NGL Fractionation Train 1

NGL Fractionation Train

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Workshop
Recovery of natural-gas liquids (NGL) from natural gas is quite
common in natural gas processing. Recovery is usually done to

Produce transportable gas (free from heavier hydrocarbons

Fast Track to page 6. which may condense in the pipeline)
Meet a sales gas specification
Maximize liquid recovery (when liquid products are more
valuable than gas)

HYSYS can model a wide range of different column configurations. In

this simulation, an NGL Plant will be constructed, consisting of three
columns:

De-Methanizer (operated and modelled as a Reboiler Absorber

column)
De-Ethanizer (Distillation column)
De-Propanizer (Distillation column)

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

Add columns using the Input Experts

Add extra specifications to columns

Prerequisites
Completed Modules 1, 2, and 3

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Process Overview
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Column Overviews
DC1 - De-Methanizer

DC2 - De-Ethanizer

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DC3 - De-Propanizer

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Building the Simulation

Defining the Simulation Basis
1. Start a new case.
2. Select the Peng Robinson EOS.
3. Add the components: N2, CO2, C1 - C8.
4. Enter the Simulation Environment.

Adding the Feed Streams

1. Add a Material Stream with the following data:

In this cell... Enter...

Name Feed1

Temperature -95C (-140F)

Pressure 2275 kPa (330 psia)

Flowrate 1620 kgmole/h (3575 lbmole/hr)

Component Mole Fraction

N2 0.0025

CO2 0.0048

C1 0.7041

C2 0.1921

C3 0.0706

i-C4 0.0112

n-C4 0.0085

i-C5 0.0036

n-C5 0.0020

C6 0.0003

C7 0.0002

C8 0.0001

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2. Add a Material Stream with the information provided below:

In this cell... Enter...

Name Feed2

Temperature -85C (-120F)

Pressure 2290 kPa (332 psia)

Flowrate 215 kgmole/h (475 lbmole/hr)

Component Mole Fraction

N2 0.0057

CO2 0.0029

C1 0.7227

C2 0.1176

C3 0.0750

i-C4 0.0204

n-C4 0.0197

i-C5 0.0147

n-C5 0.0102

C6 0.0037

C7 0.0047

C8 0.0027

Adding the Unit Operations

Open case Optional4.hsc De-Methanizer

Start with the De-Methanizer.
The De-Methanizer is modelled as a reboiled absorber operation, with
two feed streams and an energy stream feed, which represents a side
heater on the column.

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1. Add an Energy stream with the following values:

In this cell... Enter...

Name Ex Duty

Energy Flow 2.1e+06 kJ/h (2.0e+06Btu/hr)

2. Double click on the Reboiled Absorber button in the Object

Palette. The first Input Expert view appears.
3. Complete the view as shown below:

Reboiled Absorber Column

Button

The Input Experts provide the

new user with step by step
instruction for defining a
column.

4. Press the Next button to proceed to the next page.

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5. Supply the following information to the Pressure Estimates page.

If you are using field units, the values will be 330 psia and 335
psia, for the Top Stage Pressure and Reboiler Pressure,
respectively.

The Next button is only

available when all of the
necessary information has
been supplied.

6. Press the Next button to proceed to the next page.

7. Enter the temperature estimates shown below. In field units, the
top stage temperature estimate will be -125 oF, and the reboiler
temperature estimate will be 80 oF.

Temperature estimates are not

required for the column to
solve but they will aid in
convergence.

8. Press the Next button to continue.

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9. For this case, no information is supplied on the last page of the

Input Expert, therefore simply press the Done button.

The basic Reboiled Absorber

has a single DOF. The default
specification is the Boilup
Ratio.

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When you press the Done button, HYSYS will open the Column
Property View window. Access the Monitor page on the Design tab.

Before you converge the column, make sure that the specifications are
chosen as shown above. You will have to enter the value for the Ovhd
Prod Rate specification. The specified value is 1338 kgmole/h (2950
lbmole/hr). Once this value is entered, press the column Run button to
converge the column.

What is the mole fraction of Methane in DC1 Ovhd?

