Foam-over consequences

Some cooling is necessary, or the rapid evolution of gas will cause the reaction mixture to foam over with consequent loss of material. 

In the coke drum, foaming occurs. As the bulk viscosity of the resid increases and solids form, a stable foam front forms, and it rises higher in the drum as the coke level rises. If left unchecked, the foam will reach near the top of the coke drum and potentially go overhead, or foam-over . The consequences of a foam-over are immediate and disastrous. The transfer line from the coke drum to the fractionator becomes fouled with coke particles. As the solids reach the fractionation tower, the suction screens in the bottom of the tower become plugged. Finally, the finer particles of coke that pass through the suction screens will... 

In addition, even where foaming is not a specific problem in a boiler, carryover may occur, especially in lower pressure boilers with very high TDS (i.e., over 10,000 to 15,000 ppm TDS) because of the collapse of surface bubbles. This leads to BW aerosol generation and entrainment of the spray in steam. Under these circumstances, antifoam agents such as polyamides are useful in preventing these entrainment problems. Furthermore, the antifoaming action of polyamides is often enhanced by protective colloid materials such as tannins, and consequently, formulations containing polyamide emulsions in an alkaline tannin base are available. 

Similar results have been obtained in the study of foam breakdown by alcohols using a special device for determination of the rate of contact foam breakdown [69]. The foam was supplied at constant rate over the surface of organic solvent. The rate of foam breakdown was determined in two regimes impulse and continuous. In the impulse regime, after contacting the organic solvent, the foam expanded as a consequence of its breakdown in the contact zone and detached itself from the solvent surface. Since the foam was constantly supplied, after a... 

If the foam phase is thought of as a pseudo continuous fluid with an apparent viscosity Vapp = it follows that Papp is greater than that of the aqueous liquid phase. (For the tests here, values of Uapp were on the order of 1 to 50 times that of water). Because of this, when foam and liquid move through a porous medium under an applied pressure drop, the foam, being the most viscous phase, must occupy a larger region of the pore space. Consequently, as observed, the gas saturation is increased over that of non-dispersed phase flow and the liquid permeability is correspondingly decreased. 

Aqueous solutions of these peaked materials show lower toxicides, lower viscosities, lower gel temperatures, and remain fluid over a wider concentration range. In spray-drying operations, there is less evolution of volatile material, since they contain less unreacted hydrophobe than conventional materials. They wet cotton more efficiently, show higher initial foam heights (but lower foam stability), reduce interfacial tension against mineral oil more efficiently and effectively than the corresponding conventional types. When sulfated to produce AES, the product has less non-POE alkyl sulfate and, consequently, less skin irritation and a greater tendency to thicken upon salt addition. 

If a C02 flood is already underway, then the initiation of C02-foam mobility control will cause changes in the injection schedule. Because in many cases C02 is the major adjustable operating expense, this alone may call for an increase in rate of outlay. On the other hand, industry experience has shown that other things being equal, the rate of oil production is proportional to the rate of injection of C02, which would be greater by a factor of 2—3 in a high-C02-fraction foam flood over a 2 1 WAG flood. Consequently, this cost increase can be expected to be matched quickly by increasing production. 

Binks and Murakami report unique behavior in NP-induced phase transformation in particle-stabilized air-water systems that is not demonstrated by surfactants. It was seen that by altering silica-NP (20-30 nm) hydrophobicity at constant air water ratio or by changing the air water ratio at fixed NP wettability, phase inversion could be induced from air-in-water to water-in-air foams (Fig. 12) [36]. This investigation thus demonstrates that control over interfacial assembly of NPs leads to the formation of stable NP-shelled hollow spheres, thus resulting in the formation of stable foams, dispersions, and powders with far reaching consequences in opening new avenues for advanced encapsulation (Fig. 13). 

As a result, this equation is usually the only one needed for liquid or solid aerosols. Figure 6.18 shows several sets of experimental data compared with the Einstein equation. In practice once cp reaches between 0.1 and 0.5, dispersion viscosity increases significantly and can also become non-Newtonian (due to particle/droplet/bubble crowding or structural viscosity). The maximum volume fraction possible for an internal phase made up of uniform, incompressible spheres is 0.74, although emulsions and foams with an internal volume fraction of over 0.99 can exist as a consequence of droplet/bubble distortion. Figure 6.18 and Equation 6.33 illustrate why volume fraction is such a theoretically and experimentally favoured concentration unit in rheology. In the simplest case, a colloidal system can be considered Einsteinian, but in most cases the viscosity dependence is more complicated. 

Metallic or ceramic foams exhibit several advantages over randomly packed beds [34]. Open cell foams consist of a network of interconnected rods, which delineate cavities (called cells). Metallic foams have porosities of up to 95% and ceramic foams of 75-85%. Examples are shown in Figure 11.16. Foams are characterized by considerably lower pressure drops than packed beds for comparable characteristics (such as specific geometrical surface area) and at a given superficial fluid velocity consequently, they are an interesting alternative to catalyst pellets. 

Liguras et al. investigated autothermal reforming of ethanol over ruthenium and nickel catalysts on structured supports such as ceramic foams and monoliths [212,213]. Conditions chosen were an O/C ratio of 0.61 and an S/C ratio of 1.5. The reaction was performed at a very high pre-heating temperature of the monoliths and consequently substantial conversion occurred even upstream of the reactor, which created a hot spot of up to 950 °C in the monoliths. A ceramic monolith coated with 5 wt.% ruthenium formed in addition to carbon oxides methane as the main byproduct, but there were also small amounts of acetaldehyde, ethylene and ethane [212]. When the S/C ratio was increased to 2.0, the by-products could be suppressed. Increasing the O/C ratio had a similar effect and also suppressed the methane formation. The ruthenium catalyst showed stable conversion for a 75-h test duration. Nickel/lanthana catalysts containing 13 wt.% nickel on a lanthana carrier showed similar performances with respect to activity, selectivity and stability [213]. 

Kinetics can be screened in a screw impeller-stirred reactor (SISR) [7] (Figure 9.8). The reactor system comprises a screw impeller that pumps the liquid upwards, and the high exit velocity of the liquid results in an effective foam formation in the top section of the reactor. A slug flow (Chapter 6) is thus established in the monolith channels. Consequently, the liquid and gas are pumped from the lower section to the upper section of the reactor, over and over again. In fact, the concept resembles that of a loop reactor. Cylindrical monoliths are placed in the stator of an SISR (Figure 9.8), and a foam of gas and liquid is forced through the monolith channels by a screw. 

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