control of the fractionator top pressure for a delayed coking unit

Journal of Applied and Industrial Sciences, 2015, 3 (1): 12-23, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE)

Research Article 12

Control of the Fractionator Top Pressure for a Delayed Coking Unit in

Khartoum Refinery

Salah Eldeen F..Hegazi 1, Gurashi Abdallah Gasmelseed

2 , Mohammed M .Bukhari

3

1Department of Chemical Engineering,Faculty of Engineering ,Jazan University,Saudi Arabia .

Email :[email protected] 2 Department of Chemical Engineering, Faculty of Engineering, University of science &technology -Sudan-

Email :[email protected] 3 Department of Chemical Engineering, Faculty of Engineering, University of Bengahzi –Libya

(Received November 02, 2014; Accepted February 28, 2015)

Abstract

-The control of the fractionator top Pressure for

delayed coking unit in Khartoum refinery, by keeping

it in certain limits is essential to adjust disturbances

which lead to deviation from the desired values. The

aim of this paper is to select the suitable controller and

the appropriate tuning method. The control loop is

designed depending upon controllability and

performance. The column top pressure cascaded with a

reflux rate. The mathematical model for the loop was

determined using MATLAB toolbox system

identification by designing Graphical User Interface

(GUI). The same was used to identify the transfer

function for each loop. Having identified the transfer

functions, the loop was closed and the characteristic

equation was obtained from the overall transfer

function using Routh-Hurwitz together with the direct

substitution method to get the ultimate gain and

ultimate period, from which the adjustable parameters

were determined using Z-N criterion. These were used

to investigate the offset upon a unit step change in the

set point using proportional only (P), proportional

integral (PI) and proportional integral derivative

controller (PID).The controller that gave the minimum

offset is found to be proportional integral derivative

controller ( PID) and has been selected. The procedure

is repeated using Root-Locus and Bode criterion. In

each case the controller that gave the lowest offset was

selected. The three methods of Routh, Roots Locus and

Bode were compared according to their offset and

found to be within good accuracy. However, for good

initial estimate of the adjustable parameters, it was

recommended to apply proportional gain (kc=2.5,

integral time ( =0.75min ) and derivative time(

for system tuning, with respect to

the PID controller. Having closed the loops in

conventional system, the same was transformed to

digital computer control. The analysis of the offset

investigation and system stability were performed on

(Z-domains). The results are in agreement with

conventional analysis.

Index terms: Delayed coking, Roots Locus, Bode ,

tuning, conventional.

I. INTRODUCTION

Control of Delayed coking unit Coker

Many oil refineries have nowadays integrated or are

planning to develop a fairly complex unit called Delayed

Coking Unit (DCU). The purpose of DCU is upgrading

petroleum residue by thermal cracking[1] Most aspects of

the DCU have been addressed previously modeling and

simulation optimization process control [2];

[3];[4],[5;[6];[7],[8];[9].

Delayed coking is one of the most difficult

refinery units to operate and control. The unit takes

vacuum residue (fresh feed), heats it and injects it into the

main fractionator bottom, where it is mixed with an

internal reflux recycle of heavy cracked material. The total

fresh and recycled feed is then heated in the coker furnace

to a high cracking temperature. Coke remains in the drums

and is periodically removed. That is the main reason for

the coker being a difficult unit to operate. Twice daily

filled coke drums are switched off for coke removal and

empty drums are connected. The drum that was just filled

then goes through a cycle of steaming out, cooling,

opening, coke removal, closing, steaming and pressure

testing, heating and finally reconnecting to the furnace and

fractionator. [10]

Al-fula crude is pumped from the surge tanks unit through

several heat train exchangers and into the fractionator. The

flow rate of coker feed is controlled upstream of the

exchangers by the flow controller leading to the

fractionating tower. Coker feed is preheated through three

heat exchangers prior to entering the fractionator.

Controlling the temperature of the Product quality is

controlled by throttling the amount of reflux pumped back

to the tower at various control points.[11]

Problem Statement

During the operation of the delayed coking unit

in Khartoum refinery, the pressure at the top of coking

tower is raise in severe leads to breakout the valve and

lack of control. The adjustment of the operating top

pressure of the delayed coking unit in its desired value by


selection the best techniques of control is the aim of this

paper.

Paper objectives

General Objectives

To develop a mathematical model using graphical

user inter-phase (GUI). In order to test the

performance of delayed coking unit at Khartoum.

Refinery.

To control of the process using controller that gives

the best performance .

To compare between the performance and best

selection tuning method.

Specific Objectives

To determine the transfer function from model using

system identification.

To determine the optimum parameters; Kc, τI and τD .

To tune the feedback controllers used in delayed

coking unit.

To compare the accuracy of Bode criteria, Root Locus

method and Routh test.

II. MATERIALS AND METHODS

System Identification

System Identification is the art and science of building

mathematical models from measured input-output

data. To examine the measured data, and create new

data sets from the original one by various

preprocessing .Then the models will be estimated

using the obtained data.

