Distillation-Flash Drums

The previous example (air polluted with benzene) involved a mixture of two components with very different boiling points. We now consider separating a mixture in which the components have comparable volatility - for example, a mixture of benzene (which boils at 80°C at 1 atm) and toluene (which boils at 111°C at 1 atm). 

Given an operating pressure in the flash drum, what are the compositions of the vapor and liquid streams Or, at what pressure should we operate the flash drum to achieve a desired separation What is the best separation we can expect from this simple unit  

We perform the same experiment with the other extreme, pure toluene. Some of the data we measure are given in Table 4.2. 

Pressure (atm) Phase(s) Vapor phase Liquid phase  

With this new map comes new terminology. The horizontal line from the liquid border to the vapor border is a tie line. The mol fraction of component i in the liquid phase is defined as Xi. The mol fraction of component i in the vapor phase is defined as y. It follows that 

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Heater Detol reactors Flash drum Stabilizer Clay treater Distillation... 

A system with liquid recycle would naturally occur when the vapor-liquid equilibrium is such that a simple flash drum cannot be used and a distillation column (or... 

Distillation with vapor product. When a partial condenser is used, the flash drum plays the role of a vapor/liquid separator. In the setup known as a stabilizer there is only vapor distillate, while the liquid is returned as reflux. The column has a pasteurization section when a gaseous stream leaves at the top, while the... 

For example, when we consider the design of specialty chemical, polymer, biological, electronic materials, etc. processes, the separation units are usually described by transport-limited models, rather than the thermodynamically limited models encountered in petrochemical processes (flash drums, plate distillations, plate absorbers, extractions, etc.). Thus, from a design perspective, we need to estimate vapor-liquid-solid equilibria, as well as transport coefficients. Similarly, we need to estimate reaction kinetic models for all kinds of reactors, for example, chemical, polymer, biological, and electronic materials reactors, as well as crystallization kinetics, based on the molecular structures of the components present. Furthermore, it will be necessary to estimate constitutive equations for the complex materials we will encounter in new processes. 

Part of the stream is washed countercurrently with a feed sidestream in the vent H2 absorber (9) for benzene recovery. The absorber overhead flows to the hydrogen purification unit (10) where hydrogen purity is increased to 90%+ so it can be recycled to the reactor. The stabilizer (11) removes light ends, mostly methane and ethane, from the flash drum liquid. The bottoms are sent to the benzene column (12) where high-purity benzene is produced overhead. The bottoms stream, containing unreacted toluene and heavier aromatics, is pumped to the recycle column (13). Toluene, C8 aromatics and diphenyl are distilled overhead and recycled to the reactor. A small purge stream prevents the heavy components from building up in the process. 

The stream defined below is heated to 100°C to be partially vaporized in a flash drum before entering a distillation column. The fraction vaporized is controlled by the flash drum pressure. Calculate the required pressure at 100°C to have 20% mole vaporization, assuming Raoult s law applies. What are the products flow rates and compositions The constants for the Antoine Equation 2.19 are given for each component, with the pressure in kPa and the temperature in K. 

This example is intended to demonstrate the process dynamics methodology as implemented on a single equilibrium stage. A stream of light hydrocarbons is sent to a distillation column where the C3 s and lighter components are separated from the C4 s. Since the feed composition fluctuates substantially, it is sent to a flash drum located upstream of the column in order to attenuate the composition fluctuations and thereby improve the column controllability. The vapor and liquid products from the flash drum are then sent to different trays in the column. 

The thermodynamics of this process are described in detail in references (67 —72, 80,81). Let us examine a typical methanol injection system. In a typical methanol injection and recovery system for a cold-oil absorption or turboexpander plant, feed gas passes through a free-water knockout drum and into a gas-gas exchanger with methanol being sprayed on exchanger tube-sheets. Methanol inhibits hydrate formation and aqueous methanol condenses in the exchanger (and the chiller following it) and is pumped to a primary separator. The methanol-water solution is then flashed in a flash drum and filtered into a methanol still to recover methanol. Normally, methanol dissolves in the hydrocarbon liquids and is distilled as a mixture of propane and methanol. Some of the methanol is recovered as the overhead product to recover the methanol dissolved in the heavier solution, the bottoms of the methanol still (propane product or hydrocarbon liquids from the demethanizer)... 

Distillation columns, absorbers, scrubber, strippers, flash drums, etc. 

The liquid effluent from the flash drum is first degassed. The vapors consisting of methyl iodide and formate, acetaldehyde and methyl acetate, are dissolved in the azeotrope produced by the subsequent acetic add dehydration column. The degassed arid mixture is then distilled to remove the light components and the cobalt iodide, which is recycled to the reactor in the form of an aqueous slurry. This mixture is then dehydrated and purified by azeotropic distillation. The third compound employed is one of the reaction products (methyl acetate, bp10l3 = 70.4°C, water content 8.5 per cent weight). The column has about 60 trays.. ... 

Consider the process shown in Figure 23.7. An exothermic reaction A + B - C takes place in the gas phase. The product C is taken from the top of a distillation column. The unreacted raw materials A and B are both recycled to the reactor from the flash drum and the bottom of the distillation column, respectively. Compressors (C-l, C-2) are used to increase the pressure of the feed and recycled gas A. The liquid B is vaporized in a series of two heat exchanger (E-l, E-2). The reactor is a... 

