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

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Questions and Problems

6.1. 5000 BPD of vacuum residue (API ¼ 11.5) is fed into a delayed coker.

Estimate the material balance around the delayed coker and sulphur

content if the feed contains 0.38 wt% of sulphur and CCR is 12%.

6.2. Make the mass balance for the coker gas (M ¼ 22.12 lb/lb mol),

produced from the vacuum residue in problem 6.1, based on the

chemical gas composition of the delayed coker (sulphur-free gas

basis).

6.3. In the operation of coil visbreaker, if a layer of carbon of 0.5 cm

thickness is deposited on the inside of a 10 cm diameter coil with an

800 m length. The coke is combusted by air in the coil and the heat is

removed by introducing steam at 400



C and exiting at 700



C. Cal-

culate the flow rate of steam for 60 h of cleaning time. Take C

1 kcal/kg



C; Density of Coke ¼ 1200 kg/m

; heat of combustion of

carbon ¼ 8000 kca1/kg and heat of combustion of sulphur ¼

2218 kcal/kg. Assume the coke contains 90% carbon and 10% sulphur.

6.4. Make a sketch for

(a) The Coil visbreaker

(b) The Delayed coker

6.5. A thousand pounds per hour of vacuum residue is fed into a flexicoker

which has a CCR of 15%. Find the circulation rate of coke between the

reactor and the burner in order to keep the temperature of the reactor,

heater and burner (gasifier) at 1000, 1300 and 1500



F, respectively.

The low Btu gas (LBG) flow rate is 2000 lb/h. The specific heat of

carbon ¼ 0.17 Btu/lb



F and the specific heat (Cp) for the gases ¼

0.24 Btu/lb



F. The net coke production in this case is 2.0 wt%.

Assume 70% of the coke is consumed in the burner.

6.6. 100,000 BPD of crude is introduced to the crude distillation unit

(CDU). The vacuum residue (VR 1050þ



F) from the vacuum

tower goes to the coker unit. The coker produces heavy naphtha

(HN) (190–380



F) to be combined with the (HN) from the CDU

and is fed to the reformer unit. Calculate the amount (lb/h) of the

reformate produced.

Crude assay equation:

TBPðKÞ¼ 2:2263 ln

100

100  LV%



0:368

þ 1

()

260

Crude API polynomial for atmospheric cuts as:

Thermal Cracking and Coking 151

API ¼0:0004 ðLV%Þ

þ 0:0506ðLV%Þ

 2:4284 LV% þ 76:149

Crude API polynomial for vacuum cuts:

API ¼0:0004ðLV%Þ

þ 0:0506ðLV%Þ

 2:4284 LV% þ 143:2

REFERENCES

Chang, S.H., and Robinson, O.R. (2006). ‘‘Practical Advances in Petroleum Processing,’’

vol. 1. Springer Science and Business Media, USA.

Feintuch, H.M., and Negin, K.M. (2003). In ‘‘FW Delayed-Coking Process,’’ (R.A.

Mayers, Ed.) 3rd ed., Chapter 12-2. Handbook of Petroleum Refining Processes.

McGraw Hill, USA.

Gary, J.H., and Handwerk, G.E. (2001). ‘‘Petroleum Refining.’’ Marcel Dekker, New

York.

Hammond, D.G., et al. (2003). Review of fluid coking and flexicoking technologies.

Petroleum Technology Quarterly, 8(5), 27–33.

Leprince, P. (2001). ‘‘Visbreaking of residues,’’ Chapter 11, In ‘‘Conversion Processes,’’

Petroleum Refining, Vol. 3, Leprince P. ed., TECHNIP, France.

Maples, R.E. (1993). ‘‘Petroleum Refinery Process Economics.’’ PennWell Books, Tulsa,

Oklahoma.

Parakash, S. (2003). ‘‘Refining Processes Handbook.’’ Elsevier, Amsterdam.

Speight, J. (1991). ‘‘The Chemistry and Technology of Petroleum,’’ 2nd ed., Marcel

Dekker, New York.

UNISIM Design Suite R370, Honeywell Process Solutions, Calgary, Alberta, Canada.

Zhorov, Y.H. (1987). ‘‘Thermodynamics of Chemical Processes,’’ MZR Publishers,

Moscow.

152 Chapter 6

CHAPTER SEVEN

Hydroconversion

7.1. Introduction

Hydroconversion is a term used to describe all different processes in

which hydrocarbon reacts with hydrogen. It includes hydrotreating, hydro-

cracking and hydrogenation. The term hydrotreating is used to describe

the process of the removal of sulphur, nitrogen and metal impurities in the

feedstock by hydrogen in the presence of a catalyst. Hydrocracking is the

process of catalytic cracking of feedstock to products with lower boiling points

by reacting them with hydrogen. Hydrogenation is used when aromatics are

saturated by hydrogen to the corresponding naphthenes. The use of the

hydroconversion technique depends on the type of feedstock and the desired

products as shown in the Table 7.1.

