advanced control of the delayed coking unit in khartoum...

Journal of Applied and Industrial Sciences, 2014, 2 (4): 188-200, ISSN: 2328-4595 (PRINT),ISSN: 2328-4609(ONLINE)

Research Article 188

Advanced Control of the Delayed Coking Unit in

Khartoum Refinery

Tomadir A. I. Hamed*1, Gurashi A. Gasmelseed2, Ibrahim H. Elamin3.

1Department of Chemical Engineering, University of Science and Technology, Khartoum- Sudan

Department of Chemical Engineering, University of Science and Technology, Khartoum- Sudan

Email:[email protected]

Department of Chemical Engineering, University of Science and Technology, Khartoum- Sudan

E-mail:[email protected]

(Received: April 04, 2014; Accepted: July 31, 2014)

Abstract :Delayed coking units convert heavy

crude oil or topping refinery residue to more

light valuable products including diesel,

gasoline and naphtha by thermal catalytic

treatment. The residue from the fractionation

column is to 500oC in the furnace. The

thermally treated residue enters the delayed

coking tower where cracking and

condensation reactions occur to produce oil

gas and coke.

A cascade control strategy was developed to

control the pressure of the coking drum using

the flow of the heater fuel as manipulated

variable. The block diagrams of the systems

were constructed and the process transfer

functions were identified using MATLAB

Black Box model. Then the overall transfer

functions, the open and closed-loops, and the

characteristic equations were determined, and

the control systems were tuned to obtain the

adjustable parameters using Routh-Hurwitz,

Direct Substitution, Root locus, Nyquist, and

Bode methods. The adjustable parameters

were appropriately inserted into the

characteristic equation for the offset

investigation, stability analysis and response

simulation. It is found that using of PID

controller for the Primary loop provides the

highest gain than P and PI controllers and

also it eliminates the Offset.

Indexterms: Delayed coking, Thermal

cracking, Advance control.

1. INTRODUCTION

Delayed coking is a thermal cracking process

used in petroleum refineries to upgrade and

convert petroleum residuum (bottoms from

atmospheric and vacuum distillation of crude

oil) into liquid and gas product streams leaving

behind a solid concentrated carbon material,

petroleum coke.

The delayed coker is the only main process in

a modern petroleum refinery that is a batch-

continuous process. The flow through the tube

furnace is continuous. The feed stream is

switched between two drums. One drum is on-

line filling with coke while the other drum is

being steam-stripped, cooled, decoked,

pressure checked, and warmed up. The

overhead vapors from the coke drums flow to a

fractionator, usually called a combination

tower. This fractionator tower has a reservoir

in the bottom where the fresh feed is combined with condensed product vapors (recycle) to make up

the feed to the coker heater [1].

A fired heater with horizontal tubes is used in the

process to reach thermal cracking temperatures of

485 to 505oC (905 to 941oF). With short residence

time in the furnace tubes, coking of the feed

material is thereby “delayed” until it reaches large

coking drums downstream of the heater. Three

physical structures of petroleum coke: shot, sponge,

or needle coke can be produced by delayed coking.

These physical structures and chemical properties of

the petroleum coke determine the end use of the

material which can be burned as fuel, calcined for

use in the aluminum, chemical, or steel industries,

or gasified to produce steam, electricity, or gas

feedstocks for the petrochemicals industry [2].

In Khartoum refinery a delayed coking unit with

a capacity of 1 Mt/year was constructed in Phase

I and a second delayed coking unit with a

capacity of 1 Mt/year was constructed in Phase II

where the overall delayed coking capacity

reached 2 Mt/year. A set of delayed coking unit

with annual capacity of 1Mt was constructed in

Phase I, which is composed of 8 sections, these

are electrostatic desalting; coking tower; heating

furnace; fractionation column; steaming out and

venting; rich gas compression; rich gas

desulfurization, coke storage, basin-settling, base

coking, and cold coking water circulation. The

second set of delayed coking unit consists of 6

sections, these are coking tower, heating furnace,

fractionation column, rich gas compression,

absorption and stabilization, LPG desulfurization

and dethioalcoholization. The 3 sections coking

tower, heating furnace and fractionation column

are matched with the annual capacity of 1Mt;

while the electrostatic desalting, rich gas

desulfurization, steaming out venting, coke

storage basin- settling, basin cutting coking,

water circulation and cold coking water

circulation match with the annual capacity of

2Mt.


