Impurity Distribution Behavior in Caprolactam Extraction with Environmentally Benign Mixed Solvents

Abstract

In a previous study a solvent mixture of heptane containing 40 mass % heptanol was selected as an alternative in the industrial extraction of caprolactam to replace benzene, toluene, or chlorinated hydrocarbons. This work reports the equilibrium distribution ratio of caprolactam and four model impurities of organic nature, namely, cyclohexanone, aniline, n-methylcaprolactam, and cyclohexane-carboxamide, comparing the mixed solvents with toluene as a reference. The resulting phase equilibria were interpreted using the equilibrium stage model. Based on these calculations it was found that, compared to toluene, the co-extracted fraction of cyclohexanone and aniline was higher, that of n-methylcaprolactam was comparable, and that of cyclohexane-carboxamide was lower using the mixed solvent. Overall, the mixed solvent reduced the fraction of coextracted impurities by almost 10 %.

Influence of Physical Properties and Process Conditions on Entrainment Behavior in a Static-Mixer Settler Setup

“…Commercial fiber-grade caprolactam requires high purity (>99.9%w/w). 6 During the extraction step, while extracting the aqueous-caprolactam phase with toluene, entrainment of the aqueous phase into organic solvent phase can occur as a result of the reduction of interfacial tension of the two phases due to a high caprolactam concentration. The high concentration of caprolactam (60−70%w/w) in the feed reduces the interfacial tension to values less than 5 mN/m.…”

“…7 This leads to the formation of microdroplets and eventually to their entrainment in the organic phase, having a number of consequences: (1) product dilution, which means that product specifications are no longer met and the production capacity decreases, (2) the downstream separation cost increases in meeting the product specifications, and (3) an associated increase in the impurities occurs as more entrainment of the aqueous phase leads to an increase in aqueous-phase soluble unwanted components in the organic phase. 6 To illustrate the role of physical solubility, viscosity, and density difference on entrainment behavior, we have also selected the ternary EB−MBA−water system. We found by measuring the physical properties of this system that the aforementioned properties change considerably with the concentration of MBA thus allow studying the role of these properties on entrainment.…”

Entrainment Reduction in a Static-Mixer Settler Setup by Electric Field Enhanced Coalescence

“…In the caprolactam-toluene-water system, the interfacial tension decreases to less than 5 mN=m (19) depending on the concentration of the surface active component caprolactam. Commercial fiber-grade caprolactam requires high purity (>99.9%w=w) (20). During the extraction step, while extracting the aqueous-caprolactam phase with toluene, entrainment of the aqueous phase into the organic solvent phase occurs as a result of the low interfacial tension and the formation of micro droplets.…”

“…During the extraction step, while extracting the aqueous-caprolactam phase with toluene, entrainment of the aqueous phase into the organic solvent phase occurs as a result of the low interfacial tension and the formation of micro droplets. This has a number of consequences such as product dilution, decrease in product quality and production capacity, and an associated increase in entrainment of aqueous-phase soluble impurities (20). For the ethyl benzene (EB)-a-methyl benzyl alcohol (MBA)-water system, the interfacial tension can be as low as 10 mN=m (21), where MBA acts as a surface active component.…”

Extraction of Caprolactam with an Alternative Benign Solvent in a Pulsed Disc and Doughnut Column

“…In previous studies, the mixed solvent heptane-heptanol (40 mass %) was selected via an experimental screening procedure based on a relatively high distribution ratio of caprolactam, a low mutual solvent solubility, beneficial physical properties, and a low distribution ratio of impurities. [3,4] From the phase compositions it was calculated that when using the mixed solvent less theoretical stages are required in the forward extraction compared to benzene and toluene, NTS ¼ 5 relative to 9 and 11, respectively, whereas this trend was reverse in the back-extraction, NTS ¼ 10 compared to 5 and 4, respectively. The physical properties showed slightly higher viscosities, but lower densities and interfacial tensions for the mixed solvent.…”