Cascaded units

The other major change to the control scheme is in the control systems used for the feed inventory. Level controllers are used on Tanks B and C. These level controllers use the capacitance of the tanks to attenuate the fluctuations in feed flow. The feeds to the extraction column and azeotropic column are considerably dampened, resulting in a much more consistent end product. 

The dynamics and control of continuous process units that operate as a cascade of units, either in parallel or in series, have been studied extensively for many years [5-7]. A wealth of knowledge is available to help design effective control systems for a large number of unit operations when these units are run independently [6,8]. This knowledge can be directly applied to the plant-wide control problem if a number of process units are linked together as a sequence of units. Each downstream unit simply sees the disturbances coming from its upstream neighbour. 

The design procedure was proposed almost three decades ago [5] and has since been widely used in industry. The first step of the procedure is to lay out a logical and consistent material balance control structure that handles the inventory controls, i.e. levels and pressures. This hydraulic structure provides gradual and smooth flow rate changes from unit to unit. Thus, flow rate disturbances are filtered so that they are attenuated and not amplified as they work their way down through the cascade of units. Slow-acting, proportional-only level controllers provide the simplest and most effective way to achieve this flow smoothing. 

product quality control loops are closed on each of the individual units. These loops typically use fast PI controllers to hold product streams as close as possible to specification values. Since these loops are considerably faster than the slow inventory loops, interaction between the two is generally not a probleia Also, since the manipulated variables used to hold product qualities are often streams that are internal to each individual unit, changes in these manipulated variables have little effect on the downstream process. The manipulated variables frequently are utility streams that are provided by the plant utility system, i.e. cooling water, steam, refrigerant, etc. Thus, the boiler house will be disturbed, but the other process units in the plant will not see disturbances coming from upstream process units. Of course, this is only tme when the plant utilities systems have effective control systems that can respond quickly to the many disturbances that they see coming in from units all over the plant. 

Since the propagation of the disturbances in such a system is sequential down the flow path, the use of feedforward control on each unit can also help to improve product quality control [7]. 

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Spray/cascade units are available in which the hot salt is sprayed over the surface of the product. This type of system can lose a lot of heat during the spraying operation. 

Proportional-only control should be used in nonreactive level loops for cascaded units in series. Even in reactor level control, proportional control should be considered to help filter flowrate disturbances to the downstream separation system. There is nothing necessarily sacred about holding reactor level constant. 

In contrast, the Amoco model is more fundamental and is based on extensive laboratory and commercial test data that accurately reflect unit operations. Because Stratco and cascade units account for over 80% of the alkylation capacity of Amoco refineries, we developed complete computer models for each type. 

Model simulations are useful for evaluating the benefits of proposed configurational changes in units. For example, simulations suggested that modifications to the internals of cascade units would Improve performance. After these changes had been made In an Amoco commercial unit, weekly average acid consumptions as low as 0.2 Ib/gal of alkylate were recorded. 

The models may be used to compare performance between several units or unit types. For example, a week of production data from the above-mentioned modified cascade unit was compared with simulated Stratco performance. The agreement between Amoco s Stratco simulation model predictions and material balance and performance predictions provided by the Stratford Engineering Corporation had been confirmed earlier. Model adjustments allowed a comparison at constant operating conditions to be made. Table I compares the performance of the modified cascade unit with the Stratco simulation model predictions. Acid consumption is 36% lower in the modified cascade unit. The Stratco unit does show a slight research octane advantage over the cascade. However, overall economics favor the modified cascade. in this case. 

Complete simulation models have been formulated for cascade and Stratco sulfuric acid alkylation units and studies have confirmed the accuracy of the models. Application studies include cases in which model usage Identified profitable unit modifications, determined optimal unit capacity and optimal distillation tower operation, and compared the performance of cascade and Stratco units. "Isostripper" deisobutanizer operation was determined to be relatively unprofitable for sulfuric acid alkylation units and acid consumption on a modified cascade unit was found to be 36% below that expected for a Stratco unit. The examples presented suggest the broad applicability of the simulation models for improving alkylation unit operation. Use of the models not only pinpoints areas where significant improvements are possible, but also quantifies incentives needed to get them implemented quickly. 

Stratco unit with the single mixer on one end is approximated by a single mixed tank, as shown in the upper part of the figure. However, the Kellogg cascade unit has a series of compartments with mixers and olefin is sparged into each compartment to keep the concentration low so that it reacts with the isobutane rather than polymerizing. The tank-in-series model may be used to model this type of unit and this is shown in the lower part of the figure. A mass balance can be made for a stirred tank reactor readily because the composition is the same everywhere in the vessel. 

Output variables for this specification include missing mole fractions for Vm and Lin, stage temperatures, and the variables associated with the Vout stream. Loot stream, and interstage streams. The results obtained in this example are included in Table 6.1. The N-stage cascade unit can represent simple absorbers, strippers, or liquid-liquid extractors. 

For the N-stage cascade unit of Fig. 6.6, with reference to the single... 

Determine the valves to use for inventory control all liquid levels (except for surge volumes in liquid recycle systems) and gas pressures must be controlled. Select the largest stream to control levels whenever possible. Use proportional-only level control in nonreactive level loops for cascaded units. Fresh feed makeup streams are often- used to hold levels or pressures when these variables reflect the inventory of specific components in the process. There should be a flow controller somewhere in all liquid recycle loops. 

