Ind. Eng. Chem. Process Des. Dev. 1982, 21, 211-216
21 1
Liquid Ion-Exchange Process for the Recovery of Hydroxylamine from Raschig Synthesis Mixtures Styllanos SHnlades; Allen A. Tunlck, and Fred W. Koff corporate Research Center, Allied Corporation, Monistown, New Jersey 07960
Hydroxylammonium ions were extracted from commercial aqueous streams containing essentially hydroxylammonium sulfate,ammonium sulfate, and sulfuric acid with a selectivity factor of 6-10 by contacting the neutralbed feed stream with a kerosene solution of bls(Bethylhexy1) phosphate and trioctylphosphine oxide saturated with ammonia. The organic phase was then stripped with an aqueous solution of a strong acid and could be recycled after saturation with ammonia while hydroxylammonium salt could be recovered by fractional crystallization from the aqueous solution. A process for the recovery of hydroxylammonium sulfate employing countercurrent extraction, reflux, and stripping stages was conceived and partly demonstrated. M was estimated that in order to remove >99% of hydroxylammonium sulfate and produce a 99% pure hydroxylammonium sulfate solution, ten extraction, eight reflux, and two stripping stages are required.
Introduction Technical solutions of hydroxylamine are made by the Raschig process and typically contain hydroxylammonium, ammonium, and hydrogen ions in corresponding approximate concentrations 1.8 N, 4.7 N, and 1.9 N. The counterion is largely sulfate with small amounts of nitrate also present. The technical solutions are commonly called “hydrox” solutions. In previous work (Koff et al., 1980, 1982) we described a process for the recovery of hydroxylamine from hydrox solutions using a sulfonic acid resin in the ammonium form. In the loading step of that process, hydroxylammonium ions from the solution displaced ammonium ions from the resin in an essentially statistical manner, while in the regeneration step aqueous ammonia was used to selectively displace hydroxylamine from the resin. In this way relatively pure, concentrated hydroxylamine solutions were obtained from which crystalline hydroxylammonium salts could be recovered by neutralization with an appropriate acid followed by evaporation. The process relied on the strong basicity of ammonia relative to hydroxylamine for the selective displacement of hydroxylamine by ammonia in the regeneration step. In this report we describe a process for the recovery of hydroxylammonium salta from neutralized hydrox solution which is based on the selective extraction of hydroxylammonium ions by liquid ion exchange (Figures 1and 2). When the loaded extractant is subsequently stripped with an aqueous solution of a strong acid, relatively enriched solutions of the corresponding hydroxylammonium salt are obtained from which the crystalline salt can be recovered by fractional crystallization. Moreover, we show that by employing several reflux stages it is possible to obtain relatively pure hydroxylammonium salt solutions from which the crystalline salt can be obtained by simple crystallization. Experimental Section Chemicals. Hydroxylamine sulfate and ammonium sulfate used to make simulated hydrox solutions were MCB reagent grade; authentic hydrox solution 1.78 N NH30H+,4.72 N NH4+,and 1.94 N H+ was obtained from Allied Chemical Corp. Sulfuric acid and ammonium hydroxide were B and A reagent grade. The following chemicals were obtained as shown: kerosene, Aetna Chemical Corp.; HDEHP, Union Carbide Corp.; TOPO,
