Xanthation of Starch by Continuous Process

XANTHATION OF STARCH BY A CONTINUOUS PROCESS C. L . SWANSON, A N D C.

T. R . N A F F Z I G E R , C . R . R U S S E L L , B . T . H O F R E I T E R ,

E. R l S T

Northern Regional Research Laboratory, Peoria, Ill.

Downloaded by NANYANG TECHNOLOGICAL UNIV on August 26, 2015 | http://pubs.acs.org Publication Date: March 1, 1964 | doi: 10.1021/i360009a008

A rapid, continuous procedure for making starch xanthates was developed to make additives suitable for improving strength properties of pulp and paper products. Starch xanthates having degrees of substitution from 0.07 to 0 . 4 7 have been made in a small-scale continuous mixer-reactor system. Data were developed on the effect of mole ratios of starch, CS2, and aqueous NaOH, order of addition, temperature, and discharge pressure on degree of substitution, reaction efficiency, and power consumption. Starch xanthates were discharged as viscous pastes of 53 to 61% solids following 2-minute mixing. Conversion of CS2 to xanthate was 87 and 80% complete, respectively, for starch xanthates of D.S. 0.07 and 0.17 analyzed within 10 minutes after discharge. For products of D.S. 0.1 0 to 0.29 conversion was 90 to 93% upon standing 1 hour after discharge. In general, maximum efficiency is favored by increasing the temperature, adding CS2 to starch before NaOH, increasing the NaOH-CS2 ratio, and decreasing discharge orifice size.

HE effectiveness of cross-linked xanthates of starch and Tcereal flours for improving the production and properties of paper and insulation board has been demonstrated in recent exploratory studies (3, 5). Among the advantages reported are reductions in degree of pulp refining required, improved drainage rate characteristics of pulp slurries, and markedly increased wet and dry tensile strengths of finished products. The cereal xanthates employed in the exploratory studies were prepared bv batch techniques not appropriate for industrial practice. Therefore, studies were initiated to develop a practical continuous reaction system to make cereal xanthates. Cornstarch was employed in the present study; however. trials have shown that other starches and cereal flours may be processed similarly. The lower order of crystallinity of starch, as compared with cellulose, makes xanthation of starch relatively easy and thus more adaptable to a rapid and continuous process. Procedures and equipment used commercially to make viscose are not suitable for xanthation of cereal products (7, 2. 6, 9). A continuous mixer-reactor system for starch xanthation should provide for mixing viscous. high solids reaction masses under high shear. metered addition of solid and liquid reactants, and control of reaction time and temperature. ‘4n experimental xanthation system incorporating the above provisions \z as assembled to obtain data for the present study.

Experimental

Xanthation System. A Model 50 Ko-Kneader with an interrupted screw reciprocating past fixed pins was selected as substantially meeting the requirements previously mentioned. T h e 1.95-inch-diameter screw operates within a cylindrical barrel, A , 2 inches in diameter by 17l/2 inches long (Figure 1). Sixty-two removable pins, B, arranged in three rows a t 120’ intervals around the barrel, section A A , project into the mixing chamber. The hollowshaft screw, C, rotates a t a speed of 19 to 93 r.p.m. and reciprocates inch on each revolution. The interruptions in the thread of the screw permit the screw to pass by the pins as it reciprocates. As a result, some of the material is held back by the pins on the forward thrust and moved back on the reverse thrust of the screw. Thus the starch and reagents are thoroughly mixed in passing through the barrel. 22

