Flushing and removal of a viscoplastic fluid from pipes

Palabiyik et al. (2018, AIChE Journal, 64, 1517-1527) studied the water-based purging of a range of fast moving consumer goods (FMCG) products from pipes wherein the flow in-itially displaced a core of material then eroded the annulus of residual material on the wall. They reported that the time to clean the pipe fitted a general dependency on wall shear stress when the wall shear stress was expressed as ? * (the ratio between the yield stress and wall shear stress), with ? * > 1. The material is not expected to flow in this case, so erosive removal must involve other mechanisms, such as creep. This work reports a series of experimental and modelling investigations aimed at elucidating these observa-tions. The rheology of a toothpaste, reflecting one of the groups of products studied by Palabiyik et al., was studied. Simple viscoplastic descriptions were unable to capture the observed behaviour. Purging, and particularly the initial breakthrough of water to form a 'core', was investigated using a syringe pump, monitoring the motion of the initial plug of toothpaste and the pressure drop across the plug. We show that a mass transfer model can reproduce the scaling proposed by Palabiyik et al. for laminar flows but a similar scaling did not describe their case where the water flow was turbulent.(c) 2022 The Authors. Published by Elsevier Ltd on behalf of Institution of Chemical Engineers. CC_BY_4.0

Automated summary

This study investigates the water-based purging process of fast moving consumer goods products from pipes and identifies a dependency between cleaning time and wall shear stress. The rheology of toothpaste cannot be explained by simple viscoplastic descriptions. A mass transfer model can explain the scaling for laminar flows but not for turbulent flows.