Eastman Organic Chemicals; oleic acid, MC&B; decanol, Eastman Organic Chemicals; OPAP, Mobile Chemical Co.; TBP, Eastman Organic Chemicals; PNP, Eastman Organic Chemicals. Determination of Selectivity Factors. Measured samples of aqueous solutions containing known concentrations of ammonium sulfate, hydroxylammonium sulfate, and free base were well stirred magnetically for several minutes in a thermostatted 100-mL vessel with measured samples of extractants. The mixture was allowed to settle and an aliquot of the organic layer was taken and stripped by extraction twice with 1.00 M sulfuric acid. The sulfuric acid was then titrated with standard sodium hydroxide. Three breaks in the titration curve were found at approximate pH values of 4.0, 7.5, and 10.5 corresponding to H+, NH30H+,and NH4+,from which the amount of NH4+ and NH30H+ present in the extractant could be calculated. The NH4+and NH30H+ in the aqueous layer were found by difference and/or by hydroxide titration after addition of a known quantity of standard acid. The selectivity factor was determined by entering these quantities in the following formula
n=
[NH30H+Imp[NH4+Iaq [NH30H+Iaq[NH4+I,,g
Selectivity Factor for Hydroxylammonium vs. Sodium Ion. A 5.0-mL aqueous solution containing 0.880 g (10.73 mequiv) hydroxylamine hydrochloride, 1.085 g (18.56 mequiv) sodium chloride, and 0.188 g (4.69 mequiv) sodium hydroxide was shaken with 10.0 mL of a kerosene solution containing 1.612 g (5.00 mequiv) HDEHP and 0.967 g (2.50 mequiv) TOPO. A 5.00-mL portion of the kerosene layer was then shaken with 5.00 mL of 2.025 N H2S04. The aqueous acid layer contained 4.14 mequiv of hydroxylamine and only traces of sodium and chloride ions. The selectivity factor was, therefore, at least 100. Selectivity of HDEHP/TOPO for H+/NH30H+. A kerosene solution 0.7 M HDEHP/O.4 M TOPO, 5 mL, was shaken at 30 “C with an equal volume of 4 N hydroxylammonium sulfate. The aqueous layer was found to contain [H+] = 0.0678 N, whereas the organic layer contained [RNH,OH] = 0.0719 M (found by stripping with 1 M H2S04). The selectivity factor of HEDHP/TOPO for to 1.89 NH30H+vs. H+ was calculated to be 1.85 X 0 1982 American Chemical Society
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Stripping
Neutral'n
4 v2
A'IllTlHtrn.l
C a',,.
3
U
h"
1
Reflux
v3
L Extraction
VI+V,
y,A m
0 ,h,,
=
o
Spent Hydrox
Figure 1. Block diagram for the recovery of hydroxylammonium salt by liquid ion exchange: (1) acid to stripping; (2) enriched hydroxylammonium salt; (3) hydrox feed; (4) spent hydrox; VI, V,, V,, aqueous volume flows; U, organic volume flow; Ai, Hi,S, ammonia, hydroxylamine, and acid concentration, respectively, in aqueous phase; a,, hi,ammonia and hydroxylamine concentration, respectively, in organic phase.
Figure 2. Expanded block diagram of the extraction section showing the numbering of the aqueous and organic streams: Ai, Hi, aqueous ammonia and hydroxylamine concentrations leaving stage i; ai, hi,organic ammonia and hydroxylamine concentrations leaving stage i.
