121
I n d . Eng. Chem. Res. 1990,29, 121-125 Groves, F. R. Amine Removal from Waste Water by Ligand Exchange. Chem. Eng. Commun. 1984,31, 209-221. Groves, F. R.; White, T. Mathematical Modelling of the Ligand Exchange Process. AZChE J. 1984,30,494-496. Hassan, S. S. M.; Iskander, M. L.; Nashed, N. E. Spectrophotometric Determination of Aliphatic Primary and Secondary Amines by Reaction with p-Benzoquinone. Talanta 1985,32, 301-305. Helfferich, F. Ligand Exchange I. Equilibria. 11. Separation of Ligands Having Different Coordinative Valences. J. am. Chem. SOC.1962,84, 3237-3245. HO11, W.; Sontheimer, H. Ion Exchange Kinetics of the Protonation of Weak Acid Ion Exchange Resins. Chem. Eng. Sci. 1977, 32, 755-762. Jeffrey, M. D. Removal of Ammonia from Waste Water Using Ligand Exchange. M.S. Thesis, Louisiana State University, 1977. Satterfield, C. N. Mass Transfer in Heterogeneous Catalysis; M.I.T. Press: Cambridge, MA, 1970.
Thomas, H. C. Heterogeneous Ion Exchange in a Flowing System. J. Am. Chem. SOC.1944,66, 1664-1666. Villadsen, J. V.; Michelaen, M. L. Solution of Differential Equations by Polynomial Approrimation; Prentice Hall: Englewood Cliffs, NJ, 1978. Wilke, C. R.; Chang, P. C. Correlation of Diffusion Coefficients in Dilute Solutions. AZChE J. 1955, I , 264-270. Wilson, E. J.; Geankoplis, C. J. Liquid Mass Transfer at Very Low Reynolds Number in Packed Beds. Znd. Eng. Chem. Fundam. 1966, 5, 9-14. Yoshida, H.; Kataoka, T.; Fukikawa, S. Kinetics in Chelate Ion Exchange: I. Theoretical Analysis, 11. Experimental. Chem. Eng. Sci. 1986, 41, 2517.
Received for review J u n e 6, 1988 Revised manuscript received September 1, 1989 Accepted September 26, 1989
GENERAL RESEARCH Selectivity for High -Value Products in Process Biotechnology Kieran I. Ekpenyong* Department of Chemistry, University of Jos, Jos, Nigeria
J. Ray Walls Department of Chemical Engineering, Teesside Polytechnic, Middlesbrough, U.K.
Amides are manufactured from fatty acids by reaction with ammonia in a pressure autoclave. Under pressure, the initial formation of the ammonium soap is a fast reaction that is quickly established. The soap then quickly dehydrates to form the amide and subsequently the nitrile product. Nitriles are strongly colored and need to be removed by refining them from the amide product. This paper examines the reactor operating conditions used to optimize the amide production and minimize the subsequent refining operation in the reaction of erucic (docosenoic) acid with ammonia. The progress of the reaction with time was followed by using infrared spectroscopy for the detection of amide and nitrile groups. This showed conclusively that the reactions proceeded consecutively. Rate and equilibrium constants are reported as a function of temperature. The conclusions based on a laboratory scale were verified by using a pilot-plant autoclave. Fatty acids extracted from vegetable oils and animal fats are used widely industrially either directly or as raw materials to other chemical compounds (Gunstone, 1987). Esterification of fatty acids, which includes amidation and nitrilation of uncatalyzed and catalyzed reactions, has been discussed in detail (Reid et al., 1958). Fatty acid amides are incorporated into low-density polyethylene and polypropylene films as slip agents. Other important commercial uses are as lubricating additives for power transmission units, rubber and textile manufacture, and printing inks. The fatty acid-ammonia reaction is a well-known route to amide manufacture. The reaction proceeds through an intermediate ammonium salt (soap) which decomposes to
* T o whom correspondence should be addressed. 0888-5885/90/2629-0121$02.50/0
the amide and/or nitrile. The following reaction scheme is therefore proposed: RCOOH
+
NH3
RCOONH4
FA
(3) -2H20
RCN
AS (1)
J 1
-W
RCONH2 (2)
-Hz0
RCN
Here FA represents the fatty acid, AS the intermediate ammonium salt, RCONHzthe amide, and RCN the nitrile.
0 1990 American Chemical Society
122 Ind. Eng Chem. Res., Vol. 29, No. 1, 1990
I20
100
,.
-? z 4
50
! \ \ 0
?
L
5
5
178'C
15
12
Time
(hrs)
01 I.
2
6
8
IO
Time ( H r s )
Figure 1. Acid number (AN) and conversion as a function of time.
