US3176047A

US3176047A - Pyrolysis of hydrocarbons

Info

Publication number: US3176047A
Application number: US23116A
Authority: US
Inventors: Frederic F A Braconier; Jean J L E Riga
Original assignee: Societe Belge de lAzote et des Produits Chimiques du Marly SA
Current assignee: Societe Belge de lAzote et des Produits Chimiques du Marly SA
Priority date: 1959-03-04
Filing date: 1960-03-04
Publication date: 1965-03-30
Anticipated expiration: 1982-03-30
Also published as: CH391690A; ES256233A1; DK95473C; GB876263A; BE587790A; DE1148229B

Description

March 1965 F. F. A. BRACONIER ETAL 3,176,047

PYROLYSIS OF HYDROCARBONS Filed March 4, 1960 I United States Patent Ofiice 3,l76,ul7 Fatented Mar. 36, 1965 3,176,047 PYRULYSES 6F HYDRQCARBONS Frederic F. A. Braconier, Plainevaux, and Clean I. L. E. Riga, Liege, Belgium, assignors to Societe Beige dc lAzote et des Produits Chimiques du ll larly, Liege, Belgium Filed Mar. 4, 1960, Ser. No. 23,116 Claims priority, application Great Britain, Mar. 4, 1959, 7,536/59 2 Claims. (Cl, 266-683) This invention relates to the pyrolysis of saturated hydrocarbons to produce therefrom unsaturated hydrocarbons in a pyrolysis chamber or furnace and, more particularly, to protecting the Walls of such a pyrolysis chamber continuously from undesired accumulations thereon of carbonaceous or tarry by-products of the pyrolysis reaction.

Chemical reactions generally of the character to which this invention relates may be illustrated by the pyrolysis production of unsaturated hydrocarbonse.g., particular- 1y acetylene, ethylene, and other olefines-from the pyrolysis reaction of saturated hydrocarbons by heating the latter in a gaseous or finely divided liquid state to a high temperature for a very short time in the presence of a heating medium such as, for example, a combustion mix ture of an oxygen containing gas, such as air, and a fuel or comburent gas such as hydrogen or a hydrocarbon, under conditions effecting pyrolysis by the partial combustion of the saturated hydrocarbon starting material and/or by the subjecting thereof to the heating or thermal effect of hot combustion gases.

In such pyrolysis production of unsaturated hydrocarbons particularly on a large commercial scale, as is now Well understood, optimum or maximum efficiency of the pyrolysis reaction with optimum or maximum utilization of the saturated hydrocarbon for production therefrom of the desired unsaturated hydrocarbons demands that the heating or pyrolysis or partial combustion reaction be achieved in a pyrolysis zone very quickly and with a high degree of control of the temperature therein and with rapid quenching of the products of the pyrolysis reaction in order to stop the pyrolysis at the desired point and before there is an undesired or inefficient formation tarry or polymerized and otherwise useless by-products of the reaction.

Similarly, if such pyrolysis reaction is conducted within an enclosed combustion and/or pyrolysis chamber particularly when it is desired to conduct the reaction in a continuous commercial scale manner and with high preheating of the gaseous or vapor ingredients prior to injection thereof into the combustion or pyrolysis chamber for reaction therein-considerable, and even crucial, difficulties may be experienced within the combustion or pyrolysis chamber by the undesired deposition and accumulation on the walls thereof or". carbon or carbonaceous deposits or tarry by-products of the pyrolysis process to an extent which, indeed, may interfere with the efficiency of continuous operation of the pyrolysis reactor. Such occurrences may require periodic shutdown of the apparatus for the manual cleaning of the inside of the pyrolysis or combustion chamber to permit optimum operation thereof. Furthermore, if the walls of the pyrolysis or combustion chamber are lined with refractory material to withstand the Very high temperatures of the pyrolysis reaction, particularly under continuous operating conditions, a more or less progressive deterioration of the refractory lining may be noted, resulting in a deterioration of the desired degree of impenetrability of the lining to the gaseous materials therein, and, similarly, it metal materials are used for the inner walls of the pyrolysis or combustion chamber, some disadvantageous experience may be produced because of the inherent thermal expansion characteristics of various metals at the normal operating temperatures of such a pyrolysis reaction, as well as by, perhaps, undesirable catalytic eifects of such metal walls on the reacting materials or constituents within the combustion or pyrolysis chamber.

