Ethylene Glycol Plant

Ethylene Glycol Plant

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© 2000 AEA Technology plc - All Rights Reserved. Chem 6_3.pdf

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Workshop Ethylene glycol has many industrial uses: a feed stock for polyester resins, a hydrate inhibitor in natural gas pipelines, an all-weather antifreeze and coolant, or an industrial solvent. Ethylene oxide and water are fed to a reactor to produce ethylene glycol. The product stream is fed to a distillation column where the excess water and ethylene oxide are stripped off the top and the ethylene glycol is the bottom product.

Learning Objectives After completing this module, you will be able to:

• • • •

Simulate CSTR reactors Simulate an Ethylene glycol plant Use the recycle operation in HYSYS Choose suitable locations for recycles

Prerequisites Before beginning this module, you need to be able to:

• Add and define the simulation basis, including components, property packages, and reactions. • Add streams and operations to a simulation.

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Process Overview

Column Overview


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Building the Simulation Defining the Simulation Basis Start a New case and choose the NRTL Activity Model. The components are Ethylene Oxide (C2H4O), Water (H2O) and Ethylene Glycol (C2H6O2). Fast track to page 11.

These components can be hard to find in the long list of available components in HYSYS. Use the Formula filter and type the chemical formulas given above into the Match Cell. This will help you find the desired components quickly and easily. Go to the Binary Coeffs tab. Select UNIFAC VLE as the Coeff Estimation method to estimate the missing binary coefficients and press Unknowns Only. This will estimate the coefficients for the ethylene oxide / ethylene glycol pair.

Adding the Reactions On the Rxns tab, add the following reaction to the Fluid Package: C2H4O + H2O → C2H6O2 1. 2.

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To define the reaction, press the Simulation Basis Mgr button and select the Reactions tab. Add all of the Fluid Package components by selecting the Add Comps button and pressing the Add this Group of Components button in the window that appears. Press the Add Rxn button, choose Kinetic, and press the Add Reaction button.

In the Stoichiometry and Rate Info group, define the reaction stoichiometry of the reaction previously stated (-1 for reactants, and 1 for product comps).

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4.

Complete the Basis and Parameters pages as below:

In This Cell...

Enter...

Basis Basis

Molar Concn

Base Component

C2Oxide

Rxn Phase

Combined Liquid

Basis Units

kgmole/m3

Rate Units

kgmole/m3-s

Parameters Forward Reaction A

5.0e+06

E

5.0e+04

No reverse reaction is defined in this example.

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5.

Return to the Simulation Basis Manager by closing the Kinetic Reaction window. Return to the Rxns tab of the Fluid Package. Press the Add Set button to add the Global Rxn Set to the Current Reaction Sets group. Close the Fluid Package view.

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Press the Enter Simulation Environment button.


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Adding the Feed streams Add two feed streams with the following information. All compositions are entered as mole fractions In This Cell...

Enter...

Conditions Name

EOx Feed

Temperature

25°C (77°F)

Pressure

120 kPa (17.4 psia)

Molar Flow

105 kgmole/h (230 lbmole/hr)

Composition C2Oxide

1.0

H2O

0.0

EGlycol

0.0

Conditions Name

Water Feed

Temperature

25°C (77°F)

Pressure

120 kPa (17.4 psia)

Molar Flow

150 kgmole/h (330 lbmole/hr)

Composition C2Oxide

0.0

H2O

1.0

EGlycol

0.0

Save your case!

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Adding the Unit Operations CSTR Reactor

CSTR button

Add a CSTR operation and enter the following information. Be sure to assign the Global Rxn Set as the Reaction Set for the separator to model the reactor. In This Cell...

Enter...

Connections Name

Reactor

Inlets

EOx Feed Water Feed

Vapour Outlet

Rx Vapour

Liquid Outlet

Rx Liquid

Energy

Rx Coolant

Parameters Pressure Drop

10 kPa (1.5 psi)

Volume

2 m3 (71 ft3)

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On the Reactions tab, select the Global Rxn Set and Rxn-1 as the Reaction.

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Specify the temperature of stream Rx Liquid to be 65°C (150°F).

From the Reaction results, what is the % Conversion of Ethylene Oxide? __________

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Ethylene Glycol Tower The Ethylene Glycol tower will be modelled as a Distillation Column. Add the column with the following information: In This Cell...

Enter...

Connections Name

T-100

No. of Stages

10

Inlet

Rx Liquid, Stage 5

Condenser Type

Partial

Ovhd Vapour

Vent

Ovhd Liquid

Distillate

Bottoms Liquid

Bottoms Product

Condenser Energy

Cond Duty

Reboiler Energy

Reboiler Duty

Pressures Condenser

100 kPa (14.5 psia)

Condenser Delta P

0 kPa (0 psi)

Reboiler

105 kPa (15 psia)

Specs Vapour Rate

0.0 kgmole/h (0.0 lbmole/hr)

Reflux Ratio

3.0

Reboiler Temperature

150°C (300°F)

Note: HYSYS generates column temperature estimates if none are provided.

Save your case!

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Installing the Recycle A Recycle operation is a mathematical unit operation and is installed as any other. It has an inlet (calculated) stream and an outlet (assumed) stream. The operation is called/calculated whenever changes to the inlet stream fall outside of the converged tolerance.

In general, a Recycle operation is required for material transfer and not for thermal recycles. Always supply a guess or starting point for the outlet stream of the Recycle, never the inlet. A guess close to the solution will result in a faster convergence time.

