Alternative solvents for a method of reclaiming styrene and other products from polystyrene based materials

A method of pretreating polystyrene-containing materials to form a solution of polystyrene in a processing solvent from which the styrene in the polystyrene in the materials is reclaimed. The materials are mixed with an environmentally acceptable pretreating solvent having a lower boiling point than the processing solvent, typically at a location remote from the reclamation plant. The pretreating solvent is selected from the group consisting of d-limonene, l-limonene, dipentene, and blends thereof. Prior to actual processing to reclaim styrene, the pretreating solvent is substantially replaced with the processing solvent. The pretreating solvent may be recovered for reuse.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a method of reclaiming styrene and other products 
from polystyrene based materials. In particular, the invention relates to 
use of solvents approved by governmental agencies to be of low toxicity in 
a pre-treatment container used to receive the materials to be 
depolymerized. 
2. Description of Related Art 
U.S. Pat. No. 5,502,263 (the '263 patent) describes a process for recycling 
polystyrene trash using a manufactured liquid, "styrene oil" (defined in 
the '263 patent), as a solvent for the polystyrene during the processing. 
A preferred embodiment of that recycling process involves placing a 
dissolving tank, equipped with a removable lid and partially filled with 
styrene oil, in each industrial or commercial site which routinely 
discards polystyrene, especially foamed polystyrene. An employee of each 
site would place its discards into the local dissolving tank as they are 
discarded, thus eliminating the need for voluminous storage space for 
discards between trash collections, and also substantially reducing the 
cost of trash collection. Moreover, the use of styrene oil as an aid in 
collecting the discards is doubly useful, as it is not only a strong 
solvent for polystyrene, but also greatly assists in the later processing 
of the polystyrene into styrene monomer. 
Although styrene oil is believed to have a very low toxicity, its toxicity 
has not yet been officially established by the Environmental Protection 
Agency or OSHA. Should a significant delay occur in obtaining approval for 
the use of styrene oil in dissolving tanks, which are frequently opened 
within occupied facilities, the use of this particularly efficient method 
of trash collection may be precluded for some time. Therefore, there 
exists a need for a pre-processing method using a solvent that already has 
been governmentally approved for use within occupied facilities. 
SUMMARY OF THE INVENTION 
The invention is a pre-processor subsystem, or collector system, which uses 
an alternate solvent, i.e., a solvent other than styrene oil, to dissolve 
polystyrene. The polystyrene then is transferred from the alternate 
solvent to the styrene oil for depolymerization. 
The alternative solvent is used for the collection of polystyrene discards 
and trash at the collection sites. The resulting solution is collected, 
typically by tank trucks, all in accordance with the collection process 
described in the '263 patent. If desired, the pre-processor subsystem of 
the invention also can be co-located with the polystyrene processing plant 
described in the '263 patent. 
An alternative solvent used to collect and transport dissolved polystyrene 
may not be suitable for use in place of or with styrene oil when 
polystyrene-containing solution is heated in the depolymerization step in 
the practice of the method of the '263 patent. The entirety of the '263 
patent is incorporated by reference herein. Typically, such alternate 
solvents decompose or degrade at a temperature lower than the temperature 
at which the depolymerization step of the method of the '263 patent is 
carried out. 
In accordance with the method of this invention, the polystyrene-containing 
alternate solvent is treated so as to transfer the dissolved polystyrene 
to styrene oil before depolymerization of the polystyrene. In particular, 
a suitable alternate solvent is compatible with that part of the 
polystyrene processing system described in the '263 patent before the 
depolymerization of the styrene and is amenable to treatment to cause 
significantly complete transfer of the dissolved polystyrene from the 
alternative solvent to styrene oil. Thus-transferred polystyrene then is 
depolymerized in accordance with the depolymerization portion of the 
method of the '263 patent. The invention also relates to the method of 
using the subsystem. 
