US2833644A - Resolution of alkali metal amalgams - Google Patents

Description

May 6, 1958 J. M. AVERY 2,333,644

RESOLUTION OF ALKALI METAL AMALGAMS Filed Nov. 17, 1955 FIGURE 2.

the ionized salt.

United States PatentO RESOLUTION OF ALKALI METAL AMALGAMS Julian M. Avery, Greenwich, Conn., assignor to Ethyl Corporation, New York, N. Y., a corporation of Delaware Application November 17, 1955, Serial No. 547,466

5 Claims. (CI. 75-66) This invention relates to the resolution or separation of the components of dilute alkali metal amalgams. More specifically, the invention relates to a new and novel process for the separation of dilute sodium amalgams into fractions including a fraction greatly enriched in sodium metal, and a separated mercury enriched fraction having v fice , out process which comprises, in its broadest terms, the

sure.

appreciably lower sodium metal content than the initial amalgam.

The manufacture of the alkali metals, especially sodium in a form suitable for use for their metallic content as such (as opposed to the manufacture of caustic aqueous solutions (has been a problem which has intrigued the cells of the Downs type such as is disclosed in U. S..

Patent 1,501,756. Mercury cathode cells on the other hand are well known as permitting a highly effective process for forming amalgams of the alkali metals by depositing from aqueous solutions the metal cations of In such operations, the cathodic product is a very dilute amalgam of the sodium metal.by dilute meaning appreciably below one weight percent concentration, usually of the order of about 0.2 to not over about 0.3 weight percent in the case of sodium. Because of this extremely low concentration, the cathodic product from such cells has heretofore been employed only in subsequent reaction with aqueous liquids to form a high grade of aqueous caustic solution. Unfortunately, the inherent economies of the electrolysis itself have not been realizable when sodium or an enriched sodium fraction is the other concentrated in themercury content and available for return to the electrolysis operation for re-deposition of additional alkali metal values. A sodium enriched fraction is suitable, for certain purposes, as a replacement for pure sodium metal, which finds numerous applications in chemical processing as a strong reducing agent. For example, amalgams are eficctive for the preparation of high molecular weight alcohols by reduction of fatty acid derivatives such as glycerides, and the.production of metal alcoholates by reaction with a suitable anhydrous alcohol. a

It is therefore an object of the present invention to provide a new and highly efiicient method of accomplishing the foregoing resolution into a sodium enriched stream and an amalgam greatly denuded of sodium content. Hereafter, for clarity, this denuded stream is to be referred to as the mercury enriched stream. It will be understood that, throughout the discussion following, all the streams of the process normally contain appreciable weight percentages of mercury and that the terms en: riched' and denuded are of relative significance. A fur ther object is to provide a process'which accomplishes vaporization of a mercury rich fraction from a dilute amalgam feed, the heat involved in said vaporization being derived by the liquefaction of the same mercury rich vapor stream after compression to a moreelevated pres- I This compressed stream is recondensed in indirect heat exchange relationship with the boiling liquid from which the mercury rich vapor is vaporized. This boiling liquid, as willbe apparent from the description hereafter yields the enriched sodium fraction and is withdrawn from the vaporizing chamber for subsequent usage and processing in various ways. As'described in more detail hereafter, the highest degree of economies, obtainable in the most effective and preferred species, at provided by supplementing the above described main operation with further indirect liquid-liquid heat exchange relationships, and in addition a small amount ofdirect heat is frequently provided to the system .as isdescribedhereafter, and particularly during start up operation. The sodium enriched stream may contain, in theory, up to about 25 percent by weight sodium. However, this limitation is not easily attained for practical reasons, and in fact, a more realistic top level of concentration is to provide enrichment up to about 15 weight percent sodium. Furthe'r, in practice, the preferred range of concentration of the sodium enriched product stream is from about 3 to about 5 weight percent sodium. This corresponds to, for a feed amalgam having 025 weight percent sodium, removal of 91 and 95 percent of the initially present mercury, respectively. I I

