US2139721A

US2139721A - Concentration of nitric acid

Info

Publication number: US2139721A
Application number: US12563A
Authority: US
Inventors: Carl Fred
Original assignee: EI Du Pont de Nemours and Co
Current assignee: EIDP Inc
Priority date: 1935-03-23
Filing date: 1935-03-23
Publication date: 1938-12-13
Anticipated expiration: 1955-12-13
Also published as: GB472493A

Description

Dec; 13, 193s.A F. CARL 2,139,721

l K CONCENTRATION NITRIC ACID Filed March 23, 1935 Sheets-Sheet 1 v hamm/05 I LLM INVENTOR. Fred Carl ATTORNEY.

CONCENTRATION OF NITRIC ACID fred Caz- ATTORNEY.

Patented Dec. 13, 1938 UNITED STATES CONCENTRATION OF NITRIC ACID Fred Carl, Wilmington, Del., assignor to E. I. du Pont de Nemours & Company, Wilmington, Del., a corporation of Delaware Application March 23, 1935, Serial No. 12,563

11 Claims.

The present invention relates to an improved process of concentrating nitric acid, and more particularly to such a process in which a minimum amount of dehydrating agentis required.

6 This invention also relates to the denitration of spent nitration acids and to the production of concentrated residual acid therefrom.

It has long been recognized that dilute nitric acid can not be concentrated above 68% strength by distillation without the use of a dehydrating agent. For this purpose, sulfuric acid is most commonly employed, but other materials such as anhydrous sodium nitrate, calcium nitrate, sodium polysulfate, phosphoric acid and arsenic acid have also been suggested.

Of these dehydrating agents, sulfuric acid has been employed in two general types of procedures, namely, (1) batch processes and (2) continuous processes. According to the batch procedure, a mixture of sulfuric acid and dilute nitric acid is placed in a simple retort and the nitric acid distilled. The rst fractions of the distillate are concentrated acid, but, since successive fractions are progressively weaker, the batch procedure has been largely replaced by more efficient continuous methods.

The continuous process of Pauling (disclosed in U. S. Patent 1,031,865), for example, introduces a mixture of strong sulfuric acid and weak nitric acid continuously at the top of a packed tower. At the same time, a countercurrent of steam is introduced at the base of the tower, and the interaction of the ascending steam and the descending acids generates the heat required to volatilize the nitric acid. Subsequent processes along the same lines have covered methods of generating the steam from materials employed within the system.

The most serious objection to any method employing steam is the diluting effect of the watervapor on the residual sulfuric acid. To minimize or completely avoid this effect, hot gases other than steam have been proposed as counter-current heating media. Thus steam mixed with air, air followed by a final operation using steam, or air used alone, have been suggested. Nitrous gases have also been advanced for the same reason. While these media produce a stronger re- 50 sidual sulfuric acid, they have not been widely employed in the art because of the somewhat greater efficiency of steam as a heating and denitrating medium. All known processes of concentrating nitric acid, therefore, are disadvantageous either in the poor thermal efficiency of the counter-current medium or in the diluting effect of this medium on the residual sulfuric acid.

The object of my invention is a new and improved continuous process for concentrating nitric acid. A further object is a thermally efficient process which produces a stronger residual sulfurie acid. A still further object is a process of concentrating dilute nitric acid without the use of steam as a counter-current heating medium. An additional object is a process of denitrating spent nitration acids. 4Other objects will become apparent as the invention is hereinafter described in detail.

AI have found that the foregoing objects are accomplished by the use of a thermally efficient gaseous counter-current medium which does not dilute the sulfuric acid, but, on the contrary, actually increases its concentration. I have found that sulfur trioxide is satisfactory as the counter-current medium. 20

'Ihe dilute nitric acid and the strong sulfuric acid are rst preheated according to my invention and then added separately to the top of the dehydrating tower. The heat content of the separate acids, plus the heat liberated when the two acids mix, is sumcient to volatilize much of the nitric acid in the mixture. The remaining nitric acid is then driven off by introducing gaseous sulfur trioxide into the bottom of the tower. In this manner a large amount of the heat usually lost is employed, and the resulting sulfuric acid residue is stronger, since sulfuric acid is formed in the process. The advantages of this method will now be apparent, since the resulting residual sulfuric acid according to this procedure is of greater strength than that resulting from prior art methods.

