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

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The equilibrium conversion is calculated as follows

moles of cyclohexane ¼ 1 bbl

42 gal

1 bbl

264:17 gal

0:779 g

1 mol

84:16 g

¼ 1471:63 mol

moles of hydrogen ¼

10;000 SCF

1 bbl



1 mol

379 SCF



1bbl of CHðÞ¼26:39mol

We will assume X moles is transformed to benzene (B) and 3X to hydrogen

). The equilibrium constant can be expressed as:

K ¼

The mole fractions of products are:

moles of B ¼ X

moles of CH¼ 1471.63  X

moles of H

¼ 3X þ 26.39

Total mole (1498 þ 3X)

The equilibrium constant can be expressed as:

K ¼

ðÞy

ðÞ

6:55  10

1498 þ 3X



3X þ 26:39

1498 þ 3X



20ðÞ

1471:63  X

1498 þ 3X



solving X ¼ 1464 mol

Equilibrium conversion of CH ¼

moles of CH reacted

moles of CH fed

1464

1471:63

¼ 0:995

Barrels of benzene produced are:

¼ 1464 mol

78:11 g

1 mol



0:879 g



264:17 gal



1 bbl

42 gal



¼ 0:818 bbl

Catalytic Reforming and Isomerization 101

5.2.6. Reaction Kinetics and Catalysts

The catalyst used for reforming is a bifunctional catalyst composed of platinum

metal on chlorinated alumina. Platinum acts as the centre for the dehydrogena-

tion reaction, and chlorinated alumina acts as an acidic site to promote structure

changes, such as cyclization of paraffins and isomerization of the naphthenes.

Recently additional elements have been added to platinum to promote

additional properties for the catalyst. Iridium (Ir) is added to boost activity,

Rhenium (Re) is added to operate at lower pressures and Tin (Sn) is added

to improve yield at low pressures. The use of Pt/Re is now most common

in semi-regenerative (SR) processes with Pt/Sn is used in moving bed

reactors. The quantity of chlorine used is approximately 1 wt% of the

catalyst and the quantity of platinum is from 0.2 to 0.6 wt%. Impurities

that might cause deactivation or poisoning of the catalyst include: coke,

sulphur, nitrogen, metals and water. Because of these problems, the

reformer feed has to be severely hydrotreated to remove most of these

impurities, and the reformer should be operated at high temperature and

low pressure to minimize coke deposition.

Paraffin and naphthene dehydrogenation reactions are very rapid and

usually occur in the first reactor. The isomerization of paraffin and naphthenes

is fast, whereas hydrocracking is slow and takes place in the last reactor. The

effect of operating conditions on reaction rate and other properties is shown

in Table 5.1.

5.2.7. Process Technology

There are several commercial processes available for reforming. These

include Platforming (UOP), Powerforming (Exxon), Magna forming

(Engelhard), Catalytic reforming (IFP), Rheniforming (Chevron) and

Ultra forming (Amoco). The old technologies are fixed bed configuration.

Moving bed technology has also recently been introduced.

5.2.7.1. Semi-regenerative Fixed Bed Process

The schematic flow diagram of this process is shown in Figure 5.5. The

name semi-regenerative comes from regeneration of the catalyst in the fixed

bed reactors after shut down by burning off the carbon formed on the

catalyst surface.

Reactions such as dehydrogenation of paraffins and naphthenes which

are very rapid and highly endothermic (Table 5.1) occur in the first reactor,

with high temperature drop. Reactions that are considered rapid, such as

paraffin isomerization and naphthens dehydroisomerization, give moderate

temperature decline in the second reactor. Furthermore, slow reactions

such as dehydrocyclization and hydrocracking (Table 5.1) give low

temperature decline in the third reactor.

102 Chapter 5

Heavy

Naphtha

Heater

Reactor

Compressor

Recycle H

Net H

Stabilizer

Separator

Off gases

C3’s and C4’s

Reformate

Figure 5.5 Semi-regenerative (SR) fixed bed reforming process

Catalytic Reforming and Isomerization 103

Table 5.1 Thermodynamic and kinetic comparison and effect of operating condition on main reactions and products (Gary and

Handwerk, 1994)

Reaction

type

Reactive

rate

Heat

effect

Re action

equilibrium

Pressure

effect

Temperature

effect

production RVP

Product

density Yield Octane

Naphthene

dehydrogenation

Very

rapid

Very

endo

Yes þ Produce þ  þ

Naphthene

dehydroisomerization

Rapid Very

endo

Yes þ Produce þ  þ

Parafﬁn isomerization Rapid Slight

exo

Yes None Slight No þ Slight  Slight þþ

Parafﬁn

dehydrocyclization

Slow Very

endo

No þ Produce þ  þ

Hydrocracking Very slow exo No þþ þþ Consume þ  þ

RVP is Reid vapour pressure.

