Chemical cycle is one of the important
methods of polymer material cycle, which refers to the degradation reaction of
polymers under the action of heat and chemical reagents to form low-molecular
weight products, which can be further utilized, such as monomer
repolymerization, oil products can be further processed. At present, the main
methods of chemical cycle are chemical degradation. Chemical degradation can be
divided into depolymerization, pyrolysis, hydrogenation and gasification.
The present situation of chemical cycle
development of polymer materials
1.
Step - by - step polymer
material
The progressive polymer materials mainly include polyester
and polyurethane, which are represented by polyethylene terephthalate. Mainly
used for film, fiber and fabric, beverage bottle, etc. Waste materials can
react with polyols in the presence of catalysts and the products can be
condensed with unsaturated polyacids to make unsaturated polyresin. Different
esters can be obtained by alcoholysis with different alcohols, either as
monomers or as plasticizers. PET can be hydrolyzed under acidic or alkaline
conditions. It can be hydrolyzed under normal pressure in strong acid (such as
sulfuric acid and nitric acid) medium. The hydrolysis rate is fast. If it is
hydrolyzed in an aqueous solution of alkaline (such as NaOH), 3-sh should be
reacted at 21 2500C and 1.4-2.0mpa. After the reaction, TPAO weak alkali (such
as hydrogen and oxygen) can be precipitated, and can also be used to hydrolyze
PET waste to obtain monomer. Saponification reaction under atmospheric pressure
has been applied in silver and TPAO polyurethane is from recycled PET film
condensation polymer materials, can be hydrolyzed into the pluralistic alcohol
and amine, with a special extruder hydrolysis, dibasic acid can be obtained by
the purification of the product and diamine, diamine and reaction with
phosgene, preparation of diisocyanate, used for foam production. But the cost
of this process is large and the recovery benefit is not high. PU alcoholysis
is a widely used method at present. PU waste materials can be alcoholyzed to
obtain polyol mixture, which cannot be separated effectively at present, but
this product can be used as a component in the manufacture of foam plastics and
elastomers.
2.
Additive polymer material
Polystyrene (ps) in addition to used as paint, binder, but
also used to crack styrene ps under the action of heat can be cracked into
styrene, the yield of 65%, above. Japanese scientists have cracked PS in
solvent method, decomposed it at 400 to 500 ℃ for 1 20min, and
the condensate obtained can be distilled into styrene with a purity of 96%.High
purity styrene can be obtained by fractionation using metal oxide as catalyst
in melting state (>350 ℃).In addition, lead alloy is used as heating medium to crack ps. Under
the action of appropriate catalyst or irradiation, polyolefin can undergo
chemical reactions to form materials with good performance, such as
polyethylene (PE), which can be crosslinked with crosslinking agents (such as
peroxides) to produce PE materials with good performance. Waste polyolefin can
be chlorinated, and the chlorinated polyolefin can be used in binders and
coatings. In addition, polymer polymerization to make oil is a common cycle
method.
3.
Hybrid polymers and composites
A common method of making use of a variety of polymer
mixtures is cracking them to make oil. The mixture is cracked at high
temperatures to produce gas and oil, which can be used as fuel or refined
directly at the refinery. Chemical plants have high requirements on the
organochlorine content of oil, generally no more than 10 * 10-6. However, the
organochlorine content of oil obtained from cracking of plastic mixture can
reach (50 to 200) * 10-6, so it is very important to dehalogen before cracking
or during cracking. In addition, waste plastics often contain heavy metal
element compounds, cracking oil refining to consider the poisoning of the
catalyst. Most of the resins of composite materials are thermosetting resins
such as unsaturated polyvinegar resin, epoxy resin and phenolic resin, etc. The
waste materials of composite materials are not only used as powder filler,
combustion to take heat and chemical auxiliary fuel, but also used for cracking
and recycling oil products and raw materials. For example, after the
decomposition of glass fiber reinforced plastics at 380 ℃ at
atmospheric pressure of 50 ℃, the further decomposition at 450 ℃ and 550 ℃ can obtain
oil products. Due to the large amount of residue, a special decomposition
furnace needs to be designed to complete the pyrolysis process, and the
research work is still under way. Another example is the pyrolysis of phenolic
resin in the experimental vulcanization bed (722 ℃), the products
include aliphatic hydrocarbon (mass fraction is 5 · 24%),
phenol (8, 25%), carbon black (42).2%), gas (24, 3%), etc.
Chemical cycle process and equipment
1.
Reaction Still
Reaction kettle is a common chemical
equipment for chemical circulation, and its matching equipment is condenser,
storage tank, distillation tower, etc. The products of raw materials (such as
polyolefin) degraded in the reaction kettle can be monomer, chemical raw
materials, etc., such as PET pyrolysis and then polymerization into PET, or
unsaturated polyvinegar resin can be produced. According to the demand, the
reactor can be designed into a tank reactor to facilitate heating and slag
removal. The reactor can also be designed as a tubular reactor, which can
increase the cracking temperature, shorten the time, and continuously crack,
suitable for the cracking of ps, PMMA and other polymers.
2.
Fluidized bed reactor
Fluidized bed reactor is a kind of
bed reactor. W. kanaminsky et al., Germany, used propane combustion to heat
carrier gas or water vapor, and used heater to heat sand and carrier gas to 500
° c at 00 ° c. The carrier gas should be enough to promote sand fluidization in
the reactor. The polymer is squeezed into the fluidized bed by the extruder,
and the polymer material is cracked in the fluidized bed. The resulting gas and
carrier gas are separated by condensation and separation, and the pyrolysis
products are obtained. Fluidized bed cracking apparatus has the advantages of
fast heating, high efficiency, even cracking temperature and closed system. Cracking
reaction is best carried out under inert carrier gas. If air is used as carrier
gas, the product is easy to be oxidized and the thermal energy of the obtained
oil is 10700 lower.
3.
Extrusion cracking equipment
Extrusion cracking equipment is
composed of two extruders in series. The first extruder has a vent hole. The
waste polymer materials were cracked at low temperature on the first extruder.
The main purpose was to remove HCI from the waste materials. The intermediate
products of cracking are then entered into the second extruder for
high-temperature cracking, which turns the polymer into a low-molecular
compound or oil or gas. After separation, HCG cracking reaction can be carried
out continuously, which can be decomposed mechanically and thermally. In
addition, new devices are being researched and developed.
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