IT'RILE AND AC YLONITRIL Production by Oxidation of Methallyl- and Allylamine LESLIE M. PETERS, KENNETH E. MARPLE, T. W. EVANS, S. H . McALLISTER, A N D R . C. CASTNER S H E L L DEVELOPMENT C O M P A N Y , EMERYVILLE. CALIF.
W
IDESPREAD
interest Considerable interest has developed during t h e past few The catalyst consists of a years i n t h e use of unsaturated nitriles for t h e preparation during the past few silicon carbide porous aggreof synthetic rubbers, various other polymers, and plastiyears in the use of unsatugate upon which is chemically cizers for polyvinyl compounds. This paper describes a rated nitriles for the prepadcposited a thin silver mirror process for t h e production of methacrylonitrile and acryration of synthetic rubbers, corresponding to about 1% of lonitrile by t h e vapor phase catalytic oxidation of methalresins, and fibers prompted qilver by weight. the investigation of the prolyl- and allylamines. Air is used as t h e oxidant a t about In practice the reactants duction of niethacrylonitrile 450'to 600OC. over a silver catalyst prepared by t h e chemiare preheated separately, cal deposition of silver mirrors on a carrier. Yields of mixed, and passed through an and acrylonitrile by various nitriles of t h e order of 90% are obtained. T h e amines methods. The present paper adiabatic fixed bed of the are readily obtained by t h e ammonolysis of t h e correcatalyst. The reaction is exdescribes the vapor phase oxidation of methallyl- and allylsponding unsaturated chlorides. Details are given of a tremely rapid and exothermic, amine, these being derived pilot plant for t h e production of 200 pounds of nitrile per producing about 175,000. from the corresponding unday. Development of t h e process is discussed, from laboB.t.u. per pound mole of saturated organic chlorides. ratory scale through pilot plant, and t h e effects of process amine oxidized. ApproxiThis investigation was mately 1.2 moles of oxygen in variables are established on t h e basis of both laboratory initiated in the laboratory and and pilot plant data. Crude nitriles of 93 t o 96% purity the form of air are supplied are obtained. These can be used as such for m a n y appliper mole of amine and sufficarried through t o a pilot cations or purified further by fractionation. plant scale. Construction of cient steam added t o function the pilot plant was carried as a diluent; thus the temthrough during the mar years perature rise is limitcd on as part of a broad program of these laboratories in the field of passage through the catalyst bed. Ordinarily from 6 to 12 synthetic rubber. pounds of steam per pound of amine suffice for this. Khile acrylonitrile is used fairly extensively in industry, its A temperature of about 450" C. is required to initiate the homolog, methacrylonitrile, is comparatively little known. reaction. At this temperature mixtures of amine, steam, and air This is largely because, in contrast to acrylonitrile, no reasonably are unrtable, and to prevent loss of amine by noncatalytic oxidaefficient process for commercial production of methacrylonitrile tion and thermal decomposition, it is necessary to preheat the has been developed. I t is believed that the future will find many components separately and mix just before entering the catalyst uses for methacrylonitrile in the preparation of homopolymers and chamber. In general, it is best to limit the maximum temperature in the bed to about 600" C . although in the oxidation of Iwide variety of copolymers which are characterized by their ease of solubility, good processing characteristics, and resistance methallylamine successful operation has been achieved with only slight loss of yield a t temperatures as high as 800' C. t o heat and aging. The purpose of this papel is to discuss in detail only the process A contact time of as little as 0.02 second is sufficient to effect for oxidizing methallyl- and allylamines t o the coiresponding complete conversion of the amine. Considering the nature of the reaction and the temperatures involved, a high order of nitriles. Processes for the production of the unsaturated chloselectivity is achieved and yields of amine to nitrile of 90% are rides, methallyl chloride, and allyl chloride, From the cori-esponding olefins, have been described ( 2 , 5 ) . obtained without difficulty. The major by-products obtained are ammonia and carbon dioxide although a number of other D E S C R I P T I O N OF PROCESS organic compounds are formed in lesser amounts. I n the case of methacrylonitrile synthesis, small quantities of acetone, The conversion of unsaturated amines to the corresponding methacrolein, hydrogen cyanide, acetonitrile, propionitrile, nitriles is accomplished by passing a mixture of amine, air. and acrylonitrile, and isobutyronitrile have been found; these agsteam over a silver catalyst a t a temperature in the range 450" yiegate about 3% of the methacrylonitrile produccd. In acryloto 600' C. At these temperaturrs the predominant reaction is nitrile synthesis, low boiling aldehydes, hydrogen cyanide, acethe direct oxidation of the amine group to form the nitrile and tonitrile, and propionitrile aggregate about 5% of the total two molecules of water. nitrile produced. Boiling points of materials associated with R the proces;: and physical constants of pure methacrylonitrile R I are F ~ ~ O nT Tin Tables I and 11. CH~-&--CH~--~H~ oi--+ CH~=C-C=S + ?H?O
