US2386359A - Gas purification - Google Patents

Description

Oct. 9, 1945. F. D. SCHREIBER GAS PURIFICATION Filed Sept. 22, 1942 Patented Oct. 9, 19.45

yiJNrrEo STATES PATENT OFFICE ons PUnmoA'rroN Fredrick D.' schreiber, Dumont, Pa. Application September 22, 1942, Serial No. 459,315

' '2 clams. (ci. 23-2) l sorption of the acidic constituents from the gases by the use of a dilute solution of sodium carbonate followed by release or liberation of the HzS or HCN in an actification or regeneration step. Thus, in accordance with the so-called Seaboard process, the solution containing the absorbed constituents is blown with air or other inert gas. This process has the. disadvantage of low eiciency and fails to liberate the acidic components in suitable form for further utilization. Subsequent attempts to. improve the foregoing process have resulted in the actication of the solution by utilization of steam for the purpose of expediting and carrying to greater completion the gas elimination. Yet more recently the use of absorbing solutions having a relatively higher carrying power for I-IzS and HCN has been developed inthe so-called phenolate process. While this latter process has been found advantageous inthe treatment of gases containing relatively large amounts of HzS, it has not been characterized by optimum results when applied to coke oven gas containing less than, for example, 500 grains HzS per hundred cubic feet. When attempt is made to actify the phenolate solution under vacuum conditions the HzS and HCN are recovered in a form contaminated by components of the solution. In short, the process is relatively unsuitable to the production from coke oven gas of relatively pure acidic gases of the foregoing class in such form that they may be utilized subsequently in connection with production processes such, for example, asin the contact sulfur acid process and in the preparation of commercial cyanides.

In accordance with the present invention there is provided an improved process for the separation of HzS and HCN relatively free from undesirable impurities, which process is operable in general upon fuel gases, including refinery gases, but particularly in connection with the coke oven gases and the like. In accordance with this process, 1t has been discovered that a weak carbonate solution is highly efficient in absorbing the acidic constituents from the fuel gas and that the absorbed solution is adapted to be actiiied by heat and under a vacuum to result in high eiiiciency operation provided that` accumulation of naphthalene and light oils is prevented in the system. i In short, it has been lobservedlthat, the accumulation of the foregoing products seriously interferes with the proper and elicient operation of the process, and their substantially complete elimination offers a marked improvement in operation of the absorption and actication steps. While elimination of naphthalene and light oils may have hitherto occurred to some extent in the final cooler, nevertheless under the conditions commonly prevailing a substantial loss of acidic gases occurred so that the advantages of high yield and improved operation were never fully obtained.

lIn accordance with the present invention, however, the final cooler is operated in such a manner that the cooling water is recycled in a closed system in which cooling takes place under conditions such that none of the acidic gases are permitted to escape. 'I'hus the cooling water is recycled in a closed system, being continuously subjected'to the action of a tar or oil scrubber and precooled for passage through the final cooler. In this treatment it will beappreciated'that the Water becomes saturated with HCN and HzS whereby these acidic gases ultimately tend to remain in the coke oven gas upon passage through the final cooler and thus are passed on to the absorption tower without loss.

In accordance with the present invention the gas from the coke oven battery, for example, .is rst passed through primary coolers and separators for pitch and tar in the usual manner and then is conveyed through the sulfuric acid saturator for the purpose of removing the contained ammonia. Vapors from the ammonia still, normally including HzS and HCN, may be conveniently combined at the inlet to the sulfuric acid saturator. Substantially all, that is at least 98 per cent, of the ammonia contained in the gas is thus recovered in the saturator in the form of ammonium sulfate.-