I b E C PRODUCT RESEARCH A N D DEVELOPMENT

The mixing action can be altered by changing speed, size of discharge orifice, or temperature. Orifices of two sizes were employed : the smaller had an annular discharge opening of 0.37 square inch in area and the larger, 1.91 square inches. The smaller the discharge orifice, the greater were the internal pressure, degree of mixing, and power consumption. Reaction mixture temperature could be controlled by independent circulation of fluids of appropriate temperature through the hollow shaft of the screw and through three separate sections jacketing the mixer barrel. The unit was incorporated in a continuous mixer-reactor system (Figure 2) for producing cereal xanthates by providing: a variable-speed screw feeder for introducing the starch by gravity to the barrel hopper; axially bored pins, A and B, located one fourth and one third of the barrel length from the hopper for injecting fluid reactants into the mixer-reactor from diaphragm pumps; a terminal block, C, allowing a convenient choice of the order of addition of CS2 and N a O H solution into the mixer-reactor barrel; a thermocouple well consisting of a hollowed pin, D, located near the discharge end of the barrel; a temperature recorder; an automatic mixing valve controlling temperature of water to jacket and screw shaft; a recording watt-hour meter connected to the explosion-proof main drive motor; and a plastic hood connected to an exhaust system to assure an atmosphere with less than 20 p.p.m. of CS2 within the working area around the mixer-reactor unit. Two high-pressure, 2500-p.s.i. diaphragm pumps were provided, one capable of delivering 0.6 to 29 ml. of CS2 per minute and one suitable for delivering 14 to 140 ml. of N a O H solution (2.2 to 5.7.Y) per minute. Stainless steel tubing and valves were used to connect the pump outlets to the drilled pins in the mixer-reactor barrel. Rotameters were inserted in feed lines for continuous indication of flow rates of liquids from the pumps to the mixer-reactor. Materials. Commercial cornstarch was used for the xanthation studies. .4nalyses: retained on 325-mesh screen, 49%; fat (dry basis) by acid hydrolysis, 0.5 to 0.77,; nitrogen (dry basis), 0.04 to 0.057,; and moisture, 9.5 to 10.0%. The term “mole of starch” is defined as the molecular weight of the repeating anhydroglucose unit (AGU)-Le., 162 grams. N a O H was technical grade in aqueous solutions of from 2.2 to 5.75. CS2 was reagent grade, although technical grade gave equal results. Xanthation Procedure, The feeder was regulated to deliver starch a t about two thirds the capacity of the mixerreactor when the latter was operated a t a speed of about 90 r.p.m. Based on the starch delivery rate. the pumps were adjusted to deliver the CS? and S a O H solution to the mixerreactor a t rates sufficient to achieve the desired molar ratios of reactants.

I

I

/A


-#' Small Orifice\

1

a Figure 1.

Design of mixer-reactor

Blower

..Polymethyl Methacrylate .. . m

I

/:

-.

Var.- -Speed - - - - Dr. :

f bReagent Pumps Figure 2.

Mixer-reactor assembly for continuous xanthation VOL. 3

NO. 1

MARCH 1964

23


A reaction was started with only starch and alkali being fed into the mixer-reactor. As soon as the starch and alkali mixture started to discharge, CS2 was introduced and final adjustments of delivery rates were made. Temperatures of the reaction mixtures were controlled by adjusting the temperature of the water flowing through the mixer-reactor jackets and screw shaft. After 15 to 30 minutes of operation, depending on the reactant delivery rates, samples of the discharge were removed for analyses. The production rate for reaction mixtures, all of which contained about 40y0 starch, was approximately 6 pounds of xanthate per hour (dry basis). The average reaction period in the mixer, the interval between injection of the final reagent and discharge of the product, was 2 minutes as determined from the time required for a red dye to pass through the barrel. The degree of substitution (D.S.) of products was determined a t three time intervals following discharge from the mixerreactor. An “initial” determination was made within 10 minutes after discharge. Further analyses were made after aging in sealed plastic bags a t room temperature for 1 hour and 3’/z hours. Determination of Degree of Substitution. Xanthate degree of substitution was determined by titration as previously described ( 5 ) . For each analysis, approximately 18 grams (dry basis) of starch xanthate were dispersed with a Waring Blendor in about 340 grams of a 50: 50 mixture of ice and water. Alkali concentration in the dispersion was determined by titrating a 100-gram sample with 1-V HtSO4 to the methyl orange end point. Starch concentration in the dispersions was then calculated from the alkali concentration and the NaOH-starch ferd ratio. The calculated starch concentrations agreed well with values found by periodically determining reducing sugar content of starch xanthate hydrolyzates by the Somogyi method (8). The starch xanthate solutions were hydrolyzed in 4 5 HC1 a t 95’ C. for 20 minutes and neutralized before starch analysis.

