Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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PROCESS FOR PURIFYING A LIQUID CONTAMINATED BY FILAMENTARY MOLECULES
The present invention relates to a method for the purification of a caustic
5 solution contaminated by hemicelluloses in accordance with the preamble of claim 1,
as well as to a device for executing the method in accordance with the preamble of
claim 5.
Production steps are known in various branches of industry, wherein
caustic solutions, for example caustic soda, are employed for carrying out a reaction
10 and are recovered at the end of the reaction, contaminated by dissolved substances.
The contaminated liquids are subsequently purified and returned to the process cycle
for the renewed performance of the reaction. Production installations of this type are,
for example, by breweries, producers of nonalcoholic beverages such as fruit juices,
dairies, producers of aluminum, manufacturers of various catalyzers, but also by15 producers and processors of pulp and viscose.
Caustic solutions contaminate by hemicelluloses mainly occur in the two
last mentioned branches of industry, for example caustic soda is recovered which is
contaminated with dissolved technical hemicellulose.
Inaccordancewith K. Gotze, "Chemiefasern nachdemViskoseverfabren"
20 [Chemical Fibers in Accordance wit the Viscose Method], 3rd, ed. page 120, "technical
hemicellulose" is understood to be in general all those polysaccharides of pulp which
are soluble in caustic soda, mainly in the steeping or mash liquid, and which therefore
theoretically are removed in the viscose process. Chemically considered, these are
decomposed pulp, as well as degradation products of native hemicelluloses which were
25 generated in the course of the wood digestion process during pulp production.In the course of further treatment of pulp, for example when producing
viscose products and pulp derivatives, the first step generally is mercerizing, i.e. a Iye
treatment of the pulp material with strong alkaline solutions which generally contain
more than 17% of sodium hydroxide. The largest portion of the hemicellulose and the
decomposition products of the pulp are dissolved in the course of the Iye treatment.
One method practice for recovering the caustic soda is purification of the steeping
liquids by dialysis. This method results in high investment and maintenance costs.
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Further than that, it is also necessary to concentrate the large amounts of purified,
highly diluted NaOH solution for their renewed use, which is mainly accomplished by
the evaporation of water.
The method described in DE-A-2,433,235 seeks to remedy this
disadvantage. In connection with this method for removing hemicellulose from
circulating alkali solutions containing hemicellulose, one or several organic compounds
are added to the alkali solution, which at atmospheric pressure have a boiling point
below 100~C, for example aliphatic alcohols, such as methyl alcohol or ethyl alcohol,
and which are easily soluble in water as well as in alkaline solutions. The precipitated
hemicellulose is then separated from the mother Iye, the organic compound(s) is (are)
recovered from the mother Iye by distillation and the purified Iye is returned to the
process cycle.
An improvement of this method is described in DE-A-3,405,208. In this
improved method the hemicellulose is also precipitated from the alkalyzing Iye produced
during the viscose preparation, using aliphatic alcohols. To improve the degree of
effectiveness of hemicellulose precipitation, in a first step the alkalyzing Iye is
concentrated by ullrafilLration to a hemicellulose content of 90 to 140 g/l.
To avoid the addition of aliphatic alcohols, which leads to a more
elaborate production process and the addition of alcohol for separation by distillation,
another attainment of the object is proposed in US-A-4,270,914. There, the
hemicellulose is removed from the caustic soda cycle by means of ultrafiltration,
wherein a separation limit (cut-off) of 10,000 Dalton (= g/mal) is disclosed. This molar
mass information means that 90% of all molecules of a test liquid which have this value
of 10,000 Dalton are retained, wherein molecules of a greater molar mass are retained
at larger percentages, and molecule with smaller molar mass are retained to a lesser
degree or not at all.
A method for the removal of hemicellulose by means of ultrafiltration is
also described in US-A-3,556,992. The liquid to be purified is passed through ananisotropic membrane, wherein the working pressure in a first step is selected such that
the hemicellulose forms a gel on the membrane surface. The working pressure is
subsequently increased in order to compress the gel until the flow through the
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membrane has been stabilized. In the last step the working pressure is then reduced
to the value of the first step. The pressures used correspond to classical values in
ultrafiltration .
A membrane separation method for the recovery of caustic soda is
described in FAT SCI. Technol., vol. 94, No. 10, 1992, Echterdingen, pages 401 to 403.
The employment of nanofiltration or ultrafiltration is proposed, depending on the molar
mass of the substances to be filtered. Ultrafiltration is employed for separating
hemicellulose or other polymers with molar masses between 50,000 and 150,000
Dalton, nanofiltration is used for substances of lesser molar mass, for example for
silicates with molar masses of 500 Dalton.