104 Chapter 5

The temperature and concentration profile in each reactor is shown in

Figure 5.6. To prevent catalyst coking, the hydrogen partial pressure is

maintained at a level such that the hydrogen-to-hydrocarbon ratio by

weight (H

/HC) is greater than 25 for monometallic catalyst. This is

done by recycling some of the hydrogen produced (Figure 5.5). Some

light hydrocarbons (C

–C

) are separated from the reformate in the

stabilizer. At the top of t he stabilizer res idual hydrogen and C

to C

are

withdrawn as condenser products, which are then sent to gas processing,

and part of the liquid product (C

and C

)isreturnedfromthereflux

=60 vol%

=30 vol%

=10 vol%

Aromatics

Paraffins

Naphthenes

Figure 5.6 (A) Furnace and reactor layout for fixed bed reforme r (B) Variation of tem-

perature in the reactors. (C) Variation in effluent compositions; P

, initial Paraffins;

, initial Napthenes and A

, intial Aromatics (Martino, 2001)

Catalytic Reforming and Isomerization 105

drum back to the stabilizer (Figure 5.6). The main product of the column

is stabilized reformate, which is sent to the gasoline blending plant.

A slight modification to the semi-regenerative process is to add an extra-

reactor to avoid shutting down the whole unit during regeneration. Three

reactors can be running while the forth is being regenerated. This modified

process is called the ‘‘cyclic fixed bed’’ process.

5.2.7.2. Continuous Regenerative (moving bed) CCR Platforming

UOP Process

In this process, three or four reactors are installed one on the top of the

other. The schematic flow diagram of the continuous regenerative process

(CCR) is shown in Figure 5.7 (Martino, 2001). UOP has licensed this

process under the CCR Platforming process (Martino, 2001). The effluent

from each reactor is sent to a common furnace for heating. The catalyst

moves downwards by gravity from the first reactor (R1) to the forth reactor

(R4). The catalyst is sent to the regenerator to burn off the coke and then

sent back to the first reactor R1. The final product from R4 is sent to the

stabilizer and gas recovery section.

The process can be operated at lower hydrogen partial p ressure

¼ 3 bar) compared to the semi-generative process (P

¼ 35

bar), with a reformate yield gain of nearly 10 vol%. Table 5.2 gives a

comparison of the operating conditions for the three reforming

processes.

5.2.8. Material Balance in Reforming

5.2.8.1. Material Balance Using Emperical Correlations

Catalytic reforming data base compiled by Maples (1993) were correlated

using multiple regression. Yield correlations for the reformer were devel-

oped as given in Table 5.3. The correlation coefficients were in the range of

0.990 – 0.999.

5.2.8.2. Material Balance Using Conversion Criteria

If detailed analysis of the reformer feed is known, the feed conversion can be

calculated from the conversion data for each class of compounds as shown in

Tables 5.4 and 5.5.

5.2.9. Process Simulation of Reformer by Equilibrium

Reactions

The process simulator UNISIM can be used to calculate the material balance

around each of the three reactors. Energy balance can also be calculated for each

reactor, furnace and other units in the schematic diagram given in Figure 5.5.

Simulation of equilibrium of dehydrocyclizatio n and hydrocracking reactions

using the process simulator are illustrated in examples E5.4 and E5.5.

106 Chapter 5

Feed

Gas Recovery

Furnace

Debutanizer

Reactor

Exchanger

Separator

Catalyst

Regenerator

Furnaces

Reformate

−

Figure 5.7 Continuous regenerative reformer (CCR), UOP Platform ing process

Catalytic Reforming and Isomerization 107

Table 5.2 Typical operating conditions of three reforming processes (Martino, 2001)

Catalyst P (bar)

/HC

(mol/mol)

Space

velocity

1

)RON

Semi-

generative

ﬁxed bed

Monometallic >25 >25 1–2 90–92

Bimetallic 12–20 4–6 2–2.5 81–98

Cyclic bed Bimetallic 15–20 4 2 96–98

Continuous

moving

bed

Bimetallic 3–10 2 2–3 100–102

>104 for

aromatic

production

Table 5.3 Reformer correlations

Correlation Equations

wt% ¼12.1641 þ 0.06134  C

vol % þ 0.099482  RON

(5.8)

wt% ¼ 11.509  0.125  C

vol % (5.9)

wt% ¼ 16.496  0.1758  C

vol % (5.10)

wt% ¼ 24.209  0.2565  C

vol % (5.11)

