Column distillation external balances

In this chapter we introduced the idea of distillation columns and saw how to do external balances. At this point you should be able to satisfy the following objectives ... 

Can we do the internal stage-by-stage calculations first and then solve the external balances To begin the stage-by-stage calculation procedure in a distillation column, we need to know all the conpositions at one end of the column. For ternary systems with the variables specified as in Table 5-1. these conpositions are unknown. To begin the analysis we would have to assume one of them. Thus, internal calculations for multiconponent distillation problems are trial and error. This is a second major difference between binary and multiconponent problems. 

Step 2. Design column 2. The feed to column 2 is the bottoms from column 1. It is a saturated liquid at 1.0 atmosphere. Specify the bottoms flow rate that will give you the desired purity of distillate and bottoms (do a very accurate external balance). Very small adjustments in the bottoms flow rate maybe necessary to meet the specifications. Find an approximate (L/D) (N2 = 50 and NF2 = 25 is sufficient start with L/D = 1.0 and move down). AspenPlus has convergence problems near the value of (L/D)j. ... 

On the specification sheets for both distillation columns the lines from the decanter should be input as feed on stage 2 (first actual stage in the column). Use ketde-type reboilers. Set the bottoms rate for the butanol column at the values calculated from the external mass balances. Initially, set Col-1 with N =... 

The resulting Makeup flow rate will be extremely small since losses of solvent are small (after all, no one wants to drink ethylene glycol with their alcohol or their water). If you immediately use this value of solvent makeup as a feed to the system. Aspen Plus will not converge. Start with the value you were using previously and rapidly decrease it (say by factors of roughly 5 or 10). Until the solvent makeup stream is at the desired value for the external mass balance, the extra ethylene glycol will exit with the distillate (water product) from column 2. This occurs because Bottoms flow rate in column 2 is specified and the only place for the extra ethylene glycol to go is with the distillate from column 2. 

Scheme J. This scheme directly adjusts the column material balance by manipulation of the distillate flow. The main advantage of this scheme is that it has the least interaction with the eneigy balance. In terms of a McCabe-Thiele diagram, this means that the slopes of the column operating lines can be held constant in spite of energy balance upsets. This independence ftom energy balance upsets is achieved by the scheme s ability to maintain a constant internal reflux even for variations in external reflux subcooling. When the temperature of the external reflux varies, the external reflux adjustment to maintain accumulator level offsets temporary internal reflux variations. If the accumulator level loop responds rapidly, the dis-tuibanoe will not propagate down the column, and the column s overall material balance remains undisturbed.

There is the crude distillation column (node No. 101) with further redistillation of naphtha 102. The tank 103 serves as the storage of light fuel oil LFO which is used also as the fuel in the furnace (included here in the distillation system 101) heating the crude oil. The crude oil is imported from the cmde terminal 01, all fractions are shipped to the tank farm 03. There is also further processing of the refinery gas 02. Note that codes of all external nodes not belonging to the balanced system start with zero. 

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