“…Online measurement may not be practical due to the lack of economically priced, reliable and sterilisable transducers (Veloso et al, 2009, Farza et al, 1999, Gonzalez et al, 2001, Dochain, 2003, Bastin and Van Impe, 1995. Previously, indirect techniques such as filtering and calorimetric balances have been introduced to manually obtain non-measurable variables but it is reported that the techniques are time consuming, require the involvement of personnel and may prove unstable and expensive (Gonzalez et al, 2001). An alternative strategy is to consider using available online data to estimate non-measurable variables using an observer or soft sensor (Veloso et al, 2009, Bastin and Van Impe, 1995, Cheruy, 1997, Aguilar-López and Maya-Yescas, 2005, de Assis and Filho, 2000.…”