University of Florida Department of Chemical Engineering

A Dynamic Simulation of the Unit Operations Lab West Column Undergraduate Honors Thesis

Timothy Heneks 7-31-2013

Abstract

The Unit Operations Laboratory is primarily used for hands on teaching labs and contains,

among other equipment, the West Column. This particular distillation tower is treated in

experiments as a continuous column. This projects aim was to simulate the column using the

process design software UniSim both in the Steady State and Dynamics modes. The steady

state model utilized data and calculations previously collected and recorded by students to

ensure an accurate simulation. Once completed, the steady state model was converted to a

dynamic model for the purposes of adding detail that would better mirror the equipment seen in

the laboratory and allowing for controllers to be introduced and manipulated. The process of

building this dynamic model is much more in depth than that of steady state Thus, it required

quite a bit of trial and error, estimation and creativity to construct a dynamic model that closely

resembles that which is physically present in the Unit Ops Lab. Once the bare dynamic model

was complete, several control strategies were implemented within the software without success

and the model that was created seemed to be unstable. Key learnings were still obtained and

possible causes were identified for this instability.

Introduction

The West Column of the Unit Ops Laboratory is used as a teaching aid for the University

of Florida Chemical Engineering Department for ECH4404L or Unit Ops 2 lab. It facilitates the

continuous distillation experiment within the course and is similar, but not the same as, the East

Column used for batch distillation. The two primary components of the West Column feed are

isopropyl alcohol (IPA) and ethanol with some small percent of methanol in the mixture. This is

pumped down from a tank on the third floor to the column that rises from the first to the second

floors. The tower is 24 trays with a reboiler below and total condenser overhead. This column

will be simulated in the process design program UniSim which is essentially equivalent to

HYSYS first in steady state and later in dynamic mode. The purpose is to create a suitable

dynamic simulation of the distillation system in order to better understand how the column

reacts to any changes introduced. The ultimate goal of this project is to thoroughly model a

control scheme that stabilizes the system under varying conditions. Currently, students running

this column manually control all valves (some done from the control room) to achieve a steady

state. This is written with the expectation that some prior knowledge of the UniSim or HYSYS

software and its workings is known on the part of the reader and that the reader has a thorough

knowledge of chemical engineering processes.

Methods and Results

Steady State Simulation

This steady state simulation, shown in Figure 1, is a close replication of the West

Column of this Departments Unit Ops Laboratory. The simulation was modeled by using data

obtained from one of the previous groups of the ECH 4404L. After inputting all streams and

solving the column, efficiencies and stream compositions were varied to find the closest

possible model for the given data. The parameters for each of these streams is laid out in the

workbook, Figure 2.

Figure 1: Steady State Simulation of West Column

Figure 2: Workbook for Steady State Simulation

To gather data, this group first deter mine d a mini mum re flux ratio, Rmi n, by running the column under full refl ux a nd applying the McCabe -Theile method. Appendi x A contains the M cCa be-Theile drawing s and full set of data. This also allowed the groups to deter mine an overall effi ciency of t he col umn. This group then ra n the column at a spe cifie d reflux ratio, R, w hich must be larger than Rmin. I n this ca se, the refl ux ratio was to be 16. T he column was the n run at constant Fee d rate and composition fr om a tank on the third fl oor whi ch was approximate d to be well mi xed. The feed of methanol, ethanol and isopropanol was pumpe d to the column and entere d into the eight h tray from the bottom. After a mple time passe d to allow the column to rea ch steady state, sa mple s were collected from sa mple ports at the fee d, distillate and bottoms streams. Compositions were deter mine d using a gas chr omatograph. Figure 3 shows the re sults obtained:

Figure 3: Steady State data gathered on West Column when R = 16

Comparing this experimental data to the above modeled values, it is clear that this is a

somewhat accurate steady state simulation to use as foundation for a dynamic model. Given

that some of the parameters were unknown, the following assumptions were made:

Since the feed is pumped down from at least one floor above to the tower, a head of 6.5

psi was added to the initial pressure of 14.7 psia.

An electric heater is installed on the feed line before the control valve and it will be

assumed to heat the feed to roughly 100 F.

Condenser pressure assumed to be 15 psia and reboiler pressure to be 17 psia.

Pressure drops for heat exchangers assumed to be roughly 0.5 psi.

The pressures and pressure drops assumed above will become increasingly important as the

simulation is transitioned into a dynamic state.

Setting Up the Dynamic State Simulation

As mentioned above, dynamic processes must first be modeled in the steady state.

However, once the steady state simulation is completed, more detail can be added and useful

tools such as control loops can be implemented. This would be the most useful function of the

dynamic model, to evaluate how the column would react to a disturbance or a change of inputs.

Since the dynamic model deals with changing flows, temperatures, levels, pressures,

etc., it is necessary and important to take measurements of all vessels and to accurately detail

where equipment, valves, and instrumentation are spaced in relation to each other. Before

working on this model, a detailed P&ID was drawn up and certain pertinent length and volume

measurements were taken from the column, trays, sight glass, condenser and reboiler. The

values are of course estimates in most cases as insulation covers most of the piping and

equipment near the condenser, reboiler and column. These measurements were then inserted

to the model appropriately to help make it as accurate as possible. Figure 4 below gives the

measurement values utilized.

