Evaporation internal reflux

In the present estimation, a continuous dehydrogenation reactor in which decalin is supplied to the catalyst at various feed rates without internal refluxing is assumed. Here all the condensable products and unconverted decalin were removed from the reactor to the condensation part (see Figure 13.22). Now, the stationary rates of hydrogen generation (VH), naphthalene formation (VN), and evaporation of unconverted decalin (VD) are defined as the magnitudes per area of the catalyst layer (mol/m2h). All these rates are expressed from mass balance as follows . 

The tray temperatures in our preflash tower, shown in Fig. 4.4, drop as the gas flows up the tower. Most of the reduced sensible-heat content of the flowing gas is converted to latent heat of evaporation of the downflowing reflux. This means that the liquid flow, or internal reflux rate, decreases as the liquid flows down the column. The greater the temperature drop per tray, the greater the evaporation of internal reflux. It is not unusual for 80 to 90 percent of the reflux to evaporate between the top and bottom trays in the absorption section of many towers. We say that the lower trays, in the absorption section of such a tower, are drying out. The separation efficiency of trays operating with extremely low liquid flows over their weirs will be very low. This problem is commonly encountered for towers with low reflux ratios, and a multicomponent overhead product composition. 

Steam coils at the top of the tower maintain the specified temperature gradient across the rotating disc contactor. The higher temperature in the top section of the rotating disc contactor results in separation of the less soluble heavier material from the deasphalted oil mix and provides internal reflux, which improves the separation. The deasphalted oil mix leaves the top of the rotating disc contactor tower. It flows to an evaporator where it is heated to vaporize a portion of the solvent. It then flows into the high pressure flash tower where high pressure solvent vapors are taken overhead. 

Heat can be added to or removed from stirred-tank reactors via external jackets (Figure 7.5a), internal coils (Figure 7.5b) or separate heat exchangers by means of a flow loop (Figure 7.5c). Figure 7.5d shows vaporization of the contents being condensed and refluxed to remove heat. A variation on Figure 7.5d would not reflux the evaporated... 

Method B Phenoxypropionyl hydrazide (125.9 g, 0.7 mol) is suspended in 650 ml ice-water and concentrated hydrochloric acid (123 ml, 1.47 mol) was added. The mixture is stirred for 20 min and toluene (350 ml) is added. A solution of sodium nitrite (53.1 g, 0.77 mol) in 200 ml water is added over a period 15 min. The internal temperature is kept below 15°C and if necessary, ice is directly added to the reaction mixture. After the addition is completed the mixture is stirred for a further 1 hour and filtered through Celite. The solid is washed with 30 ml toluene and the filtrate is separated. The aqueous layer is extracted with 200 ml toluene and the combined toluene solutions are dried over MgS04. The dried toluene solution is filtered and added dropwise to a preheated flask at 95°-100°C. Nitrogen evolution occurs as the solution is dropped in. After the addition is complete, the reaction mixture is heated to gentle reflux until nitrogen evolution has ceased. The reaction mixture is cooled to room temperature and can be used directly in subsequent reactions. 1 ml of the reaction mixture is withdrawn and evaporated to dryness, and the weight of the residue is measured. This provides an estimate of the concentration of isocyanate per ml of reaction mixture. 

Procedure Pipet 1-mL portions of the Standard Solution and of the Assay Preparation into separate 100-mL, round-bottom boiling flasks. Add 1.0 mL of Internal Standard Solution to each flask, and evaporate each mixture to dryness on a water bath at 60° with the aid of a rotary evaporator. Dissolve each dry residue in 1 mL of pyridine, and add 1 mL of acetic anhydride to each flask. Boil each solution under reflux for 1 h to complete the acetylation. Separately inject l- xL portions of the derivatized solutions from the Assay Preparation and the Standard Solution into the gas chromatograph and measure the peak responses. Calculate the percent of Xylitol, on the as-is basis, by the formula... 

