U.S. patent application number 16/062681 was filed with the patent office on 2018-12-27 for process for continuously converting mixed waste plastic into waxes and liquid fuels by cracking.
The applicant listed for this patent is SOLVAY SA. Invention is credited to Dominique BALTHASART, Miriam CERRO-ALARCON, Avelino CORMA, Michel GARRAIT, Philippe MARION, Jes s MENGUAL, Marco PICCININI, Stephane STREIFF.
Application Number | 20180371325 16/062681 |
Document ID | / |
Family ID | 54936879 |
Filed Date | 2018-12-27 |
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United States Patent
Application |
20180371325 |
Kind Code |
A1 |
STREIFF; Stephane ; et
al. |
December 27, 2018 |
PROCESS FOR CONTINUOUSLY CONVERTING MIXED WASTE PLASTIC INTO WAXES
AND LIQUID FUELS BY CRACKING
Abstract
The present invention relates to a process for continuously
converting mixed waste plastic into waxes and liquid fuels by
cracking. The process comprises the steps of feeding a mixed waste
plastic stream to a cracking reactor where the mixed waste plastic
is catalytically cracked in the presence of a catalyst and
circulating the catalyst between the cracking reactor and a
regenerator where the catalyst received from the cracking reactor
is regenerated and heated by burning coke and/or other combustible
material deposited on or mixed with the catalyst.
Inventors: |
STREIFF; Stephane;
(Shanghai, CN) ; BALTHASART; Dominique; (Brussels,
BE) ; PICCININI; Marco; (Brussels, BE) ;
MARION; Philippe; (Vernaison, FR) ; GARRAIT;
Michel; (Charly, FR) ; CORMA; Avelino;
(Valencia, ES) ; CERRO-ALARCON; Miriam; (Valencia,
ES) ; MENGUAL; Jes s; (Carcaixent, ES) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
SOLVAY SA |
Brussels |
|
BE |
|
|
Family ID: |
54936879 |
Appl. No.: |
16/062681 |
Filed: |
December 15, 2016 |
PCT Filed: |
December 15, 2016 |
PCT NO: |
PCT/EP2016/081310 |
371 Date: |
June 15, 2018 |
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C10G 1/10 20130101; C10G
2300/4081 20130101; C10G 11/187 20130101; C10G 65/12 20130101; C10G
1/086 20130101; C10G 1/083 20130101; C10G 2300/708 20130101; C10G
47/36 20130101 |
International
Class: |
C10G 1/08 20060101
C10G001/08; C10G 11/18 20060101 C10G011/18; C10G 47/36 20060101
C10G047/36; C10G 65/12 20060101 C10G065/12 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 18, 2015 |
EP |
15201142.5 |
Claims
1. A process for continuously converting mixed waste plastic into
waxes and liquid fuels by cracking, the process comprising the
steps of: feeding a mixed waste plastic stream to a cracking
reactor where the mixed waste plastic is catalytically cracked in
the presence of a catalyst; circulating the catalyst between the
cracking reactor and a regenerator where the catalyst received from
the cracking reactor is regenerated and heated by burning coke
and/or other combustible material deposited on or mixed with the
catalyst; wherein the heat balance between the cracking reactor and
the regenerator is adjusted depending on changes in the chemical
composition of the mixed waste plastic.
2. The process according to claim 1 wherein the catalyst is a
mixture of at least two different catalysts having different
catalytic activity and the heat balance between the cracking
reactor and the regenerator is adjusted by adjusting the ratio of
the at least two different catalysts in the mixture of
catalysts.
3. The process according to claim 1 wherein the heat balance
between the cracking reactor and the regenerator is adjusted by
diluting the mixed waste plastic with a diluent.
4. The process according to claim 3 wherein the mixed waste plastic
in the feed stream is diluted with waxes and/or liquid fuels.
5. The process according to claim 4 wherein the waxes and liquid
fuels are obtained from the cracking process.
6. The process according to claim 1 wherein the heat balance
between the cracking reactor and the regenerator is adjusted by
introducing a combustible stream into the regenerator.
7. The process according to claim 1 wherein the heat balance
between the cracking reactor and the regenerator is adjusted such
that the cracking reactor and the regenerator are maintained in a
steady state heat balance.
8. The process according to claim 1 wherein the heat balance
between the cracking reactor and the regenerator is adjusted such
that a predetermined ratio of waxes to liquid fuels is obtained
from the cracking reactor.
9. The process according to claim 1 wherein temperature and flow of
the circulating catalyst are adjusted to maintain a pre-selected
temperature in the cracking reactor.
10. The process according to claim 1 wherein the chemical
composition of the mixed waste plastic changes over time.
11. The process according to claim 1 wherein the mixed waste
plastic comprises more than 50% by weight of polyolefin and
polystyrene based on the total weight of the mixed waste
plastic.
12. The process according to claim 1 wherein the mixed waste
plastic is subjected to a dry pretreatment prior to subjecting it
to the cracking reactor.
13. The process according to claim 1 wherein the mixed waste
plastic prior to feeding it to the cracking reactor has a water
content of less than 20% by weight, based on the total weight of
the mixed waste plastic.
14. The process according to claim 1, further comprising the step
of reducing the content of air and/or oxygen in the mixed waste
plastic prior to feeding it to the cracking reactor.
15. The process according to claim 1 wherein the oxygen content in
the cracking reactor is lower than 10 vol. % of the gas phase in
the reactor.
16. The process according to claim 13 wherein the mixed waste
plastic prior to feeding it to the cracking reactor has a water
content of less than 10% by weight, based on the total weight of
the mixed waste plastic.
17. The process according to claim 15 wherein the oxygen content in
the cracking reactor is lower than 5 vol. % of the gas phase in the
reactor.
