Organic Fouling Deposits

Operating parameters such as temperature, pressure, and contact time, all of which increase fouling reaction rates, ordinarily are set by processing conditions. Additional factors are stream 

Another factor that increases fouhng is the presence in the process streams of trace quantities of certain active metals such as iron, nickel, vanadium, and particularly copper. These metals are present because of their original occurrence in the crude streams, or from corrosion of process equipment constructed from the metals or their alloys. Surfaces of these metals are also active catalysts for fouling reactions. Here again, the interdependence of corrosion and fouling is illustrated, since metal contaminants resulting from corrosion in up-stream units may be reduced by the use of corrosion inhibitors. 

Oil-soluble dispersants are widely used to alleviate both organic and inorganic fouling problems. The object is not to prevent the initial formation of coke nuclei and other insoluble particles in the stream, but to reduce their tendencies to agglomerate into larger precipitates that can settle out of the process stream and deposit on and in various places in the equipment. A test for effectiveness of materials as anti-foulants, based on their ability to disperse carbon black in hydrocarbons can be established. 

Commercial materials recommended for use as anti-foulants in processing industries contain combinations of dispersants, anti-oxidants, metal deactivators, and/or corrosion inhibitors. The choice of the best material for a given application should be determined by effectiveness and cost. Screening tests to differentiate between alternative materials will be described below. Because of the wide variety of streams requiring treatment, many commercial anti-foulants have been developed for different applications. The situation is similar to that of corrosion inhibition and no universal remedy is available. 

An additional important property of anti-foulants is high-temperature stability. Temperatures above 200 °C (400 °F) are common and applications in the range of 315 to 345 °C (600 to 650 °F) are not unusual. 

---

Li, J. X., Sanderson, R. D., Hallbauer, D. K., and Hallbauer-Zadorozhnaya, V. Y. (2002b). Measurement and modeling of organic fouling deposition in ultrafiltration by ultrasonic transfer signals and reflections. Desalination 146, 177. 

From these estimates it is clear though, that monolayer adsorption can only account for a small proportion of the organic carbon deposited. Monolayer adsorption was chosen as the the change in charge due to adsorption is large which would reduce further adsorption. It is an objective of the fouling studies to measure and understand the mechanisms of the deposits observed. 

The XPS analysis of unwashed membranes showed that ESCA could identify the organic fouling layers of the membranes as well as the surface structure. Peak fitting regimens determined whether the materials discovered were foulants deposited (as indicated by the N (Is) data) on the surface or materials that had become chemically bound to the surface. To be chemically bound to the surface, the foulant must show an oxidation state indicative of a bond with membrane substituents. If the foulant showed a chemical composition not associated with the membrane, it was determined to be lying on the surface (physisorbed) and not chemically attached. This was the case for calcium on LFCl (Ca (2pl, 2p3)BE 350.3-346.9 eV). However, this finding does not preclude the possibility of calcium complexation with functional groups of the RO membranes used... 

Biological Fouling Deposition and growth of macro-organisms and microorganisms on the heat transfer surface. It usually happens in water streams. 

The above analysis indicates that the high concentrations of sulfur-containing deposits and corrosion products were caused by the influence of large organisms. Bacterial contributions to corrosion and associated fouling were minimal. 

Condenser leaks permit dissolved solids, oxygen, organics, and suspended solids to enter the CR system and from there to enter the FW system. This is a very serious problem and leads to feed system fouling and deposition and the depassivation of metal surfaces, which in turn induces pitting corrosion to occur. 

The type of membrane cleaning required depends on both the type and degree of fouling experienced, but typically it is either organic (bacterial slimes, natural organics, or process foulants and nutrients) or inorganic (silica, carbonate, sulfate, or phosphate deposits). 

Several cleaning methods are used to remove the densified gel layer of retained material from the membrane surface. Alkaline solutions followed by hot detergent solutions are indicated for organic polymer colloids and gelatinous materials fouling. Ferrous deposits, t3 pical in water treatments, are usually removed with a citric or hydrochloric wash. [35]. 

Heat transfer may be degraded in time by corrosion, deposits of reaction products, organic growths, etc. These effects are accounted for quantitatively by fouling resistances, l/hf. They are listed separately in Tables 8.4 and 8.6, but the listed values of coefficients include these resistances. For instance, with a clean surface the first listed value of V in Table 8.4 would correspond to a clean value of U = 1/(1/12 — 0.04) = 23.1. How long a clean value could be... 

Corrosion takes many forms but is always an electrochemical process, whereas deposition and fouling are often combinations of both chemical and physical processes, involving inorganic and organic contaminants, the effective control of which tends to be much more of an art than a science. 

---