If it is attempted to alleviate the foregoing problems by providing a liquid screen of water or other material continuously flowing down and along the inner walls of the pyrolysis or combustion chamber, some advantageous or enhanced effects are obtained, but, even with such a technique, difiiculties may arise with regard to partial or momentary interruptions of the Water screen by carbonaceous deposits, and such difiiculties may be enhanced by the fact that the tendency to deposit or the quantity of carbonaceous deposits may increase along the pyrolysis chamber as the reactions therein progress, so that, even with the provision of the water screen surrounding the combustion or pyrolysis zone, the maximum tendency for carbonaceous deposits may occur at a point along the pyrolysis or reaction chamber at which the water screen is least stable or continuous or dependable.

Similarly, if the foregoing difiiculties are attempted to be alleviated by introducing a curtain of steam, rather than water, along or around the inner wall of the pyrolysis or combustion chamber, some difilculty may be experienced in that the desired continuity of the protection screen or curtain of steam requires the injection of a substantial proportion of steam into the pyrolysis or combustion zones to an extent which may disadvantageously dilute or interrupt or otherwise afiect the reactions therein, as well as, perhaps, disrupting the high degree of control or regulation of the thermal conditions within the pyrolysis or combustion zone desired for optimum or maximum eliiciency of the pyrolysis reactor, while the attempted injection of a quantity of steam less than will disadvantageously interrupt the reactions may produce, particularly in large commercial scale reactors, a discontinuous or otherwise ineffective curtain of steam for the protection of the heated inner walls of the pyrolysis or chamber from undesired accumulations of carbon or tarry or carbonaceous by-products thereon and/or the formation of such byproducts or accumulations in the first place.

According to this invention, however, there is provided for such a pyrolysis reaction or production of unsaturated hydrocarbons a system for preventing undesired accumulations of carbonaceous deposits on the walls of a pyrolysis chamber and avoiding other disadvantageous interruptions in the eificiency of the pyrolysis reaction by externally cooling the walls of the pyrolysis chamber while continuously injecting steam along the internal surface thereof for condensation thereon to form a substantially continuous and homogeneous screen or curtain of water thereover and around the pyrolysis zone as a result of the external cooling of the Walls; and, as a further feature of this invention.

With the foregoing objects and advantages in view, this invention may now be more particularly described, and other objects and advantageous showings thereof will be apparent from the following description, the accompanying drawing, and the appended claims.

In the drawing there is illustrated, somewhat diagrammatically in axial section, combustion chamber and pyrolysis apparatus embodying and for practicing this invention.

Referring to the drawings, a pyrolysis furnace or reactor apparatus generally of the character to which this invention relates is illustrated in axial vertical section, it being understood that the apparatus is of generally cylindrical form and/or circular cross section. Such apparatus is illustrated as comprising a burner or distributor member 1 above a substantially cylindrical combastion chamber indicated at 2, which, in turn, leads into a substantially cylindrical pyrolysis chamber indicated at 3. In the illustrated construction, the axis of the coaxial combustion and pyrolysis chambers Z and 3 is, preferably, vertical with the burner or distributor member 1 at the top thereof. Proposed in the line of gas flow to burner 1 and chambers 2. and 3, there are provided inlet ducts 4 and 5' for the introduction into combustion chamber 2 of, respectively, a comburent fuel gas and oxygen or oxygencontaining gas. The hydrocarbon to be pyrolyzed is injected through an inlet duct 6 (or a plurality thereof) through a plurality of injector nozzles 16 spaced around the circumference of combustion chamber 2 and axially spaced below the burner member 1 adjacent the junction of chamber 2 with chamber 3.

Burner or distributor l is, preferably, made of steel, and is provided with an annularly relieved groove or notch area indicated at S. Annular ducts 9 and 10, respectively, place inlet ducts 4- and 5 in direct flow communication with combustion chamber 2 in the area of the relieved annulus 8 for the flow of fuel gas and oxygen, respectively, through inlet ducts i and 5 and annular ducts 9 and 19 for admix are into a combustible and heat- :oducing product in combustion chamber 2. in the of burner 1, all as indicated by the flow arrows in the drawing.

