Distillation columns maximum vapor flow

In addition to entrainment flooding, we need to be concerned with the turndown ratio, the maximum vapor flow divided by the minimum vapor flow. This ratio is important because it determines the flexibility of our distillation column. In an ideal world, we would want to operate our separation at full capacity all the time. In a real world, we will not always have a high, steady demand for our product, or we may have a fluctuating feedstock. In this real world, we may want to run at less than full capacity without the possibility of problems like weeping. 

Whether for a distillation, absorption, or stripping system the material balance should be established around the top, bottom, and feed sections of the column. Then, using these liquid and vapor rates at actual flowing conditions, determine the flooding and maximum operating points or conditions. Then, using Figures 9-21B, -21E, or -21F, establish pressure drop, or assume a pressure drop and back-calculate a vapor flow rate, and from this a column diam-... 

In the development of a new process, it will be necessary to fractionate 910 kg/h of an ethanol-water solution containing 0.3 mole fraction ethanol. It is desired to produce a distillate containing 0.80 mole fraction ethanol, with negligible loss of ethanol in the residue, using a sieve-tray distillation column at atmospheric pressure. Preliminary calculations show that the maximum vapor-volumetric flow rate occurs at a point in the column just above the feed tray, where the liquid and the vapor contain 0.5 and 0.58 mole fraction of ethanol, respectively. The temperature at that point in the column is about 353 K the local slope of the equilibrium curve is m = 0.42. The liquid and vapor molar flow rates are 39 and 52 kmol/h, respectively. 

Dtotai is calculated from the Rayleigh equation calculation procedure, with F set either by the size of the still pot or by the charge size. For an existing apparatus the distillate flow rate, D in kmol/h, cannot be set arbitrarily. The column was designed for a given maximum vapor velocity, Uf]ood> which corresponds to a maximum molal flow rate, Vj ax (see Chapter 101. Then, from the mass balance around the condenser,... 

In this chapter, all of the process flow rates were considered to be constrained by zero flow and the maximum flow allowable by the valve size and span of the flow measurement. The main column constraint because of flooding is associated with the vapor traffic and pressure drop across the trays or packing in the distillation column. The mass transfer rate limit for stripping light key impurity from the bottoms stream was presented. The mass transfer rate limit for absorbing heavy key impurity from the overhead vapor stream was also presented. 

In yet another case, the vapor traffic going up the column is creating the maximum pressure drop that can be operated successfully through a distillation tray or through a distillation column packing. A higher flow rate of vapor would hold liquid up in the column, and it would be flooded. The column must be kept below flooding. This is another constraint. 

The maximum operational is the greatest vapor flow rate attained before loss of normal separation efficiency of the packing (see Chapter 7). In vacuum distillations, massive entrainment of liquid upward in the vapor phase throughout the packed bed will limit the maximum operational capacity (maximum C ) because it reduces the separation efficiency. However, if liquid entrainment only occurs at the top of the bed, as long as the entrained liquid carried into the condenser is of the same composition as the reflux liquid, the separation is not impaired. Thus, some entrainment in the operation of the column is acceptable and simply constitutes a recycle of liquid in addition to the usual reflux. 

For atmospheric pressure or vacuum distillations, the vapor flow rate is limited by the loss of normal separation efficiency due to entrainment of liquid upward in the vapor phase. The amount of entrainment increases rapidly above a threshold value for the particular system. As the vapor flow rate is increased further, the mass of entrained liquid becomes sufficient to reduce the concentration profile established in the column. The maximum operational Cs has been defined as the greatest vapor flow rate attained before loss of normal separation efficiency. Figures 7-6 and 7-7 give a prediction of the maximum capacity for IMTP random packings and Intalox structured packings, respectively, as limited by liquid entrainment. 

The separation of close boiling components in a mixture has been accomplished commercially by cascading several evaporations and condensations of such mixtures in a device known as a distillation or rectification column. There are two types of columns, namely, packed columns and plate columns. The former are vertical cylinders filled with a variety of packings that provide a large surface area per unit volume to promote maximum contact between the downward liquid flow and the upward vapor flow. Since this type of column can encounter poorer vapor-liquid contact than the plate column, it is only seldomly used in cryogenic separation systems. 

---