Figure 2. 1 / A ,
Amide and nitrile formation reactions are temperaturedependent. In this report, we discuss our investigation of the erucic acid (EA)-ammonia reaction in which erucamide (EM) and erucyl nitrile (EN) were obtained as products.
at this temperature, cooled to room temperature, and titrated against standard 0.5 M NaOH solution, using phenolphthalein as the indicator. NH3 was then passed through, and sampling undertaken at chosen time intervals thereafter. Initial and instantaneous acid concentrations were determined from the titration samples. Runs were made at 145,155,178,206,225,255,274,303,and 313 "C with fresh acid charge in each case. Temperatures were recorded to within f l . O "C accuracy, generally crosschecked with a chrom-A1 digital thermometer. The acid concentration in the laboratory reactor varied between 2.5 and 0.5 mol dm-3 for 10 h and in the pilot scale between 1.34 and 0.25 mole dm-3 in 21 h. Reaction product mixtures obtained at temperatures higher than 200 "C were additionally analyzed gas chromatographically for their nitrile content, while the infrared spectra of reaction samples at 313 "C were also obtained from reaction samples taken hourly.
Experimental Section The apparatus consisted of a glass reactor fitted with a stirrer and thermometer and heated by means of a heating mantle. The gas mixture (N2/NH3)was generally scrubbed over mineral oil before entering the reactor. The unreacted ammonia leaving the reactor was washed over distilled water, while the exit gas was vented off. The pilot-scale reactor consisted of a steam-heated 60-L autoclave, fitted with a stirrer and pressure and temperature control devices. N2 and NH, gas lines, meeting at a Tjunction and fitted with nonreturn valves, led directly into the reactor. Runs were generally carried out at reactor pressures of 18-20 psig and temperatures of 138-140 "C. The NH3 flow rate for the laboratory scale was 0.1 L/ min; that of the pilot scale (not measured) was considerably higher. The reagents consisted of industrial-grade erucic acid, containing predominantly (89.5%) CZ2fatty acid with a double bond a t the CI3-Cl4 carbons. Standard aqueous 0.5 M NaOH solution was prepared from analytical-grade NaOH pellets. Phenolphthalein was a 1% solution in neutral methanol. For a typical run, the reactor was charged with acid (120-130 g for the laboratory scale, 25.5 kg for the pilot scale) and heated under a N2 blanket to the desired temperature. An appropriate sample volume was withdrawn
-
1/A, versus time plot.
Results and Discussion Figure 1 shows a plot of the acid numbers (AN) and conversions as a function of the reaction time at the temperatures indicated. The experimental data obtained at 145,155, and 178 "C were subjected to analysis and found to fit very closely the second-order integrated rate law: 1 / A , 1/Ao = k2t (1') where A, is the instantaneous acid concentration, A , the initial acid concentration, k 2 the reaction rate constant, and t the reaction time. A plot of 1/A, -. l / A o against t is shown in Figure 2, from which k 2 has been determined.
Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990 123
J
1
3360
2i40
1720
I475
W a v e number
(cm-'1
Figure 3. Infrared spectrum of samples a t 313 "C after 2 (a), 3 (b), and 6 h (c) Table I. Amide (EM) and Nitrile (EN) Concentrations at 313 OC duration, h EM, % EN, % 1 60.4 39.6 2 29.8 70.2 22.1 77.9 3 4 15.6 84.4 5 13.7 86.3 6 13.6 84.4 7 12.8 87.2 Table 11. Infrared Absorption Band Positions of Reactants and Products g r o w absorbing origin band Dosition, cm-' c=o COOH of EA 1720 c=o CONHz of EM 1650 c=c EA, EM, EN 1425, 1475 C=N EN 2240 N-H CONHz of EM 3190, 3360
Figure 3 shows typically the infrared spectra of samples at 313 "C after 2,3, and 6 h, respectively. The respective absorption bands relevant to this discussion are shown in Table 11. In Figure 4 is shown the EA/EM/EN interconversions at 313 "C. The EN selectivity as a function of reaction time and temperature is shown in Figure 5. Table I shows the amounts of EN formed as a function of reaction time absorption band at 313 "C. Table I1 gives the infrared (Et) positions of reactants and products and Table I11 the kinetic parameters of the reaction. Considering the very close fit of the data points as plotted according to eq 1 (Figure 2) and the relatively higher concentration of acid (EA) with respect to NH, in the original mixture, the conclusion can be made that equimolar concentrations of EA and NH, are reacting. The amide (EM) constitutes the predominant reaction product for reactions below 200 "C. This is confirmed by the results of Figure 5, which shows a nitrile concentration of well below 10% at 206 "C, the percentage difference being due to the amide. The product selectivity is in favor of erucyl nitrile with an increase in reaction temperature above 200 "C (Table I and Figure 5). The spectral band assignments (Table 11) are well-correlated in the literature
.
