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Vol. 15, No. 7
The Removal of Small Amounts of Carbon Monoxide from Gases by Passage through Heated Granular Soda Lime’” By Robert E. Wilson, C. A. Hasslacher, and E. Mastersons MASSACHUSETTS INSTITUTE
Of.
TECHNOLOGY, CAMBRIDGE,MASS.
HE removal of small tions of carbon monoxide The problem of the complete removal of carbon monoxide from gas amounts of carbon over soda lime, but they streams is o j importance in a number of industriat applications, monoxide from gas were not primarily interparticularly in the purification of hydrogen and nitrogen for the streams is of importance in ested in the completeness of synthesis of ammonia. Several references and patents have mena number of industrial its removal. Two later tioned the use oj soda lime at high temperaturesjor this purpose, but applications. A frequent papers6 presented the repractically no quanfitatioe data are oflered with regard to the removal cause necessitating its comsults of a more detailed of small amounts, or the e$ect of the composition oj the soda lime. plete removal is its prostudy which indicated that This paper describes a seriesbf experiments on gas containing 2 nounced tendency to poison the presence of moisture per cent carbon monoxide in nitrogen, at temperatures varying from the catalyst in various catwas necessary to give a 250” to 550” C., and using soda limes of varying composition. The satisfactory rate of absorpalytic reactions. results show that soda limes with high caustic soda contents are better tion, and that a soda lime Probably the largest scale than those with low, or than lime alone. The former gioe substancontaining 50 per cent soapplication in which this tially complete removal of carbon monoxide at around 400’ C. dium hydroxide gave very problem is important is in Small amounts of moisture are helpful in increasing the eficiency. rapid absorption, with the the nitrogen fixation indusbut do not appear to be essential. Experiments in the presence of production of formate, a t a try. The hydrogen for hydrogen are inconclusioe because the hydrogen used reacted slightly temperature a t 220” t o making synthetic ammonia with the iodine pentoxide used to determine carbon monoxide. 230” C. At higher temperis generally produced from The fundamental reaction for the removal of carbon monoxide at atures the absorption was water gas, the carbon monthe higher temperatures is apparently: even more rapid, but reoxide being removed either CO 2NaOH = NazCOs HZ suIted in the production of by catalyzing its decomequivalent amounts of hyaosition into carbon dioxide knd carbon a t elevated temperatures, or, more frequently,,by drogen and carbonate. Apparently, the reaction may prooxidizing i t with steam in the presence of a catalyst such as ceed in steps ferric oxide a t temperatures between 400” and 600” C.4 NaOH CO = HCOONa (1) Neither of these processes is capable of the complete re2HCOONa = NanCOs CO Hs moval of the carbon monoxide, and a number of other with the readsorption of the carbon monoxide by the first methods have therefore been developed for taking care of reaction, but a t any rate the net effect is the last 2 or 3 per cent. Ammoniacal solutions of cuprous 2NaOH CO = NazCOs H) (2) formate or cuprous carbonate have been used on a large which is obviously closely related to the reaction mentioned scale,4 but the cost of the solutions is high and it is difficult to secure complete removal. Differential oxidation processes previously for the removal of carbon monoxide: Hz0 CO = COz Hz (3) have also been tried with some success, although these reactions are very difficult to control on a large scale if more except that the equilibrium is displaced to the right by the than a trace of carbon monoxide is present. absorption of the carbon dioxide by soda lime. Attempts have also been made to make use of the reaction Based on this reaction, a British patent’ was granted in 1889 for rendering water gas nonpoisonous by passing it through CO NaOH = HCOONa a layer of soda lime several feet thick at a “good red heat.” which has been successfully utilized for the production of In 1912, a German patent* was issued for a process of resodium formate. While this reaction proceeds fairly rapidly moving carbon monoxide and dioxide from water gas by passa t temperatures around 250” C. and pressures in the neighbor- ing over lime or soda lime heated to a temperature somewhat hood of 50 atmospheres for gases rich in carbon monoxide, below a red heat. A similar patentg was issued during the it is not adapted to the complete removal of small amounts. same year to other parties. None of the patents gives any Furthermore, the handling of hot caustic solutions in such a significant quantitative data, however. way as to bring them into effective contact with gas under An obscure reference by Engelslo gives the best discussion high pressure is a very difficult proposition from a mechanical of the fundamentals of the reaction between carbon monoxide standpoint, and as yet has not been worked out in a satis- and calcium hydroxide alone at the higher temperatures. He factory manner.4 recommends a temperature around 500” C . , with just enough There are several references in the literature to the use of steam to prevent dehydration of the calcium hydroxide. He granular soda lime for carrying out a similar reaction. Thus, finds that the rate of reaction is markedly catalyzed by as far back as 1877, Merz and Tibirica5 were able t o form some “iron” (ferric hydroxide?) due to its previously mentioned calcium and some sodium formate by passing high concentra- catalytic effect on Reaction 3. 1 Presented before the Division of Industrial and Engineering ChemisI n spite of these interesting statements, and the obvious try at the 64th Meeting of the American Chemical Society, Pittsburgh, Pa.. advantages of using a hard, granular absorbent such as soda
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September 4 to 8, 1922. 2 Published as Contribution No. 65 from the Research Laboratory of Applied Chemistry, M. I. T. a Most of the work described in this paper was carried out by the two junior authors as part of their theses for the degree of Bachelor of Science from M. I. T. 4 THIS JOURNAL, 12 (1920), 851. 6 Bar., 10 (1877), 2117.