Description
[0001] This application claims priority to European application No.
15201142.5--filed on Dec. 18, 2015--, the whole content of this
application being incorporated herein by reference for all
purposes.
[0002] The present invention relates to a process for continuously
converting mixed waste plastic into waxes and liquid fuels by
cracking. The process comprises the steps of feeding a mixed waste
plastic stream to a cracking reactor where the mixed waste plastic
is catalytically cracked in the presence of a catalyst and
circulating the catalyst between the cracking reactor and a
regenerator where the catalyst received from the cracking reactor
is regenerated and heated by burning coke and/or other combustible
material deposited on or mixed with the catalyst.
[0003] In view of the increasing importance of polymers as
substitutes for conventional materials of construction such as
glass, metal, paper, and wood, the perceived need to convert
non-renewable resources such as petroleum and dwindling amounts of
landfill capacity available for the disposal of waste products,
considerable attention has been devoted in recent years to the
problem of recovering, reclaiming, recycling or in some way reusing
waste plastic.
[0004] It has been proposed to pyrolyze or catalytically crack the
waste plastic so as to convert high molecular weight polymers into
volatile compounds having a much lower molecular weight. The
volatile compounds, depending on the process employed, can be
either relatively high boiling liquid hydrocarbons useful as fuel
oils or fuel oil supplements or light to medium boiling
hydrocarbons useful as gasoline-type fuels or as other
chemicals.
[0005] The cracking reaction is endothermic. At the same time, the
catalyst used in the cracking reaction is contaminated with coke
and other combustible material. It is therefore common to circulate
the catalyst between the cracking reactor and a regenerator where
the catalyst is regenerated and heated by burning the coke and
other combustible material. This burning reaction is exothermic and
when the thus heated catalyst is recycled into the cracking reactor
provides the energy for the cracking reaction. For that reason,
catalytic cracking/regenerator units are often referred to as being
"heat balanced".
[0006] It is for example common to operate the reactor at constant
raw material feed modulating the catalyst to feed ratio in order to
keep the temperature constant. In an alternative approach, for
example US 2014/0228204 suggests periodically purging a small
portion of the used catalyst and make that up with fresh catalyst
in order to maintain the catalyst activity at a constant level.
[0007] U.S. Pat. No. 5,216,149 discloses a method for controlling
the pyrolysis of a complex waste stream of plastics to convert thus
stream into useful high value monomers or other chemicals, by
identifying catalyst and temperature conditions that permit
decomposition of a given polymer in the presence of others, without
substantially decomposition of the other polymers. This method
makes it easier to purify the monomer from the easier to decompose
plastic. This process does, however, not allow to react on changes
in the waste stream of plastics over time.
[0008] U.S. Pat. No. 5,904,879 suggests recycling portions of the
hot cracked oil stream to a melting vessel to serve as the melting
medium for the waste plastic materials. The hot oil stream is
recycled at a set weight ratio of hot oil stream to bulk waste
plastic materials.
[0009] The known processes intend to maintain a balance between the
endothermic cracking reaction in the cracking reactor and the
exothermic combustion reaction in the catalyst regenerator. These
methods are effective as long as the waste plastic feed has a
constant chemical composition, i.e. the waste plastic consists of
only one type of polymer or a mixture of polymers which is constant
over time. However, the known actions are not effective if mixed
waste plastic is employed in a continuous process. This is because
mixed waste plastic composition varies over time according to the
market. Moreover, the exact plastic content of mixed waste plastic
varies with the geographic origin of the waste and also the time of
origin. For example, the composition of post-consumer waste plastic
during summer time is different to the composition of the waste
plastic in winter time. However, a varying composition of the mixed
waste plastic creates a heat imbalance in the cracking
reactor/regenerator system because different polymers require
different amounts of energy for cracking and thus varying
compositions of mixed waste plastic will require increasing or
decreasing amounts of energy in the cracking reactor. As
consequence thereof, the prior art processes which for example aim
at maintaining the catalyst activity at a constant level, require
an auxiliary heat sink and/or an auxiliary heat supply if mixed
waste plastic is to be cracked in a continuous process. This
decreases the energy yield of the process, increases the CO.sub.2
footprint and increases the overall cost.
[0010] There is therefore still a need for a process for cracking
mixed waste plastic which does not have the above disadvantages. In
particular, it would be desirable to have a process which can
continuously crack mixed waste plastic which changes its
composition over time without the requirement of any auxiliary heat
sink or auxiliary heat supply. Furthermore, it would be desirable
to have a process for continuously cracking mixed waste plastic
which can balance the changes in energy required for the
endothermic cracking reaction which are due to the changes in the
composition of the mixed waste plastic.
[0011] It has now been found that the above problems can be solved
by adjusting the heat balance between the cracking reactor and the
regenerator depending on changes in the chemical composition of the
mixed waste plastic. The present invention therefore relates to a
process for continuously converting mixed waste plastic into waxes
and liquid fuels by cracking, the process comprising the steps of
[0012] feeding a mixed waste plastic stream to a cracking reactor
where the mixed waste plastic is catalytically cracked in the
presence of a catalyst; [0013] circulating the catalyst between the
cracking reactor and a regenerator where the catalyst received from
the cracking reactor is regenerated and heated by burning coke
and/or other combustible material deposited on or mixed with the
catalyst; characterized in that the heat balance between the
cracking reactor and the regenerator is adjusted depending on
changes in the chemical composition of the mixed waste plastic.
