Gas-liquid equipment

Figure 11-4 Self-inducing gas-liquid equipment. (Part from Middleton, 1997 reproduced by permission of Butterworth-Heinemann.)...

Fixed-bed reactors in the form of gas absorption equipment are used commonly for noncatalytic gas-liquid reactions. Here the packed bed serves only to give good contact between the gas and liquid. Both cocurrent and countercurrent operations are used. Countercurrent operation gives the highest reaction rates. Cocurrent operation is preferred if a short liquid residence time is required. 

Heterogeneous reactions of industrial significance occur between all combinations of gas, liquid, and solid phases. The solids may be inert or reac tive or catalysts in granular form. Some noncatalytic examples are listed in Table 7-11, and processes with solid catalysts are listed under Catalysis in Sec. 23. Equipment and operating conditions of heterogeneous processes are covered at some length in Sec. 23 only some highlights will be pointed out here. 

Due to the nature of batch operations, transferring and charging of process materials is a common activity. This can entail gas, liquids, and/or solids handling via open equipment. This may include pumping of liquids from drums or dumping of solids from other containers into an open vessel, shoveling material into a dryer, or making temporary connections such as at hose stations. 

Tlie rates of gas-liquid reaetions are surfaee area dependent. Henee in tlie spontaneous eombustion of oil impregnating fibrous thermal insulation on hot equipment, oxidation is faeilitated by the large exposed surfaee area and, sinee the dissipation of heat is restrieted, the temperature ean rise until the oil ignites spontaneously. 

Stressed, such as heat-affected zones near welds, in areas of high acid-gas concentration, or at a hot gas-liquid interface. Therefore, stress-relieving all equipment after manufacturing is necessary to reduce corrosion, and special metallurgy in specific areas such as the still overhead or the reboiler tubes may be required. 

Chlorooctane (14 9 g, 0 I mol), potassium fluoride dihydrate (47 g, 0 5 mol), hexadecyltributylphosphonium bromide (5 1 g, 0 01 mol), and water (30 mL) are mixed in an autoclave equipped with a magnetic stirrer and heated to 160 °C (bath temperature) for 7 h After this time gas-liquid chromatographic analysis (10% Carbowax 20M on Chromosorb) shows a 95% conversion to a mixture of 1-fluorooctane (82%), octenes (6%), and 1-octanol (7%) The organic layer is separated, washed with water, washed with concentrated sulfuric acid, washed once again with water, dried over calcium chloride, and distilled to give 10 g (77%) of ] -fluorooctane... 

Packed towers are used as contacting equipment for gas-liquid and liquid-liquid systems. Figures -1A and B present a cross-section of a typical unit. The shell is usually q lindrical, although square wooden, light metal, or reinforced plastic towers are used. The basic unit consists of ... 

Figure 9-44. Gas-liquid hold-up data for ceramic rings and saddles. Used by permission of Leva, M. Tbwer Packings and Packed Tower Design, 2nd ed., U.S. Stoneware Co. (now, Norton Chemical Process Equipment Corp.) (1953).

Later publications have been concerned with mass transfer in systems containing no suspended solids. Calderbank measured and correlated gas-liquid interfacial areas (Cl), and evaluated the gas and liquid mass-transfer coefficients for gas-liquid contacting equipment with and without mechanical agitation (C2). It was found that gas film resistance was negligible compared to liquid film resistance, and that the latter was largely independent of bubble size and bubble velocity. He concluded that the effect of mechanical agitation on absorber performance is due to an increase of interfacial gas-liquid area corresponding to a decrease of bubble size. 

Efficient contact is produced between the phases in agitated gas-liquid contactors and, therefore, this type of equipment can also be useful for those absorption and stripping operations for which conventional plate or packed towers may not be suited. It may also be useful where the operation involves the contact of three phases—say, gas, liquid, and suspended solids. The latter application could be represented by the low-pressure polymerization of ethylene with solid catalysts (F5). 

Gal-Or (G4) has recently reported bubble-size distribution data in air-water dispersions. The equipment used to evaluate the bubble-size distribution is a new type of multistage gas-liquid contactor without pressure drop in each stage, in which the gas is drawn in from the bottom of the vessel. Typical bubble-size—cumulative-volume data are given in Fig. 2.f The data show that for 99% of the bubbles, 0.1 < 1.4 mm. The surface mean radius a32... 

Mixers for gas-liquid contacting are often equipped with multiple impellers on a single shaft with the gas injected below the lowest impeller. Large commercial systems may use relatively deep cylindrical vessels. The usual ratio of liquid-depth to vessel-diameter lies between 1 and 3 (R3), and the maximum recommended is 4 (P3). In such deep systems, the question arises as to whether the use of multiple impellers would provide more effective contact than a single impeller. It must be kept in mind that if the impellers are spaced too widely the result may be ineffective agitation between the fields of action of the impellers. On the other hand, if they are too close, interference may occur between the flow streams from the adjacent impellers. 

The purity of 1 and 2 is assessed by analytical gas-liquid chromatography (GC) on a Hewlett-Packard 5890 gas chromatograph equipped with a flame-ionization detector and fitted with a 50 m x 0.2 mm HP-5 fused silica glass capillary column using linear temperature programming from an initial temperature of 150°C for 5 min to a final temperature of 200°C for 10 min at a rate of 5°C/min. 

Consideration will now be given to the various flow regimes which may exist and how they may be represented on a Flow Pattern Map to the calculation and prediction of hold-up of the two phases during flow and to the calculation of pressure gradients for gas-liquid flow in pipes. In addition, when gas-liquid mixtures flow at high velocities serious erosion problems can arise and it is necessary for the designer to restrict flow velocities to avoid serious damage to equipment. 

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