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Abstract

This project models and details the modelling process of two chemical processes. It utilizes Honeywell's UNISIM(formerly HYSIS) to create steady state and dynamic models

Acknowledgements

Richard Ramlogan

Wayne Clarke

Table of Contents

Acknowledgements v

Table of Contents vi

List of Abbreviations and Symbols viii

List of Tables xi

CHAPTER 1: BACKGROUND, OBJECTIVES AND SCOPE 12

1.1 Introduction 12

Chapter 2: Literature Review 14

METHODOLOGY 16

Approach 16

Steady State Simulation 17

Process 1 - Temperature Control Module 21

Steady State Simulation of Temperature Control Accessory 22

3.Defining feed streams 23

4. Installing unit operations 23

Modelling the Plate Heat Exchanger 25

Port Diameter 25

Port Spacing 25

Challenges 26

Distillation Column 27

The Distillation Process 28

DECE Distillation Column 29

Distillation Lab 31

Calculations 33

Steady State Simulation of Distillation Column 40

3.Defining feed streams 41

Installing Unit Operations 42

Valve Unit Operation 45

Distillation column 48

Modelling of the heat exchangers 54

RESULTS 62

DISCUSSION 67

CONCLUSION 69

Bibliography 70

APPENDICES 71

List of Abbreviations and Symbols

PHE

Plate Heat Exchanger

DECE

Department of Electrical and Computer Engineering

VLE

Vapour- liquid equilibrium

LLE

Liquid-Liquid Equilibrium

VLLE

Vapour-Liquid-Liquid Equilibrium

List of Figures

Figure 1: Guidelines for Choosing the Fluid Package. Adapted from (wwilcox 2008) 19

Figure 2: PCT- 13, Schematic Diagram of Temperature Control Module 22

Figure 3: Plate Arrangement 24

Figure 4: Basic Distillation 28

Figure 5: P&ID of the DECE Distillation Column 29

Figure 6: Tank Rating Sizing 43

Figure 7: PFD of inputs to tank to create the feed stream 43

Figure 8: Pump Design Parameters 45

Figure 9: Distillation Column Input Expert Window used for modelling Page 1 of 4 49

Figure 10: Distillation Column Input Expert Window Page 2 of 4 50

Figure 11: Distillation Column Input Expert Window Page 3 of 4 51

Figure 12: Distillation Column Input Expert page 4 of 4 52

Figure 13: Distillation Column Editing Window 52

Figure 14: Distillation Column Monitor 53

Figure 15: PFD with Distillation column inserted 54

Figure 16: Heat Exchanger apparatus for preheating the feed 56

Figure 17: Continuous Distillation implemented via Recycle blocks 58

Figure 18: Completed PFD for the DECE distillation Column Process 59

Figure 19: Simulated DECE column in Dynamic mode 60

Figure 20: Steady State Simulation 62

List of Tables

Table 1: Cold Stream Specifications 23

Table 2: Heat Stream Specifications 23

Table 3: Distillation Column Conventions & Descriptions (Honeywell 2005) 27

Table 4 32

Table 5 32

Table 6: Distillation Stream Specifications 42

Table 7: Heat Exchanger Design Parameters 55

Table 8: Design Parameters for HX Bott.1 56

Table 9: Design Parameters for HX-Tops 57

Table 10: Heat Exchanger Geometry for HX-Bott.2 57

Table 11: Results of Steady State Model @ Reflux = 6.14 62

Table 12: Results of Steady State Model Tops 20 L/hr @ Reflux = 1.07 63

Table 13: Results of Steady State Model Tops 30 L/hr @ Reflux = 0.62 63

CHAPTER 1: BACKGROUND, OBJECTIVES AND SCOPE

1.1 Introduction

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Project Title

UNISIM simulation of DECE Pilot Plants

Project Category

Type I - Research

Background

Today's industrial plants are intricately designed and have complicated control schemes. Control is typically provided by a Distributed Control System (DCS) [1] . Utilizing a dynamic process modeller such as UNISIM provides a highly accurate means of predicting plant behaviour to process stimuli. A combination of both technologies would give yield to optimum performance. Simulations also provide an excellent learning environment for students by reducing learning time and increasing experimentation possibilities by removing limitations such as fear of damage to personnel and equipment. It would enable students to approach actual industrial plants with prior knowledge of its workings and controls and thus increasing student confidence in plant manipulation.

