Batch Distillation with Rectification

A batch distillation with rectification involves charging the still with the material to be separated and carrying out the fractionation until the desired amount has been distilled off. The overhead composition will vary during the operation, and usually a number of cuts will be made. Some of the cuts will be the desired products, while others will be intermediate fractions that can be recycled to subsequent batches to obtain further separation. 

Although batch distillation is covered in a subsequent separate section, it is appropriate to consider the application of RCM and DRD to batch distulation at this time. With a conventional batch-rectification column, a charge of starting material is heated and fractionated, with a vapor product removed continuously. The composition of the vapor prodiic t changes continuously and at times drastically as the lighter component(s) are exhausted from the stiU. Between points of drastic change in the vapor composition, a cut is often made. Successive cuts can be removed until the still is nearly diy. The sequence, number, and limiting composition of each cut is dependent on the form of... 

Batch with Constant Reflux Ratio, 48 Batch with Variable Reflux Rate Rectification, 50 Example 8-14 Batch Distillation, Constant Reflux Following the Procedure of Block, 51 Example 8-15 Vapor Boil-up Rate for Fixed Trays, 53 Example 8-16 Binary Batch Differential Distillation, 54 Example 8-17 Multicomponent Batch Distillation, 55 Steam Distillation, 57 Example 8-18 Multicomponent Steam Flash, 59 Example 8-18 Continuous Steam Flash Separation Process — Separation of Non-Volatile Component from Organics, 61 Example 8-20 Open Steam Stripping of Heavy Absorber Rich Oil of Light Hydrocarbon Content, 62 Distillation with Heat Balance,... 

For the production of relatively pure products by batch distillation, the stillpot is usually augmented by a multistage rectification column with reflux. Shown in Figure 12.19 is a flow diagram for a typical batch distillation process. The feed in this example contains components A, B, and... 

The batch distillation of binary mixtures will be considered fpr the cases of (1) no rectification, (2) rectification without liquid holdup in the column, and (3) rectification with holdup. 

This mode of batch rectification requires the continuous adjustment of the reflux to the colunrn in order to achieve a steady overhead distillate composition. Starting with a kettle obviously rich in the more volatile component, a relatively low reflux ratio will be required to achieve the specified overhead distillate composition. With time, the reflux ratio must be continuously increased to maintain a fixed overhead composition. Ultimately, a practical maximum reflux is reached and the operation normally would be stopped to avoid distillate contamination. 

For 1 hour vacuum rectification tower 21 operates in the self-serving mode, and then starts separating benzene, which is collected in collector 24 (from there it can be sent to the synthesis again into batch box 3). After the distillation of benzene residual pressure of 107 GPa is created in the rectification system after the constant mode is established, the intermediate fraction is separated into receptacle 25. If the methylphenyldichlorosilane content in the intermediate fraction exceeds 5%, this fraction can be sent for repeated rectification in tank 20. After the intermediate fraction, the main fraction, methylphenyldichlorosilane, is separated into receptacle 26. The fraction with the density of 1.1750-1.1815 g/cm3 and chlorine content of 36.9-37.8% is separated. The separation is conducted as long as reflux is extracted. From receptacle 26, technical methylphenyldichlorosilane flows into collector 27. 

Fig. 31 shows a diagram of the preparation of triacetoxymethylsilane with acetic anhydride. The acetylation of methyltrichlorosilane can be carried out in reactor 6, a steel enameled cylindrical apparatus with an agitato-rand, a water vapour jacket and rectification tower 3 filled with Raschig rings. The reactor is loaded with necessary amounts of methyltrichlorosilane and acetic anhydride from the batch boxes, the agitator is switched on and the jacket is filled with vapour. The process ends with the complete distillation of the fraction which boils below 58 °C. The reactor is still filled with triacetoxymethylsilane with an impurity of unreacted acetic anhydride. The product is collected in receptacle 7. 

Out of collector 13 the base salve solution of trimethylborate is sent into tank 16, where at 200 °C trimethylborate is distilled. The distilled fraction, which contains 88-90% of trimethylborate, is collected into collector 6 and sent into the tank of rectification tower 7 the base salve from tank 16 is sent through container 17 back into batch box 5. During rectification all methyl alcohol is separated in the form of azeotropic mixture with trimethylborate and collected in collector 19 trimethylborate remains in the tower tank. The azeotropic mixture is sent through collector 11 for repeated extraction into tower 12 the ready product, 98.5-99.5% trimethylborate, is sent from the tank of tower 7 into collector 18. 

Fig. 93. Production diagram of diethyl tin dicaprylate 1, 5, 16, 27, 33 - reactors 2, 3, 6, 15, 18, 26, 29, 32, 35 - batch boxes 4, 7, 11, 17, 20, 23, 28, 34 - coolers 8, 21 - collectors 9 - distillation tank 10 - rectification tower 12 - freezer 13, 14 -receptacles 19, 22 — apparatuses with agitators 24 - oil gate 25 - fire-resistant apparatus 30, 37 - nutsch filters 31 - shelf draft 36 -dehydrator 38 - container...

When the synthesis ends, as shown by pressure fall and constant temperature in the reactor, the valve between reactor 1 and cooler 4 opens, and ethyl mercaptan is distilled from the reactor. It self-flows into collector 6 through coolers 4 and 5. Collector 6 serves as a kind of settling box for tank residue. From there, ethyl mercaptan enters apparatus 7 and is pumped with immersed batching pump 8 into rectification tower 10. After ethyl mercaptan has been distilled, the tank residue in reactor 1 is cooled down after that, it is pressurised with nitrogen under 3 atmospheres into... 

Calculate, by modified McCabe-Thiele methods, residue composition and distillation time for binary batch rectification with constant reflux for a given number of ideal stages, boil-up rate, and specified average distillate composition. 

Example 6.19 Binary Batch Rectification with Constant Distillate Composition... 

As with distillation in batch, it is necessary before undertaking a continuous rectification to select the conditions, namely ... 

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