Distillate Rate

This mode of operation demands constant rate of distillate throughout. This means that, for constant reflux ratio operation, the vapour load to the condenser is also constant. Boston et al. (1981) and Holland and Liapis (1983) considered this type of operation. 

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Feed Composition. Feed composition has a substantial effect on the economics of a distillation. Distillations tend to become uneconomical as the feed becomes dilute. There are two types of dilute feed cases, one in which the valuable recovered component is a low boiler and the second when it is a high boiler. When the recovered component is the low boiler, the absolute distillate rate is low but the reflux ratio and the number of plates is high. An example is the recovery of methanol from a dilute solution in water. When the valuable recovered component is a high boiler, the distillate rate, the reflux relative to the high boiler, and the number of plates all are high. An example for this case is the recovery of acetic acid from a dilute solution in water. For the general case of dilute feeds, alternative recovery methods are usually more economical than distillation. 

The original column normally has less than 7 mol percent i-Cs in the overhead and less than 3 mole percent n-C4 in the bottoms product when operating at a distillate rate of D/F = 0.489. Can these product purities he produced on the smaller column at D/F = 0.489 ... 

Example 3 Calculation of TG Method The TG method will he demonstrated hy using the same example problem that was used above for the approximate methods. The example column was analyzed previously and found to have C -I- 2N + 9 design variables. The specifications to be used in this example were also hstedat that time and included the total number of stages (N = 10), the feed-plate location (M = 5), the reflux temperature (corresponding to saturated liquid), the distillate rate (D = 48.9), and the top vapor rate (V = 175). As before, the pressure is uniform at 827 kPa (120 psia), but a pressure gradient could be easily handled if desired. 

The value of 6 is found by solving the following nonlinear equation, where D is the specified distillate rate ... 

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. 

Types of columns and packings. A slow distillation rate is necessary to ensure that equilibrium conditions operate and also that the vapour does not become superheated so that the temperature rises above the boiling point. Efficiency is improved if the column is heat insulated (either by vacuum jacketing or by lagging) and, if necessary, heated to Just below the boiling point of the most volatile component. Efficiency of separation also improves with increase in the heat of vaporisation of the liquids concerned (because fractionation depends on heat equilibration at multiple liquid-gas boundaries). Water and alcohols are more easily purified by distillation for this reason. 

Multicycle vacuum distillations have been assessed ". The distillations were effected at 700°C. Data on the effect of distillation rate and of fraction distilled on the purity of the sample are collected in Table 1. These data show that the technique is effective in removing the less volatile impurities As, Co, Cu, Cr, Fe, Ga, Mn and Sb from Mg but has little effect on more volatile species, Ba, Zn and Zr. Increase of the distillation rate or the fraction distilled leads to a decrease in the effective purification. Double (99% fraction) distillation gives a product of similar purity to that of a single (72% fraction) distillation . Single (78% fraction) distillation of a Mg sample (assay 99.9%) unusually rich in Mn (300 fig g" ) at 3.5 g h gave a decrease (Xl0 ) in Mn content (to 0.025 fig g ) a similar value (0.04 fig g" ) was obtained from a doubly (99% fraction) distilled sample. This technique gives Mg with assays of 99.9995%... 

Hvoo, Hloo = molar enthalpies of the vapor and liquid at the rectifying pinch D = distillate rate... 

The distilling flask, collection flask, and column are continuously evacuated with a high-vacuum system. When the bulk of the pentane and residual hexane have distilled away, the temperature of a silicone oil bath surrounding the distillation flask is raised from room temperature to 125° over about a one-hour period. When the distillation rate diminishes appreciably, the temperature is slowly raised to 150° and maintained there until no more liquid is obtained. The distillation flask is cooled to room temperature before air is admitted to the system. The distilled product weighs 86.5 g. (89% yield checkers report 89 %t) and melts at 32 to 32.5°. In this state of purity, (bromomethyl)-carborane is suitable for most uses, but it may be further purified by crystallization from pentane or methanol. For example, crystals obtained by chilling a solution of 86.5 g. of the car-... 

This study used the Junk et al. (5) procedure for resin cleaning beginning with a water backwash. The remaining steps consisted of sequential Soxhlet extraction of 24-h duration for each of the following solvents methanol, acetonitrile, and ethyl ether. This process was followed by a final rinse of 2 bed volumes of 1 N sodium hydroxide. Ethyl ether was replaced by whatever sample eluting solvent was to be used. Approximately 700 mL of wet resin was Soxhlet extracted with 900 mL of solvent. The solvent distillation rate was approximately 30 mL/min. After extraction, each of the unconcentrated solvents was analyzed by capillary GC with a flame ionization detector (FID) and GC-mass spectrometry (GC-MS). 

THE VENDOR SHALL GUARANTEETHE MECHANICAL OPERATION OF THE EGUPMENT AND SYSTEM PROVIDED FOR A MINIMUM OF ONE YEAR AFTER START-UP. HLR EXPECTS PROCESS GUARANTEES ON THE SPECIFIED DISTILLATION RATES. CONDENSER PmFORMANCE.AM> MAXIMUM PRESSURE DROPS. ENTRAINMENT, AIR LEAKAGE RATES AND UTILITY CONSUMPTIONS SHALL BE SPECIFIED. ALLPROOUCT AND PROCESS SPECIFICATIONS WHICH THE VENDOR CANNOT GUARANTEE SHALL BE ENUMERATED. 

The two prime objectives of this process, maintenance of good odor and color in the distillate and proper bottoms yield, are achieved by effective control over vacuum, temperature, and distillation rate. 

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