Fig. 1: Delayed Coking Unit flow sheet

The farction heavier than coker gas oil (CGO) in

the feedstock oil flows to the column bottom

together with the condensated fraction (called

circulation oil) of overhead oil gas from the

coking tower. At a temperature of 366ºC, the

mixture is pumped to the radiation section of the

furnace by heating furnace radiation section

feeding pump to be promptly heated up to 500ºC.

Then it enters the coking tower via four way

valve, where cracking and condensation reactions

occurred to produce oil gas and coke. The

mixture at high temperature oil gas transferred

from coking tower top to the duckbilled type tray

in the lower section at the bottom of the

fractionation column. Circulation oil fraction is

condensated, the rest large amount of oil gas

passes through washing plate to enter the

vaporization stage where it rises to the

distillation stage above the CGO collection pot to

separate the cuts such as coking rich gas,

gasoline, diesel oil and CGO [3].

One of the most useful concepts in advanced control

is cascade control. It is one of the most successful

schemes for enhancing the performance of single-

loop control [4], A cascade control structure has two

feedback controllers with the output of the primary

(or master) controller changing the set point of the

secondary (or slave) controller. The output of the

secondary goes to the valve, as shown in fig. 2.

There are two purposes for cascade control:

1. To eliminate the effects of some disturbances

2. To improve the dynamic performance of the control

loop

Fig. 2: Cascade Control Loop

There are many reasons for installing advanced process control

(APC) in a delayed coking unit, in order to maximize the

throughput in compliance with all given product qualities and

security limits. Unit security as well as product yields would be

improved. APC monitors all the process variables of the unit and

reacts on changes instantly inside the pre-specified limits [5].

The objectives of this study are to: develop a cascade control

strategy for the Coking Drum of the Delay Coking Unit, Identify

the Transfer Functions of control loops, Stability analysis of the

control loops, Controllers tuning, Offset investigation, and

response simulation.

II. METHODLOGY

Standards tests for raw material and products were carried out by

ASTM D, the raw material standard tests are shown in table 1 as

example.


Table 1: Raw material, parameters and methods of determination [3]

190

System Control, Tuning, and Stability analysis

A control strategy was developed as shown in figure

3, the block diagrams were constructed, the transfer

functions were identified using System

Identification Toolbox in MATLAB and the

characteristic equations were obtained. These

characteristic equations are used for tuning, stability

analysis and simulation responses. A system is

considered unstable if, after been disturbed by an

input change, its output takes off and does not return

to the initial state of rest.

The stability analysis of a system can be treated in a

unified way independently if it is closed or open.

The location of the poles of a transfer function gives

the first criterion for checking the stability of a

system: If the transfer function of a dynamic system

has even one pole with positive real part, the system

is unstable

The criterion of stability for closed loop systems

does not require calculation of the actual value of

the roots of the characteristic polynomial. It only

requires that if any root is to the right of the

imaginary axis [6].The Routh-Hurwitz test is a

numerical procedure to determine how many roots

of a polynomial are in the Right Hand Plane and

how many are on the imaginary axis. It doesn't give

specific root locations but performing the test is

generally far easier than factoring [7]. As we change

gain, we notice that the system poles and zeros

actually move around in the S-plane. This fact can

make life particularly difficult, when we need to

solve higher-order equations repeatedly, for each

new gain value. The solution to this problem is a

technique known as Root-Locus graphs.

