Internal recycling

A schematic diagram of a six-vessel UOP Cyclesorb process is shown in Figure 15. The UOP Cyclesorb process has four external streams feed and desorbent enter the process, and extract and raffinate leave the process. In addition, the process has four internal recycles dilute raffinate, impure raffinate, impure extract, and dilute extract. Feed and desorbent are fed to the top of each column, and the extract and raffinate are withdrawn from the bottom of each column in a predeterrnined sequence estabUshed by a switching device, the UOP rotary valve. The flow of the internal recycle streams is from the bottom of a column to the top of the same column in the case of dilute extract and impure raffinate and to the top of the next column in the case of dilute raffinate and impure extract. 

Fig. 8. Combined flow reactor models (a) parallel flow reactors with longitudinal diffusion (diffusivities can differ), (b) internal recycle—cross-flow reactor (the recycle can be in either direction), comprising two countercurrent plug-flow reactors with intercormecting distributed flows, (c) plug-flow and weU-mixed reactors in series, and (d) 2ero-interniixing model, in which plug-flow reactors are parallel and a distribution of residence times dupHcates that...

Mechanical Mills with Mir Classifiers. To improve the end fineness and achieve a sharper topsize cutoff point, many mechanical impact mills are fitted with integral air classifiers (Fig. 13). These can be driven separately from the mill rotor or share a common drive. The material to be ground is introduced into the mill section of the machine, where impact size reduction takes place. The airflow through the machine carries the partially ground product to the air classifier, which is usually some form of rotating turbine. The speed of rotation determines which particle size is internally recycled for further grinding and which is allowed to exit the machine with the airflow. Machines are available up to 375 kW and can achieve products with essentially all material <20 fim. 

Opposed Jet Mills. These mills are, in some ways, similar to the fluidized-bed machine however, in this case two opposed nozzles accelerate particles, causing them to collide at a central point (Fig. 16). A turbine classifier is again used to separate the product that has achieved the desired fineness from that which must be internally recycled for further grinding. 

In the two-stage process, nitrification occurs under aerobic conditions in the second stage. The nitrified mixed Hquor from the second stage is internally recycled to the anoxic first stage, where denitrification occurs. 

Jankowski et al (1978) discuss in detail the great variety of gradientless reactors proposed by several authors with a pictorial overview in their paper. All of these reactors can be placed in a few general categories (1) moving catalyst basket reactors, (2) external recycle reactors, and (3) internal recycle reactors. 

The older internal recycle reactors of Berty et al (1969), and Berty (1974) are shown on Figures 2.4.3 a, b. The reactor of Romer and Luft (1974) uses no mechanical moving parts. The recirculation is generated by the feed gas as it expands through a nozzle. A major disadvantage of using a jet is that feed rate and recirculation rate are not independent. Due to the low efficiency of jet pumps, recycle rates are quite low. 

The operational characteristics of the older Berty reactors are described in Berty (1974), and their use in catalyst testing in Berty (1979). Typical uses for ethylene oxide catalyst testing are described in Bhasin (1980). Internal recycle reactors are easy to run with minimum control or automation. 

Various experimental methods to evaluate the kinetics of flow processes existed even in the last centuty. They developed gradually with the expansion of the petrochemical industry. In the 1940s, conversion versus residence time measurement in tubular reactors was the basic tool for rate evaluations. In the 1950s, differential reactor experiments became popular. Only in the 1960s did the use of Continuous-flow Stirred Tank Reactors (CSTRs) start to spread for kinetic studies. A large variety of CSTRs was used to study heterogeneous (contact) catalytic reactions. These included spinning basket CSTRs as well as many kinds of fixed bed reactors with external or internal recycle pumps (Jankowski 1978, Berty 1984.)... 

The ROTOBERTY internal recycle laboratory reactor was designed to produce experimental results that can be used for developing reaction kinetics and to test catalysts. These results are valid at the conditions of large-scale plant operations. Since internal flow rates contacting the catalyst are known, heat and mass transfer rates can be calculated between the catalyst and the recycling fluid. With these known, their influence on catalyst performance can be evaluated in the experiments as well as in production units. Operating conditions, some construction features, and performance characteristics are given next. 

