Fahim M.A., Sahhaf T.A., Elkilani A.S. Fundamentals of Petroleum Refining

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for coke

¼ k

þ k



f ð8:25Þ

Experimental data reported by Ancheyta-Juarez and Murillo-Hernandez

(2000), which are shown in Table 8.7, can be used to evaluate the reaction

rate constants.

Example E8.3

Use the experimental data given in Table 8.7 with f equal to 1.0. Estimate the

reaction rate constants for the three-lump model.

Solution:

For a three-lump model and f =1

¼k

þ k

ðÞy

ðE8:3:1Þ

¼ k

 k



ðE8:3:2Þ

¼ k

þ k



ðE8:3:3Þ

Initial conditions at t = 0 are at y

=1,y

= 0 and y

= 0. Initial guesses for k

, k

and k

are assumed. Equations (E8.3.1), (E8.3.2) and (E8.3.3) are solved numer-

ically in the Excel worksheet. The resulted values for y

, y

and y

, at time

corresponding to the space velocity in Table 8.7, are compared with the

experimental composition listed in Table 8.7 . The Solver in the Excel work-

sheet is used to minimise the difference by changing the values of k

, k

and k

The resulting k values are

=23h

1

, k

= 3.1 h

1

and k

= 7.5 h

1

Predicted yields are plotted as solid lines versus conversion in Figure E8.3.The

experimental data are also plotted from Table 8.7 as symbols in the same ﬁgu re.

Table 8.7 Experimental data at 548.9



C and catalyst to oil ratio (C/O) ¼ 4

(Ancheyta-Juarez and Murillo-Hernandez, 2000)

Space

velocity (h

1

)

Conversion

(wt%)

VGO

(wt%)

Gasoline

(wt%)

Gas

(wt%)

Coke

(wt%)

10 82.38 17.62 54.16 21.08 7.14

20 71.18 28.82 48.65 16.81 5.72

30 62.04 37.96 43.85 13.6 4.59

60 49.26 50.74 37.67 8.85 2.74

Fluidised Catalytic Cracking 221

8.14. Concentration and Temperature Profiles

in the Riser

It is possible to calculate the concentration profile for each component in

the riser reactor by differential material balance in the riser. If the four-lump

model discussed earlier is considered,itispossibletoderivethefollowing

equation for each lump VGO, gasoline, gases and coke (Jia et al., 2003):

For VGO

¼

þ k

½y

ð8:26Þ

For gasoline

¼

k

þ k

ðÞy



ð8:27Þ

For gases

þ k



ð8:28Þ

For coke

þ k



ð8:29Þ

0.2

0.4

0.6

0.8

0 0.2 0.4 0.6 0.8 1

Conversion, wt fraction

Yield, wt fraction

VGO

Gasoline

Gas+Coke

Predicted

Figure E8.3 Experimental and predicted gasoline, gas and coke yields

222 Chapter 8

Where

= Cross-sectional area of the riser

= Hydrocarbon gases void fraction

= Hydrocarbon gases mass flow rate in the riser (kg/s)

f = Catalyst activity

= Density of gas phase in the riser (kg/m

)

z = Axial distance in the riser (m)

Numerical integration of equation (8.26) to (8.29) will give the yield

profile of each component along the length of the riser. The value e, f, r

and m

have to be updated along the length of the riser.

Updating these values along the riser length can be carried out using the

following equations:

cat

c þ r

cat

ð8:30Þ

where c = u

, u

is the velocity of the gas (m/s) which is gas flow rate

divided by riser cross-sectional area, u

is the particle velocity in the riser and

is defined as

cat

1  e



ð8:31Þ

The hydrocarbon gases flow rate is calculated from

¼ m

VGO

þ y

ðÞ ð8:32Þ

The density r

has to be updated as a function of temperature change in the

riser, which is calculated from a temperature profile generated from the

energy balance equation.

The coke content of the catalyst C

, varies with coking time according

¼ 2:43  10

3

0:2

ð8:33Þ

And the deactivation function with respect to coke content can be

determined:

f ¼

1 þ 69:47 100C

ðÞ

3:8

ð8:34Þ

Coking time t

can be calculated at each riser interval as

cat

c þ

1  y

ðÞr

cat

ð8:35Þ

The energy balance equation gives

Fluidised Catalytic Cracking 223

¼

ðm

cat

pcat

þ m

ðk

þ k

Þy

þðk

þ k

Þy



ð8:36Þ

The boundary conditions are at z ¼ 0, y

¼ 1, y

¼ y

¼ 0, T

¼ T

and t

¼ 0.