The root locus analysis is another criterion of

stability. The root loci are merely the plots, in the

complex plane, of the roots of the characteristic

equation as the controller gain is varied from zero to

infinity. Bode and Nyquist diagrams of the open-

loop transfer functions are used to study the stability

characteristics of a closed-loop system. A feedback

control system is unstable if the amplitude ratio of

the corresponding open-loop transfer function is

larger than 1 at the crossover frequency, this is

known as the Bode stability criterion. The Nyquist

stability criterion states that: if the open-loop

Nyquist plot of a feedback system encircles the

point (-1, 0) as the frequency takes any value from -

∞ to +∞, the closed-loop response is unstable [6].

Tuning cascade control systems is more complex

than tuning simple feedback systems, if only

because there is more than one controller to tune.

However, this does not mean it is difficult. Because

the inner loop by itself is a simple feedback loop,

this controller should be tuned as fast as possible-

avoiding instability, of course. The objective is to

make the inner loop fast and responsive in order to

minimize the effect of upsets on the primary

controlled variable. Tuning this system then comes

down to tuning the primary controller [7].

Item Analyzed Item Method

1 API degree N/A

2 Density (20ºC), g/cm3 ASTM D- 4052

3 Kinematic vis.

50/80ºC, mm2/s ASTM D- 445

4 Freezing point ºC ASTM D- 97

5 Flash point (open) ºC N/A

6 Conradson carbon residue, m% ASTM D- 189

7 Water content,m% ASTM D-4006

8 Salt content, mgNaCl/L ASTM D-6470

9 Acid value, mgKOH/g ASTM D-664

10 Sulfur content, m% ASTM D- 4294

11 Nitrogen content, m% ASTM D- 4175

12 Gum, m% ASTM D- 381

13 Asphaltene, m% ASTM D- 2006

14 Wax content, m% ASTM D- 9766

15 Fe/Ni/Cu/V/Pb/Ca/Mg/Na, ppm ASTM D- 5185

16 Characteritic factor N/A

17 Crude type N/A

18 BKP－distillate

at 240ºC, m% ASTM D- 1160


Fig. 3: Control strategy physical diagram

Transfer function Identification:

The primary or Master loop:

The secondary or slave loop:

III. RESULTS AND DISCUSSION

The properties of mixed crude oil {light crude

(Fula-North-AG)/viscous crude (Fula-North-B)

=1:3} in the six districts of Sudan are shown in

Table 2. The basic characteristics of the crude oil

are high salt and calcium contents, high acidic

value, water content, high density, high viscosity

and high light component, and low sulfur content.

Tables 3.1 through 3.5 show the properties of the

oils.

Table 2: Properties of Mixed Crude Oil [3]

Item Analyzed Item Fula－North－B

(viscous crude)

Fula－North－AG

(light crude)

Mixed Crude*

1 API degree 18.07 33.1

2 Density (20ºC), g/cm3 0.9428 0.8596 0.936

3 Kinematic vis.

50/80ºC, mm2/s 1946.68/309.12 15.8/4.716 (50/100ºC) 267/117 (80/100ºC)

4 Freezing point ºC 2 13

5 Flash point (open) ºC 168 －

6 Conradson carbon residue,

m% 7.96 2.3

7 Water content,m% 1.12 6.40 2.35

8 Salt content, mgNaCl/L 683 1024

9 Acid value, mgKOH/g 13.82 0.09 10.455

10 Sulfur content, m% 0.15 589 ppm 0.1272

11 Nitrogen content, m% 0.29 1034 ppm

12 Gum, m% 13.69 －

13 Asphaltene, m% 0.18 －

14 Wax content, m% 13.50 －

15 Fe/Ni/Cu/V/Pb/Ca/Mg/Na,

ppm

97.9/18.3/1.2/0.9/0.1/1652.

0/8.5/264.0

16 Characteritic factor 12.0

17 Crude type Naphthenic- middle base Low sulfur middle base

18 BKP－distillate

at 240ºC, m% 8.3

* = Calculated data

The main products of the unit are purified dry gas, LPG, stabilized gasoline, coking diesel oil, coker gas oil and petroleum coke.


