This invention relates generally to steam distillation of ammonia and ammonium ion compounds from aqueous solutions of such compounds and relates more particularly to the removal of ammonia from weak ammonia liquor produced during a coking operation.
The vapors given off during thermal decomposition of coal to produce coke for use in the steel industry contain various components including tars, light oils, phenols, naphthalenes, hydrogen cyanide, hydrogen sulfide, carbon dioxide, ammonia and water. These vapors are customarily initially cooled in a primary cooler. During such cooling the excess moisture condenses and absorbs ammonia, ammonium compounds and other lesser contaminants and is then known as weak ammonia liquor. The weak ammonia liquor, or WAL, may be recirculated through the primary cooler several times until the WAL becomes saturated with ammonia and other constituents and must be treated to remove the ammonia.
For perhaps as long as 100 years direct contact countercurrent steam stills have been used to remove ammonia from the weak ammonia liquor generated during the coking of coal. As is more fully described in the U.S. Steel publication, "The Making, Shaping and Treating of Steel", 9th ed., Harold E. McGannon, ed. (1971) pp 165 et seq, the most prevalent process for extracting ammonia from weak ammonia liquor is the Semi-direct Process. In this process, weak ammonia liquid is first stream distilled in a so-called "Free" leg to remove the "free", or thermally decomposable, ammonia, i.e. ammonium compounds which are readily dissociated by heat. Exemplary "free" ammonia compounds are ammonium carbonate, ammonium sulfide and ammonium cyanide, which compounds when decomposed, form ammonia and carbon dioxide, ammonia and hydrogen sulfide, and ammonia and hydrogen cyanide, respectively.
Following the first steam distillation in the free leg, the once distilled weak ammonia liquor is combined with an excess of an aqueous slurry of calcium hydroxide, or "milk of lime" in a "Lime" leg. By combination of the predistilled liquor with milk of lime slurry, the "fixed", or non-thermally decomposable, ammonium ion compounds contained in the liquor, i.e. ammonium chloride, ammonium thiocyanate and ammonium sulfate, are subjected to an alkaline environment where a chemical reaction takes place in which the ammonium ion is converted to ammonia and water. The resulting weak ammonia liquor-milk of lime (WAL-MOL) suspension or slurry, containing sufficient lime, both solid and dissolved, to give a CaO to NH.sub.4.sup.+ molar ratio of not less than 1:2, is then allowed to overflow into the "Fixed" leg of the ammonia still, where a direct contact countercurrent flow of steam extracts the hydrated ammonia from the descending liquor slurry. As the ammonia is driven from the descending WAL-MOL slurry, the equilibria present in the slurry, shown in equations I and II EQU ca(OH.sub.2) .revreaction. Ca.sup.2+ + 2OH.sup.- (I) EQU nh.sub.4.sup.+ + oh.sup.- .revreaction. nh.sub.3 + h.sub.2 o (ii)
are shifted to the right, with the result that solid Ca(OH.sub.2) dissolves continuously as the liquor descends, and substantially all the ammonium ion present in the liquor is converted to ammonia and driven off.
The free and the fixed legs of the ammonia still are of similar construction and are each comprised of an upright column having internally disposed horizontal plates or trays. Each plate is equipped with gas-liquid contacting means through which ascending steam may pass. The gas-liquid contacting means are conventially either sieve holes or bubble cap assemblies.
In operation liquor enters the top of the still column and flows from tray to tray, either through downcomers in each plate or through dual flow sieve tray orifices, countercurrently with ascending steam and vapors to the bottom of the column where an effluent port allows the residual ammonia liquor, or so-called still bottoms, to be discharged to a storage vat prior to further processing.
The fixed leg and the free leg of a conventional ammonia still are normally interconnected in such a way that the stream introduced at the bottom of the fixed leg, passes directly from the top of the fixed leg to the bottom of the free leg and upwardly to the top of the still where the steam, along with entrained ammonia and acid gases, leaves the still. Once this steam/ammonia/acid gas stream leaves the top of the free leg, it may be routed to a dephlegmator to condense a portion of the steam and thence either to a sulfuric acid saturator to produce ammonium sulfate or to an incinerator for the combustive destruction of the gases.
One major drawback in the use of such a conventional ammonia still is the tendency of the distillation column to become plugged or fouled. Once a column has been in operation for a period of time, solid calcium compounds tend to accumulate in and around the gas-liquid contacting means, i.e. the bubble caps or sieve tray openings, thereby restricting and eventualy interrupting or interfering with upward flow of steam, particularly through the free leg. Such interruption or interference, of course, reduces the efficient intercontact of steam and liquor and therefore reduces the efficiency of ammonia removal from the liquor. When this occurs, the tower must be cleaned, a process which requires that it be taken out of service, dismantled and the calcium solids removed from the tray openings. Such removal is not only costly in man hours spent cleaning, but also necessitates either adequate storage facilities for the liquor accumulated during the still down-time or, alternatively, a duplication of ammonia still facilities whereby one facility may be used while the other is being cleaned.
Furthermore, since the still fairly rapidly becomes fouled or plugged between cleaning stages, the still is inevitably operated for a large percentge of the time between cleaning stages in a partially blocked condition. Ammonia stills for the distillation or stripping of weak ammonia liquor derived from coal coking operations have in the past, therefore, been designed with considerable excess capacity and with relatively low liquid to gas ratios. This is accomplished by designing the individual plates larger so that more total open area for passage of steam through the plates is available than would be necessary during unfouled operation. The increased open area is provided to insure the passage of adequate steam for efficient stripping of ammonia from the liquor at all times, both when the column is clean and when the column is partially fouled. This arrangement, however, results in relatively more steam overall being required for operation of the still, with a consequent high consumption of steam compared to that which would be required if the still were designed only for that amount of steam which is required for efficient stripping during unfouled operation. In other words, if no fouling and plugging occurred, a still could be designed for the relative passage of less steam per amount of liquor passed through the still. In most cases this would mean that the still would require a lesser diameter with respect either to its height, or more particularly with respect to its rated liquor throughput or capacity, because the total surface area of each tray or plate could be reduced. The usual molar liquid/gas ratio in conventional ammonia stills is in the range of 3 to 5.
One solution to the fouling problem is described in the March, 1975 issued of I&SM in an article by A. C. Naso and J. W. Schroeder entitled "A New Method of Treating Coke Plant Waste Waters", beginning at page 34. In their process, caustic soda is substituted for milk of lime and is added to the liquor prior to distillation. By this substitution, calcium ions are eliminated entirely from the process, thereby obviating the problem of calcium solids formation in the still. While such a solution is effective in eliminating column fouling, the cost of caustic soda as compared with that of lime renders the use of such a process economically unsatisfactory. Although the authors of this article claim that their process can be made economically feasible, this cannot be accomplished without extensive replacement or redeisgn of existing ammonia still equipment.
In light of the foregoing, there exists a need for a process which will eliminate the problems associated with free leg fouling and which will do so economically.