Module:2
Lecture:9 Phthalic acid
Dr. N. K. Patel
Module: 2
Lecture: 9
PHTHALIC ACID
INTRODUCTION
Phthalic acid, C6H4(COOH)2 is an aromatic dicarboxylic acid. It is an
isomer of isophthalic acid and terephthalic acid. Although phthalic acid is of
modest commercial importance, the closely related derivative phthalic
anhydride is a commodity chemical produced on a large scale.
Auguste Laurent first synthesized phthalic acid by oxidizing
naphthalene tetrachloridein 1836. He believed that the resulting substance is
naphthalene derivative and named it as naphthalic acid. Later on Jean
Charles Galissard de Marignac determined its correct formula, Laurent
named as phthalic acid. In the nineteenth century it was manufactured by
oxidation of naphthalene tetrachloride with nitric acid, or, better, oxidation of
the hydrocarbon with fuming sulfuric acid, using mercury or mercury(II) sulfate
as a catalyst.
Phthalic acid was the first commercial product among the various
benzene polycarboxylic acids. It is obtained via phthalic anhydride. Phthalic
anhydride (C6H4(CO)2O) is the organic compound and it is the anhydride
form of phthalic acid. It was the first anhydride of a dicarboxylic acid which is
utilized commercially.
In 1930, vapour phase oxidation of naphthalene was carried out in the
presence of vanadium and molybdenum oxide catalyst to produce phthalic
anhydride while in 1896 liquid phase oxidation of naphthalene was carried
out by sulfuric acid in the presence of mercury salt which formed anhydride.
Widely available phthalic anhydride is converted to phthalate esters.
Derivatives like 1,3-cyclohexadiene was produced by the reduction of
phthalic acid with sodium amalgam in the presence of water.
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Module:2
Lecture:9 Phthalic acid
Dr. N. K. Patel
MANUFACTURE
Phthalic anhydride
1. Phthalic anhydride from naphthalene
Raw materials
Basis:1000kg phthalic anhydride from naphthalene
Naphthalene
1134kg
Air
830,000cu.ft
Reaction
Manufacture process
Naphthalene
Water
Salt
cooler
Melting
kettle
Primary
air
Vaporizer
Flue
gases
Water
Flue
gases
Water
Reactor
Vapor
cooler
Secondary
air
Air Air
Air
Water
Flaker
Air
condenser
Melt
tank
Column
Air
Phthalic
anhydride
Steam
Waste
Figure: Manufacture of phthalic anhydride from naphthalene
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Lecture:9 Phthalic acid
Dr. N. K. Patel
Block diagram of manufacturing process
Diagram with process equipment
Naphthalene was charged into melting kettle where it was melted and
pumped to a vaporizer, where it was vaporized with bubbling primary
preheated air through the molten material.
These primary air and naphthalene vapour mixture was passed from
reactor where additional (secondary) air was added to bring the airnaphthalene ratio to 18 - 22:1 by weight. At 357 – 4540C temperature and
0.1 – 0.6sec contact time throughout the reaction was maintained. Reactor
consisting of multiple tubes filled with vanadium pentoxide as a catalyst. Heat
of reaction mass was removed either by boiling mercury under suitable
pressure or by pumping salt across the tube bank. In the reactor the
naphthalene was oxidized to phthalic anhydride, carbon dioxide, and water.
Hot vapours coming out from reactor first cooled in vapour cooler that
reduced the gas temperature to just above the dew point (1260C) then led
to air condenser where crude phthalic anhydride crystallized on the walls
which was further melted in melt tank using steam. Then melted mass was
purified using distillation and flake of phthalic anhydride was obtained which
was sent for packaging.
2. Phthalic anhydride from o-xylene
Raw material
Basis: 1000kg phthalic anhydride from o-xylene
O-xylene
880kg
Air
744,000cu.ft
Reaction
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Lecture:9 Phthalic acid
Dr. N. K. Patel
Manufacture process
Purified air and o-xylene were first heated in vaporizer and preheater.
