Process for recovery of products from a waste stream in the manufacture of acrylonitrile

Process for recovering reactants and products e.g., acrylonitrile and hydrogen cyanide from an aqueous stream obtained in the ammoxidation of propylene which comprises heating an aqueous stream containing the afore-said compounds in a heat exchanger by circulating the stream at a velocity of at least 6 ft/sec through the exchanger while maintaining the stream in the liquid state to increase the temperature of the stream to at least 105.degree. C and thereafter rapidly reducing the pressure of the heated stream sufficiently to vaporize 0.2-5% thereof and thereafter returning the vapor to the acrylonitrile process for recovery of energy used to heat the aqueous waste; usable materials e.g., acrylonitrile and hydrogen cyanide and compounds such as ammonia which can be used to neutralize acid streams and subsequently either incinerating the unvaporized material directly or using the unvaporized material as fertilizer after appropriate treatment.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention is directed to a process for treating the aqueous 
waste from a process for the production of acrylonitrile by ammoxidation 
of propylene. 
2. Description of the Prior Art 
Typical processes for the preparation of acrylonitrile by the ammoxidation 
of propylene are described in U.S. Pat. Nos. 2,904,580; 3,876,508 and 
3,936,360. These processes involve combining propylene, ammonia and air 
and passing that mixture over a suitable catalyst to produce 
acrylonitrile. The off gas from the reaction is initially directed to a 
cooling tower where the excess ammonia is neutralized with aqueous acid. 
Most of the desired products, e.g., acrylonitrile pass overhead through 
the cooling tower while the remaining products and by-products are 
absorbed in, or carried with, the aqueous solution exiting the base of the 
tower. This aqueous solution is then directed to a distillation column 
(waste water column) where most of the volatile materials, e.g., residual 
acrylonitrile and hydrogen cyanide are taken overhead. The tails from this 
distillation column, or waste water column, contain high boiling 
impurities, ammonium sulfate and reduced amounts of acrylonitrile, 
hydrogen cyanide and ammonia. This stream is particularly illustrated and 
described as the discharge through conduit 10 from column C of FIG. 1 in 
U.S. Pat. No. 3,876,508. Typically, this stream exhibits pH of 4.5-6.0 and 
contains (in percent by weight based on the total weight of the stream) 
2-6% ammonium sulfate, 0.1-0.7% HCN as cyanohydrins, 0.01-0.5 
acetonitrile, 0.01-.1% acrylonitrile, 0.001-.1% propionitrile, 0.001-0.05% 
acetaldehyde and 2.0-5.0% high boilers, e.g., nitriles such as 
fumaronitrile and higher molecular weight compounds such as polymers of 
acrylonitrile, acrolein and hydrogen cyanide. The remainder of the stream 
is essentially water. 
The economical disposition of the aqueous waste stream from the above 
described acrylonitrile process has been extensively described in the art. 
Most of the processes are directed to the recovery of purified ammonium 
sulfate for use e.g., as a fertilizer. U.S. Pat. No. 3,711,597 discloses a 
process for the recovery of ammonium sulfate by adding nitric acid to a 
specific concentration and thereafter evaporating the resultant mixture at 
40.degree.-120.degree. C following which ammonium sulfate is separated 
from the evaporated solution. U.S. Pat. No. 3,902,859 discloses the 
recovery of ammonium sulfate by concentrating the aqueous waste solution 
to the maximum degree possible while maintaining the salts in solution and 
thereafter adding an alcohol to precipitate the ammonium sulfate. U.S. 
Pat. No. 3,408,157 teaches the addition of mineral acid to the aqueous 
waste to precipitate heavy organics following which the material is 
filtered and the concentrate treated to precipitate relatively pure 
ammonium sulfate. U.S. Pat. No. 3,756,947 discloses a process for treating 
a waste water containing nitriles and cyanide by passing the waste through 
an activated sludge containing a specific form of bacteria. The removal of 
troublesome hydrogen cyanide from the waste stream by contacting with 
formaldehyde at a pH less than or equal to 3 is described in German Pat. 
No. 2,202,660. The waste stream has also been extracted with acetonitrile 
to remove organic matter prior to recovery of relatively pure ammonium 
sulfate as set forth in U.S. Pat. No. 3,607,136. Many prior techniques, 
for example, the process disclosed in U.S. Pat. No. 3,404,947, are 
concerned with disposing of the aqeuous waste stream by incineration. 
