Process mixing

There are three general types of motion involved in mixing  

Molecular diffusion results from concentration gradients and takes place spontaneously. Eddy motion is caused by turbulent flow, and bulk flow is caused by large-scale matter flows. In polymer processing, the viscosity of polymer melts is very high. How is laminar and eddy motion owing to turbulence is absent. Molecular diffusion is too slow. Convection is thus the dominant mixing process. 

There are three general categories of mixtures that can be created  

The morphological development of these polymer mixtures is determined by two possible mechanisms  

4 Self-cleaning single-screw extruder design (from Osswald and Hernandez-Ortiz, 2006). 

This chapter discusses the thermodynamics of mixing processes and processes described by reaction equations (chemical equations). It introduces the important concepts of molar mixing and reaction quantities, advancement, and the thermodynamic equilibrium constant. The focus is on chemical processes that take place in closed systems at constant pressure, with no work other than expansion work. Under these conditions, the enthalpy change is equal to the heat (Eq. 5.3.7). The processes either take place at constant temperature, or have initial and final states of the same temperature. 

Most of the processes to be described involve mixtures and have intermediate states that are nonequilibrium states. At constant temperature and pressure, these processes proceed spontaneously with decreasing Gibbs energy (Sec. 5.8). A nien the rates of change are slow enough for thermal and mechanical equilibrium to be maintained, the spontaneity is due to lack of transfer equilibrium or reaction equilibrium. An equilibrium phase transition of a pure substance, however, is a special case it is a reversible process of constant Gibbs energy (Sec. 8.3). 

A mixing process is a process in which a mixture is formed from pure substances. In the initial state the system has two or more separate phases, each containing a different pure substance at the same temperature and pressure. The final state is a single-phase mixture at this temperature and pressure. 

The process is illustrated schematically in Fig. 11.1 on the next page. When the partition is withdrawn, the two pure liquids mix spontaneously at constant pressure to form a single homogeneous phase. If necessary, heat transfer is used to return the phase to the initial temperature. 

Indifferent Substances The chemical potential of homogeneous mixtures can be applied so that reactions between mixed phases can be treated exactly like reactions between pure substances. As an example the chemical drive Amix for the mixing process of two substances that are indifferent to each other should be determined. Because the conversion numbers va and vb coincide with the mole fractions Xa and Xb, the conversion formula simplifies to 

As usual, the chemical drive corresponds to the potential drop from reactants to products. When we calculate the potential //m of the homogeneous mixture M = Ax, Bxb in the manner discussed in the last section [see Eq. (13.7)], 

Side Effects in the Ideal Case Changes of volume and entropy in mixing processes work just like those of chemical processes discussed in Chap. 8. Let us again consider a homogeneous mixture of two indifferent substances A and B. Because 

Indifferent behavior occurs on the molecular level when the interactions between particles (such as in dilute gas mixtures) are nonexistent or when they are of the same size independently of the type of particles. 

Real Mixtures Let us now turn to real mixtures where interactions cannot be 

The spiral die, commonly used to extrude tubular blown films, eliminates weld line effects and produces a thermally and geometrically homogeneous extrudate. The polymer melt in a spiral die flows through several feed ports into independent spiral channels wrapped around the circumference of the mandrel. This type of die is schematically depicted in Fig. 3.18. 

most processes involve some form of mixing. As discussed in the previous section, an integral part of a screw extruder is a mixing zone. In fact, most twin screw extruders are primarily used as mixing devices. Similarly, the plasticating unit of an injection molding 

Naturtal rubber and polybutadiene Polyamides (e.g., PA 6 and PA 66) Polyphenylene ether (PPE) and polystyrene 

Polyethylene and polyisobutylene Polyethylene and polypropylene (5% PE in PP) Polycarbonate and polyethylene teraphthalate 

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TATTERSON Fluid Mixing and Gas Dispersion in Agitated Tanks TATTERSON Scale-up of Industrial Mixing Processes VVILLIG Environmental TQM... 

Although the right-hand sides of Eqs. (8.27) and (8.28) are the same, the former applies to the mixture (subscript mix), while the latter applies to the mixing process (subscript m). The fact that these are identical emphasizes that in Eq. (8.27) we have calculated only that part of the total entropy of the mixture which arises from the mixing process itself. This is called the configurational entropy and is our only concern in mixing problems. The possibility that this mixing may involve other entropy effects—such as an entropy of solvation-is postponed until Sec. 8.12. 

The energy of interaction between a pair of solvent molecules, a pair of solute molecules, and a solvent-solute pair must be the same so that the criterion that = 0 is met. Such a mixing process is said to be athermal. The solvent and solute molecules must be the same size so that the criterion... 

