Reactor wall reaction

In this accident, the steam was isolated from the reactor containing the unfinished batch and the agitator was switched ofiF. The steam used to heat the reactor was the exhaust from a steam turbine at 190 C but which rose to about 300°C when the plant was shutdown. The reactor walls below the liquid level fell to the same temperature as the liquid, around 160°C. The reactor walls above the liquid level remained hotter because of the high-temperature steam at shutdown (but now isolated). Heat then passed by conduction and radiation from the walls to the top layer of the stagnant liquid, which became hot enough for a runaway reaction to start (see Fig. 9.3). Once started in the upper layer, the reaction then propagated throughout the reactor. If the steam had been cooler, say, 180 C, the runaway could not have occurred. ... 

Purely aqueous polymerization systems give copolymers that are not wetted by the reaction medium. The products agglomerate and plug valves, nozzles, and tubing, and adhere to stirrer blades, thermocouples, or reactor walls. These problems do not occur in organic media or mixtures of these with water. 

Although it appears that methyl ethyl ketone [78-93-3] caimot be the principal product in butane LPO, it has been reported that the ratio of methyl ethyl ketone to acetic acid [64-19-7] can be as high as 3 1 in a plug-flow-type reactor (214). However, this requires a very unusual reactor (length dia = 16, 640 1). The reaction is very unstable and wall reactions may influence mechanisms. 

Typically, reactors require some type of catalyst. Reactors with catalyst can be of the fixed-bed style for fiuid-bed types. Fixed-bed reactors are the most common. The feed often enters the reactor at an elevated temperature and pressure. The reaction mixtures are often corrosive to carbon steel and require some type of stainless steel alloy or an alloy liner for protection. If the vessel wall is less than 6 mm, the vessel is constmcted of all alloy if alloy is provided. Thicker reactor walls can be fabricated with a stainless overlay over a carbon steel or other lower alloy base steel at less cost than an all-alloy wall constmction. 

Meccaniche Modeme (Busto Arsizo, Italy) acquired rights to the original AUied Chemical Company s concentric SO sulfonation unit (272). Table 10 shows comparative data on AUied s U.S. design, and that for Meccaniche Modem s concentric design, indicating equivalency. This unit has a substantial space between reactor walls, hence it requires a 6 m length to complete reaction. The process has been scaled up to 4.0 t/h capacity. 

EPM and EPDM mbbers are produced in continuous processes. Most widely used are solution processes, in which the polymer produced is in the dissolved state in a hydrocarbon solvent (eg, hexane). These processes can be grouped into those in which the reactor is completely filled with the Hquid phase, and those in which the reactor contents consist pardy of gas and pardy of a Hquid phase. In the first case the heat of reaction, ca 2500 kJ (598 kcal)/kg EPDM, is removed by means of cooling systems, either external cooling of the reactor wall or deep-cooling of the reactor feed. In the second case the evaporation heat from unreacted monomers also removes most of the heat of reaction. In other processes using Hquid propylene as a dispersing agent, the polymer is present in the reactor as a suspension. In this case the heat of polymerisation is removed mainly by monomer evaporation. 

Hot spot develops in reaction medium. Temperature excursion outside the safe operating envelope, possibly resulting in a runaway reaction or decomposition. Potential mechanical failure of reactor wall. 

To accelerate the polymerization process, some water-soluble salts of heavy metals (Fe, Co, Ni, Pb) are added to the reaction system (0.01-1% with respect to the monomer mass). These additions facilitate the reaction heat removal and allow the reaction to be carried out at lower temperatures. To reduce the coagulate formation and deposits of polymers on the reactor walls, the additions of water-soluble salts (borates, phosphates, and silicates of alkali metals) are introduced into the reaction mixture. The residual monomer content in the emulsion can be decreased by hydrogenizing the double bond in the presence of catalysts (Raney Ni, and salts of Ru, Co, Fe, Pd, Pt, Ir, Ro, and Co on alumina). The same purpose can be achieved by adding amidase to the emulsion. 

