Note: Descriptions are shown in the official language in which they were submitted.
2073~70
W092/08773 PCT~EPg1/~594
Process for recovering sulphurated styrene residues
1 The present invention relates to a process for recovering
sulphurated styrene residues.
Particularly, the present invention relates to a process
for recovering the sulphurated styrene residues obtained
in the purification of styrene produced via catalytic de-
hydrogenation of ethylbenzene.
As is known, styrene obtained by catalytic dehydrogenation
of ethylbenzene at high temperatures can be recovered from
the crude reaction liquid through fractionated distillation
using a set of fractionation columns.
Since styrene tends to polymerize at the relatively high
1~ distillation temperatures required, it is well known that
during this step a certain conversion of the monomer into
polymeric materials takes place, resulting in a loss of
desired product.
To overcome this drawback, a styrene polymerization inhibi-
tor is generally employed. A very effective and preferably
utilized inhibitor is sulphur. In fact this element, as
compared with organic compounds such as dinitrophenols,
mono- and dinitrophenols containing alkyl substituents in
the aromatic nucleus, nitrous phenols, etc., offers consider-
able technological, process and economic advantages. It
is known, in fact, that the above organic compounds, be-
sides being more expensive, have a high toxicity, can give
rise to corrosion of the apparatus due to their acidic resi-
due content~and can be explosive in the anhydrous state.
However, the use of sulphur as styrene polymerization inhi-
bitor results in the presence of this element in the resi-
dual material of the final distillation column. Therefore,
the residual material obtained from said distillation column
contains:
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W092/08773 PCT/EP91/~94
1 - low-boiling hydrocarbons having a boiling point lower
than 200C such as styrene, cumene, alpha-methyl styrene,
methyl-ethyl-benzenes, methyl-vinyl-benzenes, butyl-ben-
zenes, etc.;
- high-boiling hydrocarbons having a boiling point higher
than 200C, generated in the dehydrogenation section in
the form of polynuclear aromatic compounds:
- polymeric materials such as polystyrene and sulphurated
polystyrene; and
- sulphur-
The sulphur is generally present in a total amount ranging
from about S to about 30%, preferably from about 10 to about
20% by weight calculated on the total residues.
These residues have a very low commercial value and the
prior ar~ evidences that their elimination (disposal) causes
serious problems.
In fact, the combustion of these residual materials involves
ecological and corrosion problems, particularly owing to
the emission of SO2.
Thus, it is an object of the present invention to provide
Z5 a process for recovering the residual styrene distillation
materials containing sulphur as polymerization inhibitor,
which does not show the above drawbacks.
It has now been found that this object is achieved by ad-
mixing said residual sulphurous materials with a gasoil,
subjecting the resulting mixture to a thermal cracking pro-
cess at a temperature of at least (and preferably higher
than) 400C and subsequent fractionated distillation and
hydrodesulphurization of the intermediate fractions having
a boiling point ranging from about 140 to about 390C.
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W092/08773 PCT/EP91/~594
l Thus, it is an object of the present invention to provide
a process for recovering the sulphurous residues obtained
from the purification of styrene (obtained by catalytic
dehydrogenation of ethylbenzene), which comprises the steps
of:
a) adding said sulphurous residues to a gasoil;
b) subjecting the resultin~ mixture to a thermal cracking
process at a temperature of at least 400C;
c) carrying,out a fractionated distillation of the mixture
so treated;
d) subjecting the distillate fraction having a boiling tem-
perature ranging from about 140C to about 390C to a
catalytic hydrodesulphurization process; and
e) recovering elemental sulphur from the sulphurated hydro-
lS gen so obtained.
The sulphur so obtained can be either re-used as styrene
polymerization inhibitor or utilized in its conventional
fields of use.
Those skilled in the art will appreciate the importance
of the process according to the present invention, which
permits to recover the sulphur-containing residues of sty-
rene without causing atmospheric pollution or other problems
which are of considerable technological importance in view
of the great amount of styrene produced. In fact, styrene
is widely used as monomer for producing resins, plastics,
elastomers, synthetic rubbers and the like.
The process of the present invention is preferably used
for recovering the sulphurous residues coming fro,~ the puri-
fication of styrene which has been obtained through the
conventional catalytic dehydrogenation of ethylbenzene;
however, its application is not limited to said specific
styrene source. In other words, the process of the present
invention is applicable to any styrene-Contain1ng feedstock
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W092/08773 PCT~EP91/~594
1 which is polluted by various high-boiling polymeric and
non-polymeric materials and which, in particular, contains
sulphur as polymerization inhibitor.
The gasoil utilized in the process of the present invention
typically is one produced by means of processes for the
vacuum-distillation of oil products and generally has a
boiling point ranging from about 390 to about 550C.
