improved control strategy of residue fluidized-bed...

Journal of Applied and Industrial Sciences, 2014, 2 (2): 46-57, ISSN: 2328-4595 (PRINT), ISSN: 2328-4609 (ONLINE)

Research Article 46

Abstract-Catalytic cracking to produce products of higher octane

number is rather important and needs always to be under

investigation and development. It is not an easy process, but it is

difficult due to the deactivation of the catalyst, deposition of coke

and poisoning by heavy metals. The manipulative and controlled

parameters of the regenerator(s), reactor and firing air may be

interacting. In this study two catalyst regenerators are designed

for control loops with single-input single output (SISO). A ratio

controller for air/fuel and a temperature controller are used to

adjust the air/fuel ratio and their rate. Where the temperatures in

the two regenerators were controlled by manipulating the rate of

the inlet hot air. Levels, temperature and steam rate were also

controlled as well as the top pressure in the regenerators through

manipulation of the exit flue gas. The control strategy was

developed, the overall transfer functions were identified and the

characteristic equations were used for stability analysis, tuning

and response simulation, as shown in Tables 1 through 7 and

Figures 2 through 13.

Index Terms -: Catalytic cracking, Tuning and Stability

1. INTRODUCTION

Petroleum refinery is an industrial process plant where crude

oil is processed and refined into more useful products such

as petroleum naphtha, gasoline, diesel fuel, asphalt

base, heating oil, kerosene and liquefied petroleum gas [1].

Fluid catalytic cracking is one of the most important conversion

processes in a petroleum refinery, it also occupies very

significant position in the refinery because of its economic

benefits, and the process incorporates most phases of chemical

engineering fundamentals, such as fluidization, heat/mass

transfer, and distillation. The heart of the process is the

reactor-regenerator, where most of the innovations have

occurred since 1942 [2].

Heavy Oil vapors are cracked to gasoline and fuel oil plus

low-molecular-weight Paraffins and olefins by contact with

very hot particles of fine zeolite–silica–alumina catalyst. The

catalyst provides energy for vaporization of the feed and for the

endothermic reactions. A few percent of the feed forms

carbonaceous deposits on the catalyst, rapidly decreasing its

activity, so frequent regeneration is necessary. Spent catalyst is

continuously removed from the reactor and sent to the

*Corresponding author Email: [email protected]

regenerator, where air is introduced to burn off the ‘‘coke,’’

reheat the catalyst, and restore its activity. In early versions of

the FCC process, the reactor and regenerator were fluidized

beds placed side by side as seen in Fig. 1.

Figure1: FCC Unit (Peter 2003)

Catalyst from the reactor passed down by gravity through a

stripper, where up flowing steam displaced the hydrocarbon

vapors and maintained the solid in a fluidized state. The catalyst

then flowed in a transfer line to a point below the beds, where it

was picked up by the air stream and carried into the regenerator.

It is important to separate catalyst and vapors as soon as they

enter the reactor, otherwise the extended contact time of the

vapors with the catalyst in the reactor housing will allow for

non-selective catalytic recracking of some of the desirable

products. The extended residence time also promotes thermal

cracking of the desirable products [2]. Catalyst from the

regenerator flowed through another transfer line to a tee

junction, where it joined the oil feed and passed up into the

reactor.

Other versions of FCC units had different methods of

controlling the solid flow between reactors in one

Improved Control Strategy of Residue Fluidized-bed

Catalytic Cracking Unit

Sahar .A. Salih*1, Mustafa. A. Mustafa

2, Gurashi. A. Gasmelseed

3

1Faculty of Graduate studies, University of Karary, Khartoum-Sudan

2 Department of Chemical Engineering, University of Khartoum, Khartoum- Sudan

Email: [email protected] 3Department of Chemical Engineering, University of Science and Technology, Khartoum- Sudan

Email:[email protected]

(Received: December 01, 2013; Accepted: April 04, 2014)

http://en.wikipedia.org/wiki/Industrial_process

http://en.wikipedia.org/wiki/Factory

http://en.wikipedia.org/wiki/Crude_oil



http://en.wikipedia.org/wiki/Petroleum_naphtha

http://en.wikipedia.org/wiki/Gasoline

http://en.wikipedia.org/wiki/Diesel_fuel

http://en.wikipedia.org/wiki/Asphalt



http://en.wikipedia.org/wiki/Heating_oil

http://en.wikipedia.org/wiki/Kerosene

http://en.wikipedia.org/wiki/Liquefied_petroleum_gas

mailto:[email protected]

mailto:[email protected]