Objective

The objective of the project is to repair a process plant and to use UNISIM to provide an accurate model of the selected chemical processes for the teaching and learning environment.

Scope

This project shall cover the modelling process using UNISIM, the acquisition of modelling requirements, the challenges in the process and the conversion from a steady state model to a dynamic model.

Chapter 2: Literature Review

2.1 UniSim

UniSim provides a means of predicting plant response to input stimuli. (Honeywell 2009) asserts that UniSim offers simulation solutions that improve plant design, operations and optimization using industry-specific unit operation models to obtain steady-state simulation of processes that can be extended into transient models.

UniSim allows process models to be created via piecewise addition of units downstream. Enabling UniSim's solver permits UniSim to automatically perform a steady state calculation of initial values along the stream as units are added. (Mueller 2008)

All information concerning a unit operation can be found on the tabs and pages of its property view. Each tab in the property view contains pages which pertain to the unit operation, such as its stream connections, physical parameters (for example, pressure drop and energy input), or dynamic parameters such as vessel rating and valve information.

2.2 Temperature Control Accessory, PCT 13

2.3 DECE Distillation Column

Lab

Articles

PCT 13 Manual

UNISIM Manual

PHE Help Manual

Theories

Standards

The Trinidad and Tobago OSHA Act #1 of 2004

The Control of Substances Hazardous to Health Regulations 1988

Risk Analysis

WBS

UNISIM Familiarization

Process Familiarization

Repairing the Model

Modelling

Model Testing

Tweaking the Model

Creation of Learning Module

METHODOLOGY

Introduction

Why I'm building this. How is it going to help

Currently, the process plants in the department are failing, out of date, lacking

Safety concern

No. Of students to one plant

Simulations can build student competence and confidence. Control objectives can be implemented free of charge and without fear of damage to equipment. Student simulations can yield solutions for the improvement in operation, design, ..

Approach

Analysis of the process plant

Analysis of

Creation of a steady state simulation via piecewise addition of downstream unit operations. This attempts to validate the input parameters against that of the plant equipment and data to evaluate the model's reliability. Wherever there was insufficient information, use of heuristics supplemented the simulation requirements.

Movement into dynamic mode

Assessment of Process controllability under dynamic conditions

Steady State Simulation

Steady state simulation is used for design, analysis and optimization in the process industry. In the academic sphere, it can be used to promote understanding of processes. It grants students access, control and the chance to experiment with process equipment in the field beforehand even if the equipment is inaccessible due to location or feasibility. Steady state modelling utilizes equations that govern relationships between elements in the modelled system and attempts to find a state of equilibrium and are therefore time independent. The model consists of the physical property data of the chemical components of the input streams and interconnected unit operations. UniSim allows for graphical illustration and configuration in the form of a process flow diagram. This allows students to focus on the engineering prospect of the process instead of mathematical computations.

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The steady state model was developed by the following steps:

1. Selecting the Calculation Unit Set

2. Selecting a Basis

3. Defining feed streams

4. Installing unit operations

1. Selecting the Calculation Unit Set

The calculation unit set refers to the units in which UniSim does its calculation. There are three standard selections: SI, EuroSI, and Field. The properties of a specified unit set can be modified according to the user's preferences

2. Selecting a Basis

A basis comprises the chemical components used in the model and the thermodynamic fluid property package. The property package comprises the pertinent information for flash and physical property calculations and would determine how the model behaves. The selection of an appropriate property package is of paramount importance in process modelling to ensure accurate simulations and to avoid convergence problems and erroneous errors.

There are many property packages. The choice of property package is determined by the components to be modelled, the nature of, pressure and temperature ranges in the process. Figure 1 illustrates a decision tree used as a guideline for choosing the property package.