[0014] The present process has the following advantages: [0015] be
less selective on the plastic sorting process. By accepting a lower
quality of plastic materials, more recoverable materials can be
recycled on the sorting steps. [0016] Currently the remaining
wastes, after the sorting process, are mainly composed of plastics,
paper, board, glass, food residue, etc. This fraction called solid
recovered fuel (SRF) can only be valorized as energy in the cement
furnace due to their high calorific value. Removing more plastic
from the incoming feed allows an access to a higher volume of raw
material, but also, to decrease the calorific value of the SRF and
thus valorized as energy in the conventional incinerators. [0017]
Less plastics will end up in landfill sites [0018] Less CO.sub.2 is
emitted per unit of fuel and Capex of the unit is lower.
[0019] In the context of the present invention, waxes are to be
understood as a mixture of hydrocarbons optionally comprising
heteroatoms, such as O, N, etc., being solid at room temperature
(23.degree. C.) and having a softening point of generally above
45.degree. C. Liquid fuels are to be understood as combustible
liquid hydrocarbons optionally comprising heteroatoms, such as O,
N, etc., being liquid at room temperature, such as gasoline,
kerosene and diesel oil.
[0020] The invention therefore allows producing valuable chemicals
from mixed waste plastic, such as post-consumer waste plastic, off
spec plastic, industrial scrap plastic and the like.
[0021] A plastic is mostly constituted of a particular polymer and
the plastic is generally named by this particular polymer.
Preferably, a plastic contains more than 25% by weight of its total
weight of the particular polymer, preferably more than 40% by
weight and more preferably more than 50% by weight.
[0022] Other components in plastic are for example additives, such
as fillers, reinforcers, processing aids, plasticizers, pigments,
light stabilizers, lubricants, impact modifiers, antistatic agents,
inks, antioxidants, etc. Generally, a plastic comprises more than
one additive.
[0023] Plastics suitable in the process of the present invention
are for example polyolefins and polystyrene, such as high density
polyethylene (HDPE), low density polyethylene (LDPE), polypropylene
(PP) and polystyrene. Mixed plastics mostly constituted of
polyolefin and polystyrene are preferred. In this context "mostly
constituted" is to be understood such that the concentration of the
polyolefin and the polystyrene in the mixed plastic is above 50% by
weight, more preferably above 75% by weight, each based on the
total weight of the dry mixed plastic. The mixed plastic may be
constituted of polyolefin and polystyrene. Preferably, the mixed
plastic contains less than 99.5% by weight, more preferably less
than 99% by weight of polyolefin and polystyrene, based on the
total weight of the dry mixed plastic.
[0024] Other plastics, such as polyvinylchloride, polyvinylidene
chloride, polyethylene terephthalate, polyurethane (PU),
acrylonitrile-butadiene-styrene (ABS), nylon and fluorinated
polymers are less desirable. If present in the waste plastic, they
are preferably present in a minor amount of less than 50% by
weight, preferably less than 30% by weight, more preferably less
than 20% by weight, even more preferably less than 10% by weight of
the total weight of the dry waste plastic. Preferably, the
individual content of any less desirable plastic is less than 5% by
weight, more preferably less than 2% by weight based on the total
weight of the dry waste plastic.
[0025] Usually, waste plastic contains other non-desired
components, namely foreign material, such as paper, glass, stone,
metal, etc.
[0026] In the process of the present invention the chemical
composition of the mixed waste plastic will change over time. In
this context, "chemical composition" refers to the polymers present
in the waste plastic. The above described additives and non-desired
components in this context do not add to the chemical composition
of the waste plastic. In other words, the chemical composition
changes if the polymers constituting the waste plastic or their
ratios to each other in the waste plastic change.
[0027] As different polymers require different amounts of energy
for cracking, a change in the chemical composition of the mixed
waste plastic results in a heat imbalance in the cracking
reactor/regenerator system. In this regard, "cracking reactor" is
to be understood as that part of the system where heat is consumed
(mainly due to the endothermic cracking reaction but also for other
parts of the process, such as the evaporation of the resulting
products). "Regenerator" is to be understood as that part of the
system where heat is produced by burning coke and/or other
combustible material deposited on or mixed with the used catalyst
received from the cracking reactor.
[0028] According to the invention the heat balance between the
cracking reactor and the regenerator is adjusted, preferably
automatically adjusted, depending on changes in the chemical
composition of the mixed waste plastic. Thus, the invention allows
to avoid heat imbalances in the cracking reactor/regenerator
system.
[0029] The temperature obtained in the reactor in case of adiabatic
operation is the result of balance between the hot catalyst (flow
and temperature) and the cold fluxes (plastic mix heating, cracking
and vaporization of the products). The overall cracking reaction is
endothermic and its extend results from the kinetic mainly
influence by the catalyst nature, catalyst amount (catalyst/plastic
ratio) and the temperature. The catalyst nature influence the
selectivity and activity of the cracking (in term of gaseous,
gasoline, diesel, kerosene, wax and coke and unconverted plastic
fractions) the catalyst amount and the temperature.
[0030] In the process of the present invention the heat balance
between the cracking reactor and the regenerator can be adjusted by
adjusting the above parameters. The present inventors have,
however, found that in preferred embodiments the heat balance can
be adjusted by using a mixture of at least two different catalysts
having different catalytic activity and adjusting the ratio of the
at least two different catalysts in the mixture. Thus, the overall
catalytic activity of the mixture can be increased or decreased
depending on the changes in the chemical composition of the mixed
waste plastic.
[0031] Adjusting the heat balance between the cracking reactor and
the regenerator can be accomplished by adjusting the ratio of at
least two different catalysts having different catalytic activity
in a mixture of catalysts being circulated between the cracking
reactor and the regenerator. Such mixture of at least two different
catalysts can comprise highly active FCC catalysts known to the
person skilled in the art and being commercially available.