Upon admixture and combustion of the oxygen from inlet 5 and annular duct Sillwith the fuel gas from inlet 4 and annular duct 3, a heat-producing flame is established generally in the annular area 3 of burner l, which flame is operative for the pyrolysis reaction of the saturated hydrocarbon injected from inlet ducts through injection nozzles in to produce the desired pyrolysis reaction in the pyrolysis chamber 3, which reaction is quenched at the desired point or" completion thereof and in known manner as by horizontally directed water sprays '7.

Preterably, the axes of too outlet portions of annular ducts 9 and i meet at substantially a right angle, and the annular walls of relieved notch or annulus 8 in burner 1 are, preferably divergent as indicated in the drawing and symmetrically inclined with respect to the vertical axis of combustion chamber 2 at an angle of about 45. annular ducts 9 and are provided with a plurality of outlets (instead of a continuous annular outlet) into combustion chamber 2, preferably the number of outlets communicating between each annular duct 9 and 1G and combustion chamber 2 is equal.

As indicated in the drawing, there is also provided a steam or vapor feed line 12 more or less centrally of bur er 1' and leading to a distributing space 11 from which an annular slot or passage 33 communicates directly into combustion chamber 2 but centrally of and at a lower point than relieved annular notch or space 3, and, preferably, annular slot 13 is inclined toward the wall of the combustion chamber 2 at an angle of from about 35 to St) with the longitudinal vertical axis of the cylindrical pyrolysis apparatus. An additional steam or vapor injection or inlet duct is provided leading to an annular inlet notch 14, also in the steel plate of burner 1 but around the periphery thereof, and the inlet angle of art-- nular slot M is inclined toward the axis of the pyrolysis apparatus at an angle of about to During operation, then, steam is simultaneously injected into the central portion of combustion chamber 2 (through inlet 12 and inclined annular passages 13) and peripherally of combustion chamber 2 through inlet 15 and annular passages 14.

The injection of the hydrocarbon to be pyrolyzed, as noted, is provided for by inlet ducts 6 through injector nozzles 16, a plurality of which are symmetrically distributed around combustion chamber 2 substantially in a single circular horizontal plane therethrough, as indicated, and each of the injector nozzles 1'6 has an outlet of small diameter, with respect to ducts s, and is inclined,

l preferably, to inject the hydrocarbon to be pyrolyzed at angle of about 30 with respect to a radius of combustion chamber 2 for producing a swirling eilect upon injection of the iydrocarbon to be pyrolyzed into the flame or combustion gases from the annular flame at burner l in combustion chamber 2.

Below combustion chamber 2 is provided a cylindrical pyrolysis chamber 3 of substantially the same inside diameter as chamber 2 and defined by cylindrical metal walls 21, which are preferably provided with surrounding coolant jacket 22 for the circulation of coolant therein, as through inlet 23 and outlet 24, for cooling the inner surface of the walls 21. Around the upper portion of the chamber 3 substantially adjacent the juncture of chamber 2 with chamber 3 and below the hydrocarbon iii-Let nozzle 16 is provided an annular passage 18 into which steam is introduced through inlet l7. An annular slot 25 com mnicates betwec passage 13 and combustion cha nber 3 for the assage of steam into chamber 3, and

such outlet or slot 25 is positioned, as by bat'tles or guides and 2G, to direct steam passing therethrough downely adjacent the inner surface of walls 21. l the operation of a pyrolysis furnace or reactor such as the foregoing, a fuel gas, such as hydrogen or a hydro- :n rich gas, which may be pro-heated, is fed into the ombustion chamber 2 through conduit 4 and annular duct 9. Oxygen, which also may be pie-heated, is fed into combustion chamber 2 through conduit 5 and annular duct ill. These gaseous reactants are preferably provided with substantially equal flow rates, and are directed into combustion chamber 2 in such manner that the individual streams thereof meet at an angle of approximately resulting in local and ciiicient and rapid mixing of the gases and the formation of a ring of short ilarnes in a direction generally parallel to the axis of combustion chamber 2 and generally in the area of relieved groove 8 in burner 11.

Steam is introduced through conduit 12 and is distributed in space 11 for cooling or protecting at least from the intense heating of the central area of burner l the oxygen-hydrogen flame. The steam is then introduced into combustion chmnber 2 through annular slot 13. Steam is also introduced through conduit 15 and injected into combustion chamber 2 through annular slot 14 cornrnunicating therewith. Such streams of steam injected into combustion chamber 2 through annular slots 13 wd 14 meet at an angle of from about 70 to about and substantially completely surround the flame ring formed in the area 8 by combustion of gaseous reactants from ducts 9 and ill, and form a curtain or screen pro tecting the walls of combustion chamber '2 against the direct heating elleot of the high temperature heat radiated by the flames at burner l.