For this reason, operation around atmospheric pressures is typical. Space velocity should he high to avoid the reaction of ammonia with oxygen on the reactor walls, which produces nitrogen and water, and results in lower conversions. The concentration of ammonia must he kept helow the inflammahility limit of the feed gas mixture to avoid explosion. Optimum nitric acid production was found to he obtained at approximately 900°C and atmospheric pressure. 

In which 100 cc could be polymerized. We used a pressure gage, rated from 0 to 140 pounds per square Inch. There were 3 type J thermocouples - one In the center of the solution, one In the reactor wall, and the third near the heater outside the reactor. The experiments were conducted In a high pressure bay and observed on closed circuit television. The Initial polymer concentrations of the test reactants were either 0 or 15 or 30 percent by weight. An electric heater controlled the ambient temperature of the nitrogen - purged reactor, and supplied heat to Initiate the reaction. 

No slip Is used as the velocity boundary conditions at all walls. Actually there Is a finite normal velocity at the deposition surface, but It Is Insignificant In the case of dilute reactants. The Inlet flow Is assumed to be Polseullle flow while zero stresses are specified at the reactor exit. The boundary conditions for the temperature play a central role in CVD reactor behavior. Here we employ Idealized boundary conditions In the absence of detailed heat transfer modelling of an actual reactor. Two wall conditions will be considered (1) adiabatic side walls, l.e. dT/dn = 0, and (11) fixed side wall temperatures corresponding to cooled reactor walls. For the reactive species, no net normal flux Is specified on nonreacting surfaces. At substrate surface, the flux of the Tth species equals the rate of reaction of 1 In n surface reactions, l.e. 

After gas-phase oxidation reaction finished, the reactor wall surfece was coated with a thick rough scale layer. The thickness of scale layer along axial direction was varied. The scale layer at front reactor was much thicker than that at rear. The SEM pictures were shown in Fig. 1 were scale layers stripped from the reactor wall surface. Fig. 1(a) was a cross sectional profile of scale layer collected from major scaling zone. Seen from right side of scale layer, particles-packed was loose and this side was attached to the wall surface. Its positive face was shown in Fig. 1(b). Seen from left side of scale layer, compact particles-sintered was tight and this side was faced to the reacting gases. Its local amplified top face was shown in Fig. 1(c). The XRD patterns were shown in Fig. 2(a) were the two sides of scale layer. Almost entire particles on sintered layer were characterized to be rutile phase. While, the particle packed layer was anatase phase. 

Many other, less obvious physical consequences of miniaturization are a result of the scaling behavior of the governing physical laws, which are usually assumed to be the common macroscopic descriptions of flow, heat and mass transfer [3,107]. There are, however, a few cases where the usual continuum descriptions cease to be valid, which are discussed in Chapter 2. When the size of reaction channels or other generic micro-reactor components decreases, the surface-to-volume ratio increases and the mean distance of the specific fluid volume to the reactor walls or to the domain of a second fluid is reduced. As a consequence, the exchange of heat and matter either with the channel walls or with a second fluid is enhanced. 

Catalysts and their carriers are provided in micro channels by various means and in various geometric forms. In a simple variant, the catalyst itself constitutes the micro-reactor construction material without need for any carrier [2-A], In this case, however, the catalyst surface area equals that of the reactor wall and hence is comparatively low. Accordingly, applications are typically restricted to either fast reactions or processing at low flow rates for slow reactions (to enhance the residence time). 

Similar findings were made by BASF in studies investigating an undisclosed gas-phase reaction in capillaries made of quartz, catalyst material and reactor-wall material [105]. The dimensions were chosen in such a way that they match the of surface-to-volume ratio of a fixed-bed reactor used previously for the same reaction. A quartz capillary shows no conversion, whereas reactor-wall material actually has a greater activity than the catalyst itself Hence BASF came to the, at first sight, surprising conclusion that in their production process it was the reactor wall, and not the catalyst, which catalyzes the reaction. The reactor wall was 70 times more active than the catalyst it needs a temperature increase of about 100 °C to have both at equal conversion. 

