INDUSTRIAL AND ENGINEERING CHEMISTRY
900
Vol. 23, No. 8
High-pressure Rectification I-Vapor-Liquid Equilibrium Relations at High Pressures' L. W. T. Cummings RESEARCH LABORATORY OF APPLIED CHEMISTRY,
DEPARTMENT OF CHEMICAL ENGINEERING, hfASSACHUSETTS INSTITUTE OF TECHNOLOGY,
CAMBRIDGE, MASS.
The effect of high pressure on the vapor-liquid ary. A section through the equilibrium relation is such that rectification of binary higher operating pressolid at constant temperature mistures of most common occurrence has the following s u r e s in rectification gives the relation between the limitations. As the operating pressure approaches the requires that the limitations pressure and composition of critical pressure of the component of lowest critical of this process be clearly dethe liquid and gaseous phases pressure, separation becomes increasingly difficult. fined. Since the vapor-liquid a t that temperature. Figure Above this range of pressure and up to the maximum equilibrium r e 1a t i on is the 2 shows this relation for sevpressure which the two-phase system exhibits, separabasis of design of rectificaeral different temperatures. tion may be effected only within limited ranges of contion equipment, the effect of The figure exhibits graphicentration. Above the maximum pressure no separapressure on this relation will cally the increasing limitation tion by rectiacation is possible. indicate the limitations to be on the tGo-phase region as the encountered. temperature increases from No direct meaourements of the vapor-liquid equilibrium the critical temperature of the more volatile component to relations have been made at pressures merging into the that of the less volatile component. critical region. They may be derived, however, from the A horizontal line, designating constant pressure, on a given data of investigators interested in the equation of state of pressure-composition diagram represents a non-variant sysbinary mixtures. tem, and its intersections with the liquid and vapor curves The data of Caubet (1) on the pressure-temperature rela- indicate the composition of liquid and vapor in equilibrium. tions of carbon dioxide and sulfur dioxide mixtures a t constant Vapor-liquid equilibrium relations a t several temperatures have been constructed by the appropriate choice of pressures on the corresponding pressure composition diagrams and are shown in Figure 3. The constant-temperature vapor-liquid equilibrium relation at high pressure exhibits a maximum and a constant boiling point, and is in marked contrast with the relation at low pressures since it is discontinuous a t the constant - b oil i n g point mixture. Any m i x t u r e richer in the more v o l a t i l e c o m p o n e n t exists only as a homogeneous p h a s e ; h e n c e r e c t i f i c a t i o n can proceed no further. composition are shown in Figure 1. The solid lines represent The diagonal in Figthe loci of the boiling points, and the broken lines the loci ure 3 is the limit of of the dew points of particular mixtures, the compositions concentration beof which are indicated as mol. fraction of sulfur dioxide. yond which no recThe vapor-pressure curves of the components flank the dia- tification can t a k e gram, The critical temperature, T,,and the cricondentherm place. At tempera(critical condensation temperature), T,, of mixture 2 are tures above the critiindicated. The former is the temperature a t which the oal temperature of liquid and vapor phases have become identical, and the the more v o l a t i l e latter is the highest temperature at which any liquid may be c o m p o n e n t of a condensed from a particular gaseous mixture. Between binary mixture, the these temperatures condensation may be accomplished only shape and limitation partially on account of retrograde condensation. The en- of the vapor-liquid velope curve is the locus of the critical temperatures of all equilibrium diagram mixtures. at c o n s t a n t temThe data of Figure 1 may be represented by the surface of a perature depend ensolid, the coordinates of which are pressure, temperature, tirely upon how near and the composition of the liquid and gaseous phases. The the temperature apgeneralized surface has been described completely by Rooze- proaches the critical boom ( 7 ) . The interior of the solid is a two-phase region, temperature of the the exterior a single-phase region, and the surface the bound- less v o l a t i l e component. 1 Recdved April 18. 1931.
