Ind. Eng. Chem. Res. 1991,30, 2292-2298
2292
Pitchai, P.; Klier, K. Partial Oxidation of Methane. Catal. Rev.-Sci. Eng. 1986,28,13-88. Rytz, D.; Baiker, A. Catalytic Partial Oxidation of Methane in a Flow Reactor. Manuscript in preparation, 1991. Stroud, H. J. F. U.K. Patent 1,398,385, 1975. Tripathy, N. Slow Combustion of Methane at Elevated Pressures. Isr. J. Chem. 1975, 13, 190-196. Wilms, M. Methode zur kinetischen Kontrolle der Methanoxidation.
Ph.D. Dissertation, Technische Hochschule Aachen, 1989. Yarlagadda, P. S.; Morton, L. A.; Hunter, N. R.; Gesser, H. D. Direct Conversion of Methane to Methanol in a Flow Reactor. Znd. Eng. Chem. Res. 1988,27, 252-256.
Received for review October 2, 1990 Revised manuscript received May 29, 1991 Accepted June 22, 1991
MATERIALS AND INTERFACES Determination of Hansen Solubility Parameters for Selected Cellulose Ether Derivatives Wesley L. Archer Specialty Chemicals TS&D, Larkin Laboratory, Dow Chemical U.S.A.,Midland, Michigan 48674
This paper describes a method for determining the partial solubility parameters of six Methocel and six Ethocel cellulose ether products. A simple computer spreadsheet has been devised to calculate the partial solubility parameters from the laboratory data. The cellulose ethers are characterized in terms of nonpolar, polar, and hydrogen-bonding behavior. Methocel A, E, F, K, and J all have similar solubilities and are thus assigned solubility parameters in the range of 6d = 17.4-18.2, 6, = 14.6-16.5, and 6h = 15.5-19.4 in units of MPa1l2and with a radius of interaction of 10.3. Methocel 311 has been assigned the values of 6d = 17.1, 6, = 9.8, 6 h = 13.2 MPa1/2and a radius of interaction of 10.2. A very high methoxyl and hydroxypropyl substitution in the Methocel311 product reduces the inherent polarity and hydrogen-bonding nature of the basic cellulose structure. Increased organic-like solubility is shown by the six Ethocel ethoxy ether cellulose derivatives. The range of Hansen partial solubility parameters are nonpolar (6d) = 16.6-17.3, polar (6,) = 6.6-8.3, and hydrogen bond (6h) = 8.5-10.0 MPa1/2.
Introduction New insights into cellulose ether solubility-application relationships have come from a study that made use of Hansen's solubility parameter model. Cellulose ethers are linear polymers of condensed anhydroglucose r i n e forming a carbohydrate skeleton to which methyl and hydroxypropyl groups are attached by an ether linkage. The solubility profiles of six different types of Methocel cellulose ether products have been determined. (Methocel is a trademark of The Dow Chemical Company.] The solubilities are characterized in terms of nonpolar, polar, and hydrogen-bonding components. Rowe (1986) has used solubility parameters to examine certain pharmaceutical uses of cellulose ethers like (hydroxypropy1)methylcellulose, (hydroxypropyl)cellulose,and ethylcellulose. The solubility of wood lignin (cellulose derivative) in organic solvent blends has been examined by Schuerch (1952), Lindberg (1967, 1968), and Ekman and Linfberg (1966) using the Hansen solubility parameter model. Turbak et al. (1980) have noted that solubility parameters are one factor to consider when selecting a solvent blend for cellulose. The three Hansen solubility parameters for microcrystalline cellulose have been determined by a gassolid chromatography method by Phuoc et al. (1987). A similar study by Phuoc et al. (1986) determined the solubility parameters of lactose. Ethylcellulose and cellulose acetate membranes have been characterized by Klein and Smith (1972) and Mulder et al. (1982) using solubility parameter profiles. Solubility
