Vessels continuous operations

Equations (1-1) and (1-2) are true in the general case and can be used to study several modes of reactor operation (e.g. batch, semi-batch, continuous, start-up procedures, etc.). If the assumption is made that the reactor is a vessel continuously operating full, i.e. overflow CSTR, then the right hand side (RHS) of equation (1-2) is zero and (1-1) is considerably simplified to yield ... 

In principle, at least, any mixer may be coupled with any settler to provide the complete stage. There are several combinations which are especially popufar. Continuously operated devices usually, but not always, place the mixing and settling functions in separate vessels. Batch-operated devices may use the same vessel alternately for the separate functions. 

The vertical vibratoiy mill has good wear values and a low-noise output. It has an unfavorable residence-time distribution, since in continuous operation it behaves like a well-stirred vessel. Tube mills are better for continuous operation. The mill volume of the vertical mill cannot be arbitrarily scaled up because the static load of the upper media, especially with steel beads, prevents thorough energy introduction into the lower layers. Larger throughputs can therefore only be obtained by using more mill troughs, as in tube mills. 

Figure 4-4 shows a semi-batch reactor with outside circulation and the addition of one reactant through the pump. Semi-batch reactors have some reactants that are charged into the reactor at time zero, while other reactants are added during the reaction. The reactor has no outlet stream. Some reactions are unsuited to either batch or continuous operation in a stirred vessel because the heat liberated during the reaction may cause dangerous conditions. Under these... 

Consider a thin layer solid bowl centrifuge as shown in Figure 4.20. In this device, particles are flung to the wall of the vessel by centrifugal force while liquor either remains stationary in batch operation or overflows a weir in continuous operation. Separation of solid from liquid will be a function of several quantities including particle and fluid densities, particle size, flowrate of slurry, and machine size and design (speed, diameter, separation distance, etc.). A relationship between them can be derived using the transport equations that were derived in Chapter 3, as follows. 

Batch crystallizers are widely used in the chemical and allied industries, solar saltpans of ancient China being perhaps the earliest recorded examples. Nowadays, they still comprise relatively simple vessels, but are usually (though not always) provided with some means of agitation and often have artificial aids to heat exchange or evaporation. Batch crystallizers are generally quite labour intensive so are preferred for production rates of up to say 10 000 tonnes per year, above which continuous operation often becomes more favourable. Nevertheless, batch crystallizers are very commonly the vessel of choice or availability in such duties as the manufacture of fine chemicals, pharmaceutical components and speciality products. 

WTiile his cxpcrilnciits yielded acceptable gasolines, serious problems arose as Burton attempted to scale up his process. The short cycles forced on producers by extensive coking worked against continuous operations. The carbon buildup interfered with heat transfer from the furnace to the petroleum and resulted in the formation of hot spots and in turn damage to the vessel. Pressure control was also a problem. 

Bonded silver linings are fabricated for mild steel or copper vessels. They are soldered in situ to the walls of the vessel by means of a special tin-silver solder. The melting point of this solder is approximately 280°C, and 200°C is recommended as the maximum continuous operating temperature for linings bonded with it. Since the whole of the silver is firmly adherent to the vessel, bonded linings are suitable for operation under vacuum conditions, and provide excellent heat-transfer characteristics. 

Chapter 2 treated multiple and complex reactions in an ideal batch reactor. The reactor was ideal in the sense that mixing was assumed to be instantaneous and complete throughout the vessel. Real batch reactors will approximate ideal behavior when the characteristic time for mixing is short compared with the reaction half-life. Industrial batch reactors have inlet and outlet ports and an agitation system. The same hardware can be converted to continuous operation. To do this, just feed and discharge continuously. If the reactor is well mixed in the batch mode, it is likely to remain so in the continuous mode, as least for the same reaction. The assumption of instantaneous and perfect mixing remains a reasonable approximation, but the batch reactor has become a continuous-flow stirred tank. 

The fed-batch scheme of Example 14.3 is one of many possible ways to start a CSTR. It is generally desired to begin continuous operation only when the vessel is full and when the concentration within the vessel has reached its steady-state value. This gives a bumpkss startup. The results of Example 14.3 show that a bumpless startup is possible for an isothermal, first-order reaction. Some reasoning will convince you that it is possible for any single, isothermal reaction. It is not generally possible for multiple reactions. 

Decanters are normally designed for continuous operation, but the same design principles will apply to batch operated units. A great variety of vessel shapes is used for decanters, but for most applications a cylindrical vessel will be suitable, and will be the cheapest shape. Typical designs are shown in Figures 10.38 and 10.39. The position of the interface can be controlled, with or without the use of instruments, by use of a syphon take-off for the heavy liquid, Figure 10.38. 

For the individual rotor types, different vessels are available. The 16-vessel rotor, dedicated for the performance of standard reactions at up to 240 °C, offers 100 mL PTFE-TFM liners. Applying different pressure jackets allows continuous operation of these vessels to a maximum of 20 or 40 bar, respectively. The 8-vessel rotor is designed for high-pressure reactions, offering PTFE-TFM liners or quartz vessels, which enable reactions to be performed at 300 °C and 80 bar for several hours. 

For continuous operation of a CSTR as a closed vessel, the general material balance equation for reactant A (in the reaction A vr C +. .. ), with a control volume... 

A schematic representation of this reactor model is shown in Figure 22.2. Particles of solid reactant B are in BMF, and fluid reactant A is uniform in composition, regardless of its flow pattern. The solid product, consisting of reacted and/or partially reacted particles of B, leaves in only one exit stream as indicated. That is, we assume that no solid particles leave in the exit fluid stream (no elutriation or entrainment of solid). This assumption, together with the assumption, as in the SCM, that particle size does not change with reaction, has an important implication for any particle-size distribution, represented by P(R). The implication is that P(R) must be the same for both the solid feed and the solid exit stream, since there is no accumulation in the vessel in continuous operation. Furthermore, in BMF, the exit-stream properties are the same as those in the vessel Thus, P(R) is the same throughout the system ... 

Baking is currently performed by continuous operation. Modern variations involve using heated crushing or milling equipment, such as kneader dispersers or oscillating mills at approximately 200°C [9]. This technique significantly improves the reaction control over a batch process. If baking is performed by continuous process, phthalonitrile only remains within the reaction vessel for a very short period of time (between 3 and 20 minutes). It is important to remember that the temperature may not exceed 250°C. The product which evolves from this process is usually purified by acid treatment. 

For continuous operation, where fresh wash-liquid enters the vessel continuously and liquor is withdrawn through a filter screen, then a mass balance gives ... 

For continuous operation with two liquids, where the vessel is operated full and consequently in the absence of vortex, very limited tests with flat-bladed turbines (F3) have tentatively indicated that (a) the effect of extent of baffling on power requirements may be different from that found in open vessels, but has not yet been established (b) at all except low agitator speeds the ratio of phases contained within the vessel is the same as the ratio in the feed mixture and (c) the power is independent of rate of flow through the vessel. But these conclusions are all based on measurements with small vessels and with only two systems. They need confirmation under a much wider variety of conditions. 

The more finely the liquids are dispersed within one another, the more slowly will they settle, either in a separate decanter for a continuous operation or in the same vessel for a batch process. Most stable emulsions, those which settle and coalesce only very slowly if at all, are characterized by maximum particle diameters of the dispersed phase of the order of 1 to 1.5 microns. Presumably one could estimate through Eq. (6) what agitator speeds would produce such droplet sizes, but such calculations are not likely to yield completely useful results. For example, it has been observed on several occasions that the settling ability of some liquid dispersions passes through a minimum as agitator speed is increased. 

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