Exploratory trials showed that a moisture content of 30 to 50% in the reaction mixture was required for adequate mixing. To eliminate effects that varying moisture contents might introduce, aqueous solutions of technical grade S a O H were introduced a t appropriate normalities and feed rates to give KaOH-starch mole ratios of 1.00, 0.75, 0.50, and 0.25 and moisture content of reaction mixtures of 43 =k 4% (standard deviation). Variation in the NaOH-starch mole ratio within the prescribed moisture limits was achieved by use of NaOH solutions of varying normality. Thus a built-in interaction between the mole ratio of alkali to starch and normality of alkali was introduced that prevented assessment of the independent contributions of these variables. Therefore, in analyzing the effects of NaOH, only variations in the NaOH-starch mole ratio were considered. T o facilitate interpretation and presentation of data, quadratic equations relating the independent variables to chemical efficiency and power consumption !\.ere computed. These equations permitted pertinent relationships to be presented graphically; therefore. only a sample of typical primary data is given (Table I). Analysis of Results

General Equations. Quadratic equations were prepared relating the independent variables to each dependent variable. The method of least squares was used to determine the constants and coefficients in equations of the type:

V=

60

4-b i x i

+

+ b 3 ~ i 4~z + (bg + +

b2Xz

6 7 ~ 1 x 3f 6 ~ x 2 ~ 3

Variables. A knowledge of the relationship between chemical efficiency of xanthation and reaction variables is needed to devise a n economical continuous process. Therefore, a n experimental design was developed to obtain data on effects of independent variables-mole ratios of reactants, relative orders of addition of CS2 and N a O H to starch, discharge orifice area-and reaction temperature on the dependent variables-percentage conversion of CSZto xanthate (chemical efficiency), xanthate degree of substitution, and power consumption. Figure 3 gives the combinations of variables studied. Although 80 combinations of variables are indicated, replications resulted in a total of 144 runs.

, 1.0

, ,

Moles NaOH

;io

0.r

Moles CSz 0.50

0.25

0.125

0.50

, , , ,I, Of0

0.50

0.25

0.y5

0.125

csz

26‘C.

Figure 3.

24

38°C.

26°C.

38°C.

26°C.

38°C.

0.25

0.0625

26°C.

38°C.

Combinations of reaction conditions investigated

I&EC P R O D U C T RESEARCH A N D DEVELOPMENT

611x2

+

+

66x3

4-

X

bi2~3)

- 1.273~4)”~

where for the dependent variables

V

=

chemical efficiency for initial and 1-hour reaction periods and power consumption

and for the independent variables

+ +

x1 = 1 log M S a O H xz = 2 log Mcsz x3 = temperature, O C . x4 = orifice area, square inches M = moles of CS2 or N a O H per mole of starch

CSS-Starch Mole Ratio 1 .o 1 .o 0.50 0.25 0.13

0.50 0.50 0.13

1st.

610x1

f b6xz2 (4

Table 1.

One Mole Starch

biXi2

Typical Xanthation Dataa on Degree of Substitution (D.S.) and Chemical Efficiency (Eff.)b Znitialc D.S. Eff.

7 Hr. D.S. Eff.

31/2-Hr. N a O H , .Vormality D.S.

1 MOLENaOH/MoLE STARCH 43 0.55 55 0.50 0.43 57 0.64 0.47 47 0.57 62 0.40 80 0.39 0.31 0.23 92 0.28 112 0.21 0.09 69 0.11 85 0.10

5.9 6.4 5.2 5.6 5.2

0.5 MOLE NaOH/MoLE 38 0.25 38 0.23 0.19 0.06 46 0.10

3.0 5.2 3.0

0.19

STARCH 50 0.30 46 0.28 77 0.09

0.25 MOLENaOH/MoLE STARCH 0.13 0.05 38 0.10 77 0.08 2.3 0.06 0.05 83 0.04 67 0.06 2.0 a CS2 added before N a O H ; oriJce 0.37sguare inch; temperature 38’ C. 70conversion of CS2 to xanthate. c 70-minute b Chemical eficiency: auerage.

T h e general equations contain no term for order of reagent addition, because this variable cannot be quantitated. Therefore, for each of the three dependent variables, two separate equations were prepared representing the two orders of reagent addition.