Examples for highly active FCC catalysts are crystalline
microporous zeolites like ZSM-5, ZSM-11, ZSM-22, ZSM-23, ZSM-35,
ZSM-48, ZSM-50, TS-1, TS-2, SSZ-46, MCM-22, MCM-49, FU-9, PSH-3,
ITQ-1, EU-1, NU-10, silicalite-1, silicalite-2, boralite-C,
boralite-D, BCA and mixtures thereof. Less active FCC catalysts are
also known to the person skilled in the art. Examples for less
active FCC catalysts are SiO.sub.2 (sand) and kaolin. Preferred
materials comprise at least 60% by weight, preferably at least 70%
by weight and even more preferably at least 80% by weight of
silica-equivalent of an oxidic compound based of silicon like
Silica (SiO.sub.2), kaolin, etc. FCC catalysts can be fresh
catalysts, equilibrated catalysts (such as spent catalysts), or
mixtures thereof.
[0032] The influence of different catalytic activities of different
catalysts on the cracking of different polymers is now explained in
more detail with reference to the attached figures which show
in
[0033] FIGS. 1A and 1B the effect of the reaction temperature on
the cracking of LDPE in the presence of SiO.sub.2,
[0034] FIGS. 2A and 2B the effect of the reaction temperature on
the cracking of HDPE in the presence of SiO.sub.2,
[0035] FIGS. 3A and 3B the effect of the reaction temperature on
the cracking of HDPE in the presence of ECAT-1C, and
[0036] FIGS. 4, 5A, 5B and 5C the effect of the reaction
temperature on the cracking of PP in the presence of SiO.sub.2.
[0037] FIG. 1A shows the conversion rate of LDPE over time at
425.degree. C. and 450.degree. C., respectively. It is apparent
that at 450.degree. C. the conversion is nearly complete after
about 50 minutes. FIG. 1B depicts the selectivity of the conversion
at 450.degree. C. Surprisingly, with increasing conversion rate the
gas fraction decreases, the gasoline fraction slightly decreases
and the wax fraction (diesel (D) and gasoil (GO)) increases.
[0038] Basically, the same effects are observed when cracking HDPE
in the presence of SiO.sub.2 (see FIGS. 2A and 2B).
[0039] Interestingly, the above effects are not observed when using
a catalyst having higher catalytic activity than SiO.sub.2. This is
demonstrated by using the highly active equilibrated FCC catalyst
ECAT-1C for cracking HDPE. The results are shown in FIGS. 3A and
3B. FIG. 3A shows that even at low temperature the conversion is
fast. However, as can be seen in FIG. 3B, at all temperatures the
selectivity between gases, gasolines and waxes is about the same
and in particular the gasoline fraction is much higher than the wax
fraction which is contrary to the effect observed with the less
reactive catalyst SiO.sub.2.
[0040] Finally, FIGS. 4, 5A, 5B and 5C relate to the cracking of PP
using SiO.sub.2 as catalyst. FIG. 4 demonstrates that the
conversion is fast and nearly complete at 425.degree. C. already.
However, as shown in FIG. 5A, at 400.degree. C. the main fraction
is the gasoline fraction while at 450.degree. C. (FIG. 5C) the main
fraction is the wax fraction.
[0041] Thus, catalysts of a different catalytic activity have
different effects on the cracking of different polymers. As a
consequence thereof, it is possible to adjust the heat balance
between the cracking reactor and the regenerator by adjusting the
ratio of different catalysts having different catalytic activity in
a mixture of catalysts circulated between the cracking reactor and
the regenerator.
[0042] For example the plastic mix is introduced at a defined
temperature and flow in a reaction chamber operating adiabatically.
A hot catalyst stream is introduced in the reaction chamber at a
relative flow rate versus the plastic mix in such a way to obtain
the fixed temperature. The catalyst, the coke and the unconverted
material are sent to a regenerator where air is introduced and the
coke and unconverted material are burnt rising the temperature of
the catalyst. A quantity of the catalyst is extracted from the
regenerator and replaced by an amount of lower activity or higher
activity catalyst in such a way to keep the temperature level in
the reaction chamber at the desired value. Without willing to be
limited by a theory, increasing the low activity catalyst content
will induce an increase of the catalyst flow to keep the average
activity resulting in an increase of the heat supplied and increase
the reaction chamber temperature while increasing the high activity
catalyst fraction will induce a decrease of the catalyst flow
resulting in a decrease of the heat supplied and decrease the
temperature in the reaction chamber allowing to control the
temperature at the desired level.
[0043] In an alternative embodiment the inventors found that the
heat balance between the cracking reactor and the regenerator can
be adjusted by diluting the mixed waste plastic with a diluent.
Such diluent participates also to the heat balance of the system
and therefore is a suitable parameter for adjusting the heat
balance between the cracking reactor and the regenerator. Preferred
diluents are waxes, liquid fuels and mixtures thereof. Most
preferred the waxes and liquid fuels are obtained from the cracking
process. Diluting can take place in the feed stream i.e. prior to
feeding the mixed waste plastic into the cracking reactor, or in
the cracking reactor, i.e. after feeding the mixed waste plastic
into the cracking reactor.
[0044] Preferred diluents are liquid products from the cracking
process having an atmospheric boiling point in the range of 25 to
250.degree. C., more preferably in the range of 50 to 150.degree.
C. The diluent is generally used at a ratio of 0.1 to 10 kg per kg
of mixed waste plastic, preferably at a ratio of 0.5 to 5 kg per kg
of mixed waste plastic and more preferably at a ratio of 0.7 to 2
kg per kg of mixed waste plastic.
[0045] The diluent is introduced preferably at a temperature below
its flash point. When the mixed waste plastic is not melted, the
diluent and the mixed waste plastic may form a slurry. When the
mixed waste plastic is melted, the diluent allows reducing the
viscosity of the melt, easing by this way its cleaning and
processing. By cleaning removal of foreign materials is
understood.
