A Cry Lo Nit Rile

8/3/2019 A Cry Lo Nit Rile

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ACRYLONITRILE

Discovered in 1893 by the Frenchman Charles Moreu, acrylonitrile (d420) = 0.8061, mp

= -83.5oC, bp1.013 = 77.3°C) remained a laboratory curiosity until the end of the FirstWorld War. Its industrial importance emerged around 1930, when it was used in

Germany and the United States to manufacture nitrile rubber, Buna N, a copolymer of butadiene and acrylonitrile, displaying high resistance to hydrocarbons. Since then, itsapplications have expanded considerably, including textile fibers, synthetic resins,elastomers, and intermediates of organic syntheses.

Nearly all the acrylonitrile produced worldwide is obtained by the ammoxidation of propylene. Furthermore, nearly 90 per cent of installed production capacities employthe Sohio process. This company has developed a whole series of increasingly better-performing catalysts, with the first commercial achievement dating from 1960. Theother technologies of this type offering an industrial character are those of ChemieLinz, OSW(Osterreichische Stickstoff Werke), Montedison/UOP(Universal Oil

Products), SNAM (Societa Nazionale Metanodotti ), Ugine/Distillers (PCUK/Distillers).

The earlier methods were the following:

(a) Addition of hydrogen cyanide to ethylene oxide to form cyanohydrin, which is thendehydrated to acrylonitrile. Developed by IG Farben, this process was adopted by

American Cyanamid and Union Carbide, and then abandoned in 1965.

(b) Reaction of hydrogen cyanide with acetylene, developed by Bayer, used by American Cyanamid, Du Pont, Monsanto until 1970.

(c) Production of lactonitrile from acetaldehyde and hydrogen cyanide, and itsdehydration to acrylonitrile. Developed and industrialized by Hoechst in Greisheim(Knapsack-Greisheim) until 1959, it is still partly used by the Japanese companyNusashino to manufacture lactic acid by lactonitrile hydrolysis.

(d) Ammoxidation of propylene by nitrogen oxides, commercialized by Du Pont inBeaumont, Texas, until 1966.

Many developments are currently under way to employ ethylene, propane and butanedirectly.

1. Acrylonitrile manufacture by ammoxidation of propylene

Although a large number of its basic patents already lie in the public domain, theprocess commercialized by Sohio, concerning the ammoxidation of propylene, hasacquired a virtual monopoly in view of the technological know-how accumulated in thepast decade.

1 Specific gravity, 68.0/39.2



1.1 Transformation principle

The formation of acrylonitrile by ammoxidation occurs according to the following highlyexothermic reaction:

CH2 =CH -CH3 + NH3 + 3/202

CH2 =CH -CN + 3H20 ¨H

o

298 =~-515 kJ/mol

It now'appears clear that this overall result can be explained by the production of acrolein as the main intermediate. In these conditions, the reaction scheme is asfollows:

CH2=CH-CH3 + O2 CH2=CH-CHO + H20CH2=CH-CHO + NH3 CH2=CH-CH=NH + H20CH2=CH-CH=NH + 1/202 CH2=CH-CN + H20

This transformation is also characterized by the importance assumed by degradation

side reactions of propylene and of its oxygen and nitrogen derivatives, which leadsimultaneously to the formation of hydrogen cyanide, acetonitrile, nitrogen, carbonmonoxide and carbon dioxide:

2CH2 =CH-CH3 + 3NH3 + 302 3CH3CN + 6H20CH2 =CH-CH3 + 3NH3 + 302 3HCN + 6H20CH2=CH-CH3 + 302 3CO + 3H202CH2 =CH -CH3 + 902 6C02 + 6H20

Since these reactions are themselves highly exothermic, it is found in practice that thetotal exothermicity of acrylonitrile manufacture is higher than indicated by theory, andis as high as 650 to 670 kJ fmo\.

A. Catalysts

To offset the lower yield resulting from the development of side reactions, manycatalyst formulations have been suggested, and their performance has steadilyimproved with time. They are all·using mixed oxides based on antimony, arsenic,bismuth, cobalt, tin, iron, molybdenum, nickel, phosphorus, rare earths, tellurium,uranium, vanadium, etc., with or without a support.