Where moulded case circuit breakers are chosen for a plant in favour of fuses the coordination of cascaded units becomes a little more difficult than with fuses. This difficulty arises from the fact that these circuit breakers have a definite or near definite minimum time limit to their time-current characteristic. This causes the lower part of the circuit breaker protection curve to be almost horizontal at a low value of time, typically in the range of 0.003 and 0.01 second. 

Other variations in the different zones of cascade reactors, which have not been reported, include residence time (space velocity), acid/hydrocarbon ratio, and character of the dispersions. The decreases in RON for alkylates in the latter zones are also partly due to degradation reactions. It has been shown (16,17) that TMPs degrade in the presence of sulfuric acid. It has also been indicated (14) that both the quality and quantity of allgrlates are reduced by such degradations the quality was estimated to reduce by perhaps 0.11 RON in cascade units. In Stratco units, the decrease was perhaps 0.07 RON this lower value is because of lower residence times. Degradation reactions of TMPs are also thought to be more pronounced in cascade reactors because of increased amounts of mixing of TMPs with olefins. 

Fig. 33. High voltage cascade units for VDU terminals arranged in the vacuum vessel in which the circuits are encapsulated in a two-component silicone elastomer. (Wigo GmbH and IBM Corp.)...

Use of the cascade cycle makes for a complex plant. However, the utilization of a series of refrigerants in the manner described improves the reversibility of the process, and makes it thermodynamically more efficient. To liquefy 1,000,000 scf/day would take about 460 bhp in a well-designed cascade unit. About 90 of the sweet gas charged to the unit is liquefied, with 10 going to fuel for steam boilers, compressors, electric generators, etc. 

The two procedures primarily used for continuous nitration are the semicontinuous method developed by Bofors-Nobel Chematur of Sweden and the continuous method of Hercules Powder Co. in the United States. The latter process, which uses a multiple cascade system for nitration and a continuous wringing operation, increases safety, reduces the personnel involved, provides a substantial reduction in pollutants, and increases the uniformity of the product. The cellulose is automatically and continuously fed into the first of a series of pots at a controlled rate. It falls into the slurry of acid and nitrocellulose and is submerged immediately by a turbine-type agitator. The acid is deflvered to the pots from tanks at a rate controlled by appropriate instmmentation based on the desired acid to cellulose ratio. The slurry flows successively by gravity from the first to the last of the nitration vessels through under- and overflow weirs to ensure adequate retention time during nitration. The overflow from the last pot is fully nitrated cellulose. 

The sputtering yield is proportional to the number of displaced atoms. In the linear cascade regime that is appUcable for medium mass ions (such as argon), the number of displaced atoms, E (E, is proportional to the energy deposited per unit depth as a result of nuclear energy loss. The sputtering yield Y for particles incident normal to the surface can be expressed as foUows (31). 

Gaseous diffusion cascades for uranium enrichment have also been built in the United Kingdom, France, the former USSR, China, and, more recendy, in Argentina. 

There are areas (22) where selenium levels in the soil are very low these include regions of volcanic activity like that adjacent to the Cascade mountains in the Pacific Northwest states of the United States and the central north island of New Zealand. There, because the heat of emption volatilized the selenium, the residual soil parent material is virtually devoid of selenium. Other areas of low soil-selenium reflect leaching of selenium out of the top soil, as in the Canterbury plain on New Zealand s south island. Areas of selenium deficiency have negative implications for animal and human health. 

Other burners are used for low capacity operations. A cascade or checker burner, ia which molten sulfur flows down through brick checkerwork countercurrent to a flow of air, is used ia small units with a sulfur trioxide converter to condition gases entering electrostatic precipitators at boiler plants operating on low sulfur coal. A small pan burner, which is fed with soHd, low carbon sulfur, is used to produce sulfur dioxide for solution ia irrigation water to control the pH and maintain porosity ia the soil. The same type of burner is used to disiafect wastewater ia this case sulfur dioxide is used iastead of chlorine. 

It can be assumed that P,Jp, and for the cascade have been specified, and that the cost of feed and the cost per unit of separative work, the product of separative capacity and time, are known. The basic assumption is that the unit cost of separative work remains essentially constant for small changes ia the total plant size. The cost of the operation can then be expressed as the sum of the feed cost and cost of separative work ... 

Equations 27 and 28 can be used in conjunction, along with the corresponding equations for the stripping section, to produce an ideal plant profile such as is shown in Figure 4 where F is plotted against for the example of an ideal cascade to produce one mol of uranium per unit time enriched to 90... 

For the example considered above, the total cascade upflow is found to be 33 x 10 mols per unit time. 

The second term in brackets in equation 36 is the separative work produced per unit time, called the separative capacity of the cascade. It is a function only of the rates and concentrations of the separation task being performed, and its value can be calculated quite easily from a value balance about the cascade. The separative capacity, sometimes called the separative power, is a defined mathematical quantity. Its usefulness arises from the fact that it is directly proportional to the total flow in the cascade and, therefore, directly proportional to the amount of equipment required for the cascade, the power requirement of the cascade, and the cost of the cascade. The separative capacity can be calculated using either molar flows and mol fractions or mass flows and weight fractions. The common unit for measuring separative work is the separative work unit (SWU) which is obtained when the flows are measured in kilograms of uranium and the concentrations in weight fractions. 

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