A PROCESS FOR PURIFYING CELLULOSE ETHERS

TECHNICAL FIELD

The invention relates to a process for purifying cellulose ethers.

More particularly, the invention relates to a process for purifying carboxymethyl cellulose from an aqueous solution.

STATE OF THE ART

Sodium carboxymethyl cellulose (CMC or carboxymethyl cellulose) can be defined as the sodium salt of the water-soluble anionic polymer, which is produced by replacing and etherifying the hydroxyl groups in the cellulose ring with the carboxyl groups.

CMC is a water-soluble cellulose derivative. It is a frequently used chemical due to its nontoxicity, high water solubility, and light and thermal stability. CMC is generally the sodium salt of carboxymethyl cellulose. The cellulose is dissolved in water by chemical reaction based on its raw material. By binding carboxymethyl groups to the cellulose chain, hydration of the molecule is possible and this provides water solubility. Substitutions are irreversibly bonded to the cellulose main chain by ether bridges. Thus, CMC ether is in the cellulose group. CMC, which is easily soluble in hot and cold water, can be produced with different physical and chemical properties. These features affect the performance of the CMC used in applications and play an important role in determining product cost.

CMC is a white-cream colored powder material. This product has a wide range of applications in food, medicine, coating, and detergents. There are also areas of use such as drilling, cosmetics, textile, paint, and glue.

Figure 1 shows the molecular structure of CMC. Because CMC has an acid function, CMC is an anionic polyelectrolyte. CMC has many interesting properties when dissolved in aqueous solutions. However, this varies depending on CMC quality and solution conditions.

CMC is prepared by the reaction between cellulose and monochloroacetate (or monochloroacetic acid) from cellulose macromolecules and is obtained by partial substitution of hydroxyl groups at positions 2, 3, and/or 6 with carboxymethyl groups. Cellulose is carefully selected to meet the finished product specifications. First, it is treated with sodium hydroxide and alkaline cellulose is obtained in the cellulose-sodium hydroxide reaction. In this step, it is very important to homogeneously convert cellulose into alkaline cellulose. This step is generally known as mercerization. Since alkaline cellulose becomes reactive against monochloroacetic acid, monochloroacetic (MCA) or sodium salt of monochloroacetic acid is given to the environment as the free acid. After completion of the different reaction steps, the product comprises about 25-35% w/w of the by-product salts (sodium chloride and sodium glycolate).

CMC which is dried without a purification process is classified as technical quality/class. An additional step is required to remove the sodium chloride and sodium glycolate salts contained in it, and generally, the solvent is used in the step. Methanol, ethanol, and isopropyl alcohol are preferred as solvents. Generally, the purification process is performed using different aqueous concentrations of these solvents. During this purification, quite a lot of solvent is used. Ethyl alcohol with a concentration of 70-75% is generally used in the industry. During purification with ethyl alcohol, 40% of the CMC is lost together with the washing solution.

US 2,617,800A discloses a method for purifying carboxymethyl cellulose using ion-exchange resin. By the said method, purified carboxymethyl cellulose is obtained by separating byproducts such as sodium chloride and sodium glycolate. However, the said method is not suitable for industrial use. More effective and efficient purification processes are needed to both eliminate the use of solvents and minimize product loss.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a process for purifying cellulose ethers, particularly carboxymethyl cellulose, which meets the aforementioned needs and provides some additional advantages, the cellulose ethers that can be obtained by the said process, and the use of the said cellulose ethers.

The primary object of the invention is to provide a process for reducing product losses and increasing yield during the purification of carboxymethyl cellulose.

Another object of the invention is to provide carboxymethyl cellulose with a reduced impurity ratio.

Another object of the invention is to provide a process to reduce the use of solvents in the purification of carboxymethyl cellulose.

To achieve all the objects mentioned above and which will emerge from the detailed description below, the present invention provides a process for purifying cellulose ethers from an aqueous solution, the said process comprising the step of ultrafiltration of the said aqueous solution.

In a preferred embodiment of the invention, the said cellulose ether is carboxymethyl cellulose.

In another preferred embodiment of the invention, the said aqueous solution further comprises sodium chloride, sodium glycolate, and/or sodium acetate as a byproduct.

In another preferred embodiment of the invention, the said process comprises the step of passing the said aqueous solution through an ultrafiltration membrane module.

In another preferred embodiment of the invention, the said aqueous solution is passed through the said ultrafiltration membrane module by the cross-flow method.

