Fouling limitations

First we check the concentration (C,) at which the flux becomes fouling-limited ... 

SNCR and SCR use ammonia or urea to reduce NO in the combustion gases to elemental nitrogen. A significant common consideration with both of these processes is ammonia slip, i.e., unused ammonia in the downstream flue gas. However, ammonia slip, which exists whether ammonia or urea is the reagent, can be effectively controlled. The acceptable amount of ammonia slip is usually dictated by fouling limitations and by permit requirements. Regulators often set 10 to IS ppm at the stack as the maximum allowable ammonia concentration. 

Reboiler temperature increases with a limit often set by thermal decomposition of the material being vaporized, causing excessive fouling. 

Gun Propellents. Although the stresses on individual gun propellant grains are less severe because of the small size, these propellants must withstand much higher weapon pressures and accelerations. Formulation options are usually more limited for gun propellants than for rocket propellants because the products of combustion must not foul or corrode a gun, should have a low flame temperature, and should exhibit minimum flash and smoke characteristics. Gun propellants are examined microscopically for porosity, are tested for mechanical characteristics, and fired in closed bombs to determine the burning characteristics. 

Oxygen difluoride must be regarded as a highly poisonous gas, somewhat more toxic than fluoriae. It has a foul odor with a limit of detectabiUty of 0.1—0.5 ppm. Repeated exposure of rats to 0.5 ppm OF2 produced death repeated exposure to 0.1 ppm, however, caused no discernible effects. 

Tubular Modules. Tubular modules are generally limited to ultrafiltration appHcations, for which the benefit of resistance to membrane fouling because of good fluid hydrodynamics overcomes the problem of their high capital cost. Typically, the tubes consist of a porous paper or fiber glass support with the membrane formed on the inside of the tubes, as shown in Figure 24. 

In reverse osmosis, most modules are of the hollow-fine fiber or spiral-wound design plate-and-frame and tubular modules are limited to a few appHcations in which membrane fouling is particularly severe, for example, food appHcations or processing of heavily contaminated industrial wastewater. 

High Water Velocities. The abiUty of high water velocities to minimize fouling depends on the nature of the foulant. Clay and silt deposits are more effectively removed by high water velocities than aluminum and iron deposits, which are more tacky and form interlocking networks with other precipitates. Operation at high water velocities is not always a viable solution to clay and silt deposition because of design limitations, economic considerations, and the potential for erosion corrosion. 

Coal deposits from east of the Mississippi River generally have acidic mineral constituents, ie, they are richer in siUca and alumina and tend to produce higher melting ash mixtures. These materials do not soften until above 1000°C and have limited problems with deposition on the inside walls of the boiler (slagging) or on the superheater tubes inside the boiler (fouling). 

Heat Recovery Equipment. Factors that limit heat recovery appHcations are corrosion, fouling, safety, and cost of heat-exchange surface. Most heat interchange utilizes sheU and tube-type units because of the mgged constmction, ease of mechanical cleaning, and ease of fabrication in a variety of materials. However, there is a rich assortment of other heat exchangers. Examples found in chemical plants in special appHcations include the foUowing. 

The inside of the convection tubes rarely foul, but occasionally the Hquid unsaturates in feedstocks tend to polymerize and stick to the walls and thus reduce the heat transfer. This soft coke is normally removed by mechanical means. In limited cases, the coke can also be burnt off with air and steam. Normally, the outside surface of the convection section fouls due to dust and particles in the flue gas. Periodically (6 to 36 months), the outside surface is cleaned by steam lancing. With Hquid fuel firing, the surface may require more frequent cleaning. 

There are situations where thermal oxidation may be preferred over catalytic oxidation for exhaust streams that contain significant amounts of catalyst poisons and/or fouling agents, thermal oxidation may be the only technically feasible control where extremely high VOC destmction efficiencies of difficult to control VOC species are required, thermal oxidation may attain higher performance and for relatively rich VOC waste gas streams, ie, having >20 25% lower explosive limit (LEL), the gas stream s explosive properties and the potential for catalyst overheating may require the addition of dilution air to the waste gas stream (12). 

Membrane Limitations Chemical attack, fouling, and compaction are prominent problems with RO and NF membranes. Compaction is the most straightforward. It is the result of creep, slow cold flow of the polymer resulting in a loss of water permeability. It is measured by the slope of log flux versus log time in seconds. It is independent of the flux units used and is reported as a slope, sometimes with the minus sign omitted. A slope of—0.001, typical for noncelhilosic membranes, means that for every threefold increase in log(time), 10 seconds, a membrane looses 10 percent of its flux. Since membranes are rated assuming that the dramatic early decline in permeability has already occurred, the further decline after the first few weeks is veiy slow. Compaction is specific to pressure, temperature, and envi-... 

Table 12-4 is a summary of liquid fuel speeifieations set by manufaeturers for effieient maehine operations. The water and sediment limit is set at 1% by maximum volume to prevent fouling of the fuel system and obstruetion of the fuel filters. Viseosity is limited to 20 eentistokes at the fuel nozzles to prevent elogging of the fuel lines. Also, it is advisable that the pour point be 20 °F (11 °C) below the minimum ambient temperature. Failure to meet this speeifieation ean be eorreeted by heating the fuel lines. Carbon residue should be less than 1% by weight based on 100% of the sample. The hydrogen eontent is related to the smoking tendeney of a fuel. Lower... 

There are at least three reasons for on-stream cleaning. The first is to restore the system s capability. If the unit is a driver, its maximum horsepower will probably drop as it becomes dirty. Cleaning will restore this limit. If the machine is a dynamic compressor, fouling may reduce its head, and therefore, the maximum gas flow rate. Cleaning will restore the capacity limit. 

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