[0046] For example the plastic mix is introduced at a defined
temperature and flow in a reaction chamber operating adiabatically.
A hot catalyst stream is introduced in the reaction chamber at a
defined relative flow rate versus the plastic mix. An auxiliary
hydrocarbon liquid at a defined temperature is introduced in the
reaction chamber at a flowrate such that the temperature in the
reaction chamber reaches a desired value. The catalyst, the coke
and the unconverted material are sent to a regenerator where air is
introduced and the coke and unconverted material are burnt rising
the temperature of the catalyst. Without willing to be limited by a
theory, it is thought that the excess heat is absorbed mostly by
the evaporation of the auxiliary liquid allowing to control the
temperature in the reaction chamber.
[0047] Alternatively, a mixture of the plastic mix with a recycled
hydrocarbon liquid is introduced in a reaction chamber operating
adiabatically. A hot catalyst stream is introduced in the reaction
chamber at a defined relative flow rate versus the plastic mix. The
catalyst, the coke and the unconverted material are sent to a
regenerator where air is introduced and the coke and unconverted
material are burnt rising the temperature of the catalyst.
[0048] In a further alternative embodiment the inventors found that
the heat balance between the cracking reactor and the regenerator
can be adjusted by introducing a combustible stream into the
regenerator. Such combustible stream participates also to the heat
balance of the system and therefore is a suitable parameter for
adjusting the heat balance between the cracking reactor and the
regenerator.
[0049] Preferred combustible streams are hydrocarbon stream from
the cracking process having an atmospheric boiling point lower than
250.degree. C., more preferably lower than 50.degree. C. The
gaseous stream produced after the condensation of the liquid
fraction of the cracked gas is particularly preferred. The
combustible stream is generally used at a ratio of 0.001 to 1 kg
per kg of mixed waste plastic, preferably at a ratio of 0.002 to
0.25 kg per kg of mixed waste plastic and more preferably at a
ratio of 0.005 to 0.15 kg per kg of mixed waste plastic.
[0050] The combustible stream is introduced in the regenerator by
any way known in the art. Examples of introduction are dedicated
dip pipe and spray nozzle. Alternatively, the combustible stream
could be mixed with the coked catalyst stream before introduction
in the regenerator. A gaseous combustible gas stream recovered at
the end of the condensation used as fluidizing medium to transport
the coked catalyst is particularly preferred.
[0051] All three measures (mixing catalysts, using a diluent and
introducing a combustible stream) may be taken alone or in
combination of two or three of these measures.
[0052] In the process of the present invention, the waste plastic
can be subjected to a pretreatment. The pretreatment includes for
example size reduction and foreign material removal. In the prior
art processes in particular foreign material removal is often
conducted in the presence of or using water. The thus obtained
pretreated waste plastic is rather wet and requires either time and
energy consuming drying or results in potential problems during
further processing, such as corrosion. The present inventors
additionally found that also the heat balance and coke formation in
the cracking reactor can be impaired by the presence of water which
vaporizes thereby cooling the contents of the cracking reactor in
addition to the endothermic cracking reaction. Thus, water entering
the cracking reactor together with the waste plastic requires
additional energy or may even lead to an un-uniform cooling within
the reactor which may result in undesired coke formation on the
catalyst particles. This may result in an increase in temperature
when burning the coke in the regenerator which in turn may have an
adverse influence on the overall energy balance of the system.
Therefore, in a preferred embodiment of the present invention the
pretreatment of the waste plastic is conducted as dry
pretreatment.
[0053] Dry pretreatment is to be understood as a pretreatment in
the absence of additional water. In this context, "additional
water" is to be understood as water being present in addition to
the water or moisture in the waste plastic before pretreatment.
Thus, as pretreatment the waste plastic is for example not washed
with water or separated from foreign material by floatation in
water or an aqueous liquid.
[0054] Furthermore, dry pretreatment in the context of the present
invention excludes any liquefying (e.g. melting or dissolving) of
the waste plastic. Any liquefying step can optionally be conducted
in addition to the dry pretreatment for example after the dry
pretreatment and prior to subjecting the waste plastic to
cracking.
[0055] Suitable dry pretreatment steps are for example size
reduction by grinding or shredding and foreign material removal by
separation by cycloning, air or gas elutriation, sieving and
magnetic separation.
[0056] For example, during pretreatment the size of the mixed
plastic pieces can be reduced to a suitable value to be handled.
Waste plastic is available as "volume particles" having significant
length in three directions, as "surface particles" having
significant length in two directions and a much lower thickness or
"line particles" having one major length and two minor dimensions.
Examples of "volume particles" are pieces of shoe soles, car
bumpers, residual plastic pieces from extrusion, etc. Examples of
"surface particles" are pieces of bottles, bags, etc. Examples of
"line particles" are wires, filaments, etc. For the volume
particles the size if to be understood as the two larger dimensions
of the particles, for surface particles, the larger of the two
larger dimensions and for line particles, the larger dimension.
Preferably, the waste plastic particles after size reduction have a
maximum size of less than 100 mm, preferably less than 50 mm.
Typical minimum size is 0.05 mm, preferably 0.1 mm. Suitable
apparatuses for size reduction are known in the art.
[0057] Usually, the waste plastic includes some free water. "Free
water" is to be understood as non-chemically bonded water. Usually,
the water content of waste plastic is less than 20% by weight,
preferably less than 10% by weight, each based on the total weight
of the waste plastic. Since the pretreatment in the process of the
present invention is a dry pretreatment, the water content of the
waste plastic before and after pretreatment can be the same or the
dry pretreatment can even reduce the water content of the waste
plastic. The above preferred water contents of the waste plastic
are preferably the water contents of the pretreated waste
plastic.
