Freeze concentration (FC) by suspension crystallization is a complex process due to the combination of a scrapedsurface heat exchanger, crystallizer, and wash column. Following the study of a three-in-one structure of a multipass FC published earlier this year, modeling and experiments of the heat and mass transfer of this freeze concentrator are presented in this paper. The experimental assessment of the system performance, including the measured values of the heat transfer coefficient, ice production rate, and energy efficiency as well as their correlation with the concentration ratio and partition coefficient are presented in this article.
Freeze concentration (FC) is a nonthermal processing technology of liquid food, in which a portion of the water in the aqueous solution is frozen and converted into relatively pure ice crystals, after which it is removed from the liquid phase to concentrate the remaining solution. It can be used to concentrate or pre-concentrate heat-sensitive aqueous solutions, such as milk (Habib and Farid, 2007; Sanchez et al., 2011), fresh fruit juices (Orellana-Palma et al., 2017; Petzold et al., 2013; Bayindirli et al., 1993), other liquid foods (Moreno et al., 2014a), and biological solutions (Moreno et al., 2014b). Studies have shown that compared with evaporation concentration, FC has the advantage of producing less thermal denaturation of the solution, and thus, it can better maintain the original flavor, nutrition, and color of liquid foods (Benedetti et al., 2015; Moreno et al., 2014c; Miyawaki et al., 2016a). Moreover, the latent heat of water freezing is almost one seventh of the latent heat of water evaporation (Lide and Haynes, 2010), which offers potential for energy savings for de-watering of aqueous solutions.
Ice crystals can be formed from aqueous solutions in two ways: progressive crystallization (Miyawaki et al., 2005, 2016b; Zambrano et al., 2018) and suspension crystallization (Huige and Thijssen, 1972; Qin et al., 2007). In the former, the water freezes on the cooling surface, forming an ice layer progressively and a concentrated liquid phase. Thus, it is also known as layer crystallization. This method of FC is called progressive FC. When the ice layer extends to the entire vessel to form an ice block, it is known as block FC (Moreno et al., 2014c; Zambrano et al., 2018). The advantage of this technology is the low equipment cost and simple operation management. However, the ice layer has a poor heat transfer coefficient of less than 0.1 kW m 2 K 1 (Qin et al., 2003a; Pronk et al., 2010; Hasan et al., 2017), and a huge cooling surface area is required for practical applications. In addition, the ice layer tends to entrain liquid sacs and causes severe solute loss (Miyawaki et al., 2016a, 2016b; Samsuri et al., 2015).