CA1042325A - Sulfur removal - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Sulfur deposits on the walls of HF process equipment can be removed by periodic treatment with SO3, preferably as 20 to 35% oleum.

Description

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~ ROUND OF THE rNVE~TION
-Hydrogen fluorlde i~ produced commercially by the reactlon of a fluorlne-contalning mineral, e.g., fluorspar, with mineral ~cld, ~ulrurlc ~cid, ~t tempera-tures v~rying ~rom lOO~ to 400C. The reactlon product ; from the lnltlal reaetlon 18 then cooled, eondensed ~nd subsequently re~ined to produce anhydrous hydrogen fluorlde In processes such a8 above, the inltlal reactlon products will contaln contamlnants, lncluding sulfur and sulfur-formlng lmpurlties. m ese contamlnants cause pro~lems ln the subsequent ~teps of conden~lng and recove~ng the hydrogen fluorlde by belng deposlted upon the piping an~ he~t trans~èr surfaces or on filter surfaees.
~hen thls equlp~ent is opened to permlt removal of the d~poslts, re~idual HF can be released lnto the atmosphere.
Furthermore, the removal of the deposits 18 a costly and tlme-consuming operation.
- In assignee's Canadian appllcatlon Serial No ; 20 182 818, of ~.~. Ehllg, flled October 9, 1973, (now . Canadian patent l 003 620~, a proce~s 18 descrlbed whereln by the use Or eertaln coollng techniques, the buildup Or sulrur coatlng on the heat transfer ~urraces o~ the HF
process equlpment 18 reduced.
~o~ever, thls process 18 not ~ldely appllcable to exlstlng HF proce~ses a~ a change ln process equ~pment may be requlred ln ~any lnstances SUMMARY OF THE D~V~N~ION
I have dl~covered th~t the bulldup of ~ulfur 30 deposlts on HF proee~ equlpment, partleularly on th~ heat tran~rer sur~ace ~hereln the HF 18 cooled, can be remov~d

