Matar Sami, Hatch Lewis F. Chemistry of petrochemical processes
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The second step is the vapor phase reaction of methylal with isobutene to
produce isoprene.
2-Butene in the C
4
mixture also reacts with methylal but at a slower
rate to give isoprene. 1-Butene reacts slowly to give 1,3-pentadiene.
From Propylene (Goodyear Process)
Another approach for producing isoprene is the dimerization of propy-
lene to 2-methyl-1-pentene. The reaction occurs at 200°C and about 200
atmospheres in the presence of a tripropyl aluminum catalyst combined
with nickel or platinum.
Crude Oil Processing and Production of Hydrocarbon Intermediates 107
The next step is the isomerization of 2-methyl-1-pentene to 2-methyl-2-
pentene using an acid catalyst.
2-Methyl-2-pentene is finally pyrolyzed to isoprene.
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110 Chemistry of Petrochemical Processes
Chapter 3 1/22/01 10:58 AM Page 110
CHAPTER FOUR
Nonhydrocarbon
Intermediates
INTRODUCTION
From natural gas, crude oils, and other fossil materials such as coal,
few intermediates are produced that are not hydrocarbon compounds.
The important intermediates discussed here are hydrogen, sulfur, carbon
black, and synthesis gas.
Synthesis gas consists of a nonhydrocarbon mixture (H
2
,CO) obtain-
able from more than one source. It is included in this chapter and is fur-
ther noted in Chapter 5 in relation to methane as a major feedstock for
this mixture. This chapter discusses the use of synthesis gas obtained
from coal gasification and from different petroleum sources for produc-
ing gaseous as well as liquid hydrocarbons (Fischer Tropsch synthesis).
Naphthenic acids and cresylic acid, which are extracted from certain
crude oil fractions, are briefly reviewed at the end of the chapter.
HYDROGEN
Hydrogen is the lightest known element. Although only found in the
free state in trace amounts, it is the most abundant element in the uni-
verse and is present in a combined form with other elements. Water, nat-
ural gas, crude oils, hydrocarbons, and other organic fossil materials are
major sources of hydrogen.
Hydrogen has been of great use to theoretical investigation. The struc-
ture of the atom developed by Bohr (Nobel Prize Winner 1922) was
based on a model of the hydrogen atom. Chemically, hydrogen is a very
reactive element. Obtaining hydrogen from its compounds is an energy-
extensive process. To decompose water into hydrogen and oxygen, an
energy input equal to an enthalpy change of +286 KJ/mol is required
1
:
111
Chapter 4 1/22/01 11:00 AM Page 111
H
2
O
r
H
2
+
1
/
2
O
2
∆H = +286 KJ/mol
Electrolysis, and thermochemical and photochemical decomposition of
water followed by purification through diffusion methods are expensive
processes to produce hydrogen.
The most economical way to produce hydrogen is by steam reforming
petroleum fractions and natural gas (Figure 4-1).
2
In this process, two
major sources of hydrogen (water and hydrocarbons) are reacted to pro-
duce a mixture of carbon monoxide and hydrogen (synthesis gas).
Hydrogen can then be separated from the mixture after shift converting
carbon monoxide to carbon dioxide. Carbon oxides are removed by pass-
ing the mixture through a pressure swing adsorption system. The shift
conversion reaction is discussed in relation to ammonia synthesis in
Chapter 5. The production of synthesis gas by steam reforming liquid
hydrocarbons is noted later in this chapter.
Recently, a new process has been developed to manufacture hydrogen
by steam reforming methanol. In this process, an active catalyst is used
to decompose methanol and shift convert carbon monoxide to carbon
dioxide. The produced gas is cooled, and carbon dioxide is removed:
CH
3
OH(g) + H
2
O(g)
r
CO
2
(g) + 3 H
2
(g)
112 Chemistry of Petrochemical Processes
Figure 4.1. A process for producing hydrogen by steam reforming of hydrocar-
bons:
2
(1) reforming furnace (2,3) purification section, (4) shift converter, (5) pres-
sure swing adsorption.
Chapter 4 1/22/01 11:00 AM Page 112
This process is used to produce relatively small quantities (0.18–1.8
MMscfd) of highly pure hydrogen when methanol is available at a rea-
sonable price.
In the petroleum refining industry, hydrogen is essentially obtained from
catalytic naphtha reforming, where it is a coproduct with reformed gasoline.
The use of hydrogen in the chemical and petroleum refining industries
is of prime importance. Hydrogen is essentially a hydrogenating agent.
For example, it is used with vegetable oils and fats to reduce unsaturated
esters (triglycerides). It is also a reducing agent for sulfide ores such as
zinc and iron sulfides (to get the metals from their ores).
Hydrogen use in the petroleum refining includes many processing
schemes such as hydrocracking, hydrofinishing of lube oils, hydrodealkyla-
tion and hydrodesulfurization of petroleum fractions and residues. Hydro-
cracking of petroleum resids is becoming more important to produce lighter
petroleum distillates of low sulfur and nitrogen content to meet stringent
government-mandated product specifications to control pollution.
In the petrochemical field, hydrogen is used to hydrogenate benzene
to cyclohexane and benzoic acid to cyclohexane carboxylic acid. These
compounds are precursors for nylon production (Chapter 10). It is also
used to selectively hydrogenate acetylene from C
4
olefin mixture.
As a constituent of synthesis gas, hydrogen is a precursor for ammo-
nia, methanol, Oxo alcohols, and hydrocarbons from Fischer Tropsch
processes. The direct use of hydrogen as a clean fuel for automobiles and
buses is currently being evaluated compared to fuel cell vehicles that use
hydrocarbon fuels which are converted through on-board reformers to a
hydrogen-rich gas. Direct use of H
2
provides greater efficiency and envi-
ronmental benefits.
