JANUARY, 1940
INDUSTRIAI, AND ENGINEERING CHEMISTRY
Acknowledgment The authors wish to express their gratitude to A. S. Brunjes of the Lummus Company and to Mr. Beamer of the Standard Alcohol Company for the supply of raw materials. Literature Cited (1) Bua, H. E., and Aldrin, E. E., S. A . E.Journal, 39,333 (1936). (2) Degering, E. F., J . Chem. Education, 13, 494 (1936).
(3) Fife, H. R., and Reid, E . W., IND. ENQ.CHEM.,22,513 (1930).
125
(4) Hodgman, C. D.,Handbook of Chemistry and Physics, 22nd ed., p. 672, Cleveland, Chemical Rubber Publishing Co., 1937. ( 5 ) International Critical Tables, Vol. 111, p . 33, New York, McGraw-Hill Book Co., 1928. (6) Othmer, D.F., IND. Esa. CHEY.,20,743 (1928). (7) Perry, J. H., Chemical Engineers’ Handbook, 1st ed., p. 308, New York, McGraw-Hill Book Go., 1934. PARTof a thesis presented by H. C. Miller in June, 1938, t o the faoulty of t h e Towne Scientific School of t h e University of Pennsylvania in partial fulfillment of t h e requirements for t h e degree of master of science.
SEPARATION PROCESSES The Principle of Minimum Dilution in Design of New or Unusual Processes MERLE RANDALL AND BRUCE LOYGTIN University of California, Berkeley, Calif.
I
S PREVIOUS papers of this series (3-9) exact methods of
analysis of separation processes were presented, based on the use of the molal property V S . mole fraction type diagram, together with methods of transferring the exact results to the more familiar y us. x type diagram. BoSnjakovid (1) also recently published numerous applications of heat content us. composition diagrams. At present few data are available for construction of the phase diagrams needed in the exact method, particularly for systems in which the heat content is an important factor. The authors are now engaged in collecting and correlating what data are available, and intend to publish i t in the form of convenient diagrams. The methods which have been reviewed and proposed are particularly helpful in a qualitative sense even when a lack of data does not permit them to be used quantitatively. It is possible only to sketch rough outlines of the general features of the necessary diagram where data for a heat content us. mole fraction diagram are lacking. Such a rough sketch will not allow us to determine graphically whether a given equipment operating in a given way will separate a particular raw material into specified products with a specified expenditure of heat. However, i t will permit us to decide qualitative, rather than quantitative, questions, such as whether a proposed change in the flow sheet of a process will improve or diminish the separation efficiency. It is particularly important to have general qualitative principles as guides in attacking new or unfamiliar problems. Without such principles a solution is reached only by groping. The molal property vs. mole fraction type diagrams offer considerable assistance in this respect, because of the simplicity with which each condition of material or heat balance governing the behavior of the process is represented by its distinct graphical construction. In studying a new or unusual process, i t is a relatively simple problem to set up the correct design diagram corresponding to a proposed flow sheet. The authors’ preferred procedure consists in first setting up the constructions which represent the over-all material balance (i. e., a center of gravity construction for the entering and product streams) and
the broader features of the process, and in filling in the details (4) subsequently. In determining the general features of the diagram, no attempt is made to have the location of individual points correspond to any particular compositions or stream ratios; these parameters are left free to be chosen after the general layout of the construction has been determined. The details are filled in in an orderly fashion, proceeding from one
To obtain the highest thermodynamic efficiency in a separation process, those streams which must be mixed should be potentially as nearly at equilibrium as possible. This principle is illustrated by application of graphical methods to the study of cases in which the principle is not adhered to. A number of faulty types of interconnection of separation units frequently found in existing industrial separation processes are pointed out. Application of the principle allows ready detection and elimination of such faulty connections, and permits a process to be easily reduced to the simplest and most effective form. end of the flow sheet to the other. As the details present themselves, it is easy to decide whether the unit or equip ment whose functioning they represent is serving a useful purpose or is really acting to lose the effect gained in some other stage in the process.
