A proposed predictive methodology for the evaluation of caking effect on different types of commercial sugars

Abstract This paper presents preliminary results of an alternative experimental methodology under development to predict the caking effect often observed in bulk solids after time storage. This methodology is based on 1) the principle that the particle is sheared after the shear stress overcome a maximum value; 2) the maximum shear stress depends on the particles physical properties and their chemical nature; 3) Prorating is a well-known and often used empirical procedure to minimize the scatter at shear tests; and 4) that the static friction coefficient depends on particle-particle contact time and could be described by the phenomenological state-and-rate model formulated by Dieterich, Rice, and Ruina. The proposed methodology aims to evaluate the increasing of the static coefficient of friction with rest time, through “stop-and-go” shear experiments and according to Dieterich, Rice and Ruina model. Finally, the increase in shear stress with time is used to extrapolate the instantaneous yield locus into a predicted time yield locus, using the prorating technique We tested four types of commercial sugar and the results of time yield locus, obtained from experiments, have seemed similar to those predicted using the proposed methodology. We obtained satisfactory results for the short rest times tested in this preliminary study, which could be extended to long periods.

摘要 本文报告了一种正在研发中的替代实验方法的初步研究成果，该方法用于预测散装固体颗粒（bulk solids）经长时间储存后常出现的结块效应（caking effect）。该方法基于以下四项核心原则：1）当剪应力（shear stress）超过临界最大值时，颗粒将发生剪切变形；2）最大剪应力（maximum shear stress）取决于颗粒的物理性质与化学本质；3）分载法（prorating）是一种广为应用的经典经验方法，可有效降低剪切试验的数据离散性；4）静摩擦系数（static friction coefficient）与颗粒间接触时间（particle-particle contact time）相关，可通过由Dieterich、Rice与Ruina提出的现象学状态-速率模型（phenomenological state-and-rate model）进行表征。

本文所提出的方法旨在通过启停式剪切试验（stop-and-go shear experiments），并依据Dieterich、Rice及Ruina模型，探究静摩擦系数随静置时间的变化规律。最后，借助分载法，利用剪应力随时间的增长特性，将瞬时屈服轨迹（instantaneous yield locus）外推得到预测的时效屈服轨迹（time yield locus）。

本研究测试了四种商用食糖，实验测得的时效屈服轨迹结果与本文所提方法的预测值具有良好的一致性。本初步研究针对较短静置时间取得了令人满意的实验结果，该方法可进一步拓展至更长储存周期的应用场景。