The eiiluent gas leaving the saturator, normally at a temperature in the order of 50 to 609v C., passes to the finalcooler, where its temperature is reduced, preferably by direct cooling, to 20 to 30 C. The foregoing steps are in general analogous to the corresponding portions oftheprocess disclosed in my Patent No. 2,219,713, dated October 29, 1940. It is important to note, however, that in accordance with the present process the final cooler is modified in such a manner as to eliminate naphthalene from` the gases and to this end a directly acting coolant is passed preferably countercurrent to the gases in such a manner not only to reduce their temperature but to take up available naphthalene. The coolant water is then cycled in a closed system comprising a water cooler, preferably indirect, and a tar or oil scrubber for recovering naphthalene therefrom. The indirect cooler may operate to maintain the water at a temperature of, for example, C. for direct passage to the iinal cooler. The 4water accumulating at the bottom of the ilnal cooler is removed or passed through the tar or oil scrubber and then returned to the indirect cooler. It will be appreciated that such additional moisture as accumulates in the system `through condensation and the like may be periodically withdrawn from a sump and returned to the ammonia still.

It is particularly important to note that in the foregoing system the HzS and HCN build up in the water of the final cooling system until the water becomes saturated therewith. Accordingly the cooling system after saturation of the wateris completed operates to return all of the HzSfand HCN to the gases leaving the final cooler and passing to the scrubber for removing the acidic gases. To this end it is necessary that the water oi the iinal cooler recirculating sys'- tem, pass in a closed circuit wherein the acidic constituents are retained. It is thus advantageous to use an indirect cooler in the recirculating system and just subsequent to the removal of naphthalene from the water in the oil or the scrubber.

Asia result of the foregoing the gases in cooled condition, substantially freed from naphthalene and ammonia, are suitable for treatment to recover the I-IrS, HCN and CO2, subject iirst to removal of the light oils. Therefore, in accordance with the present invention, the gaseous eiiiuent is fed to a benzol scrubber unit. The details of this element of the system are relatively unimportant due to the fact that the criterion here involved requires merely an eiiicient removal of light oil constituents. -Therefore conventional benzol scrubbers in series arrangement are suitable. It is important to note, however, that the disposition of the units, as above, results in a product substantially freed of ammonia', naphthalene and light oils and thus capable of resulting in a novel improvement in operation of the absorbing and activating system.

Accordingly the resulting cooled gas enters an absorber constituting part of a hot vacuum actiiication gas purifying system employing dilute sodium carbonate solution as aforesaid, as the absorbing medium. It will be understood that the absorbing solution is preferably cooled to at least about C. before being applied to the absorber and 4thus presented into contact with the gases at a temperature favoring the absorption of .the HzS, HCN and CO2. vThe effluent gas from the absorber is highly puried with respect to acidic constituents and may be discharged through a gas holder or other discharging system.

The solution containing the absorbed acid constituents is then passed through heat exchangers to the actifier where the acidic gases are removed by heating under a vacuum. Conveniently the heat is supplied by steam coils operating indirectly to heat the solution substantially to its boiling point. It is important to note that operation is decidedly advantageous when utilizing indirect heating since direct steam would tend to dilute the solution. thus interfering with operation as a substantially completely closed system. The gases liberated in the actiiier comprise a relatively high purity mixture of HzS and HCN as well-as CO2. 'I'his product is withdrawn by the vacuum pump and may be utilized in various ways although I prefer to treat it in accordance with the method disclosed in my application Serial. No. 459,316 tiled of even date herewith, for the purpose of separating the several constituents and forming relatively high purity products.

In order that my invention may be more readily appreciated, it may now be described brieiiy with reference to the accompanying drawing, which is a :dow sheet settingl forth the parts of the process more or less diagrammatically. In accordance with the. ilow sheet, the reference `numeral 2 indicates a saturator of conventional type to which sulfuric acid is supplied through an inlet il. Coke oven gas or the like is introduced into the saturator through a pipe 6, into which flow ammoniastill vapors from pipe 8. The ammonium sulfate formed in the saturator may be withdrawn continuously or from time to time through an outlet l0.