e Merz and Tibirica, Ber., 13 (ISSO), 23; M e n and Weith, I b i d . , 1s (ISSO), 718. 1 Brit. Patent 10,164. a D . R. P. 248,290, to the “SociBtd G6nCral des Nitrures” in Paris. Brit. Patent 13,049 (1912), to the “Chemische Fabrik GrieshamElectron.” 10 C. A , , 14 (1920),599.
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July, 1923
I N D UXTRIAL A N D ENGINEERING CHEMISTRY
lime, rather than loose, hydrated lime or molten caustic, so far as the writers are aware, soda lime has never been utilized commercially for the removal of carbon monoxide, and no quantitative study has been made of its absorption efficiency as a function of its composition and temperature. Previous investigations in this laboratory11on the efficiency of various kinds of soda lime as an absorbent for other gases had shown that its absorption efficiency varied greatly with composition, and that a low caustic soda content (about 4 per cent sodium hydroxide, hereinafter referred to as the "low alkali" soda lime) and a fairly high moisture content (12 to 18 per cent, depending on the gas to be absorbed) gave far better efficiency than ordinary, commercial, highalkali soda limes, and had the additional advantage of not deliquescing in moist atmospheres. It was therefore decided to undertake a quantitative investigation of the absorption of carbon monoxide by soda lime, studying particularly the effect of temperature, moisture content, and alkali content of the soda lime on its efficiency in removing small amounts of carbon monoxide from gas streams. The treatment of more concentrated mixtures, such as water gas, was considered to be of less practical interest, because the chemical cost would be high compared to other met hods mentioned above by which the bulk of the carbon monoxide may be converted to carbon dioxide.
APPARATUS AND PROCEDURE
It appeared from the literature as summarized above, that soda lime a t temperatures in the neighborhood of 230" C. would have promising possibilities, in that it should not only absorb the carbon monoxide, but give formates as a valuable by-product. The first experiments, therefore, were carried out a t temperatures obtainable in an oil bath. The apparatus consisted essentially in a gas holder containing 98 per cent nitrogen and 2 per cent carbon monoxide (the latter made by the action of sulfuric acid on sodium formate) which fed through a train comprising: (a) A carefully calibrated flowmeter with inclined manometer arms. ( b ) A wash bottle containing alkaline pyrogallol to remove any trace of oxygen (and incidentally any carbon dioxide). (c) A humidifier, consisting of two gas wash bottles containing water, and immersed in an electrically heated bath whose temperature was controlled so as to give any desired moisture content (O'.2 to 40 per cent by volume) to the gas passing through, the exit tube being heated to prevent any condensation. ( d ) A Pyrex preheating coil immersed in another electrically heated and stirred oil bath (later a lead bath). (e) A long armed U-tube of Pyrex glass containing a 13-cm. layer of soda lime and immersed in the same bath as (d). (f) A calcium chloride drying train. (9) A connection through which a gas sample could be slowly drawn off into a gas buret. (h) A vacuum regulator (about 6-in. water suction) and thence t o the vacuum line.
