Example 7-8 Liquid Overpressure

Determine the rupture disk size required to relieve the following operating condition  

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Figure 7-34. Safety relief valve specification for liquid overpressure example.

Note that the recommended value for p is not always conservative. In some cases, heat input may be so high that the safety valve cannot vent all the generated vapor. In such cases, the internal pressure will rise until the bursting overpressure is reached, which may be much higher than the vessel s design pressure. For example, Droste and Schoen (1988) describe an experiment in which an LPG tank failed at 39 bar, or 2.5 times the opening pressure of its safety valve. Note also that this method assumes that the fluid is in thermodynamic equilibrium yet, in practice, stratification of liquid and vapor will occur (Moodie et al. 1988). 

A simphstic view of a FIA analytical system is outfined in Fig. 2.19. The liquid flow can be obtained in a number of ways, most commonly by using a peristaltic pump. In addition, gravity-feed systems, overpressure systems on liquid vessels and simple and double piston pumps normally associated with HPLC systems, are often used. The interrelation of the pumping system and the bore size of the transport tubing to a great extent modify the theoretical considerations involved and the operation of the FIA regime. In common with CFA, the minimization of dead volumes—in detector cells and between T pieces for example—is particularly important. Injections of the sample into... 

Filtration separates components according to their size. Efficiency depends on the shape and compressibility of the particles, the viscosity of the liquid phase and the driving force, which is the pressure created by overpressure or by vacuum. Filtration can be performed either as dead-end filtration, where the feed stream flows perpendicular to the filter surface (Lee, 1989) or as tangential flow filtration, where the feed stream flows parallel to the filter and the filtrate diffuses across it. Examples of the former are the continuous rotaiy vacuum dram filter, where a rotaiy vacuum filter has a filter medium covering the surface of a rotating drum and the filtrate is drawn through the dram by an... 

A certain minimum pressure is required in PHSE to maintain the extractant in the liquid phase. Fortunately, the overpressure need not be too high. For example, 20 atm is sufficient to keep -hexane (atmospheric boiling point 68.7°C) in the liquid state at 209°C. The minimum pressure required can be estimated from straightforward equations as described by Hass and Newton [17],... 

The term explosion is best defined as a process that involves a sudden release of energy resulting in a rapid and significant buildup of overpressure. Explosions can be categorized into physical/mechanical and chemical explosions. For example, an explosion caused by a sudden release of compressed gas is a physical explosion. A chemical explosion is caused by a chemical reaction(s), which could be combustion, exothermic decomposition or exothermic reaction. Chemical explosions can occur in gas, liquid or solid phase. Chemical explosions that occur in liquid and solid phases are sometimes called condensed phase explosions. Explosive explosions fall in this category. 

In this section we are interested in predicting the uncontrolled energy release of an explosion that occurs without a chemical reaction. That is, we are interested in energy released from an explosion that results from the bursting of an overpressurized tank, or the rapid depressurization of a hot liquid that leads to its partially boiling (flashing) to a vapor-liquid mixture. We do not consider chemical explosions, for example, the detonation of TNT or a natural gas explosion, both of which involve multiple chemical reactions and require the estimate of properties of mixtures that we have not yet considered. Chemical explosions are considered in Chapter 13. 

With the aid of the columns and other apparatus described it is possible to separate liquid mixtures continuously at normal and reduced pressures as well as at small overpressures (chap. 5.4.5) as sharply a.s in batch distillation. This is clear from the distillation curves of the products obtained in the continuous distillation of a mixture of crude fatty acids in the C4 to Ci range (Fig. 168). A further example shows that by the use of control devices it was possible to separate a mixture of phenols into its main components with great constancy. Fig. 169 illustrates the results of the first separation in this sequence, which was made between the ortho- and Twcfa/pom-cresol fractions. 

For process equipment, designers need to specify necessary safety features and the tests for meeting requirements. For process equipment, there should be fail-safe features. Fire protection, overpressure, excess heat, runaway reactions, dust control, exhaust ventilation, dangers of flammable liquids, leaks, sensing devices to report status are all examples of important safety features. Designers and purchasers need to consider access for setup, maintenance and cleaning. There may be a need for access by stairs, fixed ladders or platforms as part of large equipment. 

If the vaporization intensity is independent of the thermophysical parameters of the LPC gas and is determined only by the initial temperature of the liquid, one may observe the dependence of the shock intensity on the nature of a test gas in the LPC. For example, as is obvious from q. (8), the use helium instead of air at the same temperature T leads to a decrease in the shock overpressure Ap2 by about 2.5 times. 

An example of a run which is typical of those made with the prototype system is described to illustrate the use of the experimental data. This run used oxygen gas to overpressurize liquid oxygen and included a 14 minute hold period after completion of the pressurization. 

For example, an LNG mixture of 80 % methane and 20 % ethane has a boiling temperature of about 114 K, while the boil-off gas will contain more than 95 % methane with a condensing/boiling temperature of say 112 K at the purge-gas overpressure. The margin in temperature is close at 2 K, but acceptable provided the ethane content in the purge gas does not rise above that of the stored liquid mixture (see Fig. 5.4). 

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