The combustion gases produced by the flames meet the screen or curtain or steam rapidly and mix homogeneously therewith. This resulting mixture of hot combustion gases and water vapor then passes downwardly from cornbustion chamber 2 into pyrolysis chamber 3, at the top of which the hydrocarbon to be pyrolyzed, preferably after pro-heating, is injected through nozzles 16. in this manner the hot gases and the hydrocarbon reactant are intimately and rapidly mixed, with the hydrocarbon being subject-ed to intense thermal shock and rapid temperature rise immediately upon being introduced through nozzles 16 into the hot combustion gases and steam from combustion chamber 2. The desired pyrolysis react-ion, then, continues as the mixture of the gases passes through pyrolysis chamber 3 until the reaction is arrested by quenching the reacting gases with horizontally directed sprays of water from spray mechanism '7, in known manner.

Additional steam is introduced into the pyrolysis chamher 3 from inlet 17 and annular passage 18' and inlet slot 2'5 between baflle l9 and it so that it is directed along the internal surface to wall 21 of chamber 3. Also,

a cooling liquid such as water is circulated in jacket 22 to cool wall 21. As a result of this cooling of wall 21, steam around the upper portion of wall 21 condenses thereon and forms around the inside of the pyrolysis chamber 3 a water screen or curtain separating the hot reacting gases in chamber 3 from direct contact with the surface of wall 21 thereof. Thus, the deposition and accumulation of carbon or other combustion products on the inner surface of wall 21 is substantially avoided, and the wall-s are maintained clean and free of undesired deposits even after long periods of continuous operation.

As will be noted from the foregoing, this invention provides for spontaneously forming a water curtain or screen along the internal surface of wall 21 of pyrolysis chamber 3 by the condensation of steam on the walls as cooled by coolant circulating through water jacket 22. The resulting water screen or curtain is in the form of a substantially stable and homogeneous and continuous film or layer of substantially uniform distribution circumferentially around chamber 3, and so the necessity for utilizing special mechanical devices or liquid pressure means for maintaining a continuous and uniform distribution of water over the surface of Wall 21 is avoided, while forming the screen by condensation of steam aids in obtaining a continuous water film originating at the top of wall 21 and of somewhat increasing thickness axially along wall 21 toward the bottom thereof. Since the proportion of carbon or carbonaceous byproducts in pyrolysis chamber 3 increases (or the tendency to deposit on the walls increases) progressively from the upper part of the chamber where the pyrolysis reaction commences toward the lower end thereof, the progressive thickening of the water screen surrounding the pyrolysis zone within walls 21 automatically compensates for the progressive increase in carbon products formation as the reaction proceeds toward the lower end of the pyrolysis zone, so that the water screen or curtain is at all points along the pyrolysis zone adapted and maintained, not only to protect walls 21 thereof, but also to entrain and carry olf particles of carbon or other heavy by-product materials from the gases in the pyrolysis zone before such particles or materials have an opportunity to settle or accumulate on wall 21.

Also, by injecting steam, instead of water, into the pyrolysis chamber, in accordance with this invention, for forming a thermal screen or curtain surrounding the hot gases reacting therein, the calorific yield or thermal etficiency of the entire pyrolysis reaction is increased as a result of the reduction in heat losses by the pre-heated hydrocarbon to by pyro-lyzed and in the heat losses of the pyrolysis chamber itself. Thus, the heat lost to walls Z1 by contact of the water curtain therewith is primarily heat from the steam injected into the pyrolysis zone, and not the combustion heat which it is desired to utilize for a pyrolysis reaction, and, since it is steam and not water which is injected, no heat is lost from the pyrolysis reaction itself for the purpose of vaporizing injected water as would otherwise be the case. At the same time, the temperature gradient or differential between the hot central portion of the pyrolysis zone and the cool walls 21 surrounding the pyrolysis chamber is reduced, and quenching of hot reactants at peripheral areas of the pyrolysis zone is substantially avoided.

