Partial and Total Condensers

The condenser is the stage where overhead vapors are condensed and liquid is returned to the top of the column as reflux. The condenser is partial if only part of the vapor is condensed and refluxed and the remainder leaves the condenser as vapor distillate. This type of condenser adds one equilibrium stage to the column trays because it holds a vapor phase and a liquid phase at equilibrium with each other. A total condenser is one where the entire overhead vapor is condensed (cooled to the bubble point temperature or subcooled to a lower temperature), part of the condensate is returned as reflux, and the remaining part is taken as liquid distillate. This type of condenser does not count as an equilibrium stage because no vapor-liquid separation takes place in it. The liquid distillate composition is identical to the composition of the vapor leaving the column top tray. 

A third type of condenser is often used, one which is a partial condenser but where only part of the condensate is refluxed and the remaining part is drawn as liquid distillate along with the vapor distillate. This type of condenser constitutes an equilibrium stage. The two overhead products have distinct compositions, being the equilibrium vapor and liquid phases. A column with vapor and liquid overhead products may be regarded as a special case of a multi-product column since the liquid overhead is essentially a side product from the condenser, or top stage. 

The partial condenser with a vapor product and all the liquid being refluxed is represented by an equilibrium stage. It adds to the column one degree of freedom the condenser heat rejection rate. 

The total condenser consists of a stage with heat rejection, and a splitter that splits the condensed vapor into a reflux and a liquid product. The stage heat rejection rate is one degree of freedom, and the splitter split ratio is another. Hence, a total condenser adds two degrees of freedom to the column. 

The partial condenser with vapor and liquid products is represented by an equilibrium stage and a splitter, each of these units having one degree of freedom The heat rejected at the equilibrium stage and the splitter split ratio. This type condenser also adds two degrees of freedom to the column. 

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Subroutine conden is used to calculate all condensers. It operates in very much the same fashion as subroutine eeboil, except that more possibilities occur. If reflux has been fixed, the amount of flfees is examined to see if it is larger than fixre. If not the product is set to zero and temperature is determined by bubble point. If the condenser is a two product condenser with a set temperature, tcset, this temperature is accepted if it is below the bubble point, indicating subcooled reflux. If flfees is large enough, the remainder after subtraction of fixre is taken as the appropriate product, and temperature is determined either by isovFL or BUBPT for the cases of partial and total condenser respectively. For two-product condensers, the division of the total product at tcset must be determined and calculation transfers to statement 21. 

According to the way in which the reflux is produced we can distinguish between column heads for partial and total condensation [88]. A column head for partial condensation has beendesoribed in a previous part of the book (Fig. 171). In low-temperature distillation dephlegmators are commonly used since the product is often withdrawn in the gaseous state. The column heads are designed to meet this requirement (c/. chap. 5.3.1). As was stated in section 5.2.3, total condensation is the soundest method and it is the one most used in laboratory distillation, esjiecially if large reflux ratios have to be maintained. 

A variety of other configurations and modifications of the basic design shown in Figures 3-6 and 3zZ are possible. Valve trays (see Chapter lOi are popular. Downcomers can be chords of a circle as shown or circular pipes. Both partial and total condensers and a variety of reboilers are used. The column may have multiple feeds, sidestream withdrawals, intermediate reboilers or condensers, and so forth. The column also usually has a host of temperature, pressure, flow rate, and level measurement and control devices. Despite this variety, the operating principles are the same as for the sinple distillation column shown in Figure 3-6. 

As indicated in Fig. 1-5, the overhead vapor V2, upon leaving the top plate, enters the condenser where it is either partially or totally condensed. The liquid formed is collected in an accumulator from which the liquid stream Lx (called reflux) and the top product stream D (called the distillate) are withdrawn. When the overhead vapor V2 is totally condensed to the liquid state and the distillate D is withdrawn as a liquid, the condenser is called a total condenser. If V2 is partially condensed to the liquid state to provide the reflux Lx and the distillate D is withdrawn as a vapor, the condenser is called a partial condenser. The... 

The rectifying section consists of the partial or total condenser and all plates down to the feed plate. The stripping section consists of the feed plate and all plates below it including the reboiler. When the total molar flow rates do not vary from plate to plate within each section of the column, they are denoted by Vr (vapor) and Lr (liquid), in the rectifying section and by Vs and Ls in the stripping section. The feed rate F, distillate rate D, bottoms rate B, and reflux rate Lj are all expressed on a molar basis. 

Material-balance diagrams for top plate and condenser (a) top plate (6) total condenser (c) partial and final condensers. 

Graphical construction for top plate (a) using total condenser b) using partial and final condensers. 

Example 8.7 A complex distillation column, equipped with a partial reboiler and total condenser and operating at steady state with a saturated liquid feed, has a liquid side stream draw-of in the enriching section. Making the usual simplifying assumptions ... 

Return to the design of the toluene hydrodealkylation process, as it is presented in Section 4.3. In the reactor section, after heuristics are utilized to set (1) the large excess of H2 in the hydrodealkylation reactor, (2) the temperature level of the quenched gases that enter the feed-product heat exchanger, and (3) the temperature in the flash vessel, the simulator is used to complete the material and energy balances and to examine the effects of these heuristics on the performance of the reactor section. In the distillation section, after heuristics are used to set (1) the quahty of the feed, (2) the use of partial or total condensers, (3) the use of cool-... 

A favored model for histone binding to DNA proposes that histone binds only to the major and not the minor groove of DNA. In this arrangement, the basic part of the histone is in an extended configuration, whereas the nonbasic parts are partially or totally condensed into an a-helix [75]. 

