Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method and
apparatus for the continuous automatic analysis of the
crystallization point of liquid substances, especially of
aromatic hydrocarbons.
2. Description of the Prior Art
A method and an apparatus for the automatic
continuous analysis of the filterability point of liquid
substances, in particular of gas-oils during manufacture,
has previously been described in a U.S. patent specification
by applicant.
The method consists of passing a substance to be
analyzed through a measuring circuit at a constant flow rate,
the temperature therein being maintained at a value sufficiently
below the assumed filterability point of the substance, heating
means being provided along the flow path of the liquid substance
in said circuit, and automatically turning on and off said
heating means respectively at two characteristic temperatures
detected by measuring the pressure drop or difference in pressure
of the substance at the inlet and at the outlet of the circuit,
the variation of said pressure drop being effected as a function
of temperature, the values thus measured of the pressure drop
being utilized as control data for respectively turning on and
off the heating means, when the temperature at the outlet of
the substance reaches one of the two characteristic temperatures,
the first corresponding to a temperature slightly below the
filterability point, whilst the other corresponds to a tempera-
ture sufficiently above said filterability point, and in that
the temperature of the substance at the outlet of the circuit is
recorded continuously, which temperature constitutes a
representative value of the filterability point of the substance.
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A preferred embodiment of the apparatus for
carrying out the aforementione~ method comprises a measuring
circuit constituted by a capillary tube of stainless steel
traversed by the substance to be analyzed, whose flow rate is
kept constant by a small volumetric pump, the capillary tube
having a large portion of its length immersed in a cooling
liquid bath contained in a measuring well, the inlet and the
outlet of the capillary tube in the well being connected,
outside of the well, to two electrical leads of which one is
directly connected to the power supply whilst the other is
connected to differential pressure-responsive switching means,
mounted between the inlet and the outlet of the capillary tube,
whilst a thermosensitive element is positioned at the outlet
of the measuring well along the flow path of the substance to
be analysed and is connected to a recording device.
The aforementioned patent application also describes
a method and an apparatus for the continuous measurement of the
filterability temperature limit, the filterability point
corresponding to the appreciable diminution of the flowability
of a gas-oil when the temperature drops.
Devices have also already been described, enabling
the determination of the crystallization temperature of a
liquid. Reference may notably be made to U.S. Patent No.
3,577,765, which relates to a method and an apparatus for
determining automatically the crystallization temperature of
a flowing liquid and more particularly, of aqueous solutions,
for example of sodium benzenesulfona~e whose crystallization
is not of the "clean" type, that is to say it manifests an
increase in viscosity of the cooled liquid stream.
Such a method is not suitable for substances of
high purity, such as, for example, benzene, paraxylene, or
orthoxylene, since these substances having a crystallization
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of the "clean" type, exhibit a random undercooling phenomenon,
described in more detail in the description which follows, and
this phenomenon is a handicap for the automatic continuous
analysis of the crystallization point.
OBJECTS AND GENERAL DESCRIPTION OF THE INVENTION
It is therefore an object of the present invention
to provide an improved method and apparatus for the automatic
continuous analysis of the crystallization point of liquid
substances.
It is a further object of the present invention to
provide a method and an apparatus specially adapted to the
automatic continuous analysis of the crystallization point of
substances with crystallization of the "clean" type.
It is another object of the present invention to
provide a method and an apparatus for overcoming the above-
mentioned drawbacks of the prior art method and apparatus,
notably in that it overcomes the phenomenon of random under-
cooling or supercooling of a substance with "clean" crystalliz-
ation in the course of cooling.
~0 In cold testing, gas-oils undergo chanses of state.
It is therefore a further object of the present invention to
provide a method and an apparatus well adapted to the contin-
uous measurement and definition of the characteristics of a
gas-oil.
It is a further object of the present invention to
provide a method and an apparatus for the continuous measure-
ment of the crystallization point of a gas-oil and more
particularly of an aromatic hydrocarbon.
Other objects and advantages of the method and
apparatus according to the present invention will emerge from
the description which follows.