Identification: get the system transfer function

Get the closed loop characteristic equation of the

system.

Investigate offset for the system by calculation using

Routh to get ultimate gain (Ku) ,ultimate period (Pu),,

usepropotional gain (Kc,),integral time ( τ ) and

derivative time (τ ) from Z-N table.

1. Get the system performance using P,PI and PID,

upon a unit step change in set point.

2. 5-Repeat step1 to step3 for other loops.

3. Compare the performance

4. Select the controller that gives the best performance.

5. Repeat using Bode Method and Root Locus method.

Select for each the controller that gives the best

performance.

III. RESULTS AND DISCUSSION

Control of the column top pressure

The top pressure of the fractionator of the delayed coking

unit (DCU) in Khartoum Refinery is the primary control

loop and the reflux flow rate is the secondary control

loop. The MATLAB tool box system identification by the

Graphical User Interface was used to determine the model

and the transfer functions of the cascade control loops.

13 Different tuning methods are used for investigating the

system stability.

Frequency Dynamic System Analysis of the top pressure

Data for pressure control

Table (3.1): Operating records for Column top pressure

and reflux rate

Pressure, MPa Reflux flow(T/h)

0.093 68

0.095 68

0.0109 74

0.0111 73

0.011 73

0.0114 74

0.0113 74

0.0114 74

Transfer functions of the Top pressure and the reflux

rate

To keep the pressure of the column at designed

value through controlling the reflux rate of the column, the

MATLAB software supported with GUI (system

identification) the transfer function was obtained. The

transfer function in z-domain is:

The same is transformed into S-domain:

The System Performance for Closed Loop:

P-Controller

For closed loop transfer function using P-

controller only:

(3)

Routh-Hurwitz Method

The characteristic equation is

+(6.7 + 0.35 K) + (25 -4.5 k) S +33.6 +5.05k=0

(4)

Routh Array :

S3 1 25 – 4.5 k

S 2 6.7+0.35k 33.6+5.05k

S1 (25 - 4.5k) -


S0 33.6+5.05k

25 - 4.5 k -

=0

The ultimate gain Ku = 4.1

The characteristics equation is

+(6.7 + 0.35 K) + (25 -4.5 k) S +33.6 +5.05k=0

(5)

14

putting

s=i.w(6)

-I

w=4.3 rad/sec, The ultimate period Pu=2π/ω=1.46

min

Using Ziegler Table; Kc=2.1 for P-controller only.

The step response curve is illustrated inFigure(3.1)

below

Fig.(3.1):The final value of closed loop response of the system using P-controller only

The Offset for Closed Loop Transfer Function

The Transfer function is:

)

The offset = 0.24 – 1 = -0.76

S 2 6.7+0.35k 33.6+5.05k

S1 (25 - 4.5k) -

S0 33.6+5.05k

25 - 4.5 k -

=0

PI-Controller

For closed loop transfer function using PI-controller

τ

Using Z-N table; Kc=1.85 and =1.2


3.8 The Offset for Closed Loop Transfer Function

R(t)=1.0(14)

R(s) =

τ

The offset =0.28-1= -0.72

The step response curve is illustrated below

Fig.(3.2): The final value of closedresponse of the system using PI-Controller

PID-Controller

For closed loop transfer function using PID-controller

From Z-N table; Kc=2.5, ,

The Offset for closed loop transfer function for PID

controller

r(t)=1.0 (19)

R(s) =

(20)

,The offset =1-1= 0.0

The step response curve is illustrated in Figure (3.3)


16

Fig.(3.3): The final value of closed response of the system using PID-Controller using Routh test.

Fig.(3.4): The final values for different type of controllers using Routh test

Fig.(3.5): The final values and overshoots for different type of controllers using Routh test.


For the system performance using Routh test as a

tuning method and proportional controller. Figure(3-1)

illustrates the offset =-0.76.Using PI controller reduce

the offset to -0.72 illustrated in Figure(3.2).

Adding derivative controller to PI eliminates the offset

to zero according to Figure(3.3).The

overshoot=28.6%.Due to elimination of the offset the

is PID –controller is selected.

Controller Tuning Using Root-Locus Plots

17

To determine the ultimate gain and ultimate period from

the root locus, this can be realized by the following

methods:

Using the close –loop transfer function

And OLTF is given as:

The

OLTF=

(24)

Draw the roots locus using MATLAB software

Fig.(3.6): Root- Locus Analysis for controlling top pressure of the column

Expand equation

+(6.7 + 0.35 KC) + (25 -4.5 kC) S +33.6 +5.05k=0

(25)

Putting

s=i.w (26)

Equating the imaginary part to zero gives:

-

Equating the real part to zero gives

-

The equation becomes:

(28)

Substituting into equation (28)

=4.1

Using Ziegler Table; Kc=2.1 for P-Controller.

The transfer function is:

r(t)=1.0 (30)

=

(31)

The offset =0.24-1= -0.76

The step response curve is illustrated Fig.(3.7)


18

Fig.(3.7):The final value of closed loop response of the system using P-controller only.