+
2046
INDUSTRIAL A N D E N G I N E E R I N G C H E M I S T R Y
November 1948
furic acid and stripping off the nitriles and other volatile products. Acidification of the reactor products is an important step in the recovery. It prevents loss of the nitrile through reaction with the by-product ammonia or unreacted amine, and a t the same time serves as a convenient method for removing in one step the basic by-products formed in the process. I n the absence of acid, reaction of the unsaturated nitriles with ammonia or amines takes place readily resulting in the formation of high boiling nitrogen bases. Methacrylonitrile of 96% purity or acrylonitrile of 93% purity is obtained directly from the stripping operation. Further purification is accomplished by fractionation according to the quality of product required. Examination of tho boiling points in Table I shows that removal of water and nonnitriles is accomplished easily, but that separation of the b r product nitriles requires more rigid fractionation. For most applications the latter is not necessary; in the case of methacrylonitrile only 2% of by-product nitriles are present, of which half is acrylonitrile. For manv uses, where the presence of a small amount of water is not objectionable, thc crude product obtained directly from the strippers is often suitable. The primary amines used in the synthesis of the nitriles were prepared by the ammonolysis of the corresponding unsaturated chlorides, methallyl chloride (8) and allyl chloride. This process, which also has been carried through on a pilot plant scale, consists of reacting the unsaturated chloride with a large excess of ammonia at a temperature of about 100" C. and a pressure of 1000 pounds per square inch. The reaction products, consisting of amine hydrochlorides and ammonium chloride, are treated with caustic soda and the products recovered by distillation. Yields of primary amine of the order of 85% are obtained readily. Preparations of the unsaturated chlorides by the substitutive chlorination of olefin have been in commercial or semicommercial practice for several years (3,5 ) . A summary of the main reactions involved in the synthesis of the nitriles from the parent olefins is given in Table 111.
Table I . Compounds Associated w i t h Production of Methacrylonitri le and Acrylonitrile Arranged According t o Boiling Point Azeotrope with Water B.P. Anhydrous, 0
26 52.7 52.9 56.4 68.4 77.3 78.7 82 90.3 97 103
Hydrogen cyanide Acrolein Allylamine Acetone Methacrolein dorylonitrile Methallylamine Acetonitrile Methacrylonitrile Propionitrile Isobutyronitrilo
Table I I .
c.
B.p.,
c.
No 52.4 No No 63.9 70.6 78.4 76.1 77 81.8 83.2
W%ht water azeotrope 2.6 azeotrope azeotrope 6.7 12 4.1 16 16 26 30
Properties of Methacrylonitrile
Molecular formula. Structural formula Boiling point a t 760 mm. Melting point Flash point (tag open cup) Suecific gravity (vac.)
Refractive index Viscosity a t 20' C. Surface tensien a t 20' C. Solubility~
CaHaN CHI
2047
Molecular weight 67 09
HD=C-CE-N 90.3' C. -35.8O C. 55O F. d2" 0.8001 di0 0.7896 Coefficient of expansion (20' to 2 5 - C.) = 0.00133 per C. na2 1,4007 n32 1.3954 0 392 centipoives 2 4 . 4 dvnes/om.
D E V E L O P M E N T OF T H E PROCESS Vepor pressure
to
c.
0 10 20 30 40 50 60 70
80 90 90.3 100
Laboratory Experiments. The early exploratoi y n-ork which led to the development of this process (6) was carried out in the laboratory using rather simple equipment. The major portion of the work was concerned with the oxidation of primary allyland methallylamines, with some runs using other unsaturated amines. In a typical run an aqueous solution of the unsaturated amine was metered into a n aluminum preheater held a t 180" to 200" C. The amine-water vapor stream thus produced waa mixed with a gas stream containing the required amount of oxygen and this mixture then passed into a reactor where the actual oxidation took place. The product from the reactor wa9 passed through a series of condensers, cold traps, and water scrubbers, and the remaining gases collected. Several different types of reactors were studied before the design shown in Figure 1 was adopted. In early norlc this reactor was made of glass but later i t was fabricated from steel. The reaction was initiated by heating the reactor with a gas burner. I t was generally necessary when using the steel reactor to supply some heat to maintain the desired temperature The
p(mm.) 16.00 28.48 48.32 78.58 123.12 186.62 274,59 394, It, 550.45 753.62 760 1011.80
-
+
l o g p ( m m . ) = 6.98021 1274'959 t o C. 220.734 d t / d p = 0.0433' C. per mm. a t the boiling point L a t e n t heat of vaporization = 7.6 X 103 cal./mole a t the boiling point Methacrylonitrile is miscible with acetone, octane, and t o h e n e a t 20° to
25" C.