and the inverse factor 530 to 540. Countercurrent Extraction. An extraction apparatus consisted of a vertical glass tube, of 1.2 cm internal diameter, 130 cm long, closed a t the bottom and open a t the top and equipped with four ports, A, B, C, D, located correspondinglya t 5,10, 116, and 124 cm from the bottom. A glass rod, 0.5 cm diameter and 140 cm long, was positioned on the long axis of the tube and was supported by X
means of a polytetrafluoroethylene bearing at the bottom and a greased glass bearing a t the top. Two pieces of nichrome wire, 0.64 mm thick and 120 cm long, were attached along the length of the rod on opposite sides thereof by means of ten equally spaced loops of nichrome wire of the same thickness. The rod/wire assembly could be spun by means of a variable speed motor attached at the top of the rod. The total capacity of the apparatus up to port D was 130 mL. In order to start an extraction, 115 mL of kerosene was placed in the apparatus, then 15 mL of a kerosene solution 0.75 molar in HEDHP and 0.5 M in T O P 0 previously saturated with anhydrous ammonia at atmospheric pressure and essentially ambient temperature were introduced through port B. The rod/wire assembly was started spinning at about 360 rpm while the kerosene solution of HDEHP/TOPO was introduced through port B at the rate of 2 to 3 mL/min and an aqueous solution, which was made by dissolving 4.45 equiv of ammonium sulfate, 1.19 equiv of hydroxylammonium sulfate, and 0.29 equiv of ammonia in sufficient water to obtain total volume equal to 1 L, was introduced through port C a t the rate of 0.5 to 1 mL/min. Simultaneously, aqueous raffinate was collected through port A and kerosene extract through port D. The operation was continued for 77 min, a t which time 210 mL of the kerosene solution and 55 mL of the aqueous solution had been fed to the apparatus. The temperature during extraction was about 25 "C. The aqueous raffinate collected contained 184 mequiv of ammonium ion and 8.5 mequiv of hydroxylammonium ion. The kerosene extract was stripped with 1M sulfuric acid which was titrated and found to contain 41.0 mequiv of ammonium ion and 37.7 mequiv of hydroxylammonium ion. Thus the apparent selectivity for hydroxylamine in this extraction is equal to (184 X 37.7)/(8.5 X 41.0) = 19.9, a far greater value than obtained in single-stage extraction. It is estimated that the apparatus used provided 4-5 theoretical extraction stages. This example demonstrates that countercurrent extraction can be used to extract hydroxylammonium ion almost quantitatively from an aqueous solution containing a mixture of hydroxylammonium and ammonium salts. Crystallization from Ammonium Sulfate/Hydroxylammonium Sulfate Solution. Hydroxylammonium sulfate (5.00 g, 30.5 mmol), ammonium sulfate (5.00 g, 31.8 mmol), and 5 mL of 1 M H2S04(5.00 mmol) were heated at 40 "C until a clear solution was obtained. The solution was cooled for 2 h at -5 "C and the white crystals that formed were filtered, washed with methanol/water, and dried in vacuo, yield 1.74 g. Titration by acid/base showed that the crystals were 98% pure hydroxylammonium sulfate.
Results and Discussion We have found that when a mixture of hydroxylammonium and ammonium sulfate is made alkaline by addition of ammonia and then contacted with a kerosene solution of certain organic acids, hydroxylamine is extracted in preference to ammonia by the organic phase. The final composition of the two phases is the same if alternatively the same amount of ammonia is added first to the kerosene solution of the acid and the resulting ammonium salt solution is then contacted with the aqueous solution containing the neutral hydroxylammonium and ammonium salts. The latter case is a clear example of liquid ion exchange, involving the exchange of ammonium ions from the organic phase with hydroxylammonium ions from the aqueous phase. The following equilibria are relevant in the separation of hydroxylammonium salts from hydrox solutions via
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982 213 Table I. Extraction of Hydroxa by HDEHP/TOPO PHC
VorgIVaq
[HDEHP], M
[TOPO], M
T,"C
selectivity factor
7.34 6.88 7.24 7.24 7.24 7.24 6.99 6.82 7.49 7.24 7.24 7.24 7.25 7.25 6.89
1.60 1.70 2.06 1.70 1.70 1.70 1.95 2.12 1.94 1.79 1.58 1.88 1.83 1.85 1.64
0.500 0.541 0.541 0.541 0.682 0.541 0.639 0.708 0.637 0.574 0.688 0.747 0.783 0.887 0.821
0.250 0.285 0.285 0.350 0.360 0.285 0.387 0.478 0.384 0.317 0.435 0.397 0.428 0.480 0.500
20.0 24.1 24.1 24.1 24.1 38.2 33.9 40.8 33.7 33.1 35.4 40.0 33.7 34.0 33.3
3.8 5.3 7.3 6.8 7.4 6.5 9.8 9.5 7.2 7 .O 7.8 8.5 9.0 9.6 9.0
Simulated hydrox solution containing total ammonia 4.36 N and total hydroxylamine 1.68 at initial pH 8.46. HDEHP/TOPO in kerosene solution. pH adjusted with concentrated sulfuric acid and measured before extraction.
