US1899627A - Dehydrating caustic alkali solutions - Google Patents

Description

Patented Feb. 28, 1933 UNITED STATES PATENT. o1=1=1cE Y CLARENCE E. MILLER, OF CANTON, NORTH CAROLINA, ASSIGNOR TO THE CHAMPION m3 Drawing.

FIBRE COMPANY, OF CANTON, NORTH CAROLINA,

A CORPORATION OF OHIO IDEHYDRATING CAUSTIC ALKALI SOLUTIONS carbon monoxide or other constituents of the alkalimetal hydroxides and particularly so 7 gaseous combustion products from the commonly-used solid, liquid or gaseous fuels. The invention is especially applicable in the evaporation or dehydration of solutions of in those cases in which such hydroxides are preparedv by electrolysis of an aqueous solution of analkali metal salt with simultaneous evolution of hydrogen as a by-product.

It heretofore has been proposed to dehydrate electrolytically produced aqueous solutions of alkali metal hydroxide, e. g., sodium hydroxide, by first subjecting the cell liquor to evaporation in multiple effect evaporators, with removal of salt as the evaporation progresses, the said treatment being continued until the intermediate product repre sents about a 50% solution of the hydroxide, and then transferring the so-obtained 50% solution to a large open evaporating pot hea ed by a coal fire, oil blast, or the like, in which pot the solution is heated at atmospheric pressure either to dehydration or to a concentration higher than the gaseous or vaporous products of evaporation being conducted by means of a hood from the pot to apoint removed therefrom. The efiiciency of heat transfer in this commonly employed method is known to be very low and, inasmuchas the surface for evaporation is limited, the method is a very slow one. Furthermore, contamination of the evaporated proclucts with the products of combustion of the fuelemployed, with formation of carbonates of the metals, is practically unavoidable. So expensive and inefiicient is this commonly employed method that investigators have proposed further evaporating partially evaporated, e. g., 50%, solutions of alkali metal hydroxides in closed jacketed vessels through the jackets of which highly heated vapors are conducted, the vaporous evaporation products being removed from the vessels by any appropriate means such as, for instance, a vapor system operated at subatmospheric pressure. While such method may result in Application filed August 19, 1031. Serial No. 558,187.

relatively very speedy dehydration of the solutions and an increased efliciency, the complicated special apparatus therefor makes the method expensive. An object of the present invention is the provision of a simple yet highly economical process for evaporating or dehydrating solutions of metallic hydroxides, which method may be practiced with the use of comparatively simple and inexpensive evaporation equipment. Among other objects thereof is the provision of a process foreva'poratingor dehydrating solutions of metallic hydroxides which is not only very rapid but also effects avoidance of contamination of the evaporated product. w r p These, and other, objects of invention are found to be attained by exposing the alkali metal hydroxide solution to be evaporated, preferably in thin layers or in finely'divided form or in any other form providing re1atively large layers of the solution, directly in contact with burning hydrogen. That ,is to say, the invention resides in the evaporation of aqueous solutions of alkali metal hydroxides or their equivalents by direct contact combustion of hydrogen. High evaporation efliciency is attained thereby and, since the products of the combustion are steam and nitrogen, there is thereby eliminated any possibility of contaminating the evaporated product with the combustion products. Obviously, this process finds particular applicability in the evaporation ordehydration of alkali metal hydroxide solutions obtained by the electrolysis of solutions of alkali metal salts with simultaneous evolution of hydr0- gen, in that by the. process forming the basis of the present invention there is provided an economically important outlet for the by product hydrogen. a