studies with ethylcellulose used in film coatings in the pharmaceutical industry have been reported by Rowe (1986), Sakellariou et al. (1986a,b), Entwistle and Rowe (1979),and Kent and Rowe (1978). The solubility profiles of several Ethocel (Dow Chemical Co.) products (ethoxycellulose ether structures) as determined in our laboratory will also be detailed in this paper. This paper will include a short review of the Hansen solubility parameter model, comments on literature references and solubility work with cellulose derivatives, a description of our laboratory work and the solubility parameters determined for six Methocel and six Ethocel products, and finally some comments on the use of these solubility data to predict formulated product performance. Solubility Parameter Theory The concept that solubility is related to the internal energy of solvents and solutes (e.g., polymers) was first proposed by Hildebrand in 1916. The theory states that molecules subject to the same internal pressure are most effective in attracting and interacting with each other where the resultant heat of mixing will equal zero. Internal pressure is the energy required to vaporize 1 unit volume of a material. The internal energy/molar volume ratio E / V (where V = molecular weight/density) is referred to as the cohesive energy density (CED). By definition the solubility parameter is the square root of the CED value
0888-5885/91f 2630-2292$02.50f 0 0 1991 American Chemical Society
Ind. Eng. Chem. Res., Vol. 30, No. 10,1991 2293 Table I. Profile of Methocel Samples Methocel type methoxy, % A4M 27.5-31.5 E4M 28.0-30.0 F4M 27.0-30.0 K4M 19.0-24.0 E6 premium 28.0-30.0 J4M 16.5-20.0 311 24.0-28.0
dP
4
. L - ==--l,’ ’ ---------*’
-- ----‘+-I--
/
.
hydroxypropyl, % 7.0-12.0 4.0-7.5 7.0-12.0 7.0-12.0 23.0-32.0 23.0-27.0
derivatives. If the value from eq 3 is less than the radius of interaction for the polymer VjR < jR), the solvent or solvent blend will dissolve the polymer.
\ I
Experimental Section The Methocel products are cellulose ethers that possess varying ratios of hydroxypropyl and methyl substitution, Figure 1. Three-dimensional display of solubility parameters. a factor that influences solubility of these cellulose ethers. During the manufacture of cellulose ethers, the cellulose where T = absolute temperature, R = gas constant, V = fibers from either wood pulp or cotton linters are treated molar volume, H = enthalpy of vaporization, and E = with caustic solution which in turn is treated with methyl energy of vaporization. The CED is the energy that binds chloride, yielding the methyl ether of cellulose. For (hythe molecules in 1 unit volume of liquid or solid. The 6 droxypropy1)methylcelluloseproducts, propylene oxide is value has the unit dimensions of (calories/volume)1/2. Current usage favors the SI units where 1 ~ a l l / ~ / c = m ~ / ~ used in addition to methyl chloride to obtain hydroxypropyl substitution onto the anhydroglucose units. In 2.05 MPa1/2(megapascals). Solubility theory predicts that identifying the Methocel products, an initial letter idendissolution occurs when a solute (polymer) is surrounded tifies the type of cellulose ether. The letter A identifies by a solvent of similar CED value. methylcellulose products. The letters E, F, J, and K Hansen (1969) has pioneered the concept that the total identify different (hydroxypropy1)methylcellulose products cohssive energy term and thus the total solubility paramwith varying degrees of substitution as shown in Table I. eter (6,) arises from a nonpolar interaction (6d), a polar The Ethocel products used in this study had an ethoxyl interaction (6p), and a hydrogen-bonding component (6h). substitution of 4 5 5 2 % on the cellulose backbone where Hansen’s contribution extends the solubility parameter ethyl chloride is used to react with the caustic-cellulose model to solutions where the heat of mixing is greater than reactive complex. or equal to zero. The three partial solubility parameters An array of 39 solvents with varying solubility paramare related by the expression eters were available to determine the solubility profile of 6, = [ad2 + ;6 + 6h2]1/2 (1) the Methocel and Ethocel ether samples. Thirty- four solvents were used in all the Methocel ether testa except The solubility parameters of a great number of solvents Methocel A, where only 17 solvents were needed. An array are known and have been tabulated by Barton (1983,1990). of 29 solvents were used for characterization of the Ethocel Solubility parameters of solvent mixtures can be calculated ether derivatives. The solubility behavior of the test from the equation