# CFD Analysis of a Tubular Heat Exchanger for the Conditioning of Olive Paste
## 3 Results and Discussion
### Temperature Variation in Different Scenarios
Figure 7a shows the shear rate, which decrease moving through the center of the tube, since the velocity variation between the fluid layers at the center is lower than that near the wall. As a consequence, the dynamic viscosity distribution increases at the center due to the pseudoplastic behavior (viscosity increases when the shear rate decreases), as shown in Figure 7b. Figure 6: Path lines and velocity values at paste flow rate of 3000 kg/h, (**a**) of olive paste, (**b**) water. The absence of heating uniformity of the olive paste leads to further considerations about the olive oil quality. Indeed, generally, the olive paste is heated at a temperature ranging from 24 to 27 \({}^{\circ}\)C or, in any case, on pre-set temperature values not to be exceeded by connecting in series more than one module. This range of temperature is strictly related to the olive oil quality that is consumed worldwide due to its appreciated sensory attributes and health benefits due to its fatty acid and richness in volatiles and phenolic compounds. To ensure the high content of phenolic and volatiles compounds, the extraction conditions and the temperature must be optimized. To this end, various studies have shown that malaxation at temperatures above 27 \({}^{\circ}\)C does not significantly increase the oil yield but leads to a gradual and significant reduction in the phenolic content as reported in [38; 39; 40; 41; 42]. In particular, in [40], it was found that temperatures ranging from 22 to 30 \({}^{\circ}\)C produced an increase in phenols of about 26%, while the C6 aldehydes decreased by about 18%. In [43], the authors analyzed four different varieties of olives identifying the optimal values of temperature and oxygen concentration in the malaxer to maximize the main chemical parameters related to the oil quality, such as the phenolic and the volatile compounds. The authors found for each cultivar the following optimal temperature and oxygen levels: 33.5 \({}^{\circ}\)C and 54 kPa of oxygen (Peranzana), 32 \({}^{\circ}\)C and 21.3 kPa (Ogliarola), 25 \({}^{\circ}\)C and 21.3 kPa (Coratina) and 33 \({}^{\circ}\)C and 21.3 kPa (Itrana). These results demonstrate the importance of optimizing the homogeneity of temperature to enhance the quality parameters. Figure 7: Shear rate (**a**) and dynamic viscosity (**b**) inside the heat exchanger with a paste flow rate of 3000 kg/h. The non-uniform heating of high-viscosity fluids in a tube-in-tube heat exchanger suggests the importance of providing for the insertion of some mixing device able to stir the paste and ensure a better temperature distribution in the product. In addition, such a device could represent a good aid to prevent the fouling of inner surface of the heat exchanger. To this purpose, the use of a scraped surface heat exchanger (SSHE), which has a rotor coaxially inserted in the internal tube equipped with scraping blades, may be appropriate for olive paste pretreatments. In fact, SSHE models may provide a suitable solution to enhance the heat transfer in highly viscous or sticky fluids, where due to the low Reynolds number values, the fluid advances in a laminar regime, limiting the heat exchange as reported in [44; 45; 46]. An SSHE equipped with alternate blades particularly suited to treat high viscous fluids (hazelnut paste) has been developed in [23]. However, the hazelnut paste does not reach the viscosity values of the olive paste, especially at low shear rates. Furthermore, the typical SSHE blading should be carefully studied to avoid undesirable effects due to the presence of the pits.