I
C
0
-
0
C 0 C
u
\
0
I
4
5
6
Time
(Hours)
7
Figure 4. Reaction profile of EA, EM, and EN at 313 OC.
(Pasto and Johnson, 1969).'Significant in Figure 3 is the increasing intensity of the amide NH and carbonyl stretching absorption bands (3190, 3360, and 1650 cm-l, respectively) as the reaction progresses. These bands attain a maximum after 3 h of reaction at 313 "C (Figure 3). The maximization of the NH and CONHz carbonyl bands is accompanied by a decrease in the carboxyl carbonyl (COOH) absorption band at 1720 cm-', thus indicating the gradual consumption and depletion of EA in the reaction mixture. In all cases, the band at 1425 cm-' due to the C13-C14 double bond of EA remained unchanged.
124 Ind. Eng. Chem. Res., Vol. 29, No. 1, 1990
Table 111. Kinetic Parameters mode
temD. K
10%. L/(mol-min)
K
(A) Amide Formation
lab scale lab scale lab scale pilot scale pilot scale E = 10.2 kcal/mol A = 4.1 x 105 mode
418 1.60 12.1 428 2.42 20.6 172.1 451 4.90 396 8.10 413 9.20 lab scale lab scale temp, K i04k,, min-I (B) Nitrile Formation lab scale 479 2.5 lab scale 528 15.0 lab scale 586 112.2 E = 19.0 kcal/mol" E = 11.1 kcal/molb A = 1.2 x 105
"This work. *Cavalli et al. (1987).
The kinetic data obtained at 313 "C fitted very closely the mathematical model for a first-order reaction in which EN is formed from EM, namely, (2' 1 In (A,/Ao)= k,t
O
;
2
7
L
5
f Time
7
e
(hcs)
Figure 5 . EN selectivity as a function of time and temperature.
Equally significant is the absorption band at 2240 cm-' due to the CN (nitrile) functional group of erucyl nitrile (EN). In principle, the EN concentration in the reaction mixture would be expected to be linearly related to the intensity of the 2240-cm-' band and could thus be used for EN determination (Ekpenyong and Okonkwo, 1983; Buswell and Link, 1964). It is to be noted, however, that at higher concentrations this linearity is not generally observed, which is also the case here. Also observable in the spectra is the rapid drop in the band intensities of NH and CONH, within a period of 3 h, with the 2240-cm-' band absorption remaining intact. This indicates the gradual conversion of the amide (EM) to the nitrile (EN) during this period. The end product, a dark-brown liquid, predominantly EN, was obtained, in clear contrast to the amide product, which is solid at room temperature. The preponderance of the nitrile product a t higher temperatures (>200 "C) was confirmed by gas chromatographic analysis and is shown in Table I and Figure 5. The experimental data explain unequivocally the conversion of EM to EN in a consecutive reaction step (reaction 2 in the reaction scheme and also Figure 3). An important requirement that rules out the predominance of reacton 3 in the reaction scheme is as follows. If reaction 3 predominated over reaction 2, then the IR spectra of the samples would be expected to display all other absorption bands except those due to the NH and CONH, functional groups, which are present in EM but not in EN. This is not the case, and this fact lends strong support to our conclusion of consecutive EN formation from EM (Figure 3). The consecutive nature of the EM/EN conversion reaction, which is so rapid at the reaction temperature of 313 "C as to limit complete display of features for EM formation and decomposition, is nevertheless brought out in the plot shown in Figure 3.
Here A. is the initial amide concentration and A , the concentration a t time t. A plot of In (A,/Ao)against t enabled the determination of k l from the slope. Also by use of values obtained at other temperatures, an Arrhenius plot enabled the determination of the activation energy ( E )and the preexponential factor ( A ) . Pilot-scale studies which yielded the amide principally gave results comparable to the laboratory scale. The order of magnitude of the activation energy for nitrile formation in the laboratory scale (19.0 kcal/mol) compares favorably to that of benzonitrile formation (11.1 kcal/mol) in a catalyzed system (Cavalli et al., 1987). These are shown in Table 111.
Conclusion Erucic acid and ammonia react to form predominantly ercamide in a second-order reaction at temperatures below 200 "C. A t higher temperatures, selectivity in product formation favors erucyl nitrile. Mechanistically, erucyl nitrile is formed consecutivelyfrom erucamide rather than in a parallel reaction step. Minimization of product coloration resulting from nitrile formation requires that amide production reactions be carried out a t well below 200 "C. Industrially, in fact, a temperature range of 185-190 "C is used.
Acknowledgment We acknowledge laboratory equipment fabrication by Les Alexander and the use of research facilities of the Chemical Engineering Department at Teesside Polytechnic, England.