The amount of sulphurous residues obtained from the styrene
purification which can be added to the gasoil is not criti-
cal; generally, amounts exceeding 0.1% by weight, with re-
spect to the gasoil, can be advantageously utilized; amounts
ranging from 0.5 to 10% by weight are preferred.
According to the process o~ the present invention, the mix-
ture composed of gasoil and residual sulphurous materials
is subjected to a thermal cracking process, which preferably
comprises heating the mixture to a temperature of about
465 to about 500C at a pressure of about 10 to about 50
bar in a furnace equipped with an inner heating coil.
The residence time of the mixture in the furnace usually
varies from about 1 to about 15 minutes.
The cracking reaction is preferably completed in a subsequent
soaker, where said mixture is kept for about 10 to about
30 minutes.
At the soaker outlet the mixture is preferably cooled to
a temperature of about 380 to about 400C and then is fed
to a conventional fractionation column, wherein the differ-
ent product fractions are separated.
; as From the column head separator, a stream of light gases,
generally comprising hydrogen, ethane, propane and butane,
.
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W092/08773 PCT/EP91/~S94
s
1 along with sulphurated hydrogen, is obtained; these light
gases can be compressed and conveyed to known aminic scrub-
ing systems for the recovery of sulphurated hydrogen and
then to the sulphur recovery system.
The fractions which boil in the gasoline range(about 70
to 140C) can be either washed and admixed with other com-
ponents in order to obtain gasolines or can be sent to the
catalytic hydrodesulphurization units where, in the presence
of hydrogen and a catalyst based on Co-Mo or Ni-Mo and at
a temperature of about 250 to about 350C and a pressure
of about 20 to about 80 bar, they are first freed from
foreign matters and then sent to octane conversion units
(isomerization and reforming).
The intermediate distillates having a boiling point in the
range of from about 140 to about 390C and comprisin~ kero-
sene (boiling point 140 to 240C) and gasoil (boiling point
240 to 390C) are subjected to catalytic hydrodesulphuri-
zation.
The catalytic hydrodesulphurization process is well-known
and comprises admixing the intermediate distillates with
hydrogen and then heating the resulting mixture to a tem-
perature of about 350 to about 420C at a pressure of about
20 to about 80 bar in the presence of a catalyst, preferably
based on Co-Mo, for about 1 to about 60 minutes. After the
heating treatment, the unreacted hydrogen and the sulphurated
hydrogen formed are separated from the mixture. The obtained
residual product is utilized in the mixtures usually sold
for being burnt in diesel engines or for heating purposes.
The sulphurated hydrogen is preferably recovered by means
of aminic washings and converted into elemental sulphur
by means of conventional techniques such as, e.g., the Claus
process.
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W092/08773 PCT/EP91/OOS94
1 The residual portion which has not been converted in the
thermal cracking process, discharged from the fractionation
column bottom and having a boiling point higher than about
390C, may be subjected to a stripping treatment and utilized
as heavy fuel, according to conventional techniques.
The addition of the residual sulphurated styrenic materials
to the gasoil to be subjected to the thermal cracking process
results in various surprising and unexpectable advantages.
In fact, it has been found that the pyrolysis reactions
are enhanced during the cracking step with formation of
useful products. Therefore, by adding styrene residues and
by operating under otherwise identical temperature and pres-
sure conditions it is possible to obtain an enhancement
of the pyrolysis reaction with a consequent higher yield
of useful fractions or, the yield of useful fractions being
the same, it is possible to carry out the cracking at lower
temperatures ~ T of about 14 to 15C), thereby reducing
the fouling of the cracking furnace heating coil by 60%,
with a consequent increase in the number of operation days
without interruptions for cleaning the coil.
Another advantage resulting from the use of the sulphurous
styrene residues in the gasoil cracking process is the con-
version of the high-boiling products contained in said resi-
dues into oil fractions having a higher added value.
Furthermore, the process of the present invention permits
to suppress the emission of SOz connected with the combustion
of the styrene residues derived from the styrene production
processes, resulting in obvious ecological advantages, and
to considerably reduce or even eliminate the consumption
of sulphur utilized for the styrene inhibition.
To permit a better understanding of the present invention
and to reduce the same to practice, the following examples
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W092/08773 PCT/EP91/0~94
1 are given hereinafter for illustrative and exemplifying
purposes; however, they are not to be construed as a limi-
tation of the invention.
In said examples, reference is made to the attached figure 1
which shows a schematic view of a possible embodiment of
the process of the present invention.