substance was judged as soluble (S),partial solution/hazy S(mixture) = C$161 4 1 ~ + 6 ~... (2) (H), or insoluble (I) after contacting the cellulose ether where C$ is the volume fraction of a mixture component. sample with a solvent for some 18 h (overnight). Testa The solubility parameters of solids or polymers are used 0.25 g of sample in a 20-mL (60-mm height, 27.5-mm available in standard references like Barton’s handbooks diameter) bottle equipped with a polyseal cap that conon solubility parameters (1983, 1990). Estimates of the tained 10 mL of test solvent. The test bottles were placed polymer solubility parameters as determined from a group in a large ball-mill jar with adequate padding, and the jar contribution theory based on the chemical structure are was placed on a roller overnight. Placement of the vertical also described in Barton’s books. The most exact solubility axis of the bottles 90° to the top/bottom axis of ball-mill parameter determination uses the solubility behavior of jar allows the test bottle contents to tumble as the mill jar the polymer in a series of solvents with varying degrees rolls. After the test period the samples are judged for of polarity and hydrogen-bonding ability. solubility behavior as indicated previously with a rating The total solubility parameter of a material is a point of S, H, or I. Areas of product solubility were then plotted in three-dimensional space where the three partial soluin various manners and the data used to calculate the bility parameter vectors meet as shown in Figure 1. The partial solubility parameters for the particular cellulose distance (A)between the two total solubility points (S is ether. for solvent and P is for polymer) determines the degree Results of interaction and solubility for the solvent and polymer. Partial solubility parameters of polymers are best deHigher solubilities are expected as the A value between points P and S decreases. termined by means of a series of solubility testa with a The distance in space between two sets of parameters selected array of solvents. Dante et al. (1989) have suc(i.e., how closely they are matched) can be represented by cessfully used a computer program to calculate the soluthe term radius of interaction, or ‘jR, and calculated by bility parameters of resins and in turn match the resin solubility with a suitable solvent blend. These researchers ’1R = (4(’6d - J6d)2 (i6p - J6p)2 (‘ah - i6h)2)1/2 (3) a t Exxon determined the resin solubility behavior in an In eq 3, the j terms correspond to the parameters of the array of 39 test solvents that display an increasing degree solute and the i terms to the parameters of the solvent. of hydrogen-bond character (hexane to water). The radius of interaction terms will be used later to calIn 1988, the Exxon workers characterized both Methocel culate the solubility parameters for the Methocel ether 311 (a highly substituted (hydroxypropy1)methylcellulose) dh
+
+
+
2294 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 Table 11. Summary of Solubility Parameters for Several Types of Cellulose Ethers solubility Daram, MPa1/2 hydrogen radius cellulose ether nonpolar polar bond interaction 1 Methocel A4M 18.0 15.3 19.4 8.2 2 Methocel E4M 17.4 14.9 19.3 7.6 3 Methocel F4M 18.2 16.5 15.5 7.7 4 Mathocel K4M 18.0 15.3 19.4 8.2 17.6 14.6 18.2 7.8 5 Methocel J4M 6 Methocel 311 17.3 9.9 13.5 8.7 7 Exxon detn 17.6 9.8 13.1 6.8 Methocel 311 8 Exxon detn 17.6 10.2 15.4 9.4 Klucel H 9 Dow detn 17.2 9.8 13.5 7.3 Klucel H
and Klucel H (a highly substituted (hydroxypropy1)cellulose structure) in their computer aided solubility parameter program. [Klucel is a trademark of Aqualon.] Both cellulosic compounds had very similar partial solubility parameters and were recommended as good thickeners for paint-stripper formulations. The solubility data from our laboratory work were evaluated by use of a 123 Lotus spreadsheet. We also took the seven best solvent pairs reported by Hercules (1984) for Klucel H and calculated the partial solubility parameters using our 123 Lotus spreadsheet method. The data detailed in Table I1 demonstrate the usefulness