__________

Although the column is converged, it is not always practical to have

flow rate specifications. These specifications can result in columns
which cannot be converged or that produce product streams with
undesirable properties if the column feed conditions change.

An alternative approach is to specify either component fractions or

component recoveries for the column product streams.

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1. Go to the Specs page on the Design tab of the Column Property

View.

2. Press the Add button in the Column Specifications group to

create a new specification.
3. Select Column Component Fraction from the list that appears.

4. Press the Add Spec(s) button.

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5. Complete the spec as shown in the following figure.

6. When you are done, close the window.

The Monitor page of the Column Property View shows 0 Degrees of

Freedom even though you have just added another specification. This
is due to the fact that the specification was added as an estimate, not as
an active specification.

7. Go to the Monitor page. Deactivate the Ovhd Prod Rate as an

active specification and activate the C1 in Overhead specification
which you created.

What is the flowrate of the overhead product, DC1

Ovhd? __________

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Once the column has converged, you can view the results on the
Performance tab.

Pump
The pump is used to move the De-Methanizer bottom product to the
De-Ethanizer.

Install a pump with the following information.

Pump Button In this cell... Enter...

Connections

Inlet DC1 Btm

Outlet DC2 Feed

Energy P-100-HP

Worksheet

DC2 Feed Pressure 2790 kPa (405 psia)

Ensure that the pump button is checked ON. When a pump is

specified with an outlet pressure specification, the ON button must be
checked to avoid consistency errors.

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De-Ethanizer
The De-Ethanizer column is modelled as a distillation column, with 16
stages, 14 trays in the column, plus the reboiler and condenser. It
operates at a pressure of 2760 kPa (400 psia). The objective of this
column is to produce a bottom product that has a ratio of ethane to
propane of 0.01.

1. Double click on the Distillation Column button on the object

palette and enter the following information.

In this cell... Enter...

Distillation Column Button
Connections

Name DC2

No. of Stages 14

Feed Stream / Stage DC2 Feed / 6

Condenser Type Partial

Overhead Vapour Product DC2 Ovhd

Overhead Liquid Product DC2 Dist

Bottom Product DC2 Btm

Reboiler Duty DC2 Reb Q

Condenser Duty DC2 Cond Q

Pressures

Condenser 2725 kPa (395 psia)

Condenser Delta P 35 kPa (5 psi)

Reboiler 2792 kPa (405 psia)

Temperature Estimates

Condenser -4C (25F)

Reboiler 95C (200F)

Specifications

Overhead Vapour Rate 320 kgmole/h (700 lbmole/hr)

Distillate Rate 0 kgmole/h

Reflux Ratio 2.5 (Molar)

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2. Press the Run button to run the column.

What is the flowrate of C2 and C3 in DC2 Btms?

C2__________, C3__________, Ratio of C2/C3__________

3. On the Specs tab, press the Add button to create a new

specification.
4. Select Column Component Ratio as the specification type and
provide the following information:

In this cell... Enter...

Name C2/C3

Stage Reboiler

Flow Basis Mole Fraction

Phase Liquid

Spec Value 0.01

Numerator Ethane

Denominator Propane

5. On the Monitor tab, deactivate the Ovhd Vap Rate specification

and activate the C2/C3 specification which you created.

What is the flowrate of DC2 Ovhd? __________

Valve
A valve is required to reduce the pressure of the stream DC2 Btm before
it enters the final column, the De-Propanizer.

Add a Valve operation and provide the following information:

In this cell... Enter...

Valve Button Connections

Feed Stream DC2 Btm

Product Stream DC3 Feed

Worksheet

DC3 Feed Pressure 1690 kPa (245 psia)

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De-Propanizer
The De-Propanizer column is represented by a distillation column
consisting of 25 stages, 24 trays in the column plus the reboiler. (Note
that a total condenser does not count as a stage). It operates at 1620 kPa
(235 psia). There are two process objectives for this column. One is to
produce an overhead product that contains no more than 1.50 mole
percent of i-C4 and n-C4 and the second is that the concentration of
propane in the bottom product should be less than 2.0 mole percent.
Distillation Column Button
1. Add a distillation column and provide the following information:

In this cell... Enter...

