Ind. Eng. Chem. Res. 1989,28, 622-630
622
Registry No. DLA, 6144-28-1; SAE 1010, 12725-33-6; cupronickel 70-30, 11114-43-5; zinc, 7440-66-6.
Literature Cited ASTM Committee G-1 Standard Practice for Preparing, Cleaning, and Evaluating Corrosion Test Specimens. ASTM Stand. 1981, 1501-1504. Biddle, T. B.; Meehan, R. J.; Warner, P. A. Standardization of Lubricity Test. Report AFWAL-TR-87-2041, 1987; Air Force Wright Aeronautical Laboratories, Aero Propulsion Laboratory (AFWAL/POSF), Wright-Patterson AFB, OH. Black, B. H. M.S. Thesis, Southeastern Massachusetts University, North Dartmouth, MA, 1986. Black, B. H.; Hardy, D. R.; Wechter, M. A. J. Chromatogr. 1988,437, 203-210. Dacre, B.; Wheeler, P. A. Adsorption of Lubricity Additive Components, Part 3-Kinetics of Adsorption. Technical Report AC/ R/33, 1980; Royal Military College of Science, Shrivenham, Swindon, Wiltshire, England; Contract AT/2160/025, Procurement Executive, Ministry of Defense, London. Dacre, B.; Savory, B.; Wheeler, P. A. Adsorption of Lubricity Additive Components, Part 1. Technical Report AC/R/ 13, 1977; Royal Military College of Science, Shrivenham, Swindon, Wiltshire, England; Contract AT/2160/025 ENG D, Procurement Executive, Ministry of Defense, London. Dacre, B.; Wheeler, P. A.; Savory, B. Adsorption of Lubricity Additive Components, Part 2. Technical Report AC/R/BO, 1979; Royal Military College of Science, Shrivenham, Swindon, Wiltshire, England; Contract AT/2160/025/ENG D, Procurement Executive, Ministry of Defense, London. Goodger, E.; Vere, R. Aviation Fuels Technology; MacMillan: Houndmills, Basingstoke, Hampshire, England, 1985. Grabel, L. Lubricity Properties of High Temperature Jet Fuel. Report AD-A045467,NAPTC-PE-112,1977; Naval Air Propulsion Test Center, Trenton, NJ.
Hardy, D. R.; Black, B. H.; Wechter, M. A. J . Chromatogr. 1986,366, 351-361. Hillman, D. E.; Paul, J. I.; Cobbold, D. G. Recent Developments in the Petroleum Industry; Applied Science: Barking, Essex, England, 1977. Masters, A. I.; Weston, J. L.; Biddle, T. B.; Clark, J. A.; Gratton, M.; Graves, C. B.; Rone, G. M.; Stoner, C. D. Additional Development of the Alternate Test Procedure for Navy Aircraft Fuels. Report NAPC-PE-16OC, 1987; United Technologies Corporation, Pratt & Whitney; Contract N00140-84-C-5533, Naval Air Propulsion Center, Trenton, NJ. Moses, C. A.; Callahan, T. J.; Cuellar, J. P., Jr.; Dodge, L. G.; Likos, W. E.; Naegeli, D. W.; Valtierra, M. L. An Alternate Test Procedure to Qualify Fuels for Navy Aircraft; Report NAPC-PE-l45C, 1984; Southwest Research Institute; Contract N00140-80-'2-2269, Naval Air Propulsion Center, Trenton, NJ. Petrarca, J., Jr. Lubricity of Jet A-1 and JP-4 Fuel (as Indicated by Wear Friction). Report AD-784772, AFAPL-TR-74-15, 1974; Air Force Aero Propulsion Laboratory, Wright-Patterson AFB, OH. QPL-25017-14 Qualified Products List of Products Qualified Under Military Specification MIL-1-25017, Inhibitor, Corrosion/Lubricity Improver, Fuel Soluble (Metric), 1984; U.S. Air Force, ASD/ ENES, Wright-Patterson AFB, OH. Vere, R. A. Soc. Automot. Eng. 1969,2237-2244, SAE Reprint Paper 690667. Wechter, M. A. Quantitative Determination of Corrosion Inhibitor Levels in Jet Fuels by HPLC. Final Report, 1986; Southeastern Massachusetts University; Contract N00014-85-M-0248, Naval Research Laboratory, Washington, DC. Wheeler, P. A.; Dacre, B. Adsorption of Lubricity Additive Components, Part 4 (Final Report). Technical Report AC/R/44, 1980; Royal Military College of Science, Shrivenham, Swindon, Wiltshire, England; Contract AT/2160/025, Procurement Executive, Ministry of Defense, London. Received for review June 15, 1988 Accepted January 23, 1989