of the simple spreadsheet calculations since our Klucel H values and those for Methocel311 are close to the reported Exxon values. Shareef (1986) in a recent paper characterized four inorganic pigments in terms of partial solubility parameters. Shareef’s data treatment method was simplified and transferred to a 123 Lotus spreadsheet. The evaluation critique requires one to find the center and radius of the sphere whose volume is minimum and which best encloses all of the given points (solvents showing soluble or partial soluble behavior). The center of this sphere as determined by the nonpolar, polar, and hydrogen-bond vectors represents the partial solubility parameters of the test polymer. The spreadsheet calculations determine the distance (Dij)between two points in the sphere of influence where the coordinates of the sphere (center point and radius) allow inclusion of the polymer solvents in the sphere space and nonsolvents are excluded. The Dij between two points is equal to Dij = [ ( X i - Xj)’ + (Yi - Yj)2 + (Zi - Zj)2]’/2 (4) All combinations of the solvents yielding a soluble or partial soluble behavior are calculated so as to arrive at the largest (Dv)values. The vector between two points giving the maximum separation distance can be considered the diameter of the sphere, and the radius denoted as the radius of interaction is CR = Dij(max)/2. In turn the coordinates of the center of the sphere are xi + xj Yi + Yj zi + zj c, = *- 2 , cy=--.2 , c,=- 2 These three coordinates are the three partial solubility parameters for the test polymer. The Lotus spreadsheet used to calculate the partial solubility parameters for Methocel J4M ether is detailed in Table 111. The basis for these calculations was the soluble and partially soluble behavior of Methocel J4M in the test solvents. Again in order to calculate the solubility parameters, one uses eq 4 to calculate the Dij value or the
distance in 3-D space between each solvent or partial solvent. The next task is to search the columns of Dij calculations for maximum values, e.g., >11.0. Twentyseven binary solvent combinations with large Dij values were selected, and the radius of interaction and the center of the sphere were calculated. The final values for Methocel J4M were taken as the average of each of the twenty-seven values listed for the solvent pairs. Table I1 lists the solubility parameters for six types of Methocel ethers. The Exxon determined values for Methocel 311 and Klucel H are also included for comparison along with the calculated values for Klucel H using our spreadsheet method. Only Methocel 311 has an appreciably different solubility profile of the six Methocel samples tested. The high methoxy and hydroxypropyl substitution in the 311 product makes this Methocel ether unique in solubility behavior (see Table I). As one proceeds from “A” chemistry to “J” chemistry the number of solvents showing solubility for the Methocel ether increases. Methocel A shows solubility in only three solvents, dimethyl sulfoxide, benzyl alcohol, and water. Methocel E, F, and K show solubility in formamide and the three solvents listed for Methocel A. The higher hydroxypropyl substitution of Methocel J allows the addition of Methyl Cellosolve (trademark of Union Carbide) solvent to the list of four soluble solvents. As noted before, Methocel 311 is soluble in a total of eight solvents and partially soluble in a large array of additional solvents. The solvents that dissolve each type of Methocel ether are listed in Table IV. A summary of Hansen solubility parameters for cellulose derivatives from our study and earlier literature values, Rowe (1988), is shown in Table V. The term “delta A”, which combines the polar and hydrogen-bonding components, is used to sort the derivatives from most “water-like” to the least “water-like”. Methocel A, E, F, K, or J with the partial reaction of hydroxyl groups falls below cellulose and considerably above the less polar cellulose acetate. The highly substituted Methocel311 has combined polarity and hydrogen-bond character similar to cellulose acetate along with a similar solubility behavior. A