Figure 1: Guidelines for Choosing the Fluid Package. Adapted from (wwilcox 2008)

3. Defining feed streams

Once the components and the thermodynamic package are selected the feed streams are defined by specifying the process conditions and the composition. In order to define a stream in UniSim it is required to specify two process variables (temperature, vapour fraction, pressure etc.), flow rate and composition. The other conditions of the stream are estimated by HYSYS.

4. Installing unit operations

Unit operations refer to standard plant equipment such as vessels, columns, heaters, coolers and heat exchangers. UniSim uses a 'degrees of freedom' approach, to automatically perform calculations as soon as unit operations and property packages have enough required information. (Honeywell 2005)

Process 1 - Temperature Control Module

The first chemical process pilot plant selected was the Temperature Control Module, PCT 13 which analyzed the temperature gradient across a Plate Heat Exchanger. The process as seen in Figure 2 was studied until its process was understood.

The temperature Control Module comprises a miniature plate heat exchanger specially designed by Armfield, a heating fluid reservoir, circulating pump, variable area flowmeters, manual flow control valves and a motorised flow control valve. The units are mounted in a support frame which was designed to stand on a bench top. Hot water from the heated reservoir was continuously circulated in counter-current flow to cold process fluid provided externally. The cold process fluid was water at 31oC. Temperatures and flow rates of the inlet and outlet streams of the heat exchanger were monitored via thermocouples and flowmeters. Power to the heater was controlled by time proportioning or by on/off control. The flow of the heating fluid was controlled by the proportional motorised valve or by on/off control of the circulating pump. The unit was designed to run with an electric console.

Figure 2: PCT- 13, Schematic Diagram of Temperature Control Module

Source: (Armfield 2004)

Steady State Simulation of Temperature Control Accessory

Constraints: No product specifications available

Selecting the Calculation Unit Set

The default SI calculation unit set was customized so that the flow rate was input in cm3/min in accordance with the calibration of the flow meters

Selecting a Basis

The only chemical component in the temperature control accessory's process was water. By analysis of the process its nature and its chemical components and in accordance with (Walas 1990)'s Chemical Engineering heuristics the ASME steam fluid property package was selected.

Defining feed streams

There exists two streams in the Temperature Control Accessory's model: the cold stream and the heat stream.

The specifications for the cold stream were given as shown in Table 1.

Table 1: Cold Stream Specifications

Temperature

25 oC

Pressure

101.3 kPa

Flowrate

150 cm3/min

Composition

1.0

*Composition expressed in mole fraction of Water

The specifications for the heat stream were as follows

Table 2: Heat Stream Specifications

Temperature

25 oC

Pressure

101.3 kPa

Flowrate

150 cm3/min

Composition

1.0

*Composition expressed in mole fraction of Water

4. Installing unit operations

Heat Plate exchanger

Description

Created by Armfield

Diagram

Figure 3: Plate Arrangement

Source: (Armfield 2004)

Modelling the Plate Heat Exchanger

The geometry of the plate is defined by six key parameters. To determine these parameters the plate heat exchanger was opened and examined.

Port Diameter

Using a vernier calliper the internal diameter of the port in the plate was calculated as 6.8 mm

Program Limitation: The range of port diameters within the program was restricted to a minimum of 20 mm

Port Spacing

Heat Transfer Area

Plate thickness

Chevron Angle

Challenges

Data

Lack of chevron angles in plate exchanger

Broken flow gauge

Incapability in modelling program

Limited knowledge of dynamic influences of assumptions made in the modelling process

Distillation Column

Distillation is a widely used technique for the separation of dissolved liquids in chemical process industries for applications such as the rectification of alcohol and the fractionation of crude oil. This project simulates via UNISIM the DECE Distillation column that separates ethanol from water.

The following table comprises conventions and descriptions of the basic distillation column.

Table 3: Distillation Column Conventions & Descriptions (Honeywell 2005)

Column Component

Convention/Description

Tray Section

A UniSim Design unit operation that represents the series of equilibrium trays in a Column.

Stages

Stages are numbered from the top down or from the bottom up, depending on preference. The top tray is 1, and the bottom tray is N for the top-down numbering scheme.

Overhead Vapour Product

The overhead vapour product is the vapour leaving the Condenser.

