Fouling thickness

In this section it has generally been assumed that in addition to deposition processes, there are removal mechanisms occurring at the same time. Such a situation may not apply to all fouling processes. In some examples the mass transfer of fouling precursors towards the surface may be the only transport mechanism removal processes may be non-existent. The general shape of the curve of fouling thickness with time (Fig. 1.1) may arise, not because of the interaction of the two mass transfer processes, but because the deposition process is suppressed, i.e. an auto retardation of the deposition process till it becomes virtually zero. Such effects might arise due to electro-chemical conditions (see Chapter 6), or solution saturation conditions near the deposit/fluid interface. 

This technique was used to measure the fouling thickness deposited on a tubular membrane. Bentonite as a particulate model solution was filtered during fouling characterization experiments. The maximum CFV and solid concentration used for this analysis were 0.3 m s and 375 mg L , respectively [72]. The limitations of the technique are the inability to visualize the cake structure and the requirement of light adsorption into the lumen, reducing the visualization potential for MBR study. Extended information can be found in Chapters 11 and 15. 

PT Measurement of fouling thickness dining the filtration of synthetic wastewater on tubular membrane Unable to detect the individual bacterial cell interactions [63]... 

Detection of corrosion and defects under a layer of protective coatings, rust and foulings to 8-10 mm in thickness. 

When fouling is present or possible, ultrafiltration is usually operated at high Hquid shear rates and low pressure to minimize the thickness of the gel polarization layer. 

The Nomograph Part 3 (Figure I0-43C) may be used in a number of ways. For example, what will the fouling resistance, R(, be after an arbitrarily chosen time, t, or it can calculate the thickness of a fouling deposit after an arbitrarily chosen time t, providing the thermal conductivity of the deposited material is known. It can calculate thermal conductivity of a deposit, providing thickness is known, or estimated. 

Since no serious problems exist with corrosion, the materials of constmction can be cheaper and the thicknesses reduced. Fouling coefficients do not need to be incorporated into the design calculations, as the surfaces remain clean indefinitely. This in itself provides a secondary benefit in that the pitching of fins may be reduced without the risk of reducing flow. 

The values of /"fh and ffc (the film resistances for the hot and cold fluids, respectively) can be calculated from the Dittus-Boelter equations previously described and the wall metal resistance / from the average metal thickness and thermal conductivity. The fouling resistances of the hot and cold fluids /"dh and are often based on experience, but a more detailed discussion of this will be presented later in this chapter. 

The hole sizes used vary from 2.5 to 12 mm 5 mm is the preferred size. Larger holes are occasionally used for fouling systems. The holes are drilled or punched. Punching is cheaper, but the minimum size of hole that can be punched will depend on the plate thickness. For carbon steel, hole sizes approximately equal to the plate thickness can be punched, but for stainless steel the minimum hole size that can be punched is about twice the plate thickness. Typical plate thicknesses used are 5 mm (3/16 in.) for carbon steel, and 3 mm (12 gauge) for stainless steel. 

Example 15.4 A reboiler is required to supply 0.1 krnol-s 1 of vapor to a distillation column. The column bottom product is almost pure butane. The column operates with a pressure at the bottom of the column of 19.25 bar. At this pressure, the butane vaporizes at a temperature of 112°C. The vaporization can be assumed to be essentially isothermal and is to be carried out using steam with a condensing temperature of 140°C. The heat of vaporization for butane is 233,000 Jkg, its critical pressure 38 bar, critical temperature 425.2 K and molar mass 58 kg krnol Steel tubes with 30 mm outside diameter, 2 mm wall thickness and length 3.95 m are to be used. The thermal conductivity of the tube wall can be taken to be 45 W-m 1-K 1. The film coefficient (including fouling) for the condensing steam can be assumed to be 5700 W m 2-K 1. Estimate the heat transfer area for... 

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