summary of the Hansen solubility parameters for six Ethocel ether derivatives from our study is given in Table VI, where increasing hydroxyl substitution gives decreasing polar and hydrogen-bonding character as shown by the delta A values. The Ethocel ether partial solubility parameters were derived in a manner identical with that for the Methocel ether derivatives. The Ethocel products have more hydrophobic solubility (organic-like) than cellulose acetate derivatives, which are in turn more hydrophobic than the methyl and hydroxypropyl ether derivatives of cellulose. Cellulose ethers like Methocel311 and Klucel H stand midway between the Methocel A-J types and the ethoxy ethers like the Ethocel types in organic solvent solubility. A graphical representation of the hydrogen-bond and polar character of various cellulose derivatives is detailed in Figure 2. The progression from a highly hydrogen bonding and polar character of cellulose to the methyl and hydroxypropyl ether structures with water-like solubility to the future capping of the hydroxyl groups with acetate groupings is shown in the figure. Increased organic-like solubility is obtained in the (ethoxy)cellulose structures that do not have any water-like solubility.
Discussion The solubility behavior of the lignin moiety found in wood has been related to the polarity and hydrogen-
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2295 Table 111. Calculation of Solubility Parameters for M e t h o d " J4M OF OPTlbdllbhspLuBu I l Y PAR-
1. Generate solvent table which reflects solvents that give total or partial solubility. Calculation of radius of interaction for each possible solvent pairs. Select solvent pairs which give the largest radius values, e.g., > = 11. 3. Generate solvent pair table with lookup formula from original table. The average solubility 2.
-
values are calculated for each solvent pair and all averages used to figure the final solubility parameters values for the METHOCEL species. CALCULATIONOF RADIUS OF INTERACTION
SOLVENTS THAT GNE SOLUBLE (S) OR
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10.2 13.7 19.0 17.6 16.4 7.4 22.3 26.0 23.3 29.3
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Trademark of The Dow Chemical Co. Table IV. Liquids That Dissolve Methocel Methocel type solvents A E F K J ethylene dichloride x Methyl Cellosolve butyrolactone DMSO x x x x x benzyl alcohol x x x x x methanol x x x x formamide water x x x x x
20 18
311 X
x
l6 14
0 Water
1
Methocel' F 0 Methocel' A,K Methocel' J 0 'Methocel' E Cellulose 0 Acetate
X
,
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x x X
om
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x x
bonding characteristics of solvents. The lignin structure is comprised of chains of phenylpropane units having large numbers of phenolic, alcoholic, and methoxy groups as well as some carbonyl and carboxyl groups. Schuerch (1952) has found that the ability of a solvent to dissolve or swell lignin increases as the hydrogen-bonding capacities of the solvent increases. Lindberg (1968) has plotted the areas of partial and total solubility for Kraft process lignin in terms of total solubility parameter vs hydrogen-bonding capacity. Teas (1968) used Hansen's three partial solu-
Cellulose a
00
35
70
10.5
140
175
210
245
28.0
315
0
HYDROGENBONDING
Figure 2. Hansen solubility parameters for cellulose derivatives. *, trademark of the Dow Chemical Co.; **, trademark of Aqualon Inc.
bility parameter values to plot an area of solubility-nonsolubility for lignin. Cellulose acetate has been well characterized in terms of solubility behavior. Barton's handbooks (1983,1990)
2296 Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 Table V. Hansen Solubility Parameters for Cellulose Derivatives solubility param, MPa112 material total nonpolar polar lactose 39.9 19.6 26.2 microcryst cellulose 39.3 19.4 12.7 17.4-18.2 14.6-16.5 Methocel A, E, F, K, or J 29.0-30.6 18.6 12.7 cellulose acetate 25.1 Methocel311 24.1 17.3 9.9 Klucel H 24.0 17.2 9.8 Ethocel 20.4-21.0 16.6-17.3 6.6-8.3 water 31.7 19.5 17.8
delta Aa 35.0 33.8 22.6-24.7 16.8 16.7 16.7 10.8-13.0 25.0 .
