Short contact times

Figure Bl.12.10. CP data from the two carbons in glycine as a fiinction of contact time. The signal for short contact times is shown in the inset where the effect of the different Tjs values can be clearly seen.

The bottoms from the solvent recovery (or a2eotropic dehydration column) are fed to the foremns column where acetic acid, some acryflc acid, and final traces of water are removed overhead. The overhead mixture is sent to an acetic acid purification column where a technical grade of acetic acid suitable for ester manufacture is recovered as a by-product. The bottoms from the acetic acid recovery column are recycled to the reflux to the foremns column. The bottoms from the foremns column are fed to the product column where the glacial acryflc acid of commerce is taken overhead. Bottoms from the product column are stripped to recover acryflc acid values and the high boilers are burned. The principal losses of acryflc acid in this process are to the aqueous raffinate and to the aqueous layer from the dehydration column and to dimeri2ation of acryflc acid to 3-acryloxypropionic acid. If necessary, the product column bottoms stripper may include provision for a short-contact-time cracker to crack this dimer back to acryflc acid (60). 

Dehydrogenation of Propionates. Oxidative dehydrogenation of propionates to acrylates employing vapor-phase reactions at high temperatures (400—700°C) and short contact times is possible. Although selective catalysts for the oxidative dehydrogenation of isobutyric acid to methacrylic acid have been developed in recent years (see Methacrylic ACID AND DERIVATIVES) and a route to methacrylic acid from propylene to isobutyric acid is under pilot-plant development in Europe, this route to acrylates is not presentiy of commercial interest because of the combination of low selectivity, high raw material costs, and purification difficulties. 

A major step in the production of nitric acid [7697-37-2] (qv) is the catalytic oxidation of ammonia to nitric acid and water. Very short contact times on a platinum—rhodium catalyst at temperatures above 650°C are required. 

Today, the air oxidation of toluene is the source of most of the world s synthetic benzaldehyde. Both vapor- and Hquid-phase air oxidation processes have been used. In the vapor-phase process, a mixture of air and toluene vapor is passed over a catalyst consisting of the oxides of uranium, molybdenum, or related metals. High temperatures and short contact times are essential to maximize yields. Small amounts of copper oxide maybe added to the catalyst mixture to reduce formation of by-product maleic anhydride. 

Outside the realm of typical hydrocarbon pyrolysis is the high temperature pyrolysis of methane. In one variant of this process, which has only been commercialized to produce acetylene (with some BTX), methane reacts in an electric arc at about 1500°C (17) with very short contact times. At higher temperatures or with a catalyst and added hydrogen, BTX is produced with fairly high selectivity (18). 

U. Single water drop in air, liquid side coefficient / jy l/2 ki = 2 ), short contact times / J 1 lcontact times dp [T] Use arithmetic concentration difference. Penetration theory, t = contact time of drop. Gives plot for k a also. Air-water system. [lll]p.. 389... 

When deactivation occurs rapidly (in a few seconds during catalytic cracking, for instance), the fresh activity can be found with a transport reac tor through which both reac tants and fresh catalyst flow without slip and with short contact time. Since catalysts often are sensitive to traces of impurities, the time-deac tivation of the catalyst usually can be evaluated only with commercial feedstock, preferably in a pilot plant. 

High selectivity of the nitrile is favored by short contact time. 

In delayed coking, the reactor system consists of a short contact-time heater coupled to a large drum in which the preheated feed soaks on a hatch basis. Coke gradually forms in the drum. A delayed coking unit has at least a pair of drums. When the coke reaches a predetermined level in one drum, flow is diverted to the other so that the process is continuous. 

Short contact time in the riser and in the reactor dilute phase... 

The last boundary condition results from the assumption that for the relatively short contact times occurring in real systems, the effect of diffusion at b is negligible, and, therefore, a change in concentration at this point results only from chemical reaction. 

Amides are very weak nucleophiles, far too weak to attack alkyl halides, so they must first be converted to their conjugate bases. By this method, unsubstituted amides can be converted to N-substituted, or N-substituted to N,N-disubstituted, amides. Esters of sulfuric or sulfonic acids can also be substrates. Tertiary substrates give elimination. O-Alkylation is at times a side reaction. Both amides and sulfonamides have been alkylated under phase-transfer conditions. Lactams can be alkylated using similar procedures. Ethyl pyroglutamate (5-carboethoxy 2-pyrrolidinone) and related lactams were converted to N-alkyl derivatives via treatment with NaH (short contact time) followed by addition of the halide. 2-Pyrrolidinone derivatives can be alkylated using a similar procedure. Lactams can be reductively alkylated using aldehydes under catalytic hydrogenation... 

Considerably trailing behind VjOj but only slightly behind CeO is FejOj, which shows an onset temperature for HjS release of approximately 520 C. This means Fe Oj is not expected to be able to adequately catalyze step 3 in the FCCU riser environment because of short contact time [5], even though the temperature at the very bottom of the riser exceeds 530 C. In fact, the result of our pilot plant test of such a catalyst is in agreement with this assessment. Thus, it is quite clear that the onset temperature for HjS release is more critical than the rate of take-off in determining the catalyst efficiency for step 3. 

The reaction was performed over the iron phosphate catalyst by changing the feed rate of oxygen from zero to 350 mmol/h, while fixing the sum of feed rates of oxygen and nitrogen at 350 mmol/h. The feed rate of pyruvic acid was fixed at 10.5 mmol/h. The yields of citraconic anhydride obtained at a temperature of 230°C and a short contact time of 0.52 s (amount of catalyst used = 2 g) are plotted as a function of the feed rate of oxygen in Figure 3. 

Concerning the reaction pathway, two routes have been proposed the sequence of total oxidation of methane, followed by reforming of the unconverted methane with CO2 and H2O (designated as indirect scheme), and the direct partial oxidation of methane to synthesis gas without the experience of CO2 and H2O as reaction intermediates. The results obtained by Schmidt and his co-workers [4, 5] indicate that the direct reaction scheme may be followed in a monolith reactor when an extremely short contact time is employed at temperatures in the neighborhood of 1000°C. However, the majority of previous studies over numerous types of catalysts show that the partial oxidation of methane follows the indirect reaction scheme, which is supported by the observation that a sharp temperature spike occurs near the entrance of the catalyst bed, and that essentially zero CO and H2 selectivity is obtained at low methane conversions (<25%) where oxygen is not fully consumed [2, 3]. A major problem encountered... 

The short contact times required, typically being considerably below, also favor operation in micro channels. Hence high reaction rates and high exofhermicity are characteristic of the oxidation of ammonia. 

Whitehead, K. and McCue, K. A. (2010). Virucidal efficacy of disinfectant actives against feline calicivirus, a surrogate for norovirus, in a short contact time. Am. J. Infect. Control 38, 26-30. 

The aqueous solution flows from the jet nozzle to a receiving capillary with no overflow into the outer stream, resulting in short contact times of around 0.05 s. Analysis is implemented by flowing the outer organic phase continually through a closed loop and monitoring concentration changes spectrophotometrically. The apparatus used by Freeman and Tavlarides employed capillaries with internal diameters of 2 mm, and the... 

The second section of this volume describes several potentially new liquefaction processes which may have higher efficiencies than today s developing technologies. The theme of the Storch Award Symposium, featured throughout these six chapters, was new process potentials through the use of short-contact-time thermal processes followed by catalytic upgrading. 

A New Outlook on Coal Liquefaction Through Short-Contact-Time Thermal Reactions Factors Leading to High Reactivity... 

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