Vapor-liquid separators Flash tanks

The attainment of optimum rate at relatively low [H2O] is a significant benefit for the iridium system, since it results in less costly product purification. A typical configuration for an iridium-catalyzed methanol carbonylation plant is shown in Figure 2. The feedstocks (MeOH and CO) are fed to the reactor vessel on a continuous basis. In the initial product separation step, the reaction mixture is passed from the reactor into a flash tank where the pressure is reduced to induce vaporization of most of the volatiles. The catalyst remains dissolved in the liquid phase and is recycled back to the reactor vessel. The vapor from the flash tank is directed into a distillation train, which removes methyl iodide, water, and heavier byproducts (e.g., propionic acid) from the acetic acid product. At the relatively high water levels used in the rhodium-catalyzed Monsanto process, three distillation columns are typically required. In the Cativa process, a lower water concentration means that the necessary product purification can be achieved with only two columns. 

The reactor system may consist of a number of reactors which can be continuous stirred tank reactors, plug flow reactors, or any representation between the two above extremes, and they may operate isothermally, adiabatically or nonisothermally. The separation system depending on the reactor system effluent may involve only liquid separation, only vapor separation or both liquid and vapor separation schemes. The liquid separation scheme may include flash units, distillation columns or trains of distillation columns, extraction units, or crystallization units. If distillation is employed, then we may have simple sharp columns, nonsharp columns, or even single complex distillation columns and complex column sequences. Also, depending on the reactor effluent characteristics, extractive distillation, azeotropic distillation, or reactive distillation may be employed. The vapor separation scheme may involve absorption columns, adsorption units,... 

Liquid propylene, gaseous carbon monoxide and hydrogen, and a soluble cobalt catalyst are fed to a high-pressure catalytic reactor. The reactor effluent goes to a flash tank, where all of the solution constituents are vaporized except the catalyst, which is recycled to the reactor. The reaction products are separated from unconsumed reactants in a multiple-unit process, and the product stream, which contains both butyraldehyde and /i-butanol, is subjected to additional hydrogenation with excess hydrogen, converting all of the butyraldehyde to butanol. 

The separation of a volatile component from a liquid process stream can be achieved by way offlash distillation. It is referred to as a flash since the more volatile component of a gas mixture rapidly vaporizes upon entering a tank or drum that is at a lower pressure and/or a higher temperature than the incoming feed. If the feed is considered to be cold, a pump and heater may be required to elevate the pressure and temperature, respectively, to achieve an effective flash (see Figure 26). As the feed enters the tank/drum, it may impinge against the wall or an internal deflector plate, which would promote liquid-vapor separation of the feed mixture. 

The liquids that are separated from the gas stream in the first separator may be flowed directly to a tank or may be stabilized in some fashion. As was discussed in Chapter 2 of Volume 1, these liquids contain a large percentage of methane and ethane, which will flash to gas in the tank. This lowers the partial pressure of all other components in the tank and increases their tendency to flash to vapors. The process of increasing the amount of intermediate (C3 to C5) and heavy (C + ) components in the liquid phase is called stabilization. In a gas field this process is called condensate stabilization and in an oil field it is called crude stabilization. 

In many areas of the unit, separation and processing vessels were operated at 3S psig (343.3 kPa). Hydrocarbons discharged from these vessels were flashed to stock tanks, and all vapors were vented to the atmosphere. Following consolidation of these conventional tank batteries, rich vapors were recovered by vapor-recovery equipment at the central treating facility. Total recovery over the life of the unit is projected to be 133,000 bbl (21 144 m3) of liquids and 262 MMcf (7.4 x I06m3) of gas. 

An equimolar liquid mixture of n-pentane and n-hexane at 80 C and 5.00 atm is fed into a flash evaporator at a rate of 100.0 moVs. When the feed is exposed to the reduced pressure in the evaporator, a substantial amount is vaporized. The temperature in the tank is maintained at 65 C by adding heat. The vapor and liquid phases, which are in equilibrium with each other, are separated and discharged... 

Flash drums are often used as liquid surge tanks in addition to separating liquid and vapor. The design procedure for this case is discussed by Watkins (196Z) for petrochemical applications. The height of the drum above the centerline of the feed nozzle, h, should be 36 in. plus one-half the diameter of the feed line (see Figure 2-14i. The minimum of this distance is 48 in. 

To calculate the required amount of heat, estimate a temperature at the low-pressure separator. The higher the operating pressure of the separator, the higher the temperature necessary to lower the stock-tank s vapor pressure will he. Using the assumed temperature and pressure, run a flash calculation to determine the composition of the liquid. This liquid must be cooled before leaving the separator to go to the stock-tank. 

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