to the outlet of the reactor. Xs a consequence, a large number of terminal states must be considered to calculate stage by stage from the outlet of the reactor to the inlet. Nomenclature
c = conversion. fraction of compound .i\ that reacts
X
= -
Ca = concentration of compound A CB = concentration of compound B Cn = concentration of compound D cp = specific heat of gas mixture D = compound D d = tube diameter e = friction factor E = energy of activation g = gravitational constant H = heat of reaction ko = Arrhenius constant x-1 = forward reaction velocity constant k2 = reverse reaction velocity constant
heat transferred = tube length L = total tube length t = time T = temperature of mixed gases = flow rate of A entering reactor. mole, time product rate of compound B, mole, time or product rate of compound D, or mole,/time of reactant .4that reacts z = compressibility factor of mixed gases P = density of mixed gases P = constant, limit on amount of compound ,4 that reacts A = Lagrangian multiplier
T C’
References
( 1 ) h i s , R., “The Optimal Design of Chemical Reactors. A Study in Dynamic Programming,” Academic Press, New York, 1961. ( 2 ) Bellman, R., “Adaptive Control Process,” Princeton University Press, Princeton, N. J., 1962. (3) Bellman, R., “Dynamic Programming,” Princeton University Press, Princeton, N. J., 1957. ( 4 ) Bellman, R., Dreyfus, S., “Applied Dynamic Programming,” Princeton University Press, Princeton, N. J., 1962. ( 5 ) Roberts, S. M., ”Dynamic Programming in Chemical Engineering and Process Control.” Academic Press, New York, 1963.
= equilibrium constant = kl , x2
f = maximum or minimum of objective function J = objective function M = molecular weight of gas mixture 11
Q =
1
w x =
A = compound .4 A , = cross-sectional area of tube B = compound B
R,
P = pressure in reactor
= total rate of gas flowing in reactor, mole, time = T t 7
RECEIVED for review January 28, 1963 ACCEPTEDSeptember 24, 1963
+X
RASCHIG SYNTHESIS OF HYDRAZINE Innvestigation of Chloramine Formation Reaction s. G
J
R
.
M
.
E L L I S , Department of Chemical Engineering, T h e L7nimrsity of Birmingham, Birmingham, England
. v . J E F F R E Y S , Dfpartment of Chemical Engineering, Faculty of Technology, University of ,bfanchester,
. T.
W
H A R T 0 N , E. I.
.Lfanchester, England
du Pant de JVernours @ Co., Wilmington, Del.
The chloramine formation stage of the Raschig synthesis of hydrazine has been studied by reaction of ammonia and sodium hypochlorite in a glass apparatus. Two factorial experiments were carried out to study the effect of eight variables. In the first experiment the chloramine content of the reaction mixture was estimated by ultraviolet spectrophotometer; in the second, nitrogen evolution was measured in a specially designed gas collector. Ammonia and sodium hypochlorite concentration and mole ratio of NHB to N a O C l exerted the greatest influence on chloramine yield. These have been further studied over a wide range of concentrations and the results expressed in the form of chloramine yield and concentration contours. In general, chloramine yield increases with decrease in ammonia and hypochlorite concentrations and increase in mole ratio; increased yields are accompanied by decreased chloramine product concentrations.
an important inorganic chemical that finds widespread use in the form of its hydrate and its various organic derivatives. It is an excellent rocket propellant when used in conjunction with powerful oxidizing agents such as liquid oxygen, and this accounts for a large tonnage at the present time. Other varied uses have been discussed by IVharton (73). Raschig ( 1 7 , 72) in 1907 first showed that hydrazine could be produced in quantity by the action of sodium hypochlorite on ammonia to produce chloramine, which then reacted \$ith further ammonia to form hydrazine. YDRAZINE IS
NH3 18
+ NaOCl
+
NHiCl
+ KaOH
I&EC PROCESS D E S I G N A N D DEVELOPMENT
(1)
KH?