[0058] Usually, the waste plastic is used in bulk with air enclosed
in the bulk plastic. The present inventors found that also the
presence of air and in particular oxygen in the waste plastic can
adversely affect the cracking process. In particular, the presence
of oxygen can be dangerous due to the risk of uncontrolled
temperature increase or even explosion in the crack reactor.
Therefore, it is preferred that the process comprises the further
step of reducing the content of air and/or oxygen in the waste
plastic prior to subjecting it to cracking. The content of air
and/or oxygen in the waste plastic can for example be reduced by
mechanical compression, applying vacuum, diluting the air by an
inert gas, purging the waste plastic with an inert gas, and/or
contacting the waste plastic with an oxygen scavenger. Suitable
inert gases are nitrogen, carbon dioxide or combustion gases,
combustion gases being preferred.
[0059] Furthermore, when a pneumatic transportation is used, which
is preferred, as a transportation gas a suitable inert gas may be
used. Suitable inert gases are nitrogen, carbon dioxide or
combustion gases, combustion gases being preferred.
[0060] Preferably, the content of air in the waste plastic prior to
subjecting it to cracking is lower than 10 g/kg of dry waste
plastic, preferably lower than 5 g/kg of dry waste plastic.
[0061] In a further embodiment, the cracking is conducted at an
oxygen content in the cracking reactor which is lower than 10 vol.
% of the gas phase in the reactor, preferably lower than 5 vol. %
of the gas phase in the reactor. The amount of oxygen in the
reactor can be reduced by reducing the amount of air and/or oxygen
in the supplied waste plastic.
[0062] After pretreatment, the plastic waste can be introduced in
the cracking reactor by any suitable means known in the art. In one
embodiment, the waste plastic is in solid state. For introducing
solid waste plastic into the cracking reactor suitable means are
screw conveyer, belt conveyer, pneumatic transportation, bucket
elevator and flexiscrew (transitube). Screw conveyer and pneumatic
transportation are preferred. Pneumatic transportation is
preferably made using the inert gas as defined above. Preferably,
by pneumatic transportation using an inert gas the oxygen content
of the atmosphere surrounding the particles is reduced.
[0063] Before reacting, the waste plastic can be liquefied.
Liquefaction can be conducted by any known means. Suitable means
are heating, dissolving with a suitable solvent or a combination of
heating and dissolving. Heating can be direct heating, indirect
heating or a combination of both. Suitable direct heating are
steaming, contact with hot gas, contact with hot liquid, and
contact with hot solid. Suitable indirect heating are heat transfer
through a surface, mechanical friction, etc. Heat transfer through
a surface is the preferred indirect heating method.
[0064] During heating the waste plastic, if desired, can be
azeotropically further dried with a suitable liquid. Examples of
suitable liquids are hydrocarbons and in particular hydrocarbon
mixes. Particularly suitable is a hydrocarbon mix produced by the
pyrolysis of the waste plastic. Preferably, the light fraction of
the fuel produced by the pyrolysis can be used to azeotropically
further dry the waste plastic.
[0065] The liquefied waste plastic is usually a viscous liquid. In
certain embodiments it can be convenient to reduce the viscosity of
the liquefied waste plastic by adding a suitable diluent. A
suitable diluent is a hydrocarbon mix, such as a hydrocarbon cut. A
hydrocarbon cut is a mixture of hydrocarbons of various molecular
weight mostly constituted with H and C optionally with minor amount
of heteroatoms. Hydrocarbon cuts of any origin are suitable.
Preferred is a hydrocarbon cut from the pyrolysis of plastic.
Gasoline, kerosene, diesel or wax cut from pyrolysis of plastic or
mixtures thereof are particularly preferred. Even more suitable are
gasoline or wax cut from catalytic pyrolysis of mixed waste
plastic. Gasoline cut is to be understood as a mixture mostly
constituted of hydrocarbons having an atmospheric boiling point in
the range of from 25 to 250.degree. C., preferably of from 40 to
250.degree. C., more preferably of from 50 to 150.degree. C.
Kerosene cut is to be understood as a mixture mostly constituted of
hydrocarbons having an atmospheric boiling point in the range of
from 100 to 350.degree. C., preferably of from 150 to 250.degree.
C. Diesel cut is to be understood as a mixture mostly constituted
of hydrocarbons having an atmospheric boiling point in the range of
from 250 to 500.degree. C., preferably of from 250 to 350.degree.
C. Wax cut is to be understood as a mixture mostly constituted of
hydrocarbons having an atmospheric boiling point over 300.degree.
C., preferably over 350.degree. C. In this context, "mostly
constituted" means that the cut is constituted of at least 95% by
weight of said hydrocarbons, preferably by more than 99% by weight
of said hydrocarbons.
[0066] Hydrocarbon cut is an organic phase which may contain water
either dissolved, separated and/or in the form of an emulsion. The
water content is preferably less than 5% by weight, more preferably
less than 2% by weight. Particularly suitable is a hydrocarbon mix
produced by the pyrolysis of waste plastic, in particular mixed
waste plastic. During the melting of the waste plastic some
decomposition may occur. For example, small molecules may be
released during the polymer decomposition. Such small molecules
often contain heteroatoms. Heteroatoms are atoms other than
hydrogen and carbon. Examples of heteroatoms are O, Cl, Br, F, S
and N. Addition of a scavenger for the heteroatoms can be useful to
avoid corrosion induced by such heteroatoms and/or avoid fuel
contamination. Examples of scavengers for heteroatoms are minerals,
such as lime, soda lime, magnesia, silico alumina, alumina,
silica.