The Recycle installs a theoretical block in the process stream. The feed into the block is termed the calculated recycle stream, and the product is the assumed recycle stream. The following steps take place during the convergence process

• HYSYS uses the conditions of the assumed stream (outlet) and solves the flowsheet up to the calculated stream (inlet). • HYSYS then compares the values of the calculated stream to those in the assumed stream. • Based on the difference between the values, HYSYS modifies the values in the calculated stream and passes the modified values to the assumed stream. The calculation process repeats until the values in the calculated stream match those in the assumed stream within specified tolerances.

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Recycle the Water Stream The stream Distillate contains mostly unreacted water. This water can be recycled to the reactor in order to reduce the feed water requirements (as well as eliminate a waste stream). Open the case OptRecy.hsc

Install a Recycle block operation. The inlet stream is the Distillate and the outlet stream should be mixed with the pure Water Feed stream, before entering the Reactor. Delete the specified molar flow rate of the Water Feed stream. This value will be back-calculated from the outlet of the mixer. Specify the molar flow of the mixer outlet stream, Water to Reactor, as 150 kgmole/h (330 lbmole/hr). Once the flow rate in this stream is set, HYSYS will automatically calculate the flowrate of the Water Feed stream. If we had not deleted the specified value, HYSYS would report a consistency error, because a the two values would be in conflict with each other. What value does HYSYS calculate for the flowrate of the stream "Water Feed"? __________

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Parameters tab Tolerance page

The smaller the tolerance value, the tighter the tolerance. Generally it is a good idea to start with the default tolerance until you have a converged solution and then tighten the tolerance.

HYSYS allows you to set the convergence criteria or tolerance for each of the Recycle variables. In this example, leave everything at the default.

Numerical Page This page contains the options for the two types of Recycles, Nested and Simultaneous.

• Nested - this type of recycle gets called whenever it is encountered during calculations. Use this type if you have a single Recycle or if you have multiple recycles which are not connected. • Simultaneous - all recycles set at Simultaneous will be called at the same time. Use this option if your Flowsheet has multiple inter-connected recycles. Change the Maximum Iterations number to 20.

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Monitor tab This page displays convergence information as the calculations are performed. Any variable that changes between iterations is displayed in this table.

Worksheet tab The Recycle WorkSheet page displays the Inlet and Outlet stream information. In this instance, notice that the Inlet and Outlet streams have the same values. This is because before we installed the Recycle, the Inlet stream was already calculated by HYSYS. When the Recycle was connected, the known Inlet conditions were automatically passed to the Outlet stream to serve as the starting guess.

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Exploring with the Simulation Exercise 1 Create a Case Study to plot the Ethylene Glycol molar flow in the liquid product against the Reactor Outlet Temperature. The temperature range can be between 45 and 110 °C (100 and 240 °F), with a step of 10 °C (20 °F). Be sure to "Ignore" the Recycle operation before starting the Case Study. What temperature produces the maximum Ethylene Glycol flow? __________ What problems will be encountered if the temperature exceeds approximately 115 oC? __________ Remember to save the file under a different name if you wish to save it!

Exercise 2 Set up an Adjust operation to make sure the molar flow of Ethylene Glycol in the Rx Liquid stream is equal to 110 kgmole/h (240 lbmole/ hr), by adjusting the EOx Feed molar flow with a step size of 1 kgmole/h (2 lbmole/hr). What EtOx feed rate is required to produce the specified molar flow of Ethylene Glycol? __________

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Advanced Modelling Because the Recycle operation is a mathematical representation of a physical process, its location in a simulation is a particularly important one. The location of the tear stream can often determine success or failure to converge a recycle

Choose a Tear Location to Minimize the Number of Recycles Reducing the number of locations where the iterative process is required will save on the total convergence time. Choosing the location of the Recycle will depend on the Flowsheet topology. Attempt to choose a point such that specifying the assumed stream will define as many streams downstream as possible. It generally occurs downstream of gathering points (mixers) and upstream of distribution points (tees, separators, and columns).

Choose a Tear Location to Minimize the Number of Recycle Variables

A very poor choice of a tear stream is a stream with an Adjust operation controlling one of its variables.

Variables include vapour fraction, temperature, pressure, flow, enthalpy and composition. Choose the tear stream so that as many variables as possible are fixed, thus effectively eliminating them as variables and increasing convergence stability. Good choices for these locations are at separator inlets, compressor after cooler outlets and trim heater outlets.

Choose a Stable Tear Location The tear locations can be chosen such that fluctuations in the recycle stream have a minimal effect. For example, by placing the tear in a main stream, instead of the physical recycle, the effect of fluctuations will be reduced. The importance of this factor depends on the convergence algorithm. It is more significant when successive substitution is used.

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Recycle Exercises Choosing the Right Location When installing Recycle operations in a HYSYS simulation, it is vital that right location for the operation be chosen. Several guidelines were given on a previous page, and several different problems will be given here. Note that some of these flowsheet may require more than one Recycle operation.

Flowsheet 1 Where should the Recycle be placed in this flowsheet and why? Assume that you know the following information:

• • • • •

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Temperature and Vapour Fraction of "Cond Out". Pressure drop and Duty of "Chiller" operation. Pressure of "Chiller Out" stream. Pressure drop of "Condenser" Operation. The Mixer is set to "equalize all."


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Flowsheet 2 Where should the Recycle be placed in this flowsheet and why? Assume that the Feed is fully defined, Shell and Tube Side pressure drops are known, as well as the Column Feed temperature.

Flowsheet 3 Where should the Recycle be placed in this flowsheet and why? Assume the Feed is completely defined, shell and tube side pressure drops for E-100 and E-101, and the temperatures of streams 3 and 4 are known.

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Flowsheet 4 Where should the Recycle be placed in this flowsheet and why? Assume the Feed is completely defined, and the shell and tube side pressure drop for E-100 is known.

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Ethylene Glycol Plant

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