Fouling Tendency

Convection tube spacing is important when the fuel is residual oil or coal, especially co with low asn-fusion or high ash-fouling tendencies. The amount of the ash and, even more important, the characteristics of the ash must be specified for design. 

FTIR can be used to screen membranes for fouling tendencies prior to the first ultrafiltration experiment. Screening can be done by means of a simple static adsorption test. Membranes showing greater static adsorption are expected to foul more during ultrafiltration and are disfavored. Figure 8 illustrates the FTIR results... 

Some chemical product manufacturers claim a need for higher dose rates (say, up to 10 ppm) to minimize scaling and fouling tendencies, but this is usually prohibitively expensive and may well prove to be an overdose. 

May be able to handle streams containing solids or with fouling tendencies. 

The deposition of crystalline scales, air-borne contaminants, biofilms, etc. tends to be higher on previously fouled or corroded surfaces than on clean surfaces. Also, deposition and fouling affect both the rates and mechanisms of corrosion on a clean metal surface. It can therefore be useful to obtain subjective information on deposition and fouling tendencies in a cooling system, provided that the methods are simple and produce results quickly. The use of blank coupons inserted in a bypass corrosion rack can often provide this support information. 

The best type of valve unit depends on the corrosive and fouling tendencies of the service, as some valve units tend to pop out of their seats in corrosive services, while others tend to stick to their seats in fouling services. 

Maintenance, fouling tendency, and effects of corrosion are least in sieve trays, although these are not much greater for valve trays. 

Fouling tendency Low Low to moderate High. Tends to collsot solids Extremely low, Suitable where fouling is extensive and for slurry handling. 

Discuss the applicability of sieve trays. The sieve tray is probably the most versatile contacting device. It should be considered first for the design of a tray column. It has the lowest installed cost of any equilibrium-stage-type device, its fouling tendencies are low, and it offers good efficiency when properly designed. 

The HP stripper and deethanizer bottoms (of different quality) are fed to a two-column dual pressure depropanizing system (14,15) for C3 cut separation from the C4 cut and heavies, thus giving a low fouling tendency at minimum energy consumption. 

Spacings are from 6.35 to 31.75 mm (in 6.35 mm increments) with 9.5 mm the most common. Stud densities are 60 x 60 to 110 x 110 mm, the former the most common. The width (measured to the spiral flow passage), is from 150 to 2500 mm (in 150 mm increments). By varying the spacing and the width, separately for each fluid, velocities can be maintained at optimum rates to reduce fouling tendencies or utilize the allowable pressure drop most effectively. Diameters can reach 1500 mm. The total surface areas exceed 465 sqm. Materials that work harder are not suitable for spirals since hot-forming is not possible and heat treatment after forming is impractical. 

Several researchers have investigated the possibilities of membranes for the removal of dispersed water-based ink pigments from wash effluent [121-126]. Generally, membranes, in particular ultrafiltration membranes, have been found to completely remove ink pigments from effluent streams. It has also been observed that the permeate flux and the fouling tendency depend on operational conditions and effluent composition. For instance, coagulation pretreatment [125], feed water acidification [121], and surfactant addition [123] have been found to improve the flux and decrease fouhng. 

The simplest design is a dead-end operation, as shown in Fig. 16A. As the feed is forced through the membrane, the concentration of rejected components in the feed increases and accumulates at the membrane interface, hence the permeate quality decreases with time. Therefore, for industrial applications, a cross-flow operation, as shown in Fig. 16B, is preferred for its lower fouling tendency comparing to the dead-end mode. 

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