Special Features of the Process

The carbohydrate chemist is familiar with the general aspects of the process for the manufacture of crystalline dextrose because of the important position this compound occupies in the whole carbohydrate field. The special features of the process, however, are less familiar and it is hoped that this discussion will afford a better idea of modern methods. 

Special Features of the Process of Dynamic Adsorption Layer Formation in the... 

At 0 -> n the process of dynamic adsorption layer formation is complicated by three factors the expansion of the diffusion boundary layer, the necessity to consider surface diffusion, and the retardation of the surface taking place at 0 -> r even when the complete remaining surface is essentially not stagnant. The appearance of all the three factors is qualitatively different at high and low surface activity, respectively. The special features of the process of adsorption layer formation at 0 -> tt are considered by Dukhin (1965) for each case separately. 

The fractionation of air into nitrogen and oxygen is a classical process with a great industrial importance, even after a history of over 100 years (Baldus et al. 1983). The special feature of the process is the very low temperature level required for the operation of the distillation colunm, approximately -180 to -190°C. Hence, it is not possible to heat and cool the colurrms by steam and water, respectively. 

Chiral LA are rarely used in the constmction of chiral six-membered cyclic nitronates by the Diels-Alder reaction of olehns with a-nitoralkenes (96, 158), in spite of the potential efficiency of the process. Apparently, this is associated with the absence of known common features of the process and, as a consequence, with the necessity to perform special investigations for optimization in each particular case. 

Abstract This review reports on the study of the interplay between magnetic coupling and spin transition in 2,2 -bipyrimidine (bpym)-bridged iron(II) dinuclear compounds. The coexistence of both phenomena has been observed in [Fe(bpym)(NCS)2]2(bpym), [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) (bpym = 2,2 -bipyrimidine, bt = 2,2 -bithiazoline) by the action of external physical perturbations such as heat, pressure or electromagnetic radiation. The competition between magnetic exchange and spin crossover has been studied in [Fe(bpym)(NCS)2]2(bpym) at 0.63 GPa. LIESST experiments carried out on [Fe(bpym)(NCSe)2]2(bpym) and [Fe(bt)(NCS)2]2(bpym) at 4.2 K have shown that it is possible to generate dinuclear molecules with different spin states in this class of compounds. A special feature of the spin crossover process in the dinuclear compounds studied so far is the plateau in the spin transition curve. Up to now, it has not been possible to explore with a microscopic physical method the nature of the species... 

The special feature of the spin crossover process in all bpym-bridged dinuclear compounds studied so far is the occurrence of a plateau in the spin transition curve. A reasonable assumption to account for this observation is that a thermal spin transition takes place successively in the two metal centres. However, it cannot be excluded that spin transition takes place simultaneously in the dinuclear units leading directly from [HS—HS] pairs to [LS-LS] pairs with decreasing temperature. Therefore, two possible conversion pathways for [HS—HS] pairs with decreasing temperature may be proposed [HS—HS]<->[HS—LS]<->[LS—LS] or [HS-HS] [LS-LS]. The differentiation of the existence of the [LS—LS], [HS—LS], and [HS—HS] spin pairs is not trivial and has recently been solved experimentally by utilisation of magnetisation versus magnetic field measurements as a macroscopic tool [9], and by Mossbauer spectroscopy in an applied magnetic field as a microscopic tool [11]. 

It should be noted that there are special features of the 9-substituted anthracenes that may introduce complications. The materials dimerize also in solution, yielding exclusively the centrosymmetric dimers, which are analogous to 87. Further, the few 9,9 -disubstituted dianthracenes that have been prepared are very unstable. Thus the head-to-head approach of two monomers is apparently an energetically unfavorable process. This is not so for the head-to-tail approach, and it is therefore not surprising that structures in which there are overlapped monomers related by centers of symmetry yield topochemical dimers. 

In this review of LCM we will try to be general when possible, but we are forced to specialize to the RTM process in many cases. In particular the special features of the S-RIM process (high-speed mixing, materials, etc.) are almost completely left out of the discussion. For readers with a special interest in this topic we recommend the book by Macosko [3]. 

The study and control of a chemical process may be accomplished by measuring the concentrations of the reactants and the properties of the end-products. Another way is to measure certain quantities that characterize the conversion process, such as the quantity of heat output in a reaction vessel, the mass of a reactant sample, etc. Taking into consideration the special features of the chemical molding process (transition from liquid to solid and sometimes to an insoluble state), the calorimetric method has obvious advantages both for controlling the process variables and for obtaining quantitative data. Calorimetric measurements give a direct correlation between the transformation rates and heat release. This allows to monitor the reaction rate by observation of the heat release rate. For these purposes, both isothermal and non-isothermal calorimetry may be used. In the first case, the heat output is effectively removed, and isothermal conditions are maintained for the reaction. This method is especially successful when applied to a sample in the form of a thin film of the reactant. The temperature increase under these conditions does not exceed IK, and treatment of the experimental results obtained is simple the experimental data are compared with solutions of the differential kinetic equation. 

A special feature of a process of branched polymer formation is the possibility of the appearance in the reactor of a macroscopic structure whose size is comparable with that of the reactor. The main peculiarity of such a phenomenon, known as gelation, is that the range of conversions... 

Special features of the isothermal unmixing process are the following ... 

The special features of the different routes of administration are dealt with in separate sections of this chapter, after a brief summary of the general properties of biological membranes and drug transport, a knowledge of which is important in understanding all absorption processes. It is impossible to be comprehensive in this one chapter, but we will concentrate on factors unique to the routes discussed, such as the properties of the vehicle in topical therapy, and the aerodynamic properties of aerosols in inhalation therapy, to give a flavour of the different problems that face formulators. Where attempts have been made to quantify absorption, equations are presented, but the derivations of most equations have been omitted. 

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