Although the above described operation is susceptible of numerous different andhighly effective embodiments, in general it is preferredthat the vaporizing operation be carried out as a single stage vaporizing step In addition, a] highly preferred embodiment involves an entirely pressurized system which, surprisingly, is particularly effective. Another type of embodiment is especially effective for low'pressure operation and employs a particular contacting technique for providing the advantages of the high pressure operation.

t The details of the invention and of the best modes of its operation will be readily appreciated from the detailed description and working examples given hereafter, and from the accompanying figures wherein:v

Figure 1 is a schematic representation of a typical apparatus set up suitable for carrying out preferred embodiments of the process, and

Figure 2 is a diagrammatic detail portion showing details of the construction of a vaporizing chamber particularly' suitable for performing certain embodiments of the process.

The mode of operating the process in the broadest form is most readily described with reference to Figure '1, which. illustrates schematically the main units of apparatus and their interrelationship for most embodiments of the process. Referring to Figure l, the principal units are a v'aporizin'g chamber 11, a compressor 21,- and a heat transfer coil 14 within the vaporizing chamber 11.

electrical energy, when required. The compressor 21.

supplies .by either positive displacement or centrifugal force, increased pressure to the vapor, the compressed vapor .being discharged through a high pressure vapor line 13, which feeds a heat transfer coil'or 'exchanger 14 21. A supplemental heater The actual work input to the compressor and heat input to the heater will be somewhat higher than these values;

whichis positioned within the vaporizing space 11 in such a manner as tobe in contact with and provide heat to liquid therein. In giving up ,heat to liquid in the vaporizer 11,. the compressed vapor is condensed in the coil and discharged through the hot condensate line 15.

In the most effective forms of the process, the sensible heat content of, the hot condensateis utilized in afheat exchanger 31 to. preheat fresh amalgam, feed to the op eration, the fresh. feed being introduced through a line 32 to the heat exchanger 31. The condensate so cooled is discharged through line 33 and is then available for recycling .tothe electrolysis operation. Preheated feed amalgam after leaving the heat exchanger 31 passes through a line 32. to an additional heat exchanger 17.

A liquid line 16 from the body of the vaporizer 11 is provided to lead a sodium-rich phase, at the boiling temperature thereof, from the vaporizer 11 to the heat exchanger, wherein additional preheating of the amalgam feed is provided. The thus cooled concentrated enriched sodium phase is discharged from this supplemental heat exchanger .17 through line 18 and is available for subse- 'quent operations. p p

' i To further illustrate the process, the following examples describe typical continuous operations at various 1 conditions. The first example illustrates a low pressure embodiment, that is, approximately 1 atmosphere pres- V sure.

Example I An amalgam feed of 2,520,000 parts by weight is fed at a uniform rate through feed line 32 and the heat exchanger 31. The amalgam contains 0.25 weight per- 5 cent sodium. The initial temperature is a nominal or ambient temperature of about 70 F. After preheating in the heat exchangerBl, the amalgam is at a temperature of about 680 F. The heated amalgam then passes to the body of the vaporizer wherein a boiling concen-.

trated amalgam is maintained, having a 3.5 weightpercent sodium. concentration. The vaporizerll operates with a boiling temperatureof; about 755 and apres- I sure of about 1 atmosphere, plus minor pressure ditferi entials necessary to overcome fluid'flow friction restrictions. The vaporizer converts to thevapor phase 2,346,- 000 parts by weight of the mercury, there being substantially .no vaporization of the sodium content. The

vaporized mercury then passes through line 12 ,to,the"

compressor 21, and is there raised by a compression ratio of about 2.5, to an absolute discharge pressure of about 35 pounds per square inch absolute. This compressed mercury then passes through the heat exchanger line 14 and condenses at the higher pressure and at a temperature of about 1300) F. In addition to being liquefied, the mercury discharged from the coil 14 is subcooledto about 770 F., or. 15 F. above the boiling amalgam in the vaporizer ,11. In addition, heatis supplied-by the electrical'heater 19.. After passing through the heat exchanger coil 14 in the vaporizer, the temperature of the condensate is about 770 F. This hot mercury stream then passes through line 15 to the heat exchanger Bland is reduced in temperaturetherein'to about 78 F.