The products leaving the catalyst bed are cooled to about Since by
25" C. and separated into gas and liquid phases. pressure about one third of virtue of vapor the nitrile produced remains 'in the permanent gases, these are passed through an absorution tower where the nitrile is removed by solution in a high boiling organic solvent and recovered by fractionation. The liquid phases resulting from cooling the reactor products contain about two thirds of the nitrile produced along with practically all of the ammonia and other basic materials occurringas by-products. Recovery of the nitrile from the liquid, which is mostly water, is accomplished by acidifying slightly with sul-
Table 1 1 1 .
M a i n Reactions i n Synthesis of Unsaturated Nitriles R
R
Chlorination + Clz --+ HzC=&-CH&I
HaC=(!-CHa Olefin
I
H2C=c--CH2C1 f + Unsatd. Chloride
+ HCI
Unsatd. Chloride
R
xaoHp
Ammonolysis
--+
D
+
HIC=LcH2--NH2 NaCi Unsatd. Bmine
+ HzO (2)
D
LL
LY
H2C=L-cH2--NH2 Unsatd. Nitrile
(1
R
Oxidation + o2----+-
+
H3C=L-czN 2 Unsatd. Nitrile
(3)
2048
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 40, No. 19
and 28 inches in length. A 0.25-inch thermocouple well gases entering the reactor were preheated to 180" C. and by also of stainless steel, extended the full length of the tube heat exchange in the reactor they reached the required tcmpcraFor auxiliary studies when longer residence tinies were desired, ture before striking the catalyst bed. a second tube 1 inch in diameter pas used. The first 12 inchet Once the course of the reaction had been established in thiof the tube weie used as a mixing section for the feed streams and Laboratory reactor an investigation was carried out to determinr contained no catalyst. Following the mixing section, a 12-inch the effect on the yield obtainable of such variables as: ratio of length m-as packed with catalyst, retained in place by 18-8 wire reactants, catalysts, screens. Reaction products passed directly from the reactor tube temperature, oxythrough a stainless steel condenser into a glass separator. Here the gen sources, and cooled products were separated into gas and liquid phases. From catalyst poisons. this separator the gases ITere passed through a series of cold traps The results obcooled by ice, and by solid carbon dioxide t o condense volatile tained in several constituents, and then through two gas washing bottles contypical runs are taining acid and high boiling hydrocarbon, respectively, to given in Table IV. TO CONDENSERS remove ammonia and any remaining organic materials. At the [t was found that in completion of a run the liquid products from the separator and the laboratory reac ttaps and the contents of the acid absorber were combined anrl tor, using allyl reserved for analysis. amine, the molal Several types of silver catalyst were studied in the unit, par ratio of oxygen to ticularly TT ith respect to their mechanical stability on prolonged amine should a t all INLET use in a packed bed. Metallic silver, while perfectly satistinies be greater factor) from the standpoint of activity, was found t o be mechanithan 1.0, whereas callv unsatisfactory because of swelling and disintegration to a with methallylponder with continued use. Several forms of silver were used amine the ratio such as meens, tubing, foil, lathe turnings, and small rings, but CATALYST could be less than BED all eventually disintegrated with use. Far superior were chemicaY 1.0 without serideposits of silver on a suitable porous carrier; of these silicon ously affecting the Figure 1. Reactor Used i n Exploracarbide appeared to be particularly satisfactory. Consequently yield. I n the latter tory Laboratory Work this material was chosen for the pilot plant studies. case, however, unStability of Amine Feeds. One factor which influenced the deconverted methalsign of the preheater system of the pilot plant was the rclativr lylamine was present in the product, and had to be recovered. inatability of the amine feeds when heated to the temperatures Because of the high reactivity of the allylamine with acrylorequired to initiate the oxidation. I n the laboratory process nitrile it was almost impossible to recover unreacted allylamine. development studies it n-as found that serious decomposition of The use of 20 to 30% aqueous solutions of the primary amines amine occurred if the reactants mere preheated together. Rates gave the highest yields. of decomposition for mixtures of methallylamine, steam, and air i n the course of the study a large number of oxidizing catalysts in ratios which are used as feed to the pilot plant are showii in were tried. Of all the catalysts silver metal gave by far the Figure 3. These experiments were carried out a t atmospheric best and most reproducible