Sieve tray extractors

Fig. 9.17 Residence times of (a) continuous and (b) drop phase in a liquid pulsed sieve tray extractor according to measurements (squares and crosses) and according to calculations by the dispersion model (drawn lines). Measured and calculated response curves agree well for the continuous phase, but not for the dispersed phase. (From Ref. 14.)...

Siemens-Martin furnace-regenerator, 590 Sieve tray extractors, 483 capacity, 484,487 diameters, 483, 487 efliciency. 483.487 pulsed, 478,483,487 sizing example, 486 Sieve trays, 428 assembly in a tower, 428 comparison with other types, example, 431... 

Sieve tray extractors are popular in the chemical and petrochemical industries. The trays minimize axial mixing, which results in good scale-up from laboratory data. The dispersed phase drops re-form at the each perforation, rise (or fall) near their terminal velocity, and then coalesce underneath (or above) the tray, as shown in Figure 14.14(d). The coalesced layer is important to prevent axial mixing of the continuous phase and to allow re-formation of the drops, which enhances mass transfer. The continuous phase passes... 

The sieve tray extractor is amenable to mechanistic modeling. Seibert and Fair (1993) provide a rigorous model for the prediction of efficiency. 

Poor interface control allows the main liquid-liquid interface to leave the extractor. This may result from inadequate size of interface flow control valves, or operation with internals that provide inverse control responses such as those observed with sieve tray extractors. (See Process-Control Considerations. )... 

Liquid Distribution Good initial distribution is not as essential in a sieve tray extractor as it is in a packed extractor, since the trays provide redistribution. While the same distributors used in packed columns are... 

EnveTope-style segmental downcomers (Fig. 15-39) often are used in commercial-scale sieve tray extractors instead of circular or pipe-style downcomers. The area of an envelope downcomer is given by... 

High Cross-flow of the Continuous Phase Miniplant tests of sieve tray extractors are often performed prior to the final design of a commercial-scale column. The design often is scaled up based on superficial velocities of the dispersed and continuous phases calculated from the volumetric flow rates and the column cross-sectional area. However, in scaling up one must be careful about the cross-flow velocity (V eow) of the continuous phase. A value may be estimated from... 

The Graesser contactor takes an extreme position in the diagram. It has the highest separation efficiency (10 stages per meter) but the lowest capacity (1-2 m/h). The other extremum takes the static sieve tray extractor with only one equilibrium stage per meter and up to 50 m/h capacity. The capacity of a pulsed sieve tray column is as high as 30 m/h with a separation efficiency of 5 to 6 stages per meter. 

There are two different types of energy input in sieve tray or perforated tray columns (Fig. 6-39) in a pulsed sieve tray extractor (PSE) the liquid column is pulsed in a swing tray extractor (STE) fixed sieve trays are mounted on a swinging axis which is driven by an infinitely variable, directly coupled geared motor, including crank shaft and connecting rod. The top and bottom section are expanded to allow the phases to rest and then separate. 

The raw material which is pre-heated to around 115 °C, is fed at atmospheric pressure to the extraction column (e.g. rotating disc contactor, sieve tray extractor), where the aromatics are preferentially dissolved by sulfolane. The solvent is separated from the aromatics mixture at a temperature of around 190 ""C and a pressure of just 1 bar. The sulfolane which boils at 287 °C, remains at the base of the column and is recycled to the top of the extraction column. The aliphatics are recovered by counter-current washing with sulfolane and subsequent distillation. 

The two-phase systems of interest here are SCF-liquid and SCF-solid. Lahiere et al. (1987) have studied the transfer of ethanol or propanol from an aqueous solution into supercritical CO2. The experimental extraction behavior in a sieve-tray extractor was compared with that predicted from a model used for common liquids. This model for subcrit-ical liquids uses overall mass-transfer coefficients obtained from the individual film coefficients by the conventional sum of the two resistances approach. Lahiere et al. (1987) observed reasonable correspondence between experimentally observed values and the performance predicted from conventional models. For SCF-solid systems with pure solids, Debenedetti and Reid (1986) have observed that the mass-transfer coefficient for the SCF phctse is very strongly influenced by natural convection. This happens because the SCF has very low viscosity and yet has a high density, leading to a much more important role of natural convection than in normal liquids. 

---