CA1044676A - Process for pulverizing coal to ultrafine size - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
A process for pulverizing coal to ultrafine size of a few microns, comprising conveying the coal through successive pulverizers with an inert gas, separating the ultrafine coal from the inert gas, cooling and recyling the inert gas and producing make-up inert gas by water scrubbing and alkaline or monoethanolamine scrubbing boiler flue gases to remove particulate, carbon dioxide, thereby leaving essentially nitrogen inert gas. Control of the final ultrafine particle size is achieved by regulation of the flow of conveying gas through the final pulverizer.
Reject material streams are taken from the pulverizers to improve the quality of the product pulverized coal, which subject streams are burned in a furnace so as to recover their heat value.

Description

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~0~676 This invention relates to a process for pulverizing coal to ultrafine size.
While attempts have been made in the past to pulverize coal to such size, these have not been successful, either because of the very high consumption of power for the process, the impurities of the resultant product, or the insufficiency of the comminution for effective use.
The largest single use of coal is as a fuel in utility and industrial boilers. In both instances, the larger steam generation units burn the coal in the pulverized form. Nominal partlcle sizes of coal as fired are 74 microns for bituminous coal ~`-and 44 microns for anthracite. These finenesses achieve efficient -combustion characteristics while still permitting collection of the ash particles before emission of the flue gas to the atmosphere.
Various types of pulverizers are used to achieve the finenesses desired and each type has been empirically developed to a high degree of efficiency for the purpose intended. Pul-verization to particle sizes finer than stated might improve com-bustion efficiency slightly but at the expense of pulverizer 2a pawer requirements and difficulty of ash separation. Accordingly there has been no development ~ ' :~ .