Chemical Efficiency

Effects of Operating Conditions. The effect of order of addition was tested by use of a n analysis of variance (7). An analysis of variance was also used to test the significance of various coefficients, as well as the over-all fit of the calculated equations. These tests determined the significance of each of the independent variables and their interactions. For unaged or initial products neither orifice size nor order of addition of CS2 and N a O H had a statistically significant effect on chemical efficiency. However, the mean efficiency was increased from 51 to 56% by raising the temperature from 26' to 38' C. Inspection of D.S. values after 1 and after 3l/2 hours of aging indicated no practical advantage for the longer period. However, chemical efficiency, particularly a t high levels of CS2 addition, was considerably higher for the products aged 1 hour than for the initial products. For 1-hour samples, all independent variables ere significant a t the 997& confidence level. Mean chemical efficiency was increased 8% by raising the temperature from 26' to 38' C. Increases in efficiency up to 17% were obtained by introduction of CS2 before N a O H in combination with the smaller discharge orifice. Operating conditions resulting in the highest chemical efficiency for samples held 1 hour were: temperature of 38' C., small orifice size, and addition of CSZ before NaOH. These conditions held also for the initial samples, although order of addition and orifice size were not statistically significant. For aged products the least effective combination of operating variables was low temperature, large orifice size, and introduction of CS2 after NaOH. Effects of Mole Ratios. T o evaluate the effects on chemical efficiency resulting from variation of the mole ratios of reactants, three of the general equations were simplified by insertion of definite values for temperature and orifice size. These simplified equations give the efficiencies achieved with operating conditions established by the analysis of variance as

The equations are of the

optimum and as least effective. form :

v = GO +

61x1

+ + 62x2

C3XlXZ

+

+

C 4 d

csx2

where values of the constants depend upon both the operating conditions and the time interval after discharge a t which the xanthate degree of substitution was determined. For the two time intervals employed, values of the constants for optimum operating conditions were:

INITIAL co = 18.49;

~1

= 103.35;

i 2

= 34.50; Cq

= 29.97;

~3

= -58.43;

C.5

= -39.19

ONEHOUR co = -8.35;

~1

= 30.17;

= 135.69;

~2

~4

=

~3

=

-98.14;

124.14; ~5 = -99.09

and, for the least effective operating conditions :

ONEHOUR co = -17.28;

~1

=

-8.22;

GZ Cq

= 142.65; =

-27.35;

~3 C.5

= 61.77; = -80.47

Data from these equations were used to prepare contours on triangular coordinates representing the relation of reactant compositions to chemical efficiency, xanthate degree of substitution, and minimum cost of reactants. The initial percentage conversions of CS2 to xanthate over the range of mole ratios of starch: N a O H : CS2 from 1 : 1 : 1 to 1 : 1/4: 1/16 are shown in Figure 4. The vertices ( A , B, and C) of the triangular diagram represent 100% CSZ,NaOH, and starch, respectively. The reaction mixture giving highest efficiency for initial products is indicated by the 87% efficiency ' starch, point. The composition a t this point is 48.2 mole % 48.2 mole % S a O H , and 3.6 mole % CS2. The corresponding mole ratio is 1 : 1 : 0.07. Similar contour diagrams for the 1-hour sample determined under both optimum and least efficient operating conditions are given in Figures 5 and 6, respectively. In areas of the triangular diagrams (Figures 4, 5 , and 6), in which the molar percentage of CS2 was approximately 10 or above, efficiency depended mainly on the CS2-NaOH ratio. Efficiency was a t a maximum when the molar percentages of

c

30

30 / 70 65 60 55 50 45 40

A

rI- - - -

+Mole

\

-----' B

% CS2

Figure 4. Effect of mole ratios of reactant on initial chemical efficiency

Figure 5. Effect of mole ratios of reactants on chemical efficiency after 1-hour aging

Optimum operating conditions


VOL. 3

NO. 1

MARCH 1964

25

c

I


A L----

+Mole

% CSz

-----‘ B \

Figure 6. Effect of mole ratios of reactants on chemical efficiency after 1 -hour aging

Figure 7. Relationship between mole ratios of reactants and initial degree of substitution