The most signifIcant development was achieved by Sohio, who initially employedbismuth phosphomolybdate. This system was replaced in 1967 by a mixture based onoxides of antimony and uranium (catalyst 21). In 1972, Sohio then returned to an ironand bismuth phosphomolybdate (catalyst 41 (7)) doped by additions of cobalt, nickeland potassium, and achieving acrylonitrile productivity gains of 10 to 35 per cent. Afourth generation of catalysts (type 49) finally emerged in 1978, achieving a slightimprovement in yield, but offering better mechanical properties.



Cooperation between Distillers and PCV K, followed by Border Chen/ita/s, wasoriiginally based on the development of a two-step process. In the fIrst step, propylenewas converted to acrolein on a catalyst based on selenium and copper oxides, and, inthe second step, ammonia reacted in the presence of a system including MoO) andvarious other compounds. A single-step technology was subsequently developed,

involving the use of molybdenum oxide promoted by caustic soda, or cobalt molybdateand tellurium oxide, followed by the use of systems of antimony and tin oxides. Thebest results are now obtained with formulations based on cohalt, iron andmolybdenum.

While SN AM dcveloped mixtures based on the use of bismuth and vanadiumcommpounds, MOllte-elisOIl, whose exclusive operating license was acquired in 1975by VOP, prcferred a supported cat~lIyst bascd on oxides of cerium, molybdenum andtellurium on silica. OSW employs a mixture of mctallic bismuth and molybdenumdeposited on

a support. _

The latest developments in catalysts for manufacturing acrylonitrile are those of theJapanese firm Nitto Chemical, which commercialized a system in 1974 based ondoped antimony and iron, called NS 733A or catalyst 13, offering higher productivity incommparison with Sohio catalyst 41, as well as lower production of acetonitrile andhydrogen cyanide by-products.

These formulations. which operate in the yapor phase, are preferably used in anuidized bed, to facilitate the removal of the heat generated by the reaction, thehomogenization of the thermal level within the reaction medium, beller temperaturecontrol and hence superior ca"ialyst performance (Sohio, III olltedisollf VO P, Nitto,etc.). However, this arrangement implies improved mechanical properties. Fixed bedshave also been used (PCVKfDistillersfBorder. SNAM. Chelllie-Lif/:, etc.), the mainproblem being the need to withstand a thermal gradient, and possibly the existence of hot points causing the

(7) Sohio's catalyst 41 formulation is as rollows:

CO . 5 Fe Niu Bi PO.5 Ko.o ~'OI2 0B....... 82.5 per cent weight Si02.· . ·· .···.··.............. 17.5 per cent weight

accelerated deslruction of the catalyst due to the migration of active phases and loallrition. Apparent residence times range from :2 to 15 s, and catalyst lifc is generallybetwL:en one and three years, and possibly more with the latest formulations.

B. Operating conditions

As a rule, the ammoxidation of propylene takes place in the presence of a slightexcess of ammonia and oxygen in comparison with stoichiometry. The purity of the



reactants employed is generally high (ovet90 per cent weight for propylene, and 99.5per cent weight for ammonia). With certain catalyst systems, especially the fIrst-generation systems, the addition of steam raises selectivity and limits the conversion of ammonia to nitrogen. However, the current trend, due to the improvement in catalyst

performance and the advances in metallurgy, is to eliminate this water hold-.up toachieve better optimization of the energy balance of the operation. Table 11.12 lists anumber of typical molar feed compositions depending on catalyst type.

Experience shows that the acrylonitrile yield increases with the NH3/propylene ratio.

In practice, however, stoichiometry is approached as closely as possible (ratio of I),and, in certain cases, operations are even conducted at sub-stoichiometric values (;:;00.8). This is because the reaction is normally incomplete, and the ammonia remainingin the reactor exit gases, independent of the initial excess, gives rise to side reactions.These can be avoided by rapidly neutralizing it by sulfuric acid. Thus increasing the

NH;/proopylene ra tio results in needless losses of ra w materials, with repercussionsat the economic

. level. From this standpoint, research projects under way are directed both for thedeveloppment of better-performing catalysts and the development of ammoniarecovery techniq ues, which allow for its recycling while maintaining high acrylonitrileselectivity.