Figure 4: Volume Measurements of Various equipment at West Column

Additionally, the gravitational energy that drives the overhead liquid through the sight

glass and into reflux and distillate streams is compensated for in this dynamic model through a

simulated pump that runs at 100% efficiency. Although this will cause a slight temperature rise

as the pressure head is created, it is assumed to be small and inconsequential to the system.

This pump is making up for the column of water that is usually about 4 feet high relative to the

equipment and top tray. Thus, a 4-foot head is applied to the pump shown in the final PFD.

After further investigation of the electric pre-heater described above, no evidence was

found that the heater is working or that the feed must be pre-heated before entering the column.

It will be neglected from the model moving forward. Other assumptions will be made in the

process of constructing the dynamic model as they appear. For example, each valve must have

a pressure drop associated with it before a pressure-flow relationship can be developed. In such

a case, sound judgment must be used to ensure reasonable estimations are made since the

pressure drop through these valves is not known. Furthermore, without a chance to thoroughly

vet the column and run experiments specifically for this investigation, no confidence can be

given to the flow, pressure or temperature at any point in the system. Thus, estimation is vital to

the success of this model.

Building the Dynamic Simulation

After constructing the steady state model, collecting relevant information from the Unit

Ops Laboratory and deciding how to make certain assumptions, the next step is switching the

software from the Steady State Mode to Dynamics Mode. At this point, the solver in the program

is turned off and no values are calculated until the time Integrator runs and calculates values as

they change with time.

To ensure that the Integrator will run and that the system is properly defined with

degrees of freedom equal to zero, each boundary stream must be defined as having a user

defined pressure or flow. Either, neither or both can be chosen within the software for each

stream depending on the needs of the model. It should be noted that all streams and unit

operations have a Dynamics tab that allows for these specs to be selected and defined as well

as sizing information for vessels, valves, heat exchangers and columns.

Once the User Defined Variables are set, it is useful to ensure that the number or

variables equals the number of equations in the background solver of UniSim. Figure 5 shows a

view of the Equation Summary tool after the streams and condenser were first defined. Notice

that although there are 209 each of equations and variables, only 5 were user defined. As the

dynamic model is built, these numbers will grow with each piece of equipment or stream that is

added.

Figure 5: Equation Summary showing degrees of freedom as 0.

For all practical purposes, Figure 5 above shows a dynamic simulation that would run on

within the program and could possibly give some useful information. However, the West Column

system is a complex and detailed setup that requires the addition of many new pieces of

equipment and new steams to accurately depict the setting.

Building a Detailed Dynamic Simulation

In order to accurately depict the West Column, the condenser will be replaced by a

cluster of new operations including a heat exchanger, accumulator (sight glass), control valves,

splitters and pumps. To do this, the Column Environment must be entered from the Parent

Environment. This means that the PFD will show only that which is boxed in around the West

Column from Figure 5. Upon entering the Column Environment, the PFD will look as shown in

Figure 6 on the next page displaying the columns tray section (as Main TS), the reboiler and

condenser as well as the streams to and from this equipment.

This basic layout shown above in Figure 6 is the default for UniSim and can be altered

simply by adding and removing unit ops and streams. For example, to more closely match the

setup found in the Unit Ops Lab, the condenser will be removed and replaced by a heat

exchanger, vessel and splitter so that the cooling comes from a stream of water rather than an

energy stream. Additionally, as mentioned above, a pump will be added immediately

downstream of the Sight Glass to provide the pressure generated from gravity. It is important to

note that the model being designed at the moment is looking to mirror what is seen on the West

Column as closely as possible. Overhead features such as vapor bypass on the heat exchanger

or vent control on the vessel will not be modeled. However, such features should be considered

in future designs as it increases the stability of the model.

Figure 6: Column Environment sub-flowsheet showing internal equipment and streams of West Column

Before making any changes to the West Columns sub-flowsheet, it is necessary to

make changes to the columns Solving Method. Currently, the solver is set on the default option

of Legacy Inside-Out. In order to run the unit ops necessary to properly model the system,

Solving Method for the West Column must be switched to Modified Inside-Out. Figure 7 shows

the dialog box in which the Solving Method has been correctly converted.

Figure 7: Dialogue box showing where to switch Solving Methods

With the columns solver issue resolved, the condenser, its duty stream and the distillate

stream can be deleted from the sub-flowsheet. In their place will be inserted a heat exchanger,

the vessel modeled after the sight glass, a pump to simulate gravity, a splitter and control valves

for reflux and distillate streams, respectively. Additionally, a water stream and control valve will

be added to condense and cool the overhead vapor as it passes through the heat exchanger.

This process must be done one element at a time ensuring that the Integrator function is utilized

regularly to give each new stream and equipment calculated values. It is also an exceptionally

bright idea to save frequently under new file names to ensure that each step in the process of

constructing the model is accessible if a mistake is found later on.