Principle of Minimum Dilution From such studies the authors have been impressed with the importance of the principle of minimum dilution, which
126
INDUSTRIAL AND ENGINEERIKG CHEMISTRY
in many cases allows a decision as to the best manner of interconnection of units to be made w i t h o u t recours e t o either graphical or numerical calculation or t o experiment (10). Any separation into purer products, beyond that occurring a t equilibrium, must be gained a t the expense of some form of e n e r g y . F r o m a thermodynamic s t a n d point a process requires the least expenditure of energy when it is reversible. Any mixing of materials which leads to spontaneous composition changes (such as interdiffusion of two completely miscible liquids N=O N=l of different composition or the changes which FIGURE 1. LOCATION OF NET bring a liquid and vapor FLOWCOMPOSITION AND ITS RELATION TO THREECLASSES of the same composition OF REFLUXING to their equilibrium compositionshill be irreversible to an extent depending on the spontaneous change of composition. Some of the energy which was expended in producing the initial separation is lost in such mixing, and must be resupplied in order to recover the lost separation. I n practice, other factors than energy expenditure enter in determining the optimum process. A completely efficient process (thermodynamically) must be infinitely slow and may require overelaborate apparatus. Thus i t may be the most extravagant of labor and equipment. As an example, the ordinary fractionating column requires the least expenditure of energy when operating a t minimum reflux, but a t the same time requires the largest number of plates and hence the most expensive equipment. Again, thermodynamically the fractionating column is essentially inefficient. The mixing of reflux streams with the vapors rising through the column produces irreversibilities which result in a much greater heat expenditure than is required by the reversible process. However, in many cases, economic factors indicate that a fractionating column even with known thermodynamic inefficiencies may be preferable to a thermodynamically more reversible process. To obtain the highest thermodynamic efficiency, those streams which must be mixed should be potentially as nearly a t equilibrium as possible. To be potentially a t equilibrium, two completely miscible liquids should have the same composition, temperature, and pressure. As an example, it is known that the best feed conditions obtain when a liquid feed is of as nearly the same composition, temperature, and pressure as the liquid on the plate a t which it is entered. The principle may be given a somewhat more specific though less precise form. In a properly designed process, there are no streams more widely separated than the final products of the process. If such a stream exists, then i t must have been irreversibly diluted, with accompanying loss of efficiency a t some intermediate stage. As a corollary of this statement, if one of the products of a process undergoes further separation, it is then obvious that there is no place in the process to which the most purified fraction may be returned without dilution and loss of efficiency. It is this form of the
VOL. 32. NO. 1
principle whose application will be tested by the following esamples.
Material Balance as Expressed by y 23s. x Diagram From the correspondences between the y us. x and H us. x type diagrams as developed in a previous paper (6),each unsaturated phase is represented in the y us. x diagram by a curve or sloping line. Each point on the line represents the composition of a saturated liquid and vapor phase which, when mixed in the proper proportions, will produce the given unsaturated phase. The proportions are given approximately by the slope of the line (with a reversal of sign). Thus a slope of - 3/4 requires approximately three moles of liquid to four of vapor; a slope of +11/* requires three moles of liquid t o a negative two moles of vapor. In the latter case two moles of vapor of the proper composition is to be extracted from three moles of liquid of the corresponding composition in order to obtain the given unsaturated phase. Point D in the H us. x diagram of Figure 1 represents a superheated vapor. Considered as the net flow point of a stepwise construction for an enriching section, it represents the composition and heat content of the vapor that would be left over as product, if a t some point in the section the liquid
~
Reflux
1-1
I
(a)
FIGURE2. CONNECTIONS WHICHRETURN A REFLUX RICHERIS LIGHTCOMPONESTTHANV.4POR DELIVERED FROM T O P OF COLUMN reflux could be extracted from the rising vapor without loss of heat to a condenser. That there would be residual vapor is indicated in the H us. N diagram by the fact that point D lies nearest the vapor curve and in the y us. x diagram by the fact that the slope of the reflux curve is less than unity. The same superheated vapor is represented in the y us. x diagram by the curve ( r 3 ) which may be considered as the reflux curve (i. e., operating line) of the construction for the enriching section. The points on the curve were obtained by transferring the liquid and vapor phase mole fractions of the rays through D to the y us. x diagram as x and y coordinates, respectively. In particular, point 2, a t which the reflux curve intersects the diagonal line y = x, was obtained by transferring the liquid and vapor mole fractions from the vertical line through D (ray labeled 2). Thus the intersection of the reflux curve with the line y = zin general gives the composition of the net flow (e. g., superheated vapor) phase which it represents.
