Note: Descriptions are shown in the official language in which they were submitted.
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WO 98/0222~; PCT/NL97/00396
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ELECTROSTATIC COALESCENCE
BACKGROUND OF THE INVENTION
The invention relates to a method for
segregating a dispersion having a continuous organic
- phase and a disperse aqueous phase from a caprolactam
production process.
DISCUSSION OF PRIOR ART
In a caprolactam production process
caprolactam is prepared from cyclohexanoxime after a
Beckmann-rearrangement in the presence of sulphuric
acid or oleum. The rearrangement-mixture is then
neutralised with ammonia whereby ammonium sulphate is
formed. Subsequently caprolactam is extracted from
water with an organic solvent, for example benzene. In
the resulting organic phase also a disperse agueous
water phase is present. This aqueous phase comprises
besides caprolactam also ammonium sulphate. The
presence of ammonium sulphate is disadvantageous in the
subsequent processing and puryfying of caprolactam.
Removing the disperse aqueous phase as much as possible
is therefore highly desired.
There are many processes in which it is
necessary to separate dispersions.
One can use settling tanks for the separation
of dispersions where the densities of the two phases
differ sufficiently. The denser phase sinks below the
less dense phase and given a sufficient time the two
phases can be separated sufficiently to be drawn off.
However, such a process reguires the use of very large
tanks taking up a correspondingly large amount of
~5 space. Furthermore, this separation procedure would be
the slowest stage of the more extensive caprolactam
production process and would therefore determine the
throughput of the complete process.
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It is also possible to use coalescors or
washing treatments to remove a disperse phase as for
example described in German patent application
1,031,308. The addition of caustic washing material in
the caprolactam production process is disadvantageous
to the quality of caprolactam.
US patent 3,528,907 describes the possibility
of separation of water dispersed in a hydrocarbon, for
example crude oil, from said oil by the use of an
electric field. However, this patent also teaches that
the voltage of the applied electric field must
definitely not exceed 1000 volts per inch, or 400 volts
per cm, because this would cause a short-circuit
between the electrodes. A disadvantage of this method
lS is that, in particular, a dispersion containing at most
only about 5 wt.% aqueous phase of the combined total
liquid phases cannot be readily separated from the
organic phase.
European patent application No. 51463
describes settling of liquid emulsions by means of
electrostatic coalescence. This process however
requires the presence of high volumes of conducting
liquid (the aqueous phase), for example 50%. Especially
insulated electrodes are needed because otherwise
short-circuiting would occur.
German patent application No. 3,709,456 and
European patent application 438790 both describe
separation of liquid membrane systems where the aqueous
phase is the continuous phase.
In general, dispersions exhibit a continuous
particle size distribution. Dispersions can be
subdivided into primary and secondary dispersions. In
the case of primary dispersions the particles forming
the disperse phase have an average particle size
> 0.1 mm. In many cases these dispersions can easily be
segregated gravimetrically with the aid of settling
tanks. In the case of secondary dispersions, however,
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the particles forming the disperse phase have an
average particle size of only ~ 0.1 mm. Segregating
these latter dispersions by means of settling tanks
takes an unacceptably long time, however, for
industrial applications. To shorten this segregation
time it is therefore possible and advantageous to
employ electrostatic coalescence according to this
invention.
GENERAL DESCRIPTION OF THE INVENTION
This invention has as its primary object the
provision of a method whereby a dispersion from a
caprolactam production process, in particular a
dispersion containing at most only about 5 wt. of
aqueous phase, can be segregated very efficiently.
This object is achieved by applying
electrostatic coalescence at an electric field of at
least 500 volts per centimetre and a frequency of at
least 5 Hz. Electric fields with a (peak)strength of at
least 1000 volts per cm, preferably at least 3000 volts
per cm, more preferably at least 5000 volts per cm and
especially at least 10,000 volts per cm can also be
advantageously used, and by so doing the segregation
results achieved become increasingly better. In spite
of use of these high electric fields, no short-circuits
occur, yet the segregation of the aqueous phase from
the organic phase is very efficient. It was found, for
example, that in continuous organic liquid phases, at a
frequency of 50, Hz 50% of the aqueous disperse phase
present can be separated after 10 minutes at 733 volts
per cm. At 6,666 volts per cm, as much as 90% of the
aqueous phase present can be segregated after 10
minutes. With a seltling tank it would take several
days to a week to achieve the same effect.
With electrostatic coalescence it is
obviously also possible to vary the frequency. Good
results according to this invention are achieved if the
....
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frequency is at least 20 Hz. Frequencies up to 500 Hz
can very well be used. Good results can also be
obtained at frequencies outside the preferred ranges.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 illustrates in schematic form one type
of apparatus which may be used in the practice of this
invention.
Fig. 2 illustrates in schematic form another
type of apparatus which may be used in the practice of
this invention.
DETAILED DESCRIPTION OF THE INVENTION
The shape of the vessel or container in which
the electrostatic coalescence can take place is less
important. The vessel may be cylindrical, square,
rectangular, etc.
The electrostatic coalescence technique of
this invention can be conducted either continuously or
batchwise. When continuous application of electrostatic
coalescence is carried out, however, care should be
taken regarding the internal surface configuration of
the vessel or the container, to ensure that laminar
flow is established; the length/diameter ratio may be
significant in this context. In particular, turbulent
flow should be avoided, since this turbulence adversely
affects segregation or, as the case may be, segregation
time requirements.
Electrodes which can be used may be made from
various materials, e.g. conductive glass, metal etc.