Internal recycle flow rate created by the blower was 620 times larger than the make-up feed rate in an actual experiment. At this recycle flow, a particulate based Rep = 3050 was achieved. The corresponding transfer coefficients were very high and gradients were negligible. 

The experimental setup uses the ROTOBERTY internal recycle reactor. The catalyst basket of this is charged with W = 35.5 g or V = 44.3 cm of OXITOX that contains 0.25 mol, i.e., 26.5 g of sodium carbonate. 

Affected by corrosion and erosion of the rotor and casing, increasing the gas-slip internal recycle. This problem is not serious for water- or oil-injected screws. 

Recent process development efforts have been devoted to more expeditious and less costly pyrochemical reprocessing of residues created by the metal preparation and purification process. We intend to establish an internal recycle which yields either reusable or discardable residues and recovers all plutonium for feed to the electrorefining purification system. This internal recycle is to be performed in a more timely and less costly operation than in the present reprocessing mode. 

A volumetric scaleup by a factor of 512 is quite large, and the question arises as to whether the large vessel wiU behave as a CSTR. The concern is due to the factor of 4 increase in mixing time. Does it remain true that tmix h/i and tmix t If so, the assumption that the large vessel wiU behave as a CSTR is probably justified. The ratio of internal circulation to net throughput—which is the internal recycle ratio—scales as the inverse of the mixing time and will thus decrease by a factor of 4. The decrease may appear worrisome, but if the increase in mixing time can be tolerated, then it is likely that the decrease in internal recycle ratio is also acceptable. 

Recycling of partially reacted feed streams is usually carried out after the product is separated and recovered. Unreacted feedstock can be separated and recycled to (ultimate) extinction. Figure 4.2 shows a different situation. It is a loop reactor where some of the reaction mass is returned to the inlet without separation. Internal recycle exists in every stirred tank reactor. An external recycle loop as shown in Figure 4.2 is less common, but is used, particularly in large plants where a conventional stirred tank would have heat transfer limitations. The net throughput for the system is Q = but an amount q is recycled back to the reactor inlet so that the flow through the reactor is Qin + q- Performance of this loop reactor system depends on the recycle ratio qlQin and on the type of reactor that is in the loop. Fast external recycle has... 

This new monomer is separated from the excess glycol and polymerized. The monomer has two hydroxy endgroups but with catalysis and temperature, it will self-condense to give ethylene glycol as the by-product. The overall result is a one-to-one reaction of terephthalic acid with ethylene glycol, but a substantial amount of glycol is internally recycled. 

These are only a few specific examples of paleodietaiy consequences of biochemical pathways. Paleodiet researchers shonld probably try to enlist the aid of metabolic biochemists in a search for other possible conseqnences of differential metabolic pathways, internal recycling of metabolites, etc. Furthermore, many of these problems will become clearer as we begin to have access to isotopic analyses of individnal amino acids or even specific carbon atoms at sites on individual AAs. 

The process needs input of lime and water next to the PVC waste. No energy input is needed since the organic condensate provides for the energy needed in the process. Energy needed for pretreatment can be up to 25-35 kWh/tonne. Downstream separation of the coke products needs another 30-40 kWh/toime. The process does not emit dioxins, metals or plasticisers. Due to internal recycling there are no aqueous waste streams. The reaction of lime with HCl forms some CO2. The coke product provides a calorific value. 

Figure 5.4-19. Internally recycled reactor (Berty reactor).

Internally recycled reactor (Berty) High temperature, high pressure catalytic processes High transport rates, intense mixing Limited ease of variation of parameters... 

To use anaerobic digesters with high internal recycling ratios to maintain a low concentration of formaldehyde inside the system. 

The T-STAR ebullated bed is shown schematically in Fig. 6. The figure demonstrates that a uniform catalyst distribution is maintained throughout the reaction chamber via the upward flow of the hydrogen, feed oil, and recycle oil. The internal recycle allows for increased conversion and assists in maintaining a uniform temperature throughout the reactor. Careful monitoring of the temperature in an ebullated bed... 

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