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l( l iO~i'76 of pulverizing equipment to produce finer particle 8ize8 in l large amounts. Some equipment has been developed to produce ; much finer particle sizes, but due to high power con~umption, ~ low output, and high cost, are economical in processing ~ only relatively higher value products, such a~ co~metics, pharmaceuticals, and foodstuffs. However, a number of proces~es and use~ are in development which would benefit ~ignificantly through the use of large quantities of coal of much finer particle size~ than used heretoforeO Pulverizing coal to a particle size of less than 10 microns would have, sm~ng many, the following immdiate uses: 1. water and liquid waste purification; 2. direct burning in combustion turbines; 30 suspension in liquids to produce colloidal fuels; 40 as a raw material for the production, by further processing, of sub-micron particles; and 5. as a feed stock for gasification processeæ facilitating direct methanation of carbon.
Besides the direct uses of ultrafine coal, pul~ierizing to smaller than 10 microns particle size will permit, in the p~ocess, re val of substantial portions of inorganic ash an~ pyritic ~ulfur. In combustion processes using the 1 to 10 micron sizes or substantially smaller sub-micron sizes, the environmental effects of the products of combustlon would be considerably reduced. ~ith such "cleaner" fuel in fine particle size~ the way i8 opened for employing combustion techniques which would minimize formation of N0x products in the combustion gases emitted into the atmosphereO Use of these techniques is not now feasible in pulverized fuel combustionO 3 .
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4tj76 According to one aspect of the invention there is provided a pro-cess for pulverizing coal to ultrafine size of less than 10 microns which comprises drying the coal, subjecting the dried coal to a coarse pulveriza-tion stage in an atmosphere of inert gas, sub~ecting the coarsely pulverized coal to a further pulverization stage in the presence of inert gas to obtain ultrafine coal, separating the ultrafine coal from the inert gas and re-cycling the separated inert gas to the pulverization stages wherein make up ~ ~
insert gas for supply to the pulverization stages is obtained from flue gas :
from a boiler by cooling the flue gas and scrubbing it to remove particulate --matter, sulfur dioxide and carbon dioxide. i~
According to a further aspect of the invention there is provided apparatus far pulverizing coal to ultrafine size, comprising means for drying coal, a coarse pulverizer for grinding the coal particles to about 74 micron size, means dissimilar to said coarse pulverizer for reducing said coal particles to less than 10 micron size, means for introducing an inert gas into said coarse pulverizer to convey the coal in continuous suspended flow through said reducing means, means for separating the resultant ultrafine coal particles from said inert gas, means for cooling and removing condensate from the separated inert gas and recycling it through said coarse pulverizer, -;
2Q a steam generator, flue gases from which are passed through the said means for drying coal to dry it before pulverization, a water scrubber for removing heat and particulate matter from said flue gases, an alkaline scrubber Por removing sulfur from said flue gases, means for removing the carbon dioxide from the resultant gases so as to leave essentially only nitrogen, and means for introducing said nitrogen into the stream of said inert gas stream to replenish losses thereof.
The invention will be further illustrated by reference to the accompanying drawings showing, by way of example, an embodiment of the invention, in which:
3Q Figures lA and lB, taken together show, schematically~ a process ~ '.
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Referring to Figures lA and lB, lump or run-of-the-mine coal as received in dumper 1 is crushed in crusher 2 to a nominal maximum size of ;~
approximately 1 inch, then passed through magnetic separator 6 and conveyor 4 and dried with inert boiler flue gases in flash coal driers 5. The dried coal is stored in coal bunkers 7 from which it is fed, at a controlled rate, through gas lock feeders 8, to the coarse (74 microns) pulverizer 9. Con-ditioned inert gas is introduced through pipe 24 into the control feeder, pressurizing the pulverizer circuits and spaces and causing a flow of inert gas through the coal from and in the bunker 7, thus purging the spaces be-tween coal lumps of air. i-The coal is conveyed from the coarse pulverizer 9, pipe 15, ;
through cyclone separator 11 and through the fine pulverizer and reductor -mlll 12 by conditioned inert gas 16a to pipe 16 and the ultrafine product filters 17 where the ultrafine material is placed in storage bunkers for end use. --: ~ , , . ;. -:
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lO~q~i76 The inert gas separated from the product i~ passed through pipe 18 and cooled by gas cooler 19, separator 20, compres~et by compressor 21, and returned through separator 22 and pipe 24 to the pulverizer inlet. A portion of this inert gas separated from the product is passed through pipe 52, compressed to a higher pressure by compressor 53 for operation of the filters 17 and returned to the main inert gas stream in the filters.
Losses of the inert gas will occur through leakage, backflow into the bunkers 7 and entrainment in the product.
To replace these losses, inert gas i8 made from the boiler flue gases used to dry the raw coal. Steps in the preparation of this iert gas consi~t of water scrubbing in water scrubber 42 to cool the wet flue gas flowing through pipe 26 and remove particulates, aLkaline scrubbing by scrubber 47 to remove gaseous sulfur compounds and scrubbing with noethanolami*e in M.E.A. scrubber 32 to remove carbon dioxide. The remaining gas is essentially pure nitrogen.
Mbre detailed descriptions of the several subsy8tems ~; 20 follv~

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~, lU ~1676 STORAGE, P Æ ~ARATION AND HANDLING SYSTEM

Coal is recelvèd either directly fro~ a mIne, by - me~ns of belt ¢onv-yor, or in rail cars if the plant i~
loc-ted r-mote from the mine Even lf the plant iB located ~d~oa~e ~to a mine nd the rotary car dumper l, yard locomotive and a portion of the car dumper hou~e could be ~ ~ elimln-t d,~ it~m~y be economic-Ily prud-nt to insta1l T~ ~ompl te rail car facilitie8 Thi8 would improve reliability ~ ..
d th`e~raw~m terial suppIy ~nd also f-cllit~te the opportùoity;for taklng;;adv-otag- of favorable purchasing 10~ ~ oiportunities Upon rec-ipt, the co-l is t~ken to the~crush r hou8e 2 where it is crushed to nominal I inch size and coov yed to~ un ion~poi-t~oere~toe coal c-o be div-rt-d ~-into~one~of~two~stre _ . ~The first ~3) of the two streams 15~ ~ permit8~tbe~co-1 to~be movet~to~a ground leve1 8to~ge pile from whicb the~co~ is ~ran-ferr-d by~bulldoz-r8 to~- main torage~pll-`.~ The~main~-torag-~pile will create p-rmanent ; ~ re-erY-;~cap city vithln~the plant rea~to be u-ed in the ven~t tbe~r~w~m~téri l ~upply i~ int-rrupt d for any reason.
20- ~ ~ The -econd~;8tre _ 4 c~rr~ies the coal~through fla8h dry-rs~5 wher-~ext-rn-1 moi-ture i8 re~oved and tbrough ` ~ m-gn tic~-p r-tnr- 6 wh re -t-el part-, ~uch a8 nuts, bolt8, etc~ ;are re~ov d and fina~lly disc~h~rges it into elevated r~ 7.

