Pressurized reactors

Szanyi J and Goodman D W 1993 Combined elevated pressure reactor and ultrahigh vacuum surface analysis system Rev. Sc/. Instrum. 64 2350... 

Commercially, urea is produced by the direct dehydration of ammonium carbamate, NH2COONH4, at elevated temperature and pressure. Ammonium carbamate is obtained by direct reaction of ammonia and carbon dioxide. The two reactions are usually carried out simultaneously in a high pressure reactor. Recendy, urea has been used commercially as a catde-feed supplement (see Feeds and feed additives). Other important appHcations are the manufacture of resins (see Amino resins and plastics), glues, solvents, and some medicinals. Urea is classified as a nontoxic compound. 

The Stainicaibon process is described in Figures 3—7. The synthesis section of the plant consists of the reactor, stripper, high pressure carbamate condenser, and a high pressure reactor off-gas scmbber. In order to obtain a maximum urea yield pet pass through the reactor, a pressure of 14 MPa (140 bar) and a 2.95/1 NH —CO2 molar ratio is maintained. The reactor effluent is distributed over the stripper tubes (falling-film type shell and tube exchanger) and contacted by the CO2, countercurrendy. This causes the partial NH pressure to decrease and the carbamate to decompose. 

At still higher temperatures, when sufficient oxygen is present, combustion and "hot" flames are observed the principal products are carbon oxides and water. Key variables that determine the reaction characteristics are fuel-to-oxidant ratio, pressure, reactor configuration and residence time, and the nature of the surface exposed to the reaction 2one. The chemistry of hot flames, which occur in the high temperature region, has been extensively discussed (60-62) (see Col ustion science and technology). 

Polysilicon. Polysihcon is used as the gate electrode material in MOS devices, as a conducting material for multilevel metallization, and as contact material for devices having shallow junctions. It is prepared by pyrolyzing silane, SiH, at 575—650°C in a low pressure reactor. The temperature of the process affects the properties of the final film. Higher process temperatures increase the deposition rate, but degrade the uniformity of the layer. Lower temperatures may improve the uniformity, but reduce the throughput to an impractical level. 

One was a water-moderated and water-cooled pressurized reactor the other was a Hquid-metal-cooled iatermediate neutron energy reactor. A land-based prototype submafine power plant called Mark I was built and tested at the National Reactor Testing Station. Argonne National Laboratory provided scientific data and Bettis Laboratory of Westinghouse Electric Corp. suppHed engineering expertise. 

Fig. 4. Higher yields from lower pressures reactor pressure A, 791 kPa (100 psig) B, 1480 kPa (200 psig) C, 2515 kPa (350 psig). Data for Arabian...

Another common situation is batch hydrogenation, in which pure hydrogen is introduced to a relatively high pressure reactor and a decision must be made to recycle the unabsorbed gas stream from the top of the reactor or use a vortexing mode for an upper impeller to incorporate the gas from the surface. 

High pressure reactions High inventories of stored pressure (e.g. in pressurized reactors or associated plant) can result in catastrophic failure of the pressure shell... 

The reaction section consists of the high pressure reactors filled with catalyst, and means to take away or dissipate the high heat of reaction (300-500 Btu/lb of olefin polymerized). In the tubular reactors, the catalyst is inside a multiplicity of tubes which are cooled by a steam-water condensate jacket. Thus, the heat of reaction is utilized to generate high pressure steam. In the chamber process, the catalyst is held in several beds in a drum-type reactor with feed or recycled product introduced as a quench between the individual beds. 

Into a pressure reactor there was charged 100 ml of methanol and 1 g of diruthenium nona-carbonyl. The reactor was closed, cooled in solid carbon dioxide/acetone, and evacuated. Acetylene, to the extent of 1 mol (26 g), was metered into the cold reactor. Carbon monoxide was then pressured into this vessel at 835-9BO atmospheres, during a period of 16.5 hours while the reactor was maintained at 100°C to 1 50°C. The reactor was then cooled to room temperature and opened. 

Reactor dilute phase (dome) pressure Reactor catalyst dilute phase bed level Reactor-stripper catalyst bed level Reactor-stripper catalyst density Spent catalyst standpipe elevation Pressure above the spent catalyst slide valve Spent catalyst slide valve AP ( 55% opening)... 

The early attempts at NMP of S in emulsion used TEMPO and related nitroxides and needed to be carried out at high temperatures (100-130 °C) necessitating a pressure reactor. Problems with colloidal stability and molecular weight control and limiting conversions were reported.215 217... 

Most of the published promotional kinetic studies have been performed on well defined (single crystal) surfaces. In many cases atmospheric or higher pressure reactors have been combined with a separate UHV analysis chamber for promoter dosing on the catalyst surface and for application of surface sensitive spectroscopic techniques (XPS, UPS, SIMS, STM etc.) for catalyst characterization. This attempts to bridge the pressure gap between UHV and real operating conditions. 

Figure 9.32. Experimental set-up (a) Machinable ceramic holders and two proton conducting pellets showing the location of catalyst, counter and reference electrodes, (b) Twenty four pellet unit, (c) High-pressure reactor, gas feed and analysis unit.43 Reprinted with permission from the American Chemical Society.

This electrochemical promotion study was novel in three respects a) The catalyst-electrode was a fully promoted industrial catalyst, (b) The study was carried out at high pressure (50 atm), (c) This was the first attempt for the scale-up of an electrochemically promoted reactor since 24 CaZro.9Irio.1O3m cell-pellets, electrically connected in parallel, were placed in the high pressure reactor (Fig. 9.32).43... 

Some reactants in atmospheric-pressure reactors must be highly diluted with inert gases to prevent vapor-phase precipitation, while generally no dilution is necessary at low pressure. However, atmospheric pressure reactors are simpler and cheaper. They can operate faster, on a continuous basis and, with recent design improvements, the quality of the deposits has been upgraded considerably and satisfactory deposits of many materials, such as oxides, are obtained. 

Variable volume (e.g., a constant-pressure reactor), V t) Variable cross section, A (z)... 

In this example, only one of the reagents has a concentration that can vaiy, and each stoichiometric coefficient is one. What happens for a more complicated reaction Consider the synthesis of ammonia carried out in a pressurized reactor containing N2, H2, and NH3 at partial pressures different from 1 bar ... 

The important message here is that the overall performance of the reactor may be improved by using an assembly of catalysts that varies though the reactor bed. To what extent such approaches will become viable depends on the cost of varying the catalysts and the savings realized by reducing the size of the high-pressure reactor. 

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