Modeling effect of the main synthesis parameters on the properties of carboxymethyl cellulose

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Carboxymethyl cellulose is the most widely used and a large-capacity cellulose ether, which is produced on an industrial scale. Carboxymethylcellulose (CMC) has a degree of substitution of 0.6-0.9; polyanionic cellulose (PAC) is a highly substituted CMC with a degree of substitution ? 0.9.

Despite the development of numerous studies on the synthesis of CMC in this field we are still particularly relevant when using the new domestic raw materials, in particular pulps prepared from hardwood and softwood trees, flax and other plant crops. Currently, with a comprehensive study of synthesis of CMC at best used experimental design method to the construction of empirical regression equations. The aim of this work is to develop a new approach to research, minimizing the amount of experimental work on the synthesis of CMC using the deterministic model of the kinetics of carboxymethylation of cellulose.

The basis Macrokinetics cellulose carboxymethylation simulation on the following assumptions:

Ш In the zone of the chemical reactions generated a uniform distribution of the reactants;

Ш A two-phase system consisting of solid and liquid reactants (cellulose, NaOH, sodium monochloroacetate, H2O, etc.) is considered as a quasi-homogeneous phase.

The paper considers the model macrokinetic cellulose carboxymethylation. The basic reaction between the hydroxyl groups of cellulose and sodium monochloroacetate flows through the third order (K1 rate constant). Adverse reaction between sodium monochloroacetate and sodium hydroxide proceeds according to second order (rate constant K2). On the basis of macrokinetic equations obtained theoretical dependence of the degree of substitution of the cellulose OH groups on the initial molar ratios of reactants and the rate constants ratio K2/K1.

In processing the experimental data carboxymethylation different pulps for a wide range of molar ratios of reagents the value of K2/K1 (relative reactivity) remains constant for the production of pulp and paper mill and certain environmental conditions. Thus, conducting the experiment for a given proportion of reagents and specific K2/K1, can be calculated by the theoretical formula for the degree of substitution of any desired ratios of reactants, minimizing the cost of the experimental work.

Among cellulose ethers produced on an industrial scale the most basic and commonly used ether includes carboxymethylcellulose (CMC) with the substitution degree of 0.6-0.9, and polyanionic cellulose (PAC), CMC substitution with the substitution degree of ? 0.9 [1].

In terms of chemistry all technological processes of carboxymethylcellulose production essentially involve interaction of sodium cellulose and sodium monochloracetate (NaMCAA) or monochloroacetic acid. The reaction can be carried out with both solid phase and suspension methods in inert organic solvents (mainly in aqueous alcohols). During cellulose carboxymethylation the main reaction of forming CMC, NaCl, H2O and side reaction of hydrolysis of Na-MCAA with the formation of glycolate sodium and NaCl [1].

In spite of numerous development results on CMC synthesis research in this field is currently important especially when using new domestic raw materials, in particular, cellulose made from softwood and hardwood, flax plant and other crops. At present, an integrated study of CMC synthesis is carried out with the method implying planning experiment and developing empirical regression equations at the most [2]. In this case, a multifactor experiment requires considerable variations of main parameters of synthesis on the properties of CMC. The purpose of this work is to develop a new approach to research, minimizing experimental efforts on CMC synthesis through deterministic models of the kinetics of cellulose carboxymethylation.

Experimental part.

Modeling of cellulose carboxymethylation macrokinetics is based on the following assumptions:

Ш in the area of chemical reactions there is an equilibrium distribution of agents;

Ш two-phase system consisting of solid and liquid reagents (cellulose, NaOH, sodium monochloroacetate, H2O and others) is regarded as the quasi-homogeneous phase.

This model is commonly referred to as quasi-homogeneous[3].

Within the framework of the quasi-homogeneous model one can write chemical reactions equations similar to formal kinetics [3].

The main reaction

Cel.-OH + NaOH + ClCH2COONaCel.-OCH2SOONa + NaCl + H2O (1)

Side reaction

ClCH2COONa + NaOHHOCH2OONa + NaCl

Below, the following definitions and ratios have been adopted .

Molar ratio:

,

,

,

where AGU stands for anhydroglucoside cellulose link.

Chemical bulk loading

,

,

,

where G, G1, G2, G3 are masses of the completely dry cellulose, NaOH, NaMCAA, water, kg;

M, M1, M2, M3 are molecular masses of AGU (0.162 kg/mole), NaOH (0.040 kg/mole), NaMCAA (0.1165 kg/mole), water (0.018 kg/mole) respectively.

,

where Gr is the mass of the reaction mixture.

The initial reagent concentration (Ci):

, , (2)

,

where Cn, C1n, C2n are initial concentrations of hydroxyl groups of cellulose, NaOH, NaMCAA, mol/kg.