The relatively ammonia-free gas is discharged through a pipe l2 into the bottom of a nal cooler ill containing a suitable packing i6 facilitating contact between the gas and. cooling water. The liquid coolant and condensate accumulating in the bottom of the cooler is discharged through a pipe i8 to a sump 20 and as the amount of liquid in the system builds up any excess may be withdrawn through an loverow line 22, preferably being sent to the ammonia. still or ammonia liquor storage tank (not shown). The continuously cycled water is withdrawn from the sump by a pump 2Q, and passed in the present embodiment, into the lower part of a tar scrubber 26. While the sump 20 is represented more or less diagrammatically it is advantageous to use a closed sump provided with a vent. While under the conditions prevailing, an insignicant proportion oi gas evolves from the water at this point, I prefer to absorb any gaseous eiiluent by introduction of a small spray of cool water into the vent. This spray may readily be apportioned to act as make up water for the system, accountving for that absorbed and removed with the tar in the tar or oil scrubber. It will be understood that the water passes upwardly in'a tortuous path, through a mass of tar introduced at the point 28 and maintained at a predetermined level by Vstandpipe 30. The tar passes out through the outlet s tandpipe 30 after having absorbed the naphthalene from the water. A heating coil in the base of the tar and oil scrubber warms the tar to a temperature of approximately 45,-55 C. in order to eiiect a good separation of the tar from the aqueous material and permit removal of tar with as low a moisture content as possible.` Accordingly the water is warmed somewhat and leaves the scrubber at a temperature somewhat' above its temperature of entry (for example 35-40 C. or thereabouts). 'I'he effluent water therefore is advantageously cooled before reintroduction to the final cooler by way of pipe 38; To this end the water from pipe 34 passes through indirect cooler 36 and thence returns to the top of the iinal cooler through pipe 38. The water introduced into the top of the cooler is accordingly not only cooled to atleast 20 to 30 C. but is maintained substantially pure with respect to naphthalene.

dled by It will be understood that for the tar scrubber illustrated there may be substituted an oil scrubber, a corresponding modiiication of structure being made as will be understood by those skilled in the art, in view of the characteristic distinctions-of the two devices. oil scrubber, it willbe necessary to introduce the water at the top portion of the device in order to eilect relative countercurrent ow of the fluids.

, The cool gas leaving the top of the final cooler passes through a pipe 39 into the bottom of the ilrst benzol scrubber Sli. embodiment and for the purpose of illustration, a pair of benzol scrubbers dll and d2 are utilized, it will be understood that any desirable arrangel ment may be provided in order to eiect the purpose of substantial separation of light oil constituents.A To this end wash oil is introduced through inlet d4 in the second benzol scrubber d2, being withdrawn at the bottom portion by pump d6, and passed into the top of the first scrubber in countercurrent relation to the oncoming gases. Gas conduit t8 connects the two scrubbers as shown.