Faulty Connection of Refluxing Equipment A casual inspection of Figure 1 might lead one to think that the separating efficiency of the plates in an enriching section would be much enhanced if some could operate in the region to the right of point 2, where the reflux curve passes below the line y = x; certainly the steps are of longer span. A column section can be made to operate in this region by re-
JANUARY, I940
'
INDUSTRIAL AND ENGINEERING CHEhlISTRY
turning liquid of the proper composition as reflux. The composition of liquid reflux and of the vapor issuing from the top of the section together define a point on the reflux curve (e. g. point 1). If this point is to lie below the line y = z, as i t must if any of the section is to function in this region, the liquid must be richer in light component than the vapor. This may be accomplished, as in an absorption tower, simply by feeding in a liquid of the required composition. It may also be accomplished by splitting the vapor into an enriched and a stripped fraction in some other equipment. The enriched fraction might then be returned as reflux, ensuring that the column would operate in the manner indicated. This procedure of returning an enriched fraction of the vapor as reflux should, according to the principle of minimum dilution, result in a loss of separating efficiency. Offhand we have been led to expect the reverse. However, from a consideration of the meaning of the net 00w point, D, i t is seen that no matter how the required reflux is obtained from the vapor, although the residual product may differ in heat content, it must have a composition corresponding to the composition of the net flow-hence, that of point 2 a t which the reflux curve cuts the diagonal, y = x. Thus by returning the enriched fraction of the vapor as reflux, not only is this enrichment wasted, but also the enrichment gained in all of the plates operating in the region to the right of point 2. Figure 2 illustrates two possible interconnections of equipment which return the enriched rather than the stripped fraction of the vapor as reflux. I n a the vapor is split into fractions by a partial condenser; in b it is split by means of a second fractionating column. Other examples might be given in which the vapor stream from the top of a column section is sent to other separating equipment, and finally an enriched rather than a stripped fraction is returned to the first section as reflux. The above considerations show that such an interconnection is to be avoided as wasteful of separating effect, in accordance with the principle of minimum dilution. Similarly, it may be shown that, according with the principle, the returning of a stripped fraction of the liquid from the bottom of a stripping- section as the bottom vapor stream is undesirable.-In general, the validity of the principle may be demonstrated by qualitative application of the graphical methods. For purposes of classification there are three Y ways of returning reflux. The reflux may be of the same composition as the vapor, or it may be an enriched (or a stripped) X fraction of the vapor. FIGURE3. DIAGRAM ILLUST h e composition' of TRATING THREECLASSES O F vapor and-reflux in the VAPORIZING EQUIPMENT three cases is reoresented, respectively,- by points 2, 1, and 3 in the y 2's. x diagram (and by the corresponding rays in the H vs. N diagram) of Figure 1. The corresponding three cases for return of vapor to a stripping section are shown in Figure 3. I n both diagrams point 1 represents the undesirable case. Such faulty connection of equipment would not ordinarily be expected in familiar processes which are simple and well understood. However, it is easy to make the mistake of connecting equipment in this fashion when the process is complex or new and unfamiliar. The authors have found examples of faulty interconnection of units in processes which are of commercial importance and in actual use. The point
127
FIGURE 4. DIAGRAM ILLUSTRATING Two CLASSES OF RETROGRADE PROCESSES
to be emphasized, which is coming to be appreciated in petroleum refining, is that attention must be given to the design of a process as a whole, as well as to the design of individual units. It is entirely possible to design each unit to operate most efficiently without having achieved the most efficient operation of the complete process, as a result of improper interrelationship of the units.
Retrograde Action of a Column Section Kormally the most volatile component concentrates a t the top of a fractionating column section. It is possible, however, to operate a column section in such a way that the relative order of the positions a t which the various components are found in the section is inverted. Such an operation may be said to be retrograde. Two examples are given by columns operating in the fashion indicated by the two stepwise constructions of Figure 4. Both are characterized by the fact that the stepwise construction lies entirely above the equilibrium curve in the y us. x diagram. A point on the reflux curve represents streams flowing a t the same interplate level, while a point on the equilibrium curve represents vapor rising and liquid flowing down from the same plate. Thus, in going down the column the direction of motion through the stepwise construction will be from equilibrium-curve point to reflux-line point along the vertical lines (lines representing liquid), and from reflux line to equilibrium curve along the horizontal lines in the y us. z diagram. When this concept is applied to the construction in which the operating line lies above the equilibrium curve, the composition of streams progresses from left to right through the stepwise construction in going down the column. Thus the more volatile component is found concentrated a t the bottom rather than the top of the column. The necessary condition for retrograde action is that the vapor streams be richer in more volatile component than the liquid streams flowing in the same interlevel by more than an equilibrium separation; i. e., that the reflux line lie above the equilibrium curve in the y us. 2 diagram. This means that in the H vs. N diagram the construction rays through the net
128
INDUSTRIAL AND ENGINEERIYG CHEMISTRY
flow (01) point must slope more to the side of the more volatile component than the equilibrium tie lines which they cut. Figure 4 shows two cases; in one the reflux line cuts the equilibrium curve, in the other it does not. The intersection of the equilibrium and reflux curves in the y us. x diagram corresponds to a coincidence of a ray through the net flow point with an equilibrium tie line in the H us. N diagram.
