Basic Distillation Problem

Optimum Reflux Ratio. The reflux ratio affects the cost of the tower, both in the number of trays and the diameter, as well as the cost of operation which consists of costs of heat and cooling supply and power for the reflux pump. Accordingly, the proper basis for choice of an optimum reflux ratio is an economic balance. The sizing and economic factors are considered in a later section, but reference may be made now to the results of such balances summarized in Table 13.3. The general conclusion may be drawn that the optimum reflux ratio is about 1.2 times the minimum, and also that the number of trays is about 2.0 times the minimum. Although these conclusions are based on studies of systems with nearly ideal vapor-liquid equilibria near atmospheric pressure, they often are applied more generally, sometimes as a starting basis for more detailed analysis of reflux and tray requirements. 

Azeotropic and Partially Miscible Systems. Azeotropic mixtures are those whose vapor and liquid equilibrium compositions are identical. Their x-y lines cross or touch the diagonal. Partially miscible substances form a vapor phase of constant composition over the entire range of two-phase liquid compositions usually the horizontal portion of the x-y plot intersects the diagonal, but those of a few mixtures do not, notably those of mixtures of methylethylketone and phenol with water. Separation of azeotropic mixtures sometimes can be effected in several towers at different pressures, as illustrated by Example 13.6 for ethanol-water mixtures. Partially miscible constant boiling mixtures usually can be separated with two towers and a condensate phase separator, as done in Example 13.7 for n-butanol and water. 

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Binary Distillation 379 Material and Energy Balances 380 Constant Molal Overflow 380 Basic Distillation Problem 382 Unequal Molal Heats of Vaporization 382 Material and Energy Balance Basis 382 Algebraic Method 382... 

These basic methods were the inspiration for scores of papers, ranging from simplifications based on various assumptions to shortened rigorous mathematical derivations applicable to more or less specific distillation problems. Among these may be mentioned those of Fenske (19). Dodge and Huffman (18), Smoker (53), Jenny (27), and Underwood (56). 

Buiten (108), studying distillation problems, observed that a mixture of pyridine and formic acid evolved carbon monoxide when heated to about 150°C. We repeated this experiment, using a 1 1 (weight) mixture of y-pycoline and formic acid and a mixture of triethylene diamine and formic acid. In both cases the gas evolved principally contained CO. In the latter case also 4% H2 -f 4% C02 was present in the CO evolved. Although these basic compounds can form salts with formic acid (as is evident from the high boiling points), the decomposition products are practically identical with those formed in acid media. 

Plant design for processes of the first group utilizes, in its initial stage, the basic chemical stoichiometric eqnations describing the chemical reactions, in order to do material balance (MB) and heat balance (HB) calculations. However, for industries of the second gronp, calcnlations are usually based on setting up the total MB and component MB, for example, the solution of binary-distillation problems involved in the setting np of two equations in two unknowns, as was presented in Chapter 6. 

Distillation is one of the most important unit operations in chemical engineering. It forms the basis of many processes and is an essential part of many others. It presents a more difficult control problem then with many other unit operations, as at least five variables need to be controlled simultaneously and there are at least five variables available for manipulation. Thus, a distillation column provides an example of a multiple-input-multiple-output control problem. It is critical that variable pairing is done appropriately between controlled and manipulated variables. The overall control problem can usually be reduced to a 2 x 2 conposition control problem since the inventory and pressure loops frequently do not interact with the composition loops. This workshop will highlight some fundamental mles of distillation control and show how a basic distillation control scheme can be selected. 

Determine the An element of the relative gain array for each of the basic distillation control configurations (i.e. the LV, DV and LB configurations). Consider only the composition control loops so that the problem reduces to a 2 x 2 system. 

The basic bacl round and understanding of binary distillation applies to a large measure in multicomponent problems. Reference should be made to Figure 8-1 for the symbols. 

While we laud the virtue of dynamic modeling, we will not duphcate the introduction of basic conservation equations. It is important to recognize that all of the processes that we want to control, e.g. bioieactor, distillation column, flow rate in a pipe, a drag delivery system, etc., are what we have learned in other engineering classes. The so-called model equations are conservation equations in heat, mass, and momentum. We need force balance in mechanical devices, and in electrical engineering, we consider circuits analysis. The difference between what we now use in control and what we are more accustomed to is that control problems are transient in nature. Accordingly, we include the time derivative (also called accumulation) term in our balance (model) equations. 

Iron and chloride catalysts are basically disposable because they are considered to be rather cheap and difficult to recover from residual products, while Ni-Mo and Co-Mo catalysts are too expensive to be considered disposable (82). Recovery of very fine particles of MoS2 by hydroclone separation has been shown to be promising (83). Disposable catalysts added at levels similar to that of ash mineral contents significantly reduce the potential recovery of oil in both distillation and extraction. This is problematic because equal volumes of oil adhere to solid particles after separation. Slurry transportation of residues suffers from the same problem. Even if the cost of the disposable catalysts is affordable, adding 1 to 5% of the catalyst to the... 

The flange leak was taped over, and the exhaust-steam pressure dropped back to 100 mm Hg. The steam required to drive the turbine fell by 18 percent. This incident is technically quite similar to losing the downcomer seal on a distillation tower tray. Again, it illustrates the sort of field observations one needs to combine with basic technical calculations. This is the optimum way to attack, and solve, process problems. 

To a slurry of undecan-6-one toluene-p-sulphonylhydrazone (5.08 g, 15 mmol) (1) in 50 ml of glacial acetic acid is added sodium borohydride pellets (c. 5.67 g, 150 mmol, 24 pellets) (2) at such a rate that foaming is not a problem (c. 1 hour). The solution is stirred at room temperature for 1 hour and then at 70 °C for 1.5 hours. The solution is then poured into crushed ice, made basic with aqueous sodium hydroxide and extracted with three portions of pentane. The pentane solution is dried and concentrated in a rotary evaporator, and the residue distilled at reduced pressure (Kugelrohr apparatus) to obtain 1.96 g (84%) of undecane. Undecane has b.p. 87 °C/20 mmHg. 

All of the microbial alcohol production processes are confronted with two basic problems, namely product toxicity and energy-consuming product separation. However, recent progress made in distillation techniques allows ethanol production at economical feasible costs. 

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