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Oct-2007

Tray technology 
enhances extraction

A case study involving the revamp of two rotating disc contactors in aromatics service highlights the most important design considerations for enhancing capacity and efficiency in high-performance extraction sieve trays

Waldo de Villiers and Jose L Bravo, Shell Global Solutions (US) Inc
Glenn Shiveler, Sulzer Chemtech USA Inc
Frank Seibert, The University of Texas at Austin
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Article Summary
Solvent extraction processes are often applied when separation by distillation is difficult or impossible. Since this technology is applied in a large variety of industrial processes, a wide range of liquid-liquid contacting devices are available. These include sieve tray, packed, spray, rotary agitated, reciprocating plate and pulsed columns, mixer-settlers, hollow-fibre contactors and centrifugal extractors. The sieve tray, spray and packed column extractors (Figure 1) are the most common types of continuous commercial extractors found in the petroleum and petrochemical industries in steady-state operation. Their characteristics are simplicity, low-to-medium efficiency, high capacity, and low capital and operating costs.

Spray columns are one of the simplest and oldest liquid-liquid contacting devices available. They feature high capacity, low cost, good fouling resistance and easy operation. However, they suffer from backmixing of the continuous and dispersed phases, which reduces efficiency, and are thus normally limited to one to two theoretical 
stages of separation regardless of the column height.

Packed columns offer high capacity and are often used when only a few equilibrium stages are required. The packing efficiency is limited because of axial mixing of the continuous and dispersed phases. Although similar to spray columns, they are more efficient because the packing elements provide for better contacting between the dispersed and continuous phases, and help reduce backmixing compared to the spray column. This increases the dispersed phase hold-up and thus the interfacial area. Random as well as structured packings are used. In commercial-scale columns with random packing, normally no more than two theoretical stages can be achieved in a single bed regardless of the bed height. When more stages are desired, multiple beds with redistribution trays are required or, in the case of structured packing, special perforated plates can be applied between packing layers.

Other factors that favour the use of packing include:
— Easier control of the main operating interface
— Packed extractors tend to have greater flexibility regarding operating flow rates compared to those equipped with trays
— Vessels less than 30 in (762 mm) in diameter are usually designed with packing
— Applications that benefit from the preferential wetting of the packing surface into a thin film for mass transfer. The selection of packing material and choice of dispersed phase is therefore important
— Phase flow ratios close to unity are optimal to avoid loss of efficiency due to axial backmixing.

The sieve tray extractor is one of the most common types of extractor, since its simple geometry, high capacity and reliability in dirty service make it attractive as a commercial device. Since backmixing of the continuous phase is limited to the inter-tray spacing, a large number of theoretical stages can be achieved just by increasing the number of trays. One disadvantage of conventional sieve trays is that their capacity is decreased when scaled to large diameters. This is due to long, continuous-phase flow paths leading to increased cross-flow velocity and associated premature flooding. Control of the main operating interface can be difficult because of the fluctuating coalesced layer heights with each tray. Other factors favouring tray use include:
— Applications that require a large number of theoretical trays (typically more than three to five)
— Fouling applications. Trays can be more easily cleaned and serviced compared to packing. Trays can be designed with rag eliminator pipes
— Sieve tray extractors can operate at phase flow ratios very different from unity while maintaining good mass transfer efficiency, especially for low interfacial tension systems
— The cost of trays is usually lower per unit vessel volume compared to packing
— Convenient revamping of rotating disc contactors (RDCs) with low interfacial tensions.

Figure 2 (Sulzer Chemtech US data) illustrates that trays and (random) packing are generally applied across the same loading range, but that trays are more often used with dispersed-to-continuous-phase ratios larger than 10. Random packing applications with phase velocity ratios greater than 10 ([Vd/Vc] >10) are typically limited to two or less theoretical stages.

High-performance sieve trays
High-performance sieve trays have been developed to address the shortcomings of conventional trays when scaled up to large column diameters. This has resulted in the use of multiple up/downcomers with short flow paths, and thus increased capacity and efficiency, such as the Shell HiFi Extraction tray (Figure 3), which scales up well since the short flow path can be maintained simply by adding more up/downcomers. As a result, the hydraulic correlations, which were developed for short flow-path trays, are still applicable.