results. Silver catalysts of many pressure in stainless steel tubes using 1.0 to 1.3 moles of oxygen types were used, but optimum results were obtained r i t h a 3ilvor (an air) and 40 to 120 rnoles of steam per mole of amine. A smd! mirror on silicon carbide catalyst ( 1 , .J). Silvw chips gave comparable results but failed mechanically after continued use. In t,he oxidation of various nnsaturated amines the optimuni temperature for the reaction using silver catalyst was found to be around 500' C. Xi lower temperatures thc reaction stopped while ai increased temperature t,he yields were lowered. Best results were obtained using air as the source of oxygen although mixtures of oxygen with other diluents could be added with only slight lowering of LEAD BATH the yield. The silver catalyst in the case of primary allylamine was inactivated easily and the rcWEHEAEW actor used had bo be scrupulously clean a t all times; 4 vith methallylamine this is seemingly not so important as n o cases of ca.talyst poisoning \?-ereencountered. Process Development Studies. Following the exLIQUID ploratory work which established the basis of the PRODUCT process, and prior to the construction of the pilot plant, a short process development study was made ? to establish reasonable limits of operation and to serve as a guide in design. This work was mainly carried out in the apparatus shown in Figure 2 . Amine, steam, and air were preheated separately and fed into a reactm tube heated by a four-unit electric furnace. Individual current control to each unit provided a satisfactory means of regulating METER temperature. The reactor tube was constructed Figure 2. Laboratory Apparatus Used in Process Development Studies of 18-8 stainless steel, 0.75 inch in diameter of Oxidation of Amines .4-
i r
T.e.
m
November 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY Table IV.
Typical Runs i n Laboratory Synthesis of Unsaturated Nitriles"
Concd. Run 2
Amine Used Allylamine Allylamine
3
Allylamine
4
Methallylamine
20.0
5
Methallylamine
30.0
6
Methallylamine
30.0
NO.
1
to
Aq.
Amine Soln., yo 20.0 20.0 30.0
2049
Catalyst Used Silver chips Silver on silioon carbide Silver on silicon carbide Silver on silicon carbide Silver on silioon carbide Silver chips
Duration of Run, Min. 155 300 117
Total Amine Used, Moles
Mole Ratio 0z:Amih
l.iO
0.504
1.39 2.65
0.500
1.03
Amine Rate, G./R;lin. 0.613
_-
Balance, Mole yo
Re-
Unaccounted
Nitrilr Yield,
85.6
85.3
forb 14.4 14.7
50.6
49.4
50.6 85.a
covered amine
Nitrile
0
1.21
n
85.6
0.78
0
%
85.3
586
0.469
3.94
1.03
9.h
76.9
13.3
493
0.585
4.07
0.83
14.9
76.0
9.1
89.4
408
0.695
3.99
0.86
11.1
79.7
9.2
89.6
These runs were made in the reactor depioted in Figure 1. Average temperature in the reaction zone was 500° C. 12 cc. Oxygen was supplied by air and was preheated along with the amine solution to 180' C. No attempt was made t o recover products other than the amine and nitrile.
tliriount of nitrile was formed as a product of reaction but the major products were ammonia, hydrogen, nitrogen, carbon dinxide, and high boiling nitrogen bases. Even amine and steam mixtures alone in the absence of air were found to be unstable. Figure 4 shows the decomposition of allylamine-steam mixtures as a function of temperature when vaporizing and preheating a 10% aqueous solution in a stainless Jteel coil. On the basis of the data obtained, it was realized that i t would be necessary t o preheat the components of the feed mixture separately and mix just prior to entering the catalyst bed. Residence time between the point of mixing and the catalyst bed proper also must be kept t o a minimum. Because of the sensitivity of the amines it was decided to preheat only the steam and air to high temperature, and merely to vaporize the amine and mix with the preheated steam. I n this way exposure of the amine to high skin temperatures in the preheater was avoided. Stability of Reactor Products. Another factor discovered in the early work, which had a bearing on pilot plant design, was the Instability of the reactor products. It was found during the handling of laboratory samples containing acrylonitrile and methacrylonitrile that reaction of the nitrile with the ammonia and amines usually present in the crude product was fast enough to cause a n appreciable loss of nitrile. The loss was completely halted by slightly acidifying the samples. This principle was followed also in the pilot plant design where the crude reaction products were acidified immediately and the nitriles recovered from the acidic solution by stripping with steam. Attempted recovery of the nitriles without acidification resulted not only in a loss of nitrile but also caused considerable mechanical dif-
7
Volume of the mealgat bed amounted
4
'4Db ..