Overhead Liquid Product

The overhead liquid product is the Distillate leaving the Condenser referred to as the top liquid product in Distillation Columns

Bottom Liquid Product

In Distillation Columns, the bottom liquid product is the liquid leaving the Reboiler.

Overhead Condenser

An Overhead Condenser represents a combined Cooler and separation stage, and is not given a stage number.

Bottom Reboiler

A Bottom Reboiler represents a combined heater and separation stage, and is not given a stage number.

The Distillation Process

The separation of liquid by distillation depends on differences in volatility between the components. The greater the relative volatilities, the easier the separation. The fundamentals of the continuous distillation process are illustrated in Figure 4. Vapour flows up the column and liquid flows counter-currently down. Part of the condensate is returned to the top of the column to provide liquid flow above the feed point and part of the liquid vaporized in the reboiler is returned to the column to provide vapour flow up the column. (Sinnott 2005)

Figure 4: Basic Distillation

Source: Adapted from (Sinnott 2005)

In the section below the feed, the more volatile liquids are stripped from the liquid while upwards of the feed stream the concentration of the more volatile components is increased i.e. enrichment or rectifying.

DECE Distillation Column

The DECE Distillation Column follows the fundamentals of the distillation process as described previously described. Figure 5 illustrates the process for the DECE distillation column for total reflux. The alcohol feed solution is firstly input into the tank. The feed stream is then pumped through the heat exchanger, HX1, to be preheated before it goes to the distillation column for rectification.

Figure 5: P&ID of the DECE Distillation Column

In the distillation column the more volatile component, ethanol, is stripped from the mixture and its concentration increases up the column. At the bottom of the column where the concentration of water would be highest, the bottoms product is drawn into the reboiler and heated. The vaporized product is sent back into the column to provide vapour flow up the column. The liquid bottoms product is then sent to the heat exchanger, HX1, to preheat the feed and then cooled. The tops product from the distillation column is sent to the condenser where it is cooled. Some of it is returned to the column to provide liquid flow down the column and the rest of the distillate is produced as the product. The DECE distillation column uses the distillate to combine it with the ethanol to form the feed again. This forms a continuous distillation process where the purity of the product increases as time progresses.

Distillation Lab

Feed at a rate of 200L/hr was pumped to the column, the tops product rate was set and the conditions allowed settling down. Samples of feed, tops and bottoms were taken and their densities measured. Appropriate temperatures and flow rates were taken and the reflux ratio measured from the heat balance on the overhead condenser. This was repeated for 2 other values of tops product flow rate.

PROCEDURE

One of the tanks was filled up with alcohol feed solution.

The product lines were checked in order to transfer the products to the appropriate tanks

The pump was started and fed into the column using the entry point on tray 4, counting from the bottom.

The cooling water was turned on to the overhead condenser and product coolers.

When the liquid level in the base of the column reached the overflow the steam supply to the reboiler was cautiously turned on.

The feed rate was set to 200 L/hr.

Once distillate was produced, tops product rate was set to 10 L/hr and conditions were allowed to settle down.

Samples of feed, tops and bottoms were taken and the density measured. Samples were cooled down to below 40oC. Temperature was measured also.

Appropriate temperatures and flow rates were taken. The reflux ratio was measured from the heat balance on the overhead condenser.

The tops product rate was changed to 20L/hr and repeated.

The tops product rate was changed to 30L/hr and repeated.

When the experiments were completed, the steam supply, the feed pump and the cooling water were shut off in that order, and the column was allowed to drain.

Results

The tables below contain the data obtained from distillation column.

Table 4

ENVELOPE

FEED FLOW RATE KG/HR

TOPS PRODUCT RATE L/HR

FEED TEMP

TOPS TEMP

RELATIVE DENSITY

BOTTOMS TEMP

RELATIVE DENSITY

I

200

10

74

61

0.92

85

0.99

II

200

20

74

65

0.94

85

0.99

III

200

30

76

65

0.95

87

0.988

Initial temperature of feed = 31

Relative density feed = 0.982

Water flow rate = 17.5 L/min @20

Table 5

REFLUX TEMP

TEMP OF VAP

LEAVING

HEAT EXCHANGER TEMP

INLET

OUTLET

45

81

29

54

58

80

29

45

60

82

29

48

Condensate flow rate:

Mass=23lbs

Time of collection = 4mins 51.13s

Temperature =93

Pressure = 15psi

Temperature of bottoms = 87

Temperature of feed = 78.5

Temperature of tops = 104

Calculations

The following calculations were done to achieve the reflux ratio of the column.

