Steam flash drum

The heat and mass balances for the flash drum can be applied to determine the amount of flash steam based on the inlet and outlet conditions. 

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The steam flash drum is a device for steam recovery. Flash steam occurs at the drum where steam condensate or boiler blowdown experiences a drop in pressure causing some of the condensate or boiler blowdown to evaporate forming steam and thus produces steam at the lower pressure (Figure 15.11). For low-pressure condensate, flash steam is negligible and thus it is not worth to recover. However, for medium-and high-pressure condensate, it is important to recover flash steam. 

Figure 3.15b. Similar to the first arrangement but with the water kept at high pressure to prevent vaporisation. The high-pressure water is flashed to steam at lower pressure in a flash drum. This system would give more responsive control of the reactor temperature. 

Flash steam recovery might also feature. Condensate or blowdown is fed to the flash drum, as illustrated in Figure 23.26. A material balance gives ... 

The flash drum pressure will be known as this will be set by the pressure of the main into which the flash steam will be recovered. The flash steam and condensate outlet enthalpies are set by saturation conditions. 

The product stream from the reactor (Bl) was flash cooled using a flash drum (B3) to separate vapors from the liquid phase. The flash models available in ASPEN-Plus determine the thermal and phase conditions of a mixture with one or more inlet streams. A separator (B6) was employed to separate C02 and steam. The resulting recycle streams no. 10 and no. 9 were sent to B13 and B7, respectively. The liquid stream (no. 14) from the flash drum was sent to a Pneumapress filter (B10), where it was separated into filter cake (stream no. 13) and filtrate (streamno. 8). This separation was done to facilitate heat extraction from the product stream for heat exchanger Bll. 

Slurry is transferred from reactor to a stirred flash drum where it is mixed with steam and hot water (=60° C, 1.5 bar). Methyl chloride and unreacted monomers are flashed off overhead and recycled, whereas particles agglomerate as coarse crumbs, the size of which is controlled by addition of zinc stearate. The suspension is then stripped to eliminate traces of volatiles, and the rubber is separated by filtration, dewatered by extrusion, dried and sent to a finishing unit for baling, packaging, and weighing. 

Flash drum (Figure 23.5d). This is similar to the flash drum system analyzed in Examples 23.1 and 23.2 with one difference instead of steam heating (see Figure 23.1a) there is a water cooling system (Figure 23.5d). Therefore, following the same procedure as in Example 23.1 we conclude that there should be... 

Ethylene feed and catalyst solution enter the bottom of the cylindrical reactor where the reaction proceeds at 125-130 C (255-265 °F) and 8-9 atm. (100-115 psig). The reactor contains internal distributors to insure good vapor-liquid distribution. Ethylene conversion is 96.7< 7o and selectivity to acetaldehyde is 9B.2 o. Even though there is a high exothermic heat of reaction, the reactor temperature is nearly isothermal because of the large quantity of catalyst solution circulated to the reactor. The reactor effluent is flashed adiabatically. Acetaldehyde product, unreacted ethylene, and flashed steam constitute the overhead vapor from the flash drum, and the catalyst solution is pumped from the bottom. 

The drum dryer is commonly used to dry viscous, concentrated solutions, slurries or pastes on rotating steam-heated drums. It can also be used to dry concentrated solutions or slurries that become more viscous or pasty because of flashing or boiling off of moisture or of irreversible thermochemical transformations of their content that occur on their first contact with the hot drum surface. " ... 

The general characteristics of dispersions from different sources static mixers, flash drums 100-100 pm solvent extraction units, mechanically agitated systems 20-300 pm high Ap across a valve, steam stripper bottoms, caustic wash drums 10-400 pm systems with interfacial tension < 10 mN/m or containing surfactant systems or secondary coalescence 0.1-25 pm. 

The feed water pump is run intermittently only to maintain the water level in the flash drum (which is a separate unit), corresponding to the amount of water evaporated. Such steaming-type economisers can be seen in a sulphuric acid plant. 

For Steam Recovery from Blowdown A flash drum eould be used to recover steam firom boiler blowdown or high-pressure condensate. In the former case, the recovered LP steam (Wp ) comes from the overhead of the drum while the bottom is sent to the sewer ... 

Application Remarks You do not need to feel intimidated with the set of equations above. You will find the task simple when you follow the top-down approach to establish steam balances from boilers to each steam header, to steam turbines and letdown valves, and to deaerator and blowdown flash drum. The top-down approach for steam balance is discussed in detail in Chapter 16. During the process of setting up the steam balances, you could apply some of the equations above for modeling the equipment and subsystems. The steam balances can be conducted readily in a spreadsheet environment. 

Finishing. The halogenated rubber solution then passes into a vertical drum where the solvent is flashed and stripped by steam and hot water. Calcium stearate is added to the slurry in this drum to prevent polymer agglomeration. A second vessel in series provides additional residence time for the solvent to diffuse from the ruhher and be vaporized. The final solvent content and the steam usage for solvent removal depend on the conditions in each vessel. Typically, the lead flash drum is operated at 105-120°C and 200-300 kPa (29-43.5 psi). Conditions in the final stripping stage are 101°C and 105 kPa (15 psi). The hexane can be reduced to 0.5-1.0 wt% with a steam usage of 2.0-2.5 kg/kg rubber. 

A process instrumentation diagram for a flash drum is shown in Fig. E9.2. Steam is condensed in a steam coil to vaporize a portion of the liquid feed, and the liquid product is removed by a pump. There are control valves for the steam flow, vapor product, liquid product, feed flow, and steam chest (which allows the steam chest to be rapidly evacuated in emergency situations). Determine whether the five valves should be fail-close (F/C) or fail-open (F/O) for safe operation, for each of three cases ... 

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