Reboil, external

Calculate the reflux heat, at Tray Dl. Reflux heat is defined as the apparent heat imbalance between external heat quantities at the point in question in the tower. These external heat quantities are denoted as Q with appropriate subscripts to signify their location. External heat input quantities are defined as the heat contained in the feed plus all heat to the system at product strippers either directly as steam or indirectly throu reboilers. External heat output quantities at a given tray are defined as the heat contained in liquid products leaving the system from points lower in the towier, the heat contained in the internal vapors of products plus steam and the heat contained by a product liquid flowing to the sidestream stripper. If the tray is nbt a sidestream draw tray, this latter quantity does not enter into the heat balance. 

Consider again the simple process shown in Fig. 4.4d in which FEED is reacted to PRODUCT. If the process usbs a distillation column as separator, there is a tradeofi" between refiux ratio and the number of plates if the feed and products to the distillation column are fixed, as discussed in Chap. 3 (Fig. 3.7). This, of course, assumes that the reboiler and/or condenser are not heat integrated. If the reboiler and/or condenser are heat integrated, the, tradeoff is quite different from that shown in Fig. 3.7, but we shall return to this point later in Chap. 14. The important thing to note for now is that if the reboiler and condenser are using external utilities, then the tradeoff between reflux ratio and the number of plates does not affect other operations in the flowsheet. It is a local tradeoff. 

Acryflc acid, alcohol, and the catalyst, eg, sulfuric acid, together with the recycle streams are fed to the glass-lined ester reactor fitted with an external reboiler and a distillation column. Acrylate ester, excess alcohol, and water of esterification are taken overhead from the distillation column. The process is operated to give only traces of acryflc acid in the distillate. The bulk of the organic distillate is sent to the wash column for removal of alcohol and acryflc acid a portion is returned to the top of the distillation column. If required, some base may be added during the washing operation to remove traces of acryflc acid. 

Dry inlet gas that has been dehydrated by molecular sieves (qv) or alumina beds to less than 0.1 ppm water is spHt into two streams by a three-way control valve. Approximately 60% of the inlet gas is cooled by heat exchange with the low pressure residue gas from the demethanizer and by external refrigeration. The remainder of the inlet gas is cooled by heat exchange with the demethanized bottoms product, the reboiler, and the side heater. A significant amount of low level refrigera tion from the demethanizer Hquids and the cold residue gas stream is recovered in the inlet gas stream. 

The steam balance in the plant shown in Figure 2 enables all pumps and blowers to be turbine-driven by high pressure steam from the boiler. The low pressure exhaust system is used in the reboiler of the recovery system and the condensate returns to the boiler. Although there is generally some excess power capacity in the high pressure steam for driving other equipment, eg, compressors in the carbon dioxide Hquefaction plant, all the steam produced by the boiler is condensed in the recovery system. This provides a weU-balanced plant ia which few external utiUties are required and combustion conditions can be controlled to maintain efficient operation. 

There are a number of ways to provide the heating or cooling medium at temperatures closer to the optimum level. One is by use of double-effect distillation, which uses the overhead vapor from one column as the heat source for another column such that the second column s reboiler becomes the first column s condenser. This basically cuts the temperature differential in half, and shows up as an energy saving because external heat is suppHed to only one of the units. 

Heat transfer by nucleate boiling is an important mechanism in the vaporization of liqmds. It occurs in the vaporization of liquids in kettle-type and natural-circulation reboilers commonly usea in the process industries. High rates of heat transfer per unit of area (heat flux) are obtained as a result of bubble formation at the liquid-solid interface rather than from mechanical devices external to the heat exchanger. There are available several expressions from which reasonable values of the film coefficients may be obtained. 

UK. = Light key component in volatile mixture L/V = Internal reflux ratio L/D = Actual external reflux ratio (L/D) ,in = Minimum external reflux ratio M = Molecular weight of compound Mg = Total mols steam required m = Number of sidestreams above feed, n N = Number of theoretical trays in distillation tower (not including reboiler) at operating finite reflux. For partial condenser system N includes condenser or number theoretical trays or transfer units for a packed tower (VOC calculations) Nb = Number of trays from tray, m, to bottom tray, but not including still or reboiler Nrain = Minimum number of theoretical trays in distillation tower (not including reboiler) at total or infinite reflux. For partial condenser system,... 

