Abstract:
A process for separating dissolved corrosive gases from aqueous solutions, e.g., scrubbing waters from a coking operation, is disclosed. The process involves directly contacting the solution with a hot liquid heat exchange medium, such as drops of heating oil, to drive off the dissolved gases. Residual ammonia in the solution is thereafter separated out by blowing steam into the treated solution.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a process for driving off dissolved corrosive gases from aqueous solutions. More particularly, this invention relates to a two-step process of first separating acid gases from solutions such as scrubbing waters from a coking plant and thereafter separating out the ammonia in the solution. 
     The watery condensates and scrubbing waters produced in coking plants and gas treatment facilitates contain hazardous and corrosive dissolved gases such as NH 3 , CO 2 , HCN, H 2  S, etc. which normally are separated from solutions by heating and blowing in steam. This requires a considerable amount of heat and steam, especially when the NH 3  contained in the water is separated simultaneously with the above-mentioned components by steam introduction in column apparatuses. In addition, other apparatus such as heat exchangers and fractionating towers associated therewith are subject to considerable corrosion by the acid gases and ammonia. 
     SUMMARY OF THE INVENTION 
     It is among the principal objects of this invention to provide a process for separating out these corrosive gases in a simple and economical fashion. 
     This object and others are achieved by providing a process for separating dissolved corrosive gases wherein a heated liquid heat exchange medium immiscible in the aqueous solution is passed in direct contact with the solution to drive off the dissolved acid gases. In the presently preferred form of the invention, the liquid medium is light heating oil which is injected at the bottom of the solution and flows upwardly as drops of oil through the solution. Thereafter, the residual ammonia in the treated solution is removed by blowing steam into the treated solution. In accordance with the principles of this invention, the acid gases such as CO 2 , HCN and H 2  S are separated out at a lower temperature than the ammonia resulting in a considerable savings in steam. That is, the acid gases are driven off at a relatively low temperature, for example, somewhat above 338° K. and at atmospheric pressure after which the solution is heated to an elevated temperature of at least about 373° K. to drive off the ammonia. 
     The higher steam temperature in the ammonia separation phase of the process causes at the head of the ammonia separator about eight times as much steam as ammonia. This steam is preferably cooled to approximately 353° K. so that approximately one part of ammonia and one part of steam can be routed for burning. Since scrubbing waters contain predominantly acid gases less steam is burned than in the prior art steam separation process. 
     If an especially distinct separation between the acid gases and the ammonia in the scrubbing water is desired, the acid gases may be driven off at an elevated pressure in the range of about five bars and at an elevated temperature, for example, 423° K. In contrast to the prior art processes and facilities, corrosion is avoided in practice of this process as well. A more distinct separation from ammonia from the acid gases is desired for environmental reasons since the clean ammonia gases can be burned with a limited steam content in special combustion devices producing nitrogen oxide in a concentration below 100 ppm. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic diagram of a process of this invention, and 
     FIG. 2 is a schematic diagram of another embodiment of this invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to FIG. 1, ammonia containing scrubbing water resulting from treatment of coking gas enters the process with an ammonia content of, for example, 18 grams per liter and at a temperature of 303° K. via line 1. The flow is split into two partial flows. A partial flow of the scrubbing water proceeds through a line 2 into a heat exchanger 3 and is preheated by the hot treated scrubbing water from the ammonia separator 8 to a temperature of about 330° K., and then passed via a line 6 to a deacidifier 5. A second partial flow of untreated scrubbing water proceeds directly, without additional heat adsorption, via a line 4 to the deacidifier 5 and is sprayed as cooling water at the acid vapors issuing from the deacidifier 5, whereby these are cooled to about 338° K. and then leave the deacidifier 5 through a line 17 at the top. 
     The scrubbing water entering the deacidifier 5 through the two lines 4 and 6 is heated in the deacidifier 5 to a temperature of 338° to 345° K. by light heating oil which flows upward in the form of drops. The acid components of the scrubbing water are separated thereby, and the hot scrubbing water, essentially free of acid components, is passed to the NH 3  separator 8 via a line 7. 