Alternate techniques for alleviating the substantial problem whereby this 
aqueous stream causes plugging of equipment when attempts are made to 
concentrate it involve the addition of ammonia or amines as disclosed in 
U.S. Pat. No. 3,468,624. More elaborate methods for treating the stream to 
recover ammonium sulfate are disclosed in British Patent No. 1,314,047 
wherein complexing agents and a solvent such as dioxane, dimethylformamide 
or a lactam are added to the aqueous waste to prevent contamination of the 
crystallized ammonium sulfate. 
None of the foregoing art discloses a process which permits the recovery of 
ammonium sulfate-organic containing solution which can be recovered for 
use as a fertilizer along with the recovery of a vapor stream containing 
unconsumed reactants such as ammonia which can be used elsewhere in the 
process and compounds such as acrylonitrile and hydrogen cyanide which can 
be recycled to the process. 
SUMMARY OF THE INVENTION 
The present invention provides a process for the recovery of reactants such 
as ammonia and products such as acrylonitrile from an aqueous stream from 
the waste water column of an acrylonitrile process as described 
hereinabove. The reactants and products along with ammonium sulfate are 
recovered by passing the stream through a heat exchanger at a velocity of 
at least 6 ft/sec usually at a velocity at a range 6-15 ft/sec and 
preferably at 8-10 ft/sec while maintaining the stream in the liquid state 
whereby the temperature of the stream is increased to at least 105.degree. 
C, usually 120.degree.-150.degree. C and preferably 
130.degree.-135.degree. C and thereafter rapidly reducing the pressure of 
the heated stream, e.g., by flashing to a degree sufficient to vaporize 
0.2-5% by weight and preferably from 0.3-1.5% by weight. The pressure in 
the vaporizer is usually maintained in the range 0-25 psig and preferably 
in the range 10-15 psig. After the stream is vaporized the volatile 
material can be returned directly to the acrylonitrile process for 
recovery of certain reactants such as acrylonitrile and hydrogen cyanide 
as well as ammonia in a form suitable for neutralization of acid streams 
in the process while the unvaporized material can be incinerated directly 
or applied as a fertilizer after appropriate treatment.

DETAILED DESCRIPTION OF THE INVENTION 
The process of the present invention is practiced using the apparatus set 
forth in the FIGURE attached hereto and made a part of the specification. 
Steam is introduced via line 1 into heat exchanger 2 and condensate is 
removed from the heat exchanger via line 3. The aqueous waste stream is 
introduced into the apparatus via line 4 and thereafter directed via line 
5 to pump 6 where it is pressurized and forced via line 7 through heat 
exchanger 2 and thence via line 8 through orifice 9. The hot stream under 
pressure is then introduced into flasher 10 which is equipped with an 
internal separating means comprising a column and hat portion identified 
as item 11. The vapor is withdrawn from the flasher via line 12 where it 
is subsequently treated to recover desirable products contained therein. 
The liquid bottoms are circulated through lines 13, 5 and 7 at a rate of 
at least 6 ft/sec through the heat exchanger 2. The circulation rate is 
maintained at a volume which is high relative to the amount of waste water 
introduced via line 4. The concentrated solution containing ammonium 
sulfate is removed continuously via line 15 so that a liquid level is 
maintained in flasher 10 at approximately location 14. 
The temperature of the waste water exiting heat exchanger 2 via line 8 must 
be maintained at least 105.degree. C to provide sufficient energy for 
vaporization and to prevent undesired precipitation of ammonium sulfate 
and/or high boilers and polymers contained in the aqueous solution. 
Usually the temperature of the stream exit the heat exchanger 2 is 
maintained in the range 120.degree.-150.degree. C and preferably in the 
range 130.degree.-135.degree. C. 
In order to minimize fouling of heat exchanger it has been discovered that 
it is necessary to maintain a minimal velocity of liquid through the 
exchanger of at least 6 ft/sec and usually a velocity in the range 6-15 
ft/sec. It is preferred to maintain a velocity through the heat exchanger 
in the range 8-10 ft/sec. 
As would be apparent to one skilled in the art the degree of vaporization 
can be controlled by either the temperature of the stream entering flasher 
10 or the pressure within flasher 10 relative to the entering stream. It 
is preferred to conduct the flasher at a pressure in the range 0-25 and 
preferably 10-15 psig in conjunction with the above discussed stream 
temperature, thus at least 30% of the entering stream (line 4) will be 
vaporized under the least severe conditions and as high as 95% can be 
vaporized by increasing the temperature and/or decreasing the pressure 
within flasher 10. It is preferred to vaporize between 80 and 95% of the 
stream (line 4) for optimum results. 
The amount of material vaporized per pass through orifice 9 is maintained 
at a low level and the desired compounds can be removed with the vapor 
while the portion of the stream in the heat exchanger is maintained under 
liquid conditions which minimize fouling of the equipment. 
