Reflux rate

Non-standard distillation equipment having up to 100 plates and operating at high reflux rates Is also used. The fractionation is very efficient and gives a precise distribution of boiling points. 

The two condensate Hquids must be used to provide reflux and distiUate streams. NormaHy, the reflux ratio, r, is chosen so that r = L jD > (j). This requires that the reflux rate be greater than the condensation rate of entrainer-rich phase and that the distiUate rate be correspondingly less than the condensation rate of entrainer-lean phase. This means that the distiUate stream consists of pure entrainer-lean phase, ie, Xj = x, and the reflux stream consists of aU the entrainer-rich phase plus the balance of the entrainer-lean phase. Thus, the overall composition of the reflux stream, Hes on the... 

Redux Ra.te, The optimum reflux rate for a distillation column depends on the value of energy, but is generally between 1.05 times and 1.25 times the reflux rate, which could be used with infinite trays. At this level, excess reflux is a secondary contributor to column inefficiency. However, when designing to this tolerance, correct vapor—Hquid equiUbrium data and adequate controls are essential. 

FIG. 13-93 Comp arisen of internal-reflux rates for three methods of providing reflux. 

Stripping gas will have the effect of requiring reduced top still temperature to produce the same reflux rate. 

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,... 

Increase in the use of main column overhead reflux rate instead of top pumparound to control the top temperature... 

Molar holdup in still Molar holdup in reflux drum Vapor boil up rate Reflux rate Relative volatilities... 

Fixed reflux, where the reflux rate is kept constant. The compositions will vary as the more volatile component is distilled off, and the distillation stopped when the average composition of the distillate collected, or the bottoms left, meet the specification required. 

Variable reflux, where the reflux rate is varied throughout the distillation to produce a fixed overhead composition. The reflux ratio will need to be progressively increased as the fraction of the more volatile component in the base of the still decreases. 

Calculation of the pressure drop and flooding rate is particularly important for vacuum columns, in which the pressure may increase severalfold from the top to the bottom of the column. When a heat-sensitive liquid is distilled, the maximum temperature, and hence the pressure, at the bottom of the column is limited and hence the vapour rate must not exceed a certain value. In a vacuum column, the throughput is very low because of the high specific volume of the vapour, and the liquid reflux rate is generally so low that the liquid flow has little effect on the pressure drop. The pressure drop can be calculated by applying equation 4.15 over a differential height and integrating. Thus ... 

The reflux rate and flow of nitrogen gas must be kept at a minimum to assure that the amount of dioxane carried over to the traps in liquid nitrogen is kept as small as possible. 

Further increases in the reflux rate, then act to increase, rather than decrease, the butane content of the overhead propane product... 

Figure 1.10 illustrates this point, from plant test data obtained in a Texas refinery. Point A is called the incipient flood point, that point in the towers operation at which either an increase or a decrease in the reflux rate results in a loss of separation efficiency. You might call this the optimum reflux rate, that would be an alternate description of the incipient flood point, neglecting the energy cost of the reboiler steam. 

K = 0.18 to 0.25 tray operation close to its best efficiency point K = 0.35 to 0.40 tray suffering from entrainment—increase in reflux rate, noticeably reduces tray efficiency K = >0.5 tray is in fully developed flood—opening a vent on the overhead vapor line will blow out liquid, with the vapor K = 0.10 to 0.12 tray deck is suffering from low tray efficiency, due to tray deck leaking... 

Unfortunately, high reflux rates are expensive. They represent an energy cost, in that reflux is actually generated from the reboiler. The... 

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