In another preferred embodiment of the invention, the said ultrafiltration membrane module comprises a membrane containing polyvinylidene fluoride. To achieve all the objects mentioned above and which will emerge from the detailed description below, the present invention provides cellulose ethers obtained by a process comprising the step of ultrafiltration of the said aqueous solution for purifying cellulose ethers from an aqueous solution.

In a preferred embodiment of the invention, the said cellulose ether is carboxymethyl cellulose.

To achieve all the objects mentioned above and which will emerge from the detailed description below, the present invention provides for the use of cellulose ethers obtained by a process comprising the step of ultrafiltration of the said aqueous solution for purifying cellulose ethers from an aqueous solution as a concentrator, binder, film forming, stabilizer, protective colloid, and water resistance aid.

To achieve all the objects mentioned above and which will emerge from the detailed description below, the present invention provides a method for preparing the purified carboxymethyl cellulose product. The said method comprises the steps of (i) providing the crude carboxymethyl cellulose product; (ii) obtaining an aqueous solution of the crude carboxymethyl cellulose product; and (iii) ultrafiltration of the resulting aqueous solution.

In a preferred embodiment of the invention, the said aqueous solution further comprises sodium chloride, sodium glycolate, and/or sodium acetate as a byproduct.

In another preferred embodiment of the invention, the said aqueous solution is passed through the said ultrafiltration membrane module by the cross-flow method.

In another preferred embodiment of the invention, the said ultrafiltration membrane module comprises a membrane containing polyvinylidene fluoride. The structural and characteristic features of the invention will be understood clearly by the following detailed description and therefore the evaluation shall be made by considering the detailed description.

DETAILED DESCRIPTION OF THE INVENTION

This detailed description describes a process for purifying cellulose ethers, especially carboxymethyl cellulose, from an aqueous solution, and CMC and other cellulose ethers obtained by the said process. The preferred embodiments of the invention are described only for a better understanding of the subject matter and without any limiting effect. The present invention is a process for purifying carboxymethyl cellulose (CMC) from an aqueous solution, comprising the step of ultrafiltration of the said aqueous solution. Carboxymethyl cellulose is an alkali metal salt of carboxymethyl cellulose, such as sodium, and will be referred to as “carboxymethyl cellulose” or “CMC” in this application.

Cellulose ethers such as CMC are used to provide rheology control to water-based systems. The said cellulose ethers are prepared by reacting the cellulose with an alkali hydroxide such as sodium hydroxide and a suitable etherifying agent such as monochloroacetic acid, ethylene or propylene oxide, methyl or ethyl chloride, etc. For example, CMC can be prepared by reaction of cellulose with sodium hydroxide and monochloroacetic acid.

The reaction product obtained as a result of the said reactions contains inorganic and organic salts such as sodium chloride, sodium glycolate, and sodium acetate as well as the said cellulose ether. The reaction product containing the said by-products shall be referred to in this application as "crude cellulose ether product" or specifically as "technical grade CMC product" for CMC. To produce purified CMC, all or at least part of the said by-products must be separated from the raw product.

A process has been found with the present invention that enables the said separation process to take place at relatively high efficiency, thereby reducing product losses. According to the said process; an aqueous solution of the resulting technical grade CMC product is prepared and the said aqueous solution is subjected to ultrafiltration. Preferably, the resulting technical grade CMC product (i.e., crude CMC product) is directly used to prepare an aqueous solution.

The process of the present invention may also be used to purify other crude cellulose ether products which are water soluble and which have the same or similar impurities as CMC during their preparation.

The membrane is defined as the selective permeable barrier between the two homogeneous phases. It is an important separation material because it is a barrier for various pollutants according to its selectivity and characteristics. The fact that the output water quality of the systems developed with the use of this material is very good, they take up little space, require little construction, allow automation, and use very few chemicals offers significant advantages.

Membranes are generally used in areas such as drinking water, domestic and industrial wastewater treatment, separation of gasses, electrochemical processes, dialysis of blood and urine in the biomedical area, oxygenation, membrane-based sensors, controlled drug release, etc. As a result, the application areas of the membranes are summarized below.

• Drinking water treatment

• Treatment and re-use of domestic and industrial wastewater

• Aqueous salt treatment

• Obtaining drinking water from seawater

• Gas separation

• Removal of hardness, organic matter, micro-pollutants, etc.

