Improvements in or relating to cyclic process for separating ammonia and hydrogen chloride from ammonium chloride

摘要

Ammonia and hydrogen chloride are separated in a cyclic process from ammonium chloride by mixing the ammonium chloride with a melt of an alkali metal or ammonium bisulphate in an amount providing 0.15-0.75 mole of NH4C1 per mole of bisulphate, circulating the NH4C1-containing melt from the mixing zone through successive hydrogen chloride and ammonia stripping zones, maintaining a temperature of 220-270 DEG C. in the HC1 stripping zone, separately removing a vapour stream and partially stripped melt therefrom, maintaining a temperature of 330-380 DEG C. in the NH3 stripping zone, separately removing a vapour stream and stripped melt therefrom, returning the stripped melt to the mixing zone, and recovering HC1 and NH3 from the vapour streams removed from the respective stripping zones. The heat may be supplied by heating the melt or preferably, by direct transfer from preheated inert thermally stable fluids, such as steam, nitrogen, or vaporized organic liquids, e.g. benzene or chlorinated aromatics, such as chlorobenzene, orthodichlorobenzene, mixed isomeric trichlorobenzenes, or chlorinated diphenyls. The bisulphate may be sodium bisulphate, or mixtures of 50-75 per cent. thereof with ammonium bisulphate, or mixtures including potassium or lithium bisulphate, or three component mixtures. The ammonium chloride may be added as solid, or as a slurry in the organic heat transfer liquid. In Fig. 1, NH4C1 is added to the bisulphate melt in mix tank 10 and the melt flows to HC1 stripping tower 13 and therethrough countercurrent to gas or vapour, preheated in heater 23, injected through 24. HC1 is recovered from the vapour passing out at 25. The PICT:0716754/III/1 PICT:0716754/III/2 partly stripped melt is withdrawn at 14 to blow case 15, whence, by means of gas or vapour entering at 18, it is elevated to top of NH3 stripping tower 17 down which it flows countercurrent to a portion of the gas or vapour from preheater 27, and thence through 28. NH3 is recovered from the vapours passing out of tower 17. The stripped melt is withdrawn at 19 to blow case 20, whence it is elevated to mix tank 10 by gas or vapour entering at 22. Some of the super heated gas or vapour may be passed to mix tank 10. In Fig. 2 (not shown) a single stripping tower is shown divided into upper and lower ceramic or refractory packed sections for HC1 and NH3 stripping respectively, wherein the upper section is atmospheric, while the lower section is under vacuum, or the upper section is under partial vacuum and the lower section under higher vacuum, and wherein the heat is supplied by withdrawing portions of the melt through coils in an external gas fired heater and returning them to different points in the lower section. In Fig. 3 (not shown) a packed stripping tower is divided into several sections by liquid seals and melt passes therethrough from the top, while steam is injected through a recycle gas heater into the bottom of the lower-most section, and the gas from the top of this section is passed through another recycle gas heater of the next section above and so on, each section being provided with a recycle gas heater. In Fig. 4 NH4C1 is mixed with hot organic heat transfer liquid from tank 312, which is water immiscible and has a boiling point above that of water but below the sublimation point of NH4C1, in tower 310. Moisture is flashed off with entraining organic liquid by distillation through cooler 318 to separator 319, whence separated organic liquid is returned to tower 310. The resultant dehydrated slurry containing say 20-25 per cent. NH4C1, is passed to mix tank 324 and mixed with bisulphate melt, which is passed as in figure 1 through HC1 and NH3 stripping towers 330, 336. Vapours of the organic liquid from tank 312, after passage through boiler 316 at say 260 DEG C. and superheater 326 at say 450-550 DEG C. are injected through lines 314, 328, 332, and 337 into tower 310, mixing tank 324 HC1 stripping tower 330, and ammonia stripping tower 336 respectively, and through lines 334 and another into vapour lifts 333 and 339 to raise the melt to the top of the ammonia stripping tower 336 and to the mix tank 324 respectively. The organic vapours are in sufficient quantity and at a sufficient temperature to maintain the desired stripping temperatures. The HC1 and NH3 are recovered from the vapours from the stripping towers by passage through primary and secondary condensing towers 340, 347, and 357, 362 respectively, which are each cooled by recirculation of condensed organic liquid, for example through pump 341 and cooler 342 in the case of tower 340. Excess organic liquid from the condensing towers is returned through a common pipe 344 to a filter 345 and thence to tank 312. Ammonia may be added to the recovered liquid at 334a to avoid presence of free HC1. The HC1 is passed finally through a sulphuric acid drier 352, and both HC1 and NH3 are passed through refrigerated coolers 353, 367 respectively to condense residual organic liquid, which is separated in separators 355, 368 and passes to return line 344, while anhydrous HC1 and NH3 are recovered at 356, 369 respectively. The refrigeration and drying steps may be replaced by use of active carbon or silica gel absorbent. Make-up bisulphate may be added as necessary, or H2SO4 may be added to the melt, if crude NH4C1 is used, which contains NaC1.