Efficiency continuous catalyst regeneration

Coke, a byproduct of cracking, is deposited on the catalyst particles. Since these deposits impair reaction efficiency, the catalyst must be continuously put through the regenerator, where the carbon is burned with a current of air. The high temperature of the regeneration process (675-785°C) heats the catalyst to the desired reaction temperature for re-contacting fresh feed in the unit. 

In any case, the use of these catalytic systems will depend on their stability. In fact, the still unresolved problem with the application of these catalysts to industrial processes is whether to prolong the life of the catalyst or to continuously regenerate the catalyst in an efficient way. 

One of the most important considerations in designing a process for converting methanol to olefins was to find the best way to remove the considerable heat of reaction. Despite the fact that we are stopping the reaction at the intermediate olefin product, the reactions leading to these intermediates give off almost 90% of the heat released in the overall MTG reaction scheme (49 vs. 56 kJ/mole of methanol converted for MTO vs. MTG). Efficient removal of the heat of reaction is one of the main reasons a fluid-bed reactor was selected for scale-up demonstration. A second advantage of the fluid-bed is that product composition can be kept constant, since optimum catalyst activity can be maintained by continuous make-up and regeneration. 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

In the synthesis of carbamates, R NH.C02R, from A iV -dialkylureas, (R NH)2CO, and dialkyl carbonates, (RO)2CO, dibutyltin oxide, Bu2SnO, acted as an efficient catalyst. The proposed mechanism (Scheme 13) involves addition of the dialkyl carbonate to Bu2SnO to give an adduct (43), which is attacked by the urea to yield a new tin complex (44) and one molecule of carbamate. Attack by dialkyl carbonate upon this complex (44) yields a further molecule of carbamate and regenerates the original tin complex (43), which can continue the catalytic cycle.42... 

Another well-recognised approach for soot abatement is the use of a supported Pt oxidation pre-catalyst (upstream of the filter) aimed at producing NO2 (from NO oxidation), which decreases the non-catalysed oxidation temperature of soot by approximately 200 K (from ca. 773 to 573 K) relative to air oxidation.76,81,94 This is one of the basic concepts involved in the so called NOx-aided CRT (continuously regenerated trap), which is proposed as one of the most efficient technologies for soot abatement (Fig. 8.4).76,95 Under practical operating conditions, the oxidation precatalyst overall converts 90% of the CO and hydrocarbons and 20-50%... 

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