An equipment or apparatus suitable for carrying out the present invention comprises a standard packed nitric acid dehydrating tower or other equally satisfactory apparatus which will accomplish the desired results. Also, if preferred, an oleum still may be used as a suitable source of sulfur trioxide in conjunction with a nitric acid dehydrator. In this case, the residual overow of the oleum still runs into the top of the packed dehydrating tower, while the vapor line conducts v gaseous sulfur trioxide to the bottom of the tower. The vapor line may be provided, if desired, with a cooled section in which a portion of the sulfur trioxide is condensed and withdrawn as liquid sulfur trioxide.

In order .to disclose my invention more fully, reference is made to the accompanying diagrammatic representations of preferred embodiments of my invention. These are to be regarded solely as illustrative only, and are not to be taken as limitations of the scope of the invention.

Referring to the drawings, Figure 1 represents a vertical elevation of a nitric acid dehydrating tower as contemplated by my invention.

Figure 2 represents a similar View of a dehydrator in combination with an oleum still constructed in accordance with my invention.

The same letters are used in the drawings and the accompanying description, to indicate corresponding parts of the respective views.

Referring particularly to Figure 1, A represents a standard type nitric acid dehydrating tower packed with an acid-resisting material B. The upper end of the tower is provided with a pipe C leading to any suitable nitric acid condensing system; a pipe D for introducing the pre-heated dilute nitric acid; and a pipe E, through which the pre-heated concentrated sulfuric acid is introduced. The base of the tower is provided with a pipe E for admitting the sulfur trioxide, and a trapped line G for discharging the residual sulfurie acid. The pipes may be fitted with valves, as desired.

In operation, dilute nitric acid and 101% sulfuric acid are introduced into the tower through the pipes D and E, respectively. These acids are pre-heated to a temperature of approximately 110 C. so that the mixing of the preheated weak nitric and the strong sulfuric acids generates sufficient additional heat to cause the temperature of the mixture to rise above the boiling point of the nitric acid, most of which distills. The residual acid, containing a small amount of nitric acid, then flows downward over the acid-resisting material B and comes in contact with an ascending counter-current of sulfur trioxide, admitted through the pipe F. The sulfur trioxide combines with the water in the mixture to form sulfuric acid, thereby generating suiiicient heat to cause the last traces of nitric acid to distill. The vapors of the nitric acid pass; out of the tower through the pipe C leading to suitable condensing coils, where approximately nitric acid is condensed. The residual sulfuric acid passes out through the trapped line G, from which approximately 74% sulfuric acid is discharged into a storage tank. The residual acid thus obtained is generally more than 6% stronger than that obtained when steam is employed as the countercurrent heating medium.

Referring now to Figure 2, the dehydrating tower of Figure 1 is connected with an oleum still I-I. Fuming sulfuric acid is introduced into the oleum still through the line J from the source of supply. Gaseous sulfur trioxide is distilled through the line K which is joined to the opening F at the base of the dehydrating tower A. A portion of the line K may be trapped and cooled, if desired, and liquid sulfur trioxide may be drawn ofi through the valve L. The residual sulfuric acid from the oleum still passes through the trapped line Minto the upper part of the dehydrating tower A through the pipe E. Dilute nitric acid is supplied to the dehydrator from a source of supply through the line D which is surrounded by a steam jacket N. The oleum still is likewise heated by the steam jacket O.

