Fractionator relative volatility

Fractionators = (relative volatility of key components) (viscosity of feed in centipoises). Relative volatility and viscosity are taken at average tower conditions between top and bottom. 

Liquid mole fraction Vapor mole fraction Relative volatility (kPa) Activity coefficent Enthalpy kj/kmol Isobaric specific heat, kJ/(kmol-K)... 

Component Feed Mole Fraction Relative Volatility... 

Flowrate (kmol/h) Mole Fraction Relative Volatility (P- 10.4 bar)... 

Component in feed Mole fraction, Relative volatility, Oy... 

The most volatile product (myristic acid) is a small fraction of the feed, whereas the least volatile product (oleic—stearic acids) is most of the feed, and the palmitic—oleic acid split has a good relative volatility. The palmitic—oleic acid split therefore is selected by heuristic (4) for the third column. This would also be the separation suggested by heuristic (5). After splitting myristic and palmitic acid, the final distillation sequence is pictured in Figure 1. Detailed simulations of the separation flow sheet confirm that the capital cost of this design is about 7% less than the straightforward direct sequence. 

Liquid mole fi actioa Vapor mole fraction Temper- ature, R Relative volatility Pressure activity coefficient Endialpy, Btii/ (Ib-mol) Heat capacity, Btu/(lb-mol- R)... 

LK = subscript for light key Nn, = minimum theoretical stages at total reflux Xhk = mol fraction of heavy key component Xlk = mol fraction of the light key component otLK/HK = relative volatility of component vs the heavy key component... 

To determine the reasonableness of the top and bottom compositions of a fractionation column, a Hengstebeck plot is fast and easy (Reference 4). First, select a heavy key component and determine the relative volatility (a) of all column components to the heavy key. The a can be otfeed or perhaps more accurately cc = (a,op oCboitom) - Plot In D/B versus In a and the component points should fall close to a straight line. If a fairly straight line does not result, the compositions are suspect. A nomenclature table is provided at the end of this chapter. 

N,n = Minimum theoretical stages at total reflux Q = Heat transferred, Btu/hr U - Overall heat transfer coefficient, Btu/hrfP"F u = Vapor velocity, ft/sec U d = Velocity under downcomer, ft/sec VD(js = Downcomer design velocity, GPM/fL Vioad = Column vapor load factor W = Condensate rate, Ibs/hr Xhk = Mol fraction of heavy key component Xlk = Mol fraction of the light key component a, = Relative volatility of component i versus the heavy key component... 

Acetonitrile serves to greatly enlarge the spread of relative volatilities so that reasonably sized distillation equipment can be used to separate butadiene from the other components in the C4 fraction. The polar ACN acts as a very heavy component and is separated from the product without much difficulty.The feed stream is carefully hydrogenated to reduce the acetylene level rerun, and then fed to the single stage extractive distillation unit. Feed enters near the middle of the extractive distillation tower, while (lean) aqueous ACN is added near but not at the top. Butenes and butanes go overhead as distillate, with some being refluxed to the tower and the rest water washed for removal of entrained ACN. 

Ethylbenzene is separated from mixed xylenes by fractionation using 360 trays and a high reflux ratio. Ethylbenzene is separated from the closest isomer paraxylene whose normal boiling point is only 3.90°F higher. The average relative volatility between ethylbenzene and paraxylene in the fractionation is about 1.06. The fractionator feed is entirely Cg aromatics which are prepared by the extraction of powerformate by the sulfolane process and by fractionation of the aromatic extract. 

Traditionally, LC and GC are used as separate steps in the sample analysis sequence, with collection in between, and then followed by transfer. A major limitation of off-line LC-GC is that only a small aliquot of the LC fraction is injected into the GC p. (e.g. 1 - 2 p.1 from 1 ml). Therefore, increasing attention is now given to the on-line combination of LC and GC. This involves the transfer of large volumes of eluent into capillary GC. In order to achieve this, the so-called on-column interface (retention gap) or a programmed temperature vaporizor (PTV) in front of the GC column are used. Nearly all on-line LC-GC applications involve normal-phase (NP) LC, because the introduction of relatively large volumes of apolar, relatively volatile mobile phases into the GC unit is easier than for aqueous solvents. On-line LC-GC does not only increase the sensitivity but also saves time and improves precision. 

Sm = total number of calculated theoretical trays at total reflux, from Equation 8-30 X]k = xlk = liquid mol fraction of light key Xhk = xhk = liquid mol fraction of heavy key Ik - hk = LK - HK= average relative volatility of column (top to bottom)... 

X = mol fraction of a component in liquid phase y = mol fraction of a component in vapor phase a = relative volatility P = constant in Equation 8-43 K = total pressure, psia L = total mols in liquid phase... 

X = mole fraction of substance in liquid y = mole fraction of substance in vapor a = relative volatility... 

Rg = sidestream component of minimum reflux S = flowrate of sidestream, mol/h Sk = flowrate of sidestream k, mol/h V = vapor flowrate, mol/h X = mole fraction of component i in liquid y = mole fraction of component i in vapor z F = mole fraction of component i in feed Zi Fj = mole fraction of component i in feed j z s = mole fraction of component i in sidestream Zj sk = mole fraction of component i in sidestream k a = relative volatility 0 = underwood parameter... 

The system is ideal, with equilibrium described by a constant relative volatility, the liquid components have equal molar latent heats of evaporation and there are no heat losses or heat of mixing effects on the plates. Hence the concept of constant molar overflow (excluding dynamic effects) and the use of mole fraction compositions are allowable. 

Assuming the equilibrium to be expressed in terms of relative volatilities a, and theoretical plate behaviour, the relation between the vapour and liquid mole fraction compositions leaving the plate is given by... 

The above methods work well for LC fractions in volatile and relatively nonpolar organic solvents. It is much more difficult to apply the same techn to reversed-phase eluents containing an... 

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