Total C

wt%

¼ 27.024  0.2837  C

vol % (5.12)

wt% ¼ 0.585  total C

wt% (5.13)

wt% ¼ 0.415  total C

wt% (5.14)

vol% ¼0:03806  RON

þ 6:292793  RON

 14:4337  K (5.15)

vol% ¼ 132.2483 þ 0.66472  RON

þ 0.297921  RON

(5.16)

vol% ¼ 142.7914  0.77033  RON

þ 0.219122  (N þ 2A)

(5.17)

SCFB H

¼ 0.0002 þ 0.48848 H

wt% (5.18)

RON

¼ research octane number of feed; RON

¼ research octane number of reformate;C

vol% ¼

volume percent of reformate yield; SCFB H

¼ standard cubic foot of H

produced/barrel of feed;

K ¼ characterization factor (T

)

1/3

/SG; T

¼ absolute mid-boiling of feed,



R;SG¼ specific gravity

of feed; N ¼ napthenes vol % and A ¼ aromatics vol %.

Example E5.2

100 m

/h of heavy naphtha (HN) with speciﬁc gravity of 0.778 has the

following composi tion: A ¼ 11.5 vol%, N ¼ 21.7 vol% and P ¼ 66.8 vol% is

to be reformed to naphtha reformate of RON ¼ 94. Calculate the yields of each

product for that reformer.

Solution:

Given RON

¼ 94 and N þ 2A ¼ 44.7%, equation (5.18) gives the following

reformer yield:

108 Chapter 5

Table 5.4 Naphthenes conversion to aromatics by dehydrogenation (Gary and

Handwerk, 2001)

Feed (naphthenes) Conversion Products (aromatics)

Methylcylohexane (MCC

) 0.98 Toluene

Cyclohexane (CC

) 0.98 Benzene

Dimethylcyclopentane (DMCC

) 0.98 Toluene

Dimethylcyclohexan e (DMCC

) 0.98 Xylene

Cycloheptane (CC

) 0.98 Toluene

Methylcycloheptane (MCC

) 0.98 Xylene

Table 5.5 Parafﬁn conversion to aromatics by dehydrocyclization (Gary

and Handwerk, 2001)

Feed (paraffi ns) Conversion Products (aromatics)

Hexane 0.05 Benzene

Heptane 0.10 Toluene

Octane 0.25 Xylene

Nonane 0.45 1,3,4-Trimethylbenzene (TMB)

Decane 0.45 1,2,3,4Teramethyl benzene

vol% ¼ 142.7914  0.77033  (94) þ 0.219122  (44.7) ¼ 80.17%

The yields for the other products can then be calculated from the correlations in

Table 5.3. The material balance for the reformer is presented in the following

table:

vol%

Volume

/h)

Density

(kg/m

)wt%

Yield

(kg/h)

Feed HN 100 100 778 100 77,800

Products

wt% 2.1051 1637

wt% 1.4871 1157

wt% 2.4012 1868

wt% 3.6441 2835

Total C

wt% 4.2784 3328

wt% 2.5029

wt% 1.7755

vol% 80.17 80.17 835 (86.086) (66,975)

Values in brackets are calculated by material balance between feed and products.

Catalytic Reforming and Isomerization 109

Example E5.3

Heavy naphtha, which has the following detailed analysis in mol%, is fed to a

reformer unit.

Cycloheptane (C

): 20%

Heptane (C

): 40%

Dimethylcyclohexan e (C

): 30%

1,3,5-Trimethylbenzene (C

): 10%

Find the composition of the products.

Solution:

The data, given in Tables 5.4 and 5.5, are used to estimate the composition of

the products as follows:

For 100 mol of feed:

^ Naphthenes

20 mol of cycloheptane (C

) will give 0.98 (20) ¼ 19.6 mol Toluene

30 mol of dimethy lcyclohexane (C

) will give 0.98  (30) ¼ 29.4 Xylene

^ Paraffins

40 mol of heptane (C

) will give 0.10 (40) ¼ 4.0 mol Toluene

^ Aromatics

1,3,5-trimethylbenzene stays as it is.

Final composition (mol%)

Cycloheptane Heptane Toluene DMCC

Xylene 1,3,5-TMB

0.4% 36% 23.6% 0.6% 29.4% 10%

Example E5.4

The following feed of 100 lb mol/h of naphtha was introduced to a reformer

Compound C

mol% feed 25 25 25 25

a 32.6  10

31.24  10

29.7  10

28.94  10

b 51.17 53.36 53.36 53.78

Knowing that:

ln K

¼ a=T þ b

is the equilibriu m constant and T in K.

110 Chapter 5