A Comparative Chromatographic Study of Liquid Adsorption and Diffusion in Microporous and Macroporous Adsorbents Yue-sheng Lint and Yi Hua Ma* Department of Chemical Engineering, Worcester Polytechnic Institute, Worcester, Massachusetts 01609
Mathematical analysis is presented for mass transfer in liquid chromatographic columns packed with microporous and macroporous adsorbents using a pore diffusion model and a pore diffusion and adsorption model. T h e numerical solutions and moment equations SLOWthat the two models are mathematically identical, but with different values for the adsorption equilibrium constant, K,, and the effective intraparticle diffusivity, De. The experimental results on De for liquids in silicalite crystals (micropores) and an activated alumina (macropores), however, suggest that the pore diffusion model is appropriate for microporous adsorbents, and the pore diffusion and adsorption model is suited for macroporous adsorbents. Experimental results also show that values of Defor liquids are of a similar order as those for gases in silicalite and are about 3 orders of magnitude smaller than those for gases in the alumina. With increasing applications of adsorption processes in liquid purification, separation, and downstream bioprocessing, the study on liquid-phase diffusion as well as adsorption equilibrium in porous solids, which had been ignored for a long time primarily because of the experimental difficulty associated with the batch methods, has gained much more attention in the past few years. Recently, with the growing availability of liquid chromatog-
* To whom
all correspondence should be addressed. Present address: Material Science Group, Department of Chemical Technology, University of Twente, 7500 AE! Enschede, T h e Netherlands. 0888-5885189 f 2628-0622$01.50/0
raphy instruments, the chromatographic techniques (e.g., HPLC) have been extended to the determinations of diffusion as well as adsorption for liquids in porous materials (Ma and Lin, 1987; Ma et al., 1988). The recently reported chromatographic studies on liquid-phase adsorption and diffusion were separately focused on microporous zeolite crystal (Ma and Lin, 1987; Lin and Ma, 1988; Ching and Ruthven, 1988), macroporous aluminas (Ma et al., 1988), and bipore structured pellets (Ho et al., 1987). A comparative study on liquid adsorption and diffusion between microporous and macroporous adsorbents, therefore, can provide a better understanding of the liquid-transport phenomena in porous adsorbents. 0 1989 American Chemical Society
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 623 Table I. Two Models for Chromatograohic Columns model A model B kinetic mechanism in micropores pore diffusion pore diffusion external fluid-particle film transfer pore adsorption external fluid-particle film transfer bulk-phase volume bed void space: v, = vc+, bed void space plus particle pore space: V, = Kucera (1965), examplesa of similar models in lit Rosen (1952), Deisler and Wilhelm (1953), Masamune and Smith (1964, 1965),, Schneider and Smith (1968),,# Rasmuson and Nertnieks (19801, Hashimoto and Smith (1973)b,# Weber and Liang (1983)b Kawazoe and Takeuchi (1974)b Raghavan and Ruthven (1983), Haynes (1975)b Chiang et al. (19841, Saminathan (1979)b Ruthven (1984),b Raghavan and Ruthven (1985)b O s
vcfb
+ Vc(l- f b ) f p
= single pore models; b = bipore models; q = models with finite pore adsorption rate.