It is advantageous in case of the separation by means of membrane
filtration that no precipitation agents and no additional energy, such as is necessary for
evaporation, need to be added. Although the membranes used in micro-filtration have
a sufficiently small cut-off for the macro-molecules occurring in pulp and viscose
production, the disadvantage occurred in actual practice that the membranes became
clogged relatively rapidly and could barely be cleaned even by means of reverse
flushing, i.e. a liquid flow opposite the permeated flow direction.
It is therefore the object of the invention to create a method for the
removal of dissolved hemicellulose from caustic soda that prevents the clogging of the
membrane.
The rnethod in accordance with the invention can be used not only for the
purification of a caustic soda flow from the hemicelluloses dissolved therein, but can
also be used for any arbitrary caustic liquids which are contaminated with substances
in the form of hemicelluloses and have molar masses of at least 10,000 Dalton.
The use of nanofiltration for purifying contaminated liquids, in particular
Iyes, is known per se. In accordance with a college course "Membranprozesse"
[Membrane Processes] of prof. Dr. -Ing. R. Rautenbach et al., Aachen, May 25 to 27,
1994, nanofiltration, the same as reverse osmosis and ultrafiltration, is a pressure-
driven membrane separation method. The separation properties of nanofiltration can
here be inserted between the known methods of reverse osmosis and ultrafiltration.
Typical reverse osmosis membranes separate contained materials of a molecular
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weight of more than 100 Dalton, while ultrafiltration membranes only retain molecules
of a molar mass above 1,000 Dalton, typically above 10,000 Dalton. The retentioncapabilities of nanofiltration lie between these values. Something similar applies to the
trans-membrane pressure differences to be used. With nanofiltration, at 0.5 to 3 Mpa
(=5 to 30 bar) they lie below the values required for reverse osmosis membranes and
above the values required for ultra-filtrates to achieve the same permeate flows.
Nanofiltration, the same as reverse osmosis, is mainly employed for
solutions containing ions, wherein monovalent anions can pass through the membrane
in nanofiltration in contrast to reverse osmosis. Nanofiltration has proven itself in those
areas in which substances must be filtered which, because of their molar mass, cannot
be separated from the liquid by means of ultra- or micro-filtration, but where reverse
osmosis would not be suitable because of its separation properties as a result of the
high osmotic pressure. If possible, however, membranes, and thus filtration methods
with the largest possible cut-offs, are always employed in order to keep the
overpressure to be applied as low as possible and the permeate flow as large as
possible.
The following publications indicate several application areas of
nanofiltration methods in accordance with the prior art.
EP-A-0 551 245 discloses method for filtering contaminated Iyes
recovered in the course of foodstuff processing by nanofiltration. To remove coarser
contaminants, micro-filtration is first performed. The nanofiltration by itself is used for
removing the particles remaining in the Iye, as well as the organic or polyphenolic
compounds.
WO 95/27681 describes a method for the purification of a Iye by means
of nanofiltration for recovering hydroxides. The substances retained on the feed side
are organic compounds with a small molar mass of at least 150 Dalton, in particular
complexing agents such as EDTA (ethylenediaminetetraacetic acid) or NTA
(nitrilotriacetic acid) of typical molar masses between 200 and 400 Dalton. This method
is employed in the food industry.
DESALINATION, vol 67,1987, Amsterdam, pages 455 to 465, discloses
a method for the purification of the wash water resulting in the course of bleaching of
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paper, wherein the chromophores are removed by nanofiltration. The pH value of the
contaminated wash water is less than that of contaminated Iyes.
CHEMICAL ENGINEERING PROGRESS, vol 90, No. 3, March 1994, New
York, pages 68 to 74, recites several areas of application of nanofiltration, such as
5 removing hardness from water, purifying ground water and the removal of dyes in paper
making .
The problems of the removal of hemicellulose are not addressed in these
documents, since they hardly occur in the area of application described here.
The JOURNAL OF MEMBRANE SCIENCE, vol. 98, No. 3, January 31,
1995, Amsterdam, pages 249 to 262, shows several experiments in which different test
liquids are purified by means of nanofiltration. The test liquids consisted of water with
sodium chloride, ammonium chloride, magnesium chloride, iron chloride, sucrose,
vanillin, lignin sulfonate, dextrane sulfonate, potato starch and/or cornstarch. No test
were performed with hemicelluloses. The first result of the experiments is of a purely
15 phenomenological nature. It was determined as a second result that polar materials do
not plug the membranes, since the dissolved ions have the same electrical charge as
the membrane and are therefore repulsed.