In the operation of this example the heat and work" inputs are as follows:

t B. t. u. per pound a mercury vaporized Work by compressor 9.7 Heat by heater 7.8 g

dependent on the efiiciency of operations.

From the foregoing example it is. seen that approximately 93 percent of the original mercury component of the amalgam feed is separated and the concentration of sodium in the product sodium enriched stream is increased by a factor of 16, thus providing a chemical material having a sodium concentration effective for use as such in supplying sodium values for chemical reaction.

7 The efliciency of the low pressure operation as described above is limited in the gross aspects because of the rela-,

temperature differential and, by decreasing the thermal driving force for heat transfer, results in an increase in theheat transfer area necessary for a given rate of production. Thus, for example, in an installation operation according to the above conditions described in Example I, a heat transfer area of about 1.5 sq. ft. per 1,000

pounds of feed per hour is required, the mean temperature differential in the vaporizing heating coil being about 75 F.

The deficiencies of low pressure operation indicated above can be circumvented by an elevated pressure operation as illustrated by the examples below.

composition and sodium enriched fraction being obtained 1 but with the following changes in conditions. The temperature of the boiling system is increased approximately 200 F. to about 965 F. The boiling pressure is about pounds per square inch gauge, and the mercury vapor is compressed to about -145 pounds per square inch gauge, the compression ratio being about 1.6. Operations under these conditions reduce the work input via the compressor to approximately 4.8 B. t. u.s per pound of mercury vaporized. In contrast to the results of Example I, however, the process provides excess heat to the extent of about 2 B. t. u.s per pound of mercury-and this can .be employed for providing process steam through various heat recuperating heat exchangers.

Example III The procedure of Example 11 is again repeated, except that in this instance, the compression step is altered to provide a compression ratio of 1.66 to 1, the pressure of the compressed mercury vapor being pounds per square inch. In addition, the condensed mercury, in being discharged through the vaporizer coil 14, is cooled to only 972 F. thus increasing the final temperature Example IV The procedure of Example II is repeated, except the compression ratio is again increased to about 1.85:1, the

discharge pressure from the compressor being pounds 3 per square inch. In this case also, the condensed mercury is subcooled to only about 987 F. before being discharged from the vaporizer heating coil 14, or to a temperature of 30 F. above the temperature of the boiling liquid in the vaporizer. In this case, the work input by the compressor is about 7.1 B. t. u.per pound of mercury vaporized but the heat input 'bythe supplemental heater is reduced to less than 0.2 B. t. 11. per pound.

From the foregoing examples it is seen that a higher pressure operation provides marked advantages in not only reducing the work input required to compress the mercury vapor, but also in appreciably reducing the amount of direct heat which must be added to supplement the operation. The benefits from increase in pressure of the vaporizing operation are surprising, in that it is well known that in all liquid-vapor systems, the enthalpy of vaporization decreases with increases in pres sure. Thus, in Example '11,. where the compressor discharge pressure is about 60 pounds greater than the vaporizing pressure, it would be expected that the enthalpy of condensation at the higher pressure would be so much lower than the enthalpy of vaporization of the mercury from the vaporizer, that extremely large heating coil surfaces would be required. This would be considered necessary to allow recuperation of some of the sensible heat of the condensed mercury. 'Actually, it is found that the vaporizing rand condensing operations at higher pressure levels are much more efiicient and provide relatively lowor heat exchanger requirements than at the low pressure ranges. I

