title: The reactive extraction of phenylalanine with Aliquat 336: buffer co-extraction equilibrium and mass transfer kinetics.
abstract: The occurrence of significant co-extraction of buffer anions by the ion exchanger Aliquat 336 is unavoidable when high levels of system buffering is required. The co-extraction will result in inaccurate equilibrium and mass-transfer characterization of such a system unless its occurrence is taken into account, making process design and control difficult. A study of the equilibrium of phenylalanine extraction using Aliquat 336, a system where high levels of hydroxyl co-extraction occurs, was used as a model case to develop a method of accounting for co-extraction in mass-transfer modeling. Analysis of the equilibrium between bulk-aqueous-phase chloride and phenylalanine concentrations during mass transfer in a stirred-transfer cell showed there to be linear equilibrium relationships between the two parameters for a given extraction system of the form C(Cl,t) = alpha(C(A,t) - C(A,0)) for forward extraction and C(Cl,t) = epsilon C(A,t) + C(Cl,0) for backward extraction. The constants of proportionality of these relationships, or the "co-extraction constants," alpha and epsilon, were shown to be related to the selectivity of Aliquat 336 for the phenylalanine anion by the relationships alpha = -(1/S + 1) and epsilon; = -(1/S(-1) + 1). The linear equilibrium relationships were used to develop two-film theory mass-transfer models for both forward and backward extraction that account for co-extraction. These showed much higher accuracy in modeling stirred-transfer-cell data than the equivalent models which ignored co-extraction.
authors.affiliation: Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, Prince Consort Road, London SW7 2BY, United Kingdom.
authors.forename: J T
authors.initials: JT
authors.lastname: Scarpello
authors.forename: D C
authors.initials: DC
authors.lastname: Stuckey
mesh_terms: D001709:Biotechnology / Q000379:methods*; D002021:Buffers; D007202:Indicators and Reagents; D007700:Kinetics; D008956:Models, Chemical; D010649:Phenylalanine / Q000737:chemistry / Q000302:isolation & purification / Q000493:pharmacokinetics*; D000644:Quaternary Ammonium Compounds; D012997:Solvents / Q000493:pharmacokinetics
chemical_list: D002021:Buffers; D007202:Indicators and Reagents; D000644:Quaternary Ammonium Compounds; D012997:Solvents; D010649:Phenylalanine; C037759:trioctylmethylammonium