Staged columns vapor velocity

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

The vapor velocity in a finite-stage contactor column can be limited by the liquid handling capacity of the downcomers or by entrainment of liquid droplets in the rising gases. In most cases, however, downcomer limitations do not set the allowable vapor velocity instead, the common design basis for choosing allowable vapor velocities is a function of the amount of gas entrainment which can result in improper operation or flooding of the column. 

An alternate approach for estimating maximum allowable velocities has been presented by Fair (see reference given in footnote for preceding paragraph) which is based on data obtained with sieve-tray and other types of finite-stage columns and takes into account the effect of surface tension of the liquid in the column, the ratio of the liquid flow rate to the gas flow rate, gas and liquid densities, and dimensions and arrangement of the contactor. In this method, the basic equation for the maximum allowable vapor velocity, equiva-... 

In addition to the critical design factors for finite-stage contactors of number of theoretical trays, maximum allowable vapor velocity, column efficiency, and pressure drop as discussed earlier, a number of other factors are of importance in the development of the design. These factors are discussed in the following sections. 

D20/day How many columns 3 m in diameter must be used in parallel on the feed stage if the maximum vapor velocity based on the empty tower is 1 m/s ... 

G2. [Note This problem is quite extensive.] Biorefineries producing ethanol by fermentation have several distillation columns to separate the ethanol from the water. The first column, the beer still, is a stripping column that takes the dilute liquid fermenter product containing up to 15% solids and produces a clean vapor product that is sent to the main distillation column. The main column produces a distillate product between about 65 mole % and the ethanol azeotrope, and a bottoms product with very litde ethanol. The calculated diameter of the main distillation column is much greater at the top than elsewhere. To reduce the size and hence the cost of the main column, one can use a two-enthalpy feed system split the vapor feed into two parts and condense one part, then feed both parts to the main column at their optimum feed locations. This method reduces the vapor velocity in the top of the column, which reduces the calculated diameter however, a few additional stages may be required to obtain the desired purity. 

The data in top section of Figure 6.22 is a user supplied data. The relative vitalities should be calculated using Antoine equation or took from other softwares. Distillate stream composition is also needed, so pre material balance calculations are required. Reflux ratio to minimum reflux ratio (R/Rmi ), column pressure and feed quality (q) need to be provided. Vapor linear velocity and stage efficiency can be left as SuperPro default values. 

---