Packed columns typical applications

Column reactors are the second most popular reactors in the fine chemistry sector. They are mainly dedicated reactors adjusted for a particular process although in many cases column reactors can easily be adapted for another process. Cocurrently operated bubble (possibly packed) columns with upflow of both phases and trickle-bed reactors with downflow are widely used. The diameter of column reactors varies from tens of centimetres to metres, while their height ranges from two metres up to twenty metres. Larger column reactors also have been designed and operated in bulk chemicals plants. The typical catalyst particle size ranges from 1.5 mm (in trickle-bed reactors) to 10 mm (in countercurrent columns) depending on the particular application. The temperature and pressure are limited only by the material of construction and corrosivity of the reaction mixture. 

Packed columns have been in use since the inception of GC and today are used in about 10% of applications, especially in the analysis of very small molecules such as fixed gases and solvents. The dimensions of packed columns are limited by the inlet pressure and fittings of the GC. Typically, packed columns are 6-10 ft long and 1/4 or 1/8-in. 

Current EPA analytical methods do not allow for the complete speciation of the various hydrocarbon compounds. EPA Methods 418.1 and 8015 provide the total amount of petroleum hydrocarbons present. However, only concentrations within a limited hydrocarbon range are applicable to those particular methods. Volatile compounds are usually lost, and samples are typically quantitated against a known hydrocarbon mixture and not the specific hydrocarbon compounds of concern or the petroleum product released. By conducting EPA Method 8015 (Modified) using a gas chromatograph fitted with a capillary column instead of the standard, hand-packed column, additional separation of various fuel-ranged hydrocarbons can be achieved. 

Gas-solid chromatography is performed with both packed and open tubular columns. For the latter, a thin layer of the adsorbent is affixed to the inner walls of the capillary. Such columns are sometimes called porous-layer open tubular columns, or PLOT columns. Figure 31-16 shows a typical application of a PLOT column. 

Figure 32-11 Typical applications of bonded-phase chromatography, (a) Soft-drink additives. Column 4.6 X 250 mm packed with polar (nitrile) bonded-phase packing. Isocratic elution with 6% HOAc/94% H,0. Flow rate 1.0 mL/min. (Courtesy of BTR Separations, a DuPont ConAgra affiliate.) (b) Organophosphate insecticides. Column 4.5 X 250 mm packed with 5-/xm Cj bonded-phase particles. Gradient elution 67% CH30H/33% H.O to 80% CH,OH/20% H2O. Flow rate 2 mL/min. Both used 254-nm UV detectors.

The transfer of mass within a fluid mixture or across a phase boundary is a process that plays a major role in various engineering and physiological applications. Typical operations where mass transfer is the dominant step are falling film evaporation and reaction, total and partial condensation, distillation and absorption in packed columns, liquid-liquid extraction, multiphase reactors, membrane separation, etc. The various mass transfer processes are classified according to equilibrium separation processes and rate-governed separation processes. Fig. 1 lists some of the prominent mass transfer operations showing the physical or chemical principle upon which the processes are based. 

Monolithic columns are an interesting recent alternative to conventional packed columns. Such columns are created by in situ polymerization from liquid precursors, usually organic polymer- or silica-based. When prepared, monolithic columns have the form of cylindrical rods. They are much more porous than typical packed particle beds, therefore they present significantly lower resistance to mobile phase flow. Consequently, these can be operated at much higher flow rates than conventional columns. The main application of monolithic columns is in high-throughput analysis. 

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