hydrogen bond 23.2 31.3 15.5-19.4 11 13.5 13.5 8.5-10.0 17.6
ODelta A is equal to the square root of the sum of the squares of the polar and hydrogen-bond components.
Table VI. Partial Hansen Solubility Parameters for Ethocel (Ethoxycellulose Derivatives) solubility param. MPa112 Ethocel hydrogen materiala total nonpolar polar bond delta A STD 10 20.9 16.6 8.3 9.7 7.7 7.7 9.6 12.3 STD45 21.0 17.0 STD 100 21.0 17.0 7.7 9.6 12.3 MED50 21.0 17.2 6.9 9.9 12.1 HE 350 21.0 17.3 6.6 10.0 12.0 HE 45 20.4 17.3 6.6 8.5 10.8
15
dplMPa1/2 10
5
OSTD, MED, and HE are used to designate the degree of ethoxyl substitution on the cellulosic polymer (Ethocel): STD = 4849.5%; MED = 45-46.5%; HE 49.5-52%.
Figure 4. Hansen parameter (b,, ethylcellulose.
cellulose trlacetate
10
- 6J solubility map for 2.25-DS
30r cellulose dlacetate
" 0-
D
25
20
dhlMPaV2
Figure 3. Hansen parameter (6, di- and triacetate.
- 6,) solubility map for cellulose
have several references on calculating the Hansen parameters for cellulose acetate derivatives. Mulder et al. (1982) have calculated the solubility parameters for a series of cellulose acetate structures with an acetate substitution of from 2.3 to 2.9. Both the polar and hydrogen-bond character decrease as the degree of acetate substitution increases. Klein et al. (1975) published the solubility envelopes of cellulose diacetate and triacetate (see Figure 3) where the polar and particularly hydrogen-bond character decreases as the hydroxyl groups are capped by acetate groups. Solvents or solvent blends falling inside the cellulose envelope are expected to dissolve the cellulose acetate. Solubility envelopes like those shown in Figure 3 are generated by plotting the solubility behavior of the polymer in a number of solvents and blends for which the partial solubility parameters are known. Klein and Smith (1972) and Klein et al. (1975) have mapped the solubility envelopes for a number of cellulose acetate and ethylcellulose derivatives. A solubility envelope for a 2.25-DS (DS, degree of substitution) ethylcellulose as developed by Klein et al. (1975) is shown in Figure 4.
5
10
15
20
25
30
Figure 5. Solubility map for ethylcellulose (EC), (hydroxypropyl)methylcellulose (HPMC), and (hydroxypropy1)cellulose(HPC).
Rowe (1986) has published a two-dimensional solubility map for ethylcellulose, (hydroxypropy1)methylcellulose (HPMC), and (hydroxypropy1)cellulose(HPC) using polarity and hydrogen-bonding data furnished by several literature solubility studies. The solubility envelopes shown in Figure 5 were constructed by plotting the position of various test solvents that dissolved or did not dissolve the particular cellulose derivative. A plus sign indicates a solvent for the HPC and HPMC derivatives, while circled dots indicate solubility for only the ethylcellulose moiety. The HPMC envelope is slightly smaller than the HPC solubility area. The ethylcellulose envelope supports the known fact that solvents with little polarity and hydrogen-bond character can be good solvents for ethylcellulose derivatives. Solubility studies in our laboratory with HPMC derivatives support, and extend, the knowledge that Rowe's study supplies.