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,` CO:IMIIIUTIO SYSTEM 10~4~i 76 ; A controlled amount of the dxy coal is fed by gravity from the storage bunkers through ~a~ lock feeders 8 to conventional bowl roller or ball pulverizing mills 9 where it is reduced to a 74 micron size. In the pulverizing process, ~me of the pyrites are removed through pipe 10. Upon being dlscharged from the pulverizers, the coal is fed through pipe lS and cyclone ~eparators 11 into high speed impact type reductor mills 12 where it is comminuted to some predetermined size between 1 and 10 microns. Within the reductor mill, the ,~ 10 coal is sub~ected to centrifugal forces which permit removal ~ of additional inorganic ash and pyrites tbrough pipe 13.

3:~ As coal is reduced from a nominal size of 1 inch to !~ a product of 100% less than some size in the range of 1 to 10 microns, there is a tremendous increase in the exposed surface area of the co,m~lnuted material. ~long with the increase in surface area there is a very marked increase i- in the activlty of the coal and if size reduction takes place in an air atmosphere, the explosive limits with respect to temperature and/or park energy may be realized inside the mechanical grinder. Therefore, to provide for safety of operations, the coal will be transported under positive pres~ure in the comminution system by inert gas-~aoh as nitrogen. The ga~ will be introduced into the ~ystem ,~ ~ at the ga- ~ck feeders 8.
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The micron sized perticles are then pneumatically carried by means of the inert gas 16a through pipe 16 to fabric bag-type dust collectors or filter~ 17. Here the particles continuously separated from the carrying lnert gas are discharged into product storage hoppers or conveyed to process equipment depending upon the application.
INERT GAS SYSTEM

To optimize the balancing between investment and operating costs, the inert gas will be recycled with make-up added to the system to compensate for losses. The inert gas -~ which is approximately 99.5Z nitrogen and 0 5X carbon dioxide ~molar composition on a dry basLs) enters the reductor mill 12 from pulverizer 9 at approxi ately 38-C. The entering gas 15 ~ will be water saturated. A relstively small portion of the gas flowlng to the inlet side of the mill backflows ~rougb the coal feeder line from the coal bunkers and is vented to the atmo9pbere. 1~13r using inert gas for m~intainiag a~sligbt positive pressure in the coal bunkers, the quantity 20~ ~ of~aLr entering the system along with the coal feed is minLmized. Tbe quantity of gas flowing througb the reductor !,ji" ~ mLlls 12 Ls establLsbed based on the speed and physical ~dimensLons of the mill components, the characteristics of tbe cla~sifying gas with respect to the den6ity and - ~ . .
vLscosity, and the characteristics of the comminuted product with respect to particle size and specific gravity. The flow rate of gas to each of the mills will be measured and ~.~ ,.

