Physico-chemical problems relating to the plant required for the manfacture of acetic anhydride from acetone Part I : Thermal cracking of acetone to ketene

Abstract

Various processes fo r the manufacture of a ce tic anhydride today are by oxidation of acetaldehyde, pyrolysis of a cetic Pie. acid and pyrolysis of acetone. Of these,/^acetone-cracking process seems to be the most economic as i t uses the cheapest raw m aterial. The ethylidene diacetate process has become obsolete because of the c o s tly raw m aterials. The lite ra tu re survey reveals that pyrolysis of acetone occurs above 550°G and is of the f i r s t order. A fre e methyl radical chain mechanism has been proposed fo r th is p yrolysis and the existence of fre e methyl rad icals has been shovn experi mentally. But there are d i f f i c u l t i e s lik e ^ o r t chain lengths and ketene decomposition which complicate the fr e e ra d ica l mechanism to an extent th atath eoretically derived rate equation does not f i t the e35)ei?imQital data. Of various m aterials o f construction reported, sta in less steel i s very much used fo r the reactor tube because o f i t s higher s ta b ility to oxidisin g and reducing atmospheres and higher melting poin t. Stainless s te e l containing n ick el has the problem o f coke-formation v;hioh can be avoided by using carbon disulphide. In the present studies, a set o f fiv e rea ction s, based on the fre e radical mechanism o f p yrolysis of acetone has been ABSTRACT formulated. This set shows a l l the products foimed in the temperature range of 600-750°0. These reactions are: GHgOOGHg ------------ + CO GHgGOGHg ------------OHgGO + GH^ 2GH2GO ^ ^ GgH^ + 2G0 20E^00 -----------GH2GGH2 + GO2 2GH2OO ------------ GH^ + G + 2G0. A thermodynamic analysis was made f o r the fe a s ib ilit y o f these reactions and i t was found that ketene fom a tion reaction is fe a sib le above 600°K while the other reaction s are fe a s ib le even at room temperature. The analysis fo r the e ffe c t o f pressure shows that conversions to ketene may increase vath decrease in pressure but i t is n u llifie d as the ketene decompo s it io n is also accelerated. The experimental set up to study th is reaction was fa b r i cated out o f sta in less steel (18^jGr-8/oNi) and con sisted of constant feed arrangement, vaporizer and preheater, rea ctor and product c o lle c t io n unit. Two ca ta lysts i . e . , carbon disulphide and diethyl sulphide were chosen fo r detailed studies which were oriented towards ca ta ly st development and formulation of su ita ble rate equations. Various parameters studied fo r their e ffe c t on conversion of acetone to ketene are ( i ) temperature le v e ls of 650°, 700° and o —3 3 7 50 G, ( i i ) space time ranging between. 1.22 x 10“ to 5 d x 10 hours, ( i i i ) catalyst concent ration o f 0.25, 0.5 and l.O^a fo r carbon disulphide and 0 .3 , 0.5 and 0.8^t> fo r diethyl sulphide. Some duplicate runs v;ere ca rried out which gave the conversions with +5/^ of the reported values. The data so obtained fo r both the ca ta lysts has been o analysed byj^differential method. The p yrolysis o f acetone and formation of ethane have been found to be of f i r s t order mth a ctiv a tion energies o f 71*8 K .ca l, and 87 .1 4 K .ca l. per gram mole fo r carbon disulphide. These a ctiv ation energies fo r diethyl sulphide are approximately 50.0 K .ca l. and 6 8 .5 K.cal„ per gram mole resp ectively. The decomposition o f ketene has been r^resented by a zeroeth order. The a ctiv a tion energies fo r ethylene and carbon dioxide foim ation are 34.65 K .ca l. and 37o62 K .ca l. per gram mole for carbon disulphide and 26.12 K .ca l. and 3 0 .5 K .cal. per gram mole fo r diethyl sulphide r e ^ e c t iv e ly . Analysis has also been made fo r the uncatalysed decomposition o f acetoneo A comparison of the performance o f the two ca ta lysts shows that diethyl sulphideotthoughOgives slig h tly h i^ e r conversions, the conversions to ketene and ketene y ie ld are much higher with carbon disulphide, vhich thus is a b etter ca ta ly st. I t was also seen that the ketene y ie ld progressively decreased as the conversion o f acetone increased. So the se le ctio n o f optimum le v e ls of temperature, catalyst concentration and conversion would be a function of minimum operating co s ts rather than the highest conversion of acetone to ketene.