The direct time domain analysis was applied in the previous HPLC studies for the determinations of liquid diffusion in silicalite crystals mainly because the studied adsorption systems are nonlinear in their adsorption isotherms (Ma and Lin, 1987; Lin and Ma, 1988). The moment method was also used to study liquid diffusion in relatively larger adsorbent particles (larger than 50 pm in diameter) due to the mathematical simplicity of this method (Ho et al., 1987; Ching and Ruthven, 1988). However, although most industrially available zeolite crystals are in very small powder form (1-5 pm) and the larger zeolite crystals cannot be easily synthesized, no study has been reported on the application of the moment method for the determinations of liquid intracrystalline diffusion in very small zeolite crystals. This was mainly due to the fact that very small particles coupled with liquids as carrier fluids could result in a rather high-pressure drop over the column, and under such conditions, it is difficult to maintain a uniform liquid flow in the column (Lin, 1988). With the extensive application of the Chromatographic techniques for bulk separation, purification, and physicochemical parameter determinations, many mathematical models have been developed for mass transfer in chromatographic columns packed with porous adsorbents. There exist two groups of models in the literature with different mass-transfer mechanisms in the adsorbent micropores, as summarized in Table I. For the adsorbents with different pore structures, however, the application of the two group models appeared to be arbitrary. By analyzing two bipore models, Lee and Tan (1987) recently suggested that the model with microporous diffusion and that with both microporous diffusion and adsorption be applied to different adsorbents depending on their pore size distribution. However, no experimental results were given. Furthermore, the existing models were mainly applied to mass transfer in gas chromatographic columns. Therefore, a comparison of the application of these models between gas chromatographic columns and liquid chromatographic columns will provide substantial information necessary for a better understanding of the transport processes occurring in a liquid chromatographic column. With different pore structure and chemical nature, the molecular sieve silicalite and aluminas represent two groups of porous adsorbents: microporous solids with pore size of the same order of magnitude as the adsorbate molecules and macroporous solids with pore size more than 1 order of magnitude larger than the adsorbate molecule size. It is interesting to use silicalite and alumina as two sample adsorbents to investigate liquid adsorptive and diffusive properties in different types of porous adsorbents. Therefore, the objectives of the present work were to (1) examine the applicability of the different models to liquid chromatographic (LC) columns packed with different porous adsorbents, (2) demonstrate the use of moment me-
thod for the determination of liquid diffusion in very small industrially available zeolite crystals and to confirm the previously, reported liquid diffusion data in silicalite crystals obtained by the direct time-domain fitting method, and (3) investigate the difference in liquid diffusive and adsorptive properties in microporous and macroporous adsorbents.
Mathematical Analysis Modeling and Moment Equations. A mass balance for an adsorbate on a differential volume of mobile phase in an LC column gives the following partial differential equation (PDE) for the mass transfer in the mobile phase:
where q is the rate of adsorption by adsorbent particles
In eq 2, C, is the concentration in the adsorbent pore, defined as C, = adsorbate mass/pore volume of particles; De is the effective intraparticle diffusivity based on the total cross-sectional area of an adsorbent particle. Other symbols are identified in the Nomenclature section. The corresponding initial and boundary conditions (Danckwerts type) for mass transfer in the mobile phase are given as C(Z,t) = 0
at t = O
(3)
aC/aZ = 0
at Z = L
(4)
C(Z,t) = C,(t)
4 ac + Luaz
at Z = O
(5)
A microscopic mass balance for the adsorbate in an adsorbent particle also yields a set of PDE's for the mass transfer of the adsorbate in the adsorbent particle. The Table 11. PDE and BC for Mass Transfer in Particles PDEa BC at r = R model
,-A,-
P'
at
with
'
cp
at
C, = K e ( ~ ) C p (8)
ill ' "-p
-=-
ar
K
*xc
-
':c
- C,)
(9)
624 Ind. Eng. Chem. Res., Vol. 28, No. 5 , 1989
resulting PDE's and the boundary conditions a t the external particle surface for the two models as described in Table I are given in Table I1 (eq 6-9). The symmetrical boundary condition is taken as the other boundary condition for the particle at r = 0
dC,/dr = 0
(10)
When the moment equations listed in Table 111 are combined, the following HETP equations are obtained for the two models:
O'L 2dP H E T P = - = - + BU k2 Pe
(21)
where for model A
with the initial conditions
C,=O
at t = O
(11)
Cp or C is related to the absolute amount adsorbed (Kipling, 1965) based on the pore volume of the adsorbent particle. By use of the method as suggested by Carleton et al. (1978), the first and second central moments as defined by
JmC(L,t)tdt (12)
k =
C(L,t) dt
are derived from these two models and summarized in Table I11 (eq 14-17). The details of the derivation of the moment equations are given elsewhere (Lin, 1988). The first-moment equation for linear chromatography can also be derived from a macroscopic approach without considering the mass-transfer mechanisms and setting up the PDE's (Lin, 1988). The resulting macroscopic moment equation is 1 k = - ( V , + mud(,) (18)
and for model B
B=
~,fi(K,+ 1)' [ I + O(K,
+ I)]'
[ d, ] +
3Kc
d,2
300,
with fi = (1 - t b ) t p / t b . The Peclet number, Pe, for liquids in the packed bed has been found to be essentially independent of carrier flow rate in the small flow rate variation range (Wen and Fan, 1975; Ma et al., 1988; Ching and Ruthven, 1988). The values of K , can be estimated by (Wilson and Geankoplis, 1966) Sh = (1.09/tb)Re0,33S~0.33 0.0016 < Re < 55 (24) Thus, according to eq 14 (or eq 16) and eq 21, plots of the experimental data of M versus 1/ U and H E T P versus U should give two straight lines. Numerical Solutions and Parametric Analysis. When the dimensionless variables defined in Table VI1 are introduced into eq 1-1 1, the following dimensionless equations are obtained for these two models: dX/d8 + d X / & $ + C) = cy d2X/d@' (25) Q = p ( x - 6 ~ ) at 4 = 1 (26) (27)
Q
where V , is the bulk phase volume (or hold-up volume) corresponding to the space where the adsorbate concentration a t equilibrium is uniform and equal to the bulkphase concentration. If equations for V , as defined in Table I and the other relations such as Q = U t b A and V , = AL are inserted into eq 18, one can obtain two firstmoment equations identical with eq 14 and eq 16 for these two models. Equating the first and second central moments for model A and model B yields the following relations: KdAI = Ke(B) -k (19) (20) The two similar equations were also reported by Lee and Tan (1987) from a moment analysis on two bipore models (with typographical errors in eq 19 and eq 21 in that paper).