[0067] During the liquefaction of the waste plastic some solid may
remain. Such solid may be material having a melting temperature
higher than the set temperature, may result from the decomposition
of the plastic material or the foreign material, or may be a
reaction product of an additive with the above heteroatoms.
Examples of solid material are char resulting from the
decomposition of paper or thermoplastics, such as ABS or PU, or
foreign material, such as glass and metal. These solids are
conveniently removed by filtration of the melted plastic. Any
filtration apparatus may be used, such as plan filters, cartridge
filters, etc. Magnetic separation can also be used.
[0068] Subsequently, the pretreated waste plastic is subjected to
catalytic cracking.
[0069] In the cracking reactor, the pretreated waste plastic is
contacted with a hot catalyst in a reaction chamber in order to
pyrolise the plastic material. The hot catalyst provides at least
part of the energy required to bring the plastic material to the
reaction temperature, to supply the heat required for the
endothermic cracking reaction and to bring the reaction products in
their state after reaction. Preferably, the hot catalyst provides
at least 60%, more preferably at least 90% of the heat required.
Adiabatic operation is particularly preferred. Optionally, a heat
exchanger can be introduced in the reaction chamber in order to
remove any excess heat. Preferably, no more than 10% of the heat is
removed. Preferably, the heat is removed by overheating low
pressure steam. Low pressure steam is a steam pressure between 1
bar absolute and 10 bar absolute, preferably between 1.5 bar
absolute and 4 bar absolute, more preferably between 2 bar absolute
and 3 bar absolute.
[0070] The pressure in the reaction chamber is usually between 50
kPa absolute and 1500 kPa absolute, preferably between 80 kPa
absolute and 1000 kPa absolute, more preferably between 100 kPa
absolute and 500 kPa absolute. A pressure above atmospheric
pressure is most preferred.
[0071] The hot catalyst is introduced into the reaction chamber in
the form of heated particles or a mixture of heated particles which
may comprise inert particles. These particles including inert
particles are designated as "hot solid". Usually, the weight amount
of hot solid is between 0.2 and 20 times the weight amount of
plastic material, preferably between 0.5 and 10 times, more
preferably between 1 and 12 times. Particularly preferred the
amount of hot solid is between 3 and 9 times the amount of plastic
material in the reaction chamber.
[0072] The residence time of the solid in the chamber may be
between 0.1 and 6000 seconds, preferably between 1 and 3600
seconds, more preferably between 3 and 1800 seconds.
[0073] In one embodiment the temperature of the hot solid
introduced in the reaction chamber is higher than the temperature
in the reaction chamber. Usually, the temperature of the hot solid
when introduced into the reaction chamber is between 100 and
500.degree. C. above the temperature in the reaction chamber,
preferably between 150 and 400.degree. C. The temperature of the
plastic material introduced into the reaction chamber is lower than
the temperature in the reaction chamber. Usually, the temperature
of the plastic material introduced into the reaction chamber is
between 100 and 350.degree. C. lower than the temperature in the
reaction chamber, preferably between 150 to 300.degree. C.
lower.
[0074] The reaction chamber ensures the contact between the plastic
feed and the hot solid and allows extracting a gaseous stream and a
condensed stream. "Condensed stream" is to be understood as solid
or liquid. Preferably, the condensed stream is a mixture of solid
and liquid.
[0075] The reaction chamber can be any type known by the skilled
person. Preferably, the reaction chamber has a continuous gas
phase. The reaction chamber may be constituted of one or several
zones having specific flow. Examples of reaction chambers and
reaction zones are fluidized bed, bubbling, bed, spouted bed,
entrained bed, etc. Fluidized bed and entrained bed are preferred.
Fluidized bed is particularly preferred.
[0076] A fluidized bed can be operated using a gas flow from the
bottom to the top in a riser or from the top to the bottom in a
downer, a downer being preferred.
[0077] The reaction chamber can also include a condensed phase-gas
separation zone. Examples of condensed phase-gas separation zones
are decantation zone, sedimentation zone, elutriation zone,
filtration zone and cyclone. Preferably, the reaction chamber is
made of at least two combined zones, more preferably, at least
three combined zones, even more preferable at least four combined
zones. One of these zones should be the reaction zone.
[0078] In a preferred embodiment, the reaction chamber is
constituted of a downer, a decantation zone, a sedimentation zone
and a cyclone zone.
[0079] Optionally, an auxiliary gas may be introduced into the
reaction chamber. The auxiliary gas may be introduced in any zone,
in particular in the reaction zone. Examples of auxiliary gases are
steam, inert gas and recycled gases. Recycled gases are preferred.
More preferably, recycled gases are mainly constituted of
hydrocarbon gases having less than 6 carbon atoms, hydrogen,
nitrogen, carbon oxide, steam, oxygen and/or noble gas. Preferably,
recycled gas contains mainly hydrocarbon gases having less than 6
carbon atoms, hydrogen and nitrogen. Also preferred, the recycled
gas contains less than 5 vol. % of oxygen, more preferably less
than 2 vol. %. Recycling gases obtained after condensation of the
gas stream extracted from the reaction chamber are particularly
preferred. Optionally, a gas stream exiting from a regeneration
chamber may be used as auxiliary gas introduced into the reaction
chamber. Optionally, the auxiliary gas may be preheated.
Preferably, the auxiliary gas is heated up to the temperature at
the bottom of the reactor. Preferably, the preheating is conducted
using the gases from the regenerator. Preferably, the gas leaving
the regenerator. In a preferred embodiment, a gas-gas heater using
the gas leaving the regenerator to preheat the auxiliary gas is
used. Preferably, the auxiliary gas is introduced at the bottom of
the decantation zone so as to flash the condensed phase from the
residual gas.