+ NH3 + NaOH
+
NzH4
+ NaCl + H20
(2)
Reaction 1 is fairly rapid, while Reaction 2 is slower and competes with a third much faster reaction which is especially sensitive to action of certain catalysts such as cupric ions. This third reaction is represented by the equation NHZCI
+ X2H4
-+
2KH4CI
+ Nr
(3)
Raschig found that protein-like materials such as glue, gelatin, and EDTA [ (ethy1enedinitrilo)tetraacetic acid] inhibited Reaction 3. In addition to Reaction 3 chloramine solutions undergo a decomposition reaction according to the following equation :
3NHiCl
+ 2SH3
-t
2NH4C1 4- IY2
(4)
GAS
COLLEClOR
\
Bodenstein ( 3 ) sho\ved that cupric ions did not affect this reaction. For a n optimum yield to be obtained in the over-all Raschig synthesis process, the optimum conditions must also be satisfied in each stage of the process. This paper presents the first part of a study of the Raschig synthesis ( 7 4 , in Ivhich the chloramine formation reaction has been investigated by reaction of ammonia and sodium hypochlorite in a glass reaction vessel.
RE E Q J A L I S
NG
Description of Apparatus
The apparatus (Figure 1) consisted essentially of a 1.0-liter reaction flask to which a second 500-ml. flask was attached by means of a 1.0-cm. glass tap. A pressure-equalizing line connected the two flasks, in order that liquid transfer benveen the two should not displace gas into the gas collector. The reaction vessel was fitted with a stirrer and a gas exit line. The gas exit line formed a ground-glass sleeve with the stirrer as shown in Figure 1 ; the shaft of the stirrer was coated with grease to form a seal against gas leakage. The length of the gas offtake prevented entrainment of liquid. The bottom of the reaction flask was fitted with a serum cap, so that liquid samples could be removed during the reaction by a 1.O-ml. hypodermic syringe graduated in 100 divisions. The constant temperature bath was constructed of galvanized iron with a Perspex window to permit observation of the reaction. The bath temperature was maintained a t 0" C. and below. by the addition of solid carbon dioxide to ethylene glycol solutions of knoivn concentration. Once the correct glycol solution had been placed in the bath. it was possible to maintain its temperature to lvithin ~ t 0 . 2C.~ of the desired value by ensuring the presence of solid carbon dioxide and agitation sufficient to maintain good heat transfer. When rhe temperature of the bath had been reduced to the desired value, very little solid carbon dioxide was required to maintain constant temperature. A small centrifugal pump. connected to the constant temperature bath. circulated coolant through a glass condenser in the gas offtake line a s illustrated. so that most of the ammonia was condensed from the gas stream passing to the gas collector. The gas collector \vas specially designed for the investigation (7). Chemicals Used
The ammonia, sodium hypochlorire, and EDTA solutions were technical grade as provided by IYhiffens. Ltd. The sodium hypochlorite \vas stored in a refrigerator to reduce decomposition. Typical Analyses of Solutions
Sodium hypochlorite Specific qravity Available chlorine content, g. ,'I. Caustic soda, q . / 1 . Sodium carbonate, 9. ;I. Iron, p.p.m. iimmonia solution. 5- l v . /w. Tetrasodium salt containing 29F0 w./\v. as acid EDTA solution
1.262 190 5 8 4.2 1 .o 19 1
Experimental Procedure
The constant temperature bath was first adjusted to the correct temperature. .4mmonia and sodium hypochlorite solutions at the correct temperature were placed in the reaction and addition flasks, respectively. EDTA inhibitor when ujed \vas added to the ammonia solution and sodium hydroxide for p H ad.justment to the sodium hypochlorite. The stirrer was sivitched on: the large-bore tap opened as quickly as possible, and the stopwatch started simultaneously. Twenty seconds were allowed for complete mixing of the contents of the flask, after which the first sample was taken by means of the hypodermic syringe. The volume of liquid sample actually removed was noted and the sample injected into distilled water in order to arrest the chemical reaction.
9CACllON
Figure 1.