X ( 8 , @ )= 0 Y(8,@,+) =0 dX/d8 = 0
> 1, eq 19 gives Ke(A!= Ke(B).) The measured intracrystalline diffusivities for liquids in silicalite (by model A) are found to be of the same order of magnitude as those for gases measured by sorption uptake and GC approaches (10-13-10-16 m2/s for C1-C4 paraffins and aromatics) (Wu et al., 1983; Chiang et al., 1984). This is probably because it is difficult to define the state of a diffusing molecule in the silicalite pore due to the fact that the size of the diffusing molecule is rather close to the pore size. Thus, both the gas and liquid molecules may exist in a similar state in the silicalite pores. It was also reported that the intracrystalline diffusivities in the X zeolite crystal for gases (sorption uptake method) (Ruthven and Doetsch, 1976) and liquids (LC methods) (Ching and Ruthven, 1988; Haq, 1983) are of the similar order of magnitude. However, the intraparticle diffusivities for liquids in the alumina (by model B) are 3-4 orders smaller than those for gases (10-5-104 mz/s for C,-C4 paraffins in an alumina with a pore size of 90 X m) (Saminathan, 1979). This is because in the alumina pore, with a pore size much larger than the size of the diffusing molecule, the liquid-diffusing molecules still being in a liquid state are different from gases. The lower values of De in alumina for liquids in comparison with those for gases are the results of the much smaller molecular diffusivity for liquids than for gases. The equilibrium results show that, in silicalite with the polar solvent (water), the adsorption equilibrium constants decrease with increasing adsorbate polarity. In the alumina with the nonpolar solvent (cyclohexane), the adsorption equilibrium constants increase with increasing adsorbate polarity. The values of Ke for most of the studied adsorption systems are much greater than unity. These measured equilibrium data are consistent with the hydrophobic nature for silicalite and the hydrophilic nature for alumina. Comparing the results of Ke and De determined by the moment method and direct time-domain fitting method, as given in Table VI, shows, in general, a fairly good agreement between the two methods, with excellent agreement for methanol since the methanol adsorption system is the only one that was in the linear isotherm range (Milestone and Bibby, 1981; Ma and Lin, 1987). For the three nonlinear systems (ethanol, 1-propanol, 1-butanol), the values of K , and De determined by the direct timedomain fitting method are greater than the values determined by the moment method. This is mainly because of the effect of the isotherm nonlinearity. For example, K, determined by the direct time-domain fitting method is the slope of the isotherm at infinite dilution, while K , determined by the moment method is an average isotherm slope between infinite dilution and injection concentration. Thus, for type I isotherms (d2C,/dC? 01, K, determined by the moment method is always smaller than or equal to K , determined by the direct time-domain fitting method, depending on the concentration of the injection sample. A detailed analysis of the effects of the nonlinear isotherm on LC responses will be given in a subsequent paper. Conclusions (1)The results of the mathematical and experimental analysis suggest that the pore diffusion model be used for the adsorption systems with pore size and adsorbate
