Optimum feed tray

7 FINDING THE OPTIMUM FEED TRAY AND MINIMUM CONDITIONS 

Now that the pressure has been determined and the product specifications attained, we need to go back and find the optimum feed tray. In addition, the minimum reflux ratio and the minimum number of trays can be determined. These will be useful for heuristic optimization, which is discussed in detail in Chapter 4. 

In most distillation columns, the major operating cost is reboiler energy consumption. Of course, if refrigeration was used in the condenser, this heat removal expense would also be 

TABLE 3.1 Effect of Feed Stage on Reboiler Heat Input 

Feed Stage Number Reboiler Heat Input (MW) Condenser Heat Removal (MW) Reflux Ratio 

---

The simplest distillation models to set up are the shortcut models. These models use the Fenske-Underwood-Gilliland or Winn-Underwood-Gilliland method to determine the minimum reflux and number of stages or to determine the required reflux given a number of trays or the required number of trays for a given reflux ratio. These methods are described in Chapter 11. The shortcut models can also estimate the condenser and reboiler duties and determine the optimum feed tray. 

Once the operating lines are plotted, the equilibrium stages are stepped off between the operating line and equilibrium curve as described in Section 5.1.3. The optimum feed tray location is at the step that crosses from one operating line to the other over their intersection point. If the number of stages is fixed, the operating lines are determined by trial and error to match the specified number of stages. 

Determine the number of theoretical trays, the optimum feed trays, and the side draw tray. Phase equilibrium data at 100 kPa are given below ... 

Step off the stages to determine the required number of trays and the optimum feed trays below the side draw. 

A distillation column is designed to separate a 70-30 mole percent mixture to produce a distillate product with 97.5% mole component 1 and a bottoms product with 20% mole component 1. The column has a partial condenser and reboiler and operates at 1 atm pressure. Determine the required number of theoretical trays and the optimum feed location at a reflux ratio of 1.5. Calculate the recovery of component 1 in the distillate. Calculate the recovery at the same distillate purity and reflux ratio if the feed is introduced three trays below the optimum feed tray. Use thermodynamic data from Problem 6.1. 

In selecting the feed tray for the main feed, it is desirable to maximize the number of stripping trays in order to minimize the reboiler duty required to achieve the specified methane stripping. However, the feed tray should not be too high. If it is, an excessive amount of ethane is lost in the overhead because of an insufficient number of rectifying trays. Table 9.2 shows reboiler duty requirements and ethane recoveries in the bottoms for different feed tray locations. The methane mole fraction in the bottoms is fixed in all cases at 0.001. The results indicate that the optimum feed tray is the third tray from the top. 

In many applications, different process streams that contain essentially the same components must undergo similar separation specifications. The process streams may have different flow rates, compositions, and/or thermal conditions. Each stream could be separated in a different column or the different streams could be first mixed together and then separated into the desired products in one column. Alternatively, the streams could be fed directly to the multistage column without prior mixing. Since the optimum feed tray depends on the feed s composition and thermal conditions, each stream is, in general, fed to a different tray. 

When both streams are fed to the same column, the above considerations still apply, but their relative importance may vary. If one of the streams is predicted to have a higher optimum feed tray than the other on account of both its composition and thermal conditions, then the optimum feed trays in the two-feed column would correspond to the same relative locations as with the streams fed separately. However, if the two factors are contradictory, as in this example, the optimum feed locations in the two-feed column may or may not correspond to the relative single-feed optimum locations. In the two-feed toluene-xylene column, the reboiler duty is minimized when feed 1 is higher than feed 2, which is contrary to the single-feed optimum feed trays. With the two-feed column, the... 

Now click the Streams page tab. A window opens on which the location of the feed tray must be given. For the moment, we set this in the middle of the column on Stage 16 (see Figure 3.28). Later, we will come back to this question and determine the optimum feed tray location by finding the tray that minimizes reboiler heat input. 

Initially, the total feed is split equally between the two columns. This is achieved in the Splitter labeled Tl on the flowsheet shown in Figure 5.33. Two Design SpecA ary are set up in each column to adjust distillate flow rate and reflux ratio to attain the 99.9 mol% product purities of all foiu streams. The optimum feed tray location is determined by finding the feed stage that minimizes reboiler heat input. In column Cl, it is Stage 19. In column C2, it is Stage 18. 

Aspen Plus UNIQUAC physical properties are used. A total condenser, partial reboiler and theoretical trays are assumed. The optimum feed tray location is determined for each selected number of total trays by finding the feed location that minimizes reboiler duty. 

LZD)jnjjj from simulations with 100 stages. Operate at 80% of flooding (based on Jim Fair s method) with sieve plates and 0.6096 m spacing between trays. Report number of trays, optimum feed tray, column diameter, L/D, Qr, and Q(.. 

Distillation operation Feeding on the optimum feed tray minimizes energy consumption for a given desired separation. However, studies [4] have shown that feeding on a non-optimum feed tray produces better dynamic controllability. This... 

Distillation columns are designed by setting the total number of trays (Nt) equal to twice the minimum number of trays Nmi and the optimum feed tray is estimated from the Kirkbride equation. 

The total number of trays is a compromise between the total equipment cost and the total utility cost. The optimum total number of trays and the optimum feed-tray location are determined to minimize TAC. The calculation procedure of Douglas is followed with the annual capital charge factor of 1 /3. The utility costs include the steam and cooling water for the operation of reboiler and condenser. The total operating costs include the utility costs and the entrainer makeup cost. The Aspen Plus simulation results for the three entrainers are summarized in Tables 9.10 to 9.12. 

Feed to be introduced on that tray which will result in the least total number of trays (optimum feed-tray location)... 

The true optimum feed-tray location is obtained by trial, altering the location and observing which location produces the smallest number of trays for the desired products. 

From the design perspective, the optimum feed tray, for a feed with only two components, is that tray where the ratio of the two components matches the ratio in the feed. If the feed is at its bubble point temperature, then the feed temperature and the tray temperature will be the same, at the same pressure. 

When this stepping procedure crosses the intersection of operating lines, this is the optimum feed tray (i.e., any other feed tray would require a greater total number of trays). Thus the feed tray Nf has been determined. 

Many columns have more than one feed. Each feed stream should be introduced onto its optimum feed tray if energy consumption is to be minimized. These feeds should not be mixed together and introduced onto the same feed tray if their compositions differ. 

---