Explain in your own words the interaction among the control loops of a flash drum (Figure 23.1b). Do the same for the loops of a distillation column (Figure 5.6). 

The reactor effluent is now at 100°F and in a flash drum operating at 450psia we can separate the liquid mixture of aromatics (unreacted toluene, produced benzene, and diphenyl) from the gaseous stream of H2 and CH4. Part of the flash liquid is used to quench the reactor effluent and the rest goed to a sequence of distillation columns. 

Fig. 3.28 presents a one-feed two-products column. This may be decomposed in feed tray, striping section, concentration section, condenser including the flash drum, reflux/distillate splitter, reflux pump and reboiler. We assume adiabatic equilibrium trays, with no heaters or coolers. The column has theoretical trays, including the feed tray, but excluding condenser and reboiler. The count of the degrees of freedom looks as follows ... 

Modelling a single tray is similar with a dynamic flash discussed before. The solution of the assembly of trays, increased with condenser, flash drum and reboiler, is a much more difficult problem, however. The equations presented below (for notations see Fig. 4.6) are known as MESH equations for modelling distillation columns at steady state enlarged with left hand terms for accumulation. 

The second column supplies benzene of high purity. One-point composition control is more robust in a recycle system. In inferential mode the heat input in reboiler controls a sensitive temperature in the column. If the sampling point is placed in the stripping section, this control loop ensures good composition control of the bottom product. The reflux is set in ratio with the feed flow rate. For moderate disturbances, this allows good purity of distillate simultaneously with high recovery. The levels in the flash drum and in the base are hold by manipulating distillate rate and bottom product, respectively. 

Step 8. The control of distillation column can be implemented mainly by stand-alone considerations. Liquid inventories in the flash and separation columns have classical structure base flow on level control. The liquid level in the flash drum is kept by the condenser cooling duty. Pressure is controlled by condenser duty in the production and recycle columns, but by vapour distillate in stabiliser. 

A flash is a single-stage distillation in which a feed is partially vaporized to give a vapor that is richer in the more volatile components. In Fig. 7.1a, a liquid feed is heated under pressure and flashed adiabatically across a valve to a lower pressure, the vapor being separated from the liquid residue in a flash drum. If the valve is omitted, a low-pressure liquid can be partially vaporized in the heater and then separated into two phases. Alternatively, a vapor feed can be cooled and partially condensed, with phase separation in a flash drum as in Fig. 7.1b to give a liquid that is richer in the less volatile components. In both cases, if the equipment is properly designed, the vapor and liquid leaving the drum are in equilibrium. ... 

Thus, the pressure falls and boiling may occur, as discussed in Sec. 5.9. Centrifugal pumps are often used to pump boiling liquids, e.g., at the bottom of many distillation columns, flash drums, evaporators, reflux drums, reboilers, condensers, etc. (see Fig. 9,7). 

The mixture of acetaldehyde and water from the flash drum is distilled in a crude aldehyde column to between 60-90% acetaldehyde solution. Light ends are removed by further distillation and pure acetaldehyde is produced in a final acetaldehyde distillation column. Chlorinated byproducts are removed as a side cut. 

Operate feed tanks at high level and product tanks at low level Use flash drums in the feed pipes to distillation columns... 

Mixers and splitters Flash drums Distillation column... 

An example of an estimate of the total capital investment for a processing plant is given in Tables 16.14 and 16.15 for an ammonia plant producing 1 billion Ib/yr. The costs are for the year 2000 at a U.S. Midwest location. The plant is part of an integrated complex. The process involves a variety of equipment, including gas compressors, pumps, heat exchangers, a catalytic reactor, a distillation column, an absorber, a flash drum, a gas adsorber, and gas permeation membrane separators. The material of construction is almost exclusively carbon steel. 

Function is to strip species from the liquid to produce a quality bottoms product for example, a solvent ready to be recycled as in glycol dehydration, amine absorption, extractive distillation or water that has been stripped of contaminants as in Sour Water strippers deodorize edible oils. Used to regenerate solvent for absorption or extractive or azeotropic distillation. Other equipment that is used for stripping include distillation. Section 4.2, gas-liquid separators. Section 5.1 and gas-liquid-liquid separation in flash drum. Section 5.4. The general characteristics of gas-liquid contacting are described in Section 1.6.1. Other operations that use this type of contactor include gas absorption. Section 4.8 reactors. Sections 6.13-6.16 and 6.19 and direct contact heat exchange Sections 3.7-3.9. 

We could, for example, consider the mathematical modeling of a flash drum separator or a distillation column. The stoichiometric equations (order of magnitude 1) stay with the enthalpy balance equations (order of magnitude 10 -10 ) and significant differences in orders of magnitude are present in the resulting nonlinear system. 

Caprolactone is continuously converted into hexanediol in a fixed bed reactor with the catalyst submerged by the liquid (flooded bed reactor). Hydrogen is charged into the bottom of the reactor in the ratio 10/1 to the caprolactone. It bubbles up through the liquid phase catalyst be mixture. The reaction conditions are 250 C and 280 bar. Backmixing, rather important in this type of reactor, and thermodynamic equilibrium impede a complete conversion. After the recovery of hydrogen in flash drums, the hydrogenated product is distilled in a series of columns where pure 1,6-hexanediol is extracted and unreacted caprolactone returned for recycle (21,22). 

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