Mixing process heat effect

The abihty to remove heat from electrophoretic systems has severely limited the maximum capacity of these systems in terms of how large or thick the systems can be. Electrophoretic separations have been performed on space flights because the effect of gravity in outer space is small and mixing from heating is negligible. Whereas electrophoresis in outer space has been accompHshed (10), the economics for a scaleable process have not (see Space processing). 

Blade shape can have a significant impact on the mixing process. A scraping profile will be useful if heat transfer is important, whereas a smearing profile will be more effective for dispersion. Ease of cleaning and ease of discharge may also be important. 

Exposures to chemicals, resulting in toxic effects or oxygen-deficient atmospheres, may arise in a variety of industrial situations. A summary of common sources is given in Table 5.18 clearly this is not exhaustive since exposure may result whenever materials are mixed, machined, heated, dispersed or otherwise processed or used. 

Note that heat effects do not necessarily indicate that a chemical reaction is taking place. Some physical processes such as mixing or dilution can generate or absorb heat. However, heat effects are very often the result of a chemical process. 

As noted earlier, virtually all liquid and semisolid products involve the unit of operation of mixing. In fact, in many instances, it is the primary unit operation. Even its indirect effects, e.g., on heat transfer, may be the basis for its inclusion in a process. Yet, mechanistic and quantitative descriptions of the mixing process remain incomplete (7-9). Nonetheless, enough fundamental and empirical data are available to allow some reasonable predictions to be made. 

Heat Requirement of the Process. Heat is required for vaporization in the extractive distillation column, and for the reconcentration of magnesium nitrate solution. Overall thermal effects caused by the magnesium nitrate cancel out, and the heat demand for the complete process depends on the amount of water being removed, the reflux ratio employed, and the terminal (condenser) conditions in distillation and evaporation. The composition and temperature of the mixed feed to the still influence the relative heat demands of the evaporation and distillation sections. For the concentration of 60 wt% HNO3 to 99.5 wt% HNO3 using a still reflux ratio of 3 1, a still pressure of 760 mm Hg, and an evaporator pressure of 100 mm Hg, the theoretical overall heat requirement is 1,034 kcal/kg HNO3. 

High-intensity inline devices are often used to mix fluids in the process industries. Such devices include simple pipes, baffled pipes, tees, motionless mixers, dynamic mixers, centrifugal pumps, ejectors, and rotor/stator mixers. In addition to their traditional application in physical processes such as mixing and dispersion, such devices can provide very effective environments for mass transfer and chemical reaction to take place. Furthermore, combining effective inline mixing with heat transfer is the basis of combined heat exchanger reactors (HEX reactors). 

The choice of reactor will be very dependent on the requirements of the chemical reaction scheme, the relative importance of mixing and heat transfer, and practical considerations (e.g., the effect of solids in the process materials of construction flexibility). A comparison of the typical performance of different designs is given in Table 5. HEX Reactors are discussed in more depth in Chapter 4. 

In practice, the process is effected as follows, Crude m- xylene, containing 60-70% of m- xylene, is mixed with sulphuric acid (sp. gr. 1.84) whereupon the temperature rises to 45°C. Then the mixture is heated to 50°C and allowed to remain at this temperature for 2 hr. Under these conditions sulphonation of the o- and m- isomers takes place. The sulphonic acids may be separated from unconverted p- xylene either by extraction with water or by expelling the p- xylene by steam distillation. 

What is the heat effect when I75(lb ) of H2 04 is mixed with 400(lbm) of an aqueous solution containing 30-wt-% H2S04 in an isothermal process at 120( F) ... 

AH a This step involves the separation of 2 kg of a 15% LiCl solution into its pure constituents at 25°C. This is an unmixing process, and the heat effect is the same as for the corresponding mixing process, but is of opposite sign. For 2 kg of 15% LiCl solution, the moles of material entering are... 

Various aspects of the effect of process scale-up on the safety of batch reactors have been discussed by Gygax [7], who presents methods to assess thermal runaway. Shukla and Pushpavanam [8] present parametric sensitivy and safety results for three exothermic systems modeled using pseudohomogenous rate expressions from the literature. Caygill et al. [9] identify the common factors that cause a reduction in performance on scale-up. They present results of a survey of pharmaceutical and fine chemicals companies indicating that problems with mixing and heat transfer are commonly experienced with large-scale reactors. 

The microanalytical methods of differential thermal analysis, differential scanning calorimetry, accelerating rate calorimetry, and thermomechanical analysis provide important information about chemical kinetics and thermodynamics but do not provide information about large-scale effects. Although a number of techniques are available for kinetics and heat-of-reaction analysis, a major advantage to heat flow calorimetry is that it better simulates the effects of real process conditions, such as degree of mixing or heat transfer coefficients. 

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