Mixed reactant

One of the major limiting factors for the time resolution of flow-hibe experiments is the time required for mixing reactants and—to a lesser extent—the resolution of distance. With typical fast flow rates of more than 25 ms [42, 43] the time resolution lies between milliseconds and microseconds. 

Undercharge/ underfeed of reactants. Possibility of unreacted mixed reactants left at end of batch, leading to a subsequent runaway reaction. 

If the phases present can be unambiguously identified, microscopy can be used to determine the geometry of interface initiation and advance, and to provide information about particle sizes of components of mixed reactants in a powder. Problems of interpretation arise where materials are poorly crystallized and where crystallites are small, opaque, porous or form solid solutions. With the hot-stage microscope, the progress of reactions can be followed in some instances and the occurrence of sintering and/or melting detected. 

This class is the simplest of all micro reactors and certainly the most convenient one to purchase, but not necessarily one with compromises or reduced fimction. HPLC or other tubing of small internal dimensions is used for performing reactions. There are many proofs in the literature for process intensification by this simple concept. As a micro mixer is missing, mixing either has to be carried out externally by conventional mini-equipment or may not be needed at all. The latter holds for reactions with one reactant only or with a pre-mixed reactant solution, which does not react before entering the tube. 

Fig. 2.5.1 Schematic of an NMR flow cell used to introduce and mix reactants and follow product formation used in a conventional 5-mm diameter NMR probe [11].

An autoclave without a bursting disk and containing the two poorly mixed reactants was wrecked by a violent explosion which occurred on heating the autoclave to 180°C [1]. This was attributed to not allowing sufficient free space for liquid expansion to occur [2], The need to calculate separate reactant volumes under reaction conditions for all autoclave preparations is stressed [3],... 

In this method (Fig. 7.3), the rapidly mixed reactants emerge through an... 

Shukla, A.K., Jackson, C.L., Scott, K., Murgia, G. 2002. A solid-polymer electrolyte direct methanol fuel cell with a mixed reactant and air anode. J Power Sources 111 43-51. 

Refers to that initial period of nonhnear product formation, commencing with the initiation of the reaction and ending when the system is at steady state. Typically, the pre-steady-state phase lasts from milliseconds to a few seconds after mixing reactants. The time course of pre-steady-state rate processes often can be evaluated using stopped-flow, temperature-jump, and mix-quench methods. 

Figure 2. Heat conduction (Seebeck effect) batch mixing calorimeter for three samples and one reference channel. After loading and establishing baselines, the assembly is inverted to mix reactants and start heat production. (Reproduced with permission from Ref. 2. 1983, Alan R. Liss, Inc.)...

The time that it takes to mix reactants or to bring them to a specified temperature may be significant in comparison to the half-life of the reaction. 

In this formulation the mixing process corresponds to a first-order reaction, which transforms unmixed (inert) reactant into mixed reactant. The rate constant for the mixing process, k, is correlated to the mixing time zm by... 

Two important questions asked in the study of chemistry are (1) "Will a reaction take place when you mix reactants A and B and (2) "How do you prepare a given compound, X "... 

Some unusual flow reactors are shown in Figure 17.14. The residence times in the units for high temperature pyrolysis to make acetylene and ethylene and for the oxidation of ammonia are measured in fractions of a second acetic anhydride is made by mixing reactants quickly in a centrifugal pump NO is formed at very high temperature in an electric furnace and ethylene is polymerized at high or low pressures in the two units shown. 

Priestnall, M., Kotzeva, V., Fish, D., and Nilsson, E. Compact Mixed Reactant Fuel Cells, Journal of Power Sources, 106,21 (2002). 

Reactive polymer processing modifies or functionalizes the macromolecular structure of reactor polymers, via chemical reactions, which take place in polymer processing equipment after the polymer is brought to its molten state. The processing equipment then takes on an additional attribute, that of a reactor, which is natural since such equipment is uniquely able to rapidly and efficiently melt and distributively mix reactants into the very viscous molten polymers. The operation is shown schematically in Fig. 1.3. 

The general equations for chemical reaction in a turbulent medium are easy to write if not to solve (2). In addition to those for velocities (U = U + uJ and concentrations (Cj = Cj + Cj), balance equations for q = A u, the segregation ( , and the dissipations e and eg can be written (3). Whatever the shape of the reactor under consideration (usually a tube or a stirred tank), the solution of these equations poses difficult problems of closure, as u S, 5 cj, cj, and also c cj, c Cj in the reaction terms have to be evaluated. The situation is even more complicated when the temperature and the density of the reacting mixture are also fluctuating. Partial solutions to this problem have been proposed. In the case of instantaneous reactions (t << Tg) the "e-quilibrium assumption" applies the mixed reactants are immediately converted and the apparent rate of reaction is simply that of the decrease of segregation, with Corrsin s time constant xs. For instance, with a stoichiometric proportion of reactants, the extent of reaction X is given by 1 - /T ( 2), a simple result which can also be found by application of the IEM model (see (33)). 

Erosive mixing followed by reaction Two unmixed reactants come into contact in a CSTR by erosion of fresh aggregates. The erosion product is either a microfluid (71) or small segregated particles of mixed reactants undergoing further interaction by molecular diffusion (108). 

Beside static mixers, there are practically no alternatives to the "ubiquitous" stirred tank, if one excepts loop reactors (167) and the somewhat special back-flow mixer (168). Imagining entirely new principles for mixing reactants is a challenge for future researchers. First estimations show that an "informed" mixing system, working as a Maxwell demon would be much more effective than our present devices (169). 

Tubular reactor advantages include their well-defined residence time distributions, turbulent mixing reactants, ease of obtaining and applying kinetic data, efficient use of reactor volume, and mechanical simplicity. However, great care must be taken when applying the correct flow model (e.g., plug... 

Reactants may be premixed or fed directly into the reactor. To the extent that the kinetics are limiting (i.e., reaction rate is slow), the rate of mixing plays a minor role in determining conversion or selectivity. If the time to mix reactants is comparable to the reaction rate, however, mixing can have a significant impact. 

---