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by contacting the deposit~ with S03. The S03 can be added to the equlpment in the vapor phase, wlth the addition being stopped when the buildup of S02 in the vapor phase ceases, or it can be added as a liquid in a ~uitable solvent wherein both S03 and S203 are dissolved, Suitable solvents include sulfuric acid (oleum~ and fluosulfonic acid (HS03F). Preferably, the S03 is added as a 20 to 35% oleum solution.
When the S03 has been added to the equipment, 10 the reaction to remove the sulfur deposits will start This reaction can proceed at ambient temperature or prefer-ably at elevated temperatures. Thus, the reaction can be preferably operated at 20 to 100eJ more preferably at 30 to 60C.
The S03 i8 maintained in the HF process equipment until the sulfur deposits are e~sentially all removed. The time ~ill vary depending upon the amount of sulfur buildup, - the~form and strength of the S03 added, and the tempera-ture. Generally, the time will be between one and slx 20 hours.
After the sulfur deposits have been removed from ~; the ~ralls of the heat tran~fer equlpment, the S03 solutlon containlng the removed sulfur as S203 and S02 i8 drained and recyoled to the HF process or ~tored in a separate tank for reuse. If desired the S03 solutlon could be fed continuously from the storage tank into the process e~uipment and back during the sulfur removal stage.
Accordingly, no dlscharge from the system is required nor need the equipment be opened, thus ellmlnatlng alr and 30 water pollution problems, Furthermore, the process does not lntroduce new chemlcals lnto the HF process, iq~ S
~ESCRIPTION OF THE INVENTION
_ _ _ _ The process of the lnvention i8 applicable to any proce~s for producing gaseous hydrogen fluoride wherein the ga eou~ crude reaction product contain9 sulfur contami-nants which deposit on the process equipment, particularly the cooling and condensing equipment, and also ~ilter sur-faces. Thus the process of the invention can be used in con~unction with the well-known commercial process for producing hydrogen fluoride by reacting fluorspar with 9ul-furic acid.
In thls process the fluor~par and sulfuric acid react to form calcium sulfate and hydrogen fluorlde. The reaction i8 endothermic and variou~ methods are u~ed for supplying the necessary heat. One par~icularly useful method of supplying this heat i~ by the addition of ~ulfur trioxide and steam to the reaction zone. This method is described in detail in U.S, Patent 3 102 78~.
Although the process of the invention is parti-cularly applicable to these commercial processes, it is ~- 20 alJo applicable to other processes wherein a gaseous, sulfur-containing crude hydrogen fluoride i8 produced, e.g., the acid attack Or other fluorine-containing ~inerals.
The following paragraphs will describe the pro-cess of the invention as it particularly pertains to the - fluorspar-sulfuric acia reaction, It i8 understood that the process of the invention is not 90 limited and can be used to remove the ~ulfur deposlts created by the cooling of crude hydrogen fluoride gases produced by other reactlons, The hydrogen fluorlde gas rrOm the rluorspar-sulfuric acld reactlon wlll normally loave the reaction 1~4~32S
zone at a temperature between 100 and 250C, usually about 170C Thls gas stream 19 prlmarlly hydrogen fluoride and saturated with H2S04 vapor; however, lt does contain impurities from the reaction zone, These impur-ities will vary with the composition of the raw materials used in the reaction.
In most operatlons the gas stream from the reactlon will contain small particles of fluorspar and/or calcium sulfate; thus initially the stream i9 fed into a gas scrubber wherein it is scrubbed with sulfuric acld to remove the dust The operation of a typical dust scrubber is described in detail in U,S. Patent 3 347 022. The ga~
leaving the dust scrubber will be at about lt~ dew point and at a temperature of 60 to 200C~ normally about 140C.
This gas stream w~ l be labeled the crude hydrogen fluoride gas stream.
This crude hydrogen fluorlde gas ætream still contains impuritie~ which include the sulfur, sulfur dioxide, sulfuric acid, silicon tetrafluorlde, fluosul-fonic acid, hydrogen sulfide, and calcium sulfate Furthermore, it is belleved that some of the components of the crude gas stream react with each other under the condltions of the cooling and other recovery steps to produce sulfur, i.e., the S02 present may react with hydrogen sulfide, It is these sulfur and sulfur-forming impurities present in this gas stream that create the problems in the subsequent recovery steps, particularly in the steps wherein the HF gas is cooled, The term "sulfur" a9 used herelnarter and ln the claims includes these 1~34;~ S
ulfur-forming compound~ as well a8 elemental ~ulfur, The sulfur particle~ ~hich normally form in the gas stream during crude recovery normally have a size of 5 microns and le~s in diameter. These tiny particles of sulfur adhere to any rel~tively cold ~urface in the HF
process egulpment, e.g., the surfaces of heat exchangers which ~re removing heat ~rom this gas ~tream, The ~urface films formed by this sulfur greatly reduce the heat trans-fer effectivene~s of the equipment.
After leaving the dust scrubber, the crude HF gas stream i9 treated in various stages to remove the undesired components and produce refined HF. It is in the cooling equipment of these puriflcation stages that the sulfur deposit~ u~ually form.
The exact nature of the cooling equlpment employed is not critical, i,e,, it can be coolers, ~urface conden-sers, direct contact condensers, wet gnæ scrubbers, refrigeration coils and the like.
Thus, the proceæs of the invention is useful in the purification process described in U.S. Patent 2 047 210. In this process the crude gas from the dust scrubber i8 initially cooled in one or two shell and tube heat exchanges from 140-170C to 5-40C, and the condensed liquid formed is removed, The gas is then contacted wlth cold liquid HF' at -~0 to 25C in contact condensers to produce a product liquid which ls subsequently distllled.
The procesæ i9 alæo useful in the contact cooling equipment described in asæignee' 9 Canadian application Serial No. 182 818 of W.H. Ehlig, filed October 9, 1973 !now Canadian Patent 1 003 620). Although wlth the use o~ the technlques described in this app~lcation, 1~4;~3~S
Jhe buildup of ~ulfur deposlt9 i8 ~lmo8t elimin~ted, The process of the invention can be used to remove sulfur collected by the filters associated with this contact cooling equipment, After the cooling equipment h~s been used for a period of tlme, sulfur deposits will build up on the walls of the heat transfer surface~, The sulfur ln these deposits can be in the ~orm of gr~nules or in an amorphou~
form.
It is these sulfur deposits that are removed by the S03 treatment of the invention.
The reaction of S03 with sulfur, i.e., 2S03 + S ~ 3So2 is known in the ~rt, e,g,, U,~, Patent 2,156,791 or British Patent 6271848, However, no one has applled this reactlon to the ~olution of a long-standing problem in the HF industry, i.e., the removal of sulfur deposits.
from the heat transfer surfaces.
The above reaction probably occurs in two step6 In the first step the S03, e.g,, AS oleum, dissolve~ the sulfur according to the following reaction:
S + S03 > S23 The washing solution then is held, optionally with heating, and the following reaction occurs:
23 + S03 > 3S02 In the process of the invention the S03 can be used in either the liguid or vapor phase, use of the liquid phase belng preferred, In v~por phase operatlons, the S03 would be fed into the equlpment to be cle~ned untll the bulldup of S02

