W.R. Paterson

The effect of Reynolds number and fluid temperature in whey protein fouling

Abstract Food fouling is a severe industrial problem. Both the chemistry and fluid mechanics of fouling from milk fluids are complex. Experiments have been conducted in a tubular heat exchanger to determine whether fouling from whey protein concentrates is controlled by a bulk or a surface reaction. Results are interpreted in terms of a simple model, in which the amount of deposit is considered to be proportional to the volume of fluid that is hot enough to produce denatured and aggregated protein. The model fits the data reasonably well, and suggests bulk processes are important in whey protein fouling.

A replacement for the logarithmic mean

Abstract A new mean has been developed as an approximation to the logarithmic mean. It may be viewed as a refinement of the arithmetic mean: the latter has been a useful approximation in economic analysis [5,6], whilst the new mean should find use both in flowsheeting programs and in rapid rating calculations a in the example above. The principal lesson is that the new mean is so tractable mathematically, and such good approximation to the logarithmic mean, th it merits serious consideration as a replacement for the logarithmic mean in many applications.

The measurement of insoluble proteins using a modified Bradford assay

A technique for determining the amount of thermally denatured, insoluble protein is described. The assay has been validated using four globular proteins, bovine serum albumin, beta-lactoglobulin, lysozyme, and ovalbumin. It consists of a resolubilization protocol, using 8 M urea and 5% 2-mercaptoethanol, linked to the Bradford dye binding assay. The resolubilization protocol was carried out at 100 degrees C to enable complete recovery of all insoluble proteins. Beta-Lactoglobulin resolubilization was completed after heating for 1 min, whereas samples of bovine serum albumin, lysozyme, and ovalbumin required heating for 1.5 min. The assay can measure protein concentrations as small as 10 micrograms, typically with standard deviations of 3%, thus comparing favorably with the standard Bradford assay. Other types of denaturation, such as chemical denaturation causing subsequent insolubility, may be studied with this technique providing that there is no interference with the Bradford assay.

Aggregation and gelation—I. Analytical solutions for CST and batch operation

Abstract This work is part of a study of model discrimination for aggregation processes characterised by continuous particle size distribution. Criteria are sought for the acceptance or rejection of proposed functional forms of the aggregation kernel, of a more fundamental nature than the goodness of fit which they might afford to a perhaps narrow range of experimental data. New analytical results for well-mixed, continuously operated vessels are contrasted with those for batch operation. It is shown that some forms of kernel result in solutions to the population balance that exhibit the mathematical equivalent of a phase-transition phenomenon, manifested as divergence of the sixth moment of the particle size distributions, of a type referred to in the literature as gelation. Kernels that predict this “mathematical gelation” for one mode of operation, e.g. continuous, need not do so for another mode, e.g. batch. It is moreover shown that, for a kernel that predicts mathematical gelation for both these modes of operation, the gelation points correspond to different values of the index of aggregation for the two modes. In addition, the gelation points are dependent on the shape of the feed or charge PSD. We propose that gelling kernels—those capable of predicting gelation—be rejected a priori as unsuitable for modelling systems that do not exhibit physical gelation, since whatever their powers for interpolation of experimental data, they are unsafe for extrapolation from one mode of operation to another, from smaller to larger values of the index of aggregation, or from one feed to another.

Fixed bed catalytic reactor modelling: The heat transfer problem

The issue of radial heat transport in fixed beds is discussed and discrepancies between observed and computed hot spots are attributed to neglect of axial dispersion of heat, which neglect also accounts for observed length-dependent radial conductivities. The Nusselt number for heat transport at the wall is inferred to be Nuw = 5.73(Dtdp)12 Pr(0.11Rep + 20.64)/Rep0.262 which is in accord with recently secured Sherwood numbers for mass transport at a packed wall. The recent success of unconventional reactor models is discussed.