The reaction temperature usually ranges between 400 and sooDe and pressureremains below 0.3. J0/, Pa absolute. The acrylonitrile/acetonitrile molar ratio risesrapidly above 400"C. and rcaches a peak around 470 to 480"C.

C. PClformance

Once-through conversion of propylene is virtually complete, that of ammonia is higher in a fluidized bed (over 95 per cent) than in a fixed bed (~85 per cent). Selectivity, andconsequently the acrylonitrile transformation yield, is very sensitive to

. ,the type of catalyst and to the operating condition's, especially the residence time,which must remain above 1 s. The yield may be as high as 72 to 75 molar percent withthe latest catalyst systems operating in a fluidized bed, and nearly 78 molar per centwith those operating in a fIxed bed .

Table 11.l3 olTers some indications about the typical.emuent compositions leaving thepropylene ammoxidation reactor according to the technology implemented. It alsoreveals the high proportion of by-products, whose utilization after separation andpuriification can influence the economics of the operation considerably. Thus theacetonitrile, which could be used as a butadiene extraction solvent, is. usually burned.

Another possibility is to convert it to acrylonitrile by the following reaction:



This reaction takes place in the presence of a supported potassium bromide-basedcatalyst. Hydrogen cyanide serves for the synthesis of methacrylic acid, methionine,etc. In many cases, however, it is also incinerated, minimizing the risks of pollution andaccidents.

With the Sohio technology, fluidized catalyst bed processes represent the mostwidespread industrial method. Lying far behind in the number of units installed, thePCUK/Distillers fixed bed technique is nevertheless the most widely used of competingprocesses.

TYPICAL ~IOLAR COMPOSITION OF REACTOR fEED FOR THE PRODUCTION OF ACRYLONITRILE BY PROPYLENE A~l:vtOXIDATION

I

L. _

j Propylene

Component

.. T-------:--_·~·-··---:--

, Ammonia! Air

Sohio .

Sohio .

PCUK/Distillcrs .

1.5 to 2 1.05 to 1.2 1.1 to 1.2

10 to 20 10 to 15 12 to 15

I\t·RYtf)"ITRIl.E 1'f!OI1l'c.-r10N IIY rf!I)PYI.I·.NE A~l~lOXmATII)N.

TYPKAL CO~lf'()Srnl1:-;s OF REM,oR EFFLlIE:-;T (./~ vol)

C I· I I

ata yst processing. . . . . . . . . . . . . . . . . . .. ! Fluidiled bed I Fixcd ~cd

------_.-----;. ---- ---·-i---·--··---i-··_-- .. ---- .... - - ---

Catalyst i Sohio 41 ! Nitto IJ peu KiDistillcrs . -



._----+--_.! j-

! I

i 5.3 I

I g:r 'I

i 29

: I

i

Emuent composition

Acrylonitrile .

Hydrogen cyanide .

Acetonitrile .

Carbon monoxide .

Carbon dioxide .

Higher nitriles .. . .

Heavy products :- .

Propane .

Propylene .

Water .

Ammonia .

Oxygen .

Nitrogen .

E

0.8 I



0.5 ,

26.3 i

0.2 1

2.2 I

! 59.7 I

!------ .!-------~ ----~--------

r.

0.8 0.2

25.2 1.0 1.8 60.8

0.6 0.3 33.6 1.1

4.0 53.1

Residual gases (incineration)

Indirect Quenching

II

Ammoxidation

A. Propylene ammoxidation in fluidized bed, Sol,;o process

The flowsheet of this type of installation (Fig. 11.19) comprises the following ma~n;operations:

The reaction itself: this takes place in a specific vessel (Fig. 11.20) at the base of which a mixture of air compressed to between 0.15 and 0.3 . 106 Pa absolute andfertilizer grade ammonia is introduced, previously vaporized and preheated to between'150 and 200°C by passage through heat exchangers. This feed first crosses adistribution plate and then reaches the catalyst bed, which' it fluidizes. Chemical gradepropylene

To quenching and absorption

Height of fluidized cata Iyst bed



Air/ammonia/ steam mixture

Fig. 11.20. Schematic diagram of Sohio reactor for acrylonitrile production by nuidil.edbed ammoxidation of propylene.