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ln the vapor ceases, At that time the cleaning operation will have been completed.
In the preferred embodiment the S03 i~ added to the process equipment as a solution in a suitable solvent Suitable solvents include sul~uric acid and fluosulfonlc acid Other solvent~ in which the S03 and S203 dissolve can be used.
When used with suitable solvents, the S03 will be present in concentrations of 10 to 45%~ preferably 20 to 35~, A preferred embodiment ia the use of 20 to 35 oleum.
The amount of S03 added will, of course~ vary depending upon the amount and type of sulfur involved, and the temperature, The only requirement is that ~ufficient S03 be added to remove the sulfur present, i.e., greater than 5 molar parts of S03 for every molar part of sulfur.
In preferred embodiments a large excess~ 5 to 20 times stoichiometric, of S03 is used and the equipment is almost completely filled with a 20 to 35~ oleum solution. Under these conditions, during the course of sulfur removal, the total amount of S03 available for reaction will only be reduced slightly. Since this solution will be reu~ed, this procedure does not create waste and insures that almost all of the sulfur will be removed.
Once the equipment has been filled, it can be heated to increase the rate of S02 formation or the S03 solution can be heated prior to being added to the equipment. Thus, the solutlon can be heated to 30 to 100C. The 20 to 35~ ôleum 801ution i8 preferably heated 3 to about 50-60C, However, 1~ a longer residence time ~4~ZS
~oes not create any problem, no he~ting i8 requlred and the reaction will occur at ambient temperatures, Experi-mental tests have determined that the reaction and solution of sulfur in oleum i9 directly proportional to the surface area. Thus, the rate of solutlon of sulfur ln 35,~ oleum is 0,005 g/min/cm2 at 53C and 0,0004 g/min/cm2 at 24C, The hold time of the S03 solution in the equlp-ment will also vary depending on the amount of sulfur and temperature, Generally, the process is conducted until the production of S02 ceases, This can be measured by standard meters or by mea~uring the residual S203 concen-- tration, In typical embodiments the hold time will vary from 1 to 24 hours. A portia~ or all of the hold time need not be conducted in the ~oce~s equipment, Thus, the eguipment can be drained and the solution held for the appropriate hold time ~ a storage tank, or the S03 solution could be recycled continuously betweén the process equipment and a storage tank, After the sulfur has been removed, th e S03 washing solution, now containing S203 and S02, is removed from the heat transfer equlpment and can be recycled to the dust scrubber, In the scrubber it would be added to the scrubbing solution and used in the HF proce~s.
Alternatively, the solution can be drained to a separate storage tank wherein makeup S03 i9 added and the solution stored ~or reuse during the cleaning cycle.
The invention will now be exemplified by the following illustrative embodiment, All pArts and percent-ages are by welght unless otherwlse lndlcated, , _g_ 1~)4Z3ZS
ExamPle A bank of two shell and tube coolers i8 used in the process for making HF. During operation the coolers cool the crude HF gas from 140C to 30C, After being in operation for twenty-four hours, sulfur deposits will build up on the internal surfaces of the coolers in the amount of approximately 45.4 to 182 kilograms.
At this time the sulfur deposits can be removed by filling the coolers with a 35% oleum solution.
Assuming 54.5 kilograms of sulfur was deposited, only about 772 kilograms of 35% oleum would be necessary to provide the 272 kilograms of SO3 needed to react with the sulfur.
Actually filling the coolers requires 7270 kilograms of oleum which provides a large excess of S03. Circulating from a large storage tank would provide an even greater exces~ SO3.
me oleum will be maintained at 30 to 50C for one hour. me oleum solution will be then drained and the sulfur deposits will have been removed from the walls.
m e drained solution can be fed to the dust scrubber, or if desired, fed into a separate ta~k, recon- -`~ stituted with S03, and then reused for sulfur removal at the appropriate time.

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Claims (10)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. In a process for producing HF by reacting a metal fluoride selected from the group consisting of alkali metal fluorides and alkaline earth metal fluorides with a mineral acid, the reaction product thereafter being cooled, where in said process sulfur deposits accumulate in the cooling equipment, the improvement comprising adding to the cooling equip-ment when said equipment is not being used to cool the reaction product, sufficient SO3 to remove the sulfur deposits and there-after emptying the equipment.

2. The process of Claim 1 wherein the metal fluoride is fluorspar and the mineral acid is sulfuric acid.

3. The process of Claim 1 wherein the SO3 is at a temperature of about 20° -100°C. while removing the sulfur

4. The process of Claim 1 wherein the SO3 is in a solvent selected from the group consisting of sulfuric acid, fluosulfonic acid and mixtures thereof at a concentration of about 10-45% by weight based upon the combined weight of SO3 and solvent.

5. The process of Claim 3 wherein the SO3 is in a solvent selected from the group consisting of sulfuric acid, fluosulfonic acid and mixtures thereof at a concentration of about 10-45% by weight based upon the combined weight of SO3 and solvent.

6. The process of Claim 4 wherein the solvent is sulfuric acid.

7. The process of Claim 5 wherein the solvent is sulfuric acid.

8. The process of Claim 4 wherein the solvent is fluosulfonic acid.

9. The process of Claim 5 wherein the solvent is fluosulfonic acid.

10. A process for removing sulfur deposits which accumulate in the cooling equipment in an HF manufacturing process consisting essentially of adding to the cooling equipment, when said equipment is not being used to cool the reaction product, sufficient SO3 to remove the sulfur deposits and thereafter emptying the equipment.