500
450
PREHEAT TEMPERATURE,%
Figure 4. Decomposition of AllylamineSteam Mixtures While Preheating
-
Aqueous a m i n e solution (10%) vaporized and preheated in 18-8 stainless steel tubing. LHSV 3
ficulty due t o the sublimation of ammonium carbonate in varioue parts in the recovery system. The reactions of the nitriles with ammonia and amines were investigated under conditions similar t o those encountered in the pilot plant, that is dilute solutions at 20" t o 25" C. Reactions were followed by titrating samples with hydrochloric acid after various periods of time using a p H meter and glasscalomel electrodes. The reactions can be followed readily in this manner because both ammonia and the allyl- and methallyl. amines are stronger bases than the products formed by thei?
"r' " '
5 !I
TIME ,SECONDS
Figure 3. Sta bi Iit y of Methal lylam ine-Steam- Ai r Mixtures in 18-8 Stainless Steel Tubes
I
Ol
'
c
'
1
1,
d
'
VOLUME HOI ADDED
"
'
c
'
I
" 10
Figure 5. Titration Curves of Samples of 0.1 Molar Aqueous Allylamine Solutions Saturated w i t h Acrylonitrile 1
-
0 min.; 2
5
-
2 mln.; 3 3 hr.; 6
--
10 min.; 4 64 hr.
-
1 hr.;
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
2050 0
1
2
REACTION TIME -MINUTES 3 4 5 6 7 8 9
1
0
I
REACTION TIME, HOURS
Figure 6.
Reaction of Amines and Ammonia w i t h Mit r i les
-
Aqueous 0.1 M base solutions saturated w i t h n i t r i l e a t 20' t o 25" C. 1 m e t h a l l y l a m i n e plus methacrylonitrile; 2 = ammonia p l u s acrylonitrile; 3 = a l l y l a m i n e plus acrylonitrile
reaction with the nitriles. In this work an exammatlon of the titration curves showed that at least two weak bases were formed. These are probably similar to the primary and secondary amines found by Wiedeman and Montgomery (9) a h o show that three bases are formed when acrylonitrile reacts with concentrated aqueous ammonia. The net reactions are as follows:
C H d H C N $. RSR,
-----f
RHS(CH2CHzCS)
+ RX\TII: --+ RN(CHACH2CK)z ~C€IF=&ITCIS + NH8 --+ N(CH2CHeCS)j
2CH24HCN
R
=
ET, allyl or mrthallyl
Thus, unless the reaction is checked, as much as 3 moles of nitrile can be lost per mole of ammonia. Typical titration curves and the rates of reaction are shown in Figures 5 and 6.
Vol. 40, No. 11
Feed System. Amine for the o-ridation unit was stored its a YO to 40VCaqueous solution in 50-gallon st,eel feed tanks and pumped continuously through a small rotameter to the vaporizer. Here the amine solution was vaporized in a coil-and-shell heat exchanger using 150-pound steam in the coil for hcating. The vaporized amine-water solution was superheat,ed somewhat and left the vaporizer a t 120" to 130" C. The vapors were mixocl first wit,h the preheated diluent steam and then with the pre. heated air just before entering the reactor. At normal rates the residence time between the point of mixing and the catalyst bed was less than 0.03 second. Diluent steam feed for the oxidizer \vas taken directly from the 150-pound service steam supply and passed through a small steel vessel filled with 0.25-inch copper rings to serve as a spray trap and aid in the removal of any sulfur that occasionally might be present,. The flow was controlled and metered by means of an orifice and manometer. Preheating of the steam was carried out in a 1-inch, 18-8 stainless steel coil heated by LL gas-fired furnace. Air feed was taken directly from the prevailing atmosphere through a compressor and filtered through glass wool to remove entrained water and oil. Flow was controlled by means of a rotameter and metered a t 50 pounds per square inch gage through a dry gas meter. Preheating was accomplished in a gas-fired furnace equipped with an 18-8 stainless steel coil similar to that used for steam preheating. Reactor. The pilot plant reactor consisted of a vertical, 48inch length of 4-inch 18-8 stainless st,eel pipe. Catalyst \\-as packed to a depth of 36 inches and held in place by shinless steel screens. In addition to an outlet and inlet a t top and bottom. the reactor TTas supplied with two auxiliary inlets a t points onet,hird and two-thirds the lengt,h of the bed. The auxiliary inlets were connected to t,he steam supply in case it became necessary to quench the reaction a t any time. Kine individual side-entering thermocouple wells were installed for measuring temperatures through the cahlyst bed. Iristantaneous temperature measurements were obtained with a I3ro~r.n
P I L O T P L A N T AND E Q U I P M E N T
A simplified flow diagram indicating the essential equipment of the pilot plant used for the production of unsaturated nitriles is given in Figuie 7. The pilot plant was constructed on a fairly small scale to permit maximum flexibility with regard to mechanical changes and also to conserve feedstocks, which were under allocation from the War Production Board a t the time of erection. The nominal capacity of the plant was 200 pounds of methacrylonitrile or 150 pounds of acrylonitrile per day. Photographs of the pilot plant are shown in Figures 8 and 9. Because of the toxic nature of the materials being handled, the plant was housed in a building walled only on three sides. This gave adequate protection from the weather without impairing proper ventilation. The preheat furnaces u-orc located directly outside the building and were enclvsed by transite fire walls. I n order t o minimize heat losses,, transfer lines were kept t o a minimum length by locating the reactor and preheaters on opposite sides of a common wall.