Ind. Eng. Chem. Res., Vol. 28, No. 5, 1989 629 molecule size in the same order of magnitude. The pore diffusion and adsorption model is more suited for the adsorption systems with pore size more than 1 order of magnitude larger than the adsorbate molecule size. (2) The parametric study shows that the effects of adsorption equilibrium constant and intraparticle diffusivity on LC response peaks are similar to those on GC. However, the effects of the axial dispersion coefficient on LC are much smaller than those on GC. (3) Intraparticle diffusivities for liquids, in the microporous adsorbent silicalite crystal, are of similar order of magnitude as those for gases. On the other hand, in the macroporous adsorbent alumina, they are about 3 orders of magnitude smaller than those for gases. Adsorption equilibrium results are consistent with the hydrophobic nature for silicalite and the hydrophilic nature for alumina. (4)The HPLC moment method is shown to be applicable to the determinations of liquid diffusion in very small crystal powders with the consistent results between moment method and the direct time-domain fitting method. Acknowledgment Partial financial support provided by the Alcoa Separations Technology Division is gratefully acknowledged. The authors also thank Alcoa and Union Carbide Corporation for providing the materials. Nomenclature A = cross-sectional area or the LC column, m2 C = adsorbate concentration in the mobile phase, kg/m3 C, = equilibrium adsorbate concentration in the bulk phase, kg/ms Co = injection sample concentration, kg/m3 C, = adsorbate concentration in the adsorbent pore, kg/m3 C, = adsorbate concentration in the adsorbed phase in the pore, kg/m3 c n o . = c O v I / v c c b i kg/m3 De = effective intraparticle diffusivity, m2/s D1 = axial dispersion coefficient, m2/s d, = particle diameter, m K , = external particle-fluid film mass-transfer coefficient, m/s K , = adsorption equilibrium constant L = length of packing in the column, m m = mass of adsorbent particles packed in a column, kg Q = volumetrical flow rate, m3/s q = adsorption rate by a adsorbent particle, kg/m3/s R = radius of adsorbent particle, m r = radial pos-itionin adsorbent particle, m t = time, s tin = injection time, =V,/Q, s U = interstitial flow velocity, m/s V , = empty LC column volume, m3 VI = injection sample loop volume, m3 V, = LC column mobile-phase volume (hold-up volume), m3 up = adsorbent particle pore volume, m3/kg 2 = column axial direction position, m Pe = Peclet number for axial dispersion, =Ud,/D1 Re = particle Reynolds number, pBud,/pB Sc = Schmidt number, pB/p~D',s Sh = Sherwood number, K,dp/DoAB Greek Letters = density of liquid, kg/m3 p = first moment (mean), s tp = porosity of zeolite crystal cb = porosity of the packed column u2 = second central moment (variance), s2 pB
fi = (1- 6 b ) c p / t b r = square impulse function L i t e r a t u r e Cited Carleton, F. B.; Kershenbaum, L. B.; Wakehan, W. A. Adsorption in non-isobaric fixed bed. Chem. Eng. Sci. 1978, 33, 1239. Chiang, A. S. C. A Non-isobaric Chromatographic Method for the Study of Fine solids. Its Application to Adsorption and Diffusion in Zeolites. Ph.D. Thesis, Worcester Polytechnic Institute, Worcester, MA, 1982. Chiang, A. S. C.; Dixon, A. G.; Ma, Y. H. The determination of zeolite crystal diffusivity by gas chromatography, I: Theoretical, I1 Experimental. Chem. Eng. Sci. 1984, 39, 1451. Ching, C. B.; Ruthven, D. M. A liquid phase chromatographic study of sorption and diffusion of glucose and fructose in NaX and K X zeolites crystal. Zeolites 1988,8, 68. Deisler, P. F.; Wilhelm, R. H. Diffusion in beds of porous solids. Ind. Eng. Chem. 1953,45, 1219. Hag, N. Chromatographic Study of Diffusion and Sorption for Gases and Liquids in Type A and 13X Zeolites. Ph.D. Thesis, University of New Brunswick, Fredericton, New Brunswick, Canada, 1983. Hashimoto, N.; Smith, J. M. Macropore Diffusion in Molecular Sieve Pellets by Chromatography. Ind. Eng. Chem. Fundam. 