VESSEL)
SERUC C A P
Chloramine reaction apparatus
The time a t which the sample \vas "quenched" was noted and taken as the time of sampling. The sample flask was shaken and the sampling process repeated. If gas was evolved during a n experiment, the volume of gas collected was measured. After the liquid samples had been diluted, each flask \\as xveighed to estimate the quantiry of distilled water used for quenching, after which the contents were analyzed. Methods of Analysis
Ammonia. Ammonia Lias estimated by acidifying a 5.0ml. sample with 50 ml. of 1.Y HC1 and titrating the excess acid with I-V S a O H solution, using methyl orange as an indicator. Sodium Hypochlorite. Ten milliliters of hypochlorire was made u p to 250 ml. \vith distilled water and to this were added 10 ml. of glacial acetic acid, 10 ml. of distilled water? and 2 grams of K I . T h e mixture was allowed to stand for 5 minutes in the dark, and titrated with 0 . 1 S Na2S203 using freshly prepared mucilage of starch as indicator. Chloramine. T h e chloramine was determined by spectrometry. A standard chloramine solution was prepared b>the method of Markwald and IVille (g)! their original apparatus being modified to include an electrical heating bath. a continuous condenser through Xvhich ethylene glycol \%-as circulated at -5' C.. and rotary vacuum pump Lsing the peak wavelength of 243 mp given by >letcalf ( T O ) , a calibration curve of percentage transmittance LIS. molarity was prepared over a concentration range of 0.1 X lop3 to 0.9 X l W 3 .I4 chloramine using a Unicam photoelectric quartz spectrophotometer. The curve obrained was exponential and the change in transmittance xvirh concentration \vas very small above a chloramine molarity of 0.4 X 1W3, Hence care \vas taken to ensure that the concentration of chloramine samples tested \vas less than this value. The lo\v concentration required by the instrument necessitared diluting the sample from the reaction vessel approximately 100 times, which provided an effective method of arresting the reaction. .L\ check \vas made to confirm that chloramine \vas stable at this dilution. The presence of ammonia an?, sodium h\-pochlorite at the concentrations used had no effect on the absorption at this lvavelength. Determination of Relative Importance of Variables
Initial Xvork and a revie\v of the literature showed that six factors probably affected the yield of chloramine (Table 1). To determine their relative importance it had been hoped to follow the chloramine concentration in the reaction vessel a t the time gas was being collected. However. at low concentrations of reagents the gas evolution was negligible. \z hile at high concentrations decomposition of the chloramine v a s too rapid for the peak concentration to be measured accurately. For this reason tivo separate factorial experiments were conducted. I n the first experiment loxv concentrations of reagents VOL. 3
NO. 1
JANUARY 1 9 6 4
19
Table 1.
levels Used for Factorial Experiments
First
.4mmonia concn., -11
2.8 5.6
Sodium hypochlorite concn., .M PH
Temp.,
8 9 6.7
0.63 1.27 1 '\NaOH added
NH3/NaOC1 mole ratio EDTA
2.1 1.6
-5
-5
20/1 6/1 Xone 10 ml. added
8/1 4/1
...
Results and Discussion
The results of the first factorial experiment are given in Table I1 in terms of the percentage yield of chloramine. The analysis of variance is given in Table 111. Standard methods were used to determine the components of variance. When the variance ratio test ( 4 ) is applied and tables given by Fischer and Yates ( 6 ) are used, the results are significant to the 0.05 level if the ratio of the mean square of the variable to the mean square of the residual is greater than 4.1 for the
a
20
41.2 80.0 19.0 13.0 0.9
17.0 45.0 19.0
RPsults of first exjeriment.
AB AC
1 1 1 1 1
1 1 1 1 1
None
The other blocks were produced by multiplying through by a symbol orthogonal to the alias subgroup not yet occurring and randomizing. Details of the randomized experiment are given in Table 11. In the analysis of the results of the first experiment NaOCl p H and EDTA were found to be insignificant. These were replaced by NaOCl age and agitation of the reactants in the second experiment. as observations made during the first experiment indicated that these factors might affect yield.
88.5 77.3 87.0 96.0 72.0 83.0 99.6 77.5
Freedom
S u m of Squares 186.0 58.0 6.7 43.0 112.0 2 6 -~ 3.7 14.3 15.9 88.4 1.5
Medium revs.
(Small letters denote variables at upper levels; otherwise they are at their lower level?.)