GAS
WASTE WATER F i g u r e l . Flow Diagram of Pilot P l a n t U n i t for Production of Methacrylonitrile and Acrylonitrile
November 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
205 1
e l e c t r o n i c potentiometer while a continuous temperature record was obtained on a 16-point recording Micromax. Flow through the reactor was upward so a drain was provided a t the bottom of the reactor for removing condensed steam during start-up periods. T h e p r o d u c t s , leaving the reactor at about 600' C., were cooled to about 25" C. by passage through a s t a i n 1e s s steel coil-in-shell heat exchanger using water &s coolant. The catalyst bed had a volume of 0.26 cubic foot and contained only 4 ounces of metallic silver on the carrier. R e c o v e r y System. The cooled products leaving the reactor were passed to a 10gallon steel a c c u m u l a t o r where the gas was separated from the liquid phases. Absorption of the nitriles and other organic reaction products from the uncondensed gases was carried out in a cadmium plated steel column 6 inches in diameter and 20 feet in height, packed with 0.5-inch carbon rings. The scrubbed gases after leaving t h e absorber were metered and vented to the atmosphere. A flame arrester on the vent line prevented possibility of a flash back in the event that inflamFigure 8. Methacrylonitrile Pilot P l a n t Oxidation U n i t mablc mixtures should be ( L e f t t o r i g h t ) Product tanks, stripper, phase separators, solvent surge t a n k , absorber, product coolers a n d vented. A continuous side a m i n e vaporizer, a n d i n s t r u m e n t panel stream of the exit gas was passed through a Leeds and FEEDSTOCKS Northrup thermal conductivity analyzer cell to indicate changes in composition, primarily carbon dioxide content. Amines. Methallylamine and allylamine feedstocks were The recovery of nitriles from the fat solvent from the absorber obtained as carefully fractionated products from the companion was accomplished in a small steel continuous fractionating pilot plant producing the amines, by ammonolysis of the correcolumn, 4 inches in diameter and 11 feet in height, packed with sponding unsaturated chlorides. Before use the pure amines were 0.5-inch carbon rings and equipped with a steam heated reboiler. diluted with water to give a 30 to 40% solution which was fed Fractionation of the nitrile from the solvent is relatively easy and directly to the oxidation unit. The diluted solutions were used no difficulty was experienced in obtaining essentially nitrilein preference to the concentrated products for reasons of safetyfree solvent and solvent-free nitrile. The stripped solvent, after that is, in the event of failure of the diluent steam flow t o the cooling, was then recycled through the absorber. A small quanoxidizer, there would be some steam present always in the feed tity of make-up solvent was required periodically to replace that to limit the temperature rise. Also in the small scale plant the lost as vapor in the exit gases from the absorber. I n the pilot increased feed rates resulting from the dilution facilitated more plant operations this amounted to about 2 pounds of solvent per accurate metering and control than would have been possible 100 pounds of nitrile produced. otherwise. Recovery of the nitriles from the liquid phases of the cooled Solvent for Absorber. The choice of satisfactory solvents for reaction products was accomplished by feeding these to a 4-inch use in the absorption system was somewhat limited because of the diameter stainless steel stripping column along with sufficient many conditions imposed. For pilot plant use the following sulfuric acid to give a slight excess of acid in the stripper bottoms. properties were desired: Stripping was accomplished by the injection of live steam into Low cost and availability in drum quantities. the bottom of the column. Water layer, resulting from conHigh solvency for nitriles. densation of the overhead nitrile-water azeotrope, was returned Chemical inertness to nitriles and other reaction products. t o the column and the nitrile layer withdrawn as product. This Thermal stability at the boiling temperature for long periods of was combined with the nitrile recovered from the absorber solvent time. to give a crude product for further purification as required. 9
INDUSTRIAL AND ENGINEERING CHEMISTRY
2052
Vol. 40, No. 11