1973,12, 353. Haynes, H. W. The determination of effective diffusion by gas chromatography, the domain solution. Chem. Eng. Sci. 1975,30, 955. Ho, C. H.; Ching, C. B.; Ruthven, D. M. A Comparative Study of Zeolite and Resin Adsorbent for Separation of Fructose-Glucose Mixture. Ind. Eng. Chem. Res. 1987, 26, 1407. Kawazoe, K.; Takeuchi, Y. Mass transfer in adsorption on bidisperse porous Materials. J . Chem. Eng. Jpn. 1974, 7, 431. Kipling, J. J. Adsorption from Solution of Non-Electrolytes; Academic Press: New York, 1965; Chapter 4. Komiyama, H.; Smith, J. M. Intraparticle mass transfer in liquid filled pores. AIChE J . 1974, 20, 728. Kucera, E. Contribution to the theory of chromatography, linear non-equilibrium elution chromatography. J. Chromatogr. 1965, 19, 237. Lee, T. Y.; Tan, C. S. Simulation of a fixed-bed adsorber consisting of bidisperse pellets. J . Chin. Inst. Chem. Eng. 1987, 18, 165. Lin, Y. S. Adsorption and Diffusion of Liquids in Porous Solids by HPLC. Ph.D. Thesis, Worcester Polytechnic Institute, Worcester, MA, 1988. Lin, Y. S.; Ma, Y. H. Liquid adsorption and diffusion of aqueous ethanols, propanols and butanols in silicalite by HPLC. ACS Symp. Ser. 1988, 368, 452. Ma, Y. H.; Lin, Y. S. Adsorption and diffusion of liquids in silicalite using HPLC. AIChE Symp. Ser. 1987, 83(No. 259), 1. Ma, Y. H.; Lin, Y. S.; Fleming, H. L. Adsorption and diffusion of polar and nonpolar liquids in aluminas by HPLC. AIChE Symp. Ser. 1988, 841(No. 2641, 1. Masamune, S.; Smith, J. M. Transient Mass Transfer in a Fixed Bed. Ind. Eng. Chem. Fundam. 1964, 3, 179. Masamune, S.; Smith, J. M. Adsorption rate studies-interpretation of diffusion and surface processes. AIChE J . 1965, 11, 34. Milestone, N. B.; Bibby, D. M. Concentration of alcohol by adsorption on silicalite. J . Chem. Technol. Biotechnol. 1981, 31, 732. Prasher, B. D.; Ma, Y. H. Liquid diffusion in microporous alumina pellets. AIChE J . 1977, 23 103. Raghavan, N. S.; Ruthven, r 'I. Numerical simulation of a fixedbed adsorption column by the method of orthogonal collocation. AIChE J . 1983,29, 922. Raghavan, N. S.; Ruthven, D. M. Simulation of chromatographic response in column packed with bidisperse structured particle. Chem. Eng. Sci. 1985, 40, 699. Rasmuson, A,; Nertnieks, I. Exact solution of a model for diffusion and transient adsorption in particles and longitudinal dispersion in nwked bed. AIChE J . 1980, 26, 686. Roseri .I. B. Kinetics of a fixed bed system for solid diffusion into spherical particles. J . Chem. Phys. 1952, 20, 387. Ruthven, D. M. Principles of Adsorption and Adsorption Processes; Wiley, New York, 1984. Ruthven, D. M.; Doetsch, I. H. Diffusion of hydrocarbons in 13X zeolite. AIChE J . 1976, 22, 882. Saminathan, M. A Study of Diffusion of Hydrocarbons in y-Alumina and Synthetic Faujasite by Gas Chromatography. Ph.D. Thesis, Worcester Polytechnic Institute, Worcester, MA, 1979.
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630
Satterfield, C. N.; Colton, C. K.; Pitcher, W. H. Restricted diffusion in liquids within fine pore. AIChE J. 1973, 19, 628. Schneider, P.; Smith, J. M. Adsorption rate constants from chromatography. AIChE J . 1968, 14, 762. Villadsen, J.; Michelsen, M. Solution of Differential Equation by Polynomial Approximation; Prentice-Hall: Englewood Cliffs, NJ, 1978; Chapters 2-4. Villadsen, J. V.; Stewart, W. E. Solution of boundary value problems by orthogonal collocation. Chem. Eng. Sci. 1967, 22, 1483. Weber, A. I.; Liang, S. Dual Particle Diffusion Model for Porous Adsorbent in Fixed Beds. Environ. Prog. 1983,2,3, 167.
Wen, C. Y.; Fan, L. T. Models for Flow Systems and Chemical Reactors; Mancel Dekker: New York, 1975. Wilson, E. J.; Geankoplis, C. J. Liquid Mass Transfer a t Very Low Reynold Number in Packed Bed. Ind. Eng. Chem. Fundam. 1966, 1 , 9.