In contrast thereto, with the method in accordance with the invention the
recognition was used that the effective cut-off of a membrane is not only a function of
20 the molar mass of the material to be separated, but also of its molecular geometry.
The special geometryofthe hemicelluloses, i.e. theirdirection-dependent
extent, leads to the plugging of pores of membranes with large pores, such as in an
ultrafiltration with a cut-off of 10,000 Dalton, for example. The molecules can penetrate
the pores but, since these pores consist of angled conduits, the molecules become
25 stuck and plug the individual pores. Pores plugged in this way cannot be cleaned by
a reverse flush, since the pressure drop of these pores is greater than with still open
pores. The purification fluid will use the open pores as a preferred flow path. In
contrast thereto, with the method in accordance with the invention the filamentary
molecules cannot plug the pores of the membranes because of the much smaller pore
30 size.
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From the point of view of the molar mass of the hemicellulose to be
separated and of the pore diameter, an ultrafiltration membrane would be sufficient, but
the filamentary molecules lead to plugging of the membrane pores and to the reduction
of the permeate flow to practically zero. By using nanofiltration membranes, the initial
5 permeate flow is less than with ultrafiltration, but remains constant. By selecting
suitable modules, which have relatively large flow cross sections forthe feed and permit
the so-called crossflow filtration, solid particles which do not permeate the membrane
are flushed out. Spiral wound modules and tubular modules, for example, have been
shown to be suitable modules for crossflow filtration.
Commercially available nanofiltration methods are employed in the
method in accordance with the invention, which have a cut-off of less than 1,000Dalton, preferably less than or equal to 300 Dalton. The membranes used are
preferably made of alkali-resistant materials.
Nanofiltration membranes in which the Donnan effect occurs are used
with the method in accordance with the invention. In accordance with the already cited
publication, the college course "Membranprozesse" of prof. Dr.-lng. R. Rautenbach et
al., Aachen, May 25 to 27, 1994, this effect occurs during the desalination of solutions
containing mono- and polyvalent anions. Although there is no solution containinganions, particularly in case of the removal of hemicellulose from caustic soda, and
therefore the Donnan effect does not occur, it has been shown that such membranes
achieve better results with the removal of hemicelluloses, than other nanofiltration
membranes .
A previous ulllarillralion forthe removal of large particles is not necessary
with the method in accordance with the invention. A single nanofiltration can beperformed with the method of the invention, or several filtration stages, in particularwith
reduced cut-offs, are connected behind each other.
A preferred variant of the method of accordance with the invention will be
described in what follows:
The method in accordance with the invention essentially consists of the
same method steps as in US 4,270,014, whose disclosure content is part of this
description. However, nanofiltration is employed in place of ultrafiltration.
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A resulting contaminated Iye with 200 g NaOH/I contains 30 g
hemicellulose/l. It is prefiltered in a filter with a pore width between 20 and 50 ~m to
remove solid particles. The hemicellulose content is reduced by a factor of 20 to
approximately 1.5 g hemicellulose/l in the permeate in the subsequent single
5 nanofiltration step. The operating parameters are pressure of 30 bar absolute on the
feed side at a temperature between 40 and 50~C. The permeate flow achieved lies
approximately at 30 V(m2h). The concentration of the caustic Iye solution in thepermeate was 10% lower than in the feed. This phenomenon can be explained in that
the hemicellulose content in the retentate (concentrate) increases and the
10 hemicellulose dissolves s defirled portion of the sodium hydroxide. The values were
measured in a spiral wound module. This is a so-called "crossflow" filtration. The
spacer employed has a nominal thickness of 50 mil (=0.050 inches=1.27 mm). In spite
of this, no cake formation was observed. This is also connected with the flow speed
along the membrane surface. The flow speed was set in such a way that the pressure
15 drop along a spiral wound module of a length of 40 inches (= approximately 1 m) does
not exceed the value of 1 bar. According to the manufacturer, the polymer membrane
used has a "cut-off" of 300 Dalton. When using this membrane for purifying an aqueous
solution containing both mano- and polyvalent anions, the Donnan effect would occur.
To check the cut-off and the integrity of the membrane, the module was tested with a
20 5% sucrose solution. Sucrose has a molar mass of 342 Dalton. The determined value
(CFeed-cpermeated/c feed) was between 0.95 and 0.96. The permeate quality obtained
allows the return of the recovered caustic soda, possibly with a slight re-topping, i.e. to
employ it again for decomposition. Since a volume of more than 80% of the caustic
soda is recovered, this method has shown itself to be extremely advantageous
25 economically. On the one hand, the consumption of fresh Iye is drastically reduced
and, on the other hand, a large portion of the disposed costs for the contaminated Iye
can be omitted.
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