of catalyst, the silvering technique used was similar t o the Brashear formula described by Gardher and Case (5) for the making of mirrors. By suitable modifications this mas adapted to the silvering of large batches of catalyst carrier. The process involves the reduction of ammoniacal silver solutions with dextrose. Common metals, including copper, have little effect in the silvering operation, and the catalysl can bc prepared in the reactor in which it is to be used 01 in any other suitable vessel. It is necessary t o use chloride-free n-ater in the preparation of the silvering solutions and to clean the carrier of grease, etc., by heating in a stream of air a t 500" to 600' C. for about 30 minutes. Thje latter operation is conveniently done in the reactor. The amount of silver deposited on the carrier depends on the quantity of silvering solution used. I n the pilot plant work sufficient solution was used t o contain silver to the extent of 2% of the dry carrier This then resulted in a finished catalyst having 1.2 to 1.4% silver. The catalyst i p not expensive. The pilot plant reactor contained only 18 pounds of finishcd catalyst (4ounces of silver) and extremely satisfactory catalyst lives were obtained. TYPICAL O P E R A T I N G RESULTS
Figure 9.
Typical operating conditions, flow rates, and yields experienced in the pilot plant were given in Table VI. Operation of the plant in most of the experimental work wae carried out on a 24-hour day, 5-day week basis. XO difficulties were associated with starting up or shutting down and the plant could be brought on stream in as little as 2 hours after the initial start-up. Operation vias extremely steady and once flows were established little attention was required. The main point of control was in maintaining the correct oxygen-to-amine ratio. In this connection the continuously recording thermal conductivity gas analyzer in the exit gas stream aided considerably as there is a direct relation between the carbon dioxide content of the exit gases and the oxygen to amine ratio. Occasionally a check for unreacted amine was made by chemical analysis.
Methacrylonitrile Pilot Plant
A m i n e solution vaporizer (left);
oxidatlon reactor showing t e m p e r a t u r e control welis (right)
Low vapor pressure t u miniinize solvent lovves in the exit gases. Boiling point lower than 185" C. SO that reboiling with 150pound steam would be possible. As a safety precaution, fire and flash points higher than atmospheric temperatures. Of readily available materials, Shell solvent TIS-16 appeared best to meet these requirements. This is an aromatic solvent of petroleum origin having a boiling range of about 130" t o 190" C. This range was a little wider than desired so the solvent was topped prior to use and the fractions boiling above 155' C. were retained for use in the absorber. PREPARATION
I
6
1
I
12 8 10 FOUNDS STEAM PER POUND AMiNE
O F CATALYST
Catalyst for pilot plant use is preparcd by chemically depositing 8 silver mirror on 4- to 6-mesh silicon carbide (Carborundum) porous aggregate. In laboratory preparations of small quantities
Figure 10. Effect of Diluent Steam Rate on Yield of N i t r i l e in Oxidation of Methallylarnine Reactants a t 460' C. entering reactor
November 1948
Table V.
INDUSTRIAL AND ENGINEERING CHEMISTRY
-.
Tvaical Pilot Plant Flow Rates and Operating Conditions
Type of Operation Feed rates to reactor: Aq. amine s o h , concn. wt. ?& rate, Ib./hr. Bteam Ib./hr. Air, sthdard eu. ft./min. lb. moledhr. 100% amine, Ib./hr. lb. moles/hr. Total water, lb./hr. lb. moles/hr. Total feeds, lb./hr. lb. moles/hr. Mole ratio: 0 ~ : a m i n e Mole ratio: Ha0:amine Weight ratio: Hz0:amne Reactor inlet pressure, Ib./sq. in. gage Reactor outlet preseure, lb./sq. in. gage Air preheat temp. (at furnace), C. Bteam preheat temp. (st furnace), C. Temp. entering patalyst bed, ' C. Residence time in total catalyst, sec. Solvent to absorber, gal./hr. 5olvent to absorber, a C. Solvent from absorber, C. Gas to absorber O C. Bolvent stripperlfeed temp., C. 5olvent stripper head temp., C. Solvent stripper kettle temp., C. Aoid to crude nitrile stripper, H&Oi lb,/hr. Crude nitrile stripper feed temp., O b. Crude nitrile stripper head temp., O C. Crude nitrile stripper kettle temp., ' C . Yield, mole % of amine fed
Methacrylonitrile from Methpllylamne 40.0 23.3
90.0
4.37 0.73 9.31 0.131 104.0 5.78 135.1 6.64 1.17 44.1 11.2 17.8 10.2 575 590 452 0.13 10 to 15 22 23 31 130 90 174 0.5 80 77 100 91.2
Acrylonitrile from Allylamine 31.3 23.8 90.0 4 . 6 to 6 . 3 0.77-1.05 7.45 0.130 106.3 5.91 136 t o 144 6 . 8 1 to 7 . 0 9 1.2-1.7 45.5 14.3 20-23 13-16 575 590 455 0.15 10 to 16 22 28 22 120 . 76 173
.
