Q, distillation feed condition

To solve Equation 9.50, start by assuming a feed condition such that q can be fixed. Saturated liquid feed (i.e. q = 1) is normally assumed in an initial design as it tends to decrease the minimum reflux ratio relative to a vaporized feed. Liquid feeds are also preferred because the pressure at which the column operates can easily be increased if required by pumping the liquid to a higher pressure. Increasing the pressure of a vapor feed is much more expensive as it requires a compressor rather than a pump. Feeding a subcooled liquid or a superheated vapor brings inefficiency to the separation as the feed material must first return to saturated conditions before it can participate in the distillation process. 

Condition of Feed (q Fine). The q line, which marks the transition from rectifying to stripping operating lines, is determined by mass and enthalpy balances around the feed plate. These balances are detailed in distillation texts (15). 

Underwood minimum reflux constant XjF = Mol fraction of component i in the feed XjD = Mol fraction of component i in the distillate q = Thermal condition of the feed Bubble point liquid q =1.0 Dew point vapor q =0 General feed q = (Ls - Lr)/F... 

Figure 6.6 shows a simple two-product distillation column and gives the notation we use for flowrates, compositions, and tray numbering. Feed is introduced on tray Nf, numbering from the bottom. There are Nt trays in the column. The molar feed flowrate is F, its composition is Zj (mole fraction of component j.k and its thermal condition is q (saturated liquid is q = 1, saturated vapor is q — 0). The heat transfer rates are Qr in the reboiler and Qc in the condenser. Distillate product... 

The next step is to plot the q-line, which intersects the diagonal at the feed composition. The slope of the q-line is computed on the basis of the feed thermal conditions at the feed tray temperature, which is dependent on the product rates. It is assumed that the product rate (distillate or bottoms) is such that the feed is almost a saturated liquid at feed tray conditions so that the q-line is close to vertical. 

A column with partial condenser and reboiler is to be used for the separation of benzene (1) from toluene (2), giving a distillate with 0.95 mole fraction benzene and a bottoms product with 0.10 mole fraction benzene. The column will operate at 105 kPa pressure and a reflux ratio of 4. The feed, at 55°C and a flow rate of 100 kmol/h containing 45 mol% benzene and 55 mol% toluene, enters the column at the fifth theoretical stage from the top. The estimated average relative volatility (benzene relative to toluene) is assumed constant, and estimated at 2.41. Based on the column conditions and thermodynamic properties, the predicted q-value is 1.2. It is required to determine the number of theoretical stages below the feed to complete the separation. 

The feed composition Zj (mole fractions j= 1, NC), the desired distillate composition Xpj (/ = 1, NC), and the feed thermal condition q are specified. The relative volatilities ot, (/ = 1, NC) of the multicomponent mixture are known. 

The feed to a distillation column can be liquid or vapor, or both, depending on the temperature, pressure, and composition of the feed. To quantify the thermal condition of the feed, the parameter q is defined as the fraction of the feed that is liquid. 

We split the saturated liquid feed into two fractions having flow rates of and introduce one fraction at the top of a distillation column, vaporize the second fraction completely and introduce it at the bottom the column. Plot the q-lines for the two streams entering the column if jtij= 0.5. Develop, under conditions of constant molal overflow, the operating line for this distillation column. Plot the line for the situation where Wtfi = Wtfi. Obtain graphically the compositions of the overhead vapor product and the bottoms liquid product for two values of the number of ideal equilibrium plates in column, N =1 and N—5. 

---