7.2. Hydrotreating

7.2.1. Objectives of Hydrotreating

Hydrotreating achieves the following objectives:

1. Removing impurities, such as sulphur, nitrogen and oxygen for the

control of a final product specification or for the preparation of feed

for further processing (naphtha reformer feed and FCC feed);

2. Removal of metals, usually in a separate guard catalytic reactor when

the organo-metallic compounds are hydrogenated and decomposed,

resulting in metal deposition on the catalyst pores (e.g. atmospheric

residue desulphurization (ARDS) guard reactor); and,

3. Saturation of olefins and their unstable compounds.

7.2.2. Role of Hydrotreating

Hydrotreating units are needed in the refinery to clean streams from

material such as sulphur, nitrogen or metals harmful to the catalysts. That

is why they are located before the reformer, hydrocracker and FCC as

shown in Figure 7.1. They are also needed to adjust the final product

Fundamentals of Petroleum Refining

2010 Elsevier B.V.

153

specification for various streams, such as light naphtha, kerosene and low

sulphur fuel oils (LSFOs).

Hence, the main role of hydrotreating can be summarized as follows:

1. Meeting finished product specification.



Kerosene, gas oil and lube oil desulphurization.



Olefin saturation for stability improvement.



Nitrogen removal.



De-aromatization for kerosene to improve cetane number, which is

the percentage of pure cetane in a blend of cetane and

alpha-methyl-naphthalene. The latter matches the ignition quality of

kerosene sample.

2. Feed preparation for downstream units:



Naphtha is hydrotreated for removal of metal and sulphur.



Sulphur, metal, polyaromatics and Conradson carbon removal from

vacuum gas oil (VGO) to be used as FCC feed.



Pretreatment of hydrocracking feed to reduce sulphur, nitrogen and

aromatics.

Table 7.1 Hydroconversion techniques

Feedstock Products Type of process

Impurities

removal

Naphtha Reformer feed Hydrotreating S

LPG Hydrocracking

Atmospheric

gas oil

Diesel Hydrotreating S, aromatics

Jet fuel Mild

hydrocracking

S, aromatics

Naphtha Hydrocracking

Vacuum gas

oil

Low sulphur fuel

oil (LSFO)

Hydrotreating S

FCC feed Hydrotreating S, N, metals

Diesel Hydrotreating,

Hydrocracking

S, aromatics

Kerosene/Jet Hydrotreating S, aromatics

Naphtha Hydrotreating S, aromatics

Lube oil Hydrotreating S, N,

aromatics

Residuum LSFO Hydrotreating S

FCC feedstock Hydrotreating S, N, CCR

and metals

Coker feedstock Hydrotreating S, CCR,

metalsDiesel Hydrocracking

154 Chapter 7

7.2.3. Chemistry of Hydrotreating

Petroleum fraction contains foreign elements, such as S, N, O, and Cl,

which can be removed by hydrotreating. Hydrotreating is also used to

transform unsaturated compounds to the corresponding saturated hydro-

carbons. Sometimes it is used to safeguard the catalyst from metals and

organo-metallic compounds by providing a cheap guard catalyst where

metals are deposited. Hydrotreating reactions can be classified as follows:

1. Desulphurization

a. Mercaptanes:

RSH þ H

! RH þ H

S ð7:1Þ

Gas from other units

Alkylation

Reformer

Hydrocracker

Gasoline

Aromatics

Kerosene

Fuel Oils

LN Naphtha

HN Naphtha

Kerosene

Atm. Gas oil

FCC

Vacuum

gas oil

Vacuum

crude

distillation

Coker

DAO

Asphalt

Coke

LPG

Olefins

Crude Oil

Crude desalter

Deasphalting

Aromatics

extraction

Polymerization

Figure 7.1 Role of hydrotreating (HT ) in the refinery

Hydroconversion 155

b. Sulphides:

S þ 2H

! 2RH þ H

S ð7:2Þ

c. Disulphides:

RSðÞ

þ 3H

! 2RH þ 2H

S ð7:3Þ

d. Thiophenes:

+ 4H

+ H

ð7:4Þ

2. Denitrogenation

a. Pyrrole:

NH þ 4H

! C

þ NH

ð7:5Þ

b. Pyridine:

N þ 5H

! C

þ NH

ð7:6Þ

3. Deoxidation

a. Phenol:

OH þ H

! C

þ H

O ð7:7Þ

b. Peroxides:

OOH þ 3H

! C

þ 2H

O ð7:8Þ

4. Hydrogenation of chlorides

RCl þ H

! RH þ HCl ð7:9Þ

5. Hydrogenation of olefins

þ H

! C

ð7:10Þ

6. Hydrogenation of aromatics

þ 3H

! Cyclohexane ð7:11Þ

Naphthalene þ H

! Tetraline ð7:12Þ

156 Chapter 7

7. Hydrogenation of organo-metallic compounds and deposition of metals

+ H

2NH

+ 2R

H + V

ð7:13Þ

Vanadium deposited as vanadium sulphide (V

)

8. Coke formation by the chemical condensation of polynuclear radicals

Coke

ð7:14Þ

7.2.4. Hydrotreating Catalysts

The hydrotreating catalyst is a porous alumina matrix impregnated with com-

binations of cobalt (Co), nickel (Ni), molybdenum (Mo) and tungsten (W).

