Exothermic mixing

Although normally straightforward for bulk process streams, it is important to select materials for skin rather than bulk temperatures in heat transfer equipment, and to evaluate the effects of mixing exotherms on local bulk fluid temperatures, such as that may occur in acid addition/dilution equipment. 

Sulfuric acid and water are said to mix exothermically since energy must be released to the environment to mix these two components at constant temperature. The temperature rise that occurs when these two components are mixed adiabatically is considered in Illustration 8.4-1. Note also that to solve this problem we have, in effect, used an energy balance without explicitly writing a detailed balance equation. We will consider the balance equations (mass, energy, and entropy) for mixtures in Sec. 8.4. H... 

This last solution should be prepared slowly as it is quite exothermic. Set all three aside in a freezer. Now prepare the mixing apparatus which will be a stainless steel "mixing bowl" suspended In the ice/salt bath made earlier. We use a stainless steel bowl here so that heat transfer will be maximal, while preventing any corrosive interaction. A glass bowl will not be sufficient for larger scale preparations as it will not conduct heat fast enough to prevent the reactants from going over IOC (at which point the Haloamide will decompose and you ll have to start over). Take the Sodium Hydroxide solution out of the freezer once it is cool, but not cold. 

Preheat a water bath on the stove (or wherever) to about 80C and place the stainless steel mixing bowl in it. Once the temperature of the solution hits about 65C, take the bowl out and set aside while stirring all the while. This is where it rearranges, and the reaction is exothermic enough to sustain it s temperature nicely. If you find the temperature climbing past 80C, immerse the bowl into some cold waiter briefly. After about 15 minutes the temperature will start to fall, at which point you should transfer the whole mess to the distilling flask. Before you continue you need to choose whether you want to make the hydrochloride salt or the aqueous solution of Methylamine, though. 

The quantity x is called the Flory-Huggins interaction parameter It is zero for athermal mixtures, positive for endothermic mixing, and negative for exothermic mixing. These differences in sign originate from Eq. (8.39) and reaction (8.A). 

Equation (8.49) accounts only for endothermic mixing. It is not too surprising that we are thus led to associate exothermic values with more specifically chemical interactions between solvent and solute as opposed to the purely physical interactions we have been describing in this approximation. 

The derivatives are hydroxyethyl and hydroxypropyl cellulose. AH four derivatives find numerous appHcations and there are other reactants that can be added to ceUulose, including the mixed addition of reactants lea ding to adducts of commercial significance. In the commercial production of mixed ethers there are economic factors to consider that include the efficiency of adduct additions (ca 40%), waste product disposal, and the method of product recovery and drying on a commercial scale. The products produced by equation 2 require heat and produce NaCl, a corrosive by-product, with each mole of adduct added. These products are produced by a paste process and require corrosion-resistant production units. The oxirane additions (eq. 3) are exothermic, and with the explosive nature of the oxiranes, require a dispersion diluent in their synthesis (see Cellulose ethers). 

A typical flow diagram for pentaerythritol production is shown in Figure 2. The main concern in mixing is to avoid loss of temperature control in this exothermic reaction, which can lead to excessive by-product formation and/or reduced yields of pentaerythritol (55,58,59). The reaction time depends on the reaction temperature and may vary from about 0.5 to 4 h at final temperatures of about 65 and 35°C, respectively. The reactor product, neutralized with acetic or formic acid, is then stripped of excess formaldehyde and water to produce a highly concentrated solution of pentaerythritol reaction products. This is then cooled under carefully controlled crystallization conditions so that the crystals can be readily separated from the Hquors by subsequent filtration. 

The Beckstead-Derr-Price model (Fig. 1) considers both the gas-phase and condensed-phase reactions. It assumes heat release from the condensed phase, an oxidizer flame, a primary diffusion flame between the fuel and oxidizer decomposition products, and a final diffusion flame between the fuel decomposition products and the products of the oxidizer flame. Examination of the physical phenomena reveals an irregular surface on top of the unheated bulk of the propellant that consists of the binder undergoing pyrolysis, decomposing oxidizer particles, and an agglomeration of metallic particles. The oxidizer and fuel decomposition products mix and react exothermically in the three-dimensional zone above the surface for a distance that depends on the propellant composition, its microstmcture, and the ambient pressure and gas velocity. If aluminum is present, additional heat is subsequently produced at a comparatively large distance from the surface. Only small aluminum particles ignite and bum close enough to the surface to influence the propellant bum rate. The temperature of the surface is ca 500 to 1000°C compared to ca 300°C for double-base propellants. 

