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

## TL;DR

This paper explores using Ca2+ and Cu2+ crosslinking to improve the drying of sugar beet pulp, reducing energy use and increasing its value as animal feed.

## Contribution

A novel crosslinking method using Ca2+ and Cu2+ is proposed to enhance SBP drying efficiency and nutritional value.

## Key findings

- Ca2+ crosslinking significantly reduces SBP moisture content by ~30–40% compared to untreated SBP.
- The Midilli–Kucuk model best describes the hot-air drying process of SBP.
- SBP binds with Ca2+ or Cu2+ through carboxyl groups, forming an 'egg-box' structure confirmed by FTIR, SEM, and XRD.

## Abstract

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.