Example E8.4

The cracking of gas oil is carr ied out through a 33 m riser height with an 0.8 m

inside diameter. Typical industrial FCC riser data (Ali and Corriou, 1997) are

listed in Table E8.4.1.

Kinetic and energy data (Abul-Hamayel, 2003) are listed in Table E8.4.2.

Typical molecular weights and heat capacities (Ahari et al., 2008) are given in

Table E8.4.3.

Solve the riser material and energy balance equations and plot the concen-

tration and temperature proﬁle versus riser height.

Table E8.4.3 Kinetic constants and energies for four-lump

model at 823 K

Molecular weight

(kg/kmol) C

(kJ/kg K)

Gas oil 333 3.3

Gasoline 106.7 3.3

Gases 40.0 3.3

Coke 14.4 1.087

Catalyst – 1.087

Table E8.4.2 Kinetic constants and energies for four-lump model at 823 K

(wt frac h)

1

(wt frac h)

1

(wt frac h)

1

(h)

1

(h)

1

121 35 21 12 3

(kcal/

mol)

(kcal/

mol)

(kcal/

mol)

(kcal/

mol)

(kcal/

mol)

28 19 15 34 30

Table E8.4.1 Typical FCC riser data

I.D.

(m)

Height

(m)

Catalyst

flow rate

(kg/s)

Feed

flow

rate

(kg/s)

Catalyst

temperature

(K)

Feed

temperature

(K)

Riser

pressure

(atm)

0.8 33 144 20 960 494 2.9

224 Chapter 8

Solution:

Utilising the operating data the r

was calculated at initial conditions of temper-

ature and pressure

PMwðÞ

2:9 333ðÞ

0:082 494ðÞ

¼ 23:84 kg=m

Gas velocity u

was then calculated at r

At z ¼ 0 of the riser

¼ 1, y

¼ y

¼ 0

¼ 20 (1 + 0 + 0) ¼ 20 kg/s

23:84 0:5ðÞ

¼ 1:67 m=s

Then u

and e

are calculated simultaneously as follows:

144

0:58ðÞ1  e



1  e

820ðÞ

23:84 144ðÞ36= 1  e



=1:67 þ 820ðÞ

The catalyst activity can be determined as

f ¼

1 þ 69:47 0:243t

0:2



3:8

Coking time t

can be calculated from equation (8.35) as

0:58ðÞ1:67= u



144 1: 67 = u



333

106:7

20ðÞ1 y

ðÞ8ðÞ

0:082T

2:9

ðE8:4:1Þ

Simple energy balance on the catalyst and the feed is performed to calculate the

riser inlet temperature at z ¼ 0, T

¼ T

¼ 821.8 K

At y

¼ 1 and y

¼ 0

¼

0:5e

ð23:84ÞðfÞ

144ð1:887 þ 20ð3:3ÞÞ

½ k

þ k

ðÞðE8:4:2Þ

Equations E8.4.1 and E8.4.2 should be solved numerically at boundary

conditions of y and values of k and H from Table E8.4.2. Gas density is then

calculated at the estimated riser temperature. Then the differential equations

(8.26) to (8.29) are solved in Excel, using the Euler method. The procedure is

repeated to calculate T

and t

then the product compositions.

Figure E8.4.1 shows the FCC riser composition proﬁles as a function of

riser height. The temperature and activity of catalyst proﬁles are shown in

Figure E8.4.2.

Fluidised Catalytic Cracking 225

0.0

0.2

0.4

0.6

0.8

1.0

0 5 10 15 20 25 30 35

Riser height (m)

Composition

(2)Gasoline

(3)Gases

(4)Coke

(1)VGO

Δz

Separator

time in the riser

Catalyst

time in

separator

Reactor

effluent

Oil

Figure E8.4.1 P roduct concentration profiles along the riser

800

805

810

815

820

825

0 5 10 15 20 25 30 35

Riser height (m)

Temperature (K)

0.84

0.86

0.88

0.90

0.92

0.94

0.96

0.98

1.00

Catalyst activity

Figure E8.4.2 Tempe rature and catalyst activity profile in the riser

226 Chapter 8

8.15. Simulation of FCC unit

The FCC unit is basically composed of the reactor and the regenerator.

At least three types of reactions take place in the riser of the reactor. The

basic equation takes place in the regenerator where coke is burned by air

to form CO

and water vapour. A UNISIM (2007) simulator is used in

the following example which is composed of a conversion reactor where

cracking, hydrogen transfer and coke formation reactions take place. The

outcome of the reactor is split into coke and products streams. Products are

sent to a fractionation unit and coke is sent to another conversion reactor

(regenerator) where all the coke is burned with 7% excess air.

