Lecture 11

Lecture 11
HYDROCONVERSION
Hydroconversion is a term used to describe all different processes in which

hydrocarbon reacts with hydrogen. It includes;
1- Hydrotreating: The process of the removal of sulphur, nitrogen and metal

impurities in the feedstock by hydrogen in the presence of a catalyst.
2- Hydrocracking: The process of catalytic cracking of feedstock to products with

lower boiling points by reacting them with hydrogen.
3- Hydrogenation: The process in which 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.
Table 7.1: Hydroconversion techniques

Hydrotreating
Hydrotreating achieves the following objectives:
1. Removal of 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).
3. Saturation of olefins and their unstable compounds.
Chemistry of Hydrotreating
1. Desulphurization
a. Mercaptanes:
b. Sulphides:
c. Disulphides:
d. Thiophenes:
2. Denitrogenation
a. Pyrrole:
b. Pyridine:
3. Deoxidation
a. Phenol:
b. Peroxides:
4. Hydrogenation of chlorides
5. Hydrogenation of olefins
6. Hydrogenation of aromatics
7. Hydrogenation of organo-metallic compounds and deposition of

metals
8. Coke formation by the chemical condensation of polynuclear
radicals
Hydrotreating Catalysts
The hydrotreating catalyst is a porous alumina matrix impregnated with
combinations of cobalt (Co), nickel (Ni), molybdenum (Mo) and tungsten (W).
The catalysts mainly have pores with a surface area of (200–300 m2/g).
 Co–Mo catalysts are used 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 removal of
nitrogen and refractory sulphur compounds.
 Ni–W catalysts are used 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
favor hydrodesulphurization (HDS) on the expense of hydrodemetallization
(HDM).
Figure 7.2 General relationship between vanadium and sulphur removal for different
Co/Mo catalyst.
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, shows that catalysts with different pore diameters can influence the
balance between hydrodesulphurization and hydrodemetallization.
Problem 7.1
Solution:
Basis: 1 mole of cyclohexane
The equilibrium constant (Keq) can be written in terms of reactants and products
partial pressure as follows:
Let x equal the fraction of cyclohexane converted, the following table can be
obtained:
Thus,
The equilibrium constant, Keq, and amount of benzene converted can be calculated
at several temperatures and pressures as shown graphically in Figure 7.3.
Figure 7.3 Effect of temperature and pressure on the equilibrium conversion of benzene to
cyclohexane
 The plots indicate that hydrogenation of benzene to produce cyclohexane

will occur at high pressures and low temperatures, whereas
 The reverse reaction in which cyclohexane is dehydrogenated to produce

benzene will be maximized at high temperatures and low pressures.
Reaction Kinetics
If we assume that the rate of a hydrotreating reaction follows n order:
Eq. 1
On integration
Eq. 2
Where t is the reaction time (h), k is the reaction rate constant (h-1), Co is the initial
sulphur content in feedstock (wt. %), C is the final sulphur content in the product
(wt. %), and n is the reaction order ≠ 1.
Equation (1) can be integrated for first order to kt = ln(C/Co).
The first order is found for the narrow cuts (naphtha and kerosene). Reaction order
n >1.0 (1.5–1.7) is found for gas oil and 2.0 for VGO or residue.
Problem 7.2
Find the catalyst volume needed for the desulphurization of VGO. The initial
sulphur content is 2.3 wt% and the final sulphur content of the product is 0.1 wt%.
The reaction rate constant (h-1) can be expressed as:
The reaction conditions are T = 415 oC and P = 5.1 MPa. The order of the reaction
was found to be n =1.7. The feed flow rate is 167,500 kg/h and has a density of 910
kg/m3.
Hydrotreating Process
 Hydrotreator is usually used to remove the impurities (mainly sulphur) so

that the hydrotreated feed can be introduced to other catalytic units (e.g.
reformer).
 The expensive platinum based catalyst used in the reformer is sensitive to
poisoning by such impurities.
 A schematic flow diagram for the hydrotreating process is shown in Figure
7.1.
 The main elements are
1. Feed Heater,
2. Reactor (Catalytic bed of Co–Mo on alumina s used)
3. Separators
4. Gas Scrubbers and
5. Treated Naphtha Fractionator
Figure 7.1 The main elements of a hydrotreating process
Process:
 The liquid feed is mixed with hydrogen and fed into a heater and the mixture
is brought to the reaction temperature in a furnace and then fed into a fixed
bed catalytic reactor.
 The effluent is cooled and hydrogen-rich gas is separated using a high
pressure separator.
 Before the hydrogen is recycled, hydrogen sulphide can be removed using
an amine scrubber.
 Some of the recycle gas is also purged to prevent the accumulation of light
hydrocarbons (C1–C4) and to control hydrogen partial pressure.
 The liquid effluent form the reactor is introduced to a fractionator for
product separation.
Table 7.1 Process parameters for hydrotreating different feedstocks (Heinrich

and Kasztelan, 2001)
Make-up Hydrogen
A certain hydrogen partial pressure should be maintained in the reactors by

recycling un-reacted hydrogen and adding make-up hydrogen to compensate for
the amount consumed.
The make-up hydrogen can be calculated by the following expression (Kaes,

2000):
Make-up hydrogen = hydrogen in feed - hydrogen consumed for chemical
requirement - hydrogen purged - amount of hydrogen dissolved in product
Hydrogen requirements for hydrotreating are classified into:
(1) Chemical requirement: This is the amount of hydrogen required to remove

impurities such as sulphur, oxygen, nitrogen, olefins and organometallic
compounds, according to the stoichiometry of these reactions.
Sometimes, it might be required to convert aromatics and naphthenes to

corresponding paraffins.
(2) Hydrogen lost due to the dissolution of hydrogen in the hydrocarbons treated.
This hydrogen can be predicted by an equation of state under hydrotreating
condition.
(3) Amount of hydrogen lost with the purging of light hydrocarbons (C1–C4) and
hydrogen sulphide (if not removed by amine treatment). This hydrogen can be
predicted using the purge gas ratio.
The purge ratio is defined as:
Table 7.2 Purge requirement of HDS processes

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Lecture 11

Hydroconversion is a term used to describe all different processes in which

1- Hydrotreating: The process of the removal of sulphur, nitrogen and metal

2- Hydrocracking: The process of catalytic cracking of feedstock to products with

3- Hydrogenation: The process in which aromatics are saturated by hydrogen to

Table 7.1: Hydroconversion techniques

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

3. Saturation of olefins and their unstable compounds.

7. Hydrogenation of organo-metallic compounds and deposition of

 Co–Mo catalysts are used for desulphurization of straight run

The reactivities of each catalyst are given in Table 7.2.

 The plots indicate that hydrogenation of benzene to produce cyclohexane

 The reverse reaction in which cyclohexane is dehydrogenated to produce

If we assume that the rate of a hydrotreating reaction follows n order:

Equation (1) can be integrated for first order to kt = ln(C/Co).

 Hydrotreator is usually used to remove the impurities (mainly sulphur) so

Figure 7.1 The main elements of a hydrotreating process

Table 7.1 Process parameters for hydrotreating different feedstocks (Heinrich

A certain hydrogen partial pressure should be maintained in the reactors by

The make-up hydrogen can be calculated by the following expression (Kaes,

Hydrogen requirements for hydrotreating are classified into:

(1) Chemical requirement: This is the amount of hydrogen required to remove

Sometimes, it might be required to convert aromatics and naphthenes to

The purge ratio is defined as:

Table 7.2 Purge requirement of HDS processes

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