Ind. Eng. Chem. Res. 1989,28, 1907-1910 Eckert, C. A.; Ziger, D. H.; Johnston, K. P.; Kim, S. Solute Partial Molar Volumes in Supercritical Fluids. J . Phys. Chem. 1986,90, 2738. McHugh, M. A. Extractions with Supercritical Fluids. Recent Developments in Separation Science; CRC Press: Boca Raton, FL, 1983; Vol. IX. Joffe, J.; Zuckbevitch, D. Prediction of Critical Properties of Mixtures; Rigorous Procedures for Binary Mixtures. Chem. Eng. Prog., Symp. Ser. 1967,63,43.
1907
Johnston, K. P. Supercritical Fluids. Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed.; John Wiley and Sons: New York, 1983; Supplement. Williams,D. F. Extraction with Supercritical Gases. Chem. Eng. Sci. 1981, 36, 1769.
Received for review April 5 , 1989 Revised manuscript received August 14, 1989 Accepted September 27, 1989
COMMUNICATIONS NazHPOaObtained by Double Decomposition of NaCl and H3P04with an Extractant A process for the production of Na2HP04 from NaCl and H3P04 has been developed. T h e double-decomposition reaction is promoted to completion by a water-immiscible primary amine. The percentage conversion can be as high as 96%. The product, disodium dodecahydrate (Na2HP04-12H20)containing less than 1% chloride ion (on dry basis), is obtained by cooling the resulting aqueous solution. Disodium phosphate (Na2HP04)is an important chemical, which can be used as a fire-proofing agent for wood and paper, a culture medium for antibiotics, and a water-softening agent. It can also be used for the production of sodium tripolyphosphate (STPP), which is one of the most important phosphates and is widely used as an ingredient in detergents and as a water-treatment agent. As with many other alkali phosphates, Na,HPO, is normally made by neutralization of phosphoric acid with sodium carbonate or hydroxide. Due to the high cost of sodium carbonate or hydroxide, some researchers have tried to use NaCl instead. The Onoda Cement Company (1966) proposed a process for preparing disodium phosphate from sodium chloride and phosphoric acid. In this process, the double-decomposition reaction between sodium chloride and phosphoric acid is carried out in a medium containing a water-miscible amine of low molecular weight. The miscibility of the amine with water results in a number of disadvantages, such as it is necessary to recover the amine adhered to the product and then a second solvent (methanol) must be used, and the mother liquor must be distilled to separate the amine for reuse. Therefore, an excessive amount of energy must be consumed. On the basis of the solvent-extraction technique, a process for the production of disodium phosphate from sodium chloride and phosphoric acid has been developed. The double-decomposition reaction in the present process is promoted by a water-immiscible extractant. The product is a crystal of disodium phosphate dodecahydrate (Na2HP04-12H20), which is obtained by cooling the resulting aqueous solution. The immiscibility of the extractant with water has the advantages of reducing the loss of the solvent and of a simple flow sheet. The experimental development of this process is described below. Experimental Section Reagents. The two amines in Table I were used in the selection of the extractant. The solvent phase was prepared by mixing the amine and diluent (odorless kerosene) in different proportions as desired. The aqueous solutions
were made of chemically pure grade reagents and deionized water. Measurement of the Distribution Ratio. The solvent phase and the appropriate aqueous solution were equilibrated by shaking in a separatory funnel for 5 min. Upon standing, the two phases separated with a clear interface, and the aqueous solution was then removed for analysis; the organic phase was reextracted twice with 0.5 N NaOH before analysis. The concentrations of chloride in both phases and the same concentrations of phosphoric acid or phosphate in the aqueous phase were determined by potentiometric titration with &NO3 and NaOH, respectively. The concentrations of phosphate in the organic phase were measured by spectrophotometry. Results and Discussion Selection of Extractant. The double-decomposition reaction in this process can be divided into the following two steps:
--
+ H3P04+ mS NaCl + NaH,P04 + nS NaCl
NaH2P04+ HCl-mS (1) Na2HP04+ HC1.nS (2)
where S is the organic solvent, the compounds with overhead bars are designated as organic phases and those without as aqueous phases. It has been found that all three kinds of amines, namely, primary, secondary, and tertiary, can be used to complete the first reaction (eq 1). However, to enforce the second reaction (eq 2) is more difficult due to the concentration of hydrogen ion, [H+],in the aqueous solution being much lower than that of the first one. The dissociation constants of H3P04 are pK, = 2.12 and pK2 = 7.21 (Weast, 1982-1983), so selecting a suitable extractant for the second reaction is a key problem. The effect of the two different amines on the conversion of NaH2P04in the second reaction is shown in Figure 1. It is obvious that much higher conversions can be achieved by using the primary amine N1923 than the tertiary amine N235 as an extractant to remove HC1 from the aqueous
0888-588518912628-1907$01.50/0 0 1989 American Chemical Society
1908 Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 Table I. Extractants type
code name
primary amine
N1923
tertiary amine
N235
structure
avMW
remarks
302
398
R , \ R-N R
similar to Alamine 336
,,I
R(R', R") = C~-CIO 100
75
m F7
50
25
s -a
B E O
N
b P 4 Q 4
&
Z
0 0
1.0
2.0 3.0 IN a C l l CNaA2P041
4.0
5.0
Mol0 r a t i o of t h e extraotant t o H,POk
Ratio of i n i t i a l N a C l concentration
2. Conversion of H3P04in the first step.
t o NaH2P04 c o n c e n t r a t i o n
Figure 1. Effect of different amines on the conveyion of NaH2P04. [NaCl] + [NaH2P04] = 2.5-2.6 M, t = 30 "C,[SI = 1.0 M, phase ratio E f R = 1.
solution. This may be due to the favorable steric structure of the primary amine. Conversion of the Reaction. Owing to the limited solubility of sodium chloride in phosphoric acid, it is necessary to carry out the double-decomposition reaction in two steps as mentioned above (eq 1and 2). In each step, the system involves the components NaC1, H3P04 (or NaH2P04), water, extractant and diluent in the feed, NaH2P04(or Na2HP04)in the aqueous phase, and HCl in the organic phase as products of the reaction. Thus, the number of components is 6, the number of phases is 2, and the number of degrees of freedom is 6. Under a given temperature and pressure, four variables have to be specified, e.g., the initial concentration of sodium chloride in the aqueous phase solution, i.e., [NaCl]; the initial concentration of phosphoric acid or sodium dihydrogen phosphate in the aqueous solution, i.e., [H3P04]or [NaH2P04];the goncentration of the extractant in the solvent phase, i.e., [SI; and the phase ratio, i.e., E / R . In order to show the effects of these four variables on the conversion of the reactions in a planar diagram, some variables must be combined as one parameter. Therefore, the conversion of these two reactions can be plotted as a function of the following two combined parameters: (1) using [NaC1]/[H3P04] at a constant concentration of H3PO4 or [NaCl]/ [NaH2P04]at a constant concentration of NaH2P04 to express the initial composition of the aqueous soluti_onfor these two reaction?, respectively; (2) using (E/R)([Sl/ [H3PO4I) or (E/R)([Sl/[NaH2P041)to express the total amount of the extractant used per mole of H3P04in the first reaction or per mole of NaH2P04in the second reaction. In Figures 2 and 3, the conversion of H3PO4 in the first reaction and the conversion of NaH2P04in the second reaction are plotted as a function of thesje two parameters, i.e., [NaCl]/[H3P04] and (E/R)([S]/[H,PO,]), or
V Oo
I
0.25
I
I
0.y
0.75
I
I
1.00
1.25
[PI -.ER CNaEpWqI
I 1.50
Hole r a t i o o f e x t r a c t a n t t o NaH2POq
Figure 3. Conversion of NaH2P04in the second step
