Feed variation

Steady-state modeling is not sufficient if one faces various disturbances in RA operations (e.g., feed variation) or tries to optimize the startup and shutdown phases of the process. In this case, a knowledge of dynamic process behavior is necessary. Further areas where the dynamic information is crucial are the process control as well as safety issues and training. Dynamic modeling can also be considered as the next step toward the deep process analysis that follows the steady-state modeling and is based on its results. 

Four generic criteria must be addressed for development of any solvent-extraction process in the nuclear or hydrometallurgical industries. Namely, the solvent must maintain its integrity over time and use. The process must accept wide feed variation, and the solvent components cannot be lost at an appreciable rate. The solvent must possess a relatively high affinity and selectivity for the component that is to be recovered from the feed stream. Also, the opposing phases in the process must be immiscible or very nearly so in order to effect efficient phase separation. Finally, the... 

Not very reliable feeding (variations of up to 10-15% from average have been observed during calibration tests). 

The flowsheet is completed with P-type controllers for level in the reflux drum and bottoms, and PI controller for pressure. These controllers ensure the basic Inventory control, but are not sufficient for quality control. Therefore, we are interested by distillate flow rate and purity faced with disturbances in the feed. Fig. 4.7 presents the open loop response to feed variation of +/- 10%. Increasing the feed to 110 kmol/hr gives an increase in purity over 99%, but a decrease of the distillate rate to less than 47.5 kmol/hr. After reset to initial conditions, the feed is reduced to 90 kmol/h. This time the distillate rate increases at 52.5 kmol/hr, but the purity drops dramatically to 86%. This behaviour seems somewhat strange, so the reader is encouraged to find a physical explanation. The need for quality control in a distillation column is obvious. This issue will be treated in the Examplel2.2. 

Figure 13.14 presents a suitable control structure for feeding. This time the reactant A is put on concentration control, while B is fed on level control. This structure keeps the advantage of measuring directly the production rate by integrating the feed variation over a period of time. However, a continuous measurement of concentration of A is needed, and this is not always obvious. Note also that the outlet reactor stream is held on flow control, in order to prevent a snowball effect. Increase or decrease in production can be achieved at best by manipulating the reactor temperature, which in turn will determine an automatic adjustment of the reactant feeds. 

Total reflux startup is most attractive in large superfractionators which use high reflux ratios (e.g., isobutane-normal butane ethylene-ethane separation) and/or in heat-pumped columns. Such columns take from a few hours to a couple of days to start up and stabilize. Due to the high reflux ratio, they are relatively insensitive to feed variations. These features make them ideal candidates for total reflux startups. Total reflux startup is least attractive when the ratio of reflux to feed is low (in such cases, most of the stabilizing can only be performed after feed is introduced), and when the column is easy and trouble-free to start up. 

Liquid maldistribution] not vertical/inadequate calming of feed/variations in weir height. 

A suitable experimental setup and feed concentration variations are chosen. The feed variations should enable one to discriminate between the suggested kinetic models. Simulation runs can be helpful for this purpose. 

The first assumption is not valid due to feed variations, which are common to many rotary dryers employed within the Australian sugar industry. In fact, it could be reasonably argued that the dryer is never at steady state and therefore the hold-up can never be uniform along its length. 

The burr size is highly sensitive to feed variations and it is beneficial to employ lower feed to minimise the burr size. 

This desirable result is mainly due to the combined pieconcentrator/recovery column dampening the large disturbance from fresh feed, thus making the feed variations to the more sensitive Cl colimin small and easier to handle. The three-column system should also have this desirable feature. However, it requires more process, instrumentation, and control equipment, and the TAC is also higher than the proposed design. 

Materials suitable as filter aids include diatomaceous earth, expanded perilitic rock, asbestos, ceUulose, nonactivated carbon, ashes, ground chalk, or mixtures of those materials. The amount of body feed is subject to optimisa tion, and the criterion for the optimisa tion depends on the purpose of the filtration. Maximum yield of filtrate per unit mass of filter aid is probably most common but longest cycle, fastest flow, or maximum utilisation of cake space are other criteria that requite a different rate of body feed addition. The tests to be carried out for such optimisation normally use laboratory or pilot-scale filters, and must include variation of the filtration parameters such as pressure or cake thickness in the optimisation. 

An important variation of this filter is based on replacing the rigid outer waU necessary for containing the feed and the cake on the rotating table by an endless mbber belt. The belt is held under tension and rotates with the table. It is in contact with the table rim except for the sector where the discharge screw is positioned, and where the belt is deflected away from the table to aUow the soHds to be pushed off the table. The cloth can also be washed in this section by high pressure water sprays. This filter, recendy developed in Belgium, is avaUable in sizes up to 250 m, operated at speeds of 2 minutes per revolution, and cake thicknesses up to 200 mm. 

A variation to the top-feed dmm filter is the dual dmm filter which uses two dmms of the same size in contact with each other and rotating in opposite directions. The feed enters into the V-shaped space formed on top of the two dmms and the cake that starts forming initially contains coarser particles due to the settling which takes place in the feed zone. This is beneficial to the clarity of the filtrate because the coarser particles act as a precoat. Erom the point of view, however, of the final moisture content of the cake the stratification of the soHds in the cake may lead to somewhat wetter cakes. Utilization of the area of the dmms is poor since there are dead spaces under the two dmms. The primary appHcation of the dual dmm filters is in dewatering coarse mineral or coal suspensions at feed concentrations greater than 200 kg/m. ... 

Efficiency of Intermediate Formation. The variation of the efficiency of a primary intermediate with conversion of the feed hydrocarbon can be calculated (22). Ratios of the propagation rate constants ( 2 / i) reactor type (batch or plug-flow vs back-mixed) are important parameters. 

Chemical Reaction Measurements. Experimental studies of incineration kinetics have been described (37—39), where the waste species is generally introduced as a gas in a large excess of oxidant so that the oxidant concentration is constant, and the heat of reaction is negligible compared to the heat flux required to maintain the reacting mixture at temperature. The reaction is conducted in an externally heated reactor so that the temperature can be controlled to a known value and both oxidant concentration and temperature can be easily varied. The experimental reactor is generally a long tube of small diameter so that the residence time is well defined and axial dispersion may be neglected as a source of variation. Off-gas analysis is used to track both the disappearance of the feed material and the appearance and disappearance of any products of incomplete combustion. 

In the most common production method, the semibatch process, about 10% of the preemulsified monomer is added to the deionised water in the reactor. A shot of initiator is added to the reactor to create the seed. Some manufacturers use master batches of seed to avoid variation in this step. Having set the number of particles in the pot, the remaining monomer and, in some cases, additional initiator are added over time. Typical feed times ate 1—4 h. Lengthening the feeds tempers heat generation and provides for uniform comonomer sequence distributions (67). Sometimes skewed monomer feeds are used to offset differences in monomer reactivity ratios. In some cases a second monomer charge is made to produce core—shell latices. At the end of the process pH adjustments are often made. The product is then pumped to a prefilter tank, filtered, and pumped to a post-filter tank where additional processing can occur. When the feed rate of monomer during semibatch production is very low, the reactor is said to be monomer starved. Under these... 

These reactions have been studied extensively, and many variations exist ia the use of HCl atmospheres to maximize the desired reactions. The dehydration of camaUite occurs with minimum hydrolysis, but results ia a mixed MgCl2—KCl cell feed. The use of magnesium chloride hexammoniate [24349-22-2], MgCl2 hNH, has been proposed, but is not practiced commercially. This salt readily deammoniates to anhydrous MgCl2. 

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