Columns with Partial Condensers

Extractive distillation with DMSO feed composition 5601------------— 

Reflux ratios are controlled by measuring distillate flow rates, sending this signal into a multiplier whose other input is the design RR and whose output sets the mass flow rate of the reflux. Note that mass units can be used here because the distillate and the reflux have the same composition. 

Bottoms level in the extractive column is controlled by bottoms flow rate. 

Bottoms level in the solvent-recovery column is controlled (loosely) by the flow rate of the solvent makeup. Very little solvent is lost, so the base level will float up and down with throughput. Sufficient surge volume must be provided in the base of this column to handle these dynamic transients. 

All of the distillation columns considered up to this point in this book have used total condensers, that is, the distillate product is a liquid. However, many industrial columns 

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Gasoline and kerosene rerunning was accomplished primarily in horizontal batch shell stills heated by direct firing or internal steam coils and surmounted by a vertical rectification column with partial condensers to supply reflux. The rectifying column in some installations was packed with iron rings, pipe fittings, earthware crocks, tin cans, or any suitable material readily available. In other units a fairly common type of column was the Heckmann bubble cap tower. 

Fig. 37. Fractionating column with partial condenser (D) (dephlegmator).

A column with partial condenser and reboiler is to be used for the separation of benzene (1) from toluene (2), giving a distillate with 0.95 mole fraction benzene and a bottoms product with 0.10 mole fraction benzene. The column will operate at 105 kPa pressure and a reflux ratio of 4. The feed, at 55°C and a flow rate of 100 kmol/h containing 45 mol% benzene and 55 mol% toluene, enters the column at the fifth theoretical stage from the top. The estimated average relative volatility (benzene relative to toluene) is assumed constant, and estimated at 2.41. Based on the column conditions and thermodynamic properties, the predicted q-value is 1.2. It is required to determine the number of theoretical stages below the feed to complete the separation. 

Example 4.4 column with partial condenser (Figure 4.12)... 

CONVENTIONAL DISTILLATION COLUMNS WITH PARTIAL CONDENSERS... 

Control Structure CS1. Figure 8.9a shows the control structure that is probably most commonly used for distillation columns with partial condensers. The main features of this structure are pressure controlled by manipulating vapor distillate flow rate and reflux drum level controlled by manipulating condenser heat removal. Reflux flow rate is fixed or ratioed to feed. 

Some distillation columns with partial condensers are constructed with the condenser installed at the top of the column inside the shell. There is usually no reflux drum. Vapor flows upward through the tubes of the condenser. The condensate liquid flows downward and drops into a liquid distributor above the top tray. These dephlegmatoi systems are frequently used when very toxic or dangerous chemicals are involved because it avoids potential leak problems with pumps and extra vessels and fittings. 

Consider now thermal coupling of the prefractionator arrangement from Fig. 5.116. Figure 5.16a shows a prefi-actionator arrangement with partial condenser and reboiler on the prefractionator. Figure 5.166 shows the equivalent thermally coupled prefractionator arrangement sometimes known as a Petlyuk column. To make the two arrangements in Fig. 5.16 equivalent, the thermally coupled prefractionator requires extra plates to substitute for the prefractionator condenser and reboiler. 

A simple model for side-rectifiers suitable for shortcut calculation is shown in Figure 11.12. The side-rectifier can be modeled as two columns in the thermally coupled direct sequence. The first column is a conventional column with a condenser and partial reboiler. The second column is modeled as a sidestream column, with a vapor sidestream one stage below the feed stage4. The liquid entering the reboiler and vapor leaving can be calculated from vapor-liquid equilibrium (see Chapter 4). The vapor and liquid streams at the bottom of the first column can then be matched with the feed and sidestream of the second column to allow the calculations for the second column to be carried out. 

If a partial condenser is linked to a rectification column, as much vapor condenses as is needed for the reflux of the column. A partial condenser is then named a dephlegmator , as shown by Fig. 2-86. The enrichment yj) — yp in the dephlegmator increases with increasing reflux ratio V = R/Djj, shown qualitatively in Fig. 2-87. 

Consider a three-product separation as in Fig. 5.11a in which the lightest and heaviest components are chosen to be the key separation in the first column. Two further columns are required to produce pure products (see Fig. 5.11a). However, note from Fig. 5.11a that the bottoms and overheads of the second and third columns are both pure B. Hence the second and third columns could simply be connected and product B taken as a sidestream (see Fig. 5.116). The arrangement in Fig. 5.116 is known as a prefractionator arrangement. Note that the first column in Fig. 5.116, the prefractionator, has a partial condenser to reduce the overall energy consumption. Comparing the prefractionator arrangement in Fig. 5.116 with the conventional... 

The dephlegmator process recovers a substantially higher purity C2+ hydrocarbon product with 50—75% lower methane content than the conventional partial condensation process. The C2+ product from the cryogenic separation process can be compressed and further separated in a de-ethanizer column to provide a high purity C3+ (LPG) product and a mixed ethylene—ethane product with 10—15% methane. Additional refrigeration for the deethanization process can be provided by a package Freon, propane or propylene refrigeration system. 

Cmde helium (containing 50—70% helium, associated hydrogen and neon, 1—3% methane, and the balance nitrogen) can easily be obtained by minor enhancements to the nitrogen rejection unit, particularly with natural gases containing 0.5% or more helium. For example, by operating the double-column condenser in a partial condensation mode, a stream of uncondensed vapor at about 50% helium concentration can be obtained. This cmde helium stream can be fed directly to helium purification and Hquefaction units. 

FIG. 13-1 Schematic diagram and nomenclature for a simple distillation column with one feed, a total overhead condenser, and a partial rehoiler. 

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,... 

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. 

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