Plug formation, avoidance

Finally, gas-solid equilibria should be studied to avoid plugging problems due in particular to hydrate formation. 

Solid magnesium must be absent to avoid the formation of diallyl via allyl magnesium bromide the insertion of a short plug of glass wool effectively removes any finely divided magnesium. 

Another method of purifying niobium is by distillation of the anhydrous mixed chlorides (29). Niobium and tantalum pentachlorides boil within about 15°C of one another which makes control of the process difficult. Additionally, process materials must withstand the corrosion effects of the chloride. The system must be kept meticulously anhydrous and air-free to avoid plugging resulting from the formation of niobium oxide trichloride, NbOQ. Distillation has been used commercially in the past. 

Although the ethyleneamines ate water soluble, soHd amine hydrates may form at certain concentrations that may plug processing equipment, vent lines, and safety devices. Hydrate formation usually can be avoided by insulating and heat tracing equipment to maintain a temperature of at least 50°C. Water cleanup of ethyleneamine equipment can result in hydrate formation even in areas where routine processing is nonaqueous. Use of warm water can reduce the extent of the problem. 

Other problems that can be associated with the high dust plant can include alkaH deterioration from sodium or potassium in the stack gas deposition on the bed, calcium deposition, when calcium in the flue gas reacts with sulfur trioxide, or formation and deposition of ammonium bisulfate. In addition, plugging of the air preheater as weU as contamination of flyash and EGD wastewater discharges by ammonia are avoided if the SCR system is located after the FGD (23). 

Elow intensification is made with the use of apparatuses in which flow follows a perfect plug flow the internal parts of the reactor have to be designed accordingly. Indeed, dead zones, that is, reactant accumulation, must be avoided not only in order to have better selectivity and yield but also to avoid formation of hot spots, which would generate safety problems. 

Unreacted NH3 in the flue gas downstream the SCR reactor is referred to as NH3 slip. It is essential to hold the NH3 slip below 5ppm, preferably 2-3 ppm, to minimize the formation of (NH4)2S04 and NH4HS04, which can cause plugging and corrosion of downstream equipment. In order to avoid the ammonia slip, and to limit the direct oxidation of NH3 to N2, the NH3/NO ratio in the feed is typically maintained below the stoichiometric values, e.g. between 0.90 and 0.95. 

While the past methods of preventing hydrate plugs have been to use avoidance with thermodynamic inhibitors such as methanol or glycols, our new understanding of how plugs form, allows us to propose economic risk management (kinetics) to avoid hydrate formation. These concepts differ in type for oil-dominated and gas-dominated systems. 

Avoidance of the hydrate formation thermodynamic conditions of temperature, pressure, or inhibitor concentration, makes it impossible for plugs to form. The calculations of thermodynamic conditions can be made with acceptable accuracy. Using the methods presented in Chapters 4 and 5 along with the CD program CSMGem provided with this book, the temperature, pressure, and inhibitor concentrations can be calculated respectively, to within 2°F, 10% in pressure and 3% of inhibitor concentration. Since the discovery of hydrate flowline plugs in 1934, such thermodynamic methods have served to provide the major method of flow assurance. 

The catalyst and oil are in plug flow and the contact time is short so that secondary reactions are avoided and catalyst deactivation by coke formation is properly simulated. The resulting product selectivity, then, is similar to commercial units. Experimental results from a laboratory scale unit can thus be translated to commercial units. 

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