Degree of OH-groups substitution :

(3)

where C is the current concentration of hydroxyl groups, mol/kg.

Let us write the following system of differential equations for reactions (1) with initial conditions:

, (4) , (5)

, (6)

, (7)

where C1, C2 are current concentrations of NaOH and ClCH2COONa, mol/kg; ф is the current time, sec;

K1 is an effective rate constant for the basic reaction, kg2/mol2C; K2 is the effective rate constant for the side reaction, kg/mol C.

Let us analyze models (4) to (7). Dividing (5) by (4), we get:

Having integrated the latter equation and initial conditions (7) and using (6) we get:

(8)

Substituting (3) to (8) we shall obtain dependency between the current degree of substitution of Z and the concentration of NaMCAA:

(9)

Completing the reaction of carboxymethylation C2 = 0 and taking into account the ratio (2) from (9) we get:

, (10)

,(11)

where r is the relative reactivity, mol/kg.

Results and discussion.

Table 1 presents experimental data on carboxymethylation of bleached wood pulp powder produced at Sovetsk pulp and paper mill (PM). Water is inert diluent in this case. Alkaline pulp was prepared at a temperature of 20 °C, carboxymethylation of alkaline cellulose was carried out at 85 °C. The degree of substitution (Z) and assay (A) were defined in the produced samples of the CMC (3-8 columns of table 1). Constant ratio r was calculated using the formulas (9-11). You can see that regardless of the mole ratio of the reagents r-value for these terms of carboxymethylation is constant r = 2.67 ± 0.03. Columns 1; 2; 9 comprise expected Z values for r = 2.67 obtained from the formulas (10)-(12) by iterations. The remaining columns are comparing experimental and expected values of the degree of substitution. Thus, if one experiment is carried out with specific values of m1, m2, m3, Z, then after defining r Z values can be calculated for any m1 and m2. In this case, you do not need numerous lengthy experiments on m1 and m2 variations using the method of experiment planning, that is, to optimize the experiment.

Array (A) can be calculated by the formula resulted from mass balance:

,

,

Similar results obtained under etherification in the fibrous sulfite pulp at Kotlas PPM are shown in table 2 (average r = 4.41 ± 0.15).

carboxymethylation cellulose reagent

Table 1. Cellulose carboxymethylation at Bratsk PPM

Table 2. Cellulose carboxymethylation at Kotlas PPM

Table 3. Cotton cellulose carboxymethylation

Figure 1. Dependence of relative reactivity on molar ratio water/pulp (1 - cotton cellulose, 2 - wood cellulose crumb)

Table 3 presents calculations r for cotton cellulose according to pilot research data [2]. Values m1 = 2; M3 = 12 were constant, the average value of the reactive capacity r = 5.38 ± 0.37.

Thus, experimental data confirm the formula (10) accuracy, which results from the scheme of the reaction (1) and Kinetic equations (4) to (7). Formula (10)-(12) reflect the influence of initial concentrations of reactants on the final degree of substitution of hydroxyl groups of cellulose.

The figure shows experimental dependence of r upon the molar ratio of water/cellulose (cotton cellulose - m1 = 2.1; m2 = 2 [2], for wood pulp crumb - m1 = 1.5; m2 = 1.3). The increase in r with the increase in m3 can be explained by the increased speed of side reaction (hydrolysis).

Conclusions.

1. The paper considers macrokinetic model of cellulose carboxymethylation. The main reaction between hydroxyl groups of cellulose and sodium monochloroacetate proceeds by the third order (kinetic constant K1). Side reaction between sodium monochloroacetate and sodium hydroxide proceeds by the second order (kinetic constant K2). Maсrokinetic equations provided theoretical dependence of OH-cellulose groups substitution on primary molecular ratios of reactants and speed constants ratio K2/K1.

2. When processing experimental data carboxymethylation of various types of cellulose for a wide range of mole ratios of reactants value K2/K1 (relative reactivity) remains constant for the production of a particular pulp and paper mill and environmental conditions. Thus, conducting an experiment under this ratio of reagents and particular K2/K1 a degree of substitution for any desired ratio of reagents can be calculated by means of the theoretical formula minimizing the costs of experimental works.

References

1. V.N. Kryazhev, O.V. Galtseva, S.I. Smirnov. CMC and PAC - traditional stabilizer of drilling mud. Oil. Gas. 2016. No.9. P. 38-43. (russian)

2. O.T. Shipina, O.K. Nugmanov, V.N. Alexandrov. Influence of production conditions on properties of Na-CMC. Ethers of Cellulose and Starch: Synthesis, Properties, Application. 2003. No.10. P.38-43. (russian)

3. O. Levenshpil. Engineering design of chemical behaviour. Transl.from English. Moscow: Chemistry. 1969. 624p. (russian)

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