Subcritical water hydrolysis of nylon 6 extract concentrate

Abstract

Nylon 6 extract concentrate, namely, wastewater produced from industrial nylon 6 extraction unit, was hydrolyzed in subcritical water to efficiently decrease the cyclic dimer (CD) content. The CD equilibrium conversion reached a maximum when the initial water content ranged from 0.40 to 0.45 g/g; specifically, a CD conversion of 94.3 % was achieved at the hydrolysis temperature of 543 K and an initial water content of 0.40 g/g. A kinetic model including both acid‐ and base‐catalyzed mechanisms was developed. The model predictions were in good agreement with the experimental results. The hydrolysis reaction constants increased significantly with increasing subcritical water content. By considering the effects of subcritical water on the individual reaction constants, the kinetic model could reproduce the experimental results over a wide range of initial water contents. Hopefully, the developed kinetic model will be applied in the design of a novel nylon 6 extract liquor recycling process.

Improved Kinetic Rate Constants for Nylon 6, Nylon 6,6, and Nylon 6/6,6 Copolymer

“…As shown in Table 7 , they used these data to estimate 20 kinetic parameters (including 10 activation energies) and 10 equilibrium parameters (including 5 reaction enthalpies). Parameter values estimated by other research groups (using smaller data sets or unpublished data) are also shown in Table 7 11,14–16,18,24,25,32,35,42,45–547 are similar for different research groups (e.g., the activation energy estimate of 78.7 kJ mol −1 for R1 reported by Arai et al11 is similar to 78.5 kJ mol −1 reported by Reimschussel et al,45 other parameter estimates differ by orders of magnitude between research groups.…”

“…Unfortunately, the K 4 a value calculated from Arai's estimated values of Δ H 4 and Δ S 4 predict an equilibrium [ CD ] at 253.5 °C which is ≈300 times larger than Heikens' measured value. Recent kinetic parameter values of Qin et al32 (obtained from experiments with a duration of ≈7 h) lead to somewhat‐better agreement with equilibrium data for [ CD ], resulting in K 4 a at 253.5 °C which is ≈10 times larger than Heikens' measured value.…”

“…Table 1 has been widely used for modeling nylon 6 production (often without transamidation reaction R6) 11–14,32–45. The model used by Arai et al for their parameter estimation study neglects reaction R6 and accounts for the catalytic influence of carboxyl end‐groups ( C ) in reactions R1–R5 using composite rate constant expressions proposed by Hermans et al8,9 In Equation , is the “uncatalyzed” rate constant for the i th reaction ( i = 1, 2,… 5), which is important when the concentration of carboxyl ends is low, and is the “catalyzed” rate constant that accounts for the catalytic influence of carboxyl end groups when their concentration is high.…”

“…In recent years, additional data have been reported that are not included in Table 6 and are excluded from the current modeling and parameter‐estimation study. These newer data were obtained at higher temperatures where thermal degradation reactions become important54 or they were performed using CD‐rich nylon 6 extract (with poorly known oligomeric composition) as a starting material 32. Over the years, a large number of mathematical modeling studies have relied on the data in Table 6.…”

Modeling Equilibrium Behavior of Nylon 6, Nylon 6,6 and Nylon 6/6,6 Copolymer

“…Table shows six reversible reactions for copolymerization of caprolactam with HMD and ADA . This reaction scheme applies for hydrolytic polymerization of caprolactam to produce nylon 6, so the first five reactions in Table (i.e., the main reactions) have been used in a large number of nylon 6 modeling studies . The sixth reaction (transamidation), which does not influence DP nor the concentrations of amine and carboxyl end groups, is included in the mechanism because reaction R6 helps to randomize the polymer chains .…”