Least effective operating conditions


CS2 were low, from about 4 to 9, and decreased as the molar percentage of CSZ was increased or decreased from this range. At constant CS2-starch mole ratio. chemical efficiency decreased as the S a O H mole fraction decreased. The chemical efficiency for initial products ranged from 40 to 87%. The efficiency after 1-hour aging ranged from 50 to 93% under the optimum operating conditions and from 37 to 77% under the least effective conditions. Degrees of Substitution. Relationships of degree of substitution obtained for the initial and I-hour reaction periods to the mole ratios of reactants are shown in Figures 7 and 8, respectively. The degree of substitution contours in combination with the appropriate superimposed efficiency curves permit selection of reactant compositions giving maximum chemical efficiency for the preparation of a given degree of substitution. A starch xanthate of given degree of substitution can be prepared from any number of CSn-starch mole ratios but with varying degrees of chemical efficiency. Each of the mole ratios can be represknted graphically by a line betlteen vertex B and side on the efficiency diagrams (Figures 4, 5, and 6). Each point of a given degree of substitution on chemical efficiency curves is conveniently located by preparation of appropriate CSZ-starch mole ratio lines. Intersection of the CS2-starch mole ratio line Ivith the chemical efficiency curve establishes the reaction mixture composition producing the degree of substitution a t a particular efficiency. The degree of substitution contours are then constructed by connection of identical degree of substitution points on the various efficiency curves. For example in Figure 7, the curve representing a degree of substitution of 0.1 as determined for the initial period intersects the 807, chemical efficiency contour a t a reaction mixture composition corresponding to about 58 mole yo starch, 35 mole 7, NaOH, and 7 mole 70 CS2. For constant mole ratios of CSn-starch, as the mole fraction of S a O H in the reactant mixture is increased, the degree of substitution values approach the experimental CSn-starch ratio. Minimum Chemical Cost, The reaction composition of minimum cost for producing a given degree of substitution does not necessarily correspond to that for which maximum

chemical efficiency is observed, because the latter is achieved only in the presence of a large excess of alkali. I n Figure 9, curves derived from initial and 1-hour data have been constructed that indicate the reactant ratios most economical for preparing starch xanthate of degree of substitution between 0.1 and 0.4. The minimum cost curves were derived from degree of substitution contours in Figures 7 and 8, as found for optimum operating conditions. The cost of chemicals per unit starch was determined for selected points on each degree of substitution curve, based on present prices of CS2 and NaOH of 4.25 and 3.1 cents per pound ( J ) ,respectively. The chemical costs of the reaction mixture composition that produces a given degree of substitution have a minimum, which \vas determined by plotting the combined CSS and NaOH costs per mole of starch against the mole ratio of the t\vo reagents. The minima determined in this manner were represented by points on the degree of substitution contours Ithich, M hen connected, gave the minimum cost curves in Figure 9. The large differences in reaction compositions yielding the maximum chemical efficiency and those giving most economical production for a selected degree of substitution can be seen by superimposing appropriate chemical efficiency curves on Figure 9. Chemical efficiencies of 60 to 807, can be obtained a t minimum cost. Figure 9 shows that a xanthate of degree of substitution 0.1 can be obtained from all reaction mixtures between the 0.1 degree of substitution cuIves for the initial and 1-hour periods by selection of an intermediate reaction time. .is expected, curves of the same degree of substitution for the two time intervals approach the same reaction mixture composition in that area of the diagram in \\hich maximum efficiency is obtained. The area betiveen the minimum cost curves for the initial and 1-hour products encompasses all compositions from which selection may be made to obtain a given degree of substitution with optimum combinations of reaction time and chemical costs. Selection of the minimum total cost to make a xanthate of a given degree of substitution requires determination of the chemical efficiency a t intermediate periods. Experimental data for intermediate periods were not obtained. However,

26

l&EC PRODUCT RESEARCH A N D DEVELOPMENT

C

C

30

30 I

70 65 60 55 50 45 4 0 35 30 25 20 15 10 5

\.


