Flash Operations

Steam is the most common liquid that is flashed in process plants, but of course, it is not the only one as many processes utilize flash operations of pure com-... 

The Stratford Engineering Company furnished many alkylation reactors and were therefore interested in efficient mixing and heat transfer in this equipment. They subsequently developed a very efficient reactor, which makes part of the recycle by a flashing operation and is applicable to both hydrofluoric acid and sulfuric acid alkylation. This flashing differs from autorefrigeration in that the effluent stream is flashed after the reaction zone instead of in the reaction zone. 

Figure 9 Spectral power distribution of a Xenon flash operated in high-power (1) and low-power (2) modes. Source Reproduced with permission from the Xenon Corp.

The main variables associated with phase relationships include the overall composition, Z , temperature, pressure, liquid composition, X , vapor composition, F, vapor mole fraction, /, and heat transferred, Q. A process in which Z, and two other independent variables are set, and equilibrium separation of the phases is allowed to take place, is called a flash operation. A general flash operation is shown in Figure 2.4. A feed stream initially at conditions T, and P, is controlled so that its final conditions satisfy two specifications. The feed is of fixed rate and composition, F and Z . A heat duty, Q, may be added to or removed from the system as required. The feed is flashed to generate a vapor product with flow rate Ft r and a liquid product with flow rate F(1 -1 /), where / is the vapor mole fraction at flash conditions and P. In general, tj/ may be equal to zero or one or any value in between. The enthalpies of the vapor and liquid products are H2 and /Z2> respectively. The type of flash operation... 

The thermal effects of absorption and stripping are demonstrated in the following example using a single equilibrium stage, that is, a flash operation with two feeds. 

With a single equilibrium stage and no reflux, the separation power in differential distillation is obviously limited. It is the equivalent of a batch flash operation. Consequently, practical applications would include the separation of wide-boiling mixtures, with low expectations on the purity of the products. 

In order to reduce the amount of heat lost as a result of the condensation of the vaporized solvent, the temperature at which flashing operations are carried out is kept as low as possible. Thus, the flash drums to which the solvent containing product streams are applied operate to produce solvent vapor at about 100°C. Heat contained in these vapors is of such low quality that economic recovery is not practical. As a consequence, such heat is extracted from the solvent by air or water cooling, and is lost to the environment. 

If the feed is a mixture of liquid and vapor, q is the fraction that is liquid. Such a feed may be produced by an equilibrium flash operation, so q = — f, where / is the fraction of the original stream vaporized in the flash. 

Dynamic flash is a simple but very useful unit in dynamic simulation. Fig. 4.2 depicts the layout of a vapour-liquid separation. A multi-component feed of molar flow rate F with the composition z, is split in vapour V and liquid L, with the composition y-, and Xj, respectively. Optionally heat may be added or removed. Initially the flash operates at steady state. The problem is to study the dynamic response at various disturbances, as changes in throughput or composition. Modelling equations are presented below. 

For the adiabatic flash operation shown below, calculate ... 

The flowsheet in Fig. 4.2 shows a typical distillation system which may be used to describe the process reactions and main characteristics of the equipment used. In addition to dimethyl ether, methyl formiate, acetone and hydrocarbons up to about Cg, the low boilers in a broader sense include also that part of the gases dissolved in the raw methanol which is not expelled when this is flashed into an interim storage tank. This flashing operation is normally performed at pressures between 5-10 bar in order to keep methanol losses down, but frequently also in order to ensiure that the raw methanol can be transferred to the light ends column without using a pump. 

When a > 1 the released vapor generated under pressure contains a higher concentration of low-boiling components than the mixture introduced into the separator. In flash operation processes, therefore, a partial separation of the feed occurs in which the separator can at best only act as one theoretical separation stage. 

A heat balance over the separator gives the amount of distillate generated during the flash operation. 

For the hydrodealkylation process, the reactor effluent. Stream 9, is cooled and separated in a two-stage flash operation. The liquid. Stream 18, contains essentially benzene and toluene. The combined vapor stream, Streams 8 and 17, contain essentially methane and hydrogen. In Example 2.3. methods to separate the hydrogen in these two streams are considered and are used to screen potential changes in the recycle structure of the HDA process. 

All of the absorbed BTEX does not end up in the regenerator vapor stream a significant fraction may flash off in the flash tank. The magnitude of this fraction depends, of course, on the pressure and temperature of the flash operation. For the previous case, flashing at 80°F and 30 psia removes almost no benzene 350 F and 100 psia removes about 23% and 350°F and 30 psia removes about 74%. 

A flash operation occurs when a mixture is expanded through a valve from a region of relatively high pressure to a region of relatively low pressure. Because of the change in temperature and pressure, a phase separation can be induced. In this type of process, the temperature and pressure at the final state are known, and while the compositions of the equilibrium vapor and liquid phases are unknown, the composition of the fluid entering the valve is known. 

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