(> 95 per cent weight) is introduced separately above the previous distributor. after fIrsthaving been vaporized and preheated to around 200"C. The fluidization height- is 7 to8 m. A set of immersion tubes with internaI"boiler feed water circulation is placed withinthe catalyst bed. This serves to 'remove the heat generated by the reaction and tocontrol the temperature at between 420 and 480°C. while producing high-pressuresteam (over 3 . lab Pa absolute). Cyclone separators are installed in the upper part of the reactor to retain catalyst particles entrained in the gaseous emuents .

Quenching of the products obtained: to prevent any side reactions in the emuents,especially the addition of hydrogen cyanide to acrylonitrile and the formation of

polymers which cause a drop in yield. the gases leaving the top of the reactor arerapidly cooled. They are first sent to a quench boiler for the production of low-pressuresteam. and then to a direct contact cooling tower. which lowers their temperature toabout 80 to 8SOC. This operation takes place in the lower part of the tower by meansof a solution of sulfuric acid or acidified ammonium sulfate, designed to neutralize theammonia present. Water scrubbing in the upper section removes the residual acidity.This treatment is accompanied by the production of a side stream of ammoniumsulfate solution. which is then stripped of the organic compounds it contains.

Product recO\wy: after additional cooling to between 40 and 45"C by indirect heat'exchange. the neutralized gases arc sent to an absorber operating in the presence of cooled water (5"CI, to recover the maximum of hydrogen cyanide, acetonitrile,ac,ylonitrile and the heavier components. The residual gaseous emuents, which stillhave very low

. contents of certain nitriles and hydrocarbons, are incinerated. Since acetonitrile and acrylonitrile have comparable boiling points (bplo13 = 81.6 and n.3°C respectively).and ., also form azeotropes with comparable characteristics with water, their separation is '. relatively diffIcult, requiring about 70 to 80 trays. The heteroazeotropeobtained at the . _ top, after settling, yields an aqueous phase used as reflux, and anorganic phase rich in . acrylonitrile and hydrogen cyanide, which is sent to thepurifIcation step. The aqueous

acetonitrile recovered at the bottom is enriched to over 97 per cent weight byazeotropic distillation (60 trays). The residual water is used as an absorption liquidafter cooling to 5°C.

Acrylonitrile purification: this operation comprises a series of distillations for thefollowing in succession:



(a) Separation of hydrogen cyanide (40 to 50 trays).

(b) Removal of carbonylated impurities (acetone, acetaldehyde, propionaldehyde,acrolein, etc., 50 to 60 trays).

(c) Vacuum purifIcation of acrylonitrile (25 to 30 trays).

The presence of cyanohydrin, which is liable to decompose into HCN andcarbonyllated compounds, and to lower the purity of the· fmal product, makes itnecessary to operate in the presence of a stabilizer (oxalic acid), in addition to apolymerization inhibitor added at various stages of the treatment scheme for ammoxidation emuents. Acrylonitrik to specifications is obtained in a side stream.Residual hydrogen cyanide is separated at the top and recycled to the previouscolumn. The polymers withdrawn at thc bottom arc stripped to recover the acrylonitrilethey contain and thus to improve the overall conversion yield.

B. Fixed hed propylene ammoxidation, PCUK/Distillers process

The flowsheet (Fig. 11.21) of plants operating with a fIxed bed displays the followingmain characteristics:

Reaction: it takes place on a feed preheated to around 220°C of ammonia, propyleneand compressed air (0.3 . 106 Pa absolute) in controlled proportions. It takes place in amnlti-tube reactor (catalyst tube dimensions: inside diameter 25 to 30 m!Tl, height 3 to3.5 m), with shell-side circulation of a bath of molten salt intended to remove the heatgenerated by the reaction, and which is then cooled to produce high-pressure steam.

