

Press the Function Help button to view the Available Spreadsheet Functions and Expressions.

Cell... Object... Variable...

B1 Cold Properties-1 Reid VP

B4 Liquid Product Liquid Volume Flow

B8 Comp1-hp Heat Flow

B9 Comp2-hp Heat Flow

B12 Steam 1 Heat Flow



D4 Gas Product Molar Flow

In order to have access to the Utility variable needed in cell B1. The Utility radio button in the Navigator Scope box must be selected.

Cell... Value... Comment...

B2 96.5 kPa (14 psia) RVP spec

B6 157.25$/m3 (25$/bbl) Oil Price

B10 0.1 $/kw-h

(2.9e-5 $/Btu)

Compression Cost

B15 1.819$/kw-h(0.0005$/Btu)

Steam Cost

D6 0.283 $/m3_gas (8 $/MSCF)

Gas Price


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Additional comments can be added, though they are not necessary. After all of the necessary variables have been imported and the formulas have been entered the spreadsheet should look something like the following.

Which Spreadsheet Cell defines the Objective Function (i.e., which cell do we want to maximize)? __________

Define the Constraint Function (RVP < 96.5 kPa) with reference to Spreadsheet cells. __________

You can change the Variable Type to Unitless for dollar value variables.


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Functions tab

The Functions tab contains two groups, the Objective Function and the Constraint Functions.

1. In the Cell area of the Objective Function group, specify the Spreadsheet cell that defines the Objective Function. Use the drop down menu in the Edit Bar to select the appropriate cell. The Current Value of the Objective Function will be provided.

2. Select the Maximize radio button

3. In the Constraint Functions group, press the Add button to define the constraint.

Parameters tab

The Parameters tab is used for selecting the Optimization Scheme.

Box - Handles inequality constraints but not equality constraints. It generally requires a large number of iterations to converge on the solution.

SQP - Sequential Quadratic Programming, handles inequality and equality constraints. Considered by many to be the most efficient method for minimization.

Mixed - Handles inequality constraints only. It is a combination of the Box and SQP methods. It starts the minimization with the Box method using a very loose convergence tolerance. After convergence, the SQP method is used to locate the final solution.

The Constraint Function is multiplied by the Penalty Value; the higher the Penalty Value, the more weight that is given to that constraint.

For more information on the Optimization Schemes, refer to the manual section 17.2 or the on-line Help.


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Fletcher Reeves - Does not handle constraints. Efficient method for general minimization.

Quasi-Newton - Does not handle constraints. Similar method to Fletcher Reeves.

1. Select the SQP method as the Scheme

2. Use the defaults for Tolerance and Number of Iterations

3. Change Shift A and Shift B to 1.0

Monitor tab

The Monitor tab displays the values of the Objective Function, Primary Variables and Constraint Functions during the Optimizer calculations.

1. Move to the Monitor tab and press the Start button to begin the optimization.

The constraint values are positive if inequality constraints are satisfied and negative if inequality constraints are not satisfied.

Save your case!


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Analyzing the ResultsOnce the Optimization is complete, examine the results and fill in the following Table.

Base Case Optimized Case

Gas Product Flow,m3/d_gas (MMSCFD)

25343 (0.8950)

Liquid Product Flow, m3/d (bbl/d)

17.247 (108.5)

Total Profit, $/d -534

Steam1 Heat Flow, kJ/h (Btu/hr)

425 000 (400,000)


315 000 (300,000)


113 000 (100,000)

Stage2 Vap Pressure, kPa (psia)

2050 (300)

Stage3 Vap Pressure, kPa (psia)

350 (50)

RVP of Liquid Product, kPa (psia)

97.85 (14.19)


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Exploring with the Simulation

Exercise 1 One thing you may notice with the Optimized solution is that the Pressure of Stage3 Vap has been decreased to 70 kPa (10 psia) which is less than atmospheric. This is not a desired condition for the inlet of a compressor. The inlet of the second compressor, Comp2, cannot be less than 125 kPa (19 psia). What is the maximum profit if you adhere to this guideline?

1.1.10 oil stabilization with optimization_3

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