United States
Environmental Protection
Agency
Industrial Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-84-028 Sept. 1984
x°/EPA Project Summary
Chemical Reclamation of
Scrap Rubber
G. C. Frazier, S. M. Chan, 0. L Culberson, J. J. Perona, and J. W. Larsen
A conceptual, commercial-scale plant
design was formulated for processing
22,500 t/yr of scrap rubber tires to
hydrocarbon fuel gases, oils, petro-
chemicals (principally ethylene and aro-
matic liquids), and carbon black. The
process is based upon molten salt (zinc
chloride) pyrolysis of the rubber, and
pyrolysis data obtained in a bench-scale
flow apparatus. An economic assess-
ment of the plant was made in terms of
late 1979 dollars, for ranges in scrap
tire costs and prices for the principal
products: carbon black and the fuel
gases and oil. Profitability at these
1979 costs and prices is somewhat
modest by chemical processing industry
standards for new processes, but any
increases in energy and carbon black
prices would cause favorable changes
in this assessment.
This Project Summary was developed
by EPA's Industrial Environmental He-
search Laboratory, Cincinnati, OH, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
Interest in the utilization of scrap tires
as a source of chemicals, fuel, and carbon
black has been increasing in recent years.
This rising interest results not only from
environmental regulations but also from
the increase in the cost of chemicals and
fuels based on petroleum and natural gas
which grew at rates much greater than
the general inflationary trend during the
late '70s. Numerous attempts are being
made around the industrialized world to
recover chemical and fuel values from
tires. Most of the current processes being
developed for this purpose rely on pyrol-
ysis of the rubber, by one method or
another, requiring the heating of the
rubber to temperatures in the range of
500-600°C, or above.
Due to the chemical and physical
nature of tlje tire itself, a number of
problems are associated with the task of
recovering chemicals from the tires. Tires
may contain fiberglass and steel wire.
The rubber is formulated to be abrasive
resistant, so grinding is difficult and
intensive energy is required. Because the
quality of the main pyrolysis product,
carbon black, seemed to deteriorate as
the pyrolysis temperature is increased,
this project sought to develop a process
for recovering chemicals from tires which
would have the potential to avoid some of
these problems. That is, a process was
sought which:
• does not require grinding of the rubber
tires in order to avoid associated costs,
and minimizes the task of separating
wire and fiber particles from the
carbon black,
• pyrolyzes the rubber at relatively low
temperatures, and
• produces chemicals and carbon black
of marketable quality.
A process route which seemed to
satisfy these process objectives in bench-
scale work was pyrolysis in molten halo
salts. Such chemistry had earlier been
used to produce hydrocarbons in the
gasoline-boiling range from coal by
Gorin's group at the Consolidation Coal
Co. (now Conoco Coal Research). This
work showed that the large coal mole-
cules were rapidly broken into molecules
in the gasoline-boiling range. Also, the
Consolidation (Conoco) research dealt
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with one of the major disadvantages of
processes of th is type—the corrosiveness
of the molten salts. Early work also
indicated that halo salts would also break
rubber into gases, a light oil, and carbon
black at temperatures in the range 350-
400°C. This project set out to develop the
bench-scale data required for the formula-
tion and economic assessment of a
conceptual, commercial plant design. The
results presented in this report are based
exclusively on the use of zinc chloride as
the heat transfer and catalytic agent.
Discussion
It is normal practice in the chemical
process industries to develop a "concep-
tual process design" in the early stages of
an actual project, in order to identify
crucial areas requiring further develop-
ment, to evaluate various process options
which may be available, to assess the
overall technical feasibility of the project,
and lastly but most importantly, to es-
tablish at the outset whether or not the
process has the potential for economic
profit. The objective of the work reported
here has been to obtain the bench-scale
experimental data required for the formu-
lation and evaluation of a conceptual
process design for the conversion of
scrap tires to chemical and fuel products
by molten salt pyrolysis, and to use this
design as a basis for estimating the
potential profitability of the process.
Experimental results and the associated
economic analysis indicate that such a
processing plant converting 22,500 t/yr
of tires has the potential for prof it at 1979
chemical prices and that energy prices of
$3-$5/MM Btu, assuming that carbon
black, the major product, is marketable at
prices in the range of at least 15 0/lb.
Although there are a number of process
steps requiring demonstration at the pilot
plant scale, the biggest unknown in our
process is the marketability of the carbon
black. A pilot plant is required to generate
sufficient quantities of this material for
testing its suitability in products such as
rubber, ink, and plastics.
Although molten salt technology is not
easy, advantages of this process lie in its
relatively low operating temperature (in
the range of 360-400°C), and in the fact
that there is no need to pulverize the tires.
However, in this process tires are chopped
for compaction purposes, in order to
increase the reactor throughput. Further,
the relatively large pieces of fiberglass
and wire, remaining after the chopping
operation, appear to be readily separated
from the carbon residue downstream of
the reactor. A test of this separability at
the pilot plant scale is desirable.