Ind. Eng. Chem. Res., Vol. 30, No. 10, 1991 2297
YUWUIIUI
/ fd
Figure 6. Klucel solubility (hydroxypropylcellulose): +, active;-, partial; 0,nonsolvent.
fd
Figure 7. Solubility envelope for wood lignin.
Blends of ethylcellulose with either a HPMC or HPC derivative are used in the pharmaceutical industry for delayed- or sustained-release films on tablets. Solubility envelopes have also been used to predict interaction of these films with various film plasticizers by Sakellariou et al. (1986a,b), Entwistle and Rowe (1979), and Kent and Rowe (1978). Cellulose ether solubilities in the past have been described in company bulletins and literature articles in very general terms, e.g., a list of active solvents, partial solvents, and nonsolvents for a particular cellulose derivative. Some solubility data furnished by Aqualon on their Klucel H product (a (hydroxypropy1)cellulose)has been translated into the more expressive plot shown in Figure 6. The amoebalike envelope was drawn to enclose the solvents (+) that afforded solubility to Klucel H. The partial solvents (-1 generally lay along the outer solubility boundary. The which extends toward the lower area of nonsolvents (0) right apex (increasing nonpolar nature) would be solvents for more nonpolar species like ethylcellulose. The wood lignin data furnished by Hansen (1967) has been translated in the fractional triangular plot shown in Figure 7. Again, the use of a visual display of the partial solubility parameters of the test solvents allows one to draw a solubility envelope for the wood lignin.
Acst6phenone
1
55% Toluene 45% MeOH
M
Figure. 8. Solubility of Methocel311 in paint Strippers. Non-MeC1, paint strippers: blend of 55% toluene and 46% methanol added to 56% acetophenone,N-methyl-2-pyrrolidone,or Dowanol PM.
On the basis of solubility and swelling properties of lignin, Hansen and Andersen (1988) list the partial solubility parameters for lignin as nonpolar = 20.6, polar = 13.9, and hydrogen bond = 15.2 with a sphere radius of 11.8. The new solubility parameter data on cellulose ether derivatives reported in this paper will allow afford workers more details on the cellulose ether solubility profiles in new application areas. One area of interest is in the w e of the solubility characteristics of Methocel 311 as help in the formulation of new paint strippers. Figure 8 details some of the paint strippers using Methocel311 and their compositions on a triangular fractional solubility plot. The final paint stripper compositions contain a blend of 55% toluene and 45% methanol which is diluted with Dowanol PM glycol ether, N-methyl-2-pyrrolidone,or acetophenone to give a blend which is then thickened with 1 wt % Methocel 311. Tie lines between the toluene/methanol composition point and one of the latter solvents than locates the solubility parameters of the final composition as indicated by the filled circles. Plots of this type enable one to suggest numerous other stripper solvent blends that fall within the Methocel 311 ether solubility envelope.
Summary Solubility parameter theory has been used previously to explore certain cellulose ether applications. Film coating in the pharmaceutical industry and the formulation of new paint strippers are two examples. Only two cellulose ethers, i.e., Methocel 311 and Klucel H, had been fully characterized in terms of Hansen's partial solubility parameter in previous industry work. Partial characterization of a HEC and a HPMC derivative has been reported by Rowe in his pharmaceutical film work. Our work has characterized Methocel A, E, F,K, and J and Methocel 311 in terms of nonpolar, polar, and hydrogen-bonding behavior. The Methocel A, E,F, K and J products all have similar solubilities and are thus assigned solubility parameters in the range of bd = 17.4-18.2, 6, = 14.6-16.5, and bh = 15.5-19.4 MPa1I2with a radius of interaction of 10.3. Only the highly substituted Methocel 311 has appreciably different solubility parameters. The parameter values for Methocel311 of bd = 17.1, bP = 9.8, and bh = 13.2 W a l l 2 are similar to the HPC derivative, Klucel H.