liquid ion exchange, where RH is an acid soluble in the organic phase NH30H+ F? NHzOH + H+ (1) NH4+ @ NH3 + H+ NH,OH+
+ RH F? RNH,OH + H+
N H 4 + + = z m + H + NH30H+ +
m4z RNH30H + NH4+
(2)
(3) (4)
(5) In these equations a bar over a formula denotes a species in the organic phase, while formulas without a bar denote species in the aqueous phase. Equation 1 represents the dissociation of hydroxylammonium ion to free base and proton. The equilibrium constant at the concentrations of interest is 7.0 X mol/L (Koff et al., 1982). Equation 2 represents the dissociation of ammonium and the corresponding equilibrium constant is 1.5 X lo4 mol/L (Koff et al., 1982). In the presence of free acidity, which is the case in the untreated hydrox solutions, both equilibria are fully shifted toward the left side. Equations 3 and 4 represent the displacement of hydrogen ion from the acid by hydroxylammonium or ammonium ion, respectively. In acidic media the equilibria lie on the left side because the organic acids of interest are generally weak acids. It is clear that extraction of hydroxylammonium ion from hydrox solutions is not feasible without prior neutralization of free acidity and in fact eq 3 and 4, when read from right to left, represent a stripping operation. Equation 5 is most relevant to the extraction of hydroxylammonium ions from aqueous solution. The extraction system consists, therefore, of hydrox solution devoid of its free acid in contact with an organic solution containing initially the ammonium salt of the acid. The most straightforward way to eliminate the free acid is to neutralize the hydrox solution by addition of ammonia. In this way protons are quantitatively transformed to ammonium ions and no new ions are introduced. Considering, however, that the extraction involves competition between hydroxylammonium and ammonium ions, it is clear that by creating more ammonium ions the task of extraction becomes harder. If sodium hydroxide is used to neutralize excess acid, the sodium ions introduced associate very weakly with the liquid ion exchanger (see below) and do not, therefore, interfere with the extraction. In this case, however, sodium sulfate is an ultimate byproduct. The most appropriate way to eliminate excess acid from hydrox solutions is treatment with a weakly basic ion-exchange resin, of the tertiary amine type. The resin
absorbs sulfuric acid without introducing any other ions and the resin can be subsequently regenerated by treatment with ammonia; this results in the formation of ammonium sulfate, a useful byproduct. Selectivity. In order for the extraction to be effective, the equilibrium constant of eq 5 must differ significantly from unity. The equilibrium constant is defined as [RNH,OH] [NH4+]
K5=
-
[RNH4][NH,OH+] Although it may be possible to find ion exchangers that selectively extract ammonium ion (K5< l),such an operation would not be desirable because ammonium ion is present in large excess over hydroxylammonium ion. What is required is a hydroxylammonium selective ion exchanger (K5> 1). We have found that solutions of certain alkylphosphoric acids in water-immiscible solvents selectively extract hydroxylamine from aqueous solutions of hydroxylammonium and ammonium salts at pH values close to 7. Moreover, the selectivity is increased in the presence of neutral phosphate esters and phosphine oxides. Particularly effective are kerosene solutions of bis(2-ethylhexyl) phosphate (HDEHP). The equilibrium constant, K5,ranges between 2 and 4 depending on pH, concentration, and other variables. In the presence of trioctylphosphine oxide (TOPO), the equilibrium constant increases significantly. A brief study, using aqueous solutions -1.7 N in total hydroxylamine and -4.4 N in total ammonia adjusted to appropriate pH by the addition of sulfuric acid, was undertaken in order to define a range of concentrations of HEDHP and TOPO that result in high selectivity. The ammonia and hydroxylamine concentrations chosen corresponded roughly to the concentrations that can be obtained by de-acidifying