Let x = mass fraction of ethanol

⇒ (1-x) = mass fraction of water

Molecular weight of water = 18.02 g

Molecular weight of ethanol = 46.07 g

Density of ethanol, = 789 kg/m3­

Density of water, = 1000 kg/m3

Moles of ethanol = x/46

Moles of water = (1-x)/18

Determination of mass fraction of ethanol

Density of a mixture of ethanol and water is given by the following equation:

..................................................................................................2

Considering a product rate of 10 L/hr:

From Table 4, the relative density of this stream is 0.92

Hence, density of this stream = Relative density

=

= 920

Substituting values into equation 1:

Mole fraction of ethanol = 0.19

Mole fraction of water = 0.81

Calculation of mean specific heat capacity of mixture, cm

..................................................(3)

Where cm = mean specific heat capacity of mixture

ce = specific heat capacity of ethanol = 2.118 kJ/kgK

cw = specific heat capacity of water = 4.18 kJ/kgK

Calculation of mass flow rate of water, M

Volume flow rate = 17.5 L/min

= 17.5 L/min / (1000 L/m3) (60s/min)

= 0.000292 m3/s

Mass flow rate, M = volume flow rate Ã- 1000

= 0.2916 kg/s

Calculation of gas flow rate, G

To calculate G a heat balance around the condenser was performed

Heat lost by vapour in condenser = Heat gained by condenser water

The vapour in the condenser

.................................................(4)

Where G = gas flow rate from the top of the column

= mean heat capacity of gas = 3.39 kJ/kgK

= temperature change in ethanol = 36 K (Table 5)

= temperature change in water = 25 K (Table 5)

= latent heat of vaporization of ethanol = 846 kJ/kg

= latent heat of vaporization of water = 2267 kJ/kg

M = mass flow rate of water = 0.2916 kg/s

G was solved using Equation 4

Calculation of Reflux Ratio, R

..........................(6)

Using tops product of relative density = 0.92 (Table 4)

Tops rate = 10L/hr

Converting to a flow in m3/s:

Mass flow rate, D =

Reflux Rate, L = Gas exiting column Rate, G - Distillate Rate, D

Recall Equation (6)

Reflux Ratio, R =

=

= 6.11

Steady State Simulation of Distillation Column

The model was constructed under the same conditions as the lab. Firstly the input streams were determined.

Constraints: No product specifications available

Selecting the Calculation Unit Set

The default SI calculation unit set was customized so that the flow rate was input in L/hr in accordance with the calibration of the flow meters

Selecting a Basis

The DECE Distillation column rectifies a mixture of ethanol and water.

In the Simulation Basis Manager under the Components tab water and ethanol C2H6O was added to the components list.

Ethanol

Ethanol's covalent bonding of its oxygen atom makes it a polar molecule. It is a non-electrolyte since it does not form any ions when mixed in water. The distillation apparatus does exceed a pressure of 10 bars (1MPa). Based on these properties the fluid package UNIQUAC was selected.

The UNIQUAC (UNIversal QUAsi Chemical) equation uses statistical mechanics and the quasi-chemical theory of Guggenheim to represent liquid structure. It provides a model for LLE (Liquid-Liquid Equilibrium), VLE (Vapour-Liquid Equilibrium) and VLLE (Vapour-Liquid-Liquid Equilibrium).A recent paper on thermodynamics claims that the UNIQUAC equation is significantly more detailed than the other fluid packages and better at representing VLE and LLE for a large range of non-electrolyte mixtures containing H2O and alcohols. (cadfamily 2003)

Defining feed streams

Laboratory

The tank was filled with the alcohol feed solution at atmospheric pressure and at room temperature i.e. 101.3kPa @ 31oC. The feed solution comprises ethanol and water such that the density of the feed stream was 982 kg/m3. The flow rate of the feed solution into the column was set to 200L/hr.