The parameter p (= 7(5 ) in gas-liquid sy.stems plays the same role as V/Aex in catalytic reactions. This parameter amounts to 10-40 for a gas and liquid in film contact, and increases to lO -lO" for gas bubbles dispersed in a liquid. If the Hatta number (see section 5.4.3) is low (below I) this indicates a slow reaction, and high values of p (e.g. bubble columns) should be chosen. For instantaneous reactions Ha > 100, enhancement factor E = 10-50) a low p should be selected with a high degree of gas-phase turbulence. The sulphonation of aromatics with gaseous SO3 is an instantaneous reaction and is controlled by gas-phase mass transfer. In commercial thin-film sulphonators, the liquid reactant flows down as a thin film (low p) in contact with a highly turbulent gas stream (high ka). A thin-film reactor was chosen instead of a liquid droplet system due to the desire to remove heat generated in the liquid phase as a result of the exothermic reaction. Similar considerations are valid for liquid-liquid systems. Sometimes, practical considerations prevail over the decisions dictated from a transport-reaction analysis. Corrosive liquids should always be in the dispersed phase to reduce contact with the reactor walls. Hazardous liquids are usually dispensed to reduce their hold-up, i.e. their inventory inside the reactor. 

The heat transfer coefficient, U, to the reactor wall (cal/h cm K) and the coolant temperature is Tj. The nitrobenzene feed rate is Nafo (mol/h), with hydrogen in very large excess. The heat of reaction is AH (cal/mol). The heat capacity of hydrogen is Cp (cal/mol K). 

A shutter can be placed in front of the substrate to prevent deposition during filament preheating. The reaction gases are injected on the lower-left side of the reactor, and directed towards the reactor wall in order to achieve more homogeneous gas flows. The (unreacted) gas is pumped out at the right side, so the gas flow is perpendicular to the filaments. 

The computational effort required to carry out the design analysis is determined mainly by the magnitude and spatial distribution of the temperature variations that are taken into account. The maximum temperature difference between the inlet and outlet of the reactor occurs when the reactor operates adiabatically. In this case, heat transfer to the reactor wall is neglected so there is no temperature variation in the radial direction. However, the temperature does vary in the axial direction, so the material and energy balance equations are coupled through the dependence of the reaction rate on temperature. If the reactor is well insulated, and/or of large... 

Brogli, F. et al., Runaway Reactions, 1981, Paper 3/M, 5-6, 10 Unstable plant-scale operation in the catalysed cyclisation by sodium hydroxide to cyclopropanecarbonitrile was investigated using a bench scale calorimeter. Crust formation on the reactor wall, which caused the erratic operation, was eliminated by using liquid alkali instead of solid. 

It was found that the steam supply to the reactor was often superheated (just prior to shutdown to 330°C) [10], Although this degree of superheat would not grossly increase the temperature of the inner reactor wall in contact with the liquid (or the bulk liquid temperature) [11], it seems probable that any reaction material splashed onto and dried out at the top of the coil-heated wall would have become heated to a much higher temperature. Further detailed work on the thermal stability of the mixture showed that a previously unsuspected very slow exothermic decomposition existed, beginning at 180°C and proceeding at an appreciable rate only above 200°C, so that the exotherm was insufficient to heat the contents of the reactor from the last recorded temperature of 158°C to the decomposition temperature of 230°C in 7.5 h [12,13,14], It was concluded that an alternative (effectively an external) source of heat was necessary to account for the observed effect, and the residual superheat from the steam at 330°C seems to have been that source. 

Hydrogenation of benzene to cyclohexane was effected in a fixed bed reactor at 210-230°C, but a fall in conversion was apparent. Increasing the bed temperature by 10°C and the hydrogen flow led to a large increase in reaction rate which the interbed cooling coils could not handle, and an exotherm to 280°C developed, with a hot spot of around 600° C which bulged the reactor wall. 

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