Adipic acid process

Adiponitrile is made commercially by several different processes utilizing different feedstocks. The original process, utilizing adipic acid (qv) as a feedstock, was first commercialized by DuPont in the late 1930s and was the basis for a number of adiponitrile plants. However, the adipic acid process was abandoned by DuPont in favor of two processes based on butadiene (qv). During the 1960s, Monsanto and Asahi developed routes to adiponitrile by the electrodimerization of acrylonitrile (qv). 

One of the methods used to isolate succinic acid from the waste stream of the adipic acid process is esterification of the mixture of succinic, glutaric, and adipic acid followed by fractionation (65—69). 

The adipic acid process we have developed involves butadiene oxidative carbonylation in the presence of methanol, a l, l-dimethoxycyclohexane dehydration agent, and a palladium(ll)/ copper(ll) redox catalyst system (Equation 1.). The reaction sequence includes an oxycarbonylation, hydrogenation and hydrolysis step(17-19). The net result is utilization of butadiene, the elements of synthesis gas, l, -dimethoxycyclohexane and air to give adipic acid, cyclohexanone and methanol. 

The main raw materials for the ARCO adipic acid process are butadiene, carbon monoxide and hydrogen. Process economics will be attractive in the next decade provided scenarios regarding the price of butadiene versus benzene come to fruitation. Table I. shows raw material cost for the ARCO adipic acid process versus conventional oxidation technology. For the time frame of this evaluation, the... 

An interesting aspect is the utilization in the process of the N2O co-produced in the oxidation of KA oil to adipic acid (Section 2.2). This allows a waste product of the integrated production cycle to be reused in the first stage of the cycle also saving on disposal costs. Solutia has recently announced, but has not yet implemented, the commercialization of an integrated adipic acid process based on this concept. 

The adipic acid process is a relatively complex process and essentially contains two plants phenol hydrogenation and KA oil oxidation. We should therefore assume at least four shift positions for each plant, say nine total. For a Northeast Asia basis, we expect that the salary cost per shift position will be lower than the typical 50,000 per year that we would assume for a U.S. Gulf Coast plant. As a first approximation this is estimated as 30,000/y. The remaining salary and overhead costs are fixed following the assumptions given in Section 6.2.4. 

A further step was taken to incorporate the phenol scheme into an overall adipic acid process. Eq. (2) summarizes one such possibility. 

Approaches for the synthesis of adipic acid are shown in Figure 2.12. The basic raw materials are benzene, cyclohexane, phenol, acrylates, and butadiene. The principal commercial processes are based on the oxidation of cyclohexane, which usually proceeds in two stages. The first step entails oxidation with air, yielding either a mixture of cyclohexanone and cyclohex-anol (process 1, Figure 2.12) or predominantly cyclohexanol (process 2, Figure 2.12). These reaction products are oxidized in the second stage with nitric acid to adipic acid. Process 1 employs a soluble cobalt oxidation catalyst [133], reaction temperatures in the range of 150 160°C, pressures between 800 and 1000 kPa, and catalyst concentrations of 0.3-3 ppm. At conversions of 5-10%, the selectivity with respect to the cyclohexanone yclohexanol mixture is about 70 80 mol %, with an alcohol/ketone ratio of about 2 1. In process 2 the oxidation is carried out in the presence of boric acid or its anhydride. This results in mixtures particularly... 

FIGURE 2.12 Block diagram of adipic acid processes. 

Single-step oxidation of cyclohexane to adipic acid (process 5, Figure 2.12) has been demonstrated [142]. This process involves a liquid-phase air oxidation using acetic acid as a reaction medium and cobalt acetate as an oxidation catalyst. The reaction temperatures are in the range of 70 90°C. At residence times of 6 10 h, conversions to about 80% were obtained with selectivities to adipic acid of 70-75%. Several alternate processes have been described for the oxidation of cyclohexane to form adipic acid [143 148]. 

Once again, modifying the reaction conditions may offer scope for improvements, one option being to settle for low conversion and recycle reactants a number of times, the approach used in the first stage of the adipic acid process mentioned above. The cost of separating reactants and recycling them will usually be an issue. 

Solutia (USA), in joint work with the Boreskov Institute of Catalysis, Russia, developed a one-step process to manufacture phenol from benzene using nitrous oxide as the oxidant (see Fig. 3.12). Nitrous oxide (a greenhouse gas) is a waste product from Solutia s adipic acid process. The preferred catalysts are acidified ZSM-5 and ZSM-11 zeolites containing iron or a silica/alumina ratio of 100 1 containing 0.45 wt% iron(III) oxide. The catalyst s half-life is 3 to 4 days, and it can be restored by passing air through the bed at high temperatures. 

The reaction of adipic acid with ammonia in either Hquid or vapor phase produces adipamide as an intermediate which is subsequentiy dehydrated to adiponitrile. The most widely used catalysts are based on phosphoms-containing compounds, but boron compounds and siHca gel also have been patented for this use (52—56). Vapor-phase processes involve the use of fixed catalyst beds whereas, in Hquid—gas processes, the catalyst is added to the feed. The reaction temperature of the Hquid-phase processes is ca 300°C and most vapor-phase processes mn at 350—400°C. Both operate at atmospheric pressure. Yields of adipic acid to adiponitrile are as high as 95% (57). 