In operation, fuming sulfuric acid (104%) is continuously passed into the steam jacketed single tube oleum still H in which it is heated t0 about 195 C. At this temperature, a portion of the free sulfur trioxide is vaporized and passes through the S03 vapor line K to the bottom of the dehydrating tower A. 'I'he resulting hot residual sulfuric acid (101%) in the oleum still H overows into the top of the nitric acid dehydrating tower A, through the trapped line M. At the same time, dilute (60%) nitric acid, heated by the steam in the jacket N to about C., is added in the correct amount to give a resulting mixed acid of about 42% HzSOi and 35% I-INOa when mixed with the 101% H2SO4 from the oleum still. The heat content of these two acids, plus the heat generated by dilution, vaporizes the greater part of the nitric acid which passes out of the dehydrating tower through the line C to the condensing system.

The mixed acids flow downward over the acidresisting packing B and comes in contact with the ascending current of S03 added at the bottom of the tower. The S03 combines vigorously with the water in the mixed acid and the heat liberated completely Vaporizes the remaining traces of nitric acid. The vapors of nitric acid leaving the still through the line C are condensed as strong nitric acid (about 95%). The denitrated residual sulfuric acid (74%) flows from the bottom of the tower through the trapped line G. The acid thus produced is substantially stronger than residual acids obtained from previous methods.

If it is desired to produce oleum distillate, the charging acid for the oleum still is increased in strength and the excess S03 is condensed and drained from the system through the valve L as liquid sulfur trioxide.

A similar procedure may be used for denitrating spent nitration acids, or mixed acids containing small amounts of nitric acid. In this case, the pre-heated spent acid is added in place of the 60% nitric acid in the above examples. Not only may the spent acid be completely denitrated by this process, but the residual sulfuric acid is partly reconcentrated by the treatment. Moreover, the traces of nitric acid in the spent acid may be recovered at the same time.

While I have described my invention with particular reference to the concentration of nitric acid, it is apparent that the same procedure may be employed to concentrate or dehydrate other volatile materials. Thus, for example, bromine in water may be concentrated in a similar manner. Likewise, other volatile acids such as acetic acid, hydrochloric acid, and the like may be obtained in an anhydrous condition by a similar procedure. Moreover, it is not essential to use sulfuric acid as the sole dehydrating agent, for any liquid dehydrating agent, such as phosphoric acid, may be employed with sulfur trioxide as the counter-current heating and denitrating medium. In general, any Volatile material may be dehydrated or concentrated in accordance with my invention, provided it is inert with respect to sulfur trioxide and sulfuric acid or similar liquid dehydrating agent.

From the foregoing detailed description of my invention, it will be apparent to those skilled in the art that many variations may be made in the process without departing from the spirit or scope of the invention. I therefore intend to be limited only as indicated in the following patent claims.

I claim:

1. The process of concentrating aqueous solutions of volatile acids selected from the group consisting of nitric, hydrochloric, and acetic acids, which comprises establishing in a common path a current of a substantially non-volatile uid dehydrating agent selected from the group consisting of sulfuric and phosphoric acids and a counter-current flow of gaseous sulfur trioxide, introducing into the common path a current of said aqueous volatile acid in co-current flow with the dehydrating agent, utilizing simultaneously the dehydrative capacity of sulfur trioxide for concentrating the dilute solution and the heat of hydration of sulfur trioxide for distilling the vapors of said volatile acid in a substantially dehydrated condition, and withdrawing the vapors of the volatile acid in concentrated form.

2. The process of concentrating aqueous solutions of volatile acids selected from the group consisting of nitric, hydrochloric, and acetic acids, which comprises establishing in a common path a current of a preheated substantially nonvolatile fluid dehydrating agent selected from the group consisting of sulfuric and phosphoric acids and a countercurrent flow of gaseous sulfur trioXide, introducing into the common path a current of preheated aqueous volatile acid in cocurrent flow with said dehydrating agent, utilizing simultaneously the dehydrative capacity of sulfur trioxide for concentrating the dilute solution and the heat of hydration of sulfur trioxide for distilling the vapors of said volatile acid in a substantially dehydrated condition, and withdrawing the vapors of said volatile acid in concentrated form.

