Fluid Dynamic Gauging Research Articles

Effect of surface treatment on cleaning of a model food soil

The effect of surface modification of 316T stainless steel on removal of a model proteinaceous food soil (baked tomato paste) was studied using fluid dynamic gauging, where the deposit is immersed in liquid and removed by stresses imposed by a well-controlled fluid flow. The surfaces studied were uncoated 316, sol–gel, diamond-like carbon (DLC), and modified DLC (SICON® and SiCAN). The surface energy and topology of the substrates were established before and after a series of 10–12 fouling and cleaning cycles. Surface modification reduced the adhesive interaction between the substrate and soil, to the extent that surface energies of 22–26 mN/m resulted in simple detachment of the deposit from the surface. No significant effect of surface roughness was observed in the range (72 nm < Ra< 700 nm, measured by AFM). The coatings exhibited varying degrees of stability towards repeated fouling and cleaning.

Solvent-based cleaning of emulsion polymerization reactors

Fluid dynamic gauging (FDG) has been used to study the kinetics of the cleaning, from various solid surfaces, of polymer layers representative of polymerization reactor foulants. Currently, solvents such as methylethylketone (MEK) are used for cleaning and it is desired to replace this with aqueous systems with less severe environmental impact. Laboratory-prepared samples of two polystyrene co-polymers and samples prepared in an industrial pilot plant were treated with two alkaline solutions, sodium hydroxide (NaOH) and sodium metasilicate, with an aqueous commercial cleaning agent (TPU) and with the organic solvent MEK. Single and composite layers were studied, and a variety of outcomes observed. The simple alkalis swelled the polymers but did not clean: MEK and TPU swelled and then cleaned off both laboratory films, the mechanism varying between cohesive breakdown and adhesive detachment for different polymer/solvent combinations. One pilot plant material behaved as its laboratory analogue, while another, which was not tested in the laboratory, left a residual film on the surface. Experiments on composite layers exhibited a rich diversity of behaviours which could be modelled as combinations of single film characteristics.

Mechanisms in the Solvent Cleaning of Emulsion Polymerization Reactor Surfaces

A new technique, fluid dynamic gauging (FDG), is introduced for studying the swelling and removal of polymer films from various surfaces. Films of two different polystyrene copolymers, labeled PX (30 kDA) and PA (125 kDa), were deposited in the laboratory on, inter alia, SS 316 plates. FDG was used to track, in situ and in real time, the films' evolving thickness during swelling and removal in three agents: NaOH, MEK, and a commercial aqueous cleaning solution, TPU. The effects of initial film thickness, solvent concentration, solvent temperature, nature of the substrate, and applied stress were investigated. Swelling profiles suggest that Type II diffusion is the governing transport process. Different modes of behavior were found. In NaOH, swelling occurred after a lengthy induction period, but no surface cleaning was detected. MEK and TPU swelled the films with a negligible induction period and then cleaned the surfaces, with the former working faster. For MEK applied to PX and PA, and for TPU with PX, the film was removed by dissolution, i.e., the cohesion of the film was attacked. By contrast, TPU applied to PA removed the film by destroying the adhesion. Application of TPU to films composed of mixed layers of PX and PA showed that the deposition sequence determines removal behavior. A simple model allows prediction of the removal kinetics of composite films from those of single-component films. The potential for substantially accelerated reactor cleaning by appropriate reactor scheduling is thereby identified.

A Method for Measuring the Strength of Scale Deposits on Heat Transfer Surfaces

AbstractThe technique of fluid dynamic gauging has been successfully applied to assess the adhesion characteristics of thin (&lt; 1 mm) layers of calcium sulphate deposits serving as mimics of mineral scales often found on heat transfer surfaces. The shear strength of the scale layers increased on ageing; layers aged for more than 14 hours could not be removed by the gauging fluid flow. The shear stresses imposed by the gauging flow have been shown to be reliably estimated by an analytical result, provided that the flow rate is measured.

Fluid dynamic gauging for measuring the strength of soft deposits

Fluid dynamic gauging (FDG) is a technique for the measurement of the thickness of soft deposit layers on solid surfaces immersed in a liquid environment, in situ and in real time. The technique has been extended using computational fluid dynamics (CFD) simulations of the laminar gauging flow to quantify the stresses imposed on the material being gauged. CFD results were validated by comparison with experimental values for (i) the hydrostatic head, (ii) the nozzle discharge coefficient and (iii) the normal stresses acting on the gauged surface. The shear stress distribution predictions were subsequently used to determine the shearing yield strength of a soft food deposit. Layers of tomato paste (∼2 mm thick) on stainless steel discs were aged by oven drying at 100 °C, and the removal characteristics studied. The shearing yield strength increased with drying time up to 3.0 h, after which the strength (13 Pa) was almost constant. These results are consistent with micro-structural images, and data reported separately by Liu et al. [Trans. IChemE Part C 80 (2002) 286] using a micro-manipulation technique. The liquid velocities required to fully remove the pastes in pipe flows were calculated from the critical shear stresses and were found to be higher than, but of the same order as, the lower limit of the range of velocities suggested for cleaning-in-place installations. FDG is therefore capable of measuring both the thickness of deposit macro-layers and their mechanical properties in situ.

Thermal Conductivity of Whey Protein Films Undergoing Swelling: Measurement by Dynamic Gauging

Films of whey protein were cleaned from stainless steel surfaces by treatment with alkaline solutions flowing through a duct. The films were monitored by the simultaneous use of a heat flux sensor and a fluid dynamic gauge, backed up by a protein assay. This combinationof techniques yields directly, for the first time, a quantitative account of the variation of a structure-sensitive property of the films, namely thermal conductivity, as their structures evolve during cleaning, particularly during the swelling stage of the cleaning process. For raw protein films—that is, for films that have not been subjected to ageing by steaming—the conditions studied fall in the ranges: NaOH concentrations, 0.3–2.0 wt%; bulk fluid temperatures, 20–50°C; and flow velocities, 0.03–0.30 ms −1 , corresponding to Reynolds numbers of 500–10,000. These resulted in swelling ratios of up to 4.4, for films which initially consisted of ca. 86% water, by volume. Thermal conductivities varied linearly with time during the swelling stage, from initial values of 0.26 Wm −1 K −1 to values at maximum swelling time of up to 0.43 Wm −1 K −1 . These latter values are substantially lower than one might expect for gels consisting mainly of water and differ substantially from estimates reported in the literature.A brief study is also reported of the effect of deposit ageing, which was simulated by holding plates in a steaming environment at 100°C for prolonged periods. In such cases, it proved possible to follow the evolution of thermal conductivity over the static and decay stages of the cleaning process.