     Steam is blown in the bottom area 9 of the separator 8 (not illustrated) whereby the scrubbing water is simultaneously heated to at least 373° K. with the aid of a thermal oil heated to 573° K. by an evaporator 10, thereby separating the so-called free NH 3  from the scrubbing water. Thus treated, the hot scrubbing water is withdrawn at the bottom of the NH 3  separator and passed to the above heat exchanger 3 where it is cooled against the partial flow of the as yet untreated scrubbing water passing through line 2 and is thereafter recirculated to scrubbing via a line 11. The H 2  O/NH 3  vapors proceed from the NH 3  separator 8 via a line 12 at a temperature of about 373° K. to a vapor cooler 13. Cooled to 333° K., the heating oil coming from the deacidifier 5 is sprayed in the top area of the vapor cooler 13 for cooling of the H 2  O/NH 3  vapors. The steam transforms into a condensate. 
     In a separating bottle 14 whose upper portion represents the vapor cooler 13, the oil is separated from the water. Heated by contact with the hot H 2  O/NH 3  vapors, the oil returns through a line 15 to the deacidifier 5 and, as previously described, causing the heating of as yet untreated scrubbing water by direct phase contact. Containing large portions of NH 3 , the steam condensate is withdrawn at the bottom of the separating bottle 14, below the oil layer, and returned to the NH 3  separator 8 via a line 16. The released NH 3  gases proceed through a line 20 to a burner 18 of a tubular furnace where also the acid gases from the line 17 coming from the deacidifier 5 are burned. The burner 18 transmits the combustion heat to a heat exchanger 19 which, in turn, through the circulation of the hot thermal oil, will give off the heat via the evaporator 10 in the NH 3  separator 8. The heat generated by burning the waste gases is thereby retrieved and utilized in the system. 
     In the second example according to FIG. 2, the NH 3  scrubbing water mentioned in the first example passes through a line 21 into a separator which for H 2  S removal employs an NH 3  circulation scrubbing and a pressure deacidification. The NH 3  scrubbing water is split in two partial flows. The one partial flow is preheated in a conventional heat exchanger 23 to a temperature of 330° K., against the hot NH 3  water withdrawn from a heat exchanger 24 working in direct phase contact, and then passed via line 26 to the oil-water heat exchanger 24. The NH 3  water is heated here to 410° K. at a pressure of about 5 bars and then passed to a pressure deacidifier. Introduced directly into the latter via a line 212 is the second, not preheated partial flow of the as yet untreated NH 3  scrubbing water, as cooling medium. The scrubbing water is rid of the acid gas components in the pressure deacidifier 25 by heating to a temperature of 423° K. 
     Thereafter still containing NH 3 , the water flows from the pressure deacidifier 25 partly back to the heat exchanger 24 where it is withdrawn at the bottom and cooled in the heat exchanger 23 against a partial flow of the as yet untreated scrubbing water flowing through the line 21, and is then recirculated to the H 2  S scrubbers via a line 22. The other partial flow of the NH 3  water is passed from the pressure deacidifier 25 through a line 27 to an NH 3  separator 28 and freed there of the NH 3  components. The scrubbing water treated here returns after the (not illustrated) cooling through a line 211 to the NH 3  scrubbers, as scrubbing water. The NH 3  vapors issuing from the latter NH 3  separator 28 are passed to a burner 218 of a tubular furnace and burned there, after cooling in a vapor cooler 213. Containing considerable portions of NH 3 , the condensate from the vapor cooler 213 is passed again to the line 22 of the H 2  S scrubbing water. 
     The above burner 218 transmits the combustion heat to a heat exchanger 219 which by way of a thermal oil circulation connects with a vaporizer 210 in the bottom area of the NH 3  separator 28 and serves to generate steam which is blown through the bottom 29 into the separator 28. Now nonhazardous, the waste gases from the tubular furnace with the heat exchanger 219 are released into the open. 
     The acid gases from the pressure deacidifier 25 are passed via a line 217 and proceed in decompressed state to a burner 218.1 of a second tubular furnace. The reclaimed heat is utilized in an evaporator 210.1 in the bottom of the pressure deacidifier 25 by way of a heat exchanger 219.1 contained in the tubular furnace and coupled with the burner 218.1, as well as via a thermal oil circulation 215.