Column internals structured packing

Simulations of an industrial scale column with structured packing have been reported by Taylor et al. (1992). They modeled a packed C4 splitter that had an internal diameter of about 2.5 m and five beds of structured packing with a total height of approximately 37 m as shown in Figure 14.33. The feed, which contains predominantly isobutane and n-butane... 

It is assumed that the mass tiansfer occurs in the layers on both sides of the phase boundary. According to the model, the mass transfer is caused by molecular diffusion. Therefore, a linear concentration profile is built up in the boundary layers. At the phase boundary itself, thermodynamic equilibrium is reached. More or less empirical correlations for the mass transfer coefficients /I and for the effective phase interface area are usually provided by the simulation program for different column internals (random packings, structured packings, and trays). The necessary diffusion coefficients can again be calculated according to Section 3.3.6. 

Some of the simplest designs are spray columns without internals and packed columns containing randomly filled and stacked packing elements, tube columns or columns with structured packing, see Fig. l-2a. 

The liquid distributor is the most important internal structure of a packed column. The distributor strongly influences packing efficiency. It must spread the liquid uniformly, resist plugging/fouling, provide free space for gas flow, and allow operating flexibility. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

The secondary structure, the mesopores, is similar to the internal structure of standard HPLC particles. This secondary structure provides the surface for retention. The standard pore size is in the order of 13 nm, resulting in a specific surface area of about 300 mVg. Due to the lower ratio of retentive structure to interstitial space, the retentivity of monoliths and the preparative loadability tends to be significantly lower than the retentivity and loadability of packed beds of 10-nm particles. Since the monolithic columns described here are made from silica, they can be derivatized in the same way and with the same technology as silica-based particles. Also, the useful pH range is the same as for silica-based particles. 

Figure 8 (a) Schematic of an RD column filled with catalytic internals CD TECH (1—catalytic balls, 2—feed, 3—distillate, 4—bottom product, 5—sieve tray) and (b) catalytic structured packing Sulzer Katapak-S. (Part a from Ref. 52.)... 

For a packed column, the principle requirements for the packing is that it should (1) provide a large surface area between the gas and liquid (2) promote uniform liquid distribution on the packing surface (3) promote uniform vapour gas flow across the column cross-section and (4) have an open structure to give a low resistance to gas flow. Many diverse types and shapes of packing have been developed to satisfy these requirements and details of the most recent ones can be found on the web sites of column internals vendors, e.g. Refs. 9 or 10. 

Most often, reactive absorption is carried out in packed or plate columns. The structure of such columns and different internals are shown schematically in Fig. 9.2. 

Furthermore, a CFD based description of single-phase and multi-phase flows in column internals is given in (Yin et ah, 2000 Yin et ah, 2002) for random packings, in (Petre et ah, 2003 Larachi et ah, 2003) for structured packings and in (Trubac et ah, 2001) for a structured catalytic packing. 

A novel family of column internals with enhanced mass transfer performance and relatively low pressure drop has been created in order to solve this problem. Most of these internals are made from corrugated sheets and have a clear structure. Therefore they are referred to as structured packings. 

Static extraction columns Spray column Baffle column Packed column (random and structured packing) Sieve tray column Deliver low to medium mass-transfer efficiency, simple construction (no internal moving parts), low capital cost, low operating and maintenance costs, best suited to systems with low to moderate interfacial tension, can handle high production rates Petrochemical Chemical Food... 

The hardware issues regarding RD columns focus on the column internals such as the packings (random or structured packing, Fig. 4), distillation trays, and downcomers. Typical hardware design information is given in the standard sources as for conventional distillation. Some of the important issues on hardware design aspects are discussed below.l ... 

Gorak (1991) conducted a number of distillation experiments with the system acetone(l)-methanol(2)-2-propanol(3)-water(4) in a column of 0.1-m internal diameter and height 0.8 m fitted with Sulzer BX structured packing. The characteristics of Sulzer BX packing are (Bravo et al., 1985). 

For distillation columns Add sufficient trays to account for disturbances and anticipated expansion of production. The expected turndown ratios affect the choice of internals in a distillation column with a large turndown ratio, suggesting the use of bubble caps a low turndown ratio may point to structured packings. At the same time, safety and hazard analysis indicates that we want a minimum of liquid holdup in the system. 

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