Because many alternative solvents are not suitable for the subsequent 
processing of polystyrene described in the '263 patent, a pre-processor 
subsystem must be used to transfer the dissolved polystyrene from solution 
in alternative solvent to solution in styrene oil, and to recover and 
purify the alternative solvent for continued use as a solvent. This 
subsystem must be contained so that styrene oil will not be released to 
the environment during the transfer operations. 
The subsystem of the invention is designed in a modular fashion so that it 
can be configured as desired to remove styrene oil contaminant from the 
reclaimed alternative solvent to a required level of purity. This 
flexibility is useful, not only because a processor then can obtain a 
preferred desired purity, but also because a governmentally-mandated 
purity standard may be enacted, or an existing standard changed, at any 
time. 
One aspect of the invention provides a method of pretreating 
polystyrene-containing materials to form a solution of polystyrene in a 
processing solvent from which the styrene in the polystyrene in the 
materials is reclaimed. The materials are mixed with a pretreating solvent 
selected from the group consisting of d-limonene, l-limonene, dipentene, 
and blends thereof, to dissolve the polystyrene and form a solution of 
polystyrene in pretreating solvent. The pretreating solvent is 
substantially replaced with the processing solvent to form the solution of 
polystyrene in processing solvent. The processing solvent has a higher 
boiling point than the pretreating solvent. 
The processing solvent preferably is styrene oil, as defined in the '263 
patent. The pretreating solvent may be recovered for reuse. 
Another aspect of the invention provides the above-described pretreating 
method as part of an overall method of reclaiming styrene from 
polystyrene-containing materials. 
A third aspect of the invention provides a method of pretreating 
polystyrene-containing materials prior to depolymerization to reclaim 
styrene from the polystyrene in the materials, wherein the materials are 
mixed with dipentene to dissolve the polystyrene and form a solution of 
polystyrene in dipentene.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The inventors have discovered that d-limonene, or with 1limonene, in its 
racemic form, dipentene, are especially suitable alternative solvents for 
use in the present invention. D-limonene, an oily solvent obtained by 
processing citrus peels, particularly orange peels, has a very low 
toxicity and has been used commercially for many years both as a solvent 
in various processes and as an orange flavoring agent in foods. D-limonene 
is relatively expensive and slightly less effective than styrene oil as a 
solvent for polystyrene, but it will serve as an alternate solvent to the 
styrene oil solvent, and it is now governmentally approved for use in an 
occupied facility. 
To clarify the following description, a short discussion of selected 
alternate solvents is useful. D-limonene is a hydrocarbon compound in a 
class called monocyclic monoterpenes, which has unusual properties. All 
monoterpenes, which have the formula C.sub.10 H.sub.16, are constructed of 
a "building block", C.sub.5 H.sub.8, which is also the formula of an 
independent hydrocarbon compound, isoprene. Isoprene is widely used to 
make many petrochemical compounds, such as man-made "natural" rubber. 
D-limonene is, in fact, a dimer of isoprene. D-limonene is one of a pair of 
"twin" compounds called enantiomers. Enantiomers are compounds having 
identical chemical formulas but chemical structures that are mirror images 
of each other. D-limonene is the dextrorotatory twin (rotates 
plane-polarized light to the right), and l-limonene is the levorotatory 
twin (rotates plane-polarized light to the left). Both of the limonenes 
occur in citrus peels, but d-limonene is much more prevalent. If 
d-limonene and l-limonene are present in equal parts, the mixture is said 
to be racemic. This racemic mixture is called dipentene. 
D-limonene is prepared by crushing and then steam-distilling citrus peels. 
The resulting distillate is actually mostly d-limonene, but it also 
contains a minor percentage of l-limonene (which is inseparable from 
d-limonene for practical purposes), and a very small amount of other 
impurities. Governmental agencies have found this mixture to be 
essentially non-toxic, and its use as a food flavoring additive has been 
approved long ago. D-limonene is expensive, however, and is not 
extensively used as a commercial solvent. 
