ACCELERATION OF THE PHASE SEPARATION OF LIQUID PHASES WHICH CONTAIN POLYMERS

ACCELERATION DE LA SEPARATION DE PHASES LIQUIDES CONTENANT DES POLYMERES

English Abstract

The invention relates to the acceleration of the separation of liquid phases
which contain polymers by using branched instead of linear solvents.

French Abstract

L'invention concerne l'accélération de la séparation de phases liquides contenant des polymères par utilisation de solvants ramifiés au lieu de solvants linéaires.

Claims

-6-
claims:
1) A method for acceleration of the separation of phases containing different
polymers,
characterized in that
a) said polymers are solved in a branched solvent or solvent mixture and
b) the resulting solvent phases which include solved polymers of different
type in
different concentrations are separated after sedimentation.
2) The method of claim 1, characterized in that said solvent is selected such
that the sedi-
mentation times of the phases in the branched solvent or solvent mixture are
shorter
than those in a non-branched solvent or solvent mixture.
3) The method of claim 1 or 2, characterized in that said polymers of
different type are
mixtures of polyolefins and said solvent is a branched alkane having a carbon
number
between 4 and 16.
4) The method of claim 3, characterized in that said solvent is a branched
alkane having a
carbon number between 4 and 10.
5) The method of claim 3, characterized in that said solvent is a branched
alkane having a
carbon number between 6 and 8.
6) The method of any of claims 1 and 5, characterized in that said solvent is
dimethylbu-
tare or methylpentane.
7) The method of any of claims 1 to 6, characterized in that the supplied
polyolefins origi-
nate from waste to be recycled.

Description

CA 02372121 2001-10-30
ACCELERATION OF THE PHASE SEPARATION OF LIQUID PHASES
WHICH CONTAIN POLYMERS
Mixed polymers may be separated by a liquid-liquid phase separation [1].
Thereby, polymers
are present in all phases, but selectively separated. Liquid phases with no
insignificant poly-
mer contents have high viscosities. Polymers are understood to be technically
or biologically
produced macromolecules having a molar weight of 1000 Daltons or more.
Polymers of dif
ferent kind are understood to be chemically or structurally different
polymers, such as HDPE,
l0 LDPE, PP or PVC.
For the economic of such a thermal separation process, the times of
sedimentation in the
separation steps are decisive, since space-time-yields are determined thereby.
Further, long
residence times contribute that decomposition reactions take place.
Liquid systems having two or more phases include a continuous phase (KP) and
one or more
discontinuous - -phases (DP) in a dispersed state. The literature [2]_
subdivides coalescence and
sedimentation of the discontinuous phase into several stages:
2 0 - coalescence of single drops to form larger drops
ascending or descending of larger drops controlled by density
unification of larger drops with already formed phase.
In all these processes, in particular the rising or settling velocities,
respectively, of single
drops are important. A comparatively simple mathematical modellation is
obtained when the
drops are considered as rigid spheres in the KP.
The modellation of the motion of sperically shaped solid particles in a
continuous phase is
made according to Clift R. et al [3] and Brauer [2J by using the equation of
motion. The
3 0 equation of motion (eq. l ) represents a term for the stationary velocity
of falling spherically
shaped particles.

CA 02372121 2001-10-30
-2-
wp= 4 Pp ~'g'dP' eq~ ~I)
3 pF ~
Therein, wP represents settling or rising velocity of particles, pp and pF
respectively, the den-
sity of the discontinuous Windex P) and the continuous phase (index F),
respectively, g the
acceleration due to gravity, dP the particle diameter and ~ the drag
coe~cient.
Equation 1 is generally valid for solid or fluid high viscous particles. The
special characteris-
tics of the phase boundary surface are described by the drag coefficient for
which the follow-
ing interrelation is valid:
I eq. (2)
x-
Re
wherein
wpda Ar eq. (3)
Re= =18 1+ 9 -I
In equation 3, vF represents the kinematic viscosity of the continuous phase
and Ar the Ar-
2 0 chimedian number. The introduction of the dimensionless Archimedian number
is useful,
. since hereby the settling velocity wP of particles in equation 1 can be
represented in explicit
form. It is:
Ar = PP
pF yF
Equation (4) teaches that only the viscosity of the continuous phase, the
particle diameter of
the discontinuous phase and the density difference between continuous and
discontinuous
phase are important for the separation. Short residence times are advantageous
to save opera-
tion and investment costs and to suppress chemical reactions.