As discussed before, pressure drops for the valves and heat exchanger are vital for

establishing pressure-flow relationships. Conductance through the heat exchanger and for the

valves (k and Cv, respectively) are calculated by UniSim for each equipment individually. This

calculation is done by specifying a flow and pressure drop, then later pressing a Size Valve or

Calculate Ks. After a proper k or Cv is calculated for the equipment, the Pressure Flow Relation

or k specification is activated while Delta P becomes unspecified, depending on whether it is a

valve or heat exchanger. Similarly, the stream that has a specified flow should be changed to a

pressure specification. Figure 8 shows an example of the dialogue box for a valve in which a Cv

has already been sized and the Pressure Flow Relation was activated. In contrast, Figure 9

shows the dialogue box for a heat exchanger where Ks have been calculated and specified.

Figure 8: Conductance, Cv, calculated for valve

Figure 9: Conductance, k, calculated for heat exchanger

Once fully complete (including control loops), only the pressures of boundary streams

should be specified. Such specifications allow for the control valves to dictate the flow as is the

case is the field. Figure 10 shows the process sub-flowsheet for the West Column after adding

all the operations necessary to accurately model the overhead section of the process. Figure 11

below it shows the values for variables of all streams and operations through a view of the

Workbook.

Figure 10: Process sub-flowsheet after modeling the overhead section of the West Column

Figure 11: Workbook showing values for the modeling of the overhead section of the West Column

Notice that the Fict V stream appears on the Workbook but not on the PFD. This is

because the vessel simulating the sight glass is obligated to have 2 outlet streams separating

vapor from liquid. To ensure that the model keeps consistent with the hardware in the lab, the

flow of vapor out of (or into) the vessel has been specified as zero. Notice also that there are no

other flow specifications, only pressure and temperature. It is worth noting that this Workbook

does not display any of the streams that lie in the Parent Environment.

Inserting Control Loops to the Dynamic Simulation

The true power of the Dynamics Mode in UniSim is the ability to create and manipulate

controls systems, most commonly the PID controller. In this simulation, a PID was added to the

Reflux Valve to attempt to control the reflux flow and another was added to the Distillate Valve

to control the level of the Sight Glass. In doing this, the bottoms stream remains at the same

constant flow as it has since the steady state model. Figure 12 shows the PFD with the PIDs in

place.

Figure 12: PFD, Integrator and Face Plates showing PI controllers on Reflux and Distillate Valves

When viewing Figure 12 above, the Face Plates in the bottom right corner are used for

each of the controllers to set the position of the valve on Manual or to apply a set point when on

Automatic.

Unfortunately, this control scenario did not yield any useful data due to the instability of

the To Condenser stream. With much effort expended, no solution was found to keep the

overhead vapor stream from blowing up or continuously increasing to infinity. This could

possibly be solved by adding a valve just past the stream and using a control loop to control the

flow. This was not done in simulation however because there is no such valve in place and the

goal of this project is to model this column as closely as possible.

Another attempt was made to control the To Condenser stream by using the reboiler

duty control valve and a PI controller. Figure 13 shows the PFD of the attempt. Again, no

success was had due to the pressure of the overhead vapor blowing up as the duty was

increased.

Figure 13: PFD showing PI controllers on Reflux valve and reboiler duty

Conclusions

After putting together a basic steady state model of the West Column that gave a rather

accurate depiction of the results obtained in lab, that steady state model was then converted

into a more detailed dynamic model. Defining the intricacies of the actual system as it physically

sits, this became an exercise in how closely the software could match the equipment that was in

the lab. Many sources of error could and most likely did creep into the model and provide for the

unstable overhead vapor situation that did not allow for the control loops to be correctly

implemented. Almost all pressure and pressure drop specifications were estimates and the

driving force for reflux and distillate flow was a pump that did not exist in the lab.

Since so much of this projects findings relied on a model fitted with control loops that

produced stable results, there is not many insights to be made with regard to the columns

reaction to change. However, key learnings were taken away dealing with the dynamic design of

columns in the future. For example, using a control valve to limit what goes to the heat

exchanger and a vent on the accumulator (sight glass) could greatly increase the stability of the

overall system. Generally, more time was spent on finding the types of control loops that did not

work than identifying ways to make this particular model stable.

Although the original goals of this project were not fully met, this experience could be a

useful tool to those looking to better understand the Unit Ops Lab West Column in the future,

especially in regards to process control. If given the opportunity, another project could be

introduced to define a more usable model that could in turn give quality projections of what to

expect when a change in the system occurs.

Acknowledgements

I would like to sincerely thank Dr. Spyros Svoronos for helping to find this opportunity for me

when I believed that there would be no avenue to complete an honors thesis. You have taught

me so much over this last year, especially with regard to UniSim, and am proud to know how

much you are doing to make this one of the premier undergrad ChE programs in the country. To

you sir, I am in constant awe of the dedication you show toward the students.

I would also like to thank Dr. Johns and Dr. Koopman for sitting on my defense committee. I

have had the pleasure of working under each of you in coursework and have great respect and

admiration for you both.

Appendix A:

Rmin Calculation

y' 0.677

x' 0.660

xD 0.910

Rmin 13.706