192

Table 3: The components of dry gas. [3]

Component Content (wt %) Component Content (wt %)

Hydrogen 1.22 Propene 1.86

Methane 51.92 Butane 0.06

Ethane 36.47 Butene 0.04

Ethene 5.98 Water 0.50

Propane 1.95 Sulfur 20mg/Nm3

Total 100.0

Table 4: Composition of LPG. [3]

Component Content (wt %) Component Content (wt %)

Hydrogen 0.0 Propene 17.99

Methane 0.0 Butane 26.50

Ethane 0.27 Butene 18.94

Ethene 0.0 C5+ 1.84

Propane 34.46 Thio-alcohol ≤10ppm

Total 100.0

Table 5: Main properties of stabilized gasoline, coker diesel oil and CGO (calculated data) [3]

Table 6: Properties of coke [3] (lab. data in Dec. 2001)

Table 7: Main properties of coker gasoline (stabilized gasoline), diesel oil and coker gas oil. [3]

Gasoline Diesel oil Coker gas oil

Density (20ºC) kg/m3 733.2 832.2 898.3

Distillate

ºC

Initial boiling

point 46 192 323

10% 76 215 351

30% 105 241 374

50% 126 264 384

70% 146 288 408

90% 169 318 440

Dry point 186 342

Acid value, mgKOH/100ml 18.0 13.8 5.56

Actual gum, mg/100ml ≯3 59 5.3（m%）

Induction period, min 669

Copper corrosion, (50ºC, 3h) Fail Fail

Sulfur content, mg/kg 244 600 0.16（m%）

Alkaline nitrogen content, mg/kg 55 414 1005

Stabilized gasoline Coker diesel oil Coker CGO

Density (20ºC) kg/m3 720 826 888

Distillate

ºC

Initial boiling point 48 131 320

5% 58 186 342

10% 63 212 358

30% 86 240 365

50% 110 264 373

70% 135 290 382

90% 160 324 396

95% 171 337 420

Dry point 183 350 430

property value

real density, g/cm3 2.101

Volatile, m% 8.8

sulfur content, m% 0.41

Ash, m% 3.33

metal content Ni/V/Na/Al/Fe/Cu/Ca

real density, g/cm3 2.101

Volatile, m% 8.8


Bromine value, g Br/100ml 45.8 24.8

Conradson residual carbon, m% 0.27

Ni/V/Na/Al/Fe/Cu/Ca，μg/g 0.1/0.1/1.4/1.1/3.0/0.1/0.6

Cetane number 51.6

Freezing point, ºC -25 20

Flash point (close), ºC 80 195 (open)

Aniline point, ºC 61.8

Kinematic viscosity, 50ºC, mm2/s 2.135 3.595(100)

Aromatic hydrocarbon, m% 22.6 22.8

Asphaltene, m% 0.1

Saturated component, m% 71.8

Ash, m% 0.004

Table 8: Composition of acidic gas [3]

Component Content, wt %

H2O 4.09

H2S 94.42

Hydrocarbon 1.49

Stability, tuning, offset and simulation

Using MATLAB the transfer function for the process of the secondary loop is:

The open loop transfer function using P controller is:

i. Routh test:

For this test the Characteristic equation of the loop is:

ii. Direct substituting:

Putting in characteristic equation to get:


Using Ziegler-Nichols table to calculate the controller parameter 194

Type of controller P 0.5 - -

PI 0.45 = 8 -

For the Primary loop:

When using P controller for the secondary loop:

Characteristic equation is:

iii. Routh test:

iv. Direct substituting:

s.

Using Ziegler-Nichols table to calculate the controller parameter

Type of controller P 0.5 - -

PI 0.45 =74 -

PID 0.6 = 44.45 =11.11

v. Root locus criterion:

Root locus plots for the Secondary loop and primary loop is shown in figure 4, and 5, respectively

root locus plot for the secondary loop

Real Axis

Imag

inary

Axis

-25 -20 -15 -10 -5 0 5 10 15 20-4

-3

-2

-1

0

1

2

3

40.9550.9780.989

0.995

0.998

1

0.70.890.9550.9780.989

0.995

0.998

1

510152025

System: T

Gain: 867

Pole: -0.00342 - 0.676i

Damping: 0.00505

Overshoot (%): 98.4

Frequency (rad/sec): 0.676

System: T

Gain: 867

Pole: -0.00342 + 0.676i

Damping: 0.00505

Overshoot (%): 98.4


0.70.89


Fig. 4: Root locus for secondary loop 195

Fig. 5: Root locus for primary loop with P controller for secondary loop

vi. Bode and Nyquist plots:

Bode and Nyquist plots for the Secondary loop and primary loop are shown in figure 6 through 9.