commercial hydrox solutions by means of ion exchange. Best results were obtained with a kerosene solution 0.7 M in HEDHP and -0.4 in TOPO which showed K5= 6-10, depending on pH, temperature, and degree of extraction (Table I). The HEDHP/TOPO system separated hydroxylamine from sodium salts with a separation factor higher than 100. Therefore, this system would be very efficient for recovering hydroxylamine salts from technical solutions in which sodium rather than ammonium ion is the contaminant. Such solutions are available through a variant of the Raschig synthesis which utilizes sodium nitrite, rather than ammonium nitrate, as a source of nitrogen. I t has been reported in a French patent (Demarthe et al., 1974) that dodecane solutions of sodium salt of HEDHP extract hydroxylamine from neutral solutions of hydroxylammonium salts. The selectivity factor of that
-
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Table TI. Extraction (at Various Extractants
- 20 "C) of Hydrox by
extractant, M, solvent HDEHP, 0.5, kerosenea oleic acid, 0.5, kerosenea oleic acid, 0.5, kerosenea HDEHP, 0.5, decanol HDEHP, 0.5, kerosenea TOPO, 0.5, kerosenea HDEHP/TOPO, 0.5/0.25, kerosene a HDEHf/TOPO, 0.5/0.25, kerosene OPAP, 0.35, kerosene PNP, 0.5, kerosene TBP, 1.O, kerosene HDEHr/TBP, 0.5/0.5, kerosene HDEHP/PNP, 0.5/0.2 5, kerosene b
Vo,/Vas
selectivity factor
4.0 1.0 4.0 1.0 2.5 2.0 2.0
3.0 2.1 2.5 1.9 4 .O no extraction 5.4
1.6
5.4
2.0 2.0 2.0 2.0
2.5 1 no extraction 4.5
2.0
3.2
a Model hydrox solution containing total ammonia 7.34 N, total hydroxylamine 2.1 N, and total free base 0.99 N, initial pH 7.21. To 1 0 m L of authentic hydrox solution added 2 mL of 1 4 N NH,OH. Resulting solution had total ammonia 6.35 N, total hydroxylamine 1.48 N , free base 0.80 N. HDEHP = bis(2-ethylhexyl) phosphate; TOPO = trioctylphosphine oxide; OPAP = octylphenyl acid phosphate, mixture of mono- and diesters; PNP = p nitrophenol; TBP = tributyl phosphate.
system appears to be less than 10. The selectivity of the HDEHP/TOPO system for hydroxylammonium over ammonium ion is indicative of a stronger association of the former with the extractant. We suggest that hydrogen bonding of the oxygen-bound proton of the hydroxylammonium ion with the oxygen atom of TOPO or the polarized oxygen atom of HDEHP may be responsible for such an association. Several other waterimmiscible acids and additives were investigated for the selective extraction of hydroxylamine from hydrox solutions but none showed the high selectivity of the HEDHP/TOPO system. These results are summarized in Table 11. Countercurrent Extraction. Although a single extraction can remove a considerable amount of hydroxylammonium ion from the hydrox solution, it is necessary to resort to multiple countercurrent extractions if an essentially complete removal is desired. Such extractions are preferably performed in a continuous flow apparatus such as a Scheibel column. The number of theoretical stages required depends on the selectivity factor, the degree of hydroxylamine removal desired, and the phase equivalent ratio. In a closed loop system incorporating extraction, reflux, and stripping stages, that ratio in turn depends on the separation factor. Assuming a separation factor of 8, we calculated (see Appendix) that nine extraction stages are required in order to extract 99% of the hydroxylammonium ion from the neutralized hydrox solution. The number of stages required, including reflux (see below), is plotted in Figure 3 as a function of the selectivity factor. We demonstrated experimentally the feasibility of countercurrent extraction by contacting in a homemade Scheibel column a simulated neutralized hydrox solution with a HEDHP/TOPO kerosene solution saturated with ammonia. The aqueous raffinate collected a t the exit of the column contained hydroxylammonium and ammonium ions in equivalent ratio 4.4/95.6 compared to the corresponding ratio of 20180 in the feed solution. This represented a 78% removal of hydroxylammonium ion and corresponded to about four extraction stages.