Choice of dispersed phase
The choice of dispersed phase can have a significant impact on the efficiency and capacity of the extractor. Normally, the phase with the higher volumetric flow rate is dispersed to maximise the interfacial area and thus the mass transfer efficiency. If capacity is more important than efficiency, the converse can be applied. There are other factors affecting this decision such as phase ratios, inventory of a hazardous or expensive material, direction of mass transfer, allowable solvent losses, wetting considerations, continuous-phase density gradients and the behaviour of a rag layer.

Physical properties
The system behaviour related to physical properties is also critical, as these properties can change significantly as extraction proceeds. Examples are interfacial tension (IFT), density difference and viscosity. The IFT is one of the most important properties for design, since it affects drop formation and coalescence. The estimation of the IFT under actual process conditions is extremely difficult due to the dynamic conditions related to mass transfer. Figure 4 illustrates the results of a dynamic measurement of IFT between a C6 range pygas hydrocarbon stream and the fat solvent stream (sulfolane) exiting the bottom of a benzene extractor. The full range of IFT values should be considered during the design of the lower trays in the extractor. Measuring the IFT between the raffinate and lean solvent would be more applicable to the design of the upper trays.

Although there are predictive methods available (Jufu et al, 1986), it is strongly recommended that IFT values covering the range of conditions expected in the extraction tower are measured. If actual samples and measurements cannot be performed, a sensitivity analysis should be done using a design range for the IFT, evaluating different points within this range.

Flooding mechanisms
The hydraulic design of sieve tray extractors has seen somewhat of a renaissance in the last two decades due in a large part to the work performed by the Separations Research Program (SRP) at the University of Texas in Austin (Seibert and Fair, 1993). This work has placed reliable design methodologies within reach of practitioners with no access to in-house design methods. At its heart lies the calculation of the dispersed-phase hold-up as a function of the superficial-phase velocities and the slip velocity (the speed of the dispersed-phase drop relative to the continuous phase). The slip velocity calculation requires knowledge of the mean drop size, which in turn is mostly a function of the system physical properties previously mentioned. A number of flooding mechanisms have been identified for tray extractors. These are:

— Excessive dispersed phase hold-up not related to problems with drop coalescence. This would represent the ultimate capacity, which would approach that of an empty or spray extractor

— Phase inversion (or dispersed-phase hold-up flooding) occurs when the dispersed-phase hold-up increases to a point where the droplets coalesce. As a result, there is competition for the continuous phase, interface-level control becomes erratic and the mass transfer efficiency is greatly reduced due to a loss of interfacial area. This can be due to increased dispersed-phase and/or continuous-phase flow or due to the dynamic action of the interface-level control system

— Large height of the coalesced layer (this is the analog of downcomer backup flooding in distillation). The height of the coalesced layer beneath the tray (light-phase dispersed) depends on the pressure drop through the sieve holes and downcomer, as well as the density difference. Flooding is defined as the point where the coalesced layer height equals the height of the downcomer

—  Inadequate coalescence of the dispersed phase (coalescence flooding) at the interface can cause entrainment of the continuous phase and limit the capacity of the extractor if the downstream equipment cannot process the entrained material. Work performed by the SRP5 determined that significant continuous-phase entrainment could occur with superficial dispersed-phase velocities above a critical value (> approx 1 cm/s) and for columns with high dispersed-to-continuous-phase ratios 
(> approx. 15:1 dispersed-to-continuous-phase ratio).

Efficiency
Predicting mass transfer efficiency in extraction is much more challenging than hydraulic capacity. It requires knowledge of the interfacial area, slope of the equilibrium curve and molecular diffusivities, and even then is only an approximation. Predictive methods are available (Seibert and Fair, 1993 and Treybal, 1963), which should be limited to preliminary designs or experimental preparation. Many industrial extractors are designed using knowledge gained from similar operating towers or pilot-scale test columns. Typical efficiencies for conventional sieve tray columns are in the 15–40% range.
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