3. The process of concentrating aqueous solutions of volatile acids selected from the group consisting of nitric, hydrochloric and acetic acids, which comprises establishing in a common path a current o-f strong sulfuric acid and a counterflowing current of gaseous sulfur`trioxide, introducing into the common path a current of said aqueous volatile acid in co-current flow with the sulfuric acid, utilizing simultaneously the dehydrative capacity of sulfur trioxide for concentrating the dilute solution and the heat of hydration of sulfur trioxide for distilling the vapors of said volatile acid in a substantially dehydrated condition, and withdrawing the vapors of the volatile acid in concentrated form.

4. The process of concentrating aqueous solutions of volatile acids selected from the group consisting of nitric, hydrochloric, and acetic acids, which comprises establishing in a common path a current of preheated strong sulfuric acid and a counter-ilowing current of gaseous sulfur trioxide, introducing into the common path a current of said preheated aqueous volatile acid in co-current ow with the sulfuric acid, utilizing simultaneously the dehydrative capacity of sulfur trioxide for concentrating the dilute solution and the heat of hydration of sulfur trioxide for distilling the vapors of said volatile acid in a substantially dehydrated condition, and withdrawing the vapors of the volatile acid in concentrated form.

5. The process according to claim 3, in which the sulfuric acid contains dissolved sulfur trioxide.

6. The process of concentrating dilute aqueous nitric acid which comprises establishing in a common path a current of sulfuric acid and a counterflowing current of gaseous sulfur trioxide, introducing into said common path a current of7 dilute aqueous nitric acid in coi-current flow with the sulfuric acid, utilizing simultaneously the dehydrative capacity of sulfur trioXide for concentrating the dilute solution and the heat of hydration of sulfur trioxide for distilling the vapors of said nitric acid in a substantially dehydrated condition, and withdrawing the vapors of the nitric acid in concentrated form.

7. The process of concentrating dilute aqueous nitric acid which comprises establishing in a common path a current of preheated sulfuric acid and a counter-owng current of gaseous sulfur trioxide, introducing into said common path a current of preheated dilute aqueous nitric acid in co-current flow with the sulfuric acid, utilizing simultaneously the dehydrative capacity of sulfur trioxide for concentrating the dilute solution. and the heat of hydration of sulfur trioxide for distilling the vapors of said nitric acid in a substantially dehydrated condition, and withdrawing the vapors of the nitric acid in concentrated form.

8. The process according to claim 6, in which the sulfuric acid contains dissolved sulfur tmoxide.

9. The process of concentrating dilute nitric acid which comprises heating fuming sulfuric acid to a temperature at which at least part of the sulfur trioxide distills, introducing the residual strong sulfuric acid resulting from this treatment, while still hot, into the upper portion of a dehydrating chamber, simultaneously introducing a pre-heated dilute aqueous solution of nitric acid into the upper portion of said chamber, passing i the gaseous sulfur trioxide distilled from said firming sulfuric acid into the lower portion of said chamber, withdrawing the vapors of concentrated nitric acid from the upper portion of the chamber, and withdrawing the' residual sulfuric acid from the lower portion of the chamber.

10. The process of denitrating sulfuric acid containing nitric aci-d, which comp-rises preheating sulfuric acid, establishing in a common path with said heated acid a counter-flowing current of gaseous sulfur trioxide, introducing into said common path a current of the acid for denitration, in co-current ow with the sulfuric acid, withdrawing vapors of nitric acid in concentrated form, and withdrawing a residual sulfuric acid in dilute form.

l1. The process of denitrating spent nitration acids which comprises heating fuming sulfuric acid to a temperature at which at least part of the sulfur trioxide distills, introducing the residual sulfuric acid from this treatment, while still hot, into the upper portion of a dehydrating chamber, simultaneously introducing pre-heated spent nitration acid into the upper portion of said chamber, passing the gaseous sulfur trioxide distilled from said furning sulfuric acid into the lower portion of the chamber, withdrawing the vapors of concentrated nitric acid from the upper portion of the chamber, and withdrawing residual sulfuric acid from the lower portion of the chamber.

FRED CARL.