Thermometers were inserted in the water bottles in the humidifier and one on each side of the U-tube. Thanks to the preheating coil, plus the rather large radiation effects on the thermometers, the recorded temperatures at the entrance and exit OS the soda lime were always very nearly the same. Some preliminary experiments around 200" C . indicated that a velocity of 50 cc. (measured as dry gas a t room temperature) per minute gave a readily measurable removal, and this rate was accordingly adopted as Rtandard for all runs, although considerably higher velocities might well have been used a t higher temperatures. An 8-hr. run a t this rate indicated no falling off in the efi11 THISJOURNAL, 12 (1920). 1000. 1 2 For the dry air runs the gas by-passed the humidifier through calcium chloride drying train.
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ciency of the soda lime, but as a factor of safety it was always renewed after the gas had been passed through for 2.5 t 0 3 hrs. Ten grams of soda lime were used, and gave a depth of about 13 cm. in the tube, which was almost exactly 1sq. em. in area. Since the gas was measured dry and a t room temperature, the apparent time of contact of gas with soda lime obviously varied considerably with temperature and moisture content, but for the most of the experiments was between 4 and 8 sec.in other words, it approximated practicable operating conditions for a commercial process. Most of the runs were made with two commercial sods limes, one containing 4 per cent and one 40 per cent sodium hydroxide. Both samples were screened to 8-14 mesh. The initial water content of the samples was not important as it very quickly reached equilibrium with the air streams. The most extensive tests were made with the low-alkali soda lime, as it was the first one studied and showed a very interesting variation with temperature. The customary procedure was to bring the humidifier and U-tube to the desired temperature and pass the gas through for about 15 min. before drawing off a sample of the gas for analysis. The quantitative determination of carbon monoxide was made by passing the sample slowly through a tube of iodine pentoxide a t 150' C., sweeping out with nitrogen and absorbing the liberated iodine in 3 per cent potassium iodide, titrating with thiosulfate. The procedure and precautions used were essentially those described in detail by Graham,13 except that it was found preferable to keep the temperature of the bath at 150" C., thus preventing any condensation of iodine in the exit tubes and speeding up the determination. At the outset of the work an old sample of iodine pentoxide was used, and even uskg all the known precautions it was impossible to get a satisfactory blank when pure air was sucked through the apparatus. Two fresh samples were later secured, which gave quite satisfactory results provided they were pretreated by passing a current of dry fresh air through them for a period of 20 hrs. at 220" to 230° C. It was found to be essential that the air used for sweeping out the tube not be taken from the room, where it was contaminated by fumes from the oil bath, nor sucked in through old rubber tubing, as high blanks would result in each case. Nitrogen from a cylinder was used for most of the work. It was also found that small amounts of hydrogen, such as those produced in absorbing the carbon monoxide, did not appreciably affect the results, but attempts to use a gas mixture containing 98 per cent hydrogen gave large and variable blanks, as pointed out later. This might possibly have been overcome by working a t somewhat lower temperatures, as recommended by Graham. Check analyses were made on practically every run, and by comparing the average of these two determinations with the original concentration (1.99 per cent carbon monoxide for most of the runs) the percentage absorption of carbon monoxide could be calculated with accuracy.
EXPERIMENTAL RESULTS WITH LOW-ALKALI SODALIME The first series of determinations (by Hasslacher) was made between 180' and 300" C . with one run a t 340" C. The two typical curves for 200" and 300" C. are shown in Fig. 1 (dotted lines). These results indicated that the rate of absorption of such dilute mixtures a t temperatures below 250' to 300' C. was so slow as to be of very doubtful commercial value for carbon monoxide removal, although for the production of formate it might be worthy of further consideration. However, the 13
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apparent rapid increase in the rate of reaction with temperature in this vicinity indicated the desirability of extending the study to much higher temperatures, for most of which work a lead bath was employed. It was then easily possible to go up to 550' C., which was the safe limit for the Pyrex glass U-tube.
