Exothermic temperature rise

Decreased exothermic temperature rise on curing" Tendency to entrap air... 

In this scheme, all monomers were added to the reaction mixture before initiation. After initiation at 50 C, the center diene block polymerized first. Depending on the amount of the diene monomer used, the temperature of polymerization could reach a maximum of 60 to 80°C during the 70 to 90 minute polymerization time. After the diene block formation was completed, the end block polymerization began as indicated by a change of color from yellowish to dark red in the polymerization mixture. A second exothermic temperature rise then took place. The end block pol3merization required about another 10 to 30 minutes, and the total time for a run was less than 2 hours. The polymers prepared have a tapered section between the end block and the center diene block. The styrene monomer could also be withheld until the diene block was polymerized. The product then was a conventional untapered triblock. Figure 4 shows the proton NMR spectra of tapered and untapered SAMS-B-SAMS prepared by these two procedures. A sharp spike on the shoulder of the styrenic peak was observed for the tapered triblock. 

One of the most important difficulties of catalyst screening is a high exothermic temperature rise due to MeOH transformation to hydrocarbon (at 450°C, = -858kJ / kg... 

In a multitubular fixed-bed reactor, the catalyst particles are packed into narrow tubes, grouped in bundles and enclosed in an outer shell (see Fig. 18.5). The tube bundles are immersed in water, which abstracts the heat and converts to high-pressure steam. The use of narrow tubes, high syngas velocities, and large catalyst particles ensures rapid heat exchange and minimizes exothermic temperature rise (Dry, 1996). The increased particle size of the catalyst is also necessary in order to avoid large pressure drops (Sie and Krishna, 1999), a problem encountered with... 

Adiabatic operation. If adiabatic operation leads to an acceptable temperature rise for exothermic reactors or an acceptable fall for endothermic reactors, then this is the option normally chosen. If this is the case, then the feed stream to the reactor requires heating and the efiluent stream requires cooling. The heat integration characteristics are thus a cold stream (the reactor feed) and a hot stream (the reactor efiluent). The heat of reaction appears as elevated temperature of the efiluent stream in the case of exothermic reaction or reduced temperature in the case of endothermic reaction. 

A. Maleic acid. Assemble the apparatus shown in Fig. Ill, 28, 1. Place 45 g. of dry mahc acid in the 200-250 ml. distilling flask and cautiously add 63 g. (57 ml.) of pure acetyl chloride. Warm the flask gently on a water bath to start the reaction, which then proceeds exothermically. Hydrogen chloride is evolved and the malic acid passes into solution. When the evolution of gas subsides, heat the flask on a water bath for 1-2 hours. Rearrange the apparatus and distil. A fraction of low boiling point passes over first and the temperature rises rapidly to 190° at this point run out the water from the condenser. Continue the distillation and collect the maleic anhydride at 195-200°. Recrystallise the crude maleic anhydride from chloroform (compare Section 111,93) 22 g. of pure maleic anhydride, m.p. 54°, are obtained. 

To a mixture of 65 ml of dry benzene and 0.10 mol of freshly distilled NN-di-ethylamino-l-propyne were added 3 drops of BFa.ether and 0.12 mol of dry propargyl alcohol was added to the reddish solution in 5 min. The temperature rose in 5-10 min to about 45°C, remained at this level for about 10 min and then began to drop. The mixture was warmed to 60°C, whereupon the exothermic reaction made the temperature rise in a few minutes to B5 c. This level was maintained by occasional cooling. After the exothermic reaction (3,3-sigmatropic rearrangement) had subsided, the mixture was heated for an additional 10 min at 80°C and the benzene was then removed in a water-pump vacuum. The red residue was practically pure acid amide... 

Rhenium hexafluoride is readily prepared by the direct interaction of purified elemental fluorine over hydrogen-reduced, 300 mesh (ca 48 pm) rhenium powder at 120°C. The reaction is exothermic and temperature rises rapidly. Failure to control the temperature may result in the formation of rhenium heptafluoride. The latter could be reduced to rhenium hexafluoride by heating with rhenium metal at 400°C. 

Temperature is the most important variable and preheating is generally necessary to 200—230°C. After air has been introduced, there is a gradual temperature rise because of the exothermic reaction, until some means is appHed to hold the temperature such as a water or steam spray on the asphalt surface to maintain a temperature of approximately 260°C. The end point can be predicted by periodic testing of the softening point. 

Hydration at Ordinary Temperatures. Pordand cement is generally used at temperatures ordinarily encountered in constmction, ie, from 5 to 40°C. Temperature extremes have to be avoided. The exothermic heat of the hydration reactions can play an important part in maintaining adequate temperatures in cold environments, and must be considered in massive concrete stmctures to prevent excessive temperature rise and cracking during subsequent cooling. 

