Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
359~3
K 9729 FF
PROCæSS FOR THE PREPARATION OF BITUMEN
The present invention relates to a process for the
preparation of bitumen, bitumen thus prepared and bitumunous
compositions ccmprising bitumen thus prepared.
Bitumens are widely used for purposes such as road
construction, roofing, the coating of pipelines, as binders for
briquettes etc. In many applications the bitumen is mixed with
aggregates and/or filler materials which render the resulting
mixture strength. For example in road construction bitumen is
mixed with sand and stones and the mixture is used as road
asphalt. It is evident that the road asphalt should be
sufficiently resistent to abrasion and fretting. So, it would
be advantageous to prepare bitumens which when mixed with filler
material and/or aggregates, show an increased resistence to
fretting.
Another imçlortant feature of bitumen is its resistanoe to
water ingress. This is especially the case when bitumen mixes
are used in applications to protect structures from water, such
as roofing, pipeline coating and road construction applications.
It has now been found that bitumen originating from a
thermally cracked hydrocarbon feedstock or bituminous
compositions containing such bitumen show excellent resistances
to fretting and water ingress.
However, it is known that bitumens obtained from thermally
cracked feedstocks have unsatisfactory ageing and stability
properties as is described in Fuel, 60 (1981) 401-404 and Fuel,
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63 (1984) 1515-1517. Therefore, such bitumens are considered to
be unsuitable for use in e.g. road asphalt.
It has now been found that a specific process for
handling a thermally cracked feedstock yields bitumen having
excellent resistances against fretting and water ingress and
showing satisfactory stability and ageing properties.
The invention therefore provides a process for the
preparation of bitumen in which a residual fraction of a thermally
cracked hydrocarbon feedstock is distilled under subatmospheric
pressure of between 2 to 120 mmHg (0.27 to 16.0kPa) at a maximum
distillation temperature of between 310 to 370C that corresponds
with the boiling point at the subatmospheric pressure of
hydrocarbons having an atmospheric boiling point of 455-540C and
at least a part of the distillation residue is recovered as
bitumen.
The reference to the hydrocarbons boiling point at
atmospheric pressure is made after conversion of a subatmospheric
boiling point in accordance with (the Maxwell-Bonnell relation
described in Ind. Eng. Chem. r 49 (1957) 1187-1196). In practice a
boiling point of a hydrocarbon ls determlned under subatmospheric
pressure. Since at many subatmospheric pressures many different
bolling points can be determined the person skilled in the art
prefers to refer to an unambiguous converted atmospheric boiling
point.
The maximum distillation temperature should not be below
the boiling point of hydrocarbons with an atmospheric boiling
point of 455C (455C/bar-hydrocarbons), since otherwise an
unsatisfactory removal of relatively light hydrocarbons would be
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obtained, which would result in relatively unstable and rapidly
ageing bitumen, just as described in the above articles from Fuel.
On the other hand, if the maximum temperature would exceed the
540C/bar-hydrocarbons boiling point the resulting residue would
be too hard to be suitable for use in e.g. road asphalt and may
give rise to incompatibllity problems when used in bitumen blends.
t
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me residual fraction subjected to the subatmospheric
distillation can be almost any fraction fro~ the thermal
cracking unit. It is advisable to send the thermally cracked
product to an atmospheric distillation unit to separate
S distillate products such as gases, gasoline, kerosene and gas
oils from the atmospheric residue. Conveniently this
a~spheric residue is sent to the subatmospheric distillation.
The atmospheric distillation is suitably carried out at a botton
temperature of from 300 to 370C. Hence, the residual fraction
sent to the subatmospheric distillation suitably has at least
80~w of components having an atmospheric boiling point of at
least 300C.
Thermal cracking is a rather simple cracking process. At a
temperature level of about 400 to 500C the longer hydrocarbons
become unstable and tend to break into smaller molecules of all
possible sizes and types. m e feedstock for thermal cracking is
generally a mixture of complex heavy hydrocarbons left over from
an atmospheric or vacuum distillation of a crude oil.