The HPS liquid consists mostly of C3 s and heavier hydrocarbons however, it also contains small fractions of Cj s, H2S, and entrained water. The stripper removes these light ends. The liquid enters the stripper on the top tray. The heat for stripping is provided by an external reboiler, using steam or debutanizer bottoms as the heat medium. The vapor from the reboiler rises through the tower and strips the lighter fractions from the descending liquid. The rich overhead vapor flows to the HPS via the condenser and is fed to the primary absorber. The stripped naphtha leaves the tower bottoms and goes to the debutanizer. Usually, at least one draw is installed in the tower to remove the entrained water. 

The debutanizer separates the feed into two products. The overhead product contains a mixture of C3 s and C4 s. The bottoms product is the stabilized gasoline. Heat for separating these products comes from an external reboiler. The heating source is usually the main fractionator heavy cycle oil or slurry. Steam can also be used. 

The 23-cm-diameter distillation column under study is used to separate ethanol and water. It contains 12 sieve trays with a 30-cm spacing (Fig. 11) as well as three possible feed locations, an external reboiler, and two condensers, which are used at the bottom and the top of the column. The second condenser is also used as a reflux drum a pump sends the reflux back to the column (tray 1) and the product to the product tank. 

An external feed of any amount, of one or more components, can be fed to any stage in the system of columns. This stage may be a plate, a reboiler, or a condenser. All external feed compositions, flows, and enthalpies are independent variables that are set in the problem description. A side stream of either vapor or liquid (or both) may be withdrawn from any stage except a reboiler or condenser, and either removed as product or sent to any stage of any column. The flow of each side stream... 

Whether or not a variable is independent may be more difficult to determine in other cases. For example, a distillation column with a side stripper is shown in Fig. 1. The side stripper in Fig. 1(a) has a reboiler and that in Figure 1 (b) is stripped with steam. Under the program, the liquid side stream which feeds the side stripper must be set. Also, in the column arrangement of Fig. 1(b) the amount and enthalpy of the steam fed must be set, since it constitutes an external feed. For illustration, we assume that the bottom product from the reboiler of the main column has... 

Notice that there is no reboiler in this flash tower. All the heat input comes from the partially vaporized crude. Both the temperature and the percent vaporization of the crude are fixed. Hence, the external heat input to the preflash tower is constant. The pounds of vapor flowing to the bottom tray must also be constant. 

Reboilers are sometimes inserted into the bottom of a tower. These are called stab-in reboilers. It is not a terribly good idea, because it makes it more difficult to fix a leaking or fouled reboiler, without opening the tower itself. However, the kettle reboiler, shown in Fig. 5.7, has essentially the same process performance characteristics as the stab-in reboiler, but is entirely external to the tower. 

Figure 3.6. Some modes of control of liquid level, (a) Level control by regulation of the effluent flow rate. This mode is externally adjustable, (b) Level control with built in overflow weir. The weir may be adjustable, but usually only during shutdown of the equipment, (c) Overflow weir in a horizontal kettle reboiler. The weir setting usually is permanent.

The procedure is outlined in Figure 13.17. The input data are listed above Box 1 and include all external material and enthalpy flows except condenser and reboiler loads, the number of trays, the reflux rate, and the reboiler load. The process is iterative, starting with estimates of temperature and vapor flow rates on each tray and making successive improvements in these values until a convergence criterion on temperatures is satisfied. 

Another pilot distillation study employed only one RPB along with external condenser and reboiler. The cyclohexane/n-heptane mixture was separated at rates up to 9 tons/h at total reflux. The system provided up to 6 transfer units (NTU) of separation in a 21-cm packing depth. The primary variable affecting... 

S4, connected to S2 by a recycle, but dynamically faster. Thus, supplementary inputs are D4-distillate flow rate, and Q4-reboiler duty. A major disturbance of the material balance can be simulated by a step variation in an external EDC feed. A second significant disturbance is the amount of impurity I3 introduced in the plant. 

Number of transfer units Pressure Vapor pressure Heat-transfer rate Condenser duty Reboiler duty External-reflux ratio... 

For example, the reboiler and condenser from the same column may be heat-integrated by using a heat pump, as discussed by Null [Chem. Eng. Prog., 72(7), 58 (1976)]. A heat pump may use an external fluid which is vaporized in the condenser, then compressed and condensed in the column reboiler (Fig. 13-75 ). Another heat pump (Fig. 13-75b) uses column overhead vapor, which is compressed and condensed in the reboiler and then returned to the top of the column as reflux. A third possibility is to use the column bottoms, which is let down in pressure and vaporized in the condenser, then compressed and fed back to the bottom of the column as boil-up (Fig. 13-75c). 

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