The treated gases pass through pipe 50 into the bottom of absorber 52 for the acidic gases. The present absorber while shown for purposes of illustration as a single section type, maypreferably comprise two or multiple section units, as will be understood by those skilled in the art. Thus in the two section type the highly actied solution is supplied to the upper .section whereas a relatively less actified solution passes to the lower section. In any event, the gas passing upwardly through the absorber, by virtue of a suitable packing ls brought into intimate contact with a sodium carbonate solution passing downwardly and introduced through pipe 56. The gas reaching the top of the tower and discharged at 56 is thus substantially completely freed from the acidic constituents absorbed by the liquid and may be passed for utilization or other treatment as hereinabove mentioned. The carbonate solution containing absorbed l-IZS, HCN and CO2 may be passed to a suitable sump or storage tank (not shown) and delivered by a pipe 58 through a heat interchanger t@ and pipe t@ to the upper part of an actier 6d. In this actier the solution is heated by steam supplied'to coils 66 at the bottom of the 'actier tower, the gaseous constituents being driven out of the solution as previously described and passing upwardly through the pipe 68 to a condenser or dephlegmator lll which condenses and returns any water carried out by the acid gases through the pipe l2 and trap ld. Particular attention is directed to the provision of a vacuum pump it in outlet pipe lll which conveys Y the liberated HON, H28 and CO2 out of the system vfor further treatment. It is by virtue of this vacuum pump that the actifier tower is maintained under a predetermined vacuum substantially below atmospheric pressure and selected to facilitate a substantially complete removal of the gaseous constituents. Advantageously the degree of vacuum is as high as possible in commercial practice. Good results however are obtained with a vacuum of the order of 25" Hg and preferably The solution resulting from the foregoing treatmen-t and collecting in the bottom of the actier being substantially completely freed of absorbed gases, is 'adapted to efficiently absorb and pick up acid constituents when returned to the absorber. To this end the actied liquid is hanpump 80, which passes it under increased In short, utilizing an While in the present pressure through the hfeat exchanger $0 from whence it moves in cooled. condition into the absorber through pipe 56. It is important to note that the pump 80 in the inletline 62 removes the sodium carbonate solution from the region of low pressure. In lthe present embodiment it must be noted that the `ow sheet contemplates a-\full scale operation wherein tower 64 may be of the order of 80 feet high. .Accordingly the upstanding inlet pipe 62, normally supplied with liquid, affords suitable means for supplying the liquid to the actiler at the required low pressure. In fact it is normally necessary in commercial operation to provide additio al pump means 82 to force the liquid through heat exchanger S0, up pipe 62sand into the activator tower.

The present invention accordingly provides a simple method and apparatus for-emclently recovering the acidic constituents of coke oven gas free from undesired impurities.` The process, moreover, is one which is adapted to tinuously-without material servicing or attention. As a result of the arrangement of the steps and utilization of the operations in the order and relationship herein disclosed the process operates with an eliciency and freedom from which is greatly superior to those processes with which I have heretofore been aware. Thus by eliminating naphthalene and light oils prior to absorption and actiiication, the operation of this system is materially improved and there is rendered possible the rapid and highly emcient removal of the desired constituents from gases in which they exist in relatively low concentration.

Moreover, the substantially complete elimination of naphthalene and light oils is accompanied by cooling of the gases to a temperature suitable for absorption. In accordance with the present -invention the cooling and naphthalene removal .bonate solution bysubjection are effected in such a manner as to obviate loss of acidic constituents at this point. This the invention accomplishes preferably by` cooling with a repeated recycling in a closed system of cooling water which is substantially saturated with respect to acidic components. Accordingly the fuel gas passing through the absorber is not only cooled to the ideal temperature for absorption by the sodium carbonate solution and in relatively puried condition but still contains all of its original acidic constituents.

The acidic gases liberated from the actiiier being of high concentration and relatively high purity are adapted for later utilization and treatment. It should be understood the foregoing data are given by way of illustra-tion only and not by way of limitation.

What I claim is:

l. lThe method of recovering acidic gases from a mixture constituting fuel gases which comprises removing a substantial amount of the naphthalene component of s aid fuel gases by cooling said gases by direct contact of said gases with a coolant and removing naph-thalene from said rcoolant by scrubbing with a naphthalene solvent immiscible with said coolant, the steps of naphthay iene removal from said gases and from said coolant being accomplished without loss of any appreciable quantity of the gases dissolved in said coolant from said coolant, removing a substantial proportion of the light oil impurities of the resulting gaseous eilluent by absorbing the acidic constituents of the resulting gas iny weak carbonate solution and, thereafter releasing said acidic constituents from said Vcar-- of the same to heat and vacuum.

operate coninterruption,

scrubbing the same,

2. The method of recovering acidic gases from a mixture constituting fuel gases which comprises continuously contacting said gases with cool water substantially saturated with respect to the acidic components of said gases whereby the naphthalene component of said gases is condensed in said cool water, removing naphthalene from said water by scrubbing th same with a naphtnalene solvent immiscible wi water, precooling said water and returning said water to 10

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