2
0
4
8
12
16
20
Selectlvlty Factor
Figure 3. Dependence of stages required and of ammonium sulfate byproduct formed on the selectivity factor. Recovery 99.5%,purity 99%: A, hydrox feed neutralized by addition of ammonia resulting in [NH,+] = 6.6 N, [NH30H+]= 1.7 N; B, hydrox feed neutralized by treatment with basic resin resulting -in [NH,+] = 4.4 N, [NH,OH+] = 1.7 N.
Reflux. Although countercurrent extraction can deplete the aqueous phase of hydroxylammonium ions to any desired degree, the resulting extract will be only partially enriched in hydroxylammonium. In fact, it can be easily seen that the hydroxyla"onium/ammonium ratio in the extract cannot be greater than the corresponding ratio in the aqueous feed solution multiplied by the selectivity factor, or -(20/80) X 8 = -67/33. (In the countercurrent experiment described in the previous section, the extract had hydroxylammonium/ammonium ratio equal to 48/52.) In order to achieve higher enrichment, it is necessary to contact the extract with an aqueous solution of hydroxylammonium sulfate produced downstream in the process. This is essentially a reflux operation, analogous to the more familiar reflux employed in distillation. Assuming a selectivity factor of 8, we calculated that eight reflux stages will be required in order to produce an extract with less than 1%ammonium content (see Appendix). Total (extraction plus reflux) stage requirements for other selectivity factors are shown in Figure 3. We did provide experimental evidence of the feasibility of reflux by showing that the hydroxylammonium/ammonium ratio of HDEHP/TOPO kerosene solutions obtained by extracting a neutralized hydrox solution increased as expected upon a single contact with aqueous hydroxylammonium sulfate. We did not, however, carry the demonstration to a countercurrent operation. Stripping. The hydroxylammonium loaded extract can be stripped by contact with a strong acid, e.g., sulfuric. Due to the relatively weak acidity of HDEHP (the pK, in methanol/H20 (51by volume) was found to be -3.0), the stripping is a facile operation. Equilibration of HDEHP (H' form)/TOPO with aqueous hydroxylammonium sulfate allowed us to measure the equilibrium constant, K3 The for reaction 3 and determine a value of 1.87 X inverse of that constant, l/K3, is equal to 535 and represents the selectivity of the extractant for proton versus hydroxylammonium ion. Given the high value of 1/K3, we estimate that two stripping stages will suffice to remove 99% of hydroxylammonium from the extract and the resulting aqueous hydroxylammonium sulfate will contain less than 1%free acidity. Process Considerations. In designing a process for the recovery of hydroxylammonium sulfate by liquid ion exchange, it is important to take account of the phase diagram of the system ammonium sulfate/hydroxyl-
Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982
215
-// I
50
1.10
,
60
,
70
,
80
I
90
I10
100
Percent Recovery of Hydroxylamine BO
90
100
Male Percent Purily of Product
Figure 4. Dependence of stages required and of ammonium sulfate byproduct formed on mole percent purity of hydroxylammonium sulfate solution produced. Calculated for selectivity factor = 8, [NH,+Io = 4.4 N, [NH30H+Io= 1.7 N, recovery = 99.6%.