As further illustrative of this invention may be noted the specific example of the satisfactory operation of a pyrolysis reactor in accordance therewith. Considering a furnace construction as generally illustrated here (or of the type disclosed in our copending application Serial No. 813,772, filed May 18, 1959, now Patent No. 3,019,271, and having this invention applied thereto) with the combustion chamber 2 having a diameter of 140 mm. and a height of 306 mm, the pyrolysis chamber 3 had a diameter of 160 mm. and a height of 103 mm, and the inner wall 2 1 of chamber 3 was made of steel and had a thickness of 3 mm. In such a furnace, 260

mfi/hr. (measured at 0 C. and 760 mm. Hg) of fuel gas were introduced through conduit 4, while 243 m. /hr. of oxygen (calculated at and measured at 0 C. and 760 mm. Hg) were introduced through conduit 5. The fuel gas was ignited in combustion chamber 2 to form a flame ring surrounded by a screen or curtain of steam, which Was injected through the annular slot 14 at a throughput of about 41 mfi/hr. Into this hot mixture of combustion gases and steam was injected 775 liters per hour of naphtha through 40 injection nozzles 16, each of winch had an orifice diameter of 4.5 mm, the naphtha having previously been mixed with 143 m. /h1. of steam and having been pre-heated to 620 C. Additional steam at the rate of m. /hr. was injected through inlet 25 from passage 18, while cooling water at a temperature of 18 C. was introduced into water jacket 22 surrounding wall 21 and circulated therethrough at the rate of 1.7 1 1 /1112, providing cooling to the extent indicated by the fact that the temperature of the cooling Water exiting from the water jacket at 24 had been raised to 68 C.

Under the foregoing circumstances, there was obtained from such reaction a resulting gas product containing 7.98% by volume of acetylene, 16.96% by volume of ethylene, and 2% by volume of propylene. Even after several months of continuous operation of such a pyrolysis reactor, it was observed that no carbonaceous deposit was formed or accumulated on the outer surface of reactor even in the area thereof between injection nozzle 16 and the opening of passage 25, and that the internal surface of wall 21 between the levels of steam passage 25 and quenching sprays 7 remained clean and free from deposits or accumulations of carbon or carbonaceous or tarry materials.

By comparison with attempts to provide some protection of the internal walls of a pyrolysis chamber as by introducing water, instead of steam, through the annular slot 25, the foregoing results indicate substantially enhanced advantages by utilizing the teachings of this invention. Thus, when water was injected, instead of steam, there was formed an annular cooled zone in the area of bafie or guide 2?) from which resulted the formation of a ring of deposited carbonaceous materials at that level of the reactor, requiring shutting down the apparatus for cleaning. Additionally, the conversion rate of the hydrocarbon to be pyrolyzed into the desired acetylene and ethylene products was significantly higher when steam was used than when water was introduced directly, such increase being of about 2% to 5% or more. That is, in the exampie given above using steam, the conversion rate of naphtha into acetylene and ethylene is about 50.6%, While a conversion rate was obtained of only about 48.2% in a comparable trial where water was introduced through passage 25 instead of steam. Under other pyrolysis conditions-eg, with other volume ratios of ethyleneto-acetyion in the formed pyrolysis gases, and particularly with such volume ratios which do not exceed 1the increase of the conversion rate is still higher-cg, a rate of about 59% when steam is used as compared to 54.2% when water is used.

It has also been observed that a screen or curtain con sisting only of steam along the inner surface of wall 7:1. is less advantageous than when a water curtain is con densed thereon from steam injected into the pyrolysis chamber. For example, to maintain a curtain of steam around wall 2-1, it is necessary to inject such a large pro portion of steam that economical disadvantages may result, and such a large amount of steam may dilute the pyrolysis reacting gases to an undesired extent. Also, attempts to inject steam in a plurality of points along the wall for maintaining a curtain of steam therearound have been found to increase undesirably the amount of steam in the pyrolysis zone with undesired dilution thereof, as well as with undesired irregularities in thermal control and the undesired production of side reactions, all of which reentrees? sult in a reduction of the conversion rate of the hydro carbon to be pyrolyzed into desired acetylene and olefine products.

It is also to be noted, in accordance with this invention, that, since the Water curtain or screen around the walls 21 is produced continuously by condensation of steam against cooled walls, the curtain possesses a property of repairing any interruptions or discontinuities therein. That is, even if a separation of desired continuous water layer occurs for one reason or another, it is repaired by immedi- .te condensation of steam from the chamber 3 and before accumulation or deposition of carbon materials on the ex posed wall 21, thus continuously protecting the internal surface of the wall.