Product benzene is separated from a benzene-toluene mixture. The feed stream, = 100 kmol/h, is a saturated liquid with composition Zp= 0.321 (mole fraction benzene) at a pressure P = 2 bar. The top product consists of a distillate stream, D = 30 kmol/h, with composition Xq= 0.885 and a corresponding benzene recovery, F = 0.827. The tower has a diameter of 0.83 m and contains 20 sieve plates with a 0.61 m plate spacing. Feed is introduced on Plate 13, and the tower operates at a reflux ratio of 2.38. No information is provided on the partial reboiler and total condenser. From a benzene balance we obtain B = 70 kmol/h and Xq= 0.079. 

When low boiling ingredients such as ethylene glycol are used, a special provision in the form of a partial condenser is needed to return them to the reactor. Otherwise, not only is the balance of the reactants upset and the raw material cost of the resin increased, but also they become part of the pollutant in the waste water and incur additional water treatment costs. Usually, a vertical reflux condenser or a packed column is used as the partial condenser, which is installed between the reactor and the overhead total condenser, as shown in Figure 3. The temperature in the partial condenser is monitored and maintained to effect a fractionation between water, which is to pass through, and the glycol or other materials, which are to be condensed and returned to the reactor. If the fractionation is poor, and water vapor is also condensed and returned, the reaction is retarded and there is a loss of productivity. As the reaction proceeds toward completion, water evolution slows down, and most of the glycol has combined into the resin stmcture. The temperature in the partial condenser may then be raised to faciUtate the removal of water vapor. 

A more complex unit is shown in Fig. 13-24, which is a schematic diagram of a distillation column with one feed, a total condenser, and a partial reboiler. Dotted hnes encircle the six connected elements (or units) that constitute the distillation operation. The variables N, that must be considered in the analysis of the entire process are just the sum of the Nfs for these six elements since here Nr = 0. Using Table 13-5,... 

FIG. 13-35 Construction for a column with a huhhle-point feed, a total condenser, and a partial rehoiler. 

Another entire column with a partially vaporized feed, a hqnid-sidestream rate equal to D and withdrawn from the second stage from the top, and a total condenser is shown in Fig. 13-36. The specified concentrations are Xp = 0.40, Xb = 0.05, and Xo = 0.95. The specified L/V ratio in the top sec tion is 0.818. These specifications permit the top operating hne to be located and the two top stages stepped off to determine the liqnid-sidestream composition Xs = 0.746. The operating line below the sidestream must intersect the diagonal at the blend of the sidestream and the overhead stream. Since S was specified to be equal to D in rate, the intersection point is... 

FIG. 13-36 Graphical solution for a column with a partially flashed feed, a liquid side-stream and a total condenser. 

Example 8 Calculation of Rate-Based Distillation The separation of 655 lb mol/h of a bubble-point mixture of 16 mol % toluene, 9.5 mol % methanol, 53.3 mol % styrene, and 21.2 mol % ethylbenzene is to be earned out in a 9.84-ft diameter sieve-tray column having 40 sieve trays with 2-inch high weirs and on 24-inch tray spacing. The column is equipped with a total condenser and a partial reboiler. The feed wiU enter the column on the 21st tray from the top, where the column pressure will be 93 kPa, The bottom-tray pressure is 101 kPa and the top-tray pressure is 86 kPa. The distillate rate wiU be set at 167 lb mol/h in an attempt to obtain a sharp separation between toluene-methanol, which will tend to accumulate in the distillate, and styrene and ethylbenzene. A reflux ratio of 4.8 wiU be used. Plug flow of vapor and complete mixing of liquid wiU be assumed on each tray. K values will be computed from the UNIFAC activity-coefficient method and the Chan-Fair correlation will be used to estimate mass-transfer coefficients. Predict, with a rate-based model, the separation that will be achieved and back-calciilate from the computed tray compositions, the component vapor-phase Miirphree-tray efficiencies. 

Pressure can also be controlled by variable heat transfer coefficient in the condenser. In this type of control, the condenser must have excess surface. This excess surface becomes part of the control system. One example of this is a total condenser with the accumulator running full and the level up in the condenser. If the pressure is too high, the level is lowered to provide additional cooling, and vice versa. This works on the principle of a slow moving liquid film having poorer heat transfer than a condensing vapor film. Sometimes it is necessary to put a partially flooded condenser at a steep angle rather than horizontal for proper control response. 

Determine top tray temperature for use in relative volatility calculations by running a dew point on the overhead rapor. For total condenser its composition is same as distillate product. For a partial condenser, run a dew point on the column overhead vapor composition as determined by a material balance around the partial condenser, reflux, and product. 

In some operations, where the top product is required as a vapour, only sufficient liquid is condensed to provide the reflux flow to the column, and the condenser is referred to as a partial condenser. When the liquid is totally condensed, the liquid returned to the column will have the same composition as the top product. In a partial condenser the reflux will be in equilibrium with the vapour leaving the condenser. Virtually pure top and bottom products can be obtained in a single column from a binary feed, but where the feed contains more than two components, only a single pure product can be produced, either from the top or bottom of the column. Several columns will be needed to separate a multicomponent feed into its constituent parts. 

Develop a mathematical model for the three-column train of distillation columns sketched below. The feed to the first column is 400 kg mol/h and contains four components (1, 2, 3, and 4), each at 25 mol %. Most of the lightest component is removed in the distillate of the first column, most of the next lightest in the second column distillate and the final column separates the final two heavy components. Assume constant relative volatilities throughout the system ai, CI2, and a3. The condensers are total condensers and the reboilers are partial. Trays, column bases, and reflux drums are perfectly mixed. Distillate flow rates are set by reflux drum... 

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