Connections

Name DC3

No. of Stages 24

Feed Streams / Stage DC3 Feed / 11

Condenser Type Total

Overhead Liquid Product DC3 Dist

Bottom Product DC3 Btm

Reboiler Duty DC3 Reb Q

Condenser Duty DC3 Cond Q

Pressures

Condenser 1585 kPa (230 psia)

Condenser Delta P 35 kPa (5 psi)

Reboiler 1655 kPa (240 psia)

Temperature Estimates

Condenser 38C (100F)

Reboiler 120C (250F)

Specifications

Distillate Rate 110 kgmole/h (240 lbmole/hr)

Reflux Ratio 1.0 Molar

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2. Run the column.

What is the mole fraction of C3 in the overhead and

bottoms products? __________ and __________

3. Create two new Column Component Fraction specifications for

the column

In this cell... Enter...

i-C4 and n-C4 in Distillate

Name iC4+nC4

Stage Condenser

Flow Basis Mole Fraction

Phase Liquid

Spec Value 0.015

Components i-C4 and n-C4

C3 in Reboiler Liquid

Name C3

Stage Reboiler

Flow Basis Mole Fraction

Phase Liquid

Spec Value 0.02

Component C3

4. Deactivate the Distillate Rate and Reflux Ratio specifications.

5. Activate the iC4+nC4 and C3 specifications which you created.

Save your case!

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Advanced Modelling
The Column is a special type of SubFlowsheet in HYSYS. SubFlowsheets
contain equipment and streams, and exchange information with the
Parent Flowsheet through the connected streams. From the Main
Environment, the Column appears as a single, multi-feed multi-
product operation. In many cases, you can treat the Column in exactly
that manner.

The Column SubFlowsheet provides a number of advantages:

Isolation of the Column Solver - The Column Build

Environment allows you to make changes and focus on the
Column without the re-calculation of the entire Flowsheet.
Optional use of different Fluid Packages - HYSYS allows
you to specify a unique (different from the Main Environment)
fluid package for the Column SubFlowsheet. This may be
useful in instances when a different fluid package is better
suited to the Column (Gas Plant using PR may contain an
Amine Contactor that needs to use the Amines Property
Package), or the Column does not use all of the components
used in the Main Flowsheet and so by decreasing the number
of components in the column you may speed up column
convergence.
Construction of custom templates - In addition to the default
column configurations which are available as templates, you
may define column setups with varying degrees of complexity.
Complex custom columns and multiple columns may be
simulated within a single SubFlowsheet using various
combinations of SubFlowsheet equipment. Custom column
examples include, replacement of the standard Condenser with
a Heat Exchanger, or the standard kettle reboiler with a
thermosyphon reboiler.
Ability to solve multiple towers simultaneously - The
Column SubFlowsheet uses a simultaneous solver whereby all
The presence of the green "Up operations within the SubFlowsheet are solved simultaneously.
Arrow" button in the Button The simultaneous solver permits the user to install multiple
Bar and the Environment: interconnected columns within the SubFlowsheet without the
Name (COL1) indicates that need for Recycle blocks.
you are in the Column
SubFlowsheet. You can enter the Column SubFlowsheet by pressing the Column
Environment button on the Column Property View. Once inside the
Column Environment you can return to the Parent Environment by
pressing either the Parent Environment button on the Column Runner
view or the Parent Simulation Environment button in the Button Bar.
Parent Simulation
Environment button

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Exploring with the Simulation

Challenge
After simulating your DeMethanizer, your boss asks you to use HYSYS
to determine the UA for the DeMethanizer Reboiler. Assume that you
have 1000 kgmole/h of saturated 100 psia steam available for the Shell
Side and there is a 5 psia pressure drop on the steam side. Your
overhead Methane spec of 0.96 (mole) must still be met.

Remember, you will need to add Water to your component list and you
will need to be in the DeMethanizer Column SubFlowsheet.

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