VOI,. 32, NO. 1
this residue from a feed of the same composition as the vapor received from the top of the retrograde section. This refluxing equipment might, for example, be an ordinary continuousfeed fractionating column, connected as shown in Figure 5b. The effect of the retrograde section is to mix the bottom product of this reflux-producing column with less pure material (the vapor fed to the retrograde section) and thus waste part of the separation obtained in the refluxing column. The retrograde section is bucking the action of the refluxing column, where it might be made to aid this action by connecting it in direct rather than retrograde fashion. The argument obviously holds as well for other values of the reflux ratio in the retrograde section.
Retrograde Action in Azeotropic Distillation
FIGURE 5. DESIGNAND FLOWDIAGRAMS ILLUSTRATIXG RETROGRADE SECTIOK
A
Hence the second case is represented in the H v s . N diagram by a stepwise construction in which the net flow point ( 0 2 ) falls in one of the two regions, L and R in which it is impossible for a ray to be tangent to either branch of the equilibrium caustic curve (4, Figure 2). I n order to obtain retrograde action, it is necessary to return a vapor to the bottom of the section which is richer in light component than the leaving liquid by more than an equilibrium separation. This may be accomplished, if the reflux ratio is greater than unity, by splitting the liquid into two fractions, one of which has the desired composition and the other is poorer in light component. Since the light component is enriched a t the bottom of the column in the retrograde case, it is the enriched fraction of the liquid which is being returned as vapor in order to produce retrograde action. This is contrary t o the principle whose application is being studied, and hence such retrograde operation should prove t o be detrimental to separating efficiency. The retrograde action may also be accomplished by returning a liquid reflux to the top of the column which is poorer in light component by more than an equilibrium separation. If the reflux ratio is less than unity, this reflux may be obtained from the vapor delivered a t the top of the column by splitting it into a stripped fraction of the desired composition for reflux, and a residual enriched product fraction. Again, since the more separated product a t the top is that richest in heavy component, the procedure used in obtaining retrograde action is contrary to the principle and hence should lead t o inefficient separation. It remains to test the principle by means of the graphical construction. Generally in retrograde action the streams leaving the column section are more nearly of the same composition than the streams entering. Hence the net effect of the retrograde process is to partially mix, rather than separate, the streams fed to it. The process effects a loss rather than a gain in separation and is detrimental rather than useful. Figure 5a represents the compositions of streams in a column operating in retrograde fashion with reflux ratio less than unity. Point D’,where the reflux line cuts the diagonal y = x , represents the composition of the residue which would remain after the reflux for the section is obtained from the top vapor. The equipment used in obtaining reflux, whatever its nature must be capable of producing the required reflux and
The discussion of retrograde action may seem only academic, until it is realized that there is a rather strong tendency to introduce retrograde sections into the flow sheets of new processes whose principles are not too familiar. One reason for the tendency is apparent when we note that the stepwise construction, in the case of a reflux line lying entirely above the equilibrium curve, is not limited as it is in the case of the direct processes. By the use of a retrograde process it would seem that the phenomenon responsible for the condition of “minimum reflux” (in which many plates near the feed stream become ineffective) might be avoided. The plausibility of the idea is even greater in the case of systems in which there is a binary azeotrope. Figure 6 gives the equilibrium curve for a system which possesses a minimum-boiling mixture corresponding to the azeotropic composition. An important example is that of alcohol-water mixtures. The stepwise construction shown is behaving in a retrograde fashion to the right of the azeotropic
I
/ I
x = M o / Fraction of Light Component in Liquid
DESIGNDIAGRAM FOR A SECTIONAT AN AZEOTROPIC COMPOSITION
FIGURE 6.