According to the invention, in its most general
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form, there is provided a method for the automatic continuous
analysis of the crystallization point of liquid substances,
particuarly of aromatic hydrocarbons, characterized in that it
consists of passing a constant flow rate of the substance to be
analysed through a measuring circuit, whose temperature is kept
at a value sufficiently below the assumed crystallization point
of said substance, heating means being provided in the flow
path of the liquid in said circuit, in that the liquid substance
to be analysed is kept in the course of cooling in contact with
an isomorphous crystalline material, and in that said heating
means are automatically turned on and off respectively at two
characteristic temperatures detected by measuring the pressure
drop or pressure difference of the substance at the inlet and
at the outlet of the measuring circuit, the variation of said
pressure drop being effected as a function of temperature, the
values thus measured of the pressure drop being utilized as
control data for turning on and off respectively the heating
means, when the temperature at the outlet of the substance
reaches one of its two characteristic values, the first
corresponding to the crystallization temperature level whilst
the other corresponds to a temperature sufficiently above said
crystallization point r and in that the temperature of the
substance at the outlet of the circuit is recorded continuously,
which constitutes a representative value of the crystallization
point of the substance.
According to another aspect of the invention, in
its most general form, there is provided an apparatus for
carrying out the aforesaid method, which apparatus comprises
a measuring circuit constituted by a stainless steel capillary
tube traversed by the substance to be analysed, of which the
flow rate is maintained constant by a small volumetric pump,
the capillary tube having a large portion of its length immersed
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in a cooling liquid bath contained in a measuring well, the
bottom of the latter being arranged in the form of a fluidYtight
crystals chamber which contains an isomorphous crystalline
material, a short portion of the capillary tube extending
through said chamber, the isomorphous material being in
contact with the substance to be analysed, by means of an
opening formed in the portion of the tube situated in said
chamber, the inlet and the outlet of the capillary tube in the
well being connected externally of the well to two electrical
leads of which one is directly connected to the differential
pressure-responsive switching means, mounted between the inlet
and the outlet of the capillary tube, whilst a thermosensitive
element is positioned at the outlet of the measuring well on
the flow path of the substance to be analysed and is coupled
to a recorder.
BRIEF DESCRIPTION OF THE DRAWINGS
In order that the invention may be more fully
understood, several embodiments of the method and apparatus
according to the invention are described below with reference
to the accompanying drawings, given of course, purely by way
of non-limiting example. In the drawings:
Figure 1 is a graph given by way of explanation;
Figure 2 is a diagram of an embodiment of the
apparatus according to the invention;
Figure 3 is a graph showing the temperature/purity
relationship for benzene;
Figure 4 is a graph showing the temperature/purity
relationship for paraxylene;
Figure 5 is a graph showing the temperature/purity
relationship for orthoxylene.
DESCRIPTION OF PREFERRED EMBODIMENTS
$here is of course a relative similarity for gas-
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oils in the course of cooling, between the change of statecorresponding to the filterability point and the crystalliæation.
However, it must be noted that there is a difference
in crystallizing substances of the "pasty" type and those with
"clean" crystallization.
Referring to Figure 1 of the accompanying drawings,
the difference between "pasty" type crystallization and "clean"
type crystallization will now be defined.
Figure 1 is a graph showing the known temperature/
pressure drop relationship in the case of a gas-oil.
Curve A shows the development of the pressure drop in
a circuit traversed by a gas-oil in the course of cooling, this
gas-oil having progressive crystallization of the "pasty" type.
Curve B shows the development of the pressure drop
in a circuit traversed by a gas-oil in the course of cooling.
this gas-oil having a "clean" crystallization. This "clean"
crystallization is the case for aromatic substances such as
benzene, paraxylene and orthoxylene.
The dashed portion of curve B corresponds to the
random undercooling which mostly precedes any clean crystal-
lization. This phenomenon is a particularly constraining handi-
cap in carrying out continuous analysis, but it is possible
to eliminate the risk thereof completely by maintaining the
liquid in the course of cooling in contact with an isomorphous
crystalline material, that is to say of the same system of
crystallization.
In the method according to the invention, generally,
the flow rate of the substance to be analysed is kept constant,
although the variation of the pressure drop is measured
progressively as the temperature of the circulating substance
drops, this substance being kept in contact with an isomorphous
crystalline material in the part where its crystallization takes
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place.
Figure 2 shows diagrammatically an embodiment of
the apparatus according to the invention.
The analyser of the crystallization point comprises
a measuring circuit cons~ituted by a capillary tube of stainless
steel.
This capillary tube 1 is traversed by a substance to
be analyzed suppliedat 21 and propelled by a volumetric pump
22 situated downstream. The tube 1 passes into a measuring
lQ well 2. This measuring well 2 is a cavity of small volume
formed in a solid mass 3 of steel. This measuring well
comprises a fluid-tight chamber 4 in its lower part, said
chamber 4 containing an isomorphous crystalline material 5,
that is to say of the same system of crystallization as the
substance to be analysed, a rhombic pyramid system, for
example, if the substance to be analysed is benzene. The
remaining part of the measuring well 2 is filled with a non-
freezable product 6.
