WO1999006817A1

WO1999006817A1 - Method for testing the quality of membranes

Info

Publication number: WO1999006817A1
Application number: PCT/EP1998/004565
Authority: WO
Inventors: Thomas Tyborski; Ralf Krack
Original assignee: Henkel-Ecolab Gmbh & Co. Ohg
Priority date: 1997-07-30
Filing date: 1998-07-21
Publication date: 1999-02-11
Also published as: EP1000339A1; AU8732298A; DE19732775A1

Abstract

The invention relates to a method for testing the quality of membranes. According to the invention, an aqueous test solution containing polyacrylic acid (s) and/or salts thereof and/or derivatives thereof is filtered through the membrane before and after said membrane has been treated with the solution to be filtered and the water rate and retention rate are determined.

Description

"Procedure for quality control of membranes"

For the selective separation of substances of different particle sizes from solutions, membrane filtration has developed into a widespread method in the past three decades due to its environmentally friendly and energy-saving procedure. The membrane filters used in this process are thin, film-like and microporous separating layers that are permeable to certain molecular sizes, while they retain substances that exceed this size (or a certain molecular weight). The filters have a foam-like network structure with cavities. Tubular membrane filter bundles made of hollow fiber or capillary membranes, which allow higher throughputs due to their large membrane surface, have proven themselves for the filtration of larger quantities of liquid. For example, cellulose derivatives, polyamides, polyvinyl chloride, polysulfones and Teflon are used as the material for the approximately 50 to 250 μm thin membranes of the membrane filters. But inorganic membranes made of aluminum oxide, carbon fibers or zirconium oxide have also been used since the late 80s. Technically relevant processes in which membranes are used include, for example, micro and ultrafiltration, reverse osmosis, electrodialysis and pervaporation. The first two methods, which are also the most technically relevant, differ in the size of the particles to be separated. While microfiltration is a process for separating colloidal or suspended particles with particle sizes between 0.1 and 10 μm, i.e. a process for separating particles from a solvent, ultrafiltration is a method for separating molecules (especially macromolecules) from solutions. The particle sizes that are retained by ultrafiltration membranes are in the order of 0.001 to 0.1 μm, corresponding to molecular weights of 10 ³ to 2D 10 ⁶ daltons. Such micro and ultrafiltration membrane systems are used on an industrial scale in a wide variety of areas, for example in whey processing (concentration of dairy proteins), beer filtration, water treatment (recovery of valuable substances, process wastewater treatment), in the pharmaceutical industry (separation problems, separation of pyrogens, Extraction of ultrapure water), in the concentration of juices and in medicine (hemodialysis on the "artificial kidney").

Signs of aging and / or possible formation of deposits on membranes require well-functioning, easy-to-use test methods that provide reliable information about the quality of the membrane. Quality controls are also necessary to determine the performance of new membranes. When determining the quality of the ultrafiltration (UF) and microfiltration (MF) membranes, two parameters are of particular importance:

The flow rate corresponds to the actual amount of liquid that penetrates through the membrane wall.

The cut off corresponds to the retention rate or separation limit in g / mol specified by the membrane manufacturer.

Suitable methods for the quality inspection of membranes must allow precise and comprehensible statements to be made about these two parameters.

The following methods for checking the quality of membranes are currently known in the prior art:

Gas or water flow through the membranes is measured to detect leaks or membrane damage. This method only provides very rough information that is difficult to evaluate.

Monodisperse macromolecules (colloidal particles of uniform size) are retained by the membrane to be checked. About retention membrane damage (leaks or blockages, e.g. due to the formation of deposits) should be recognized. The test conditions and results are not transferable to membranes of the same type. There is a risk that a non-removable monomolecular layer of these colloidal particles will form on the membrane surfaces. In addition, the test substances are very expensive and can only be detected with great analytical effort (GC-MS, HPLC, DC). Polyethylene glycol (PEG) fractions are retained by the membranes, and statements about the membrane damage can in turn be made via the retention capacity. The method using the PEG fractions is mainly used to determine comparison values for membranes from a manufacturer. As water-soluble linear macromolecules, the polyethylene glycols form strong bonds to the membrane surfaces and can therefore form fouling on the membranes, which result in reduced liquid throughput through the membrane. Detection of the test solution is also only possible with great analytical effort.

Dextran / vitamin B12, tryptophan fractions are also used as test solutions. In addition to the high analytical effort involved in determining the content of the solutions, these substances have the disadvantage that the substances are very expensive and the measurement results continue to depend on the pore structure of a membrane.