UniSim

The two feed streams in the process model were the feed stream and the utility fluid, used by the condensers and coolers.

All streams entered the process at atmospheric pressure.

The feed stream was defined such that it had the same properties of the alcoholic feed solution used in the lab.

The specifications for the three streams are as shown in Table 6.

Table 6: Distillation Stream Specifications

Stream

Feed

Cold(Utility)

Temperature

44 oC

20

Pressure

101.3 kPa

101.3 kPa

Flowrate

200 L/hr

17.5 L/min

Composition

[0.035, 0.965]

[0.0, 1.0]

*Composition expressed in mole fraction [Ethanol and Water]

Installing Unit Operations

Tank

The tank unit operation is described as possessing multiple feeds, one liquid and one vapour product stream. The tank is generally used to simulate liquid surge vessels. In this instance it was used to simulate the sump into which the feed was fed. Figure 6 shows the design requirements for the tank. The length of the tank was physically measured and input into the tank design window along with the tank shape.

Figure 6: Tank Rating Sizing

Figure 7 illustrates the tank placed onto the process flow diagram.

Figure 7: PFD of inputs to tank to create the feed stream

Pump

The feed from the tank is then pumped to the distillation column. UniSim's calculations are based on the standard pump equation for power, which uses the pressure rise, the liquid flow rate, and density:

..........................(7)

Where:

The accumulation of paint on and the wearing down of the pump's nameplate provided some difficulty in obtaining the pump's ratings.

Using some "paint thinners" the duty of the pump was obtained from the nameplate. The pump had a duty of 0.66kW. The Specification of the pump's duty in addition to the known flow rate of the feed stream allowed UniSim to calculate the other process parameters as shown in Figure 8.

Figure 8: Pump Design Parameters

Valve Unit Operation

UniSim performs a material and energy balance on the inlet and exit streams of the Valve operation. The list of variables that can be specified by the valve operation are:

Inlet temperature

Inlet pressure

Outlet temperature

Outlet pressure

Valve Pressure Drop

Three specifications are required before UniSim solves the operation. Inlet temperature and pressure are provided by the feed stream. The valve outlet pressure could not be found and therefore the valve pressure drop needed to be specified.

Gate valves were used to control the flow rates in the process. The logos on the valves determined that its manufacturer was a German company named Herose. The valves were rated PN16 on the valves' product specifications(see appendix). This meant it had a nominal pressure of 16 bars.

The valve equation was then used to calculate CV at nominal conditions

................................................(8)

Where,

The resulting value of Cv was 495.45

Therefore when

This specification allowed the valves to be solved.

The Valve Operating Characteristics was defined as Linear so that the % flow was proportional to the percentage lift.

Distillation column

Installation of the Distillation column unit proved to be difficult in building the simulation model. It consists of a series of equilibrium or non-equilibrium flash stages and has many parameters. It has a special type of sub flow sheet that contains equipment and streams, and exchanges information with the parent flow sheet through the connected internal and external streams.

By assessing the number of draws and inlet streams into the column it was determined that the distillation column comprises 5 stages. The feed was pumped to tray 4 of the distillation column. Figure 9 shows the distillation column input expert used to model the distillation column. The first of four pages requires the provision of inlet stream names, product outputs, energy streams. There is also an option for side draws for the column.

Figure 9: Distillation Column Input Expert Window used for modelling Page 1 of 4

The DECE distillation column was designed to operate at atmospheric pressure because it was intended for an academic environment. The pressure measured at the condenser was 15psi which converts to 101.3kPa ie atmospheric pressure. The pressure at the condenser and the reboiler were hence both assumed to be at atmospheric pressure.

Figure 10: Distillation Column Input Expert Window Page 2 of 4

The third window of the Distillation Column Input Expert asked for temperature estimates for the condenser, top stage and reboiler. This request was optional so as to make the convergence process (i.e. the mathematical method at which UniSim arrives at its simulated values) faster. Using the raw data obtained from the lab as shown in Table 4. The temperature values of the bottom and top feeds were used as estimates for the reboiler and condenser estimates as seen in Figure 11.