2*
of
... ...
abef. de. abdf. bc. acef. bcde. acdf
7"
D E F AD AE BC
were used and the chloramine yield was followed directly, while in the second high concentrations were used and N B evolution was used as a measure of loss of yield. Each factorial experiment was a half replicate of 32 runs confounded into four blocks of eight runs each, using the confounding ABC, ,4DE, and BCDE. Eight runs were a suitable number to conduct at any one time. The alias subgroup was ABCDEF. giving a principal block of
Block 1 1. bbaabb 2. aaabba 3. bbabab 4. abbbba 5. abbaaa 6. bababb 7. aaaaaa 8. babbab
Degrees
1
... ...
0
.4gitation
Analysis of Variance of First Series of Experiments
Direction Source of Variance
New 1 week 0
C.
Table 111.
Second
CF DE DF EF
29.3 20.4 1 71 .4 1 0.8 Blocks 3 80.7 Residual 8 46.3 Total 31 1452.8 a Values significant to 0.05 level. 1 1
Mean Squares 186.0 58.0 6.7 43.0 112.0 2.6
3.7 14.3 15.9 88.4 1.5 301 .O 328.0 13.5 27.4 1.9 29.3 20.4 71.4 0.8 26.9 5.8 1358.5
of Variance a
a b b a b
lMeGn Square Mean Square of Residual 32.10 10.oa 1.2 8.1a 19.0a 0.4 0.6 2.5 2.7 11.8 0.3 51 .8n 57.60 2.3 4.7 3.3
5 .O 3.5 12.3~ 0.1 4.63
blocks and 5.3 for the others. Column 6 of Table 111 gives these ratios, showing which variables and interactions are significant. The results indicate that the most important factor is the concentration of ammonia and this interacts significantly with the mole ratio, AE. Sothing can be said about AF, because this is confounded. The mole ratio itself is highly significant and interacts with both ammonia and sodium hypochlorite concentration. The other significant variable is sodium hypochlorite concentration, which interacts with temperature and mole ratio. The effect of temperature is just significant. while sodium hypochlorite p H and EDTA concentration are without significant effect. The results of the second factorial experiment are given in Table I1 in terms of milliliters of gas evolved per mole of sodium hypochlorite originally present in the reaction vessel. The analysis of variance of these results is given in Table IV. At the levels used in the second experiment the mole ratio is the most important variable ; this interacts strongly with ammonia concentration and sodium hypochlorite age. Both sodium hypochlorite age and concentration show high main effects, and they interact with one another. In addition, the age interacts ivith temperature, CD, and concentration with agitation and ammonia concentration. The temperature
Table It. Results of Factorial Experiments 2b Block 4 7a 2* 2b Block 3 10 Block 2 7a 16.0 5.1 25. aabbaa 100.0 3.0 17. aabbbb 82.6 9. babaaa 85.3 50.0 26. abaaba 82.8 59.0 10. abbabb 75.7 75.5 35.0 18. bbbaba 3.9 88.2 53.0 27. ababab 19. ababbb 79.0 35.0 11. bbabba 83.1 0.9 1.5 28. bbbaab 82.2 13.0 20. bbbbba 92.4 12. bbaaaa 69.0 80.2 130.0 85.0 29. aababa 74.3 46.0 21. baabaa 13. aaabab 100.0 42.0 17.0 30. baaaab 81.5 100.0 88.0 22. aabaab 14. aaaabb 84.4 71 .o 31. baabbb 76.2 26.0 0.9 23. baaaba 79.0 15. abbbab 78.0 41 .O 86.2 25.0 32. bbbbbb 31 .O 24. abaaab 76.6 16. babbba 74.5 M o l e s chloramine X 7OOYc;. b Resultj of second experiment. MI. gas evolcedper mole sodium hypoc hlorile. M o l e s sodium hypochlorite
I&EC PROCESS DESIGN A N D DEVELOPMENT
and speed of agitation are \vithout significant effect under the conditions studied in the second experiment. The results of the second experiment are of a different nature from those of the first. The nitrogen evolution must be taken as a loss of chloramine yield, so that a factor which increases Sz evolution decreases chloramine yield. Taking this into account. the important variables of the two series are grouped together in order of significance. NH,/NaOCl Mole Ratio. Both experiments demonstrate that one of the most important factors affecting chloramine yield is the mole ratio, the yield increasing with increase in mole ratio. This agrees with work published on the over-all hydrazine reaction, Lvhere many workers ( 7 , 5,8 ) have shown mole ratio to be one of the most important factors affecting hydrazine yirld. The interaction Lvith mole ratio shows that the +Id improvement is better with increase in mole ratio at low ammonia concentrations than at high values, \vhile the interaction \vith temperature shows the effect to be greater at 0' C:. than at -5' C. Sodium Hypochlorite Concentration. This is an important variable in both experiments. Reduced concentration leads to improved )-ields in each case. A possible explanation is that chloramine has a reduced tendency to decompose when present in low concentrations: enabling the forivard reaction to predominate. The agreement of interactions between the two experiments is poor, but this can be attributed to the difference in sodium hypochlorite concentrations used in the t\vo experiments. Ammonia Concentration. Ammonia concentration is highly significant in the first experiment, the yield increasing with decreasing concentration. This indicates that a small change in concentration gives a measurable yield increase only at loiv ammonia concentrations. Temperature. This is significant to a small extent in the first experiment but not in the second, higher yields being obtained a t lower temperatures. EDTA Concentration. The nonsignificance of EDTA agrees with the ivork of Bodenstein ( 3 ) >lvho shows that the
2
0
6.0 .
