Equipment packed columns

Selection of Equipment Packed columns usually are chosen for very corrosive materials, for liquids that foam badly, for either small-or large-diameter towers involving veiy low allowable pressure drops, and for small-scale operations requiring diameters of less than 0.6 m (2 ft). The type of packing is selected on the basis of resistance to corrosion, mechanical strength, capacity for handling the required flows, mass-transfer efficiency, and cost. Economic factors are discussed later in this sec tion. 

Reactions involving gaseous and liquid reactants are carried out in various types of equipment. Packed columns, spray columns and bubble columns, as well as agitated tanks are all used (Fig. 2). Trickle-bed reactors are widely used in the petroleum industry for hydrodesulphurisation and related processes. In this type of reactor, liquid and gas both flow down through a bed of catalyst particles. The liquid flows around the particles as a thin film, thereby keeping the liquid residence time short and reducing undesirable side reactions. 

In this section, the main types of absorption equipment (packed columns and tray columns) are described, and... 

Time, Cost, and Equipment Analysis time can vary from several minutes for samples containing only a few constituents to more than an hour for more complex samples. Preliminary sample preparation may substantially increase the analysis time. Instrumentation for gas chromatography ranges in price from inexpensive (a few thousand dollars) to expensive (more than 50,000). The more expensive models are equipped for capillary columns and include a variety of injection options and more sophisticated detectors, such as a mass spectrometer. Packed columns typically cost 50- 200, and the cost of a capillary column is typically 200- 1000. 

Graham, R. C. Robertson, J. K. Analysis of Trihalomethanes in Soft Drinks, /. Chem. Educ. 1988, 65, 735-737. Trihalomethanes are extracted from soft drinks using a liquid-liquid extraction with pentane. Samples are analyzed using a packed column containing 20% OV-101 on 80/100 mesh Gaschrom Q equipped with an electron capture detector. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

Of the three categories, the packed column is by far the most commonly used for the absorption of gaseous pollutants. Miscellaneous gas-absorption equipment could include acid gas scrubbers that are commonly classified as either wet or diy. In wet scrubber systems, the absorption tower uses a hme-based sorbent liquor that reacts with the acid gases to form a wet/solid by-product. Diy scrubbers can be grouped into three catagories (1) spray diyers (2) circulating spray diyers and (3) dry injection. Each of these systems yields a diy product that can be captured with a fabric filter baghouse downstream and... 

The efficiencies which may be obtained can consequently be calculated by simple stoichiometry from the equilibrium data. In the ease of countercurrent-packed columns, the solute can theoretically be completely extracted, but equilibrium is not always reached because of the poorer contact between the phases. The rate of solute transfer between phases governs the operation, and the analytical treatment of the performance of such equipment follows closely the methods employed for gas absorption. In the ease of two immiscible liquids, the equilibrium concentrations of a third component in each of the two phases are ordinarily related as follows ... 

Direct contact heal exchangers should be considered whenever the process stream and the coolant are compatible. The equipment used is basically simple and cheap and is suitable for use with heavily fouling fluids. For liquids containing solids, spray chambers, spray columns, and plate and packed columns are used. 

The gas chromatograph is better to be equipped both with a thermal conductivity detector (TCD) and with a flame ionization detector (FID). The latter is extremely useful in the analysis of organic substances at low concentrations. Packed columns are normally used, although capillary columns offer certain advantages in the analysis of a variety of products. Some of the major companies that supply gas chromatographs are ... 

Table 11.4 lists reactors used for systems with two fluid phases. The gas-liquid case is typical, but most of these reactors can be used for liquid-liquid systems as well. Stirred tanks and packed columns are also used for three-phase systems where the third phase is a catal5hic solid. The equipment listed in Table 11.4 is also used for separation processes, but our interest is on reactions and on steady-state, continuous flow. 

Gas chromatograph for fused-silica capillary or packed columns, equipped with a flame photometric detector (with sulfur filter), Hewlett-Packard, Carlo Erba, or equivalent... 

Few laboratories have developed their own facilities for packing conventional and small bore columns, preferring to rely on commercial manufacturers for their needs. This is understandable since packed columns containing any common phase can be obtained at an acceptable cost and with a guarantee of acceptable performance and stability. Column packing generally requires access to equipment not readily available in all laboratories and... 

Implementation of SFC has initially been hampered by instrumental problems, such as back-pressure regulation, need for syringe pumps, consistent flow-rates, pressure and density gradient control, modifier gradient elution, small volume injection (nL), poor reproducibility of injection, and miniaturised detection. These difficulties, which limited sensitivity, precision or reproducibility in industrial applications, were eventually overcome. Because instrumentation for SFC is quite complex and expensive, the technique is still not widely accepted. At the present time few SFC instrument manufacturers are active. Berger and Wilson [239] have described packed SFC instrumentation equipped with FID, UV/VIS and NPD, which can also be employed for open-tubular SFC in a pressure-control mode. Column technology has been largely borrowed from GC (for the open-tubular format) or from HPLC (for the packed format). Open-tubular coated capillaries (50-100 irn i.d.), packed capillaries (100-500 p,m i.d.), and packed columns (1 -4.6 mm i.d.) have been used for SFC (Table 4.27). 

It is doubtful if any design is entirely novel. The antecedence of most designs can usually be easily traced. The first motor cars were clearly horse-drawn carriages without the horse and the development of the design of the modern car can be traced step by step from these early prototypes. In the chemical industry, modem distillation processes have developed from the ancient stills used for rectification of spirits and the packed columns used for gas absorption have developed from primitive, brushwood-packed towers. So, it is not often that a process designer is faced with the task of producing a design for a completely novel process or piece of equipment. 

Preliminary design work has been done on a process to recover a valuable product from an effluent gas stream. The gas will be scrubbed with a solvent in a packed column the recovered product and solvent separated by distillation and the solvent cooled and recycled. The major items of equipment that will be required are detailed below. 

The chemical engineering undergraduate spends most of his time sizing equipment. Usually in the problems assigned the type of equipment to be used is specified. For a distillation column the student would be told whether it is a bubble cap, a sieve plate, a valve tray, a packed column, or something else, and then asked to size it for a desired separation. In other cases he would be given the size of the specific equipment and asked to determine what the output would be for a given input. 

When a piece of equipment does not respond properly, sometimes the reason may be totally unexpected. In one packed column the pressure drop was many times the design estimate. After many things had been checked, the column was opened and it was found that the workmen had stuffed the boxes in which the packing had been received, into the column along with the packing. The predicted pressure drop was obtained after the boxes were removed. 

Inlet and outlet gases were analyzed online by a Micro GC equipped with four packed columns. The liquid organic and aqueous products were analyzed using an HP 5890 GC with capillary column DB-5 and an HP 5790 GC with Porapak Q packed column, respectively. The reactor wax withdrawn periodically was analyzed by a high-temperature HP5890 GC employing an alumina-clad column. 

In designing industrial scale packed columns a balance must be made between the capital cost of the column and ancillary equipment on the one side, and the running costs on the other. Generally, reducing the diameter of the column will reduce the capital cost though increase the cost of pumping the gas through the column due to the increased pressure drop. 

From the analysis given already of the diffusional nature of absorption, one of the outstanding requirements is to provide as large an interfacial area of contact as possible between the phases. For this purpose, columns similar to those used for distillation are suitable. However, whereas distillation columns are usually tall and thin absorption columns are more likely to be short and fat. In addition, equipment may be used in which gas is passed into a liquid which is agitated by a stirrer. A few special forms of units have also been used, although it is the packed column which is most frequently used for gas absorption applications. 

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