PI-Controller


τ

Then; Kc=1.85 and =1.2

3.15 The Offset for Closed Loop Transfer Function

r(t)=1.0 (35)

The offset =0.28-1= -0.72

The step response curve is illustrated in Figure(3.8)

Fig.(3.8):The final value of closed loop response of the system using PI-Controller

PID-Controller

For closed loop transfer function using PID-controller

Taking Kc=2.5,τ τ ,

The Offset for Closed Loop Transfer Function for

PID controller

r(t)=1.0 (40)

R(s)=

(41)


The offset =1-1= 0.0

The step response curve is illustrated in figure below

Fig.(3.9): The final value of closed loop response of the system using PID-Controller

Fig.(3.10): The final values for different type of controllers using Root Locus method.

Fig.(3.11):The overshoot of closed loop response of the system for different type of controllers using Root-Locus

method.


For the system performance using Root Locus as

tuning method and proportional controller. Figure.(3.7)

illustrates the offset = -0.76.

Using PI controller reduce the offset to -0.72 illustrated

in Figure.(3.8)

Adding a derivative controller to PI eliminates

the offset to zero according to figure(4-9).The

overshoot using PID is 28.6% illustrated in

20

Figure.(3.11).Due to elimination of the offset the best

selection is PID

3.19 Controller Tunning Using Bode Method

The OLTF=

Using MATLAB commands for plotting bode diagram

as illustrated in Figure.(3-12) below

Figure (3.12):Bode diagram for controlling toppressure of the column

The results of MATLAB are:

(45)

=2.43 min

M= (47)

Log AR=log kc+log (48)

Log AR=log kc+log

AR=1; Ku=3

Using Ziegler Table ;Kc=1.5 for P-Controller.

3.17 The Offset for Closed Loop Transfer Function for P-Controller only:

The Transfer function is:

r(t)=1.0 (50)

R(s)=

(51)

The offset =0.184-1= -0.186

The step response curve is illustrated in figure below


21

Fig.(3.13): The final value of closed loop response for the system using P-controller only

PI-Controller


τ

Using from Z-N table; Kc=1.35 and =2.03

4-22 The Offset for Closed Loop Transfer Function

(55)

The offset =0.1682-1= -0.8318


Fig. (3.14): The final value of closed loop response of the system using PI-Controller

PID-Controller

For closed loop transfer function using PID-controller,

Taking Kc=1 ,

3.24The Offset for closed loop transfer function for

PID controller

(60)

(61)

The offset =1-1= 0.0



22

Fig. (3.15): The final value of closed loop response of the system using PID-Controller

Fig. 3.16: The final values of closed loop response of the system using different types of controller applying Bode

criterion.

Fig.(3.17): The overshoot of closed loop response of the system using different types of controllers applying Bode

criterion.


For the system performance using Bode criterion

as a tuning method and proportional controller.

Figure(3-13) illustrates the offset = -0.8318 .Using PI

controller reduce the offset to -0.186 illustrated in

Figure(3-14).

Adding a derivative controller to PI eliminates

the offset to zero according to figure(3.15).The

overshoot illustrated in Figure (3.17) is 0% According

to elimination of the offset the best selection is PID

using Bode method compared to Routh and Root Locus

method.

IV. CONCLUSION AND RECOMMENDATION

The delayed coking unit in Khartoum Refinery is

considered as one of the important unit for treatment of

the heavy crude oil (Al-fula crude ).The Graphical User

Interphase is used in identification and analysis the

control system. The dynamic performance of the system

is investigated ,the selection of the best mode of

controller is selected (PID-Controller). Different tuning

methods were used. These are : Ziegler, Bode, Nyquist,

Root Locus and Routh test.

From the control point of view, the delayed coking

process is a solution to the problem of decreasing

residual fuel demand. It also generates a variety of fuels

and in some cases a considerable amount of high

quality coke, while eliminating environmentally

unfriendly streams that often involve a disposal cost.

Implementing advanced process control and

optimization on a coking plant is quite a difficult task

but the results could be remarkable: energy savings,

maximized throughput, decreased CO emissions and

improved yields while increasing the overall profit of

the refinery.

Further work has to be done on the following:

1-Maximizing hot residue from crude distillation unit to

delayed coker.

2-Maximizing LPG production and fuel gas utilization.

3-Implementation of advanced process control.

4-Application of override control is essential for

controlling the base level.

ACKNOWLEDGEMENTS

The authors wish to thank the Faculty of Graduation

Studies in Gezira University, and Khartoum Refinery

Company for their help and support, this Paper.

5. References

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23

[3] G Zahedi, A Lohi, Z Karami,A neural network

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[4] Chang, A.I.&Nishioka, K.&Yamakawa, T.,

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[6] Depew, C.A.&Hashemi, M.H.& Davis, J.,

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[9] Wang, C.& Chen, Q.& Hua, B., "Analysis of

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control of the fractionator top pressure for a delayed coking unit

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