[NaC1]/[H3P04]and (E/R)([S]/[NaH2P04]).It can be seen that the relations between the conversion of H3P04 and the parameter (E/R)([S]/[H3P04]),and the conversion of NaH2P04and the parameter (E/R)([S]/[NaH,PO,]) can be divided into the following three regions. In-the first region, the values of the parameter ( E / R)([S]/[H3P04])or (E/R)([S]/[NaH2P04])are much less than the stoichiometric value, namely, below 0.5, the conversions of H3PO4 or NaH2P04are increased linearly with increasing amount of the extractant used, and a straight line having a slope of one is obtained. This means that the values of m and n indicated in eq 1 and 2 are all equal to unity. In the_ second region, as the-value of the parameter (E/R)([Sl/ [H3P041)or (E/R)([SI/~NaH2PO41)gradually approaches unity, namely, from 0.5 to 1.0, the conversions of both reactions are also increased. However, the conversion curves deviate from the straight line having a slope of one so that the following two side reactions of the coextraction of H3P04 may take place in each case: H3PO4 + S H3PO4-S (3) -+
or
2NaH2P04+ S
-
Na2HP04+ H3P04.S
(4)
Ind. Eng. Chem. Res., Vol. 28, No. 12, 1989 1909 NH,
*
60 20
30
40
60
50
Temperature,
(1)
70
OC
Figure 4. Effect of temperature on the convergion of both steps. [H,PO,] = 2.35 M, (E/R)([Sl/[H$'O,] = 092, Initial conditions: (0) [NaCl]/[H,PO,] = 1.50; (A) [NaH2P04] = 2.50 M, (E/R)([S]/ [NaH,PO,]) = 0.81, [NaC1]/[NaH2P04] = 1.40.
._I
1 p . s
I
CRYSTALLIZATION
With increasing the concentration of sodium chloride in the aqueous solution (namely, the values of the parameter [NaCl]/ [H3P0,] or [NaCl]/ [NaH2P04]),the deviations of the conversion curves from the straight line having a slope of one are decreased due to the salting out effect of sodium chloride. Therefore, it is necessary to keep an excess amount of NaCl in the initial aqueous solution so as to obtain a higher conversion and to eliminate the undesirable side reaction. However, the value of [NaCl]/ [NaH2P04] must be limited in a reasonable range so as to keep the composition of the resulting aqueous solution situated in the desired region as shown in the ternary-phase diagram of the system NaC1-Na2HP04-H20 (Makin, 1957). When a stoichiometric amount of the extractant is used, the maximum conversion of H3P04into NaH2P04is attained. A conversion of 98% in the first reaction can be obtained when the initial concentration of phosphoric acid, [H3P04],and the value of [NaC1]/[H3P04]are equal to 2.35 M and 1.7, respectively. In the third region, the value of (E/R)([SI/[H,PO,I) or (E/R)([S]/[NaH2P04])is more than unity. Because the second undesirable side reaction (eq 4) occurs at a lower value of the parameter [NaCl]/ [NaH2P04],the conversion of H3P04 into NaH2P04for the first reaction is lowered. However, the conversion of NaH2P04into Na2HP04for the second reaction is still increased. As it is highly desirable to have the second reaction go to completion, an excess amount of the extractant is needed so as to promote the transferring of the second hydrogen ion in the form of HC1 into the solvent phase. When the value of ( E / R)([S]/ [NaH2P04])reached 1.4-1.5, the initial concentration of sodium dihydrogen phosphate and the value of [NaC1]/[H3P04] being equal to 2.5 M and 1.50, respectively, the conversion of NaH2P04into Na2HP04can be reached at 298%. Therefore, the overall percentage conversion of H3PO4 in these two steps can be as high as 96%. The effect of temperature on the conversion of these two reactions, under the condition of keeping the other four variables constant, is shown in Figure 4. It can be seen that the conversions of both reactions are all decreased gradually with increasing the reaction temperature; therefore, to carry out the reaction at too high a temperature is unfavorable. However, to keep the reaction temperature somewhat higher than the ambient is necessary. Owing to the significant effect of temperature on the solubility of Na2HP04,especially in the range 30-40 OC, a slightly high temperature can permit the reaction to be carried out at a more concentrated level so as to provide a better crystalline condition for obtaining a high crop of the product. In the present process, the reaction temperature can be taken as 40 "C. Flow Sheet. According to the above consideration and the results of the bench-scale experiment, the flow sheet of the present process is proposed (Figure 5). The first portion of solid NaCl is dissolved in phosphoric acid and
Figure 5. Flow sheet of the process. aqueous stream, gas, (- - -) solid.