A conceptual, commercial-scale plant
for converting scrap rubber tires into
hydrocarbon gases, oil and carbon black
was formulated and evaluated for its
economic potential in terms of late 1979
dollars. The conceptual plant processes
2,500 t/yr of scrap tires (68.5 t/d) to 115
MMSCF/yr of gas, 1.91 MM gal/yr of oil,
and 19.9 MM Ib/yr of carbon black. The
process is of the semi-batch type, with
the tires chopped prior to their conversion
in the reactor by molten zinc chloride
pyrolysis. The economic analysis was
based upon "over the fence" sale of oil
and gas for recovery of chemical values
(both olefin gases, principally ethylene,
and aromatic liquids). The remaining gas
and oil was assigned fuel values. Carbon
black is the principal product in terms of
both quantity and economic value at
current prices.
The conceptual commercial-scale plant
design was developed primarily on a data
base generated from a bench-scale exper-
imental setup of the flow type, in a system
constructed of 316 stainless steel. Among
the data provided by these experiments
were the product yields, reactor operating
conditions (temperature and molten salt-
rubber contact times), salt makeup re-
quirements, and salt reuse data. Suitable
reaction temperatures are in the range of
680-752°F (360-400°C).
The primary advantages of this process
include:
(a) pyrolysis at moderately low temper-
atures, due to the catalytic heat
transfer action of the molten zinc
chloride salt. The carbon black pro-
duced in this relatively low temper-
ature range may have special prop-
erties, and
(b) the ability to convert the scrap
rubber to products of commercial
value, without grinding the rubber.
This fact not only saves in terms of
grinding costs, but also reduces the
problem of separating the fiberglass
tire cord and steel wire from the
carbon black.
A principal disadvantage of this type of
process is the corrosiveness of the molten
salt, but experience on this project at the
bench scale indicates this problem can be
overcome.
The economic analysis was based on a
total investment of $6.93 MM (including
20 percent working capital), straightline
depreciation over an 8-year period, 10
percent interest on total investment,
construction over a one-year period, and
marketability of all products at late 1979 |
prices (except for the steel wire and
fiberglass, both of which were assigned
no value). The profitability of the process
depends strongly on the values of the
carbon black and of the fuel gases and oil,
as well as upon the cost of the scrap, feed
tires, and less strongly on the cost of
separating (purification) the chemicals
from the gas and oil products. The most
uncertain parameters in the cost study
are the value of the carbon black, the cost
of the scrap tires, and the cost of chemical
purification. The percent return on invest-
ment (ROI) is shown in Table 1 for a
selected range in each of these param-
eters. A plant of this size (22,500 t/yr of
tires) is profitable for scrap tire costs in
the range of 20 $/t or less and with fuel
and carbon black values in the range of
3-5 $/MM Btu and 15-21 C/lb, respec-
tively. This margin of profit is probably
insufficient to attract venture capital.
Fuel and carbon black prices will need to
rise considerably for a plant of this size to
be profitable, especially if the cost of
purchasing and collecting scrap tires
should be as high as 40 $/t.
If prices increase in petrochemicals
and energy above 1979 levels, improve-
ments can be expected in the profitability
numbers given in Table 1 for this process.
There may then be sufficient incentive to
demonstrate this process at larger scale
of operations. It is recommended that
provisions for the following tests be
included in such a demonstration pro-
gram:
• Generate sufficient quantity of carbon
black for test marketing in products
such as rubber, plastics, ink, and
activated carbon. The marketability of
the carbon black is the greatest uncer-
tainty in this process, and profitability
of the plant is strongly tied to the
carbon black product and its price.
• Establish the fraction of the carbon
black which can be recovered in rela-
tively pure form.
• Establish the effectiveness of the
molten salt in the long term, confirm
the salt make-up requirements, and
establish the fraction of the salt which
must be recovered from the reaction
residue for recycle to the reactor.
• Establish the efficacy of existing screen-
ing technology for separating and
dealing with the fiberglass fluff.
• Conduct long-term corrosion testing
and generate system maintenance
data.
Despite the somewhat marginal profit
ability picture for this process at this time
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Table 1. Return on Investment (KOI) for Selected flanges in the Cost and Product Value
Parameters. Plant Capacity = 22,500 t/yr of Tires
Case
A
B
C
D
E
F
G
H
Tire Cost
$/t
0
0
0
0
20.
20.
20.
20.
Purification
Cost* %
30.
90.
30.
30.
30.
90.
30.
30.
Fuel/ Value
$/MM Btu
5.0
5.0
5.0
3.0
5.0
5.0
5.0
3.0
Carbon Black
Value, /Ib
21.
21.
15.
21.
21.
21.
15.
21.
HOI,
%
15.2
9.9
6.9
10.7
11.7
6.4
3.4
7.2
Ms a percent of the Chemicals Value.
favorable trends in the prices of its major
products might dictate that the economic
potential of this process be reassessed in
the future.
George C. Frazier, S. M. Chan, 0. L. Culberson, J. J. Perona, andJ. W. Larsen are
with the University of Tennessee, Knoxville, TN 37916.
W. W. Liberick is the EPA Project Officer (see below).
The complete report, entitled "Chemical Reclamation of Scrap Rubber," (Order
No. PB 84-162 718; Cost: $11.50, subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield. VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Industrial Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
S GOVERNMENT PRINTING OFFICE, 1984—759-015/7816
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United States Center for Environmental Research
Environmental Protection Information
Agency Cincinnati OH 45268
Official Business
Penalty for Private Use $300
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