Dipentene can be prepared by two different methods: by catalytically 
reacting turpentine at elevated temperature, or directly from isoprene. 
Dipentene made by either method is much less expensive than d-limonene. 
Since it has no desirable flavor, however, the principal use of dipentene 
is as a solvent in many processes; as such, it does not have to be highly 
refined. Historically, dipentene has been prepared from turpentine, and 
this "solvent grade" dipentene comprises about 95% dipentene and 5% mixed 
terpenes. The boiling points of these terpenes are very close to that of 
dipentene, and it is not worth the trouble of trying to remove them. 
Solvent grade dipentene has been characterized by the EPA as having 
moderately low toxicity, but not so low as pure dipentene; therefore, its 
early approval for use in the method of this invention is uncertain. The 
terpene impurities do not disqualify its use with the preferred embodiment 
of this invention, however. 
Dipentene prepared from isoprene is very pure, and has been rated by 
governmental agencies as having a toxicity comparable to d-limonene. The 
isoprene process is more recent than that using turpentine, and a 
substantial manufacturing capability to perform the isoprene process has 
not been constructed to date. The process is not complex, however, and 
facilities could be readily constructed if a demand arose. 
Although these two limonenes and dipentene can have differences in some 
properties, such as odor and flavor, they all exhibit very nearly the same 
basic physical and chemical properties, such as boiling points and 
behavior in solutions. For this reason, they are chemically 
interchangeable as solvents for use in the invention. The reasons for 
distinguishing between them are legal (governmentally approved low 
toxicity) and economic, and are significant. 
This unusual situation makes the invention more flexible in use. The 
following table may assist the skilled practitioner to select the proper 
alternative solvent for collection of polystyrene: 
______________________________________ 
Relative 
Alternate Solvent Cost Toxicity Availability 
______________________________________ 
d-limonene High Acceptable Now 
Pure dipentene Low Acceptable Soon 
from isoprene 
Solvent grade Low Moderate Now 
dipentene 
______________________________________ 
For convenience, the invention will be described as it relates to 
d-limonene. However, it should be understood that d-limonene represents 
any of the variants of the solvents d-limonene (with its impurities as 
approved for food use), l-limonene, mixtures at any ratio of the two 
limonenes, and dipentene (with its impurities as previously discussed). 
Also, in the following text, d-limonene can refer either to approved 
food-grade d-limonene as previously described, or to pure d-limonene, both 
of which will work well in the invention. 
Because a governmentally allowed concentration of styrene oil contamination 
in recycled d-limonene has not been established, and may change from time 
to time in the future as more toxicity data becomes available, the 
preferred embodiment of a pre-processor subsystem of the invention 
described in detail herein is capable of producing recycled d-limonene 
over a wide range of purity, up to 99.9+% (less than 0.1% styrene oil). 
Clearly, other preferred embodiments can be designed for narrower ranges 
of purity if desired, with consequent simplification and reduced cost. 
This flexibility will be discussed in more detail after the preferred 
embodiment subsystem has been substantially described. With the guidance 
provided herein, skilled practitioners will be able to select an 
embodiment of the invention which will provide a desired degree of purity. 
In accordance with the method of the invention, a solution of polystyrene 
in d-limonene is transferred from tank truck 201 through hose 202 to 
solvent exchange and distillation tank 203. Similarly, a supply of styrene 
oil is maintained in storage tank 204. Typically, this styrene oil was 
previously produced by the polystyrene process described in the '263 
patent. An amount of styrene oil slightly less than the amount of 
d-limonene by weight is pumped from storage tank 204 into tank 203 through 
pipe 205. Tank 204 may be either an integral part of a polystyrene 
processing plant as described in the '263 patent or an integral part of 
the pre-processor subsystem of the invention. The absolute pressure in 
tank 203 then is reduced to approximately 1.5 psia through pipe 206, 
condenser system 207, and pipe 208 by vacuum system 209, which exhausts 
its flow through pipe 210, two-way valve 211, and pipe 212 to storage tank 
213. 
