Chemical Engineering Science 59 (2004) 3117–3124 www.elsevier.com/locate/ces Solubility of amino acids and diglycine in aqueous–alkanol solutions L.A. Ferreira a , E.A. Macedo b , S.P. Pinho a ; * a Laboratory of Separation and Reaction Engineering, Escola Superior de Tecnologia e de Gest˜ ao, Instituto Polit ecnico de Braganc a, Campus de Santa Apol onia, 5301-857, Braganc a, Portugal b Laboratory of Separation and Reaction Engineering, Departamento de Engenharia Qu mica, Faculdade de Engenharia, Rua do Dr. Roberto Frias, 4200-465, Porto, Portugal Received 26 February 2004; received in revised form 4 May 2004; accepted 5 May 2004 Available online 20 June 2004 Abstract The solubility of glycine and dl-alanine were measured in the temperature range between 25 ◦ C and 60 ◦ C for the aqueous systems of ethanol, 1-propanol and 2-propanol. The analytical gravimetric method was chosen to perform the measurements and proved to be very accurate. Theoretical work was essentially focused on the application of the excess solubility approach with the NRTL model showing average relative deviations around 8.4% for correlation and 15% for predictions. ? 2004 Elsevier Ltd. All rights reserved. Keywords: Amino acid; Phase equilibria; Mixed solvents; Modelling 1. Introduction The physical and chemical properties of amino acids be- came a very important studied subject, not only because they are the basic building blocks of proteins and peptides but also for their importance in industrial processes, particularly for pharmaceutical and food industries. Recent advances of the biochemical industry draw much attention to the development of more sophisticated and ef- cient processes for separation, concentration, and purica- tion of these valuable biochemicals due to their high cost in comparison to the total manufacture cost. The choice of a suitable separating agent and the operating conditions to perform the separation of amino acids is limited, so vari- ables such as temperature and solvent composition are of ex- treme importance. Furthermore, the recovery and separation of such compounds frequently involve batch crystallization for which the solubility is the key property that should be known (Charmolue and Rousseau, 1991). The values of the solubility of amino acids in water at dierent temperatures are known due to the experimental eort developed in the 1930s by Dalton and Schmidt (1933, 1935) and Dunn et al. (1933), but only in the last decade the ∗ Corresponding author. Fax: +351-273-313-051. E-mail addresses: spinho@ipb.pt, eamacedo@fe.up.pt (S.P. Pinho). thermodynamic description of that property was properly taken into account (Chen et al., 1989; Kuramochi et al., 1996; Liu et al., 1998; Peres and Macedo, 1994; Pinho et al., 1994). Concerning the measurement of the amino acid solubility in water/alkanol mixed solvents the only work, as far as we know, taking into account the inuence of the temperature is by Dunn and Ross (1938). In fact, the ma- jority of the data available is at 25 ◦ C; water–ethanol (Cohn et al., 1934; McMeekin et al., 1935; Nozaki and Tanford, 1971), water–methanol (Gekko, 1981), and more recently, thesystemswater–1-propanolandwater–2-propanol(Orella and Kirwan, 1989, 1991), and water–1-butanol (Gude et al., 1996a). Regarding the correlation of the solubilities in this kind of systems, the works by Orella and Kirwan (1991) and Gude et al. (1996a,b) are important contributions to this subject. Orella and Kirwan used an excess solubility approach with the Wilson equation. The solubility data for each amino acid in dierent alcohol systems were correlated simultane- ously to obtain the Wilson parameters between the amino acid and each solvent, obtaining an average relative devia- tion of about 15.3% for their measured data with propanol and isopropanol. However, in order to reduce the num- ber of parameters to be estimated a constraint was estab- lished based on the fact that the ratio of the activity coe- cients of amino acids in pure solvents should be inversely 0009-2509/$ - see front matter ? 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ces.2004.05.001