Distillation towers pressure, operating

Distillation. Desalted crude oil is heated for fractional distillation between 343 and 399 degrees Celsius (650 to 750 degrees Fahrenheit). The resulting vapor and liquid mix is sent to the first distillation tower that operates at atmospheric pressure. Because of the different boiling points of the different hydrocarbon molecules of crude oil, the hydrocarbons can be separated into different fractions (also called cuts). This occurs at the distillation tower, which can be as tail as 50 meters (164 feet). The heaviest hydrocarbons remain at the bottom, while the middle and lighter ones are extracted. 

While working in a plant, a troubleshooter read a pressure gauge daily for several weeks and only realized it was inaccurate when one day the blower was down. The gauge still read about normal operating pressure. Had this have been a distillation unit, it could have been more serious. In distillation service, pressure is a more important variable than in many other unit operations. Relative volatility is a function of pressure. Pressure, or more accurately delta-P, is the best indication of the tower hydraulics. 

Extractive Distillation. In extractive distillation a fraction comprising compounds of similar volatility is vaporized and passed countercurrent to a liquid solvent stream in a packed or bubble cap tower. The operating conditions of temperature and pressure are regulated so that one or more of the components of the mixture are dissolved in the entrainer and removed in a liquid phase extract, while the remaining vapor is taken overhead and condensed or discharged as gaseous effluent. 

It is a characteristic of process equipment, that the best operation is reached, at neither a very high nor a very low loading. The intermediate equipment load that results in the most efficient operation is called the the best efficiency point. For distillation trays, the incipient flood point corresponds to the best efficiency point. We have correlated this best efficiency point, for valve and sieve trays, as compared to the measured pressure drops in many chemical plant and refinery distillation towers. We have derived the following formula ... 

Reducing reflux saves reboiler duty. Also, the lower pressure will reduce the tower-bottom temperature, and this also cuts the reboiler energy requirement. For most distillation towers, the energy cost of the reboiler duty is the main component of the total operating cost to run the tower. 

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. 

Tower 10 operates at atmospheric pressure the top part is filled with nichrome wire packing. At first, the tower operates returning the reflux until the temperature on top stabilises at 55 °C. The azeotropic mixture containing approximately 75% of trimethylborate is separated into collector 11 from the top of the tower at a speed that corresponds to that at which the solution of boric acid is fed. The tower tank is filled with 70% methyl alcohol the alcohol is reloaded into collector 9 and sent to the distillation of methyl alcohol returned to the synthesis. 

Example 6 Determination of optimum reflux ratio. A sieve-plate distillation column is being designed to handle 700 lb mol (318 kg mol) of feed per hour. The unit is to operate continuously at a total pressure of 1 atm. The feed contains 45 mol% benzene and 55 mol% toluene, and the feed enters at its boiling temperature. The overhead product from the distillation tower must contain 92 mol% benzene, and the bottoms must contain 95 mol% toluene. Determine the following ... 

Pressure drop in packed distillation tower as fimction of gas rate and operating pressure. 

A random-packed distillation tower with an inside diameter of 6 in. is being operated at a condenser pressure of 100 mm Hg. The following data are obtained during operation ... 

A special test on the 6-in. distillation tower described in Prob. 8 indicates that the total pressure drop is 2.9 mm Hg when the gas rate is 50 lb/h. For this test, the condenser pressure was maintained at 100 mm Hg, the I/G ratio was the same as shown in Prob. 8, and temperature change was negligible. Operation can be considered to be under preloading conditions. Estimate the total pressure drop at the operating conditions indicated in Prob. 8 for the 12-in. column. 

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