RETROQRADE
composition, since the azeotrope is found near the bottom of this particular portion of the column, whereas the normal tendency is for azeotrope to concentrate a t the top. At first glance, we might easily be lead to believe that here is a way to avoid the obnoxious characteristics of the azeotropic mixture. Apparently it is necessary only to connect one section of the column so that it will function in this retrograde fashion; then it will carry the mixture across the azeotropic composition and thus avoid the necessity of special “deazeotropizing” processes. That such is not the case may readily be seen from a consideration of material balance. Let it be supposed that the available raw material is not richer in light component than the azeotropic composition. This is the usual case with alcohol and water mixtures. Then the reflux to the top of the retrograde section, which is richer in light component
JANUARY, 1940
INDUSTRIAL AND ENGINEERING CHEMISTRY
than the azeotrope, must be obtained from the products of the distillation. Since it is the retrograde section which is to be responsible for producing material richer in light component than the azeotrope, it is necessary to obtain the reflux entirely from the vapor delivered by this section, by any cbnvenient means.” TGS requires first that the reflux ratio be less than unity; otherwise there will not be sufficient vapor. This being the case, the reflux line must cut the line y = 1: to the left of the azeotropic point (Figure 6). Thus the process which will obtain the required reflux from the top vapor must also be capable of yielding a residual top product whose composition lies on the opposite side of the azeotropic composition from that of the reflux. This refluxing process alone would FIGURE 7 . SCHEMATIC DIAbe sufficient to achieve GRAM OF GAY’SPROCESS the desired result of crossing the azeotropic composition. The retrograde section, if it-can be made-to function a t all, is unnecessary and hence wasteful. A more thorough investigation of other possible connections utilizing the retrograde section has shown that they cannot be made to function in the desired fashion without the aid of some other process, which is of itself capable of producing the desired result. Since the process must always depend on direct functioning equipment, as long as the reflux streams are obtained from product streams, the presence of a retrograde section can only counteract the effect of some of the direct functioning sections and is therefore detrimental. A particular example is the process proposed by Gay (8) for the separation of components in a system possessing both a minimum-boiling mixture and a region of two liquid phases. The flow sheet of the process is similar to that recently proposed by the authors for the same purpose (“), as Figure 7
129
shows. I n this process it is possible to have one of the abovefeed sections operating in retrograde fashion. This may h a p pen if a feed is entered a t too low a level in the columns. I n the design diagram Gay avoids this condition by his construction for minimum reflux. I n practice this is not sufficient since the actual column may not behave exactly as it was designed (fractional plates are not possible, etc.).
Summary The principle of minimum dilution has been tested in a number of cases by means of qualitative applications of the graphical ‘analysis. The authors have applied the principle to advantage in their work of separating the isotopic forms of water (where any separation is dear and cannot be wasted, 9) and in developing improvements on existing separation processes by the elimination of unnecessary and even detrimental steps in the process. I n general, itt has proved of particular value in reducing the number and complexity of stages in a process to a minimum. Although the principle has been illustrated by application to distillation processes, it applies by analogy in practically unaltered form to extraction and to other processes in which the reflux or recycle streams are obtained from the product streams. The application to processes of the absorption type, in which there is no recycling, must necessarily lead to results different in detail but in accordance with the principle.
Acknowledgment The clerical assistance of the Works Progress Administration (0. P. No. 665-08-3-144)is gratefully acknowledged. Bruce Longtin, the junior author, is Shell Research Fellow in Chemistry a t the University of California.
Literature Cited (1) BoSnjakoviO, “Technische Thermodynamik”, Vol. 11, Dresden and Leipzig, Theodor Steinkopff, 1937. (2) Gay, “Distillation et rectification”, Paris, Bailliere et Fils, 1935. (3) Randall and Longtin, IND.ENQ.CHEM.,30, 1063 (1938). (4) Ibid., 30, 1188 (1938). (5) Ibid., 30, 1311 (1938). (6)Ibid., 31,908 (1939). (7)Ibid., 31, 1181 (1939). (8)Ibid., 31, 1295 (1939). (9) Randall and Webb, Ibid., 31,227 (1939). (10) Treub, Rec. t m v . chim., 53, 497,688 (1934); 56, 41,510 (1937).