47


‘‘single-vessel’’ unit, the reactor was placed on top of the

regenerator. However, the biggest change came after the

introduction of very active zeolite catalysts and the realization

that much of the cracking took place in the transfer line carrying

catalyst into the reactor. Current designs feature a riser reactor,

a tall, small-diameter pipe, where all the cracking occurs as

catalyst particles are carried upward at high velocity by the oil

vapors. The gas–solid suspension is discharged through

cyclones into a vessel that serves as a stripper and a feed

reservoir for spent catalyst. The regenerator is a large reactor

(up to 18 m in diameter) with a bed depth of 10–15 m, and it is

often the largest vessel in the refinery. Some steam is fed to the

bottom of the riser to strip any hydrocarbon and maintain

fluidization until the feed oil is vaporized [3]. Gas and solid exit

the riser horizontally through swirl nozzles that make some of

the solids drop out prior to the cyclones. Steam is introduced at

several points in the multistage stripper to maximize

hydrocarbon removal. The regenerator is operated in the

turbulent regime at superficial velocities of 0.3–1.0 m/sec,

which is over 100 times the minimum fluidization velocity.

Entrainment of fines is severe, and the top of the regenerator is

crowded with many sets of two-stage cyclones. The cyclones

recover over 99.99% of the entrained solids, which are returned

to the bed through dip legs discharging below the top of the

bed. Most of the oxygen is consumed in the reactor, but the

catalyst is not completely regenerated. Because of solids

mixing, there is a wide distribution of residence times and a

corresponding distribution of carbon content on the catalyst

particles. Typically, over 90% of the carbon is burned off.

Two-stage regenerators are used in some refineries to with

operational problems due to the high nonlinearity of such

systems [3]. Both CO and CO2 are produced as the coke burns,

and some CO is oxidized in the gas phase, the rest of the CO

can be burnt to generate steam. Oxidation of CO above the bed

can lead to large, undesirable temperature increases, and some

catalysts are promoted with platinum to favor CO oxidation in

the bed [3].

Control of FCCU

The control problem of fluid catalytic cracking (FCC) units is a

challenging task due to its model complexity, non-linear

dynamics, constrained variables and cross-coupling interaction

between inputs and outputs [4]. Baker developed optimal

system of a two cascade closed-loop system which takes the

conversion percentage as the optimal variable because it is the

direct measurement to the degree of reaction and can be

calculated online from the products distribution of FCCU, used

a neural network to predict this conversion percentage online

and at real-time because there may be a large time-delay to

calculate the conversion percentage. Based on this, closed-loop

optimization is achieved by the uses of online observation for

feeds property and adaptive intelligent optimal method and the

yield of light oil increases about 0.6% [5]. ChenZiluan

Developed a design of multivariable feedback control

configurations for composition control at the riser output for

FCC units. Numerical simulations on a non-linear dynamical

model operating in the partial-combustion mode are used to

show the effectiveness of several multivariable control

configurations under disturbances and uncertainty parameters

[6]. Raluka developed a dynamic simulator of the fluid catalytic

cracking (FCC) pilot plant, The operation of the pilot plant

permits the execution of case studies for monitoring of the

dynamic responses of the unit, by imposing substantial step

changes in a number of the manipulated variables [7].

Madhusudana developed a case study of an object-oriented

model for automatic generation of a fluid catalytic cracking unit

(FCCU) reactor/regenerator is presented [8]. Bollas applied the

calculation of the optimal set points by considering the

closed-loop dynamics, focusing in particular on rigorous

handling of input saturation effects [4].

The main objectives of this study is to develop a control

strategy for tight control of the residue Fluidized-bed Catalytic

Cracking Unit (RFCCU) for improvement of performance of

the Base Case shown in Fig. 2 as well as to identify the control

functions, stability analysis, tuning and response simulation of

the RFCCU.

II. MATERIALS AND METHODS

Two control strategies were developed as shown in Fig. 1 and

Fig. 2.

Control Loops Identification

1. Temperature control, Riser temperature versus

regenerated catalyst from the second regenerator.

2. Catalyst level control versus regenerated catalyst from

the first regenerator.

3. Pressure control for second and first regenerators,

manipulating exit flue gas.

4. Steam rate versus set point.

5. Level in the reactor versus spent catalyst from the

reactor to regenerator 1.

6. Temperature in the second regenerator versus hot air

flow rate to the second regenerator.

7. Temperature in the first regenerator versus hot air flow

rate to the first regenerator.

8. Local feedback control of the hot air to the first and

second regenerator versus desired value or set point.