ammonium sulfate/water. We have shown that in a batch crystallization essentially pure hydroxylammoniumsulfate can be isolated from an aqueous solution containing initially an equal weight of ammonium sulfate. It may be advantageous, therefore, to reduce the number of reflux stages and rely on the crystallizer for the refining of the product. We calculated that a purity reduction of 5% would eliminate 4-5 reflux stages (Figure 4). The amount of ammonium sulfate byproduct would also be reduced somewhat if a less pure product is obtained (Figure 4). The calculated data do not, however, take into account the additional amount of ammonium sulfate that would be produced upon recycle of mother liquors from the crystallization for product purification. Another option is to resort to partial extraction of hydroxylammonium ions from the feed hydrox solution and utilize the partly spent liquors in the manufacture of an oxime, e.g., cyclohexanone oxime. In this way considerable reduction of the number of extraction stages would result while the amount of ammonium sulfate byproduct would remain constant (Figure 5). The ion-exchangeprocess developed earlier (Koff et al., 1982) produces about 0.8 lb of ammonium sulfate byproduct/lb of hydroxylammonium sulfate, due to the necessity of displacing hydroxylammonium with ammonium ions. The current process would produce the same amount of byproduct if only extraction and stripping stages were used. The inclusion of reflux stages, however, increases the amount of ammonium sulfate to about 1.2 lb. Conclusions 1. Kerosene solutions of HEDHP and T O P 0 extract hydroxylamine from aqueous solutions with a selectivity factor of about 8 over ammonium ion and about 100 over sodium ion. 2. Over 99% removal of hydroxylamine can be accomplished by means of a ten-stage countercurrent extraction with the HEDHP/TOPO system. The purity of hydroxylamine in the extract can be increased to >99% by an eight-stage reflux with an aqueous solution of hydroxylammonium sulfate produced downstream. 3. The extract can be effectively stripped in two stages with aqueous sulfuric acid. Essentially pure hydroxylammonium sulfate can be obtained from the sulfate liquors even in the presence of an equal weight of ammonium sulfate in solution. 4. The amount of ammonium sulfate byproduct in this process is about 50% more than that produced using our solid ion-exchange process (Koff et al., 1982).
Figure 5. Dependence of stages required and of ammonium sulfate byproduct formed on the percent recovery of hydroxylamine. Calculated for selectivity factor = 8, [NH4+l0= 4.4 N, [NH30H+Io= 1.7 N, purity of recovered salt = 99%.
Appendix Flow Considerations. Consider the extraction/reflux/stripping system shown in Figure 1. In this system an organic phase flows sequentially in a closed loop at a constant volume velocity U through extraction stages (m to l),reflux stages (1 to m?,and stripping stages (estimated to be two). Countercurrently to the organic phase flows an aqueous phase a t variable volume velocity. Assume that the velocity of the feed hydrox stream is V I ,that of the product stream is V2,and that of the acid stream entering stripping is V 2+ V,. Then the velocities of the aqueous streams through reflux and extraction are V3and Vl + V,, respectively. Countercurrent Extraction. Assume that hydroxylammonium and ammonium ions at initial combined concentration S are extracted countercurrently from aqueous solution by a liquid ion exchanger with a constant separation factor. The exchanger initially contains ammonium ions at concentration t . Assume further that the volume of each phase remains constant per stage and that a constant portion of the ion exchanger is occupied at any stage only by hydroxylammonium and ammonium ions; therefore, the sum of the two ions extracted per stage is constant. The following four equations describe the relationships of the species leaving and entering stage i of the extraction (see Figures 1 and 2). Ai Hi = S (-41)
+ ai + hi = t
(A2)