Visbreaking, i.e. reducing viscosity by breaXing of molecules,
is an important application of thermal cracking because it
reduces the viscosity of the residue obtained after the thermal
cracking considerably. Visbreaking is carried out by sending a
feed after appropriate preheat to a furnace for heating the feed
to the cracking temperature. From there the feed is fed into a
soaker dcwnstream of the furnace where most of the cracking
takes place. The soaker has suitably internal baffles to
prevent too much back-mixing. The products are gas, distillates
and residue. This residue has a lower viscosity than the feed.
Preferably such a residue, i.e. the residue of a visbroken
hydrocarbon feed, is used as the residual fraction in the
process according to the present invention. The visbreaking
conditions are suitably a pressure of from 2 to 30 bar, a
temperature of 400 to 500C and a residence time of from 5 to 60
min.
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The residual fraction is distilled under subatmospheric
pressure. This includes that it is subjected to a conventional
vacuum distillation, provided that the requirement as to the
maximum distillation temperature is met. It is, however,
preferred to subject the residual fraction to flash distillation.
In flash distillation the residual fraction is heated to a
temperature within the boiling range at a lower pressure of the
liquid and introduced into a subatmospheric flash zone to yield
distillate and residue. The residue is at least partly recovered
as bitumen.
Many subatmospheric presæures can be uæed in the process
according to the invention. Each pressure applied determines the
temperature limits within which the distillation has to be carried
out.
As stated hereinbefore, the maximum distillation
temperature is selected such that on the one hand a satisfactory
removal of relatively light hydrocarbons is obtained but on the
other hand the formation of an unacceptably hard bitumen is
avolded. Preferably, the maximum distillation temperature
corresponds with the boiling point of 460-510C/bar-hydrocarbons.
The bitumen prepared according to the invention has
satisfactory ageing and stability properties. To even improve the
oxidation stabllity the bottom fraction of the distillation is
preferably at least partly subjected to blowing before being
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recovered as the desired bitumen. The blcwing process is
generally carried out continuously in a blowing column, into
which liquid bitumen is fed and wherein the liquid level is kept
approximately constant by withdrawing bitumen ne æ the bottom.
Air is blown through the liquid mass via an air distributor at
the bottom of the column. Suitable blowing te~çeratures are
170-320C, in particulæ 220-275C.
It is known in the art to blend væ ious types of bitumen to
obtain a bitumen composition having the desired properties. me
present invention further provides bituminous compositions
comprising bitumen prepared in a pro oe ss according to the
present invention. It should, however, be avoided that such a
bituminous composition contains an overbalance of asphaltenes
sin oe in such compositions heterogeneity may occur. mere is a
chance of creating an overbalan oe of asphaltenes when a
thermally cracked residue is used as blending component, since
it is known, e.g. from the above-mentioned articles in Fuel,
that the asphaltene content in thermally cracked residue is
rather high. For, though during the thermal cracking the heavy
hydrocarbon oils are converted to lower-boiling compounds, the
asphaltenes æe concentrated in the residue. ~breover, new
asphaltenes are formed during the cracking opera~ion. The
possibility of creating an asphaltenes overbalanoe is
substantially excluded if the naximum distillation temperature
in the process according to the invention is below the boiling
point of 540C/bar-hydrocarbons, preferably of
510C/b æ-hydroc æbons. Suitably, the bituminous composition
contains from 5 to 60%w of the bitumen prepæed according to the
invention and 95 to 40~w of at least one other bitumen
component. A person skilled in the æ t will be able to select
the proper other bitumen oomponent(s) in accordance with his
desires. & itable other bitumen CQmponents include straight-run
bitumen, propane bitumen, bright stock extracts such as furfural
extracts. The components may be blown or unblown and may or may
not contain flux oils. Criteria on which the other bitumen
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components are selected comprise the volatility, density,
penetration, softening point etc, as can be determined by the
person skilled in the art.