In summary, the known methods for quality control of membranes have the following disadvantages:

The detection of the substances in the test solution is only with great analytical

Effort possible; all of the methods mentioned are very time-consuming; there is a risk of a deposit that cannot be cleaned; some of the chemicals used are very expensive; the measurement results are not on other membranes, for example those of other

Manufacturer, transferable. The object of the invention was now to provide a simple, economical and universally usable method for quality control of membranes, which makes it possible to identify damage to or formation of deposits on membranes without permanently changing them.

The object of the invention is achieved by a method for quality inspection of membranes, in which the water output and retention rate of an aqueous test solution, the polyacrylic acid (s) and / or their salts and / or their derivatives are contained, before and after the treatment of the membrane with the determined solution to be filtered. In a preferred embodiment of the invention, the test solution contains the polyacrylic acid (s) and / or their salts and / or their derivatives in amounts between 0.05 and 5, preferably between 0.1 and 2 and in particular between 0.2 and 1 wt. -%, based on the test solution.

This determination of membrane states with test solutions containing polyacrylic acid (s) and / or their salts and / or their derivatives has the following advantages over the previously known methods:

No deposits form on the membranes to be tested, i.e. membrane performance is not affected.

The proof of the polyacrylates is simple, quick and inexpensive by simple conductivity measurements.

The measurement results can also be transferred to membranes from different manufacturers.

The method is very inexpensive due to the use of inexpensive substances and the simple detection of these substances.

Both the flow rate and the retention capacity can be determined in online measurements so that membrane damage and blockages are quickly recognized. The very good water solubility of the test substances makes it possible to

Cover concentration ranges; in addition are polyacrylic acids or their

Salts or derivatives are available in many different molecular weights, which enables the method according to the invention to be used widely.

The test solutions do not cause any corrosion on the membranes examined and are therefore also suitable for sensitive membranes (for example cellulose acetate).

In the context of the present invention, a multiplicity of polyacrylic acids or their salts or their derivatives can be used alone or in a mixture with substances from the respective other groups. In the context of the present invention, derivatives of polyacrylic acids mean not only polymerized substituted acrylic acid monomers, but also, for example, copolymers of acrylic acid with other monomers in which the proportion of acrylic acid in the monomers on which the polymer is based is at least 40%. A polymer consisting of n monomer units is therefore considered a polyacrylic acid derivative in the context of this application if it has at least 0.4n acrylic acid monomers. The copolymers of acrylic acid with other monomers, for example maleic acid, can be constructed alternately, that is to say they correspond to the scheme -A-B-A-B-, but of course other linkages of the monomers are also possible, as occur, for example, in block and graft copolymers.

Polyacrylates are, for example, under the names Versicol ^® E5, Versicol ^® E7 and Versicol ^® E9 (trademark of the ied Colloids), Narlex ^® LD 30 and Narlex ^® LD 34 (trademark of the national adhesives), Acrysol ^® LMW-10, Acrysol ^® LMW- 20, Acrysol ^® LMW-45 and Acrysol ^® Al-N (trademark of Rohm & Haas) as well as Sokalan ^® PA-20, Sokalan ^® PA-40, Sokalan ^® PA-70 and Sokalan ^® PA-110 (trademark of BASF) available in the stores. Acrylic acid / maleic acid copolymers are sold under the names Sokalan ^® CP5 and Sokalan ^® CP7 (trademark of BASF). Acrylic phosphinates are available as DKW ^® (trademark of National Adhesives) and Belperse ^® types (trademark of Ciba-Geigy). In the context of the present invention, polysodium acrylates have proven to be preferred compounds. The test solution preferably contains between 0.1 and 3% by weight of polysodium acrylate, in particular between 0.2 and 2% by weight, based in each case on the test solution.

To carry out the method according to the invention, the membrane to be checked is cleaned with a common membrane cleaning agent which is compatible with the membrane. In the case of new membranes, this cleaning serves to remove any ingredients such as conditioning agents, additives, stabilizers, easily adhering contaminants or preservatives. In the case of membranes which are already in use, the cleaning is carried out only with water, in order not to remove any deposits which may be recognized by the process according to the invention. Is such a deposit formation e.g. detected by means of the method according to the invention, the effectiveness of cleaning with commercially available cleaning agents can be checked by performing the method according to the invention a second time.

Following the cleaning of the membrane, the water value of the membrane is determined, for example by allowing deionized water to flow through the membrane under a pressure of 4 bar for 15 minutes and then measuring the flow rate. The retention characteristic of the membrane can then be determined by filtering the test solution over the membrane, for example for 5 hours, the test solution being circulated for economic reasons (circulation). After a subsequent cleaning with a commercially available membrane cleaning agent, the water value can be determined again. While the water value can be measured directly as a liter flow rate per square meter of membrane area and hour, the rate of salt retention is determined indirectly via the conductivity of the test solution, based on the following mathematical relationship:

w (% SR) = (100 - LP) ^• 100% / LT w (% SR): retention rate in% salt retention

LP: conductivity value of the permeate passing through the membrane

LT: conductivity value of the test solution (measured in the feed stream)

To determine membrane damage, membrane blocking (so-called fouling) or the cleaning performance of membrane cleaners, a comparative measurement of the water output and the retention rate of the test solution is carried out before and after treatment with the product or raw material to be tested.