` 10 4~7 6 controlled to insure proper flow distribution. AB the inert gas and coaI pa~ses through the mill 12, a considerable percentage of the horsepower required to drive the mill i8 converted to heat with a resultant rise in temperature of the proce~sed material. A portion of the heat is expended in vaporizing inherent moisture from the coal material.
The system i8 designed 80 that the inert gas and comminuted ~; co-l leave the mill through pipe 16 at approximately 130-C.
and, ba~ed on complete drying of the coal which enters the mill as an assumed inherent water content of ix weight percent, the dew point o the existing Ka~ is approximately 80-C.
The gas leaving the bag filters 17 i8 pas~ed, by pipe 18, through a heat exchanger or gas cooler 19 and is cooled from approximately 130-C. by heat exchange with cooling water. As the cooling of the gas will result in partial condensation of water vapor, a phase separator 20 will be provided downstream of the cooler to remove condensate from the qstem. Subsequently, the effluent gas from the separ-tor will be boosted in pressure by an electric motor ~driven centrifugal compressor 21 from a suction pressure ; of a few inches water column to a discharge pressure, downstream of the compressor aftercooler, of approximately ~ 6 psig. Since additional condensstion of water vapor occurs ,~
in the compressor aftercooler, a phase separator 22 will be provided for removal of this condensate prior to recycling the gas to the lnlets of the mills. Supplement~l drying -~.~ _9_ ~ . :' ~4 ~ 76 of the compre~sed gas will not be required since the quantity of water reved from the stream by cooling and partial condensation will be ~ufficient to reject the entire quantity of water picked-up by the gas from the comminuted coal in the reductor mills 12. A portion of the gas stream which was used for removal of surface moisture in the flash coal dryers 5 will be taken from the flue gas circuit downstream of the particulate (42) and sulfur dioxide removal equipment (47) and will be used to provide the make-up to the inert gas recycle systemO
Flue gas containing approximately 21 percent carbon dioxide and 79 percent nitrogen (molar composition on dry basis) and water saturated will be compressed by compressor 46 from a few CM. water column to a discharge pressure, downstream of the compressor aftercooler, of approxim~tely 8 psig.
~ondensate will be removed from the stream, by separator 30, after leaving the compressor aftercooler 28 and the gas, free of entrained water, will pass through pipe 31 upwardly through a column where it will be scrubbed by MEA scrubber 32 by a 15L solution o monoethanolamine (MEA) flQwing downward in the column. Within the scrubber column, the MEA reacts with the carbon dioxide at approximately 38-C. to form a water soluble salt. The gas leaving the MEA scrubber 32 through pipe 33, which will be approximately 99.5 percent nitrogen and 0.5 percent carbon dio~ide (dry basis), will enter pipe 24 and the inert gas recycle circuit upstream c the reductor mills.

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1(~4'1676 , The carbon dioxide ri~h M~A solution leaving the bottom of the MEA tower through pipe 35 is pumped through a heat exchanger 36 and the eemperature i8 raiged to approximately lOO-C.. The heated solution i8 sent to the upper section of a stripping tower 37 and flows downwardly contacting hot vapors from the reboiler of tower 37, indicated in dash outline.
- Steam in pipe 50 will be used to supply the heat of vaporization required to generate sufficient boil-up vaporæ to strip the carbon dioxide from the rich MEA. Condensate passes through pipe Sl. The carbon dioxide vapor leaving the top of the tower 37 contains a portion of the vaporzied MEA solution.
This MEA i8 recovered by passing the hot vapor mixture through a condenser and returning the condensate to the stripping tower. The carbon dioxide is re~ected to the atmosphere at 38. The hot carbon dioxide free, lean MEA, f~o~s from the reboiler section of the stripping column and is cad}t~ by heat exchange with the rich MEA. The cool lean MEA
-~ solution i8 then pumped back by pump 39 to the absorber tower, ~hus complet$og the cycle.
~ The use of nitrogen produced from this flue gas a~ a cl-seifying gas in the mill is desirable because it is eesentially free of oxygen and it provides for safety of operation. The flow rate of gas required for this purpose i8 approximately twice that which is available from the flash dryer system. Therefore, tbe gas flowing througb the mills will be recycled and only the quantity of gas required for make-up (5 percent of the recycle stream) will be processed for carbon dioxide removal for addition to the recycle circuit.
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BO~LER PLANT AND AUXILIARY SYSTEMS