9. Ratio control of fuel/air ratio.

48


Figure2: Physical Diagram of the Base Case Control Strategy of RFCC

Figure 3: Physical Diagram of RFCCU Control Strategy

49


Mathematical models were developed for loop 1 through loop 4 and parameters of the transfer functions were cited from the

literature [9, 10].

Transfer function identification:

Loop 1:

Figure 4: loop 1 block diagram with the identified transfer functions

The chr-eq = 1+OLTF = 0 :

)1......(..............................05.108.18252.50356.3344.8s sss

Loop 2:

Figure 5: loop2 block diagram

The chr-eq is:

.......(2)........................................0.........40.85.3s21.52s30.1s

Loop 3:

Figure 6: loop 3 block diagram

50


The chr-eq is:

)3.(........................................02.97.924.2037.1044.1 ssss

Loop 4:

Figure 7: loop4 block diagram

The chr-eq is:

)4( ........................................012.37.924.2037.1044.1 ssss

Stability Analysis and Tuning:

Taking loop 1 as an example:

1- Routh –Hurwitz Technique:

The chr-equation is:

)5..(..............................019.42.428.53344.0 ssss

The ultimate gain Ku and ultimate period Pu were inserted in to Ziegler-Nicolas table and the adjustable parameters are determined

and tabulated in the following table.

ku=9.5, Pu=9.8s.

Table 1

Z.N adjustable parameters of loop1

The same is repeated for each loop and summarized as shown in the Table 2:

Type of controller kc (min)i

(min)

D

P 4.75 - -

PI 4.27 8.16 -

PID 0.57 4.9 1.22

51


Table 2

Summary of the adjustable parameters of the four loops using Routh-Hurwitz method

Loop Number mode kc (min)i

(min)

D

1 P 4.75 - -

PI 4.27 8.16 -

PID 0.57 4.90 1.22

2 P 7.96 - -

PI 7.16 0.73 -

PID 9.55 0.44 0.10

3 P 4.1 - -

PI 0.48 5.51 -

PID 0.57 33.1 0.83

4 P 5.28 - -

PI 4.75 4.43 -

PID 6.33 26.6 0.67

2- Root locus method:

Establishing the OLTF from equations 1,2,3 and 4 , Root Locus

method was applied.

The OLTF of loop 1 is:

)6.........(........................................)15)(18(1)(0.2s

2OLTF

)16.0(

sss

kc

Applying MATLAB software for loop1:

Figure 8: root locus of loop 1

52


The ultimate gain and ultimate period are:

Ku=1.91, Pu=9.99s.

Inserting the values of ku and Pu into Z-N, the adjustable parameters for loop 1 are:

Table 3

Z.N adjustable parameters of loop 1

Table 4

Summary of the adjustable parameters for loop 1,2,3 and 4 using Root-Locus method


(min)

D

1 P 0.96 - -

PI 0.86 8.33 -

PID 1.15 49.95 1.25

2 P 0.49 - -

PI 0.44 0.72 -

PID 0.59 4.3 0.11

3 P 1.06 - -

PI 0.96 5.42 -

PID 1.28 32.5 0.81

4 P 0.56 - -

PI 0.50 4.37 -

PID 0.67 26.2 0.66

3- Bode plot method

The OLTF of loop 1 is:

Applying MATLAB software for loop 1:

Type kc (min)i

(min)D

P 0.96 - -

PI 0.86 8.33 -

PID 1.15 49.95 1.25

)7..(..................................................)15)(18(1)(0.2s

2OLTF

)16.0(

sss

kc

53


Figure 9: Bode diagram of loop 1

From bode plot: ku=9.71, Pu=9.68s.

Inserting the values of ku and Pu into Z-N, the adjustable parameters for loop 1 are:

Table 5

Z-N adjustable parameters of loop1

Type kc (min)i

(min)D

P 4.86 - -

PI 4.37 8.07 -

PID 5.83 48.4 1.21

The same was repeated for loops1, 2,3and 4: Table 6

Summary of the adjustable parameters for loop 1, 2, 3 and 4 using Bode method


(min)

D

1 P 4.86 - -

PI 4.37 8.07 -

PID 5.83 48.4 1.24

2 P 8.00 - -

PI 7.20 0.72 -

PID 9.60 4.30 0.11

3 P 4.15 - -

PI 3.74 5.42 -

PID 4.98 32.5 0.81

4 P 5.28 - -

PI 4.75 4.43 -

PID 6.33 26.60 0.67

54


Offset investigation

The forcing input:

Applying the method above for loops 1, 2,3and 4 and taking unit step change in the input, the offset are determined and tabulated in

the following table:

Table 7

Offset investigation values of loops 1, 2, 3 and 4

Loop Number Method of tuning

1 Routh 0.1

Bode 0.09

R.locus 0.03

2 Routh 0.05

Bode 0.03

R.locus 0.29

3 Routh 0.10

Bode 0.11

R.locus 0.32

4 Routh 0.24

Bode 0.24

R.locus 0.60

The System Responses:

Using the highest gain from the four loops for each method and taking a step change in the input the following responses are

realized:

ss

idCt

1)(,1)(

idC

)11..(................................................................................1

)(

....(10)................................................................................1)(

(9)....................1......... valueidealid

C , (s)] y [s0s

lim C

:where

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

C C,offset

ss

idC

tCid

55


Figure 10: Response of loop 1


56




III. RESULTS AND DISCUSSION

The control system in the Reference Case as shown in Fig. 2

needs to be renewed, better by adaptive controllers as these

types of controllers can adapt themselves according to the

change of catalyst activity, temperature and pressure. These

types of controllers are costly, hence a less expensive control

system is developed in this study to replace the existing control

system in application to date as in Fig. 3. It is observed that in

the existing control system depicted in figure 3, the level inside

the regenerator and the temperature in the riser are controlled

57


by manipulating the regenerated catalyst flow rate by two slide

valves which are very interacting, at the same time the hot air

rate is not controlled.

The system depicted in Fig. 3 was developed in this study, the

transfer functions were identified and the overall transfer

functions were calculated the characteristic equations as well as

the closed loop and open loop transfer functions were

determined as seen in tables 1 through 7. Routh Hurwitz,

Root-Locus and Bode methods are used for stability analysis

and tuning. All the systems in all control loops are shown to be

stable. The tuning methods of Routh and bode give

asymptotically equal parameters but with regard to the gain

Root Locus does not in agreement with the two other methods

as shown in tables 3 and 4. The method that gives the highest

sensitivity is selected for simulation of the results, upon a step

change in the input, the results are shown in figures (10

thtough13). Summary of the adjustable parameters are

tabulated in tables (1, 2, 3 ...7), the system is recommended to

be transformed to digital control system.

IV. CONCLUSIONS

In conclusion the control system of the RFCC in the Reference

Case should be superseeded by either an adaptive control

system or by the system developed in study. It is concluded

that with the exception of root locus, the methods of stability

analysis and tuning are found to be identical and each of them

can confidently be used for tuning. The response of each loop

was stable, with minimum oscillation and very short recovery

time as seen in Figures 1 through 13.

Acknowledgement

The authors wish to thank the Graduate College for Higher

Studies and Research of Karary University for their help and

encouragement. This paper is generated from a research thesis

in partial fulfillment for Ph.D. in Chemical Engineering at the

University of Karary (Sudan).

REFERENCES [1]. Gary, J.H. and Handwerk, G.E.(1984). Petroleum Refining

Technology and Economics, Second Edition, Marcel Dekker, Inc.

[2]. Reza .S (2000).Fluid Catalytic Cracking Handbook, Second

Edition, Gulf

Publishing Company.

[3]. Peter .H (2003). Chemical Reactor Design, Marcel Dekker, Inc. -

New York Basel.

[4]. Bollas G. M., Lappas A.A and Vasalos I. A (2002). An

Integrated Riser-Reactor Dynamic Model for the Simulation of Pilot

and Commercial FCC units.

[5]. Baker.R, Swartz.C.L.E, Young. J.C, (2004). ‘framework Input’,

Computers & Chemical Engineering, Volume 28, Issue 8, Pages

1347- 1360.

[6]. ChenZiluan.W, Chen.X.M and Jiang Q.C (2003). ‘Optimal

Control of Fluid Catalytic Cracking Unit’, IFAC.

[7]. Raluka.R, Serban P.A, Zoltan K.N and Mircea.V.C 20 (2009).

dynamic modeling and nonlinear model predictive control of a Fluid

Catalytic Cracking Unit , Computers &Chemical

Engineering, Volume 33, Issue 3 , Pages 605-617.

[8]. Madhusudana.R Rao, Rengaswamy.R, A.K. Suresh& K.S.

Balaraman April (2004). ‘Industrial Experience with

Object-Oriented Modelling: FCC Case Study ’, Chemical Engineering

Research and Design, Volume 82, Issue 4, Pages 527-552.

[9]. Stephanopoulos.G (1984). Chemical Process Control: an

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improved control strategy of residue fluidized-bed...

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