Fouling in Crude Oil Preheat Trains: A Systematic Solution to an Old Problem

A major cause of refinery energy inefficiency is fouling in preheat trains. This has been a most challenging problem for decades, due to limited fundamental understanding of its causes, deposition mechanisms, deposit composition, and impacts on design/operations. Current heat exchanger design methodologies mostly just allow for fouling, rather than fundamentally preventing it. To address this problem in a systematic way, a large-scale interdisciplinary research project, CROF (crude oil fouling), brought together leading experts from the University of Bath, University of Cambridge, and Imperial College London and, through IHS ESDU, industry. The research, coordinated in eight subprojects blending theory, experiments, and modeling work, tackles fouling issues across all scales, from molecular to the process unit to the overall heat exchanger network, in an integrated way. To make the outcomes of the project relevant and transferable to industry, the research team is working closely with experts from many world leading oil companies. The systematic approach of the CROF project is presented. Individual subprojects are outlined, together with how they work together. Initial results are presented, indicating that a quantum progress can be achieved from such a fundamental, integrated approach. Some preliminary indications with respect to impact on industrial practice are discussed.

Investigation of alkaline cleaning-in-place of whey protein deposits using dynamic gauging

A novel thickness measurement technique (DYNA-PROBE) allows the study of the behaviour of layers of soft material undergoing cleaning in flowing liquids, in situ and in real time without contacting the surface. The technique is demonstrated here in a study of the swelling and removal of whey protein films related to cleaning-in-place of dairy heat exchanger fouling deposits. The experimental protocol also allows the gauge to be calibrated in situ after each test, yielding measurement accuracies of ±10μm. Cleaning experiments also featured simultaneous measurement of the mass of protein removed and the thermal resistance of the foulant layer, via a commercial heat flux sensor. Experiments performed over a range of solution concentrations (0.3–2.0 wt% NaOH), temperatures (20–50°C) and velocities (0.03–0.30 ms −1 ; Re = 500–10,000) indicated the existence of an optimal cleaning solution concentration and several other features reported by previous workers. New information provided by the technique, including the sensitivity to shear of the swollen deposit, yields new evidence permitting integration of several conflicting earlier hypotheses on protein cleaning behaviour.

Accuracy of model predictions and reliability of experimental data for heat transfer in packed beds

The reliability and accuracy of experimental data and model prediction is investigated for the effective radial thermal conductivity (λer) and the wall heat transfer coefficient (αw) in packed beds in the absence of chemical reaction. The most common type of test equipment is shown to have suffered a crucial shortcoming. Experimental and mathematical procedures are developed to recover accurate data. Several models for predicting λer and αw are compared with experimental results, for which packings of spheres, cylinders and Raschig rings were employed. This work explains and reconciles differences between the most promising theoretical approaches and data sets in the existing literature. It is concluded that modifications of the models of Zehner and Bauer and of Hennecke can be recommended for practical use for all the particle shapes mentioned above. For studies of other shapes, advice is given on appropriate apparatus and operating procedure.

Mass or heat transfer from a sphere to a flowing fluid

In the “stagnant film” model of mass transfer, many writers relate diffusivity, film thickness and mass transfer coefficient erroneously, using the result for a planar surface, rather than the correct result for a sphere. Order of magnitude calculations show that the error involved exceeds 10% in practical cases, for gases (Re<approx.1000) and for liquids (Re<approx.10). The discrepancy is particularly important for reaction–diffusion analyses which include fluid-phase reaction within the mass transfer film. The analysis presented also rationalises the form of equation most often used to correlate experimental measurements both of kg and of the heat transfer coefficient for a sphere in a flowing fluid.

Pulsed Flow Cleaning of Whey Protein Fouling Layers

This article reports on the use of intermittent pulsing superimposed on a slow steady flow to enhance the rate of cleaning of a model food soil—namely, a whey protein deposit—in a well-characterized flow geometry. Whey protein deposits were generated by recirculating 3.5 wt% WPC solutions through an electrically heated annular test section and then cleaned using recirculating solutions of 0.5 wt% NaOH, simulating industrial cleaning-in-place operations. Protein removal was monitored by local measurements of fouling resistance (at low heating power) and a total protein assay. Bulk flow velocities of 0.1–0.3 m/s and waviness ratios (amplitude of velocity pulse/baseline flow velocity) of 0.33–5.0 were studied at room temperature. Cleaning at these temperatures is a relatively slow process and allows the effect of flow regime to be followed readily. The resulting cleaning profiles showed that protein was removed in two stages: an initial rinsing stage, followed by protein swelling and gradual dissolution. Only the rinsing stage was observed in the absence of NaOH at a noticeably lower rate. Slow flow pulsing enhanced the overall cleaning rate, which exhibited a noticeable increase when the waviness of the flow exceeded unity and backflow of the fluid occurred. The results are discussed in terms of cleaning enhancement as a function of extra flow rate and extra energy input to the process.