In order to faciUtate heat transfer of the exothermic polymerization reaction, and to control polymerizate viscosity, percent reactives are adjusted through the use of inert aromatic or aUphatic diluents, such as toluene or heptane, or higher boiling mixed aromatic or mixed aUphatic diluents. Process feed streams are typically adjusted to 30—50% polymerizable monomers. 

Fresh butane mixed with recycled gas encounters freshly oxidized catalyst at the bottom of the transport-bed reactor and is oxidized to maleic anhydride and CO during its passage up the reactor. Catalyst densities (80 160 kg/m ) in the transport-bed reactor are substantially lower than the catalyst density in a typical fluidized-bed reactor (480 640 kg/m ) (109). The gas flow pattern in the riser is nearly plug flow which avoids the negative effect of backmixing on reaction selectivity. Reduced catalyst is separated from the reaction products by cyclones and is further stripped of products and reactants in a separate stripping vessel. The reduced catalyst is reoxidized in a separate fluidized-bed oxidizer where the exothermic heat of reaction is removed by steam cods. The rate of reoxidation of the VPO catalyst is slower than the rate of oxidation of butane, and consequently residence times are longer in the oxidizer than in the transport-bed reactor. 

Quench Converter. The quench converter (Fig. 7a) was the basis for the initial ICl low pressure methanol flow sheet. A portion of the mixed synthesis and recycle gas bypasses the loop interchanger, which provides the quench fractions for the iatermediate catalyst beds. The remaining feed gas is heated to the inlet temperature of the first bed. Because the beds are adiabatic, the feed gas temperature increases as the exothermic synthesis reactions proceed. The injection of quench gas between the beds serves to cool the reacting mixture and add more reactants prior to entering the next catalyst bed. Quench converters typically contain three to six catalyst beds with a gas distributor in between each bed for injecting the quench gas. A variety of gas mixing and distribution devices are employed which characterize the proprietary converter designs. 

In cases where a large reactor operates similarly to a CSTR, fluid dynamics sometimes can be estabflshed in a smaller reactor by external recycle of product. For example, the extent of soflds back-mixing and Hquid recirculation increases with reactor diameter in a gas—Hquid—soflds reactor. Consequently, if gas and Hquid velocities are maintained constant when scaling and the same space velocities are used, then the smaller pilot unit should be of the same overall height. The net result is that the large-diameter reactor is well mixed and no temperature gradients occur even with a highly exothermic reaction. 

Heat Release and Reactor Stability. Highly exothermic reactions, such as with phthaHc anhydride manufacture or Fischer-Tropsch synthesis, compounded with the low thermal conductivity of catalyst peUets, make fixed-bed reactors vulnerable to temperature excursions and mnaways. The larger fixed-bed reactors are more difficult to control and thus may limit the reactions to jacketed bundles of tubes with diameters under - 5 cm. The concerns may even be sufficiently large to favor the more complex but back-mixed slurry reactors. 

Potassium siUcates are manufactured in a manner similar to sodium siUcates by the reaction of K CO and sand. However, crystalline products are not manufactured and the glass is suppHed as a flake. A 3.90 mole ratio potassium siUcate flake glass dissolves readily in water at ca 88°C without pressure by incremental addition of glass to water. The exothermic heat of dissolution causes the temperature of the solution to rise to the boiling point. Lithium sihcate solutions are usually prepared by dissolving siUca gel in a LiOH solution or mixing colloidal siUca with LiOH. 

Styrene is manufactured from ethylbenzene. Ethylbenzene [100-41-4] is produced by alkylation of benzene with ethylene, except for a very small fraction that is recovered from mixed Cg aromatics by superfractionation. Ethylbenzene and styrene units are almost always installed together with matching capacities because nearly all of the ethylbenzene produced commercially is converted to styrene. Alkylation is exothermic and dehydrogenation is endothermic. In a typical ethylbenzene—styrene complex, energy economy is realized by advantageously integrating the energy flows of the two units. A plant intended to produce ethylbenzene exclusively or mostly for the merchant market is also not considered viable because the merchant market is small and sporadic. 

More recent process research aimed at anionic PS is that of BASF AG. Unlike the Dow Process, the BASF process utilizes continuous linear-flow reactors (LFR) with no back-mixing to make narrow polydispersity resins. This process consists of a series alternating reactors and heat exchangers (Fig. 22). Inside the reactors, the polymerization exotherm carries the temperature from 30°C at the inlet to 90°C at the outlet. The heat exchangers then take the temperature back down to 30°C. This process, which requires no solvent, results in the formation of narrow polydispersity PS. 

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