5
z I I
0
4.0
-
2.0
0
Figure 2.
0.5 1.0 1.5 2.0 SODIUM H Y P O C H L O R I T E M O L A R I T Y
2.5
Chloramine concentration and yield contours M o l e ratio NH3/NaOCI 2 to 1 -Chloramine yield contours .- -.-.- - - Chloramine concentration contours
Table IV.
Analysis of Variance for Second Series of Factorial Experiments
Degrees of Sourcr of FreeVartance dom h 1 B 1 C 1 D 1 E 1 F 1 AB 1 AC 1 AD 1 AE 1 1 BC BD 1 1 BE BF 1 CD 1 1 CE CF 1 DE 1 DF 1 EF 1 Blocks 3 Residual 8 Total a Valws sipi'jcacant
Sum of Squares 507
3,280 3.952 142 6,100 138 602 11 2 567 1,067 563 44 263 755 1 >071 219 728 764 97 150 1,194 808 25,022 to 0.05 leuel. ~
Mean Squares sn7 3,280 3,952 142 6,100 138 602 11 2,567 1,067 563 44 263 755 1,071 219 728 764 97 150 398 101 23,519
Direction o f Variance a
a a a b
a
.Mean Square Mean Square of Residual 5 .O 32.5" 39.1" 1.4 60.4= 1.4 6.0a 0.1 25.4" 10.6" 5.6 0.4 2.5 7.4" 10.6" 2.1 7.1a 7.6 0.9 1.4 3.9
chloramine decomposition reaction is not affected by the presence of cupric ions. Sodium Hypochlorite pH. Drago and Sisler (5) have reported that sufficient OH- should be present in the hydrazine synthesis solution to remove the acid formed in the chloramine reaction. Audrieth and Ro\ve (2) have shoivn that in the absence of such a n excess an increase in the OH- concentration from 0.5 to 1.0.21improves yield in the chloramine-forming reaction. This demonstrates that the sodium hypochlorite used in this study contained sufficient OH- initially to satisfy the above requirement. Agitation. T h e insignificance of agitation is explained by- the vigorous nature of the reaction bet\veen ammonia and sodium hypochlorite. Optimization Investigations of Chloramine Reaction. T h e two factorial experiments demonstrated that the most important yield-affecting variables Ivere mole ratio of NH:, to NaOC1, NH3 concentration, and NaOCl concentration. provided that the age of the sodium hypochlorite solutions \vas the same for each experiment. It is unfortunate that improving yield by any of these three factors leads to a decrease in chloramine concentration. To optimize the industrial process both chloramine yield and concentration must be taken into account, a reduced concentration leading to increased recovery costs. For this reason the above three variables rvere studied in more detail over as wide a range as possible, in order to apply cost data to the final curves. Concentrations of 0 to 11.2M ammonia and 0 to 0.25-k' sodium hypochlorite \vere used at mole ratios of 2 to 1. 6 to 1. and 20 to 1. This gave 12 series of experiments, each consisting of five runs. The results are plotred in Figures 2, 3. and 4 in the form of yield and concentration contours for mole ratios of 2 to 1. 6 to 1, and 20 to 1. respectively. The +eld contours are expressed in terms of per cent yield of chloramine. while the concentration contours are in terms of chloramine molarity. The contours on all three plots are very similar in shape and VOL. 3
NO. 1 J A N U A R Y 1 9 6 4
21
12.0
12.0 r
10.0
-
> 8.0
-
10.0
> 8.0
E (L
t
2
a
K
a
6
0
E
i
6.0
6.0.
z
4
I I a
0
z
0
I 4.0.