3
Due to the increasing demand for hydrogen, many separation tech-
niques have been developed to recover it from purge streams vented from
certain processing operations such as hydrocracking and hydrotreating.
In addition to hydrogen, these streams contain methane and other light
hydrocarbon gases. Physical separation techniques such as adsorption,
diffusion, and cryogenic phase separation are used to achieve this goal.
Adsorption is accomplished using a special solid that preferentially
adsorbs hydrocarbon gases, not hydrogen. The adsorbed hydrocarbons
are released by reducing the pressure. Cryogenic phase separation on the
other hand, depends on the difference between the volatilities of the com-
ponents at the low temperatures and high pressures used. The vapor
phase is rich in hydrogen, and the liquid phase contains the hydrocar-
bons. Hydrogen is separated from the vapor phase at high purity.
Nonhydrocarbon Intermediates 113
Chapter 4 1/22/01 11:00 AM Page 113
Diffusion separation processes depend on the permeation rate for gas
mixtures passing through a special membrane. The permeation rate is a
function of the type of gas feed, the membrane material, and the operat-
ing conditions. Gases with smaller molecular sizes such as helium and
hydrogen permeate membranes more readily than larger molecules such
as methane and ethane.
4
An example of membrane separator is the hol-
low fiber type shown in Figure 4-2. After the feed gas is preheated and
filtered it enters the membrane separation section. This is made of a per-
meater vessel containing 12-inch diameter bundles (resemble filter car-
tridges) and consists of millions of hollow fibers. The gas mixture is
distributed in the annulus between the fiber bundle and the vessel wall.
Hydrogen, being more permeable, diffuses through the wall of the hol-
low fiber and exits at a lower pressure. The less permeable hydrocarbons
flow around the fiber walls to a perforated center tube and exit at approx-
imately feed pressure. It has been reported that this system can deliver a
reliable supply of 95+% pure hydrogen from off-gas streams having as
low as 15% H
2
.
5
SULFUR
Sulfur is a reactive, nonmetallic element naturally found in nature in a
free or combined state. Large deposits of elemental sulfur are found in
various parts of the world, with some of the largest being along the
coastal plains of Louisiana. In its combined form, sulfur is naturally pres-
ent in sulfide ores of metals such as iron, zinc, copper, and lead. It is also
a constituent of natural gas and refinery gas streams in the form of hydro-
gen sulfide. Different processes have been developed for obtaining sul-
fur and sulfuric acid from these three sources.
The Frasch process, developed in 1894, produces sulfur from under-
ground deposits.
Smelting iron ores produces large amounts of sulfur dioxide, which is
catalytically oxidized to sulfur trioxide for sulfuric acid production. This
process is declining due to pollution control measures and the presence
of some impurities in the product acid.
Currently, sulfur is mainly produced by the partial oxidation of hydro-
gen sulfide through the Claus process. The major sources of hydrogen
sulfide are natural gas and petroleum refinery streams treatment opera-
tions. It has been estimated that 90–95% of the world’s recovered sulfur
is produced through the Claus process.
6
Typical sulfur recovery ranges
from 90% for a lean acid gas feed to 97% for a rich acid gas feed.
7
114 Chemistry of Petrochemical Processes
Chapter 4 1/22/01 11:00 AM Page 114
Nonhydrocarbon Intermediates 115
Figure 4-2. Permeator for gas separation.
5
Chapter 4 1/22/01 11:00 AM Page 115
USES OF SULFUR
The most important use of sulfur is for sulfuric acid production. Other
uses range from dusting powder for roses to rubber vulcanization to sulfur-
asphalt pavements. Flower sulfur is used in match production and in cer-
tain pharmaceuticals. Sulfur is also an additive in high pressure lubricants.
Sulfur can replace 30–50% of the asphalt in the blends used for road
construction. Road surfaces made from asphalt-sulfur blends have nearly
double the strength of conventional pavement, and it has been claimed
that such roads are more resistant to climatic conditions. The impregna-
tion of concrete with molten sulfur is another potential large sulfur use.
Concretes impregnated with sulfur have better tensile strength and cor-
rosion resistance than conventional concretes. Sulfur is also used to pro-
duce phosphorous pentasulfide, a precursor for zinc dithiophosphates
used as corrosion inhibitors.
Sulfur reacts with nitrogen to form polymeric sulfur nitrides (SN
x
) or
polythiazyls. These polymers were found to have the optical and electri-
cal properties of metals.
8
THE CLAUS PROCESS
This process includes two main sections: the burner section with a
reaction chamber that does not have a catalyst, and a Claus reactor sec-
tion. In the burner section, part of the feed containing hydrogen sulfide
and some hydrocarbons is burned with a limited amount of air. The two
main reactions that occur in this section are the complete oxidation of
part of the hydrogen sulfide (feed) to sulfur dioxide and water and the
partial oxidation of another part of the hydrogen sulfide to sulfur. The
two reactions are exothermic:
H
2
S +
3
/
2
O
2
r
SO
2
+ H
2
O ∆H = –519 to –577 KJ
3H
2
S +
3
/
2
O
2
r
3/x S
x
+ 3H
2
O ∆H = –607 to –724 KJ
In the second section, unconverted hydrogen sulfide reacts with the
produced sulfur dioxide over a bauxite catalyst in the Claus reactor.
Normally more than one reactor is available. In the Super-Claus process
(Figure 4-3), three reactors are used.
9
The last reactor contains a selective
oxidation catalyst of high efficiency. The reaction is slightly exothermic:
2H
2
S + SO
2
r
3/x S
x
+ 2H
2
O ∆H = –88 to –146 KJ
116 Chemistry of Petrochemical Processes
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