Fractionation System

A good understanding of control schemes is essential for understanding fractionation systems. It is not possible... 

Temperature is the hardest parameter to control in a fractionation system. It exhibits high process and measurement lag. Temperature can also be ambivalent as a measure of composition. Pressure changes are reflected quickly up and down the column. Temperature changes are not. It is typical to provide three-mode controllers for all temperature applications. 

Eor good control, design the pressure drop for the control valve between the fractionating system and the jet system for sonic velocity (approximately 2 1 pressure ratio). This means that the jets suction must be designed for half the absolute pressure of the evacuated system. 

In even a large fractionator system, under the following conditions ... 

To illustrate how the control function requires extra capacity of process equipment, let us use a typical fractionation system, as shown in Figure 1. This sample illustrates the point being made rather than recommending any particular fractionation control scheme. The best... 

Figure 1. Typical fractionation system, which illustrates how process control requires extra equipment capacity.

The fractionator shell itself should often have some extra trays. Conventional instrumentation alone cannot always be expected to handle all the things that can happen to a fractionation system, such as changes in feed composition, reboiler steam pressure, or coolant temperature (especially for an air condenser during a sudden cold front). Experience for a given service is the best guide for extra trays. 

Loss of Heat in Series Fractionation System - In series fractionation, i.e., where the bottoms from the first column feeds into the second column and the bottoms from the second feeds into the third, it is possible for the loss of heat input to a column to overpressure the following column. Loss of heat results in some of the light ends remaining with the bottoms and being transferred to the next column as feed. Under this circumstance, the overhead load of the second column may consist of its normal vapor load, plus the light ends from the first column. If the second column does not have the condensing capacity for the additional vapor load, excessive pressure could occur. 

The vapor-phase Badger process (Eigure 10-2), which has been commercialized since 1980, can accept dilute ethylene streams such as those produced from ECC off gas. A zeolite type heterogeneous catalyst is used in a fixed bed process. The reaction conditions are 420°C and 200-300 psi. Over 98% yield is obtained at 90% conversion." Polyethylbenzene (polyalkylated) and unreacted benzene are recycled and join the fresh feed to the reactor. The reactor effluent is fed to the benzene fractionation system to recover unreacted benzene. The bottoms... 

The FCC reactor pressure is usually controlled at the suction of the wet gas compressor. The reactor pressure is the wet gas compressor suction pressure plus pressure drop through the main fractionator system. 

A portion of liquid from the overhead receiver is refluxed back to the tower and the remainder is pumped on to the gas plant. The vapor from the receiver goes to the wet gas compressor. The pressure of the reactor/main fractionator system is usually controlled at the compressor suction. 

Generally, in MPPs one centrifuge or pressure/vacuum filter, one drier, and one fractionation system of reasonable capacity are installed for every two or three reactors/crystallizers. On average, about 2 to 5 m solvent storage capacity and the same amount for intermediates are needed per 1 m of reactor capacity. 

FIGURE 11.1 (a) Schematic representation of PLC of Heracleum moellendorfi fruit, crude extract (500-pl 2% solution), system Florisil/AcOEt + B plate preeluted with benzene (b) analytical HPLC of isolated fractions, system ClS/MeOH + HjO (6 4). Abbreviations B — bergaptene, I — imperatorin, Ph — phelopterin, X — xanthotoxin. (For details, see Waksmundzka-FIajnos, M. and Wawrzynowicz, T., 7. Planar Chromatogr., 5, 169-174, 1992.)... 

FIGURE 11.6 (a) Densitogram of PLC of Fumaria officinalis herb extract and (b) photocopy of isolated fractions (after spraying with Dragendorff s reagent). Numbers indicate isolated fractions. System silica/CHjClj + PrOH + AcOH (5 4 1). Plates double developed. 

Fig. 24.9. Volumes of minerals precipitated during a reaction model simulating the evaporation of seawater as a fractionating system (the flow-through configuration) at 25 °C, calculated using the Harvie-Mpller-Wearc activity model.

Reflux System Failure The quantity of reflux used in fractionation systems determines the amount of vapor generation and the consequent pressure differential through the condenser system. If a reflux system fails, lower pressures through the condensers and the vessel may result in higher pressure rise in the system as a whole. 

The most economical cyanide control method in a rehnery appears to be upstream control using polysulhdes. Sodium and ammonium polysulhde (APS) have been used to inhibit cyanide-induced corrosion in FCC and coker fractionation systems [63]. The polysulhde combines with cyanide, forming thiocyanate according to the reaction... 

The ground state energy of this fractional system is then obtained by... 

C and D by the direct return of uhcooled first stage reooopresslon vapors from 1, and the use of a heat exchanger (Indicated as H) to heat the liquids from 2 which are pumped to C. The gas conditioning unit A serves as the reflux condenser for this fractionation system. Two vapor pressure controls are available, the first stage pressure (on 1 and D), and the temperature of C and D which can be controlled, within limits, by adjusting the heat input, H. 

The advantages of the proposed LC-fractionation system are simplicity, high reproducibility, better selectivity in comparison to both SPE and LLE techniques, and low operating costs. However, high equipment cost is the main limiting factor of this approach. This method is intended for the examination of the small percentage of samples found to be positive for antibiotics by screening tests (72). 

A recent work has demonstrated that the formulation of reaction-diffusion problems in systems that display slow diffusion within a continuous-time random walk model with a broad waiting time pdf of the form (6) leads to a fractional reaction-diffusion equation that includes a source or sink term in the same additive way as in the Brownian limit [63], With the fractional formulation for single-species slow reaction-diffusion obtained by the authors still being linear, no pattern formation due to Turing instabilities can arise. This is due to the fact that fractional systems of the type (15) are close to Gibbs-Boltzmann thermodynamic equilibrium as shown in the next section. 

Liquid-Controlled In fractionation systems with high viscosity or component relative volatility that greatly exceeds 1, the liquid side will be controlling. This is clearly illustrated by Fig. 14-47 which shows a sharp decline in efficiency with either a rise in liquid viscosity or a rise in component relative volatility. 

These three rules are the industry standard for all fractionation systems worldwide. Further, it is not feasible to design any tray outside these rules. Figure 3.1 shows that setting only one of the downcomer widths will truly set the required widths (and areas) of all the subject... 

Continuing study of the fractionation system for the SRC-II process, both in pilot plant and engineering work, has indicated that some modification to the original fractionation system design is desirable. In the original design the slurry was passed... 

Exact theory. If particle properties can be related exactly to their observed behavior in the fractionation system, then the properties can be deduced directly from experimental results without the uncertainties of calibration procedures. 

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