Vol. 15, No. 7
An experiment was made a t a temperature of 420' C. t o determine whether or not hydrogen was produced as the result of the absorption of carbon monoxide, and i t was found that approximately one equivalent of hydrogen was produced for each equivalent of carbon monoxide absorbed, thus verifying the belief that the fundamental reaction involved was essentially that indicated above (Reaction 2). Tests on the effiuent gas a t several temperatures showed only a faint trace of carbon dioxide. RESULTS WITH OTHERABSORBENTS Experiments were next undertaken on the high-alkali (40 per cent sodium hydroxide) soda lime, and it was at once found that the absorption a t intermediate temperatures was much more complete than with the low-alkali soda lime, and that the efficiency of removal did not begin to drop off seriously until about 375" C. The results of these experiments are also shown on Fig. 2. Runs with straight hydrated lime were attempted, but i t was impossible to form it into satisfactory granules or to suck the desired amount of gas through 10 g. of the powdered material. Therefore, 8 to 14-mesh quicklime was used instead, and an attempt made to hydrate it, a t least superficially, by passing moist air through for some time a t 300" to 400" C. It was found very difficult to obtain check results with this material, but the average of several observations a t three different temperatures is shown by the dotted line on Fig. 2. Contrary to the result with soda lime, the efficiency did not remain constant, but dropped off fairly rapidly with time. This may be illustrated by a typical run made a t 550" C., using in this case dry gas. ' Gas samples were taken about every 5 min. TABLE I Run 30, Temperature, 550' C., 0 Per cent Moisture Carbon Monoxide Absorbed Sample Per cent 1 88.3 2
The second series of results (obtained by Masterson) are shown by the set of full lines on Fig. 1. It will be noted that low temperature results are quite reasonably consistent with those obtained a year earlier with different apparatus and different samples of soda lime, etc. The individual points on the curves were not secured by any regular increase in temperature, but runs a t higher temperatures were alternated with runs a t lower and intermediate temperatures. While the shape of the curves varies somewhat from one temperature to another and indicates that there is quite an appreciable experimental error, as might be expected, there can be no real doubt as to the essential conclusions which may be drawn from the work. For example, all the curves in Fig. 1 indicate-contrary to some statements in the literaturethat dry gas is absorbed with good efficiency, especially a t the higher temperatures, although small amounts of moisture seem to be distinctly helpful in increasing the absorption efficiency, around 4 per cent by volume in the gas apparently being about the optimum for most temperatures. Higher moisture contents appear to injure rather than improve the efficiency. Fig. 2, based on the same data as Fig. 1, shows the effect of temperature on absorption efficiency for three different moisture contents. It is very interesting-to note the sharp rise in efficiency between 320" and 440" C. The fact that formates decompose rapidly around these temperatures to form carbonates may have some bearing on the sharp increase, although the writers obtained no definite evidence of the production of formates as an intermediate step in the reaction. Soda lime used a t the higher temperatures showed large amounts of carbonates on removal from the tube.
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It seems probable that the cause for this rapid drop in efficiency might be the slow dehydration of calcium hydroxide, which has quite an appreciable vapor pressure a t this temperature. It is evident that in the presence of dry air the desired reaction cannot take place with calcium oxide, but requires calcium hydroxide. Even with 4 per cent moisture in the gas, however, variable results were obtained, and a similar tendency to drop off in efficiency was observed.
INDUXTRIAL A N D ENGINEERING CHEMISTRY
July, 1923
It appears from these results that caustic soda is the active agent in carrying out the desired reaction, but that the caustic soda thus converted to carbonate is slowly regenerated by reacting with the lime. An exactly similar behavior has been observed in the case of soda lime used for absorbing carbon dioxide at ordinary temperatures. Since hydrogen is a product of the reaction, and the freeenergy change a t the higher temperatures is probably not great (reversal becomes pronounced a t still higher temperatures), it seemed desirable to determine whether or not the presence of large amounts of hydrogen would seriously retard the absorption of the carbon monoxide. A few runs were made with a special hydrogen-carbon monoxide mixture, and the efficiencies obtained a t a given temperature appeared to be quite appreciably lower than in the presence of nitrogen, but upon investigating this point further it was found that the hydrogen itself was giving a very large blank with the iodine pentoxide used for the analysis, thus indicating small amounts of carbon monoxide in the effluent gas. This may have been due to the presence of oil vapors in the cylinder hydrogen, or to a reaclion of hydrogen itself with iodine pentoxide a t this temperature, which is somewhat higher than that recommended by Graham. Unfortunately, time was not available to determine the cause and remedy of the difficulty, but from the size of the blanks with the straight hydrogen the efficiency seems to be nearly, if not quite, as good in the presence as in the absence of hydrogen.