The foregoing discussions in the examples are directed to the most effective embodiment of the process, i. e., a single-stage vaporizing operation, conducted either at low pressures, or preferably as described, at pressures of at least 85 and preferably about 100 pounds p. s. i. a. N-umerous other embodiments of the process will be highly effective, and in some situations will be preferred for specific installations. For example, if desired, a plurality of sequential vaporizing stages can be employed. These can include what are hereafter termed as forward feed or, alternatively, backward feed systems. The characteristics of the forward feed system are that in the forward feed system, a plurality of Vaporizers, say for example three, are employed wherein the liquid phase from a first stage is transferred to the second stage and, the liquid from the said second stage is transferred to a third stage. Concurrently vapor from the first stage is condensed in a heating coil used to supply vaporizing heat in the second stage, and vapor from the second stage is employed in the third stage condensing coil, vapor from the last stage being compressed and returned to condense in the first stage. In contrast, in a backward feed system the sequence of liquid and vapor flows is reversed. Thus in a three-stage system, the vapor from the third stage 1s condensed in the heat transfer coil in the second, vapor from the second stage is condensed in the heat transfer coil from the first stage, whereas the liquid sequence flow is the same as in the forward feed system. The following illustrates the relative merits of the several systems, the comparison being on the basis of a maximum vaporizing pressure of 100 pounds. In a single stage system, the work input, as already shown, can be vin the order of about 4.8 B. t. u.s per pound of mercury evaporated. In a two-stage backward feed system the work input can be about 4 B. t. u.s per pound of mercury, and in a three-stage backward feed system the work input can be as low as 3.2 B. t. u.s, whereas if the forward system is used the work input is slightly higher at about 3.4 B. t. u.s. The foregoing data apply when the system is operated to provide at least a minimum temperature differential between the boiling liquid and condensed mercury vapor of about 8 F. Generally, although as indicated above, multiple-stake systems result in appreciable reduction of work input for a given amount of separation, in practice a multiple-stage system has severe disadvantages in that, in a three-stage system a compression outlet temperature of over about 1200 F. is required and this temperature level puts a strain in available materials of construction. In the case of a double effect system, it is feasible to opcrate with a compressor outlet temperature below 1200 F., but in such instancestthe pressure of operation of the highest pressure-stage'is necessarily not overabout 50 p. s. i. a. or 35 pounds p. s. i. -g. It is found 'that'in such instances the conventional type of vaporizing operation is highly undesirable because the high density of the boiling amalgam liquid system results in appreciable .boiling point rise if any significant liquid depth is encountered in the space. Accordingly, the preferred conditionsfor operation of a multiple-stage .unit are a two-stage vaporizing system with the Vaporizers operating atnot more than about 35 pounds p..s. i. g. I I

However, it is to be understood that if it is desired to employ a multiple-stage system orv a single-stage system under conditions which would ordinarily result 'in'a compression outlet temperature. above 1200 F., .a technique which can be used to alleviate the problem of compression outlet temperatures being above 1200 F., is to feed liquid mercury to the compressor inlet in amount not greater than that necessary to take up the sensible heat of the overhead mercury vapor going to the compressor. A still further technique which can be employed is theme of external cooling of the overhead 'mercury vaporyto compensate for such sensible heat. I

Any of various types of compressing devices can be employed toprovide the necessary increase in pressure of the mercury vapor. It is preferred to employ rotary, single-stage compressors for this purpose, although positive displacement machines are'also emp'loyable. a 2

As mentioned above, one of the limitations on the process, particularly in multiple-stage operations, arises because of the high density of the amalgam feed and the mercury rich product stream. It is found that, in the vaporizing operation, the boiling actually occurs at the interface of the liquid-heat exchanger surface. If the heat exchange surface is submerged an appreciable depth below the level of the boiling liquid, the density of the liquid results, in effect, in the boiling step itself being at appreciably higher temperature than the temperature corresponding to. the pressure of the vaporized mercury discharged from the vaporizer. This result, in turn, reduces the effective temperature dififerential available for heat transmission in the vaporizer. This feature is overcome as illustrated by the examples, by conducting the vaporization at an elevated pressure, preferably of the order of about to pounds per square inch gauge. In such instances, a liquid depth of up to about one foot above the heat exchange surface can be tolerated. It is found that this liquid depth, at a vaporizing pressure of 100 pounds, absolute, reduces the mean temperature diffcrential only about 5 F. In contrast, when the vaporizing is carried out at substantially atmospheric pressure, a liquid depth of 1 foot above the heat transmission surface results in a loss in mean temperature differential of 31 F., or more than 6 times the temperature difierential loss at the elevated pressure level.

The disadvantages or limitations attendant in lower pressure operations can be circumvented, at least in part, by a special technique which avoids the necessity for a pool of boiling liquid. Instead, a moving film of the boiling liquid is passed over the heat transfer surface, the film being of the order of about or A of an inch in thickness or less. Suitable apparatus for implementing this embodiment is shown in Figure 2.