• Obtaining process water

• Bioenergy and biogas production

• Metal removal and recovery

• Obtaining high purity water for semiconductor production and energy sectors

• Food and beverage sector (milk and cheese production, beer, wine, and alcohol production)

• Separation of olefin/paraffin, recovery of phenol and aromatic components, dehydrogenation in the oil industry

• Medical applications such as hemodialysis, blood oxygenators, plasma separation, controlled drug transport, etc.

• Bacteria/virus separation

• Separation and recovery of proteins and enzymes

• Condensation of protein solutions

Membranes are divided into four main classes according to pore diameters: microfiltration (MF), ultrafiltration (UF), nanofiltration (NF), and reverse osmosis (RO). Separation characteristics also vary according to pore diameters. The pore diameters of MF membranes range from 0.05 to 2.0 pm. MF membranes with pore diameters in this range are very effective in the removal of suspended solids, turbidity, bacteria, particles, and similar substances.

In UF membranes, the term "molecular weight cut off" ("MWCO") is generally used instead of the concept of "pore diameter". MWCO is a performance-related parameter and refers to the molecular weight value where the minimum removal rate is 90%. For example, a 41 UF membrane with an MWCO value of 10 KDa can hold 90% of substances with a molecular weight of 10 KDa and above. As the MWCO value decreases, the pore diameter of the membrane decreases. In other words, membranes with a lower pore diameter have a lower MWCO value. However, since the MWCO parameter shows the membrane's performance in retaining a material, this parameter is also affected by other factors such as the size of the molecule, polarity, and the interaction of the pollutant with the membrane. In addition, factors such as pore size distribution and surface porosity on the membrane surface are also important. UF membranes can separate protein, enzyme, virus, organic matter, and the like.

The pore diameters of NF membranes vary between 0.005 and 0.001 pm. These membranes are highly effective in the removal of bivalent salts, organic dye, pesticide, hardness, and similar parameters.

Reverse osmosis membranes (RO) are mostly defined as non-porous structures. RO membranes, on the other hand, can separate monovalent salts, metal ions, and similar substances.

Ultrafiltration (UF) is a type of membrane filtration that takes place at the molecular level and where liquids are pushed against the semi-permeable membrane by hydrostatic pressure. In the process of the present invention, the aqueous solution is passed through an ultrafiltration membrane module for ultrafiltration. When the pore diameter was evaluated, it was found that the ultrafiltration membrane was the most suitable membrane for purifying cellulose ethers, especially carboxymethyl cellulose.

The said ultrafiltration membrane module preferably comprises a membrane containing polyvinylidene fluoride fiber.

The purified cellulose ether according to the invention can be used as a condenser, binder, film forming, stabilizer, protective colloid, and water resistance aid.

EXAMPLES

Example 1 - Preparation of UF membrane module

To purify the CMC with UF, firstly a laboratory-type module was prepared. For this module, 5 PVDF (polyvinylidene fluoride) fibers were used and these fibers were placed in a hose with an inner diameter of approximately 7-10 cm. Then, a T-tube connection was attached to the two ends of the hose and the two ends of the hose were cured with polyurethane and closed. The fibers were exposed by cutting a little from the two closed ends. With these processes, a laboratory-type UF module was prepared.

Example 2 - Purification process according to the invention To carry out the purification process, firstly technical quality CMC was taken and analyzed. As a result of this analysis, it was found that it includes approximately 21 % sodium chloride and 4% sodium glycolate. In other words, technical quality CMC has been shown to be approximately 74-75% purity. A 0.5% aqueous solution of this CMC was then prepared. This prepared solution was pressed into the module via the dosing pump by cross-flow method. As the solution passed through the module, it was observed that there was water output from the fiber output point. Thus, the sodium chloride ions in the water in the solution we prepared were removed together with the water coming out of the fiber outlet of the module by passing through the pores on the fibers. CMC, on the other hand, could not pass through the pores of the fiber because it is a large molecule. As the flow continued, the concentration of the initially prepared 0.5% CMC solution increased, and most of the water was removed through the fiber. Then, CMC in a slurry state was dried. Salt analysis was performed again for the dried CMC and the product was found to contain 0.4% sodium chloride in the analysis. Thus, the purity of CMC with a purity of 73-74% was increased to 95-96%.

Example 3 - Conventional purification process (by washing with solvent)

The analyzed technical grade CMC was taken and purified with solvent. For this, two different solvents were tested in different concentrations. The test results are found in the following table. As can be seen in Table 1 , ethyl alcohol is very successful in purification and will be used to produce high-purity CMC. However, 40% of CMC has been lost after washing with ethyl alcohol. In addition, a lot of ethyl alcohol is used.