Example 1
Through a feeding pipeline (1) there were fed 1,200 t/day
of gasoil from vacuum distillation, having a boiling range
of from 390C to 522C, a sulphur content of 2.3~o by weight
and a density (lS/4) of 0.928 kg/l. To the above charge
there were added, through line (2), 3~ by weight of styrene
lS residues derived from the production of styrene via catalytic
dehydrogenation of ethylbenzene. The styrene residues had
the following composition:
- hydrocarbons having a boiling point below 200C 20~
- hydrocarbons having a boiling point above 200C 35%
- polymeric products 33%
- sulphur 12
The gasoil-styrene residues mixture was heated to about
300C and fed to a furnace (3) equipped with a heating coil
(4). In furnace (3) the mixture was heated to 495C and
kept there for about 7 minutes.
The pressure at the outlet of coil (4) was maintained con-
stant at 15 bar, while the pressure at the inlet, when the
operation was started with a clean coil ~4), was 25 bar.
3~ The mixture leaving furnace (3) was then conveyed to a soaker
(5) operating at a pressure of about 15 bar, the residence
time therein being about 15 minutes. Due to the endothermal
cracking reaction the tempexature decreased from 495C at
the inlet to about 440C at the outlet.
The mixture was then cooled to about 390C and fed to a
conventional fractionation column (6). The uncondensed light
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W092/08773 PCT/EP91/~594
1 gas leaving the head (7) of the column (6) was compressed
and sent to an aminic scrubbing unit to recover the sul-
phurated hydrogen and to subsequently convert it into sulphur,
following well-known and conventional processes.
The liquid gasoline recovered in the upper portion (8) of
column (6), was sent to the gasoline desulphurization unit
and then subjected to the well-known isomeri~ation and re-
forming processes.
Kerosene having a boiling point range of from about 140
to about 240C, leaving the upper middle portion (9) of
column (6), and gasoil having a boiling point ranging from
about 240 to about 390C, leaving the lower middle portion
(10) of column (6), were mixed together and introduced into
a desulphurization reactor (13) after hydrogen (12) had
been added thereto in an amount of about 0.3~ by weight
calculated on the kerosene/gasoil mixture. In the desulphuri-
zation reactor (13), the kerosene/gasoil mixture was heated
to 37UC at a pressure of 57 to 58 bar and in the presence
of a ~o-Mo catalyst. The sulphurated hydrogen-containing
gases leaving reactor (13) through line (14) were subjected
to an aminic scrubbing, and from said gases sulphurated
hydrogen was recovered and re-converted into elemental sul-
phur in a Claus plant.
The desulphurized mixture leaving the reactor (13~ through
line (15) was stored.
The residues at the bottom (11) of the fractionation column
(6) were utilized as fuels.
During the test, the pressure at the inlet of furnace (3)
gradually rose until reaching a value of 38 bar after 61
days of operation.
At day 61, the run was discontinued and the coil (4) was
cleaned,following conventional modalities of decoking operat-
ions.
The obtained yields are reported in the following Table I.
as
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W092/08773 pcT/Ep9l/oo5s4
1 Example 2 (comparison test)
Example 1 was repeated without addition of 3% of styrene
residues.
The obtained yields are reported in the following Table I.
TA8LE
I I
EXAMPLE 1 EXAMPLE 2
YIELDS AT THE C0LUMN OUTLET .
A) Gas ~ by weight1.4 1.0
B) Gasoline % by weight4.1 3.1
C) Kerosene ~ by weight6.5 5.0
D) Gasoil ~ by weight23.1 14.9
E) Residues X by weight64.9 76.0
DAYS OF OPE~ATION OF COIL (4) 61 85
TOTAL 8+C+D PR0~UCED DURING
THE OPERATIO~ (t) 25,400 23,460
.From the above data it is apparent that under identical
operating conditions the presence of styrene residues results
in a considerable increase in the plant productivity.
ao Example 3
Following the procedure of example 1, the gasoil-styrene
residues mixture was treated in furnace (3) at 485C and
at the same inlet and outlet pressures.
The obtained results are reported in the following Table II.
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1 Example 4 (comparison test)
Example 3 was repeated without addition of 3% of styrene
residues.
The obtained results are reported in the following Table II.
TABLE II
I I
EXAMPLE 3EXAMPLE 4
. .
YIELDS AT THE COLUMN OUTLET
.
A) Gas O by weight 1.1 0.8 .
B) Gasoline % by weight 3.4 2.4
C) Kerosene X by weight 4.9 3.8
D) Gasoil % by weight17.6 13.2
E) Residues % by weight 73 . 79.8
DAYS OF OPER~ION OF COIL (4) 106 140
TOTAL B+C+D PRODUCED DURING
THE OPERAllON ( t) 33,930 32,600
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