Cleaning insoluble viscoplastic soil layers using static and moving coherent impinging water jets

Abstract

Impinging liquid jets are widely employed in cleaning operations to remove residual soiling layers from walls and other surfaces of process vessels. Insoluble viscoplastic soiling layers represent challenging soils to clean as removal is primarily by hydraulic forces. The rheological behaviour of a commercial petroleum jelly was investigated and shown to exhibit significant creep below its critical stress. The removal of thin (< 1 mm) layers of petroleum jelly from glass and Perspex surfaces by coherent water jets impinging normally on vertical walls were studied experimentally. The jet clears a roughly circular area, forming a berm of removed material at the cleaning front. The shape of the berm was measured and found to depend on the ratio of the height of the water film and the initial thickness of the soil layer. The data were compared with the adhesive removal of viscoplastic soils proposed by Glover et al. (J. Food Eng., 2016, 178, 95-109) with the momentum flow rate calculated using the results in Bhagat and Wilson (Chem. Eng. Sci, 2019, 152, 606-623). The asymptotic approach to a cleaning limit observed in experiments with static nozzles required modification of the model: a semiempirical term which represents the transition to a creeping regime is presented. The modified model allowed results obtained using static nozzles to predict the shape of the region cleaned by a jet from a similar nozzle moving across a soiled plate. The influence of process conditions on model parameters is discussed.

Modelling the cleaning of viscoplastic layers by impinging coherent turbulent water jets

“…material. Data such as those in Figure 3, reproduced from Fernandes et al [17], showed that the local cleaning rate for an immiscible viscoplastic soil is not determined by a shear driven mechanism (as in [12,13]) since the shear stress imposed by the liquid film is smaller than the critical stress of the soil.…”

“…Neither of these models capture creep effects that occur at shear stresses below the critical stress [44], which was cited for the strong curvature at lower observed by Fernandes et al [17]. Creep is expected to occur in cleaning experiments when the soil layer is exposed to the shear stresses and forces imposed by the liquid film for long periods.…”

“…The displacement of immiscible Newtonian liquids, such as oils, by water jets has been investigated previously [12,13]. The removal of non-Newtonian soils show more complex behaviour and has been investigated for shear thinning [14], viscoelastic [15,16] and viscoplastic [1,17] layers. Cleaning viscoplastic materials that are insoluble in the cleaning solution is particularly challenging.…”

“…The cleaned radius, , is the radial position where the residual layer thickness is negligible. Cases (a) and (b) were investigated experimentally by Fernandes et al [17] and are commonly observed on the side walls of processing tanks and vessels. Case (c) is more likely to be observed at the base of such vessels.…”

“…Wilson et al [4] investigated the removal of starch layers from flat surfaces and found that their data fitted Equation ( 1) with = 0. Glover et al Fernandes et al [17] measured the shape of the berm of soil generated at the cleaning front for a different petroleum jelly. The petroleum jelly was hydrophobic and cohesive, and so it collected in a berm of dislodged material which moved steadily downstream with the cleaning front [35].…”