When the vacuum in solvent exchange tank 203 is established, the tank is 
purged with an inert gas to substantially purge oxygen therefrom, and then 
heated to approximately 100.degree. C. by a heat transfer system 214 (not 
shown). The d-limonene in tank 203 begins to vaporize at the low pressure, 
and the vapor is drawn through pipe 206, condenser 207, and pipe 208 by 
vacuum system 209 in order to maintain the pressure in tank 203 at 1.5 
psia. 
The condensate in the vacuum system 209 is exhausted via pipe 210, two-way 
valve 211, and pipe 212 into storage tank 213. The styrene oil in tank 203 
has a boiling point of approximately 215.degree. C. at 1.5 psia, so only a 
small amount of styrene oil will vaporize during this distillation. As the 
ratio of d-limonene to styrene oil diminishes, the temperature of the 
liquid in tank 203 must be slowly increased in order to maintain an 
efficient rate of vaporization of d-limonene. The temperature is raised to 
approximately 125.degree. C., when approximately 50% of the d-limonene has 
been vaporized. The purity of the initial d-limonene vapor is about 99.4% 
(0.6% styrene oil contaminant), and at this 50% point is about 98.8%, so 
the content of storage tank 213 is about 99.1% pure d-limonene. If 99.1% 
pure d-limonene is an acceptable degree of purity for use in industrial 
and commercial polystyrene dissolving tanks, or for any other use the 
operator contemplates, the 99.1% pure d-limonene in storage tank 213 is 
delivered through hose 227 to tank truck 228 or another satisfactory mode 
of transportation for delivery to the polystyrene collection sites, or for 
other use. 
If the minimum acceptable degree of purity for use in dissolving tanks is 
higher than 99.1%, the d-limonene in storage tank 213 must be further 
purified. The content is pumped via pipe 229 to the center of a fractional 
distillation column 230. The fractional distillation column operates at a 
pressure of 1.5 psia, between temperatures of about 100.degree. C. and 
125.degree. C. Approximately 99.99% pure d-limonene is drawn off the top 
of the column through pipe 231 to storage tank 232, and a small amount of 
a mixture of roughly 50% d-limonene and 50% styrene oil is drawn off the 
bottom of the column through pipe 233 to storage tank 234. Skilled 
practitioners are able to design and operate such a distillation column. 
The 99.99% pure d-limonene in storage tank 232 is delivered periodically 
through hose 235 to tank truck 228 for delivery to the polystyrene 
collection sites or to other users. Clearly, the contents of storage tanks 
213 and 232 can be blended to achieve intermediate standards of purity. 
The mixture of d-limonene and styrene oil in tank 234 is returned to 
solvent exchange tank 203 via pipe 236 for use in a later batch along with 
the charge of d-limonene/polystyrene solution from hose 202 and the 
styrene oil from pipe 205. 
D-limonene is not very stable at elevated temperatures, and it is not 
desirable to raise its temperature above about 125.degree. C. if it is to 
be repeatedly vaporized and reused. Accordingly, vaporization beyond this 
point is accomplished by steam distillation, using superheated steam 
generated in steam generator system 220, which receives its energy from 
heat transfer system 214 (not shown). After two-way valve 211 is switched 
to pipe 215, steam is introduced at approximately 125.degree. C. and 1.5 
psia into the bottom of tank 203 through pipe 221, where it bubbles up 
through the liquid and captures some d-limonene and a small amount of 
styrene oil, both as vapor. The mixed vapors are drawn into pipe 206 as 
before, condensed, and sent through two-way valve 211 and pipe 215 into 
water-separator tank 216. Steam distillation is continued at 125.degree. 