HE use of increasingly
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August, 1931
INDUSTRIAL AND ENGINEERING CHEMISTRY
901
MOL PCRUNT CARBON UOXlK IN CACH P H U L
Although the relation at constant temperature is important in a thermodynamic treatment, practically it is simpler to carry out distillation processes at substantially constant pressure, and the derivation of the relation at constant pressure follows. A plane of constant pressure will intersect the messure-temDerature-comDosition surface in the region under consideration to give temperature-composition diagrams as represented in Figure 4. The temperature-composition diagram is continuous for all concentrations below the critical pressure of carbon dioxide, but becomes increasingly limited as the maximum pressure which the two-phase system exerts is approached. Lines of constant temperature on a given teniperaturecomposition diagram intersect the liquid and vapor boundary curves at concentrations of liquid and vapor which are in equilibrium. The vapor-liquid equilibrium diagrams at constant pressure in Figure 5 were constructed by selecting a
sufficient number of constant-temperature intersections. Below the critical pressure of both components the equilibrium relation is continuous through all concentrations, quite similar to the relation at low pressures. However, a t pressures above the critical pressure of either component the equilibrium relation is discontinuous a t the concentration at which the relation intersects the diagonal of the diagram. Since only a homogeneous phase exists a t concentrations beyond the intersection of the relation with the diagonal, rectification beyond such concentrations cannot take place. At a total pressure of 74 atmospheres or below, it is possible to rectify all mixtures of carbon dioxide and sulfur dioxide, although the separation becomes more difficult at the higher pressures because the equilibrium relation approaches the diagonal as the pressure increases. Between 74 and 95 atm o s p h e r e s rectification can be effected only within certain restricted concentrations. For example, at a total pressure of 90 atmospheres it is not possible to rectify mixtures containing less than 33 or more than 72.5 mol. per cent of carbon dioxide. Above 95 atmospheres total pressure, which is the maximum pressure which the two-phase system exhibits, no rectification can take place. The coincidence of the maximum pressure with the critical pressure of one of the components depends on the similarity of the components composing the system. Figure 6 shows several constant-total-pressure v a p o r-li q u i d equilibrium relations for the system composed of oxygen and nitrogen. Here the maximum pressure and the critical pressure of the less volatile component, oxygen, are identical. This case results in an equilibrium relation which is discontinuous in the critical region only for mixtures richest in the more volatile component, as in contrast with the system previously considered, in which the maximum pressure was higher than the critical pressure of either com-
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INDUSTRIAL AND ENGINEERING CHEMISTRY
ponent and the equilibrium relation was discontinuous for mixtures both rich and lean in the more volatile component. Binary mixtures having an envelope curve with a minimum exhibit equilibrium relations which are discontinuous in the middle ranges of concentration and, in addition, may be discontinuous in concentrations either rich or lean in the more volatile component. Such systems, together with those in which immiscibility occurs, are comparatively rare and are not considered further. The data of Kuenen (4) on unknown mixtures of ethane and butane indicate that mixtures of the normal paraffin hydrocarbons have a maximum pressure higher than the critical pressure of either component and that their equilibrium relations in the critical region probably present limitations similar to those of the carbon dioxide-sulfur dioxide system. The equilibrium relation a t 10 atmospheres in Figure 6 is that of Dodge (2, 3) and was determined by analyzing directly liquid and vapor in equilibrium. The constant-pressure equilibrium relations at higher pressures were obtained by the method outlined from three dew-point and boiling-
Vol. 23, No. 8
point curves determined by Kuenen and collaborators (6,6). The figure shows qualitatively that the direct and indirect methods are in agreement. The limitations imposed upon rectification by pressure are summarized as follows: Rectification a t constant total pressure may be effected with increasing difficulty a t pressures up to the critical pressure of the component of lowest critical pressures; to a limited degree between this pressure and the maximum pressure; and not at all above the maximum pressure. Literature Cited (1) Caubet, Compt. rend., 1S0, 828 (1900). (2) Dodge, Chem. M e t . Eng., 88, 622 (1928). (3) Dodge and Dunbar, J . A m . Chcm. Soc.. 49, 591 (1927). (4) Kuenen, Proc. Roy. SOC.Edinburgh, 21, 433 (1895-97). (5) Kuenen and Clark, I b i d . , No. 150 (1917). (6) Kuenen, Verschoyle, and Van Urk, Comm. Phys. Lab. Leiden, No. 161 (1922). (7) Roozeboom, “Heterogenen Gleichgewichte,” Vol. 11, Part 1 (1904).