The shape of the electrodes employed is not essential
to the invention. Examples include rod-shaped or plate-
shaped electrodes. The electrodes may be either
insulated or noninsulated. The use of noninsulated
electrodes may make suggest an advantageous cost
difference for the procurement. On the grounds of
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safety and/or energy consumption, however, one may
alternatively opt for insulated electrodes.
The temperature at which segregation with the
aid of electrostatic coalescence can take place is not
really essential to the invention and may preferably be
at room temperature. While, in general higher
temperatures favor a more rapid achievement of the
phase segregation, it is generally necessary to guard
against the temperature rising too high, such that the
dispersion to be segregated starts to boil or lead to
generation of a gas, thereby inducing turbulence and
resulting reformation of the dispersion.
It is of course always possible to apply the
electrostatic coalescence under elevated pressure in
order thus to prevent the dispersion from boiling.
Both alternating current and direct current
can be used. When alternating current is applied the
pulse can be either sinusoidal or rectangular,
sawtooth-shaped, or a combination of these, there is
only little apparent effect on the segregation results.
The term organic phase as used in this
patent, refers to one or more organic compounds
composed of carbon and hydrogen, for example linear and
cyclic hydrocarbons, and which may also contain other
atoms such as oxygen, sulphur, nitrogen, halogens etc.
Examples of such compounds include benzene, toluene,
cyclohexane, heptane, dimethyl sulphoxide, chloroform,
trichlorethane, etc.
In addition to water, the aqueous phase may
also contain other compounds, for example salts or even
organic compounds dissolved in the aqueous phase. The
aqueous phase may even contain up to 60 wt.~ of organic
compound(s)~ for example caprolactam. Nonetheless, in
the context of this invention, the "aqueous phase" is
considered to be aqueous provided that its water
content is at least 20 wt.%.
The aqueous phase may also contain water
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soluble salts, for example between 0.1 and 5 wt.%
ammonium sulphate.
The now segregated aqueous disperse phase can
be separated from the organic continuous phase by means
which will be apparent to those skilled in the art, for
example by drawing off the aqueous layer, by
decantation, and the like. It is possible to recycle
the separated aqueous phase into the caprolactam
production process.
The preparation of caprolactam involves
various preparation steps in which such dispersions are
formed. These then generally require an effective
liquid/liquid separation technique. Examples at various
process steps include the cyclohexane oxidation, the
preparation of hydroxylammonium salts, the top and
bottom streams from extraction columns.
EXAMPLES OF THE INVENTION
DETAILED DESCRIPTION OF THE DRAWINGS
The invention will now be explained in more
detail with reference to the following examples,
without being limited thereto.
ExamPles
An Ultra-Torrax7 T50 mixer was used to
disperse water having caprolactam dissolved therein
(50 wt.) in benzene as the organic phase with 2 wt. of
water phase dispersant based on the total mixture, at
5000 revolutions per minute (rpm) over a period of
5 minutes. Within 1 minute, this dispersion was
introduced into an electrostatic coalescer, the
frequency and the voltage being subsequently set. The
examples employed alternating current in each case and
were carried out at room temperature.
The coalescer used in Examples I-III is
schematically illustrated in Figure 1, where 1 is an
insulated glass electrode, 2 is a glass jacket
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containing 3 sulphuric acid as the outer electrode and
4 is the voltage source with frequency controller. The
gap between electrodes 1 and 3 was 1.5 cm.
A coalescer as employed in Examples IV and V
is depicted in Figure 2, where 10 is a thermostatted
vessel within which a glass vessel 20, (100:30:100 mm)
has been placed which is sealed by means of a cover 30
through which two noninsulated stainless steel
electrodes 40 have been positioned. The gap between the
electrodes was 1.5 cm. After various intervals, samples
of the dispersion were taken and analysed.
The experiments were carried out at room
temperature. To determine how much water had separated,
or the concentration of aqueous phase in the
dispersion, the turbidity was measured in accordance
with the 'Deutsche Einheitsverfahren C2, DIN 38404,
Teil 2', this is also a measure for the percentage of
the water phase which had separated. Via a calibration
curve the degree of turbidity was related to the
percentage of water which had separated.
ExamPles I-III
Examples I-III were carried out as described
above, the frequency was 50 Hz, the electric field was
sinusoidal. The voltage was varied and is shown in
Table I, together with the corresponding separation
results.
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Table I
Example Voltage wt.% sepa- wt.% sepa- wt.% sepa-
(Volts/cm) rated after rated after rated after
1 min 5 min 10 min
I 733 15 40 51
II 3333 42 68 80
III 6666 60 88 90
Table I clearly shows that the higher the
voltage, the better and more complete the separation
result.
Examples IV and V
These examples were carried out as described
above, the frequency was 50 Hz and the voltage was
varied. The voltage field was sinusoidal. The results
are shown in Table II.
Table II
Example Voltage wt.% sepa- wt.% sepa- wt.% sepa-
(Volts/cm) rated after rated after rated after 10
1 min 5 min min
IV 733 48 70 73
V 2067 62 91 92
Comparative ExamPle A
A sample of the dispersion of Example ??? was
introduced into a cylindrical vessel allowed to settle
with no application of electrical coalescence. After 1
minute, only 6% of aqueous layer had separated, after
5 minutes this amounted to 9% and after 10 minutes only
to 12%.
This invention has been described with
reference to specific embodiments thereof but the scope
and extent of rights under the applicable patent laws
is limited only by the terminology of the following
claims.