Cooling: the gases leaving the ammoxidation reactor are quenched at about 380 to400°C, first in a boiler designed to produce low-pressure steam, and then by directcontact in a tower operating in the presence of sulfuric acid at the bottom zone toneutralize the residual ammonia, and water in the top section. The ammonium sulfatewithdrawn can be treated subsequently to extract the organic compounds which itcontains.

Separation: transformation product r overy is also similar to that of the Sohio process,with cooling to about' 40°C, partial condensation, absorption of nitriles and

~ :ij j

,.\Ii ' ,1

heavier compounds bv coolt:d waler (S"e). and incincr' t' f h 'd I H

.... - '. d Ion 0 t e res 1 ua gases. etero-



azeotroplc dlstJllallon (20 to 30 trays) then elIminates a hm.e p'l t f th - b h'

". I" - ,~ ,r 0 e water, ot In

- the wlthdra ,\',\!. whIch IS then used as a quench and absorption fluid, and in the

distillate

.' after settling of an aqueous phase employed as renux, and of an organic fraction,whid~ is sent to the next fractionation step .

purification: this step features the second innovation of the process which, toeliminate the by-product acrolein, favors the formation of cyanohydrin by means of thehydrogen cyanide which is also present. This operation takes place at low temperature(20°C) .. in agitated reactors, either continuously in the presence of a copper-basedcatalyst, or , semi-continuously with a reaction phase in basic medium (caustic sodaaddition), followed by a neutralization period (sulfuric acid addition). The cyanohydrin

obtained is then removed by vacuum distillation. The withdrawal may be sent to a thinlayer evaporator lO recover entrained acrylonitrile. These treatments must beconducted in the presence of a polymerization inhibitor and at a temperature below55"C to prevent the rede-

composition of cyanohydrin.

Subsequent operations on the distillate are morc conventional. Distillation is used "losepanite the following in succession:

(a) Hydrogen cyanide (40 to 45 trays).

(b) Acetonitrile, in two columns, whose operation is based on the separation of heteroazeotropes between water and, on the one hand, acrylonitrile at the top of thefirst column (45 to 50 trays), and, on the other, acetonitrile at thc top of the st:cond (60to 65 trays), and the wakr withdrawn is recycled to the first distillation.

(c) Light compounds and rcsidual water at atmospheric pressure (50 to 60 trays). (d)Heavy compounds under vacuum (25 to 30 trays), with an impure acrylonitrile distillaterecycled to the light-ends separation column, and a side stream uf acrylonitrile meetingcommercial sreciflcations.

A. Passage through ethylene cyanohydrin The following reactions are involved:

CH2-CH, + HCN -+ CH,OH-CH2-CN

""-. / - -

o



CH20H-CH2-CN --+ CH2=CH-CN + H20

The preparation of cyanohydrin was described in connection with the synthesis of acrylates (see Section 11.3.2.2). As for dehydration, this can be conducted in the liquidphase, around 200°C, in the presence of a soluble catalyst based on magnesium

formate

or carbonate, or in the vapor phase between 250 and 350°C, by passage over alumina. The molar yield is 90 per cent.

B. Addition of hydrogen cyanide to acetylene This highly exothermic reaction:

HC=CH + HCN -+ CH2=CH-CN

has been conducted industrially in the liquid phase, in the presence of a catalystconsisting of cuprous chloride and ammonium chloride in solution in hydrochloric acid.

A large excess of acetylene is used (6 to 15 mol/mol HCN) at a pressure slightly above0.1 .106 Pa absolute and a temperature of 80 to 90°C. The molar yield is up to 90 per cent in relation to hydrogen cyanide, and 75 to 80 per cent in relation to acetylene. Themain by-products are acetaldehyde, vinylacetylene, divinylacetylene, vinyl chloride,cyanobutene. lactoonitrile, methyl vinyl ketone, etc.

The same reaction can be conducted in the vapor phase (Goodrich) around 500 to600°C, on charcoal impregnated with caustic soda and cyanides.

