Terephthalic Acid (92-7)
Dimethyl terephthalate (DMT) was the first material to be used as a feedstock in
manufacturing polyethylene terephthalate (PET), the major constituent of polyester fibers.
The catalytic liquid-phase oxidation of para-xylene to terephthalic acid, known as the Mid-
Century process, was jointly developed in the 1950s by Standard Oil of Indiana, now
AMOCO, and Scientific Design. In 1965 an AMOCO pilot plant in Joliet, Illinois, began
employing a hydrogenation step to purify terephthalic acid.
A typical, generic purified terephthalic acid (PTA) process (e.g. similar to those employed
by AMOCO, ICI, Tecnimont, and Mitsui) comprises two separate steps - the production of
crude terephthalic acid by catalytic oxidation of para-xylene and the purification of the
crude acid by catalytic hydrogenation and crystallization. Crude terephthalic acid is
obtained by oxidation of para-xylene in the presence of catalyst and acetic acid solvent.
The main impurity (4-carboxybenzaldehyde) is transformed to water-soluble para-toluic
acid during hydrogenation.
Technologies offered by former Mid-Century licensees such as ICI and
EniChem/Tecnimont incorporate the same major processing steps as the AMOCO
process. The EniChem/ Tecnimont process is considered technically equivalent to the
AMOCO process, and reaction conditions in the oxidation and hydrogenation steps are
Mitsui Petrochemical offers a metals/bromide-catalyzed PTA process that consists of its
process for producing crude terephthalic acid at relatively mild oxidation conditions
coupled with the AMOCO purification process, which it is able to sublicense by
arrangement with AMOCO. SISAS has developed technology for the production of
isophthalic or terephthalic acid from the corresponding xylenes using acetaldehyde as a
promoter. Since this process does not use bromine as a catalyst promoter, less exotic
construction materials are suitable. The Huels DMT process, which is a version of what is
commonly known as the Witten process, is characterized by the simultaneous oxidation
of para-xylene and methyl para-toluate at mild temperature and comparatively low
pressure without acetic acid solvent. Glitsch Technology, a subsidiary of Foster Wheeler,
also offers a version of the Witten Process.
All of these commercial processes are discussed and economics have been developed
for each type of process. A review of world markets and producers is included in the
report along with current and forecast supply/demand figures.
Global PTA demand was 7.4 million metric tons in 1991 and is expected to reach 17.5
million metric tons in 2000, representing an average annual growth rate of 5.5 percent.
The Pacific Rim is the largest consumer of PTA, consuming over 60 percent of global
production - reflecting the significant polyester/textile industry in the region. The United
States and Western Europe are the other important PTA consuming regions. These
regions are also the major exporters of PTA to the Pacific Rim. This level of trade is
declining as new Pacific Rim capacity comes on stream.
The Pacific Rim also plays an important role in DMT demand and, similar to the PTA
pattern, DMT is exported from the United States and Western Europe. However, DMT is
a declining factor in polyethylene terephthalate (PET) production due to both economic
and environmental issues. As a consequence, the global demand for DMT was 3.9
million metric tons in 1991 and is expected to reach only 4.5 million metric tons in 2000.
U.S. demand for DMT/PTA (as PTA equivalent) in 1992 was 5.8 billion pounds, led by
fibers, bottle resin, and exports. However, demand in each of these sectors is declining
by 2 to 5 percent per year, with plant operating rates expected to fall from the low 80s to
the upper 70s by 2000.
In 1992 the PTA capacity of five producers in five locations in Western Europe totaled 1.2
million metric tons per year. Given the new/surplus PTA capacity in the Far East, West
European exports will decline. No new investments in West European capacity are
currently planned, but Chem Systems believes that a new plant will be added before the
end of the decade.
The largest demand growth is centered in Asia where growth overall is expected to be
about seven percent annually. In some areas, e.g. China and Indonesia, growth will be
much higher, while Japan is expected to show little change.