2053
460" C. was used. The main effect of increasing the maximum temperature was to increase the amount of ammonia formed. Oxygen to Amine Ratio. Another variable studied extensively in the pilot plant was the effect of variations in the ratio of oxygen to amine fed t o the reactor. It was found that this ratio was critical and that just sufficient oxygen should be used t o effect substantially complete conversion of the amine, but no excess, In practice this was easily controlled by using the carbon dioxide content of the exit gases as a guide. Too little oxygen results in low yields of nitrile due to unreacted amine whereas too much oxygen causes low yields due to carbon dioxide formation. The optimum yield of nitrile was obtained with an oxygen to amine ratio giving about 0.5% unreacted amine. *These relations are shown in Figure 11 for methacrylonitrile synthesis, where runs of over 1000 hours' duration have been made with no significant loss of catalyst activity.
0.7
7n .70 100 85.8
When synthesizing methacrylonitrile in semicommercial quatitities, continuous runs of 40 days' duration were made with practically no change in flow rates or conditions. In acrylonitrile synthesis some loss of catalyst activity takes place but spent catalyst is conveniently reactivated by resilvering. VARIABLES S T U D I E D IN T H E P I L O T PLANT
Reaction Temperature. One of the variables studied in the pilot plant was the effect of the quantity of diluent steam used for controlling the reaction temperature. While preliminary studies in the laboratory had indicated roughly the temperature ranges which might be practical, the operation of a truly adiabatic system was reserved for pilot plant study. A calculation of the theoretical heat of reaction, based on bond energies (7) and known values for the heat of combustion of hydrogen, indicated a A H of about 155,000 B.t.u. per pound mole of amine. However, in actual practice because of side reactions a AH of about 175,000 B.t.u. is realized. This heat of reaction is absorbed mainly by the diluent steam in the reaction mixture. If no steam were present a temperature rise of about 1000" C. would be expected. Under the steady conditions of plant operation it was found possible in methacrylonitrile synthesis to go to temperatures as high as 800" C. with only moderate reduction in yield. The effect of the ratio of diluent steam to amine on the yields obtained Is shown in Figure 10. A reactor inlet temperature of about
\CARBON
DIOXIDE
w UNREACTED AMINE A
I A
I
I20 I25 MOLAR RATIO, OXYGEN /AMINE
Figure 11.
I
I30
AI
13'
Effect of Oxygen: Amine Ratio on Yields in Oxidation of Methallylamine LITERATURE C I T E D
(1) Bergsteinsson, I., and Finch, H. deV. (to Shell Development Co.) U. S. Patent 2,424,085 (July 15, 1947). ENO. (2) Burgin, J., Engs, W., Groll, H. P., and Hearne, G., IND. CHEM.,31, 1413-19 (1939). (3) Fairbairn, A. W., Cheney, H. A., and Cherniavsky, A. J., Trane Am. Inst. Chem. Engrs., 43, 280-90 (1947). (4) Finch, H. deV., and Bergsteinsson, I. (to Shell Development Co.) U.S. Patent 2,424,083 (July 15, 1947).
(5) Gardner, I. C., and Case, F. A., Natr. Bur. Standards, Circ., 389 (1931).
(6) Marple, K. E., Evans, T. W., and Borders, B. (to Shell Development Co.), U.S. Patent 2,375,016 (May 1, 1945). (7) Pauling, L., "Nature of the Chemical Bond," Ithaca, N. Y . ,
Cornel1 University Press, 1939.
(8) Tamele, M. W., and Groll, H. P. A . (to Shell Development G o . ) , U. 8. Patent 2,172,822 (Sept. 12, 1939). (9) Wiedeman, 0. F., and Montgomery, W. H., J. Am. Chem. Boc., 67, 1994-6 (1945). R ~ C E I V EFebruary D 25, 1948.
E N D OF S Y M P O S I U M ON P I L O T P L A N T DESIGN AND CONSTRUCTION
