Acetic acid purification

Although acetic acid and water are not beheved to form an azeotrope, acetic acid is hard to separate from aqueous mixtures. Because a number of common hydrocarbons such as heptane or isooctane form azeotropes with formic acid, one of these hydrocarbons can be added to the reactor oxidate permitting separation of formic acid. Water is decanted in a separator from the condensate. Much greater quantities of formic acid are produced from naphtha than from butane, hence formic acid recovery is more extensive in such plants. Through judicious recycling of the less desirable oxygenates, nearly all major impurities can be oxidized to acetic acid. Final acetic acid purification follows much the same treatments as are used in acetaldehyde oxidation. Acid quahty equivalent to the best analytical grade can be produced in tank car quantities without difficulties. 

The bottoms from the solvent recovery (or a2eotropic dehydration column) are fed to the foremns column where acetic acid, some acryflc acid, and final traces of water are removed overhead. The overhead mixture is sent to an acetic acid purification column where a technical grade of acetic acid suitable for ester manufacture is recovered as a by-product. The bottoms from the acetic acid recovery column are recycled to the reflux to the foremns column. The bottoms from the foremns column are fed to the product column where the glacial acryflc acid of commerce is taken overhead. Bottoms from the product column are stripped to recover acryflc acid values and the high boilers are burned. The principal losses of acryflc acid in this process are to the aqueous raffinate and to the aqueous layer from the dehydration column and to dimeri2ation of acryflc acid to 3-acryloxypropionic acid. If necessary, the product column bottoms stripper may include provision for a short-contact-time cracker to crack this dimer back to acryflc acid (60). 

The use of more than this quantity of ammonia leads to its reaction with the diazonium salt and the formation of a brown tar, in the presence of which the diphenic acid fails to precipitate in the acetic acid purification. Less than this amount leads to low yields. The quantity employed suffices to... 

The bottoms from the solvent recovery (or azeotropic dehydration column) are fed to the foreruns column where acetic acid, some acrylic acid, and final traces of water are removed overhead The overhead mixture is sent to an acetic acid purification column where a technical grade of acetic acid suitable for ester manufacture is recovered as a by-product. The bottoms from the acetic acid recovery column are recycled to the reflux to the foreruns column. 

Acetic acid purification 18-0-05 Adsorbed phase ordering, AEL 17-0-03... 

Figure 9.10 Acetic acid purification unit of three conventional columns (Long and Lee, 2011). Source Reproduced from Long and Lee, 2011, with permission from John Wiley Sons.

Long, N.V.D., Lee, S.H. and Lee, M. (2010) Design and optimization of a dividing wall column for debottlenecking of the acetic acid purification. Chemical Engineering and Processing, 49, 825-835. 

In the mid-1960s, Paulik and Roth at Monsanto Co discovered that rhodium and an iodide promoter were more efficient than cobalt, with selectivities of 99% and 85%, with regard to methanol and CO, respectively. Moreover, the reaction is operated under significantly milder conditions such as 40-50 bar pressure and around 190 °C [8]. Even though rhodium was 1000 times more costly than cobalt at this time, Monsanto decided to develop the rhodium-based catalyst system mainly for the selectivity concerns, and thus for the reduction of the process cost induced by the acetic acid purification, even if it was necessary to maintain a 14% w/w level of water in the reactor to keep the stability of the rhodium catalyst. In addition, Paulik et al. [9] demonstrated that iridium can also catalyze the carbonylation of methanol although at a lower rate. However, it is noteworthy that the catalytic system is more stable, especially in the low partial pressure zones of the industrial unit. 

Bis [4-(3-Ethynylphenyl-N-phthalimide) ] hexafluoropropane (7) The bisimide was prepared from the reaction of 2,2-bis(3, 4 -dicarboxy-phenyl)hexafluoropropane dianhydride with two moles of 3-aminophenyl-acetylene in glacial acetic acid. Purification by chromatographing on silica gel provided the material with a mp 185-186 C, reported mp 180-186 C. 

Details of process for the acetic acid purification set together with operation conditions (T, P) are presented in Table 18.5. The operation conditions were selected using the LLE data. It should be noted that, in this table concentration is in terms of weight fraction. 

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