Example E8.5

A VGO feed of 78,428 lb/h is fed to the riser of a FCC unit at 900



F and

20 psia. Assume that the feed is composed of 94 mol% C

parafﬁn and

6 mol% n-butyl naphthalene (C

). Perform a UNISIM simulation on the

FCC unit shown in Fig. E8.5.1.

The following reactions occur in the riser:

Cracking reaction (90% Conversion)

! C

þ C

ðE8:5:1Þ

is further cracked (95% Conversion)

! C

þ iC

ðE8:5:2Þ

Hydrogen transfer reaction (100% Conversion)

þ 3iC

! 3C

þ C

ðE8:5:3Þ

Condensation reaction of coke (100% Conversion)

n-butyl naphthalene

Coke precursor

(E8.5.4)

In the UNISIM simulation deﬁne the coke as a hypothetical component

(hypo2000) with a UNIFAC structure of [(ACH)

(AC)

(ACCH

)

](Hsu

and Robinson, 2006 ).

Coke coming out from the reactor is further burned in the regenerator using air

according to the following reaction (100% Conversion):

Fluidised Catalytic Cracking 227

Distillation

Column

Out 2

Out 1

VGO

Conversion

reactor

Splitter

Products

Coke

Air

FlueGas

Reg

Dist

Products

Out 3

Regenerator

Figure E8.5.1 UNISIM flowchart for the FCC unit

228 Chapter 8

Table E8.5.1 Summary of UNISIM results

Stream

Feed

rate (lb/h) Temp. (



Stream composition (mole %)

Benz nC

Coke C

VGO 78430 900.0 0 6.0 94 0 0 0 0 0 0 0 0 0 0 0

Dist 78428 891.7 1.37 0 2.15 0.97 19.35 25.16 12.58 37.24 6.9 0 0 0 0 0

Products 76578 896.1 1.38 0 2.17 0.97 19.49 25.33 12.67 38.0 0 0 0 0 0 0

Coke 1854 896.1 0 0 0 0 0 0 0 0 1.0 0 0 0 0 0

Air 86551 100 0 0 0 0 0 0 0 0 21.0 0 0 0 0 79.0

Reg 88405 133.7 0 0 0 0 0 0 0 0 0.22 0 20.95 0 0 78.83

Flue gas 88405 1456 0 0 0 0 0 0 0 0 0 0 15.13 4.81 1.75 78.31

Out 1 18551 42.3 4.1 0 0 0 0 0 0 95.9 0 0 0 0 0 0

Out 2 34953 153 0 0 3.2 1.4 29 37 18 11.4 0 0 0 0 0 0

Out 3 23074 161.3 0 0 3.3 1.5 29.5 38.6 19.3 7.7 0 0 0 0 0 0

Fluidised Catalytic Cracking 229

Hypo2000 þ 26O

! 22CO

þ 8H

O ðE8:5:5Þ

Solution:

1) A conversion reactor is used to carry out reactions (E8.5.1) to (E8.5.3) at

900



F and 20 psia. T he product stream is called Dist (Figure E8.5.1).

2) The Dist stream is split into Hypo-2000 as a coke stream and the other

products.

3) Air is injected at 100



F and 20 psia.

4) The mixed stream (reg) is sent to the regenerator reactor where coke is

combusted producing hot ﬂue gases.

5) The product stream is sent to a fractionator.

A summary of the results for all streams is shown in Table E8.5.1.

8.16. New Technology

The major drive for FCC process improvements is the demand for

higher gases (LPG and olefins) and the decline in demand for residual oils.

Replacement of diesel fuels to gasoline will also drive FCC technology to

reduce its production in favour of lighter products. Environmental aspects

are being observed in developing new FCC unit. The following processes

have been developed and some have already been commercialised.

8.16.1. Deep Catalytic Cracking

DCC is a new FCC process using a new catalyst for heavy feed stocks to

give light olefins. The yield of olefins depends greatly on the type of

feedstock. Paraffenic feeds give the lightest propylene yield of 23 wt% and

6.9 wt% isobutylene (Hsu and Robinson, 2006).

8.16.2. Catalytic Pyrolysis Process

The catalytic pyrolysis process (CPP) is an extension of DCC but with

increased ethylene yield. The ratio of C

can be adjusted by controlling

the operating conditions. C

can be produced up to around 20 wt% while

can be adjusted between 10 and 20 wt%. It is suggested to optimise the

use of crude oil as a petrochemical feedstock by using a combination of

steam cracking (SC) and catalytic pyrolysis (CPP) as shown in Figure 8.11

(Hsu and Robinson, 2006).

230 Chapter 8