Znd. Eng. Chem. Res. 1991,30, 2298-2308
2298
Solubility studies with Ethocel ether derivatives have given Hansen partial solubility parameters in the range of nonpolar (ad) = 16.6-17.3, polar (6 ) = 6.6-8.3, and hydrogen bond (Sh) of 8.5-10.0 MPa@
Literature Cited Barton, A. F. M. Handbook of Solubility Parameters and Other Cohesion Parameters; CRC Press: Boca Raton, FL, 1983. Barton, A. F. M. Handbook of Polymer-Liquid Interaction Parameters and Solubility Parameters; CRC Press: Boca Raton, FL, 1990. Dante, M.F.;Bittar, A. D.; Caillault, J. J. Mod. Paint Coat. 1989, Sept, 46-51. Ekman, K. H.; Linfberg, J. Svom. Kem. B 1966,39,89-96. Entwistle, C. A.; Rowe, R. C. J. Pharm. Pharmacol. 1979, 31, 269-272. Hansen, C. M. J. Paint Technol. 1967,39(505), 104-117. Hansen, C. M.Znd. Eng. Chem. Prod. Res. Dev. 1969,8(l),2-11. Hanssn, C. M.;Andereen, B. H. Am. Znd. Hyg. Assoc. J. 1988,49(6), 301-308. Hercules Inc. Klucel Hydroxypropyl cellulose; Technical Information Bulletin VC-477C; 19&4; p 5. Kent, D. J.; Rowe, R. C. J. Pharm. Pharmacol. 1978,30,808-810.
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Received for review March 25, 1991 Revised manuscript received June 3, 1991 Accepted June 26, 1991
PROCESS ENGINEERING AND DESIGN Fluidized-Bed Reactor for Methanol Synthesis. A Theoretical Investigation K. M. Wagialla and S. S. E. H. Elnashaie* Chemical Engineering Department, College of Engineering, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia
Fluidized-bed technology is compared against fixed-bed technology for the production of methanol from synthesis gas. The two-phase theory of fluidization is used to model and simulate the fluidized bed. The model takes into consideration the change in the number of moles accompanying the reaction. An industrial modern quench type fixed-bed methanol synthesis converter is used as a basis for comparison. The fluidized bed resulted in a 30% improvement in the methanol production rate. The efficiency of the fluidized bed is due to the elimination of diffusional limitations, giving rise to an effectiveness factor very close to unity, and also because of the shift of equilibrium to more favorable conditions, which is due to the removal of products of reaction by diffusion from the dense phase to the bubble phase. Simulation of a fluidized-bed case with minimized effect of back-mixing shows a 51.1% improvement in production rate over the industrial fixed-bed reactor. 1. Introduction
Improvements in production efficiency of important chemicals by only a few percents can sometimes result in significant profit increases, energy conservation, and environmental protection, especially for a chemical such as methanol with a worldwide annual production rate of 3.6 X lo6 tons (Schack et al., 1989). Higher conversions in conventional methanol converters have been thwarted by equilibrium thermodynamic limitations due t~ the reversibility of the reaction(s) involved (Graaf et al., 1990). Also diffusional limitations due to relatively large catalyst particles cannot be eliminated in
* To whom correspondence should be addressed.
fixed-bed configurations by usage of smaller particles because of the attendant excessive pressure drop in the reactor. Research efforts for improvement of production efficiency have been largely directed toward the search for more active catalysts and improvement of fixed-bed-reactor design configurations to achieve better energy utilization, smaller reactor size, and higher per-pass conversion. Of late there has been a growing interest in the exploration of the technical and economic viability of reactor types other than the conventional fixed-bed design (Ozturk et al., 1988; Kuczynski et al., 1987a,b). With regard to catalysis, a very significant change occurred in the late sixties when IC1 (Imperial Chemical Industries) introduced its low-pressure methanol synthesis
0888-5885/91/2630-2298$02.50/00 1991 American Chemical Society