e)
(-*-I Solvent stream, (-)
Table 11. Material Balance Based on 358 Units of Na2HPOI*12H20 feeds product stream 1 2 3 4 5 6 7 NaCl 59.2 59.2 1.4 H3PO4 99 1 HzO 216 NH3 34.2 0.2 107 NH,Cl Na2HP0,.12H20 358
the recycled mother liquor in mixer 1. The resulting aqueous solution contacts the organic solvent in reactor 1 (at 40 OC) to complete the first reaction. After phase separation, the aqueous phase is mixed with the rest of the NaCl in mixer 2 so that the total amount of NaCl corresponds to that required to form Na2HP04. The resulting aqueous solution is then transferred to reactor 2 and is in contact with the organic solvent again (at 40 OC) until the second reaction (eq 2) is completed. Cooling the resulting aqueous solution to about 5 "C, the crystal of disodium phosphate dodecahydrate (Na2HPO4-12H20)is obtained as the product and the mother liquor is recycled to mixer 1. The organic solvent loaded with HC1 from reactors 1 and 2 is regenerated by stripping with ammonium hydroxide to form NH4C1as a byproduct, and the regenerated solvent is reused. On the basis of the results of the bench-scale experiment, a material balance of this process is given in Table 11. Product and Byproduct. Technical grade disodium phosphate dodecahydrate, with more than 96% Na2HP04 and less than 1% NaCl (dry basis), can be obtained in the present process. Fertilizer grade ammonium chloride, with more than 25% N and less than 1% P205,can also be obtained by careful control of the operation of regeneration of the loaded solvent. Conclusions The process for the production of Na2HP04from NaCl and H3P04by using a water-miscible amine can be improved by using a water-immiscibleprimary amine, N1923, as an extractant to promote the double-decomposition reaction. Extractant N1923 is efficient and selective for the separation of HC1 from the resulting aqueous solution. Satisfactory conversion of the reaction and high purity of the product are achieved in the present process. Registry No. Na2HP04,7558-79-4; NaCl, 7647-14-5; H,PO,,
Ind. Eng. Chem. Res. 1989,28, 1910-1912
1910
7664-38-2;HCl, 7647-01-0.
ed.; CRC Press: Boca Raton, FL, 1982-1983; p D-173.