C. until 99.99% of the d-limonene is removed from tank 203, and the 
polystyrene now is effectively dissolved in styrene oil. The vacuum system 
209 and steam system 220 are turned off, and tank 203 is emptied through 
pipe 222 into styrene oil solution storage tank 223. Tank 223 can be an 
integral part of the polystyrene processor of the '263 patent, or an 
integral part of the pre-processor subsystem of this invention. The 
solution in styrene oil is removed as required from tank 223 to be sent to 
the polystyrene processor of the '263 patent. 
The selections of 1.5 psia and 125.degree. C. for the distillation of 
d-limonene used in the preceding description are preferred, as these 
conditions achieve an economical balance of several important parameters. 
In general, a steam distillation system becomes more efficient in usage of 
steam (lbs of steam required per lb of d-limonene distilled) as the 
temperature rises. An upper limit on the increase in temperature is 
imposed, however, by the rapidly increasing chemical instability of 
d-limonene above about 125.degree. C., so 125.degree. C. is the best 
temperature to use. Lower temperatures would result in a slower, less 
efficient process. 
With regard to absolute pressure in the still, the amount of steam required 
to distill a pound of d-limonene decreases with decreasing still pressure 
(stronger vacuum), but a potential countering effect is the decrease in 
the condenser temperature required to maintain the desired vacuum. This 
desirable reduction in steam usage ratio is rapid down to about 1.5 psia, 
but levels out rather quickly below that absolute pressure. By 
coincidence, the steam condenser temperature required to maintain 1.5 psia 
in the still is about 46.degree. C. (115.degree. F.). This temperature is 
sufficiently above ambient temperature so that condensation can be 
achieved with an ambient temperature heat sink, either air or water. 
Operation at a lower temperature would require a refrigerated heat sink, 
resulting in significantly higher cost. In summary, 1.5 psia and 
125.degree. C. appear to be practical choices. 
Returning to water separator tank 216, the liquid mixture of d-limonene, 
water, and a slight amount of styrene oil will separate at rest naturally 
into a lower layer of water and an upper layer of d-limonene/styrene oil 
mixture. At suitable intervals, water is drawn off the bottom of tank 216 
and returned to the steam generator system 220 through pipe 224. The 
purity of d-limonene in the first steam distillate (after removal of 
water) is about 98.8%. The purity thereafter declines significantly as the 
d-limonene in tank 203 is exhausted, although the amount of styrene oil in 
the distillate is still small because the total hydrocarbon content of the 
vapor is also small. 
If the governmentally approved purity of d-limonene to be used in 
polystyrene collection tanks is sufficiently low, there is opportunity for 
blending a very high purity stream and a lower purity stream in 
proportions sufficient to obtain or slightly exceed the desired value. In 
accordance with the method of the invention, d-limonene from tank 216 is 
pumped into tank 213 via pipe 225, valve 238 and pipe 226. The blended 
mixture would then be delivered in the usual way. 
If the purity of d-limonene in tank 216 is too low to permit blending, the 
d-limonene in tank 216 must be further purified. The d-limonene in tank 
216 is pumped into the fractional distillation column 230 via pipe 225, 
valve 238 and pipe 237, where it is purified and delivered via pipe 231, 
storage tank 232, and hose 235 to tank truck 228. 
Other preferred embodiments are possible to meet other purity requirements. 
For example, if the required purity is expected to always exceed 99.1%, 
valve 211, pipe 212, storage tank 213, pipes 226 and 229, valve 238 and 
hose 227 can be eliminated, and all of the d-limonene produced can be sent 
to the fractional distillation column 230 for purification. Similarly, if 
the required purity is sufficiently low (about 95%) all of the previous 
eliminations, together with the fractional distillation column 230, 
storage tanks 232 and 234, pipes 225, 231, 233, 236 and 237, and hose 235, 
can be eliminated, and the upper part of water separator tank 216 can be 
directly connected with tank 228 by a new hose (not shown). 
It will be apparent to those skilled in the art that various modifications 
to the above-described preferred embodiment may be made without departing 
from the true spirit and scope of the invention, which is to be limited 
only by the appended claims.