Figure 2 is a segmental sectional view of a portion of a typical vaporizer for providing vaporization from a thin film of liquid. In this instance, a heat transfer tube 41 is mounted in a tube sheet 42. The upper end 43 of the heat transfer tube 41 projects above. the tube sheet 42, preferably having a sharp edge 44 forming a weir for retention of liquid amalgam 45 on the tube sheet. The space 46 defined by the underside of the tube sheet 42 and the outside wall of the heat transfer tube 41 is the condensing chamber for compressed mercury vapor.

In operation, fresh amalgam feed is passed to the pool 45 on the tube sheet 42, and generally is heated to near transfer tubef41, as a thin film 47. Infiowing down the tube wall the desiredamount of mercury is vaporized from. the amalgam zfland' isdischarged overhead as a movingvapor. The liquid, sodium enriched, fraction collects in; an appropi'iate collector, upstream at the bottom of the vaporizer. 'Further heat can be recuperated as described .in preceding examples.

It is-apparent that the procedure or techniques dis cussed above will avoid any limitations otherwise arising because. of the disadvantageous efiect of a substantial liquid depth above the heat transfer surface. Hence, this: mode of operation will be, particularly beneficial in low pressure operation andalso in multi-stage operations. i

- In starting up installations of the process, the vaporizer body or bodies are charged with the appropriate amount of amalgam and his brought up to boiling temperature by heating with the immersion heater such as the immersion heater indicatedinFigure 1.- During continuous operation, the immersionheater is employed as necessary to provide the appropriate heat balance.

Having fullydescribed the process of the invention and the mannerofits operation, what is desired to be claimed is: l

. 1.. The process of resolving a sodium amalgam comprising feeding a dilute amalgam to a boiling, sodium enriched amalgam in a boilingzone, the boilingamalgam 8, being heated by indirect heat exchange with a condensing mercury rich'vapor, boiling an enriched mercury vapor from'said boiling liquid, compressing said vapor and returning said compressed vapor for indirect heat relationship and condensing adjacent the boiling liquid.

2. The process of claim 1 further defined in that the boiling is conducted at a pressure of at least about pounds per square inch absolute.

3. The process of claim 2 further defined in that the boiling, sodium enriched amalgam contains from about 3 to about 5 weight percent sodium.

4. The process of claim -1 further defined in that the boiling, sodium enriched amalgam contains up to about 15 weight percent sodium.

5. The process of resolving a sodium amalgam into a sodium enriched liquid and a mercury rich liquid comprising passing a filmof said amalgam in out-of-contact heat exchange relationship with a condensing mercury rich vapor, and vaporizing a mercury rich vapor from said film of amalgam, then compressing said mercury enriched vapor andreturning the compressed vapor for out-of-contact condensation heat exchange with the film of amalgam.

References Cited in the file of this patent UNITED STATES PATENTS UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION.

Patent No. 2,833,644 May 6, 1958 Julian M. Avery Column 4, line 60, Example III, for "587 Fa" read 58 F. qolumn Q 5 line 68, for "multiple-stake read multiple s'bage colunm 8, lin'e' 5 for condensing" read condensation Signed and sealed this 22nd day of July 1958..

(SEAL) Attest:

A AXLINE ROBERT C. WATSON Attesting Oflicer Commissioner of Patents

Claims (1)

1. THE PROCESS OF RESOLVING A SODIUM AMALGAM COMPRISING FEEDING A DILUTE AMALGAM TO A BOILING, SODIUM ENRICHED AMALGAM ION A BOILING ZONE, THE BOILING AMALGAM BEING HEATED BY INDIRECT HEAT EXCHANGE WITH A CONDENSING MERCURY RICH VAPOR, BOILING AN ENRICHED MERCURY VAPOR FROM SAID BOILING LIQUID, COMPRESSING SAID VAPOR AND RETURNING SAID COMPRESSED VAPOR FOR INDIRECT HEAT RELATIONSHIP AND CONDENSING ADJACENT THE BOILING LIQUID.

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