Catch tank

Types of Equipment The three most commonly used types of equipment for handling emergency relief device effluents are blowdown drums (also called knockout drums or catch tanks), cyclone vapor-liquid separators, and quench tanks (also called passive scruh-hers). These are described as follows. 

Horizontal Blowdown Drum/Catch Tank This type of drum, shown in Fig. 26-16, combines both the vapor-liquid separation and holdup functions in one vessel. Horizontal drums are commonly used where space is plentiful, such as in petroleum refineries and petrochemical plants. The two-phase mixture usually enters at one end and the vapor exits at the other end. For two-phase streams with very high vapor flow rates, inlets may be provided at each end, with the vapor outlet at the center of the drum, thus minimizing vapor velocities at the inlet and aiding vapor-hquid separation. 

Cyclone Separator with Separate Catch Tank This type of blowdown system, shown in Fig. 26-17 and 26-18, is frequently used in chemical plants where plot pan space is hmited. The cyclone performs the vapor-liquid separation, while the catch tank accumulates the hquid from the cyclone. This arrangement allows location of the cyclone knockout drum close to the reactor so that the length of the relief device discharge hne can be minimized. The cyclone nas internals, vital to its proper operation, which will be discussed in the following sections. 

Cyclone Separator with Integral Catch Tank This type of containment system, depicted in Fig. 26-19, is similar to the ore-mentioned type, except that the knockout drum and catch tank are combined in one vessel shell. This design is used when the vapor rate is quite high so that the knockout drum diameter is large. 

Multireactor Knockout Drum/Catch Tank This interesting system, depic ted in Fig. 26-22, is sometimes used as the containment vessel for a series of closely spaced reac tors (Speechly et al., Trinci-ples of Total Containment System Design, presented at I. Chem. E Noith West Branch Meeting, 1979). By locating the drum as shown in Fig. 26-22, minimum-length vent lines can be routed direc tly to the vessel without any bends. 

FIG. 26-19 Cyclone separator with integral catch tank. 

FIG. 26-22 Multireactor knockout (K-O) drum/catch tank a) plan view of reactors connected to horizontal containment vessel (h) back-to-back bursting disc assembly (c) elevation of self-supporting vessel (d) elevation of horizontal vessel on roof of building (e) elevation of horizontal vessel on side of building. 

Horizontal Blowdown Drum (See Fig. 26-16.) The two main criteria used in sizing horizontal blowdown drums or catch tanks are as follows. 

Another equation for quick sizing of horizontal knockout drums/ catch tanks is presented by Tan Hydroca rhon Processing, October 1967, p. 149). He recommends the following equation for calculating the drum diameter ... 

Cyclone Separator with Separate Catch Tank (See Figs. 26-17 and 26-18.) The sizing of a cyclone knockout drum for emergency relief systems is somewhat different from sizing a cyclone separator for normal process sei vice for the following reasons ... 

Cyclone Separator with Integral Catch Tank (See Fig. 26-19.) The diameter of the knockout drum is calculated by the criteria given in the preceding section and Fig. 26-18. Since the liquid is also to be retained in the vessel, extend the shell height below the normal bottom tangent line to increase the total volume by an amount equal to the volume of the hquid carried over. 

There are usually several reactors hnked to a single catch tank. To ensure that rupture of a disk on one reac tor does not affect the others, each reactor is fitted with a double-rupture-disk assembly. The use of double rupture disks in this apphcation requires installation of a leak detection device in the space between the two disks, which... 

Design of vessel and vent line pipe supports is very important because very large forces can be encountered as soon as venting begins. Figure 4 shows the equations and nomenclature to calculate forces on pipe bends. The authors have heard of situations where vent line bends have been straightened, lines broken off, or vent catch tanks knocked off their foundations by excessive forces. For bends, the transient effects of the initial shock wave, the transition from vapor flow to two-phase flow, and steady state conditions should be considered. Transient conditions, however, are likely to be so rapid as to not have enough dura-... 

Knockout drums are sometimes called catch tanks or blowdown drums. As illustrated in Figure 8-12, this horizontal knockout drum system serves as a vapor-liquid separator as well as... 

Figure 8-13 Tangential inlet knockout drum with separate liquid catch tank.

Quench pools/catch tanks and quench towers... 

Gravity Separators Three types of gravity separators are commonly used in the chemical process industries (CPI) horizontal blowdown drum/catch tank, vertical blowdown drum/catch tank, and multireactor knockout drum/catch tank. 

FIG. 23-50 Feed distributor devices for horizontal blowdown drums/catch tanks. (Guidelines for Pressure Relief and Effluent Handling Systems, Center for Chemical Process Safely (CCPS) of the American Institute of Chemical Engineers (AlChE) copyright 1988 AlChE and reproduced with permission.). 

Vertical blowdown drum/catch tank This type of drum, shown in Fig. 23-51, performs the same function and operates on similar princi-les as horizontal separators. These separators are usually used where orizontal space is limited. The two-phase mixture enters the vessel via a nozzle on the vertical shell and is distributed by an inlet baffle chamber. 

Emergency Cyclone Separators Two types of emergency cyclone separators are commonly used, those with a separate catch tank and those with a catch tank integral with the cyclone section. 

Quench Pools/Catch Tanks and Quench Towers Two types of quench systems are commonly used the quench pool/catch tank (also called passive scrubber) and quench tower. 

Quench pool/catch tank This type of system, as shown in Fig. 23-55, is used to condense, cool, react with, and/or collect a mixture of liquid and vapors discharging from a relief device by passing them through a pool of liquid in a vessel. Feed vapor and liquid (if present) are sparged into the pool of cool liquid, where the vapors are condensed and the liquid is cooled. If the feed materials are miscible with the pool liquid, they mix with and are diluted by the pool liquid if not, the condensate, feed liquid, and pool liquid separate into layers after the emergency relief event is over. The condensed vapors, feed liquid, and quench liquid are contained in the vessel until they are sent to final disposal. 

Equipment Selection Criteria and Guidelines A number of factors should be considered to determine when to select which type ofvapor/gas/solid-liquid separator (blowdown drum or cyclone separator) to handle a multiphase stream from a relief device, and which final control or destruction equipment (emergency flare, emergency scrubber, or quench pool/catch tank) should be selected. These factors include the plot plan space available, the operating limitations of each type, and the physicochemical properties of the stream. 

Gravity Separators (Horizontal and Vertical Drums/Catch Tanks)... 

Vertical Blowdown Drum/Catch Tank (See Fig. 23-51.) Some of the basic equations used for sizing horizontal separators can be applied to vertical separators however, the superficial vapor velocity for vertical separators is based on the total vessel cross-sectional area, and the vapor velocity must be less than the terminal settling velocity for the droplet size selected as the basis of design. 

Multireactor Knockout Drum/Catch Tank The area needed for vapor disengaging is calculated by the equations given earlier in the section on horizontal blowdown drums. The diameter and length (or height) are determined by considering a number of factors ... 

A chemical reaction could continue in the knockout drum/catch tank. 

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