Drying Model and Mechanism of Sugar Beet Pulp Based on Its Crosslinking with Ca2+ and Cu2+

Sugar beet pulp (SBP) is a by-product from the sugar industry with low value. As a feed, SBP needs to be dried. However, the drying process takes too much energy, leading to potential environmental issues caused by coal use. This paper raised and tried a crosslinking method to shorten the drying process, save energy consumption, and increase the value of SBP. This paper aimed to reduce the water-holding ability of SBP while obtaining animal feed with higher nutritional value. First, the crosslinking method was used to evaluate its dryness-strengthening effect. Second, three factors were evaluated: operating temperature, solution pH, and cationic concentration. Third, a kinetic study was performed on the drying process of SBP through its crosslinking with macro-elements (Ca²⁺, Cu²⁺) using drying models; the characterization of Ca²⁺-SBP and Cu²⁺-SBP using FTIR, SEM, and XRD; and possible drying mechanisms, which were discussed using an egg box model and a simple quantum chemical calculation. Results showed that the dryness-strengthening and value-adding idea is more practical through a Ca²⁺-crosslinking method, rather than through crosslinking with Cu²⁺. Under experimental conditions, wet SBP with 2 g of dry base reacts to Ca²⁺ under optimized conditions of 1000 mg/L Ca²⁺ solution at pH 6.0 and 40 °C for 135 min, with a moisture content of 5.23 g/g as a water-holding index. Compared with SBP, the moisture content of the crosslinking SBP on a dry basis was reduced by ~30-40%. The Midilli-Kucuk model was the most suitable model to describe the hot-air drying process of SBP, while Ca²⁺ or Cu²⁺ can crosslink to the galacturonic acid in pectin and form an "egg-box" model. SBP binds with Ca²⁺ or Cu²⁺ through its carboxyl groups, as testified by a combination analysis of FTIR, SEM, and XRD. As a result, the SBP dried through the Ca²⁺-crosslinking or Cu²⁺-crosslinking method can be directly used as a feed additive with good economic benefit and without the post-treatment problem as a bio-sorbent.