Wu, P. D.; Debebe, A.; Ma, Y. H. Adsorption and diffusion of C6 and C, hydrocarbons in silicalite. Zeolites 1983, 3, 117. Received for review July 12, 1988 Revised manuscript received January 25, 1989 Accepted February 7, 1989
Polyethylene-Coated Urea. 1. Improved Storage and Handling Properties Omar A. Salman Products Department, Petroleum Petrochemicals and Materials Division, Kuwait Institute for Scientific Research, P.O. Box 24885, 13109 S a f a t , Kuwait
The effects of encapsulating urea prills with a low-density polyethylene (LDPE) film on improving the handling and storage characteristics of urea were investigated. Six tests were performed on uncoated and LDPE-coated urea samples: crushing strength, abrasion resistance, impact resistance, absorption-penetration test, caking tendency, and chemical compatibility with superphosphate. It was found that LDPE-coated urea is much superior to uncoated urea. In addition to being a controlled-release or slow-release fertilizer, its storage and handling properties are excellent. LDPE-coated urea granules are resistant to caking, abrasion, and crushing and are highly compatible when blended with superphosphate. Of the three primary plant nutrients-nitrogen, phosphorus, and potassium-nitrogen is lost most easily from the soil. Since the total loss is estimated to be between 30% and 5070,repeated application of nitrogen fertilizer becomes essential. This adds extra cost for materials and labor and causes inconvenience and a high solute concentration in the soil. One method of reducing nutrient losses is to use slowor controlled-release fertilizers. There are three types of these fertilizers: slightly soluble materials such as ureaformaldehyde; materials for deep placement such as urea super granules; and fertilizers coated with semipermeable or impermeable membranes. This paper deals with the last type, particularly polymer-coated urea. Coated fertilizers are physically prepared from granules of conventional fertilizers coated with materials that reduce their dissolution rate. Commercially available coated fertilizers can be divided into two categories: sulfur-coated urea (SCU) and polymer-coated urea. SCU has been under development by the Tennessee Valley Authority (TVA) since 1961 (Young, 1974). Sulfur was selected as a coating material because of its low cost. TVA started with coating small batches of urea in small rotating drums to continuous coating in a pilot plant with a capacity of 1 ton/h (Shirley and Meline, 1975). The first commercial polymer-coated fertilizers were developed by the Arthur Daniels Midland Co. (ADM). The main component of the coating is a copolymer of dicyclopentadiene with a glycol ester (Powell, 1968). Nutrients are released through osmotic exchange with moisture from the soil. The Sierra Chemical Co. currently produces this coated fertilizer under the trade name Osmocote. Two other polymer-coated fertilizers are produced commercially: Sierrablen and Agriform. Most of these products are based on ammonium nitrate mixed fertilizers and on single nutrients, according to the customer's requests. 0888-5885/89/2628-0630$01.50/0
In a previous paper (Salman,1988), the effect of coating urea prills with low-density polyethylene (LDPE) on reducing the release rate of urea was studied. In this paper, the physical characteristics of LDPE-coated urea compared to uncoated urea are investigated.
Experimental Procedure Materials. The urea from the Petrochemical Industries Co. had a nitrogen content of 46.0% and a particle size range of 0.5-2 mm. Low-density polyethylene (LDPE) was purchased from CDP Chemie, and laboratory-grade toluene was purchased from BDH Chemical Ltd. Apparatus and Method. A schematic diagram of the fluidized-bed equipment used for coating is shown in Figure 1. In a typical experiment, the product container is filled with about 1 kg of urea prills and is inserted into the apparatus and sealed pneumatically. The turbine is run for 5-10 min to preheat and fluidize the urea particles. Polyethylene pellets are dissolved in toluene by heating and stirring to give a polymer solution concentration of 10% by weight. The spraying process begins by starting the dosing pump and the atomizing air. Solvent vapors are collected by a tubular condensor. After the filmcoating solution is sprayed, about 200 mL of pure solvent is sprayed to clean the tubes and nozzle assembly. The final step is drying. The air temperature is raised to 80 " C , and the atomizing air is shut off. Drying time (usually 5-10 min) depends on the required moisture content. The physical properties of uncoated and LDPE-coated urea which are related to storage and handling characteristics were evaluated. Six tests were conducted: crushing strength, abrasion resistance, impact resistance, absorption-penetration test, caking tendency, and chemical compatibility with superphosphate. All tests were performed according to standard procedures (International Fertilizer Development Center, 1986). Two LDPE-coated urea products were evaluated: PCU-3 refers to 3% total C 1989 American Chemical Society