These hot vapours were mixed properly in mixer at 140 – 1500C and under a
pressure of 2atm. After that this hot vapour mixture was sent to the reactor
where oxidation takes place in presence of vanadium pentoxide as catalyst
at 5500C. Reactor contain tubes which were filled with catalyst.
Water
Salt
cooler
Flue
gases
o-xylene
Water
Preheater
Water
Flue
gases
Mixer
Vapor
cooler
Reactor
Water
Air Air
Air
Condenser
Filter
Flaker
Flue
gases
Column
Vaporizer
Flue
gases
Slack
gases
Melt
tank
Phthalic
anhydride
Steam
Waste
Figure: Manufacture of phthalic anhydride from o-xylene
Block diagram of manufacturing process
Diagram with process equipment
Exothermic heat was absorbed by circulating molten salt in the space
between tubes and shell. After oxidation of o-xylene, it was sent to a vapour
cooler where it was cooled. Then mixture was passed through a switch
condenser where phthalic anhydride was sublimed and deposit on the
surface of condenser. The deposited film of phthalic anhydride was melted
by means of circulation of steam. The molten anhydride was then distilled
under vacuum for purification and coming out through a flaker. Along with
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Lecture:9 Phthalic acid
Dr. N. K. Patel
phthalic acid, a small amount of benzoic acid and maleic anhydride were
also produced.
Phthalic acid from phthalic anhydride
Raw material
Basis: 1000kg Phthalic acid from phthalic anhydride
Phthalic anhydride
895kg
Water
105kg
Reaction
Hydrolyzer
Manufacture process
Water
Phthalic
anhydride
Dryer
Filter
Air
Recycle to
process
Hot air
Phthalic
acid
Figure: Manufacture of Phthalic acid from phthalic anhydride
Block diagram of manufacturing process
Diagram with process equipment
Animation
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Lecture:9 Phthalic acid
Dr. N. K. Patel
Phthalic anhydride either obtained from naphthalene or o-xylene
oxidation was charged into a hydrolyzer where water added from the top of
the hydrolyzer continuously. Where solution of phthalic anhydride and water
were heated and then reaction mass was charged into a filter where
phthalic acid solids were removed continuously while solution contains
unreacted anhydride along with water was recycled to the process. After
filtration phthalic acid was dried and sent to the packaging section.
Engineering aspects
 Advantages of o-xylene over napthalene
An advantage of using o-xylene compared to naphthalene is that
o-xylene gives more yield of phthalic anhydride than naphthalene
becausetwo carbon atoms of naphthalene are consumed in the formation
of CO2.
 Effect of temperature and space velocity
Vapour-phase oxidation of o-xylene was carried out by two processes,
low temperature/low space velocity and high temperature/high space
velocity. Development in catalyst resulted in higher allowable space
velocities for the low temperature case with affecting the high yield. The low
temperature process runs below temperature of 4000C is predominant.
 Design of reactor
Rectors used in the process are designed to withstand against an
explosion. This reactor has tube having inside diameter of 2.5cm. The heat of
oxidation reaction is removed by a circulating molten salt. This salt is generally
mixture of nitrate and nitrite of sodium and potassium.
 Catalyst
The catalyst combines two essential ingredients found in earlier
catalyst, V2O5 and TiO2, which were coated on an inert, nonporous carrier in
a layer 0.02 – 2.0mm thick. Instead of spherical support ring shaped support is
used to provide long life to catalyst. Half rings supports provide more catalyst
to load as less pressure though the reactor gives high yield.
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Lecture:9 Phthalic acid
Dr. N. K. Patel
PROPERTIES







Molecular formula
Molecular weight
Appearance
Odour
Melting point
Density
Solubility
: C8H6O4
: 166.13gm/mole
: White solid
: Odourless
: 210 - 2110C
: 1.593gm/cm3
: Soluble in water
USES
 Phthalic acid is used in the production of polyester resin
 Reduction of phthalic acid with sodium amalgam in the presence of
water gives the 1,3-cyclohexadiene derivative
 As a raw material for anthraquinone, phenolphthalein and
phthalocyanine pigments
 Used in synthesis of dyes, perfumes, and other organic compounds
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