Reaction Conditions. Typical iadustrial practice of this reaction involves mixing vapor-phase propylene and vapor-phase chlorine in a static mixer, foEowed immediately by passing the admixed reactants into a reactor vessel that operates at 69—240 kPa (10—35 psig) and permits virtual complete chlorine conversion, which requires 1—4 s residence time. The overaE reactions are aE highly exothermic and as the reaction proceeds, usuaEy adiabaticaEy, the temperature rises. OptimaEy, the reaction temperature should not exceed 510°C since, above this temperature, pyrolysis of the chlorinated hydrocarbons results in decreased yield and excessive coke formation (27). 

An exotherm is witnessed and the temperature rises to 70-80°. A color change from yellow to deep purple is also seen the extent of coloration varies with the purity of the sodium sulfide nonahydrate. 

It is important that the formaldehyde addition rate be balanced with the alkali content of the system and the engineering control capability. At high alkali contents, the exotherm will be more vigorous and create more load on the heat exchangers. At low alkali contents, the reaction rate may be quite slow. While this temporarily reduces the difficulty in instantaneous heat load, it may permit potentially hazardous levels of unreacted formaldehyde to accumulate. Such accumulations could become dangerous as batch temperature rises. In both cases. 

In addition to the explosive aspects of the LEL, another issue is the heat energy given off during oxidation. An estimate of the exotherm is that there will be a 25 F rise per 1 % LEL in the stream. Hence, if the process air enters the oxidizer at a given temperamre, and if the stream has a concentration of 2% LEL, then a 50 F rise in process stream temperature is expected after oxidation. If the process stream were running at a 10% LEL, then a 250 F temperamre rise would be predicted. A maximum LEL of 25% yields a 625 F temperature rise of the process stream. 

In a tubular reactor system, the temperature rises along the reactor length for an exothermic reaction unless effective cooling is maintained. For multiple steady states to appear, it is necessary that a... 

In the ease of exothermie or endothermie reaetions, seale-up may impair eonditions for heat input or removal beeause the ratio of the heat transfer surfaee area to the reaetor volume is redueed. Identieal eonditions for heat transfer in both the model and full-seale plants may be aehieved in exothermal reaetions if both have the same thermal stability eoeffieient. This requirement is obtained by introdueing external heat exehangers. Alternatively, a reaetor with a strong exothermie reaetion ean be divided into several small size reaetors. In this manner, the ratio of the external heat transfer surfaee area to the reaetor volume is inereased, thereby avoiding an exeessive temperature rise in the reaetor. 

A solution of 61 parts 4-chloro-l,l-di-(4-fluorophenyl)-l-butene in 400 parts 2-propanol is hydrogenated at normal pressure and at room temperature in the presence of 5.5 parts palladium-on-charcoal catalyst 10% (exothermic reaction, temperature rises to about 30°C). After the calculated amount of hydrogen is taken up, hydrogenation is stopped. The catalyst is filered off and the filtrate is evaporated. The oily residue is distilled in vacuo, yielding l-chloro-4,4-di-(4-fluorophenyl)-butane, boiling point 166° to 168°C at 6 mm pressure ... 

A mixture of o-methoxyphenol (57 g), glycidol (32 g) and pyridine (1 g) is warmed to 95°C at which temperature a vigorous reaction takes place. The reaction mixture is cooled to prevent the temperature rising above 110°C. When the exothermic reaction has subsided the reactants are heated at 95°C for one hour longer and then distilled under low pressure. The main fraction boils in the range 176°C to 180°C/0.5 mm. It crystallizes on cooling. Recrystallization from benzene gives the pure product, MP 78.5°C to 79.0°C. 

When the slightly exothermic reaction (rise in temperature of about 20°C) has ceased, heating is effected for 1.5 hours at 60°C. The product is then cooled to 4°C and left to crystallize for about 12 hours. The precipitate is centrifugated then recrystallized in 500 ml of absolute ethanol. 

It is more common to find that AH° and AS° have the same sign (Table 17.2, III and IV). When this happens, the enthalpy and entropy factors oppose each other. AG° changes sign as temperature increases, and the direction of spontaneity reverses. At low temperatures, AH° predominates, and the exothermic reaction, which may be either the forward or the reverse reaction, occurs. As the temperature rises, the quantity TAS° increases in magnitude and eventually exceeds AH°. At high temperatures, the reaction that leads to an increase in entropy occurs. In most cases, 25°C is a low temperature, at least at a pressure of 1 atm. This explains why exothermic reactions are usually spontaneous at room temperature and atmospheric pressure. 

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