For the assay of cocaine in Eiythroxyion coca Lam. from three locations in Peru, Turner et al.3 worked out a gas chromatographic method. The alkaloids were extracted by refluxing the powdered leaf samples (1.00 g) with ethanol (40 ml) for 15 minutes. The filtrate was evaporated to dryness under reduced pressure in a rotary evaporator, the residue dissolved in 20 ml chloroform and the alkaloids extracted into 1.5 aqueous citric acid. The pH of the solution was adjusted to 8.2 with sodium bicarbonate and the alkaloid bases extracted with chloroform. After evaporation the residue was dissolved in ethanol containing the internal standard (androst-4-ene-3,17-dione) and gas chromatographed on a packed column of 6 % OV-1 on Chromo-sorb W. A typical chromatogram is shown in Figure 9.2 and the results of the analysis in Table 9.6. 

For analysis, ca. 10-15 g wet-weight tissue was transferred to a tared, hexane washed 250 ml Isolllng flask. The Internal standard mixture was added at this point in acetone solution. A 50 ml portion of 2.0 N aqueous KOH was added to the flask containing the tissue for saponification under reflux for 4 hours (h) or until the tissue was well digested. An equal volume of saturated NaCl solution was added to the mixture, and the solution extracted with three 50 ml portions of hexane. The extracts were combined, reduced in volume with a vacuum rotary evaporator and transferred to a cleaned vial. The remaining solvent was reduced to 1 ml volume in preparation for separation into saturated and unsaturated fractions and GC analysis as described edsove for sediments. 

Sub-samples of 1 g were taken for the analysis. The PAHs were extracted under reflux with toluene/acetone (1 1 v/v, 150 mL) for 30 minutes. The efficiency of the extraction procedure was tested by adding an internal standard (indeno[l,2,3-c t/]fluoranthene) and was found to be 95 99%. An additional extraction with tetrahydrofurane did not enhance the PAH yield. The extract was evaporated to about 2 mL. A cydohexane/dimethylformamide (DMF)/water (100 mL cyclohexane, 90 mL DMF and 10 mL water) partition was performed. The cyclohexane phase was discarded and the DMF/water phase diluted with 80 mL water. This phase was re-extracted with cyclohexane. The cyclohexane phase was evaporated to a volume of 2 mL and cleaned-up over a silica column (5 g, 9.1% water). The PAHs were eluted with 80 mL cyclohexane, which was evaporated afterwards to a volume of 0.1-0.5 mL. The obtained concentrate was placed on a Sephadex LH 20 column (10 g) and eluted with propanol-2. The fraction of 0-46 mL was discarded. The fraction from 46-170 mL was evaporated to a small volume and analysed by high resolution gas chromatography (on column injection) using a fused silica capillary column of which the conditions are described elsewhere [26]. 

Comments The sample was heated by using an isomantle. Typically, refluxing of the solvent occurred at the rate of four cycles per hour. Extracts were concentrated to 10 ml using a rotary evaporator and then diluted twofold before addition of the internal standards. 

A mixture of 0.895 g (4.47 mmol) of allylic alcohol 4 (GC purity 94%), 31 mg of propanoic acid, and 8.72 g (54 mmol) of tricthyl orthoacetatc is heated until distillation begins. Distillation is continued until the head temperature reaches 120 C, then the distillation head is replaced by a reflux condenser and the solution is stirred at reflux for 3h (internal temperature 144 ( ), After cooling, the mixture is treated with aq N.iHCO, and worked up with F.t, 0 in the usual manner. The crude product is chromatographed on silica gel (50 g). The ester 6 is eluted with 9 1 and 4 1 hexane/Et,0 and evaporatively distilled giving a colorless oil yield 0.992g(82%) bp 74-78 JC/0.2 Torr (bath temp). 

Boil-outs—Conunonly used to clean BPC equipment, boil-outs entail the introduction of the solvent (it could be water) used in the just completed process, and heating it to reflux. The expectation is that the evaporation/condensation densation will result in the dissolution of any residue on the equipment in the solvent. This will remove it from the internal surfaces that are ordinarily inaccessible for direct cleaning and thus clean them. Boil-outs are also utilized as one of the last steps in preparation of equipment for the start of a process or campaign. 

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