Computer‐Aided Control of Water Activity for Lipase‐Catalyzed Esterification in Solvent‐Free Systems

Abstract

A computer system for on-line monitoring and control of the water activity (a(w)) in solvent-free media has been developed. The performance of this system was investigated by carrying out the lipase-catalyzed esterification of n-capric acid with n-decyl alcohol. A humidity sensor measured the relative humidity in the reactor headspace, which was then transmitted electrically to a digital computer that was used as a feedback controller. The water activity control was achieved by sparging either humidified air or dried air through the reaction medium at a flow rate determined by the digital feedback controller. The use of humid air and dry air for a(w) control made it possible to induce a larger a(w) gradient and thereby higher water transfer rate. As a result, the water activity quickly reached the desired a(w) values. We tested whether water activity in the reaction medium can be monitored by measuring relative humidity in the headspace. When the water activity in the liquid phase was determined from measurements of water content in the medium and compared to that measured directly with the humidity sensor, the a(w) in the reaction medium did not differ significantly from that in the headspace. This indicates that there is a near-equilibrium between the liquid medium and the exit air stream. Water activity was also successfully maintained close to the set point despite the massive production of water during the esterification process. Thus, the control system developed in this study is particularly useful for systems where large amounts of water are produced and where conventional methods make it difficult to control water activity as a result of a low water transfer rate. The effects that computer control of the water activity had on the reaction rate and yield were also examined. The reaction yield was significantly improved with water activity control. The conversions obtained at 28 h without and those with water activity control were 70% and 96%, respectively. In addition, from the fact that the final yields increased with decreasing a(w), computer-aided water activity control was performed with a set-point change. By controlling a(w) at 0.55 during initial reaction phase, followed by a step change of a(w) from 0.55 to 0 after 11 h of reaction, it was possible to enhance the final conversion to 100%.

Computer‐aided real‐time estimation of reaction conversion for lipase‐catalyzed esterification in solvent‐free systems

“…In Figure 5a, the calculated water activities of the reaction medium (open rectangle) are compared with the water activities measured in the exit air gas phase using RH sensors (dotted line). The results shown in Figure 5a again indicate that there was a near-equilibrium between the liquid mixture and the exit air gas phase during a w control, as described in our previous study (Won and Lee, 2001).…”

“…A previous study revealed that there was a near-equilibrium between the liquid mixture and the exit air gas phase during air bubbling: a w,m @ a w,out (Won and Lee, 2001). Water in the liquid medium was rapidly distributed to the enzyme particles in the reaction medium, such that the water in the liquid mixture was also nearly equilibrated with that on the biocatalyst: a w,m @ a w,e (our unpublished experimental data).…”

“…Except for the fact that another RH sensor was added, the water activity control system used in this study was the same as that used previously (Won and Lee, 2001). For a w measurement in the inlet air stream, another a w sensor with a transmitter (Novasina Hygrodat 20, Axair AG, PfaÈ kon, Switzerland) was added to the water activity control system developed in the previous work.…”

“…Although several water activity control systems have been developed, the amount of substrate used was very low and the maximum amount of water produced by the reaction was no more than 0.6±2.9 mmol (Kim et al, 1998;Pepin and Lortie, 1999;Wehtje et al, 1999). In contrast, we have succeeded in controlling a w for a solvent-free ester synthesis in which 320 mmol of substrate was used (Won and Lee, 2001).…”

“…We previously developed a computer-aided water activity control system for lipase-catalyzed esteri®cation reactions in solvent-free media (Won and Lee, 2001). In the case of enzymatic solvent-free ester synthesis, which is one of new methods that promises enhanced productivity in the ®eld of nonaqueous enzymology, it is dicult to control water activity (a w ) because a great amount of water is produced.…”