Fig. 6: Bode plot for secondary loop

root locus plot for the primary loop w ith(p secondary)

Real Axis

Imag

inary

Axis

-20 -15 -10 -5 0 5 10-150

-100

-50

0

50

100

150

0.5

0.0160.0360.0560.0850.115

0.17

0.26

0.5

20

40

60

80

100

120

140

20

40

60

80

100

120

140

System: T

Gain: 19.3

Pole: -0.0459 + 2.71i

Damping: 0.0169

Overshoot (%): 94.8


0.0160.0360.0560.0850.115

0.17

0.26

10-6

10-4

10-2

100

102

0

90

180

270

360

System: T


Phase (deg): 33.7

Phase (

deg)

bode plot for the secondary loop

Frequency (rad/sec)

-150

-100

-50

0

System: T


Magnitude (dB): -74.6

Magnitu

de (

dB

)


Fig. 7: Nyquist plot for secondary loop

Fig. 8: Bode plot for primary loop with P controller for secondary loop

-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4 0.6 0.8-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 dB

-20 dB

-10 dB-6 dB-4 dB-2 dB

20 dB

10 dB 6 dB 4 dB2 dB

Nyquist plot for the secondary loop

Real Axis

Imag

inar

y A

xis

bode plot for the primary loop w ith(p secondary)

Frequency (rad/sec)

10-3

10-2

10-1

100

101

102

103

-360

-180

0

180

360

System: T


Phase (deg): -176

Phase (

deg)

-200

-150

-100

-50

0

System: T


Magnitude (dB): -26.3

Magnitu

de (

dB

)


Fig. 9: Nyquist plot for primary loop with P controller for secondary loop

vii. Offset investigation:

Secondary loop:

For P controller

For step change in input

For PI controller


Primary loop:

For P controller for primary and secondary loops:


-1 -0.8 -0.6 -0.4 -0.2 0 0.2 0.4-0.4

-0.3

-0.2

-0.1

0

0.1

0.2

0.3

0.4

0 dB

-20 dB

-10 dB-6 dB-4 dB-2 dB

20 dB

10 dB 6 dB 4 dB 2 dB

Nyquist plot for the primary loop w ith(p secondary)

Real Axis

Imagin

ary

Axis


198

For PI controller:


For PID controller:


viii. System response:

System response for secondary loop after

introducing unit step change in the input is shown in

figure 10, and for primary loop, system responses

are shown in figure 11 through 13.

Fig. 10: system response for secondary closed loop

step change response for the primary loop w ith(p secondary)

Time (sec)

Amplit

ude

0 10 20 30 40 50 60 70 80 90 1000

0.5

1

1.5

System: T

Rise Time (sec): 0.544

System: T

Peak amplitude: 1.44

Overshoot (%): 45

At time (sec): 1.5

System: T

Settling Time (sec): 48

System: T

Final Value: 0.995

Journal of Applied and Industrial Sciences, 2014, 2 (4): 188-200, ISSN: 2328-4595

(PRINT),ISSN: 2328-4609(ONLINE)

Fig. 11: system response for primary closed loop with P controller

Fig. 12: system response for primary closed loop with PI controller

Fig. 13: system response for primary closed loop with PID controller

step change response for PI primary loop w ith(p secondary)

Time (sec)

Ampl

itude

0 200 400 600 800 1000 1200-0.4

-0.2

0

0.2

0.4

0.6

0.8

1

System: k

Final Value: 1System: k

Settling Time (sec): 767System: k

Rise Time (sec): 419

step change response for PID primary loop w ith(p secondary)

Time (sec)