Figure 11: Distillation Column Input Expert Window Page 3 of 4

The fourth window of the Distillation Column Input Expert asks for the tops output rate from the condenser and the reflux ratio.

Tops Output Rate

Following the lab's design, the tops rate was set to 10 L/hr.

Reflux Ratio

The reflux ratio was calculated to be 6.14.

Figure 12: Distillation Column Input Expert page 4 of 4

Upon completion of the fourth column input page the Distillation Column Editing Window appears.

Figure 13: Distillation Column Editing Window

The Run button was the used to start the convergence algorithm through which UniSim Design first performs iterations toward convergence of the inner and outer loops (Equilibrium and Heat/Spec Errors), and then checks the individual specification tolerances. (Honeywell 2005)

The Design Monitor page then gives a summary of the iteration process, displays the specified and current values of input specifications and the column temperature, pressure and flow profile.

Figure 14: Distillation Column Monitor

Figure 15: PFD with Distillation column inserted

Modelling of the heat exchangers

The top product was then cooled via heat exchanger. The bottom stream was then used to preheat the feed stream into the column to a temperature around 74oC before it too was cooled by another heat exchanger. The both products was cooled to temperatures below 40oC. The water used in the shell side of the exchanger had a flow rate of 17.5 L/min @ 20oC atmospheric pressure.

There are three heat exchangers to be modelled.

HX-bott.1: The heat exchanger used to preheat the feed before it enters the distillation column by transferring the heat from the bottoms product to the feed.

HX-tops: The heat exchanger used to cool the tops product below 40oC.

HX- bott.2: The heat exchanger used to cool the bottoms product below 40oC.

The table below lists the heat exchanger's geometric design parameters.

Table 7: Heat Exchanger Design Parameters

Delta P (Tube Side)

The pressure drop along the tube as the fluid passes through the heat exchanger

Delta P (Shell Side)

The pressure drop across the heat exchanger as it passes through the shell

UA

The product of the Overall Heat Transfer Coefficient, and the total area available for heat transfer.

Tube Passes per Shell

The number of times the tube travels the length of the shell to increase the length of the flow path.

Shell Passes

The number of times the fluid passes through the shell

Shells In Series

Number of heat exchangers occurring in series.

Shell TEMA Type

The Tubular Exchanger Manufacturers Association standards created for the designation of tubular heat exchangers.

Without the design specification of the heat exchangers, these parameters would be unable to be obtained. The modelling of the heat exchangers was left to the use of heuristics.

(Walas 1990) found that pressure drops in heat exchangers are 1.5 psi for boiling and 3-9 psi for other services. He also found that heat transfer coefficients for estimating purposes was 150 Btu/(hr)(sqft)(oF) for water to liquid.

Note: The academic package of UniSim in Weighted or End point Engineering design  does not provide for the specification of heat transfer area, A. There is a preset of 60.32m2 for the heat transfer area.

HX-Bott.1

The heat from the bottoms product preheats the feed stream into the distillation column. This heat exchanger consists of 8 stacked heat exchangers running in series as shown in Figure 16.

Figure 16: Heat Exchanger apparatus for preheating the feed

The table below shows the design parameters for the heat exchanger that preheated the feed. From the lab the feed stream was preheated to a temperature of 74oC from an initial temperature of 31oC.

Table 8: Design Parameters for HX Bott.1

Delta P (Tube Side)

0 bar

Delta P (Shell Side)

0 bar

UA

2700 kJ/C-h

Tube Passes per Shell

1

Shell Passes

1

Shells In Series

8

Shell TEMA Type

E

The bottoms product was used to heat the feed then it was cooled down to below 40oC.

HX-Tops

Utilizing

Table 9: Design Parameters for HX-Tops

Delta P (Tube Side)

0 bar

Delta P (Shell Side)

0 bar

UA

9000 kJ/C-h

Tube Passes per Shell

2

Shell Passes

1

Shells In Series

1

Shell TEMA Type

U

Table 10: Heat Exchanger Geometry for HX-Bott.2

Delta P (Tube Side)

0 bar

Delta P (Shell Side)

0 bar

UA

300 kJ/C-h

Tube Passes per Shell

2

Shell Passes

1

Shells In Series

1

Shell TEMA Type

U

Continuous Distillation

Since UniSim calculates on a forward path, reintegrating a calculated variable early into the path would cause the whole path to become uncalculated. To enable feedback into the beginning of the process a recycle block was used. A recycle block allows for its input to be copied for reintegration into the process.