The solid steel mass 3 comprises means connected
to a cooling unit and is heat-insulated.
The capillary tube 1 has an outer diameter of 1.5 mm
and an inner diameter of about 1.2mm, and it is shaped so that
it offers a fairly large surface in the measuring well 2 before
the entry of said tube 1 into the arystals chamber 4; the tube
1 may for example have a substantially helicoidal shape. The
capillary tube 1 penetrates the crystals chamber 4 through
a fluid-tight and electrically insulating barrel 7a and re-
emerges therefrom through a barrel 7b identical with the
barrel 7a. The portion l_ of the capillary tube 1 passing
into the crystals chamber 4 comprises a longitudinal opening 8
formed throughout the length of a generator of this portion of
tube la. This opening 8 thus permits the liquid stream cir-
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culating in the capillary tube to be placed in communicationwith the isomorphous material 5, for example, benzene, stored
in the chamber 4.
The capill~ry tube 1 has a length of about 100 cm
and represents an electrical resistance of about 1 ohm. The
inlet and outlet connections of the tube 1 are formed as
insulating joints and are connected, through a switch 10,
associated with differential pressure-sensing switch means 11,
to a potential difference 12 (mains).
The differential pressure-sensing means 11 is
mounted between the inlet 13 of the tube of the measuring
circuit on the side of its high pressure connection and the
; outlet 14 of the measuring circuit on the side of its low
pressure connection.
On the side of the outlet 14 of the measuring well,
a thermosensitive element 15 dips into the outlet section of
the capillary tube 1. This element 15 is constituted by a
platinum resistance of 100~ at 0C, technically known as
"pyrothenax"; its outer end is connected to a continuous
temperature recorder 16.
At its outlet 14 from the measuring well 2, the
capillary tube 1 is extended perpendicularly to the thermo-
sensitive element 15 and its end is provided with a drain 17.
The dynamic behaviour of the whole of the apparatus
will now be described.
At the inlet of the circuit, by means of the
volumetric pump 21, the constant flow rate of the substance to
be analysed is ensured, which permits continuous circulation of
the substance in the capillary tube 1.
The analysis unit or measuring well 2 is kept at
0C and, the circulation being established in the capillary
tube 1, the sequence of analysis cycles occurs in the manner
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described below.
The temperature of the substance to be analysed 5,
for example benzene, standing in the chamber 4, drops rapidly
since this chamber is in direct thermal contact with the bottom
of the cooling well. A~ter a possible, but single, initial
overcooling, the product 5 crystallizes in the chamber 4. The
liquid stream circulating in the capillary tube 1 is hence in
physical contact with isomorphous crystals, which avoids over
cooling during its crystallization. In fact, as soon as the
liquid benzene flowing in the tube, as a result of its pro-
gressive cooling, reaches cry~tallization temperature, there is
a solidification of the capillary, or of a considerable portion
of the latter without the appearance of a random under cooling
phenomenon preceding "clean" crystallization. The stopping
of the flow which results from the solidiication of the
capillary causes a rise in pressure at the inlet of the
capillary and results in closing the pressure-sensitive
switch 10; the differential pressure-sensing means 11 connected
to this capillary 1 then actuates the starting up of the re-
heating sequence by applying an alternating potential differenceto the terminals of the capillary 1, which then behaves as a
heating resistance.
The rise in temperature of the whole of the capillary
circuit causes fusion of the crystals occurring therein, thus
freeing the flow in the capillary, which permits instantaneous
return to the initial pressure and thus causes the stopping of
the heating.
The relatively large amount of crystals 5 immobilized
in the chamber 4, and the fact that the latter are kept at a
temperature sufficiently below their melting point, enables
their permanence to be ensured, only the layers surrounding the
capillary being liable to successive fusions and crystallization.
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~046'79Z
Liquid/solid physical contact is thus ensured constantly in
spite of the slight mobility of the transition zone.
The sequence of analyses then ~ollow, according to
the procedure described above, at the rate of about three per
minute. This frequency of recurrence is hence sufficient to
permit the analysis to be compared with continuous analysis.
The development of the temperature thus recorded
by the device 16 then presents itself in the form of a saw-
tooth, whose lower crests locate accurately the successive
values of the crystallization temperatures~
The envelope curve of these low crests enables
the development of the latter to be followed, although it is
of course possible to insert in the recording channel a memory
device enabling only the continuous tracing of this low envelope
to be recorded.
The apparatus according to the invention has been
utilized for the continuous analysis of the crystalliztion point
of benzene.