CA 02372121 2001-10-30
-3-
According to the teaching of the literature, the phase separation time can be
improved in par-
ticular by reducing the viscosity of KP. Possibilities known in the literature
are increasing the
temperature or changing the nature of the solvent which forms KP together with
the polymer,
both measures with the aim to reduce the viscosity of KP. The said measures do
not lead to
the goal in the separation of mixed polymer by liquid-liquid-phase separation,
since
the temperature does not only change the viscosity, but due to the
thermodynamics in
particular the purity/yield of the polymers to be separated. Therefore, an
improvement
of the separation time must be paid for with a deterioration of selectivity;
changing the nature of the solvent changes the thermodynamics to an extent
that the
separation temperatures are either shifted into ranges which are uneconomic or
the
phase separation does no longer exist in the range of temperatures which can
be techni-
cally reached.
This problem is solved by a method according to claim 1 in an elegant and
novel manner.
Preferred embodiments are subject-matter of the subclaims.
As an example, the thermal separation of polyolefme mixtures as polymers of
different type is
2 0 considered, with n-alkanes having five to seven carbon atoms in the
molecule as solvent. The
teachings of the patent are applicable to all other separation methods which
include the sepa-
ration of polymer mixtures by forming liquid phases.
The method described in [1] separates polyolefmes in that two liquid phases
are formed in n-
hexane at 180°C, wherein the upper light phase contains under certain
conditions polyethyl-
ene from high pressure synthesis (LDPE), and the lower one polyethylene from
low pressure
synthesis (HDPE). Due to the amount, the low viscous upper phase is KP, and
the high vis-
cous lower phase is DP in the dispersed system. The low viscosity of the upper
phase, how-
ever, does not lead to technically acceptable separation times, so that the
above mentioned
disadvantages (costs, reactions) become effective.
Experimental runs, however, have surprisingly shown that the use of branched
solvents with
same carbon number significantly and advantageously changes the separation
times. Therein,
AMENDED SHEET

CA 02372121 2001-10-30
-4-
the viscosity of KP is about the same for the non-branched and the branched
solvent, so that
the effect cannot be explained at the moment.
Branched solvents are such aliphatic materials in which at least one carbon
atom has more
than two carbon atoms as neighbors and which may also carry functional groups
along with
further C and H atoms.
To illustrate the proceedings according to the patent, the branched solvent
2,3-dimethylbutane
shall be compared with the unbranched solvent n-hexane.
Example 1
Two experimental runs were performed having the separation of a polyethylene-
polypropylene mixture as a goal. As polyolefines, HDPE (high density
polyethylene), LDPE
(low density polyethylene) and polypropylene PP were supplied in a proportion
of 15/43/42,
wherein the total content of polyolefine in solution was 20 weight %.
In the first experimental run V_1 n-hexane was used as a solvent, for the
second experimental
run V_2 2,3-dimethylbutane. The phase separation times as well as the
viscosities of the so-
lution were observed. They are represented in table 1. It is noticeable that
periods of 300 min
cannot be realized with an unbranched solvent or only in a difficult way.
The parities of the different polymers were practically identical in both
phases in both runs,
since the nature of the solvent had not been changed (in both cases, a hexane
is concerned).
However, the considerably shorter separation times in the experimental run V 2
using the
branched solvent are surprising. Even though the viscosity of KP and the
density difference
pP-pF did not change significantly in both runs, it has been detected
surprisingly that the sys-
tem with the branched solvent had been separated faster by a factor of 150
than the system
with n-alkane as a solvent.

CA 02372121 2001-10-30
-5-
ExperimentalSolvent wpeed.PolSystem SeparationViscosityViscosity
of of
~n [weighttemperaturetime of upper lower
phase phase
%] phases KP DP
V_1 n-hexane 20 180 300 Low High
V_2 2,3- 20 180 2 Low Low
dimethylbu-
tanP.
Table 1: Experimental runs to separate polyolefme with a branched
(experimental run V 2
and an unbranched solvent (experimental run V_1)
According to the teaching of the literature, these reduced separation times
lead to smaller
separation containers and therefore to a reduction of investment costs and of
residence times
at the necessary high temperatures.
The teaching of this patent can be applied to all separations of liquid phases
which contain
polymers and solvents of which branched and unbranched representatives exists.
Citation
[1] DE 199 OS 029 A, published after the priority date of the present
application
[2] Brauer, H.: Grundlagen der Einphasen- and Mehrphasenstromungen;
Verlag Sauerlander, Aarau, 1971
[3] Clift, R.; Grace, J.R.; Weber, M.E.: Bubbles, Drops and Particles
Academic Press, New York, 1978
AMENDED SHEET