The following examples are presented to illustrate but not to restrict the 
present invention. Parts and percentages are by weight unless otherwise 
noted. 
EXAMPLE 1 
A stream from a waste water column of an acrylonitrile plant was analyzed 
and found to contain 1.56% sulfate (2.15% ammonium sulfate as calculated 
from analyzed sulfate) 0.02% acetonitrile, 0.01% acrylonitrile, 0.05% 
propionitrile, 0.01% acetaldehyde and 0.36% hydrogen cyanide. pH of the 
stream was 5.0 and it contained 6.43% solids. The stream was fed to the 
apparatus described in FIG. 1 in which the flasher 10 was a cylindrical 
vessel 10 feet in diameter by 14 feet high which contained separator 
components as illustrated in the FIGURE. Two heat exchangers 2 and pumps 6 
along with the associated piping were connected to flasher 10. Each heat 
exchanger 2 had an effective heat exchange area of 3,350 sq ft and each 
pump 6 had a capacity of 5,000 gallons/min. The above described feed 
stream was introduced via line 4 to each of the pumps 6 for a total feed 
rate to the apparatus of 40,500 pounds/hr. Steam via line 1 the heat 
exchanger was adjusted to maintain the exit solution via line 8 through 
orifice 9 and thence into flasher 10 at a temperature of 
132.degree.-134.degree. C. After maintaining equilibrium and establishing 
the liquid level indicated at 14 in the flasher 10 while recirculating 
material via line 13, the overhead vapor rate via line 12 was measured to 
be 32,000 pounds/hr. The bottom flow removed from the system via line 15 
was calculated by difference to be 8,500 pounds/hr. The average holdup 
time was 2.9 hours. The overhead vapors were analyzed and found to contain 
no sulfate, 0.09% ammonia, 0.02% acrylonitrile, 0.04% acetonitrile, 0.16% 
propionitrile, 0.01% acetaldehyde and 0.26% hydrogen cyanide and exhibited 
a pH of 6.0. The bottom flow exit line 15 was analyzed and found to 
contain 8.04% sulfate, 3.4% ammonia, 0.01% acetonitrile, 0.01% 
acrylonitrile, 0.07% propionitrile and 0.08% hydrogen cyanide and 
exhibited a pH of 4.7. Acetaldehyde was below detectable levels. The 
overhead vapor stream was returned to a suitable location in the 
acrylonitrile process and resulted in the recovery of ammonia (as an 
aqueous solution), acrylonitrile and hydrogen cyanide. 
Extended operation as above described did not produce significant fouling 
or plugging of the apparatus. 
EXAMPLES 2-13 
Example 1 was repeated and the results are reported in Table 1. In each 
instance the vapors from flasher 3 were returned to the acrylonitrile 
process to recover the desired materials without adversely affecting the 
process operation. 
As should be apparent from the foregoing examples hydrogen cyanide is 
obtained via the decomposition of cyanohydrins which is formed by the 
conditions of the concentrating operation especially by the extended 
holdup time in the flasher and circulating loops. 
In addition to the above described advantages the reduction in volume 
occasioned by the concentrating reduces the cost of disposal of the waste. 
TABLE 
__________________________________________________________________________ 
Flows Vaporization Temperature 
Solution 
(thousand lb/hr) Separator % of Heater 
(.degree. C) 
Hold-Up 
Feed 
Vapor 
Bottoms.sup.1 
Pressure 
% of Feed 
Exit Flow Exit Time 
Ex. 
(line 4) 
(line 12) 
(line 15) 
(PSIG) 
(line 4) 
(line 8) 
Flasher 
Exchanger 
(Hrs.) 
__________________________________________________________________________ 
2 51 15 36 14 29 .3 .7 
3 47 19 28 7-8.sup.2 
40 .4 .9 
4 30 19 11 7-8.sup.2 
63 .4 115 127 2.3 
5 43 18 25 7.5 42 .4 115 1.0 
6 45 32 13 7 71 .6 115 1.9 
7 42 30 12 13-14.sup.2 
71 .6 123 2.1 
8 46 24 22 13-14.sup.2 
52 .5 123 1.1 
9 51 33 18 13-14.sup.2 
65 .7 125 133 1.4 
10 43 28 15 13-14.sup.2 
65 .6 125 132 1.7 
11 43 29 14 13-14.sup.2 
67 .6 124 132 1.8 
12 40 31 9 13- 14.sup.2 
78 .6 125 2.8 
13 40 31 9 13-14.sup.2 
78 .6 125 132 2.8 
__________________________________________________________________________ 
.sup.1 By difference 
.sup.2 Estimated