Mass transfer cyclone separators

Spray Dryers A spray diyer consists of a large cyhndrical and usu ly vertical chamber into which material to be dried is sprayed in the form of small droplets and into which is fed a large volume of hot gas sufficient to supply the heat necessary to complete evaporation of the liquid. Heat transfer and mass transfer are accomphshed by direct contact of the hot gas with the dispersed droplets. After completion of diying, the cooled gas and solids are separated. This may be accomplished partially at the bottom of the diying chamber by classification and separation of the coarse dried particles. Fine particles are separated from the gas in external cyclones or bag collectors. When only the coarse-particle fraction is desired for fini ed product, fines may be recovered in wet scrubbers the scrubber liquid is concentrated and returned as feed to the diyer. Horizontal spray chambers are manufactured with a longitudinal screw conveyor in the bottom of the diying chamber for continuous removal of settled coarse particles. 

In order to improve this separation and to obtain a good mass balance, we decided to study the both extraction of butylacetate or xylenes. Cyclonic separators has been built in our laboratory, they are supposed to have a good efficiency but at this moment there are no data in the litterature to calculate such separators. The dimensions and the geometry are determined so the operating parameters were temperature and pressure if we suppose we have no transfer problems, we have also enough equilibrium thermodynamic data. The results are summarized on table 1. 

To meet the supercritical conditions more suitable a mixer-settler unit has been developed by Schaffner [2], A regenerative pump was used as mixer to achieve an intense contact between the two phases and to create a large mass transfer surface area. This type of pump is capable of conveying gases as well as liquids and creates a high pressure difference which is used to separate the phases in the following fluid cyclone (fig. 1). 

Chemical engineering processes involve the transport and transfer of momentum, energy, and mass. Momentum transfer is another word for fluid flow, and most chemical processes involve pumps and compressors, and perhaps centrifuges and cyclone separators. Energy transfer is used to heat reacting streams, cool products, and run distillation columns. Mass transfer involves the separation of a mixture of chemicals into separate streams, possibly nearly pure streams of one component. These subjects were unified in 1960 in the first edition of the classic book. Transport Phenomena (Bird et al., 2002). This chapter shows how to solve transport problems that are one-dimensional that is, the solution is a function of one spatial dimension. Chapters 10 and 11 treat two- and three-dimensional problems. The one-dimensional problems lead to differential equations, which are solved using the computer. 

Despite the huge demand for biomass processes, its reactors do not present major innovations and most of them are nsed in the oil and petrochemical processes. The literature presents several systems, bnt most of them are fluidized beds, because they satisfy some important reqnirements, such as, low residence times, high heat, and mass transfer and, in particnlar, the large movement of solid particles in contact with gases or vapors, which improves the decomposition reaction and fast evolution of products (gases or vapors) in the reactor. This rapid evolution goes to phase separation in cyclones and condensers. Cyclones and separation/cooling processes will not be discussed here they have been extensively reported in the literature (Lede, 2000). 

A spray cyclone is based on the same idea as tiie bubble cyclone. A gas is introduced with a high velocity (say, 40 m/s) tangentionally into the cyclone, and liquid is sprayed in the axis (see figure 4.14). The liquid drops travel with great speed towards the wall, and flow downward as a thin film. Because of the greatly enhanced gas phase mass transfer coefficients, gas phase components can be separated on the basis of their effective liquid phase mass transfer coefficients (Schrauwen, 1986). These effects are discussed further in section 5.42.2 see eq. (5.45). 

The operational behaviour of cement kilns is also determined by the cyclone preheater, in which part of the waste gas enthalpy is transferred to the kiln feed and thus recovered for the process. In addition to the gas mass flow, which in turn is governed by the fuel energy demand as well as by the air rate, the efficiency of the preheater is mainly dependent on the dust cycles in the preheater. It is normally between 50 to 65 % and may be markedly increased by the installation of additional dip tubes. Fig. 2 (2) indicates that by increasing the separation efficiency of both lower cyclone stages from 60 to 80 % each, the preheater energy loss may be cut by about 0.15 MJ/kg of clinker. In the past the installation of dip tobes was successfully effected in numerous plants. 

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