It is evident that the bituminous compositions according to
the present invention may contain other additives such as
diluents and/or polymers, in particular styrene-butadiene or
styrene-isoprene block copolymers or atactic polypropene.
The invention will be further elucidated by means of the
following examples.
EX~MPLE I
In this Example some characteristics of thermally cracked
residues were determuned. Residue I was a thermally cracked
residue which has not been subjected to a flashing step.
Residue II is obtained after flashing Residue I at 364C/30 mmHg
(4.0kPa), corresponding to 496/bar. Residue III is obtained
after flashing Residue I at 330C/30 mmHg (4.0 kPa)
corresponding to 460C/bar and a blowing step at an air
consumption of 20-30 Nl/hg residue and at 280-300C. In a thin
film oven test (TFOT, ASTM D1754) the residues were subjected to
heat (163C) and air, and their ageing behaviour was determined.
After the test the penetration was measured and compared with
the original penetration, yielding a retained-penetration value
(in %). m e higher the retained penetration, the better is the
residue able to stand up against heat and air. The loss of
weight during the test was determined as well; and also the
change in the ~oftening point, determined by the Ring and Bell
method was measured ( R & B). me results are indicated in
Table I.
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TABLE I
Residue I II III
Penetration/25C, dmm 71 81 85
Softening point R & B, C 52 46 48
Flash point, C 210 320 316
TFOT (163C)
Loss on heating, % m/m 1.8 -0.08 0.06
Retained penetration, % 46 62 57
R & B, C 14 5 6
From comparison of the results of Residues I and II it is
apparent that bitumen prepared according to the invention has
improved ageing behaviour as shcwn by the higher retained
penetration, no loss on heating and a smaller change in the
softening point. Comparison between the results of Residues II
and III teaches that the similar characteristics can be obtained
by some milder flashing followed by blowing.
EXAMPLE II
For a number of campositions their suitability for use in
asphalt mlxes was tested. Therefore asphalt mixes were
subjected to the Marshall test, extended for retained Marshall
values upon storage of the mixes for tw~ weeks in water at 60C,
to obtain information on the sensitivity of the stability of the
mix towards water.
The mixes contained 6.0% m/m of bituminous composition,
based on 100% m/m of mineral aggregate, with a typical void
content of 2% v/v.
The bitumunous oomposition consists of a Middle East, short
residue and vacuum-flashed thermally cracked residue, flashed at
conditions corresponding to 495C/bar. The results are
indicated in Table II.
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I~BLE II
Composition cracked ShortRetained Marshall
No. residue residuevalue, %
~w %w
1 34 66 91
2 40 60 83
3 46 54 98
4 50 50 92
Similar tests were carried out with bitumlnous compositions
in asphalt muxes, which compositions consisted of propane
bitumen (PB), bright stock furfural extract (BFE) and
vacuum-flashed thenmally cracked residue (VFCR) flashed at
conditions corresponding to 500C/bar. The retained Marshall
values for the compositions are indicated in Table III.
TABLE III
No. Bituminous composition Retained Marshall
VFCR PB BFE Value, ~
%w %w %w
5 0 58 42 62
621 42 37 81
743 37 20 87
855 18 27 89
960 23 17 92
From the above results it is apparent that the bituminous
compositions according to the invention have excellent water
resistance.
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EXP~PLE III
Compositions 6 and 7 of E~a~ple II were subjected to a
fretting test in which the percentage of abraded material was
determuned after storage in water for 240 hours at 40C. The
test is described in "Proceedings of APPT, 463, v~l. 32, pp.
380-411.
The smaller the loss of material, the better was the
resistance to abrasion and fretting. The results are indicated
in Table IV.
TABLE IV
No. Bituminous composition Loss of surface
VFCR PB BFE material,
%w %w %w g
621 42 37 29.2
743 37 20 25.1
From these results it is apparent that the composition with
the higher VFCR content has even improved fretting and abrasion
resistance.
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