Jian-Zhong Zhou,* Li-Li Jiao, Yuan-Fu Su
Literature Cited Makin, A. B. Isothermal Solubility of the Ternary System Na2HPO,-NaCI-H,O at 25 OC. Zh. Neorg Khim. 1957,2, 2794-7. Onoda Cement Co. Br. Patent 1,036,207 1966. Weast, R. C., Ed. CRC Handbook of Chemistry and Physics, 63rd
Chemical Engineering Research Centre East China Institute of Chemical Technology Shanghai 200237, China Received for review March 23, 1989 Accepted August 20, 1989
Theory for the Extension of a Newtonian Filament by Gravity and/or a Take-up Force The extension of a Newtonian filament is a problem of classical interest. For the case in which viscous and gravity forces are dominant, Petrie lays out three possible solutions, one involving the hyperbolic sine, another involving the sine, and a degenerate case of both (a hyperbola). Starting with Trouton, the hyperbolic sine solution has always been selected for analyzing isothermal spinning experiments under all conditions of take-up force. That is not always correct. For the gravity-only spinning or for gravity augmented by a small take-up force, the solution must be of the sine form. A dimensionless group $ (involving the ratio of the gravity force to the viscous force, and the draw ratio) is derived that leads to the hyperbolic sine if $ 1, to the hyperbola if IF. = 1, and to the sine if $ > 1, thus giving a criterion for selecting the governing solution. The extension of an isothermal Newtonian filament with or without a take-up force is a classical problem in Newtonian fluid mechanics. Theory for low-speed isothermal Newtonian spinning provides the basis for understanding and analyzing complicated isothermal viscoelastic spinning experiments. Neglecting inertial, drag, and surface tension forces, Trouton (1906) presented the governing nonlinear differential equation that describes slow-speed isothermal Newtonian spinning. He noted that a hyperbolic sine function satisfies the governing equation, approaching a hyperbola in the limit of long filaments. The first detailed discussion was by Marshall and Pigford (1947), who showed two changes of variables by which one reduces the nonlinear equation to a linear one. A succinct review was presented by Petrie (1979), in which he identified and sketched three solutions. One involved the hyperbolic sine, another the ordinary sne, and the third, a degenerate form of the first two (a hyperbola). Petrie, however, did not suggest any criterion for choosing the form of the solution that would depend upon the imposed boundary conditions. In the literature, the hyperbolic sine solution has always been selected for analyzing the isothermal spinning experiments under all conditions of take-up force. This choice, starting with Trouton (1906), carried through the work of Matovich and Pearson (1969), Donnelly and Weinberger (1975), and Prilutski (1984). Our concern is with the analytical solutions. The choice of boundary conditions is, therefore, an important part of the problem. The initial velocity is the first condition; however, there are several choices for the second. In the most plausible cases, one imagines either an imposed take-up force or a gravity-only flow in which the force of gravity ceases to act at some point. Ziabicki (1976) identified another case by imagining that the filament “piles up”, producing a zero velocity on a stationary receiving surface. In the related experimental work, Pate1 and Bogue (1989) very briefly showed that the hyperbolic sine form cannot, in fact, accommodate the boundary conditions for problems in which the dominant force is gravity. They stated a criterion to choose the governing form of the solution. The present work expands on that analysis. Also,
the criterion to choose the governing form of the solution is derived here based solely on the operating conditions. The mathematical proof of the criterion is presented in the Appendix. Governing Equations and Solutions The governing dimensionless differential equation, neglecting the inertial, drag, and surface tension terms, is
where u* = u/uo, x* = x / L , uo is the velocity at x = 0, and L is the length of the filament. We will often use the symbol NG = (pgL2/3vuo),which is a Stokes number. It is the ratio of the gravity force to the viscous force. Marshall and Pigford (1947) described the changes of the variables that are necessary to reduce this nonlinear equation to a linear one. (One first lets p = du*/dx* and p(dp/du*) = d2u*/d(r*)2,leading to the Bernoulli equation, which is a nonlinear first-order differential equation, and the substitution z = p2, which transforms it to a linear equation.) An intermediate equation is instructive: (du*/dx*)2 = 2N&* C(U*)~ (2)
+
The character of the solution depends on the sign of C. The three possible solutions were summarized by Petrie (1979): u* =
(2)
sinh2 (A(x* + B ) )
(3) (4)
u* = (NG/2)(x*
+ B)2
(5) where A and B are constants of integration. Eauation 5 follows as a degenerate case from echer eq 3 0; eq 4 by letting A approach zero. If we restate eq 5 in terms of the dimensionless radius R *, it will be of the form R*(x* + B ) = K , leading to the designation “hyperbola solution”. I. Cases Involving Falling Filaments, with or without an Imposed Take-up Force. The physically
0888-5885/89/2628-1910$01.50/0 0 1989 American Chemical Society