Reduction. Hydrogenation of dimethyl adipate over Raney-promoted copper chromite at 200°C and 10 MPa produces 1,6-hexanediol [629-11-8], an important chemical intermediate (32). Promoted cobalt catalysts (33) and nickel catalysts (34) are examples of other patented processes for this reaction. An eadier process, which is no longer in use, for the manufacture of the 1,6-hexanediamine from adipic acid involved hydrogenation of the acid (as its ester) to the diol, followed by ammonolysis to the diamine (35). 

Since adipic acid has been produced in commercial quantities for almost 50 years, it is not surprising that many variations and improvements have been made to the basic cyclohexane process. In general, however, the commercially important processes stiU employ two major reaction stages. The first reaction stage is the production of the intermediates cyclohexanone [108-94-1] and cyclohexanol [108-93-0], usuaHy abbreviated as KA, KA oil, ol-one, or anone-anol. The KA (ketone, alcohol), after separation from unreacted cyclohexane (which is recycled) and reaction by-products, is then converted to adipic acid by oxidation with nitric acid. An important alternative to this use of KA is its use as an intermediate in the manufacture of caprolactam, the monomer for production of nylon-6 [25038-54-4]. The latter use of KA predominates by a substantial margin on a worldwide basis, but not in the United States. 

It has been known since the early 1950s that butadiene reacts with CO to form aldehydes and ketones that could be treated further to give adipic acid (131). Processes for producing adipic acid from butadiene and carbon monoxide [630-08-0] have been explored since around 1970 by a number of companies, especially ARCO, Asahi, BASF, British Petroleum, Du Pont, Monsanto, and Shell. BASF has developed a process sufficiendy advanced to consider commercialization (132). There are two main variations, one a carboalkoxylation and the other a hydrocarboxylation. These differ in whether an alcohol, such as methanol [67-56-1is used to produce intermediate pentenoates (133), or water is used for the production of intermediate pentenoic acids (134). The former is a two-step process which uses high pressure, >31 MPa (306 atm), and moderate temperatures (100—150°C) (132—135). Butadiene,... 

CO, and methanol react in the first step in the presence of cobalt carbonyl catalyst and pyridine [110-86-1] to produce methyl pentenoates. A similar second step, but at lower pressure and higher temperature with rhodium catalyst, produces dimethyl adipate [627-93-0]. This is then hydrolyzed to give adipic acid and methanol (135), which is recovered for recycle. Many variations to this basic process exist. Examples are ARCO s palladium/copper-catalyzed oxycarbonylation process (136—138), and Monsanto s palladium and quinone [106-51-4] process, which uses oxygen to reoxidize the by-product... 

Other processes explored, but not commercialized, include the direct nitric acid oxidation of cyclohexane to adipic acid (140—143), carbonylation of 1,4-butanediol [110-63-4] (144), and oxidation of cyclohexane with ozone [10028-15-5] (145—148) or hydrogen peroxide [7722-84-1] (149—150). Production of adipic acid as a by-product of biological reactions has been explored in recent years (151—156). 

V. D. Luedeke, "Adipic Acid" in Encyclopedia of Chemical Process andDesign,]. McKetta and W. Cunningham, eds. Vol. 2, Marcel Dekker, Inc., New York, 1977, pp. 128-146. 

Cyclohexane. The LPO of cyclohexane [110-82-7] suppUes much of the raw materials needed for nylon-6 and nylon-6,6 production. Cyclohexanol (A) and cyclohexanone (K) maybe produced selectively by using alow conversion process with multiple stages (228—232). The reasons for low conversion and multiple stages (an approach to plug-flow operation) are apparent from Eigure 2. Several catalysts have been reported. The selectivity to A as well as the overall process efficiency can be improved by using boric acid (2,232,233). K/A mixtures are usually oxidized by nitric acid in a second step to adipic acid (233) (see Cyclohexanol and cyclohexanone). 

A one-step LPO of cyclohexane directly to adipic acid (qv) has received a lot of attention (233—238) but has not been implemented on a large scale. The various versions of this process use a high concentration cobalt catalyst in acetic acid solvent and a promoter (acetaldehyde, methyl ethyl ketone, water). 

Adipic acid (qv) has a wide variety of commercial uses besides the manufacture of nylon-6,6, and thus is a common industrial chemical. Many routes to its manufacture have been developed over the years but most processes in commercial use proceed through a two-step oxidation of cyclohexane [110-83-8] or one of its derivatives. In the first step, cyclohexane is oxidized with air at elevated temperatures usually in the presence of a suitable catalyst to produce a mixture of cyclohexanone [108-94-1] and cyclohexanol [108-93-0] commonly abbreviated KA (ketone—alcohol) or KA oil ... 

Polymer Production. Three processes are used to produce nylon-6,6. Two of these start with nylon-6,6 salt, a combination of adipic acid and hexamethylenediamine in water they are the batch or autoclave process and the continuous polymerisation process. The third, the soHd-phase polymerisation process, starts with low molecular weight pellets usually made via the autoclave process, and continues to build the molecular weight of the polymer in a heated inert gas, the temperature of which never reaches the melting point of the polymer. 

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