A Figure 8. Relationship between mole ratios of reactants and degree of substitution after 1 -hour aging Optimum operating conditions

’---/

+Mole

\

-----\ B

% CSz

Figure 9. Minimum reactant cost curves for three levels of degree of substitution as obtained initially and after 1 -hour aging Optimum operating conditions

sufficient reaction efficiency for commercial preparation of xanthates of degree of substitution 0.2 or less may be attainable with aging periods considerably less than 1 hour. Power. Although statistically significant effects on power consumption were found by the analysis of variance for reaction temperature, order of reagent addition, and orifice size, they are of little practical importance. The difference between the highest and lowest power consumption, throughout the range of reaction conditions imposed, was 0.02 kw.-hr. per pound of product (dry basis).

mixing a t the higher temperature. These optimum conditions were used to make starch xanthates of degree of substitution 0.10 to 0.29 with chemical efficiencies in the range of 90 to 93%. Good chemical efficiencies have been achieved in the degree of substitution range previously found appropriate for use of the starch xanthate as a paper-making additive. However, additional studies will be required to establish within narrower limits the relationship between aging of the xanthates and chemical efficiency.

Summary and Conclusions

Acknowledgment

The applicability of a high-shear screw-type mixer-reactor for the continuous production of starch xanthate has been demonstrated. Starch, CS2,and N a O H were separately and sequentially metered into the unit, and the products containing 53 to 617, solids were discharged as viscous pastes after a reaction time of 2 minutes. The influence of independent variables of temperature, sequence of reactant additions, discharge orifice size, and holding time on chemical efficiency and power consumption was determined. The mole ratios of starch:NaOH: CS2 investigated ranged from 1 : 1 : 1 to 1 : 1/4: 1/16. Quadratic equations relating independent variables to chemical efficiency and power consumption were computed. From these equations thr relationships of mole ratios of reactant to chemical efficiency, degree of substitution, and minimum reagent cost for the operating conditions employed were determined and presented as contour diagrams on triangular coordinates. T h e chemical efficiency for starch xanthate production, determined within 10 minutes after discharge from the mixerreactor, was favored by increasing the reaction temperature from 26’ to 38” C. but was unaffected by either order of reagent addition or the orifice size employed. Under these conditions, starch xanthates of degree of substitution 0.07 to 0.17 were prepared u i t h chemical efficiencies ranging from 87 to SOYG, respectively. Ho\vever. maximum chemical efficiency was obtained by a I-hour aging of the reaction products prepared by introducing CS2 before NaOH, using a small orifice, and

The authors gratefully acknowledge their indebtedness to the Computing Laboratory, Biometrical Services, ARS, for computational assistance and to J. N. Boyd and SY. F. Kwolek, Biometrical Services, for statistical advice; to L. D. Miller for aid in mixer-reactor operation; to R. A. Buchanan, E. I. Stout, and E. G . Helman for chemical determinations; and to H. C. Katz for aid in computations. Literature Cited

(1) Barboza, E., U. S. Patent 3,051,560 (Aug. 28, 1962). (2) Hibbert, SY., Bunting, J. LV, (to Courtaulds Ltd.), Ibid., 3,033,000 (May 8, 1962). (3) Naffziger, T. R., Swanson, C. L., Hofreiter, B. T., Russell, C. R., Rist, C. E., Tappz 46, No. 7, 428 (1963). (4) 021, Paint Drug Reptr. 183,No. 24, 14, 27 (1963). (5) Russell, C. R., Buchanan, R. A,, Rist, C. E., Hofreiter, B. T., Ernst, A. J., T a p p i 4 5 , No. 7, 557 (1962). (6) Seaman, S. E., King, R. P., U. S. Patent 2,490,097 (Dec. 6, 1949). (7) Snedecor, G. SV., “Statistical Methods,” Iowa State University Press, Ames, 1961. (8) Somogyi, M., J . Bioi. Chem. 195, 19 (1952). (9) Yehling, G. C., Jr. (to Olin Mathieson Chemical Corp.), U. S. Patent 3,053,829(Sept. 11, 1962). RECEIVED for review November 18, 1963 ACCEPTED December 24, 1963 Division of Cellulose, SVood, and Fiber Chemistry, 145th Meeting, .4CS, New York, N. Y., September 1963. The Northern Laboratory is part of the Northern Utilization Research and Development Division. Agricultural Research Service, U. S. Department of Agriculture. Mention of firm names or commercial products does not constitute an endorsement by the U. S. Department of Agriculture. VOL. 3

NO. 1

MARCH 1964

27

Xanthation of Starch by Continuous Process

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