The difference between the initial value and the final value should not exceed 20% for the first

Performance [Im ^" h ^" ] and not greater than 2.5% in salt retention. With an increase in output of more than 20% »and falling retention, the membranes are damaged. Conversely, if the performance is reduced by more than 20% and there is increasing retention, the membrane becomes blocked.

Examples:

Tests with different polyacrylates were carried out on the following membranes:

Table 1: Membrane types

To determine the most suitable test solution, different polyacrylate solutions (0.5%) were circulated through the various membranes, the salt retention (SR) and the flow (water value, WW) being measured. The results of these orientational measurements are shown in Table 2:

Table 2: Test solutions in the membrane test

PA 1 polysodium acrylate solution (average molecular weight 20 086), solids content 35% by weight PA 2: polysodium acrylate solution (average molecular weight 24,341),

Solids content 35% by weight HP 3043: Hydropalat ^® 3043 (polysodium acrylate ex Henkel), average molecular weight 28,000 +/- 10%),

Solids content 34-36% by weight HP 3051: Hydropalat ^® 3051 (polyammonium acrylate ex Henkel), average molecular weight 28,000 - 30,500),

Solids content 39-41% by weight Nopco 44: Nopcosperse ^® 44 (polysodium acrylate ex Henkel-Nopco), average molecular weight 10,000 - 30,000),

Solids content 35% by weight HP 1706: Hydropalat ^® 1706 (polysodium acrylate ex Henkel), average molecular weight 15,000 +/- 10%),

Solids content 27-30% by weight

The flow rate decreased in all cases to a greater or lesser extent, whereby the membranes could be easily returned to the old state by cleaning with a commercially available membrane cleaning agent and after the cleaning again showed the original flux values.

To determine membrane damage, the water output and retention rate of a test solution (Hydropalat ^® 1706, 0.5% solids content) were determined using various membrane materials before and after using the membrane with a product. A 10% milk powder solution was chosen as the product, which is a common area of application for membrane filtration (concentration of milk powder). The values of the new membranes, the values of the new membranes after cleaning with a commercially available membrane cleaning agent (P3-ultrasil 91, trademark of Henkel-Ecolab, use as a 1% aqueous solution), the values of the membranes after product passage through the membrane and the values measured after subsequent cleaning. The values of salt retention (SR) and flow (water value, WW) are shown in Table 3. Table 3: Retention test

After cleaning with P3-ultrasil 91, the ETNA and FS membranes are damaged. The drastic decrease in salt retention from 91.9% to 34.0% (ETNA) and from 91.1% to 77.1% shows the damage. This damage cannot be recognized from the water values: the water value of the ETNA membrane is almost unchanged, the value of the FS membrane has dropped, which could simulate the formation of a deposit. The other membrane types are not damaged by the use of P3-ultrasil ^® 91; both the salt retention and water values are unchanged compared to the initial values. After concentration of the milk powder solution, the water value (flow) decreased as expected for all membranes. The reason for this, as mentioned above, is so-called fouling. An increase in salt retention from 34.0% to 92.9% (ETNA) or from 77.1% to 95.3% (FS) illustrates the formation of deposits and must not be misinterpreted as actual salt retention of the membrane.

After cleaning again with P3-ultrasil ^® 91, the water values rose again (detachment of the coating). The salt retention in the damaged membranes fell again from 92.9% to 81.8% (ETNA) or from 95.3% to 90. 6% (FS). The fact that the minimum values of the cleaned membrane were no longer reached before the product passed shows that the milk powder coating was not completely removed.

Claims

Claims:

1. A method for checking the quality of membranes, characterized in that the water output and retention rate of an aqueous test solution which contains polyacrylic acid (s) and / or their salts and / or their derivatives, before and after the treatment of the membrane with the solution to be filtered certainly.

2. The method according to claim 1, characterized in that the test solution, the polyacrylic acid (s) and / or their salts and / or their derivatives in amounts between 0.05 and 5, preferably between 0.1 and 2 and particularly preferably between 0, 2 and 1 wt .-%, based on the test solution.

3. The method according to any one of claims 1 or 2, characterized in that the test solution contains polysodium acrylate as the polyacrylic acid derivative.

4. The method according to claim 3, characterized in that the test solution contains polysodium acrylate in amounts between 0.1 and 3, preferably between 0.2 and 2% by weight, based on the test solution.