Nomenclature A , = initial acid concentration, mol dm-3 A , = instantaneous acid concentration, mol dm-3 AS = ammonium salt A = preexponential factor FA = fatty acid E = activation energy, kcal/mol EA = erucic acid EM = erucamide E N = erucyl nitrile k , = first-order reaction rate constant, min-'
Ind. Eng. Chem. Res. 1990, 29, 125-128 k 2 = second-order reaction rate constant, dm3 mol-’ min-’ K = equilibrium constant Registry No. EA, 112-86-7; EM, 112-84-5; EN, 73170-89-5;
125
Ekpenyong, K. I.; Okonkwo, R. 0. Determination of Acrylonitrile/ Methylmethacrylate Copolymer Composition by Infrared Spectroscopy. J . Chem. Educ. 1983,60, 429-430. Gunstone, F. D. Oils and fats; production, consumption, availability and chemical reactions. Chem. Znd. 1987,43-44. Pasto, D. J.; Johnson, C. R. Organic Structure Determination; Prentice-Hall: Englewood Cliffs, NJ, 1969; pp 121, 127, 130. Reid, E. E.; Marven, L. W.; John, W. Esterification Reactions. In Unit Operations in Organic Synthesis, 5th ed.; Groggins, P. H., Ed.; McGraw-Hill: New York, 1958; pp 694-749.
NH3, 7664-41-7.
Literature Cited Buswell, K. M.; Link, W. E. The Quantitative Determination of Small Amounts of Nitrile in fatty Acids. J. Am. Oil Chem. SOC. 1964, 41, 717-719. Cavalli, P.; Cavani, F.; Manenti, I.; Trifiro, F.; El-Sawi, M. Kinetic and Mechanistic Analysis of Toluene Ammodixation to Benzonitrile on Vanadium-Titanium Oxides. Znd. Eng. Chem. Res. 1987,26, 804-810.
Received for review January 9, 1989 Revised manuscript received September 1, 1989 Accepted October 20, 1989
COMMUNICATIONS Vapor Pressures of Pure Substances A generalized vapor pressure equation which is applicable to all substances has been developed. In addition to the critical pressure, temperature, and density (or Z,) it was necessary to introduce two other critical properties: S,, the slope of the critical density isometric for the gas phase; and K , a parameter that relates the apparent slope of the vapor pressure curve a t the critical point to the ratio S,/Z,. The equation contains two terms: the first applies to the noble gases and the second to the quantum gases, whereas all other substances contain both terms. For nonpolar and slightly polar substances, the five critical properties can be accurately calculated from vapor pressure and liquid density data in the vicinity of the normal boiling point. For any substance, the accuracy of the equation is comparable to the best available equation developed specifically for the substance. The object of this investigation was to develop an accurate generalized vapor pressure equation that uses as parameters the pressure-volume-temperature (PVT) properties at the critical point. At the critical point, thermodynamic considerations dictate that the slope of the vapor pressure curve (dP/dT)Tcis equal to the slope of the gas-phase critical density isometric (dP)dT)d,T,: (w/d ) T~,= (dp/ T )d , ~ , (1)
By use of the data for argon, hydrogen, nitrogen, propane, heptane, ammonia, and water, the following equation was developed:
Using the slope of the gas isometrics in dimensionless (reduced) form ( S ) ,we have s = (dP/dT)d/Rd (2)
For hydrogen and helium, the first bracketed term must be set equal to zero. Since S , for the noble gases is close to 1.782, the second bracketed term has a value close to zero for these gases. K is the ratio of the “apparent” slope of the vapor pressure curve at the critical point to the true slope (SC/Zc).The determination of K for nitrogen is shown in Figure 1. Note that in the critical region (T, > 0.99) the slope of the vapor pressure curve increases to a value that is 6% higher than the apparent slope. The term 0.94 in eq 5 is the value of K for the noble gases and the nonpolar diatomic gases. The apparent slope at the critical point (K(S,/ZJ)will be used instead of the true slope so the determination of K for a substance is required in order to relate the vapor pressure to the PVT properties S , and 2,. Also, how the molecular structure affects the value of K became an important part of this investigation. The integral form of eq 5 is as follows:
Since Rd = P / T Z where 2 is the compressibility factor, then (dP/dT),j(T/P) = S / Z
(3)
or (dpr/dTr)d,(Tr/pr) = S / z
(3’ )
where d,, P,, and T, are the reduced density, pressure, and temperature: d, = d / d c P, = P/P, TI = T/T,
At the critical point, (@r/dTr)Tc = (dpr/dTr)dc = Sc/zc
L
J
(4)
Thus, critical point criteria dictate that a generalized vapor pressure equation must include both S, and 2, as parameters. 0888-5885/90/2629-0125$02.50/0
3[
2
- 1](T;’
0 1990 American Chemical Society
-1
+ In T,)
(6)