Effect of Temperature on the Corrosion of Zinc’ G. L. Cox RESEARCH LABORATORY OF APPLIEDCHEMISTRY, DEPARTMENT OF CHEMICAL ENGINEERING, MASSACHWSETTS INSTITUTE OF TECHNOLOGY, CAMBRIDGE, MASS.
The corrosion of zinc in distilled water continually water and changes its physiHE corrosion rates of saturated with air is affected tremendously by temcal c h a r a c t e r i s t i c s , and metals in oxygenated perature. The most important factor influencing a t 125’ C. the hydrous oxide water a t varying temthese rates at the different temperatures is shown to is reported to have the comperatures are influenced, in be the physical nature of the corrosion-products film. position Zn(OH)*. g e n e r a l , by the manner in The experimental results are satisfactorily explained Although the above obserwhich t e m p e r a t u r e affects by a consideration of the physical properties of the v a t i o n s were made of the (1) the s p e c i f i c r e a c t i o n corrosion-products film, rate of transfer of dissolved precipitated hydroxide withrates of the various corrosion oxygen, and oxygen concentration of the corroding out reference to its formation processes, (2) oxygen solumedia. on a metal surface d u r i n g bility of the water, (3) the corrosion processes, it is rate of transfer of dissolved oxygen through the liquid, and (4) the nature of the corrosion highly probable that, if the character of the precipitated hyproducts. I n general, all other conditions being constant, droxide changes with temperature, the products of corrosion equal small increments of temperature cause approximately will likewise change, and thereby in some manner affect the the same multiplication of specific reaction rates of chemical ultimate corrosion rates. On the basis of these indications, processes, but it has been demonstrated that the corrosion tests have been made of the corrosion rate of zinc in aerated rate of iron in water of constant oxygen concentration in- distilled water a t different temperatures, and the physical concreases linearly with temperature, and that the corrosion re- dition of the corrosion-products film has been studied with the action is influenced chiefly by the rate of transfer of oxygen view of explaining some of the observed phenomena. to the metal surface ( 7 ) . The rate of transfer of oxygen will Experimental Method be deterinined largely by the nature of the corrosion products and the effect of temperature upon the rate of diffusion. The testing apparatus (Figure 1) consisted of a 4-gallon However, on account of the decreasing solubility of oxygen in Pyrex-glass jar containing distilled water continually satuwater with rising temperature, the corrosion rate of iron in rated with air, covered, and fitted with a condenser to prevent water in open systems is a balance between the rate of transfer loss of water by evaporation. No effort was made to reof oxygen to the metal surface and the oxygen solubility. move the carbon dioxide from the water or the air for Therefore, over a range of temperature, the net result is that aeration. The temperature of the water in the jar was the corrosion rate passes through a maximum. maintained by an oil bath heated with an electric immersion It is believed that this explanation will hold in general for heater and controlled by an external variable resistance. a number of active metals, provided the corrosion products When once adjusted for any particular temperature, the water of all are affected by temperature in the same manner. It is a in the jar remained substantially constant, the variation recognized fact that the hydroxide, or hydrous oxide, of zinc being not more than * 2 O C. The temperatures were noted has varying properties as the temperature is changed (2, daily, and the averages of the entire test period were used. 3 , 4 , 5 ) . At room temperature the hydrous oxide precipitates Specimens 8 x 12 x 0.076 cm. were cut from sheets of in the form of a gel with an indefinite amount of adsorbed zinc (99.9 per cent purity), and drilled with 6-mm. (‘/d-inch) water. When the precipitate is heated, it gradually loses holes near each end. The total area of the faces and edges of each specimen was approximately 190 sq. cm. The speci1 Received April 24, 1931.
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