Patented Sept. 23,1924.
- uNirsn STATES PATENT OFFICE.
CHARLES O. YOUNG, OF PITTSBURGH, PENNSYLVANIA, ASSIGNOR TO CARBIDE AND CARBON CHEMICALS CORPORATION, A CORPORATION OF NEW YORK.
rnoonss or Pnonncrnc oxnnro ACID.
To all whom it may concern:
Be it known that l, OHARLns O. YOUNG, a citizen of the United States, residing at Pittsburgh, in the county of Allegheny and State of Pennsylvania, have invented certain new and useful Improvements in Processes of Producing Oxalic Acid, of which the following is a specification.
This invention is a novel process whereby oxalic acid may be prepared, economically and with excellent yields, from carbohydrates or other appropriate oxidizable substances such as ethylene glycol. The process is not limited 'to the specific operating details given below by way of example, al-
though the conditions described are' those which have been found most advantageous from the point of view of high yields, qualitylof product, and optimum results in genera It has long been known that oxalic acid can be prepared from carbohydrates by oxidation with nitric acid. A'large excess of nitric acid over the theoretical proportion is however required, and a considerable part of the oxidizing power of the nitric acid is lost by the formation and evolution of nitrogen tetroxid according to the equat1on:
As indicated by the above equation d moles of nitric acid yield only one mole of availableoxygen.
Similarly, ethylene glycol maybe oxidized to oxalic acid by means of nin-ic acid, but such process is not eficient, owing to the loss of nitrogen tetroxid, and to the fact that the progressive dilution of the nitric acid prevents the reaction from proceeding to completion. For the above reasons the yields are low and the costs prohibitive.
In order to render possible the re-use of the dilute nitric acid solution which remains after the concentration of acid has fallen below the point necessary for the oxidation of carbohydrates, it has been proposed to introduce gaseous or liquid nitrogen tetroxid into such solution to restore the initial nitric acid concentration; the nitrogen tetroxid for this purpose being obtained from any suitable source, including the nitrous gases arising from the reaction itself. Such procedure, however, involves the isolation of nitrogen tetroxid from gaseous Application filed. April 5,
. mixtures by liquefaction processes, or otherin three steps or operating stages, to WlJbI- 1. Absorption of N O, by the water of the solution with formation of certain amounts of nitric acid; .1
2. Oxidation of glycol by the nitric acid thus formed, with evolution of more nitrogen tetroxid and lower oxids of nitrogen;
3. Oxidation of such lower oxids of nitrogen by the oxygen, together with a snficient supply of undiluted oxygen from outside sources to regenerate the original mlxture.
llt will be observed therefore that the process involves no necessary or theoretical loss of nitrogen compounds; nor does it involve'any separation or isolation of nitrogen compounds from a complex gas-mixture. Oxygen alone is consumed 111 the operation of the process, so that for practical purposes it may be considered that the oxidation is performed by oxygen, the nitrogen compounds acting as carriers. Air or other form of dilute oxy on is of course not equivalent to substantia y pure oxygen for this purpose, since it is only by the use of substantially undiluted oxygen that the volume of gases in c clic flow may be ma ntained approximate y constant, or the ncrease in volume due to inert gases be main tained within reasonable and commercial limits.
In practice the operation may for example be carried out as follows 1 An aqueous solution containing 40-50% of ethylene glycol is introduced into a. tower or other appropriate form of reaction chamber of stoneware or other resistant material, and is maintained at about (SO-80 O. A
strong stream of mixed gases N Od-O is';
continuous cyclical flow. As the reaction proceeds the gas volume tends to diminish through absorption of oxygen, and 'means are accordingly provided for introducing" oxygen into t e system to replenish that absorbed by the reaction and to maintain at all times oxygen in excess of the reaction reuirements. After the operation is contlnued for some 6-7 hours, more or less, depending on the rate of addition of the gases, the solution is withdrawn and ermitted to cool. The oxalic acid crystal izes out, is filtered ofl", and the mother liquor containin unoxidized glycol and dissolved oxalic aci is returned to the tower together with a new charge of glycol solution. The operation is then repeated.
Inasmuch as there is always some ten d ency to the building up within the system of inert gases, such as nitrogen," air, carbon dioxid, etc., it is necessary to provide an outlet for occasionally purging the system while maintaining. a substantially constant average gas volume. These waste may be. passed thmugh -an alkaline solution whereby the nitrogen oxids are caught and subsequently recoveredas alkali nitrate or nitrite. In this way loss of the valuable nitrogen oxids is minimized.
The initial sup 1y. of higher nitrogen oxids may be produced by any method desired, as, for: example from sodium nitrate by the action of sulfuric acid, or even by the addition of concentrated nitric acid to the original glycol, solution. This latter procedure illustrates the advantage of the present process as compared with the older procasses of oxidizing by means of nitric acid, for a much smaller quantity of nitric acid is adequate for charging the epresent system.
The use of catalysts capabl of accelerating the reaction is of course not excluded, but is not regarded as essential inasmuch as very satisfactory yields may be obtained. without the presence of any catalyzer. In this connection it may be remarked that the presence of small proportions of vanadium oxid dissolved in nitric acid acts to retard the oxida .tion of glycol under the conditions herein described.
The reaction when carried out as above described affords yields considerably in excess of those obtainable by the ordinary methods of nitric acid oxidation, 70-80% of the theoretical yield of oxalic acid bein readily recoverable. The reaction procee satisfactorily at pressures just sufficient to maintain the gas flow through the system. However hi her or lower pressures may be used as mayie desired.
I claim 1. Process of preparing oxalic acid from water-soluble substances capable of yielding the same by oxidation, comprising circulating a gaseous mixture of nitrogen oxids and oxygen in contact with an aqueous solution of such substance in a closed circulatin system, and supplying substantially undiluted oxygen in proportion to satisfy the reaction requirements.
2. Process of preparing oxalic acid from ethylene glycol, comprising circulating a gaseous mixture of nitrogen oxids and oxygen in contact with an aqueous solution of glycol in a closed circulating system, and supplying substantiall pure oxygen in proportion to satisfy ments.
In testimony whereof, I aflix my signature.
CHARLES O. YOUNG.
e reaction requine-