In the next section we shall examine the mixing process for molecules which differ greatly in size, building on the principles reviewed in this section. The reader who desires additional review of these ideas will find this material discussed in detail in textbooks of physical chemistry. 

In the liquid state molecules are in intimate contact, so the energetics of molecular interactions generally make a contribution to the overall picture of the mixing process. There are several aspects of the situation that we should be aware of before attempting to formulate a theory for ... 

The vaporization process requires energy both to overcome intermolecular attractions and to push back the surroundings to make room for the vapor. The quantity AU measures the former, while AH takes both into account. In connection with the mixing process, it is the contribution of intermolecular forces which we seek to evaluate, so AU is a more suitable measure of this quantity. 

In Chap. 8 we saw how the equilibrium osmotic pressure of a solution is related to AG for the mixing process whereby the solution is formed. Any difference in the concentration of the solution involves a change in AG j, ... 

Manufacture of friction elements includes the impregnation of fabrics and subsequent lamination, the wet-dough process, and the dry-mix process. Elements from the last two are prepared by compression-molding the formulation for up to an hour at 150—175°C. Thick brake elements require a carefully controUed heating-and-cooHng cycle to minimize stresses created by expansion and contraction (see Brake linings and clutch facings Fillers). 

Preparation of the vitamins in commercial quantities can involve isolation, chemical synthesis, fermentation, and mixed processes, including chemical and fermentation steps. The choice of process is economic, dictated by the need to obtain materials meeting specifications at the lowest cost. Current process technologies (ca 1997) employed for each vitamin are indicated in Table 9. 

Segments. Segments for heavy-duty use such as for medium-sized tmcks ate produced by a dry-mix process. The fiber, modifiers, and a dry novolak resin are mixed in an appropriate mixer. The blend is then formed into about a 60 by 90 cm preform (or briquet) at 3—4 MPa (400—600 psi). The briquets are hot-pressed for 3—10 min at 140—160°C and then cooled. The resin is only partially cured at this point to be thermoplastic when subsequently reheated for bending. The hot-pressed preforms are then cut to desired size and bent at 170—190°C and cured in curved molds for 4—8 h at 220—280°C. Final grinding produces the finished segments. 

In order to "cure" or "vulcanize" an elastomer, ie, cross-link the macromolecular chains (Fig. 2), certain chemical ingredients are mixed or compounded with the mbber, depending on its nature (4,5). The mixing process depends on the type of elastomer a high viscosity type, eg, natural mbber, requires powerhil mixers (such as the Banbury type or mbber mills), while the more Hquid polymers can be handled by ordinary rotary mixers, etc (see Rubber... 

Plasticizers and Processing Aids. Petroleum-based oils are commonly used as plasticizers. Compound viscosity is reduced, and mixing, processing, and low temperature properties are improved. Air permeabihty is increased by adding extender oils. Plasticizers are selected for their compatibihty and low temperature properties. Butyl mbber has a solubihty parameter of ca 15.3 (f /cm ) [7.5 (cal/cm ) ], similar to paraffinic and naphthenic oils. Polybutenes, paraffin waxes, and low mol wt polyethylene can also be used as plasticizers (qv). Alkyl adipates and sebacates reduce the glass-transition temperature and improve low temperature properties. Process aids, eg, mineral mbber and Stmktol 40 ms, improve filler dispersion and cured adhesion to high unsaturated mbber substrates. 

TATTERSON Scaleup and Design of Industrial Mixing Processes... 

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. 

If the crystallizer is not of the evaporative type but rehes only on adiabatic evaporative cooling to achieve the yield, the heating element is omitted. The feed is admitted into the circulating hue after withdrawal of the slurry, at a point sufficiently below the free-hquid surface to prevent flashing during the mixing process. 

Mixing of sohds is an important unit operation in the production of solids with consistent properties. A number of properties of the solid particles influence the mixing process, the design, and selection of mixing equipment. The second subsection elaborates on the theoiy of mixing, types of mixing equipment, and their operation. 

Consider common scrubbers, vents, sumps, drains, off-gas treatment and other opportunities for inadvertently mixing process materials. Cross-contamination potential at transfer stations should not be overlooked. 

Vertical air motions affect both weather and the mixing processes of importance to air pollution. Upward vertical motions can be caused by lifting over terrain, lifting over weather fronts, and convergence toward low-pressure centers. Downward vertical motions can be caused by sinking to make up for divergence near high-pressure centers. One must know whether the atmosphere enhances or suppresses these vertical motions to... 

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