In this connection, it is to be noted that in the electrolytic production of, say, twenty (20) tons of caustic soda there is produced by the electrolysis approximately one thousand (1000) pounds of hydrogen Which,under standard conditions, is equivalent to one hundred eighty thousand (180,000) cubic feet of hydrogen. Assuming the usual heat value of 280 'B. t.'u. per cubic foot for the hydrogen, there is available from this amount of hydrogen approximately fifty million four hundred thousand (50,400,000) B. t. u.s of heat by the complete combustion of the said hydrogen. The 50% caustic soda solution which may be obtained by the evaporation of the cell liquor from the aforesaid electrolysis (the salt being removed during evaporation to 50% strength) contains approximately twenty tons of water which, by standard calculation, requires ap-, proximately forty million (40,000,000) B. t. u.s of heat for its evaporation. It will be seen, therefore, that upon a theoretical basis the by-product hydrogen is more than sufficient to supply the heat necessary for the complete evaporation of the water present in the solution obtained from the cell liquor.'

The process forming the basis ofthepresent invention makes possible, and desirable, the

utilization of the heat contained in the products of combustion of the hydrogen in anysuitable heat exchanger, such, for instance, as

. a tubular boiler for the generation of steam for power and other purposes, the nitrogen in the gas either being allowed to escape or being employed for some useful purpose.

While it is not the intention of this application to restrict the working of the abovedescribed process to the use of any particu lar apparatus, the method being adapted to be carried out in any one of a number of types of existing apparatus used 1n chemical industries, it has been found to be preferable to inject the partially concentrated solution (e. g., 50% solution) of the alkali metal hydroxide in finely divided form, e. g., as a spray, into a closed space into which the hydrogen and a gaseous supporter of its combustion are caused to pass and there burn, which closed space is provided with suitable means for removing the dehydrated product and means for conducting away the gaseous products of combustion and the water evaporated from the'solution, the latter means preferably including a heat exchanger. The heat of combustion of the hydrogen evaporates the water from the stream of finely dixided partially concentrated solution, the

resulting dehydrated, or substantially dehydrated, caustic soda dropping to the bottom of the closed space and being removed therefrom in any suitable manner as occasion requires, and the combustion products and vaporized solvent passing out of the space. Such apparatus is already known: see Liddell, Handbook of Chemical Engineering (1922), pages 389 and 390, and therefor forms no part of the present invention.

As will be apparent from the foregoing, while it is not essential to the working of the broad invention to use as the hydrogen fuel for the dehydration the byproduct hydrogen from electrolytic cells, and while it is not essential that any of the heat contained in the combustion gases be saved, the best economic working of the invention includes both of these features.

It is to be understood that the present invention is not restricted to complete dehydration of the hydroxide, or to the use of a hydroxide solution of any specific strength or concentration, these being variables determined in part by the requirements in any specific practical operation of the'process. Furthermore, it is not essential that the contact between the partially concentrated solution and the burning hydrogen take place in exactly the manner indicated: itmay' be effected by passing the burning hydrogen and/or its hot gaseous combustion products counter currently in direct contact with a moving layer or film of the solution, or in some other suitable manner. Nor is itessenlution either to the flame itself or to the hot gaseous combustion products thereof, these being equivalents from the standpoint of the present invention. i

hat I claim is:

1. Process of evaporating a solution of an alkali metal hydroxide which comprises exposing the latter directly in contact with burning hydrogen. p 5

2. Process of evaporating a solution of alkali metal hydroxide which comprises exposing large surfaces of the latter directly in contact with burning hydrogen. I

3. Process of evaporating a solution of an alkali metal hydroxide which comprises maintaining combustion of hydrogen in an atmosphere of a gaseous supporter of combustion in direct contact with a large surface of the said solution.

4. Process of evaporating a solution of an alkali metal hydroxide which comprises directly contacting a finely divided stream of the solution with burning hydrogen.

5. Process as defined in claim 1, in which the solution so exposed is a partially concentrated aqueous solution of the hydroxide.

6. Process as defined in claim 1, in which a previously partially concentrated solution of the hydroxide is so dehydrated.

7 Process as defined in claim 1, in which the hydroxide is caustic soda.

In testimony whereof, I affix my signature.

' CLARENCE E. MILLER.