In order to ~upply heat for moisture reval, inert gas to make up system losses, regenerate MEA solution, and to heat the building, a pulverized coal-fired steam boiler 41 i8 installed~ Slightly le~s thsn theoretical air will be u~ed in the combustion so a~ to exclude oxygen in the flue gases. The boiler will be equipped to bypass, variably~ ~ome of its steam generating surface 80 as to vary the temperature af the flue gas flowing in pipe 62 to the flash coal dryer 5 to accom~date variations in the moisture in the incoming raw coal. Flue gas leaving the flash dryers through pipe 26 will be scrubbed by scrubber~ 42 with water 43 to reve heat and particulate matter, ehen with a solution of sodium hydroxide in scrubber 47 to reve ~ulfur gasee. A portion of the flue gases wlll then e~ter the MEA inert gas system through pipe 31. The carbon dioxide i8 removed from the flue gas because i~s greater density would increase windage power in the mills. The remalning larger portion of the flue gas not needed for the inert gas sy~tem will be exhausted to the atmosphere. As previously noted, it has been cleaned of particulate and ~lfur ga~e~ and contains only carbon dioxide and nitrogenO
The flue gases leaving the flash dryer will be saturated with isture at a temperature approaching 90-C., snd it is desirable to reduce thi~ moi~ture before entering the MEA scrubber 320 Accord$ngly, the liquid from the water scrubber 42 whlch follows the flash dryer will be cooled in a mechanical draft cooling tower 45.
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4 ~7 6 The boiler 41 wLll generate steam for regenerating MEA ~olution and for space heating and general plant use.
By ad~ustments of the portion o boiler gases bypassing steam generating surface and the firing rate of the boiler, S variations o~ steam demand and moisture content of the raw ~ coal can be accomodated.
;~ In the water scrubber 42, the particulate mstter from the flue gas will be removed. The liquid will pas~
through a settling tank before being pumpted through the cooling tower 45. The settling sludge consisting of flyash and fine coal particles will be air-dried and recycled into the coal entering the boiler's pulverizers. Leaving the water crubber 42, the flue gas pas-es through the ; sulfur-dioxite removal scrubber 47. In tbis scrubber, lS the su1ur dioxide is absorbed into a~dilute ~odium hydroxide solution, converting the ~olution to sodium sulfite, and -odium bi~ulfite.
The ~ulfite and bisufite are converted back to odium~hydroxide in a~coagulator reactor~by the additlon ~ calcium hydroxide. The precipitate from the resctor i8 cs1cium sulfite which is ~u~e~ a8 a sludge to settling beds or dewatered in mechanical equipment for damp hauling .
to disposal are~s.
In an effort to effect additional efficiencies, the beaviest p-rticles and pyrites from the 74 micron coal in the product pulverizers re conducted through pipes 10 and 13 and burned in the boil-r 41.

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1044~j76 ~ RECIRCULATED COOLING T Æ RS
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In the reductor plant are several souraes of heat which must be continuously removed. The main source are water j~ackets of the mills, circulating gas coolers to remove moisture emanating as inherent moi~ture of the coal,

5 ~ MEA coolers, and bearing oil cooler~ of the mills Therefore, the~plant includes a cooling water supply 56 and return ;~ -y~tom 60 circulating~through~ hu.r: exchanger 19. Only clean water clrculates through'this sy8tem. ~
Re3ection of the heat to the stmo~phere is ~ ~ accomplished~by a mechanical draft cooling tower 57.
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The~co~le~d liquid from the cooling tow-r i8 pop-d through the he-t e-chsnger 19,~_ ntioned abov-, and the heat-d ~cooling~tower water~ from thi- exch-nger i~ retùrned to the~;cooling~tower~
15~ ~ Thu~ it~will;be ~-en~that I have provided a highly ~eff~cieot~pr~oce~ nd -pp-r-tus for pulv-~rizing coal to ltr-fine;p-rticlé~ ol 1--- th-n IO microns 80 a~ to ~' ~è~nablé~8ub8equent u8e of the pulverized coal in various prooe-ses,~;such~ -6 that~involving direct methanation of 20~ ~ ~ coal~when~in powdered fonm, and other application8 : ~ n ~ r-ted~abova.