As will be understood, the optimum proportion of steam to be injected through passage 25 so as to provide, on the one hand, suifioient steam for condensation to maintain a continuous water curtain while, on the other hand, avoiding an excessive diluting or wasteful overabundance of steam, must be determined in each particular case depending upon a number of factors such as the diameter and axial extent of the pyrolysis chamber, the amount of temperature and cooling etfect of the coolant being circulated in water jacket 2-2, the pyrolysis reaction temperature, etc. Similarly, the circulation of coolant through jacket 22 must be sufiicient to provide transfer or" heat through wall 21 for the continuous condensation and formation along the internal surface of the desired water urtain or screen. Such factors are readily determined by men skilled in this art in view of the teachings and disclosure hereof. As illustrative, however, it may be noted that satisfactory results are obtained with the injection of steam through passage 25 of about 20-4O mF/hr. for each 100 mm. of periphery of pyrolysis chamber 3, and with l020 liters of cold water per mm. of steam being circulated through water jacket Z2 with an entering temperature of from 5020 C. and an exiting temperature of from 65-70 C.

Although specific embodiments have been herein shown and described, it is to be understood that they are illustrative and not limiting on the scope and spirit of the invention.

What is claimed is:

1. in a process for the production of unsaturated hydrocarbons by the pyrolysis of saturated hydrocarbons introduced into hot combustion gases in a pyrolysis zone enclosed within rigid walls onto which carbonaceous byproducts of said pyrolysis tend to deposit, the steps which comprise introducing steam into said pyrolysis zone, directing said introduced steam around the inner surface of said walls for forming a continuous screen of steam around said walls, cooling said walls for condensing said steam against said Walls forming thereover a continuous and moving Water curtain flowing along said walls axially of said pyrolysis zone for entraining and carrying away said carbonaceous by-products and protecting said walls from deposition of said lay-products thereon.

2. In a process for the production of unsaturated hydrocarbons by the pyrolysis of saturated hydrocarbons introduced into hot combustion gases in a pyrolysis zone enclosed within rigid Walls onto which carbonaceous byproducts of said pyrolysis tend to deposit, the steps which comprise cooling said walls of said pyrolysis zone to a temperature substantially below the condensation temperature of steam, introducing steam into said pyrolysis zone around the periphery thereof, directing said introduced steam around the inner surface of said walls for forming a continuous screen of steam around said walls and flowing axially of said pyrolysis zone, condensing said steam against said cooled walls forming thereover a continuous and moving water curtain flowing axially of said pyrolysis zone all along said walls for entraining and carrying away said carbonaceous by-products and protecting said walls from deposition or" said lay-products thereon, and maintaining said flow of steam at a suiiicient throughput quantity for providing said water curtain continously over and along substantially the entire inside surface of said walls.

References Cited in the file of this patent UNITED STATES PATENTS

Claims

1. IN A PROCESS FOR THE PRODUCTION OF UNSATURATED HYDROCARBONS BY THE PYROLYSIS OF SATURATED HYDROCARBONS INTRODUCED INTO HOT COMBUSTION GASES IN A PYROLYSIS ZONE ENCLOSED WITHIN RIGID WALLS ONTO WHICH CARBONACEOUS BYPRODUCTS OF SAID PYROLYSIS TEND TO DEPOSIT, THE STEPS WHICH COMPRISE INTRODUCING STEAM INTO SAID PYROLYSIS ZONE, DIRECTING SAID INTRODUCED STEAM AROUND THE INNER SURFACE OF SAID WALLS FOR FORMING A CONTINUOUS SCREEN OF STEAM AROUND SAID WALLS, COOLING SAID WALLS FOR CONDENSING SAID STEAM AGAINST SAID WALLS FORMING THEREOVER A CONTINUOUS AND SAID PYROLYSIS ZONE FOR ENTRAINING AND CARRYING AWAY SAID CARBONACEOUS BY-PRODUCES AND PROTECTING SAID WALLS FROM MOVING WATER CURTAIN FLOWING ALONG SAID WALLS AXIALLY OF DEPOSITION OF SAID BY-PRODUCTS THEREON.