Inorganic Fouling Deposits

Another common fouling problem due to inorganic deposits may occur when ammonia is used to neutralize HCl formed by hydrolysis of chlorides after crude desalting. Increasing the pH, in order to reduce corrosive potential, results in formation of the oil-insoluble salt, NH4CI. This may result in a fouling problem that can be alleviated by adding water to the affected unit, either continuously or intermittently. 

Another approach is to reduce the amount of ammonia added for neutrahzation and operate at a lower pH, and instead use organic film-forming inhibitors to control corrosion. The frequency of this approach has increased because of the development of inhibitors active over a wider pH range than 

A third solution to the problem employs neutrahzers other than ammonia, e.g. morpholine, cyclo-hexylamine, or other high molecular weight amines, which combine with mineral acids to give salts having higher oil solubility and/or dispersibility thanNH Cl. 

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The pH of cooling water naturally increases due to evaporation. Sulfuric acid is added as required to control the pH at 6.5-7.0. An excursion of high pH (9-10) will cause a rapid increase in inorganic fouling deposits in the exchanger tubes. 

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. 

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). 

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. 

Another very important operating characteristics of inorganic membranes that is not shown in Table 1.4 has to do with the phenomena of fouling and concentration polarization. Concentration polarization is the accumulation of the solutes, molecules or particles retained or rejected by the membrane near its surface. It is deleterious to the purity of the product and the decline of the permeate flux. Fouling is generally believed to occur when the adsorption of the rejected componcni(s) on the membrane surface is strong enough to cause deposition. How to maintain a clean membrane surface so that... 

Despite the aforementioned efforts, membrane flux decline due to fouling continues to be a major operational issue. Attempts have been made to modify inorganic membranes, mostly their surfaces, to render them less prone to foulant adsorption. One of the frequently encountered fouling problems in biotechnology and food applications is protein adsorption. In membrane reactor applications which are largely associated with hydrocarbons, carbonaceous deposits pose as one of the operational problems. 

There are two different levels where fouling phenomena and related effects may interfere with performance of composite inorganic or hybrid membranes. The first and the more classically reported in literature is the one of the separation process itself, which through various interactions between solution and material (adsorption, surface deposits, pore plugging) generally leads to reduced fluxes and increased retentions. The second, much more less described by authors but of the same nature and with analogous effects, concerns membrane preparation, and the possible interactions between deposited layers. Theses two aspects are linked up with the so-called formed-in-place membranes, obtained by deposition of species onto a ceramic support through cross-flow filtration. In what follows, they will be described in a unified approach. 

I n this sense, fouling means deposition of reactor debris on the particles. Scale, rust, and other corrosion products arc all possibilities, in addition to chemical components from up-stream units. Particles removed from reactors often have red-brown iron oxide crusts on the outside. Calcium compounds are also found. The most severe cases occur in processing coal and coal-derived liquids, which contain large amounts of inorganic mineral matter. 

Fouling is the (ir)reversible deposition of retained components on the membrane feed surface. These deposits can be biofllms, organic components, and/or inorganic salts. Accumulation tends to increase with time and the effect does not disappear when the process is turned off. There are some approaches to reducing the effect of fouling that will be described shortly. In Figure 9.16 we separate the flux decline in Figure 9.15 into these two effects. We can derive an equation to estimate the increase in retained solute concentration at the feed membrane surface due to concentration polarization. 

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