` ~ ~ Whi}e~ hsve illustrated and described several ~embodiment- of my inv-ntion, it will be under-tood that thes- are by way of illu8tr-tion only and that various ~ ch~ng~es snd modifictions may be contemplated in my invention and in the scope of the following claim8.

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Claims (14)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A process for pulverizing coal to ultrafine size of less than 10 microns which comprises drying the coal, subjecting the dried coal to a coarse pulverization stage in an atmosphere of inert gas, subjecting the coarsely pulverized coal to a further pulverization stage in the presence of inert gas to obtain ultrafine coal, separating the ultrafine coal from the inert gas and recycling the separated inert gas to the pulverization stages wherein make up inert gas for supply to the pulverization stages is obtained from flue gas from a boiler by cooling the flue gas and scrubbing it to re-move particulate matter, sulfur dioxide and carbon dioxide.

2. The process recited in claim 1 wherein said scrubbing includes water scrubbing to remove heat and particulate matter and alkaline scrubbing to remove sulfur gases.

3. The process recited in claim 1 wherein said scrubbing includes water scrubbing to remove heat and particulate matter and monoethanolamine scrubbing to react with carbon dioxide to form a water soluble salt.

4. The process recited in claim 1 wherein said coal is pulverized in a plurality of successive steps and the quantity of inert gas is selective-ly adjusted between any two steps.

5. The process recited in claim 4 wherein control of the final ultrafine particle size is achieved by regulation of the flow of conveying inert gas through the final pulverizer.

6. The process recited in claim 1 wherein reject material streams are taken from said pulverizing means to improve the quality of the product pulverized coal and wherein said reject streams are burned in a furnace re-covering the heat value of the reject material to a useful purpose.

7. The process recited in claim 1 wherein reject material streams taken from said pulverizer means to improve the quality of the product pul-verized coal and the reject material is processed by oxygen deficient combus-tion and said scrubbing to remove or recover sulfur, sulfur oxide gases, or sulfur salt by-products.

8. The process recited in claim 1 wherein said coal is first crushed to about 1 inch size, then coarse pulverized to about 74 micron size, and finally pulverized to a particle size of between about 1 and 10 microns.

9. The process recited in claim 8 wherein said carbon dioxide is re-moved by scrubbing said gas, after removal of sulfur, with monoethanolamine.

10. The process recited in claim 1 wherein said flue gas from the boiler is passed through the coal to dry the coal before the coal is sub-jected to pulverization.

11. Apparatus for pulverizing coal to ultrafine size, comprising means for drying coal, a coarse pulverizer for grinding the coal particles to about 74 micron size, means dissimilar to said coarse pulverizer for reducing said coal particles to less than 10 micron size, means for introducing an inert gas into said coarse pulverizer to convey the coal in continuous suspended flow through said reducing means, means for separating the resultant ultra-fine coal particles from said inert gas, means for cooling and removing condensate from the separated inert gas and recycling it through said coarse pulverizer, a steam generator, flue gases from which are passed through the said means for drying coal to dry it before pulverization, a water scrubber for removing heat and particulate matter from said flue gases, an alkaline scrubber for removing sulfur from said flue gases, means for removing the carbon dioxide from the resultant gases so as to leave essentially only nitro-gen, and means for introducing said nitrogen into the stream of said inert gas stream to replenish losses thereof.

12. Apparatus as recited in claim 11 wherein the means for removing carbon dioxide is a scrubber in which carbon dioxide is removed by scrubbing with a monoethanolamine solution, and wherein there is also provided a strip-ping tower fed by steam from said steam generator, and a heat exchanger and pump for heating and circulating the carbon dioxide and monoethanolamine solution, to the stripping tower whereby the hot vapors in said stripping tower strip the carbon dioxide therefrom and permit it to escape into the atmosphere.

13. Apparatus as recited in claim 11 wherein said separating means is a filter and wherein a gas cooler, separator and compressor are in series relationship with said filter to return inert gas therein at higher pressure and lower temperature and remove condensate.

14. Apparatus as recited in claim 13 together with a by-pass line for drawing some of the effluent inert gas from the series circuit and recycling it into said reducing means.