Of course, a relationship exists between the
crystallization temperature and the purity of the benzene,
whose graphic representation is shown in figure 3. This curve
represents the crystallization temperature in degrees
centigrade as ordinate, as a function of the purity of the
benzene in % as abscissea.
The graph of figure 3 has been calcu ~ ted from
cryoscopic constants according to the formula:
Log10 P = 2.00000 - (A/2.3026)(t~ -t~) [1 ~ B(t~ -t~]
where
P = percentage of moles of benzene
A = 0.01523 molar fraction per degree
B - 0.0032 molar fraction per degree
t p O= 5.333 ~ 0.010C
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:~0467g2
tp - crystallization point in C of the inpure benzene.
By means o~ the apparatus according to the invention,
table I given below has been recorded which enables the
temperature of crystallization~purity of benzene relationship
to be established.
TABLE I
PURITY OF THE BENZENE AS A FUNCTION OF THE
CRYSTALLIZATION POINT
Crystallization %Benzene Crystallization %Benzene
point C. point C
-
1.0 93.2 3.5 96.9
1.1 3.6 97.1
1.2 93.5 3.7 97.2
1.3 93.7 3.8 97.4
1.4 93.8 3.9 97.5
1.5 94.0 4.0 97.7
1.6 94.1 4.1 97.8
1.7 94.3 4.2 98.0
1.8 94.4 4.3 98.1
1.9 94.5 4.4 98.3
2.0 94.7 4.5 98.4
2.1 94.9 4.6 98.6
2.2 95.0 4.7 98.7
2.3 95.1 4.8 98.9
2.4 95.3 4.9 99.0
2.5 95.5 5.0 99.2
2.6 95.6 5.1 99.3
2.7 95.8 5.2 99.5
2.8 95.9 5.3 99.8
2.9 96.0 5.4 99.8
3.0 96.2 5-5 99 9
3.1 96.3
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TAB~E I (co:ntin ed~
PURITY OF THE BENZENE AS A FUNCTION OF THE CRYST~LLIZATION POINT
Crystallization ~senzene Crystallization %senzene
point C point C
.
3.2 96.5
3.3 96.6
3-4 96.8
From this recording, it appears overall that a
fluctuation in purity of 0.15% is manifested by a fluctuation
of 0.1C in the crystallization temperature.
For the continuous analysis of the crystallization
point of benzene, the extent of measurement corresponds to
2.5C to 5.5C, namely to a purity of95-5 to 99.9%. Since
the defined limit corresponds to 1/100 of the length of
measurement, it is 0.3C. According to the method of the
invention, the repeatability in measured purity is 0.05% and
the response time with a momentary variation in purity of 1%
is about 15 minutes.
According to the method of the present invention,
it is possible to carry out continuous analyses of the
crystallization points of other aromatic substances such as
paraxylene or orthoxylene, as well as any substances whose
crystallization temperatures are situated between -30C and
ambient temperature.
In figure 4, is shown the crystallization temperature
in C as a function of the % of purity of paraxylene, this
curve having been calculated from the cryoscopic constants.
Below, there is given in Table II the crystallization
points recorded by the method according to the invention, as
well as the % purity of paraxylene.
1~46792
.
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-- 14 --
~04679Z
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-- 15 --
~04679Z
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-- 16 --
1~?4679Z
In Figure 5, the crystallization temperature in C
as a function of the percentage purity of orthoxylene is
shown by a curve calculated from the cryoscopic constants.
Below, is given in Table III the crystallization
points read in a continuous analysis of orthoxylene according
to the method of the invention and the percentage purity of the
orthoxylene.
TABLE III
PURITY OF THE ORTHOXYLENE AS A FUNCTION OF THE
CRYSTALLIZATION POINT
Crystallization point % paraxylene
-29.0 90.2
-29.1 90.~
-29.2 89.7
-29.3 89.5
-29.4 89.3
-29.5 89.0
-29.6 88.8
-29.7 88.5
-29.8 88.3
-29.9 88.1
~30.0 87,8
-30.1 87.6
-30.2 87.3
~30-3 87.1
86.9
-30.5 86.5
-30.6 86.4
~30-7 86.2
-30.8 85.9
-30.9 85.7
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~04679Z
TABLE~III (continued)
Crystallization point ~ paraxylene
-31.0 85.4
-31.1 85~2
-31.2 85.0
The continuous analysis of the crystallization
point of an aromatic hydrocarbon enables the purity of the
analysed substance to be determined continuously and hence this
data to be transmitted directly to a processing system for said
hydrocarhon, for example a distillation column, in order to cause
the parameters to vary correspondingly.
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