[0080] A gaseous stream is extracted from the reaction chamber by
any means known in the art. Preferably, the gaseous stream is
extracted in a condensed phase-gas separation zone of the reaction
chamber, preferably from a cyclone zone.
[0081] The gaseous flow leaving the reaction chamber is directed to
a condenser where the heavier hydrocarbons are condensed. The
condensation can be induced by any means, for instance indirect
cooling in a heat exchanger, aerocondenser, or by direct contact
with a quench. Direct contact is preferred. Condensation can be
made in one or in several steps in series. Condensation in one or
two steps in series is preferred. Condensation made by direct
contact of the gaseous stream with a subcooled liquid is preferred.
Particularly in two steps. The first condensation step can be
conducted at a temperature sufficient to avoid the solidification
of the condensed stream. As the molecular weight of the
hydrocarbons produced by the cracking is dispersed, the use of a
direct contact condensation with the circulation of a suitable
hydrocarbon cut is preferred. Suitable hydrocarbon cuts are
kerosene, diesel, mix of kerosene and diesel and the like. The
contact can be conducted by any means known in the art. Examples of
quench means are quench tee, venturi, vessel and column. Quench
tee, vessel and venturi are preferred. A combination of quench tee
and vessel is particularly preferred.
[0082] The liquid vapor mixture obtained in the quench may be
separated by any means known in the art. Such means are gravity
liquid vapor separator, cyclone, demister, filter, etc. Gravity
separator and cyclone being preferred. A combination of a gravity
separator and a cyclone is particularly preferred. Optionally, a
fractionating column where final cooling and condensation of liquid
pyrolysis products take place, may be employed.
[0083] The uncondensed gas may be used as a fluidizing or
transporting gas or may be burnt in a combustor.
[0084] Optionally, a stripping of the liquid solid mixture
extracted at the bottom of the reaction chamber is conducted in a
fluidized bed or in an entrained bed by contacting the product with
a suitable gas stream, preferably using an entrained bed. Suitable
gas streams are overheated steam, inert gas, recycled gases from
the production, recycled gases from the regeneration, etc.
Overheated steam is preferred.
[0085] The stripped hydrocarbon and the gas are separated in a
cyclone from the entrained particles and they are supplied through
a transfer line to a quench where the waxes are condensed and
separated. The stripped solid enters in a transporting line and it
is transported to the regenerator, where coke and unconverted
plastic material are burnt, for example in a fluidized bed.
[0086] In a preferred embodiment the solid comprising the catalyst
is circulated between the cracking reactor and the regenerator.
Most preferably, the temperature and flow of the circulating
catalyst are adjusted to obtain a pre-selected temperature in the
cracking reactor.
[0087] The temperature in the regenerator usually is from 600 to
1000.degree. C., preferably from 650 to 800.degree. C. The pressure
in the regenerator may slightly exceed the pressure in the reaction
chamber. Flue gas is separated from particles entrained from the
fluidized bed in a cyclone.
[0088] The catalyst recovered as the regenerator may include
unburnt material. Unburnt material includes the mineral impurities
introduced with the plastic material. It also includes the products
of the reaction of those impurities with the impurities produced by
the cracking reaction or in the regeneration reaction. Examples of
impurities produced by the cracking reaction are HCl, HBr, HF,
SO.sub.2, H.sub.2S, CO.sub.2, etc. Preferably, the mineral
impurities are in the form of condensed matter, such as liquid or
solid. More preferably, they are solid of low dimension. By low
dimension one means less than 50 micron, preferably less than 20
micron. Fine particles obtained by abrasion of the catalyst are
included in those mineral impurities. The gases leaving the
regenerator are sent to a device allowing the separation of the
condensed phase matter. Examples of such a device are cyclone,
filter, electrostatic precipitator, quench vessel, etc. A cyclone
is preferred. The condensed phase separated includes the ashes
introduced with the plastic material, the reaction products formed
in the reactor or in the regenerator, and the fine from the
catalyst.
[0089] A more complete appreciation of the invention and many of
the attendant advantages thereof will be readily understood by
reference to the following detailed description when considered in
connection with the accompanying drawing shown in FIG. 6.
[0090] According to FIG. 6, the mixed plastic feed 1 is introduced
in the pretreatment 2 where the plastic pieces are shrunked, part
of the foreign material 3 is removed by elutriation and optionally
at least part of the free water 4 is removed. Air 5 is optionally
used for these operations. The mixed product leaving the
pretreatment 6 is introduced in the melting device 8. An auxiliary
liquid 7 is added. The product is liquefied by heating to a
predefined temperature. The gases produced by the increase of
temperature and/or by the decomposition of some components of the
plastic and/or by the reaction of decomposition products are vented
by 9. The air introduced with the mixed plastic feed is also
vented. The foreign impurities not soluble are separated by
decantation and optionally filtration giving low density 10 and
high density 11 impurities. The liquefied product 12 is sent to the
reaction chamber 14 with the hot catalyst 13 coming from the
regenerator. An auxiliary gas 12a is introduced in the reaction
chamber in order to purge the condensed matter flux 16 produced in
the reaction chamber. The vapor flux 15 is sent to the condensation
area not shown in the present figure. The condensed matter flux 16
is sent to the regenerator 19 where air 17 is injected. The
regeneration increases the temperature of the catalyst 13 which is
recycled to the reaction chamber. The gases produced by the
reaction and the ashes 18 are extracted and sent to the effluent
gas treatment not shown in the figure.
[0091] Should the disclosure of any patents, patent applications,
and publications which are incorporated herein by reference
conflict with the description of the present application to the
extent that it may render a term unclear, the present description
shall take precedence.
* * * * *