In these equations Ai and Hidenote the concentrations of ammonium and hydroxylammonium ions, respectively, in the aqueous phase at equilibrium with the corresponding concentrations, ai and hi,of ammonium and hydroxylammonium ion in the organic phase. Equations A1 and A2 are the mathematical expressions of the assumption that the sum of hydroxylammonium and ammonium ions extracted per stage is constant. Equation A3 is an expression of the material balance within a stage and states that the amount of ammonium ions entering the stage is equal to the amount leaving it. Equation A4 introduces the separation factor. From eq A2 and A4 we get tAi ai = Ai nHi
+
This system of equations cannot be solved for any arbitrary stage because it contains six unknowns for four inde-
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Ind. Eng. Chem. Process Des. Dev., Vol. 21, No. 2, 1982
pendent equations. It can be solved, however, by the following reiterative method. Assume that stage m produces a satisfactory separation. The organic stream entering that stage contains only ammonium ions at concentration t , or am+l
=t
(A6)
The aqueous solution leaving stage m contains a small amount of hydroxylammonium ions
H,,, = eS (A7) where e is a small fraction of the total aqueous concentration, e.g., e = 0.01. Once e is defined, H , can be calculated from eq A7 and A, from eq A l . Next a, is calculated by means of eq A5 and h, by means of eq A2. In order to calculate the composition of the streams leaving stage m - 1 we start by using eq A3 to calculate A,,,-l A,-1 =
(am
- a m + l ) U + Am(V1 +
V3)
643)
v1+ v 3
We then apply eq Al, A5, and A2 to calculate respectively Hm-l,am-l, and h,+ This procedure is continued until the composition of the aqueous phase corresponds to the feed stream composition. The number of reiterations required to reach that composition is equal to the number of extraction stages required to achieve the separation corresponding to the choice of e. Flow Ratio of Phases. The overall operation for hydroxylamine recovery contains extraction, reflux, and stripping stages (Figure 1). The volume velocity ratio of the organic and aqueous phases must satisfy material balance requirements. At the extraction section the amount of hydroxylamine fed must be equal to the amount leaving. Assuming that the stripping leaves no hydroxylamine in the recycled extractant we get Ho(V1 + V3) = Hm(V1 + V3) + hlU (A8) Consider also that ( A l h l ) / ( H l a l )= n and that A 1 / H 1 = Ao/Ho. Entering this equation into (A@, and also considering (A2) and (A7) we get --U - (Ho- eS)(nHo + A,) = P (A9) v 1+ v 3 nHot The right hand term of (A9) is composed of known quantities and is equated to a parameter p . In order to simplify the calculations, assume that the acid used in stripping has concentration S. Then material balance at stripping requires that
H’,,,+i(Vz + Vs) = h’,U (A10) But H’,+l = (1 - e?S and h’, = (1 -e%, where e’is a small
number defining the purity of the produced hydroxylammonium salt; therefore -- U - -S v, + v3 t Overall balance requires that the net hydroxylamine being fed to the system be equal to the hydroxylamine in the product stream and in the spent hydrox stream HoV1 = Hm(V1 + V3) + H’m+lV, (A121 Entering H,,, = eS and H’,,,+l = ( 1 - e?S and rearranging we get eSV3 + (1- e?SV2 - (Ho- eS)Vl = 0 (A13) Solving (A9) for V1and ( A l l ) for V, and entering into (A13), we get a first-order equation in V3 which has the following solution (eS (1 - e?pt - Ho)U v 3 = (A141 (S(1 - e? - H0)p Once V3is known, eq A9 can be used to calculate V1and eq A l l to calculate V,. Amount of Ammonium Sulfate Byproduct. The ammonium sulfate produced, in excess of that originally present in the neutralized hydrox feed, is equal to the following quantity (moles) ((Vl+ V3)(l - e ) V2e?S - V I A o The amount of hydroxylammonium sulfate produced is equal to the quantity (moles) V2(1 - e?S
+
+
Therefore the weight ratio R of ammonium sulfate byproduct to hydroxylammonium sulfate product is
where 132/164 is the corresponding molecular weight ratio. Recovery and Purity of Hydroxylammonium Sulfate. The yield (mole fraction recovered) of hydroxylammonium sulfate is y = V2(1- e ? S / V I H o The purity (mole fraction) is 1 - e’. Literature Cited Demarthe, J. M.; Gue, J. P.; Grieneisen, A.; Miguel, P. French Demande, 2 206 270, 1974. Koff, F. W.; Tunick, A. A.: Sifniades, S.; Belden, R. H. US. Patent 4202765, 1980. Koff, F. W.; Sifniades, S.; Tunick, A. A. Ind. Eng. Chem. Process Des. D e v . 1982, preceding article in this issue.
Received f o r reuiew September 22, 1980 Accepted September 8, 1981