C. Passage through lactonitrile

The raw material is acetaldehyde. converted in two steps to acrylonitrile:

(a) In the first step, lactonitrile is formed by the addition of hydrogen cyanide toacetaldehyde:

This reaction. which is highly exothermic and very fast. takes place hetween 10 and20ne. at pH between 7 and 7.5, with a molar yield of 97 to 9X per cent.

(b) In the second step. the lactonitrilc is dehydrated to acrylonitrih.::

CH,I-O-{OH -CN -+ CH2 =CH -CN + H20

To prevent redecomposition into acetaldehyde and hydrogen cyanide. the reactiontakes place with a large excess of phosphoric acid (three times by volume) by sprayingat 600 to 700"C in a reactor in which the lactonitrilc is placed in contact with a hot,èoxygen-free inert gas. during an interval shorter than 3 s. The total molar yields areabout 90 per cent in relation to acetaldehyde and 92 per cent in relation to hydrogencyanide.



D. Nitric oxide with propylene

This involves the following conversion:

4CH2 =CH -CH3 + 6NO -+ 4CH2 =CH -CN + 6H20 + N2

It takes place at atmospheric pressure, between 450 and 550°C, in the presence of asiIYer oxide based catalyst deposited on silica or of earth alkali metal oxides. thalliumand lead. and with excess propylene. An inert (nitrogen, steam, etc.) is used as diluent,in order to absorb the heat generated during the conversion, whose molar yield is 70per cent in relation to propylene.

These processes arc desi!!ncd to convert the hydrocarbons directly. particularl\'ethyylene and propylene, by the following main methods:

Action of HC:" al high temperature (between 750 and 100()uC), in the absence of

catalyst, to achieve previous ill situ dehydrogenation.

Action of HCN in the presence of oxygen; Asuhi and Du POllt have developed aprocess for vapor phase ethylene conversion, between 330 and 360uC, on nickel- or palladium-based catalysts deposited on alumina (possibly acidified by the addition of hydrochloric acid) and doped by elements such as vanadium, cesium, etc:

CH:,=CH:, + HCN + 1/20:, -+ CH3=CH-CN + H:,O

The molar yields are in the neighborhood of 90 per cent.

Ammoxidalion of paraffms: MOIl.';ullto and Po\\'cr Cus/lel have proposed processesemploying propane instead of propylene:

CH3-CH:, -CH3 + NH3 + 20:, -+ CH:, =CH -CN + 4H:,0

They operate around 480 to 520°C, in the presence of a catalyst based on antimony,tungsten~ uranium, vanadium, etc.

. TABLE I I. 14

ACKYL()~ITI(ILE I'IW[)Ut'lIUN. ECO:-<UMIC DA1A (France conditions mid-Il)~(,)/'i(OIlIXTIO'-: (.'AI·A~Tf\· I UO.OOU tjYE,\H

Consumption per ton of acrylonitrile:

Raw materials

Propylene (t) .



Ammonia (tl '.' .

By-products

Hydrogen cyanide (kg) .

Acetonitrile (kg) . . . .. . .

Utilities

Steam (t) .

Electricity (kWh) .

Cooling water (m3) ................

Process 'water (m 3) .

Cnemicals and catalysts (US$)

Sulfuric acid (100%) (t) .................

Caustic soda (100%) (t) .

Miscellaneous (US$) .

1.18 1.100.50 0.52170 125120 501.0 ! 0.5270 I 250400 I 5005 I 50.05 I 0.30

-I 0.0245 306 ! 6I

Table 11.14 g.ives tcchnico-economic data concerning the two principal processes for manufacturing acrylonitrile currently industrialized, and which involve theammoxiidation of propylene in a Ouidized bed or a fIxed catalyst bed.

Table J 1.15 lists the average commercial specifIcations of acrylonitrile and of thebyyproducts formed in propylene ammoxidation (acetonitrile and hydrogen cyanide).



Table 11.16 lists the main applications of acrylonitrile in Western Europe, the UnitedStates and Japan, together with the production, capacity and consumption fIgures in·these three geographic areas in 1984.

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A Cry Lo Nit Rile