The catalysts mainly have pores with a surface area of (200–300 m

/g). Co–Mo

catalysts are the most popular for desulphurization of straight run petroleum

fractions. Ni–Mo catalysts are chosen when higher activity is required for the

saturation of polynuclear aromatic compounds or for the remov al of nitrogen

and refractory sulphur compounds, while Ni–W catalysts are chosen only

when very high activity aromatic saturation is required (Speight, 2000). The

reactivities of each catalyst are given in Table 7.2. The pore size of a

catalyst should be chosen carefully because a smaller size will favour hydro-

desulphurization (HDS) on the expense of hydrodemetallization (HDM).

Table 7.2 Reactivities of hydrotreating catalysts

Catalyst

Hydro-

desulphurization

Hydro-

denitrogenation

Aromatics

hydrogenation

Co–Mo/Alumina Excellent Good Fair

Ni–Mo/Alumina Very good Very good Good

Ni–W/Alumina Good Good Excellent

Hydroconversion 157

In the case of a guard reactor, which is used to protect the main catalyst from

metal deposition, catalysts with wide pores are chosen and are generally plugged

by metal deposition. Figure 7.2,showsthatcatalystswithdifferentporedia-

meters can influence the balance between hydrodesulphurization and hydro-

demetallization (Raseev, 2003).

Catalysts are available in several different compositions and reactivity.

Cobalt–molybdenum (Co–Mo) and nickel–molybdenum (Ni–Mo) catalysts

resist poisoning and are the most universally applied catalysts for hydrodesul-

phurization of feedstocks ranging from naphtha to residue. In addition,

they promote both demetallization and desulphurization. The vanadium

deposition rate at a given desulphurization level is a function of the pore

structure of the alumina support and the types of metals on the support.

7.2.5. Thermodynamics of Hydrotreating

Hydrotreating reactions are highly exothermic and reversible. Table 7.3 lists

typical heat of reaction values for some hydrotreating reactions. The loga-

rithm equilibrium constants (K

) for several hydrodesulphurization reactions

are positive (Figure 7.3), indicating that the reactions can virtually proceed to

completion if hydrogen is present in the stoichiometric quantity. The reac-

tions given below as reaction A, B and C are examples of such behaviour.

Although the equilibrium conversions decrease with temperature rise,

1 2 3 4 5 6 7 8 9 10

Wide pore

HDM

Catalyst

Large pore

Catalyst

Small pore

Catalyst

Sulfur feed/sulfur product

Vanadium feed/vanadium product

Figure 7.2 General relationship between vanadium and sulphur removal for different

Co^Mo catalyst

158 Chapter 7

commercial hydrotreating is run at rather high temperatures to promote the

kinetics of these reactions.

A: CH

SH þ H





þ H

S ð7:15Þ

Thio

hene

+ 4H

+ H

ð7:16Þ

+ 2H

+ H

hen

lDibenzothio

hene

ð7:17Þ

012345

100

ln(K

)

1000/Temperature (1/K)

Figure 7.3 Thermodynamic limitations of HDS

Table 7.3 Heat of reaction for some hydrotreating reactions

Reaction DH (kJ/mol)

Desulphurization of sulphides 117.0

Desulphurization of disulphides 209.0

Desulphurization of thiophenes 284.2

Deoxidation of phenol 66.9

Hydrogenation of benzene 209.0

Hydrogenation of naphthene 125.4

Hydroconversion 159

The equilibrium constant of reaction A can be expressed as:

ðK

ð7:18Þ

Olefin hydrogenation is usually complete; howeve r an aromatic hydr o-

genation reaction can reach equilibrium under certain condition, as in

kerosene and gas oil hydrogenation. Hydr odesul phurizat ion c an be carried

out easie r than deni trogenation, while aromatic saturation is the most

difficult reaction.

A conversion of 0.99 for thiophene hydrogenated to butane can be

achieved at 15 bar (220 psia) and 395



C (743



F), while pyridine conversion

to pentane at the same condition is 0.97. A conversion of 0.4 is achieved for

hydrogenation of naphthalene (aromatics) to tetra-hydronaphthalene at the

same operating conditions.

The thermodynamics of hydrogenation of benzene to cyclohexane can

be studied by finding the equilibrium conversion (x

). Saturation of aro-

matic compounds is important in hydrotreating. This reaction is reversible

and the equilibrium between the forward and reverse reaction can hinder

the extent of saturation at normal operating conditions. The hydrogenation

reaction is favoured by high hydrogen partial pressures and low operating

temperatures. This can be illustrated by considering the simplest aromatic

hydrogenation/dehydrogenation reaction, the one between benzene and

cyclohexane.

Example E7.1

Consider the following dehydrogenation reaction:

clohexane (CH) Benzene (BZ) H

dro

+ 3H

Determine and plot the equilibrium conver sion as a func tion of temperature and

pressure, given the following data:

The free energy DG ¼ 200; 100  396T

where DG is in J/mol and T is in Kelvin

The equilibrium constant, K

, is related to the free energy as follows:

DG ¼RTln K

where R is the gas law constant.

160 Chapter 7