US2924515A - Preventing scale formation in slurry feeding processes by means of a mixture of alkali hydroxide and carbonate - Google Patents

Description

PREVENTING SCALE FORMATION IN SLURRY FEEDING PROCESSES BY MEANS OF A MIX- gmURE F ALKAL I HYDROXIDE AND CARBON- Ronald W. Chapman and Roger M. Dille, Whittier, Califi, assignors'to Texaco Inc., a corporation of Delaware Application November 15, 1956 SerialNo. 622,286

No Drawing.

8 Claims. (Cl. 48-206) The present invention relates to improvements in the process for treating solid particles such as minerals which involves first forming a flowable" mixture of solid particles inaa vaporizable liquid, and then passing the mixture through an elongated tubular heating zone while heating the mixture to vaporize the liquid and form a dispersion of solid particles in vapor. Such treating is employed in a recently developed fluid energygrinding process, as described in US. Patent 2,735,787, whereby minerals suchas talc or feldspar containing leachable calcium and silicon compounds can be disintegrated. It is also employed for feeding to a reaction particles of a solid material containing leachable compounds, e.g. feeding a carbonaceous solid such as coal or oil shale to a synthesis gas generator, as described in US. Patent No. 2,864,677, even with little or no concomitant grinding.

' This invention'is directed primarily to preventing or reducing the accumulation of-scale 'in tubular heating zones during such feeding operations. This beneficial result is obtained by incorporating in the flowable mix ture of solid particles and vaporizable liquid both an alkali"hydroxide and an alkali carbonate. Dissolved scale forming compounds are in this way precipitated as insoluble compounds which then' pass through the subsequent heating zone as solids instead of depositing on theiw alls during vaporization. H a

"-lnthei grinding process to which the present improvement relates, after forming a dispersion of solid particles in steam or other vapor as described above, it is passed through a succeeding zone of high velocity flow whereinthe flowing stream is subjected to turbulence and a velocity sufiiciently high to effect disintegration of the coarse particles. Velocity should exceed 25 feet p er second, but isiusually much higher, such as hundreds and even thousands of feet per second. This succeeding zone may be a section of pipe, or may include a convergentdivergent nozzle or a pair of opposed jet nozzlesh fIhen the resulting stream containing finely ground solids in suspension is dischargedfrom the high velocity zone. When grinding is the ultimate purpose, the ground prodnot. is: recovered. When gasification of coal is to be per.-

fofnied, the discharged coal particles are fed to, a suitable gasifierfor combustion and gas generation @Wate'r will be mentioned as the suspension liquid u ids also maybe used such as kerosene, alcohols, glycols, carbon tetrachloride and the like. i Q

-Much' of the development work on the foregoing operations. has involved the treating of mineral materials which contain soluble ingredients tending toform a scale ontheinternal. walls of the heated tubes when liquid is evaporated from '.the flowable mixture. Among such scale forming ingredients are certain compounds of cal.- ciu m, ,aluminum, silicon, sulphur, iron' and magnesium, which dissolve in the suspension liquid and later precipitate inthe heatertubes during vaporization of the liquid when the solubility. is exceeded. .Example'sare calcium sulfate and bicarbonate.

has matter to exemplify'the invention. However, other liq- Scale is detrimental because it causes the tube passages to become so constricted that the velocity rises to a rate high enough to erode through the tube walls. Also, when scale reduces the internal diameter of the tubes it is difiicult to maintain constant operating conditions of pressure, velocity, and flow rate during the treating oper ation. Reduced passage size also imposes overloads on feed pumps, which must operate against excessive back pressures.

Particular difficulty has been experienced when treating talc and coal, both of which may contain sulfur calcium is not deposited on the slurry particles in way, but remains in solution, it later creates plugging" and/or iron compounds, as well as silicious and bicarbonate materials. An examination of the scale in the coal feeding portion of a coal gasifying plant has shown that it is largely a calcium sulfate deposit together with coal, silica, and compounds of iron and/or aluminum.

An analysis of the scale in a tale grinding plant has indicated that in an initial portion of the tubular zone the scale is almost all calcium sulfate and calcium carbonate, but from there on the proportion of sulfate scale progressively decreases and the proportion of silicious scale increases. Such silicious scale from a talc grinding plant, upon examination, revealed that, it was composed of alternating white and grey layers, and that these layers were made up of steatite, grammatite, alpha quartz, alpha cristobalite, and other unknowns. dicates talc, silica, iron and/or aluminum.