Ampli

tude

0 20 40 60 80 100 120 140 160 180-1.5

-1

-0.5

0

0.5

1

1.5

System: n

Settling Time (sec): 121System: n

Rise Time (sec): 102

System: n

Final Value: 1

step change response for P primary loop

Time (sec)

Am

plit

ude

0 50 100 150 200 250 300 350-0.2

-0.1

0

0.1

0.2

0.3

0.4

0.5

System: v

Rise Time (sec): 61.5

System: v

Final Value: 0.34

System: v

Settling Time (sec): 171

Journal of Applied and Industrial Sciences, 2014, 2 (4): 188-200, ISSN: 2328-4595

(PRINT),ISSN: 2328-4609(ONLINE)

Delayed Coking Unit provides good processes to

upgrade the heavy crude, or to convert it to more

light valuable products.

Coke can be more utilized if it is gasified to Syngas

which is can be used to produce energy and also as a

feed for petrochemical industries.

Secondary loop gain using Routh test and Root

locus respectively, is 435, and 433.5 for P-

controller and 395.5, and 390.15 for PI-controller,

results of Routh test are identical to the results of

Direct Substitution method. Using Bode criterion

200

the value of the Gain Margin is 876, and the Phase

Margin is infinity, these results are identical to

Nyquist criterion. The offset is - 0.0038 for P-

controller and 0.0 for PI controller.

The primary loop with P- controller for the

secondary loop, the gain is 1.625, 1.463, and 1.95for

P, PI, and PID respectively using Routh test, and

9.6, 8.69, and 11.6 P, PI, and PID respectively using

Root locus criterion. The offset is - 0.64 for P-

controller and 0.0 for PI, and PID controller. A

summary for these results is shown in table 9.

Table 9: Summary for the Comparison between Stability and tuning method for Primary loop

Offset Pu Ku Method

P : - 0.612

- 3.73 Routh_Huwrtiz PI : 0

PID : 0

P : - 0.644

88.9 3.25 Direct Substitution PI : 0

PID : 0

P : - 0.2336

2.32 19.3 Root Locus PI : 0

PID : 0

IV. CONCLUSIONS

Delay Coking Unit is one of the most valuable units in the refinery

because it increases the economic benefits by converting low price

residue to valuable products with higher price.

Advance control has a great effect on the response of the chemical

plants. It increases stability and eliminate offset.

There are many methods that are used to get the adjustable

parameters such as Routh-Hurwitz, Direct substitution and there

are another three graphical method, Bode, Nyquist, and root locus.

Ziegler-Nicholas criterion is used to tune the adjustable parameters.

PID controller for the Primary loop provides the highest gain than

P, and PI controllers and also it eliminates the Offset.

ACKNOWLEDGEMENTS

The authors wish to thank the Faculty of Graduation Studies and

Scientistic Research of Karary University, and Khartoum

Refinery Company for their help and support, this Paper is

generated from a Thesis in partial fulfillment for Ph.D. in

Chemical Engineering.

REFERENCES

[1] Ellis, P. J., Paul, C. A., “Delayed Coking Fundamentals”,

Topical Conference on Refinery Processing, Paper 29, March 8-12,

1998.

[2] Meyers, R. A., “Hand Book of Petroleum Refining”, McGraw

Hill, third edition, 1986.

[3] Manual of Delayed Coking Unit at Khartoum Refinery, 2007.

[4] Jeng, J.-C., “Simultaneous closed-loop tuning of cascade

controllers based directly on set-point step-response data”, Journal

of Process Control, Elsevier, 2014.

[5] Haseloff, V., Friedman, Y. Z., et al., “Implementing coker

advanced process control”, Hydrocarbon Processing, Gulf

Publishing Company, p. 99 –103, June 2007.

[6] Stephanopolous, G., “Chemical Process Control”, Prentice-hall

of India, New Delhi, 2005.

[7] Tyner, M., May, F. P., “Process Engineering Control”, The

Ronald press company, New-York, 1968.

[8] P. C. Chau, “Chemical Process Control: A First Course with

MATLAB”, 2001.

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