POSSIBLE COMMERCIAL APPLICATIONS I n the light of the foregoing experiments it would appear that the reactions under discussion might have definite commercial possibilities for the removal of traces of carbon monoxide froin gas streams. It would probably be preferable to use a soda lime of intermediate alkalinity-say 15 per cent
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sodium hydroxide-in order to secure rapid absorption without undue cost. It would probably be found desirable to work at temperatures of 400" to 450" C. with a time of contact of 2 or 3 sec. This time of contact might have to be increased slightly to remove the very last traces of carbon monoxide, but, on the other hand, it might be reduced by working at somewhat higher temperatures, although too high a temperature (around 700" C.) would undoubtedly give trouble due to the reversing tendency of the reaction in this range. As noted previously, patents have been taken out for the use of soda lime to remove the bulk, as well as the last traces, of carbon monoxide from water gas, but the chemical cost seems to be too high to make this commercially feasible. It might, however, be practicable to take care of the bulk of the carbon monoxide by either (a) passing the gas through a fairly deep bed of high-alkali soda lime a t temperatures around 230" C. in order to produce formate as a by-product, or (b) removing the bulk of the carbon dioxide with hydrated lime-possibly in the presence of ferric hydroxide as a catalyst -and regenerating the lime by heating somewhat higher and then hydrating. One difficulty with the latter method would be in handling the powdered lime in such a way as to bring i t into effective contact with the gas and avoid trouble from clogging or channeling, but this difficulty would not be insuperable if the process were satisfactory from other standpoints. Since only lime in hydrated form appears to be effective, it would not be possible to use hard granules of quicklime. On the whole, however, it seems rather improbable that any modification of the soda lime process would be able to replace existing methods of caring for the main bulk of the carbon monoxide in water gas, especially since the oxidation with steam in the presence of ferric hydroxide produces an equivalent of the desired hydrogen for each of carbon monoxide removed.
Hydrogenated Oil for Oil Baths' By G. Ross Robertson UNIVERSITY OF CALIFORNIA, SOUTHERN BRANCH, Los ANGELES, CALIF.
"Hard" hydrogenated vegetable oil is a remarkable industrial product which seems to have been overlooked as a laboratory material, probably because it is only an intermediate in the manufacture of shortening, and never reaches the retail trade in a pure state. In this laboratory it has proved to be the oil par excellence for use in oil baths. To anyone who has put up with tarry messes and fire hazards in connection with oil baths in organic laboratories, the properties of this oil should be of interest.
"HARD SESAME OIL" Opaque white solid, resembling a very brittle laundry soap. Approximately 90 per cent hydrogenated; iodine number about 10. Melts fairly sharply about 60" C. to a clear, mobile oil. Gives an extremely weak flash in a n open pan at a minimum of 320' C.; will not burn unaided by a torch below 350' C., which is approximately the temperature at which i t gently boils with decomposition. Upon resolidification i t will not stick to either iron or glass, but cracks and falls apart into brittle fragments. Slight unpleasant odor, as of unrefined cottonseed oil. Odor is less after repeated heatings. Cost-in Los Angeles, April, 1923-17.5 cents per pound in small quantities from manufacturer. 1
Received April 26, 1923.
This preparation is much superior to vaseline, paraffin, or any other hydrocarbon, is more convenient than untreated vegetable oil, and, of course, much cheaper than fusible metal. Hard hydrogenated cottonseed oil, more widely obtainable than the sesame product, costs no more and is apparently of about the same value. A local sample, manufactured in Los Angeles, melts at 60° C. and has a flash point of 3 0 5 O C. The use of this oil has eliminated oil fires in our laboratories, despite the carelessnesg of elementary students. In commercial practice the hard oil is mixed with cheap liquid or semiliquid fats ta make the lower priced shortenings of the type of lard compound. When vegetable oil is completely hydrogenated, it becomes extremely brittle, and falls into a bulky, semicrystalline powder. Such complete saturation is unnecessary in preparing material for oil bath purposes, and for food is regardedlocnlly as undesirable. If water is spilled into the melted hydrogenated oil bath, it may easily be removed by cooling the oil, cracking the solid, and exposing to the air. Even if the watery addition has been emulsified, one needs only to crush the material into a coarse, granular mass and expose to dry air.
