Liquid Piston Reactor

Phases Gas-liquid fast reaction when the upstream equipment operates under a vacuum. The liquid piston draws the vacuum with the vapor reacting with the liquid sealing the pump. 

The example is the reaction between ketene and acetic acid to make acetic anhydride. 

Bartholomew, C.H., Farrauto R. J., Fundamentals of Industrial Catalytic Processes, 2nd edn., Wiley-Interscience, Hoboken, NJ, 2006. 

Chauvel, A., Lefebrve, G., Petrochemical Processes, Vol. 1 and 2, Gulf Publishing, Houston, 1989. 

Doraiswamy, L.K., Organic Synthesis Engineering, Oxford, London, 2001. 

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Solution of the design equations for liquid-phase piston flow reactors is usually easier than for gas-phase reactors because pressure t5q)icaUy has no effect on the fluid density or the reaction kinetics. Extreme pressures are an exception that theoretically can be handled by the same methods used for gas-phase systems. The difficulty will be finding an equation of state. For ordinary pressures, the... 

Computational Scheme for Liquid-Phase PFRs. The following is a procedure for solving the reactor design equations for a moderate-pressure, liquid-phase, piston flow reactor using the marching-ahead technique (Euler s method) ... 

Example 4.3 represents the simplest possible example of a variable-density CSTR. The reaction is isothermal, first-order, irreversible, and the density is a linear function of reactant concentration. This simplest system is about the most complicated one for which an analytical solution is possible. Realistic variable-density problems, whether in liquid or gas systems, require numerical solutions. These numerical solutions use the method of false transients and involve sets of first-order ODEs with various auxiliary functions. The solution methodology is similar to but simpler than that used for piston flow reactors in Chapter 3. Temperature is known and constant in the reactors described in this chapter. An ODE for temperature wiU be added in Chapter 5. Its addition does not change the basic methodology. 

Piston Flow in Contact with a CSTR. A liquid-phase reaction in a spray tower is conceptually similar to the transpired-wall reactors in Section 3.3. The liquid drops are in piston flow but absorb components from a well-mixed gas phase. The rate of absorption is a function of as it can be in a transpired-wall reactor. The component balance for the piston flow phase is... 

Semibatch reactors are often used to mn highly exothermic reactions isothermally, to run gas-liquid(-solid) processes isobarically, and to prevent dangerous accumulation of some reactants in the reaction mixture. Contrary to batch of)eration, temperature and pressure in semibatch reactors can be varied independently. The liquid reaction mixture can be considered as ideally mixed, while it is assumed that the introduced gas flows up like a piston (certainly this is not entirely true). Kinetic modelling of semibatch experiments is as difficult as that of batch, non-isotherma experiments. 

In a properly operated bubble-column reactor, the liquid phase can be considered to be perfectly mixed, i.e. concentrations in the liquid are the same everywhere and correspond to those in the effluent. The gas is supposed to flow like a piston, i.e. the reactor is a plug-flow reactor with respect to the gas. These two assumptions are not entirely true, but within a certain flow regime they are not far from the reality. 

A piston-flow-type reactor for dehydrogenation in continuous mode under superheated liquid-film conditions. 

In this section, several cases where there is a spread in drop size distribution will be calculated first for an ideal piston flow reactor in which all liquid parts have the same residence time distribution, and, finally, also the case of a CSTR in which there is a spread in drop size will be calculated, but only for the case of zero-order drop conversion. 

Piston flow signifies that some of the liquid passes through the reactor in plug flow. This liquid, unlike that involved in short-circuiting, has a certain residence time in the reactor. In certain operations, it is essential that the flow approach as close as possible some ideal situation, usually plug flow (e.g., in continuous, large-scale chromatographic separations). 

Pumps, such as piston pumps, can meter liquids into a reactor fairly precisely, but the chemical engineer uses a flow measurement device for greater precision. The most commonly used flowmeters are rotameters that are calibrated to translate the lifting of a float in a vertical slightly tapered tube (small diameter at the inlet of the flowmeter) into a measure of the amount of liquid delivered in a given timeframe. For greatest precision the rotameter is calibrated with the specific fluid being metered. Most modem rotameters are provided with a calibration plot that corresponds to performance. 

The FundaLux system, for instance, aspirates a liquid aliquot with a Teflon piston into an external glass cell, allows a (selectable) time (typically 2 min) to degas, measures transmission in comparison to an air blank, and releases the aliquot back to the reactor an interesting feature - specific to this instrument -is the repetitive cleaning of the optical window by the moving Teflon piston. Some problems with infections have been communicated with this device since the measuring cell is external to the bioreactor and the sensor is probably insufficiently sterilized in situ. 

The reaction of l-hexene on 1/16 1 Pt/ AlaOj (Engelhard E-302) reforming catalyst extrudates was chosen as the test reaction. Figure 1 shows a schematic of the reactor unit A Waters dual piston HPLC pump rated to provide flow rates between 6 and 600 ml/h with a pressure head up to 414 bar was used to feed 1-hexene (Ethyl Corporation CAS 592-41-6 Lot 851201). By closing the liquid shut-off valve (V3) and opening the gas shut-off valve (V2), either hydrogen or nitrogen gas may be admitted to the reactor unit. The feed gas was selected by a three-way, computer-controlled solenoid valve (VI). 

The rotating-cylinder reactor is basically a piston/cylinder dilatometer. It can be filled completely with a swollen latex, eliminating any air-liquid interface. The variation of the latex volume as a function of temperature and degree of polymerization can be monitored continuously. A thermocouple is used to follow the latex temperature within the reactor. 

The value oik a can, in certain circumstances, be determined by purely physical experiments in the reactor. For instance, kifi may be evaluated from the observed total rate of absorption in the case of a piston-like countercurrent flow of the two phases, where C% becomes a known function of CaOj or where the gas and the liquid are well stirred, so that and Cao are the same at all points. 

The flow reactor consisted of a storage bottle for the initiator/monomer solution through which N2 is bubbled, a piston pump as used in liquid chromatography, a coiled capillary (i.d. 2 mm., length 43 m., volume 135 ml.) immersed in a thermostat and a back pressure valve at the exit. 

As shown in the previous section, scaling with geometric similarity, Sr = Sl = gives constant pressure drop when the flow is laminar and remains laminar upon scaleup. This is true for both liquids and gases. The Reynolds number and the external area increase as. Piston flow is a poor assumption for laminar flow in anyfhing but small tubes. Conversion and selectivity of the reaction is likely to worsen upon scaleup unless the pilot reactor is already so large that molecular and thermal diffusion are negligible on the pilot scale. Ways to avoid unpleasant surprises are discussed in Chapter 8... 

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