The distillation process was made continuous by returning the tops product back to the tank via a recycle block.

A recycle block was also used to allow the bottoms product to heat the feed entering the column.

Figure 17: Continuous Distillation implemented via Recycle blocks

Completion of Steady State Design

Figure 18: Completed PFD for the DECE distillation Column Process

Step 5: Move from Steady State to Dynamic State

Dynamic Mode

Face plates were configured for all three controllers. And stripcharts were set up. This allows us to graphically see the Process Variable (PV), Set Point (SP) and Controller Output (OP) of the three controllers.

Figure 19: Simulated DECE column in Dynamic mode

Estimates & Guesses

UA of the heat exchanger

RESULTS

Steady State Simulation

Figure 20: Steady State Simulation

Table 11: Results of Steady State Model @ Reflux = 6.14

Parameter

Actual Plant

Steady State Model

Feed Temperature (oC)

74

74

Top Temperature (oC)

61

79

Bottom Temperature (oC)

85

95.51

Gas Flow (kg/s)

0.0182

0.01643

Reflux Flow (kg/s)

0.01565

0.01412

Distillate Flow (kg/s)

0.00255

0.0023

Feed Composition

[0.035, 0.965]

[0.035, 0.965]

Distillate Composition

[0.193, 0.807]

[0.617, 0.382]

Bottom Composition

[0.019, 0.981]

[0.021, 0.979]

Top Relative Density

0.92

0.84

Bottom Relative Density

0.99

0.99

The column parameters were then altered for a distillate product rate of 20 L/hr with a reflux ratio of 1.07.

Table 12: Results of Steady State Model Tops 20 L/hr @ Reflux = 1.07

Parameter

Actual Plant

Steady State Model

Feed Temperature (oC)

74

74

Top Temperature (oC)

65

79.82

Bottom Temperature (oC)

85

95.51

Gas Flow (kg/s)

0.0108

0.01643

Reflux Flow (kg/s)

0.00558

0.01412

Distillate Flow (kg/s)

0.00522

0.00470

Feed Composition

[0.035, 0.965]

[0.035, 0.965]

Distillate Composition

[0.129, 0.871]

[0.4894, 0.5106]

Bottom Composition

[0.019, 0.981]

[0.0102, 0.9898]

Top Relative Density

0.94

0.86

Bottom Relative Density

0.99

0.989

The column parameters were then altered for a distillate product rate of 30 L/hr with a reflux ratio of 0.62.

Table 13: Results of Steady State Model Tops 30 L/hr @ Reflux = 0.62

Parameter

Actual Plant

Steady State Model

Tops Product Rate of 20 L/Hr @ Reflux= 6.14

Feed Temperature (oC)

74

74

Top Temperature (oC)

65

80.79

Bottom Temperature (oC)

85

100

Gas Flow (kg/s)

0.0128

0.01169

Reflux Flow (kg/s)

0.00488

0.00447

Distillate Flow (kg/s)

0.00792

0.00721

Feed Composition

[0.035, 0.965]

[0.035, 0.965]

Distillate Composition

[0.108, 0.892]

[0.3718, 0.6282]

Bottom Composition

[0.039, 0.961]

[0.0022, 0.9978]

Top Relative Density

0.95

0.894

Bottom Relative Density

0.98

0.995

Dynamic Results

DISCUSSION

Analysis and Evaluation of results

Limitations

No plant specifications

Results, Description

Description of recommendations

CONCLUSION

Future Work : Running the Column

Simulators are very useful in engineering, but in some cases it is very hard to make models that are accurate enough. After all, a model still represents an ideal world of known circumstances. In order to represent the reality a model needs to be carefully validated. Only real tests can reveal deficiencies of a model. On the other hand, testing is not always possible due to risks of failure and high costs.