4.0
2.0
0
0.5 1.0 1.5 2.0 SODIUM HYPOCHLORITE MOLARITY
Figure 3.
2.5
Chloramine concentration and yield contours
0
0.5 SODIUM
Figure 4.
concentration contours
direction. They demonstrate that the over-all tendency is for chloramine yield to increase and concentration to decrease with decrease in ammonia and sodium hypochlorite concentration and increase in mole ratio.
2.5
Chloramine concentration and yield contours M o l e ratio NHs/NaOCI 20 to 1 -Chloramine yield contours _.--_._ Chloramine concentration contours
Mole ratio NHa/NaOCI 6 to 1
-Chloramine yield contours -..__. Chloramine
1.0 1.5 2.0 HYPOCHLORITE MOLARITY
ratioldoes, however? displace the absolute values of yield and concentration. Acknowledgment
Conclusions
The authors thank the directors of Whiffens Ltd., for the financial support that made this study possible.
The follo\\ing variables affect the peak chloramine concentration and yield based on the initial sodium hypochlorite. The first four have the greatest influence.
literature Cited
T h e yield increases with increasing mole ratio, NH3,;SaOCI. This variable interacts with temperature, the effect being less important as the temperature is decreased from 0’ to - 5’ C. The chloramine yield obtained using sodium hypochlorite 7 days old is less than that obtained from new solutions. Low sodium hypochlorite concentrations give improved chloramine yields. ,4 decrease in ammonia concentration gives improved yields; this effect is greatest at low concentrations. Chloramine yield is slightly higher at temperatures of -5’ C. than a t room temperature. T h e improvement is greatest at low temperatures. T h e presence of EDTA does not affect chloramine yield. Agitation does not affect chloramine yield. When the variables ammonia and sodium hypochlorite concentration are plotted over a wide range of concentrations in the form of chloramine yield and concentration contours, these contours are similar in shape and direction for the three mole ratios 2 to 1: 6 to 1, and 20 to 1. Variation of mole
22
I&EC
PROCESS DESIGN A N D DEVELOPMENT
(1) Audrieth. L. F.: Ogg, B. .k, “The Chemistry of Hydrazine,” \\-iley, New York, 1951. (2) Audrieth, L. F.: Rowe: R. .4.,J. Am. Chem. Soc. 77, 4726
(1955). (3)’ Bodknstein, M., Z . Physik. Chem. 139A, 397 (1938) (4) Brownlee, K. A . , “Industrial Experimentation,” 4th ed.. H.M. Stationery Office, London, 1960. (5) Drago, R. S., Sisler, H . H., J.A m . Chem. Soc. 77, 3191 (1955). (6) Fisher, R. .4., Yates, F., “Statistical Tables for Biological Agricultural and Medical Research,” Table V, Oliver and Bovd. London. 1948. (7) Jeffreys, G.’ V., IVharton. J. T., Provisional Brit. Patent 34125/62 (1962). (8) Jones, M . M., Audrieth, L. F., Colton. E., J . A m . Chem. SOC. 77, 2701 (1955). 19) Markwald, it-.,Wille. M.. Ber. 56, 1319 (1923). (10) Metcalf, i i . S., J.Chem. Soc. 1942, 148. (11) Raschig. F.. Ber. Deut. Chem. Ges. 40, 4587 (1907). (12) Raschig, F.; German Patent 192,783 (1906). (13) \$-harton, J. T., Birmingham l‘nic. Chem. Engr. 13, 45 (1962). (14) Wharton, J. T., Ph.D. thesis, Cniversity of Birmingham, England, 1963. ~I
RECEIVED for review February 4, 1963 ACCEPTED July 8. 1963