More in detail in accordance with the present invention, the accumulation of detrimental scale on the interior of a heating tube is greatly reduced, and in some cases completely eliminated, by introducing into the flowable mixture of solid particles in water both a small quantityof at least one water soluble alkaline hydroxide and at least one water soluble alkaline carbonate, with or without other modifying additives. pounds are thus precipitated in the slurry tank and flow as solids with the slurry, usually at least partially coating the other solid particles. It is to be understood that the term alkaline as used herein includes not only the alkali metals of group 1(a) of 'the periodic table such as lithium, sodium and potassium, but also includes the ammonium radical which is well known to be equivalent to the alkali metals in many of its properties.

In application Serial No. 582,061, filed May 2, 1956;,

by Dille and Eastman, there is disclosed a method for reducing scale by employing alkali metal carbonate alone, such as sodium or potassium carbonate. While this method went a long way toward solving the problem of scale formation, it has certain disadvantages which make it less than perfect. One of these is that when using sodium carbonate alone, the calcium hardness in the slurry water can be reduced no lower than about 10 ppm, and this is still enough hardness to give some scale deposition in the heater tubes.

In contrast, when using sodium carbonate and sodium hydroxide together the calcium concentration in the water of a coal-water slurry can be reduced to about 2 ppm. or less while requiring less sodium carbonate than if the carbonate were used alone. The effectiveness of this combination treatment is attributable to the fact that the sodium hydroxide converts all bicarbonates in the slurry water to the carbonate. Thus it converts any dissolved sodium bicarbonate to the soluble carbonate,'

which in turn reacts with any dissolved calcium not on the solid particles in the slurry tank, and in this way passes through the heating coil along with the solid particles without depositing on its internal walls. If the Patented Feb. 9,196.0

Chemical analysis in-- The scale forming com ditficultie's by depositing in the form of a spongy calcium sulfate on the internal wall of the coil.

Another disadvantage of using sodium carbonate alone is the large amount required to reduce the hardness substantially. For example, 80 pounds of sodium carbonate were required to reduce the hardness of 6000 gallons of a 46% by weight coal slurry from 600 p.p.m. down to 15 p.p.m. In contrast to this, 11 pounds of sodium carbonate and 11 pounds of sodium hydroxide together in 6000 gallons reduced the hardness from 600 p.p.m. down to 2 p.p.m.

Another disadvantage flowing from the large quantity of sodium carbonate required in the prior method is the detrimental fiuxing effect of the sodium ion on the refractory lining Within a synthesis gas generator. In such a generator a steel shell is lined with refractory material such as mullite to withsatnd temperatures over 2000 F. This tendency to flux the refractory when feeding a steam dispersion of coal to a lined generator chamber has been reduced substantially with the relatively small quantity of sodium ion required in the present invention.

Another disadvantage of using sodium carbonate alone is that one must continue to add sodium carbonate to the slurry as more hardness is leached from the coal, when the slurry is being agitated by means of air containing carbon dioxide. Sodium hydroxide along with the carbonate neutralizes the carbon dioxide of the air and prevents the soluble bicarbonate from forming, thus holding the hardness at the desired low level.

While beneficial results are obtained no matter how small the addition of alkaline carbonate and hydroxide, it is desirable to incorporate these materials in amounts sufiicient to lower the hardness of the slurry filtrate to between and 2 p.p.m. expressed as CaCOg. Normally, it is advantageous to incorporate at least /2 pound total of both materials per ton of water, and at most about 20 pounds per ton. More may be used successfully, although the improvement obtained may not be directly proportional to increase in the quantities of materials above this figure. The proportion of alkaline hydroxide to alkaline carbonate is not highly critical since any small amount of the hydroxide gives improved results over the carbonate alone. However, it is desirable not to use alkaline hydroxide in excess of the stoichiometric amount required to react with bicarbonate in the slurry water because the excess tends to react with silica in the solid particles to produce a water soluble silicate which may subsequently be deposited in the heater coil as a silicate scale. Good results are obtained when 1 part of alkaline hydroxide is present for between 0.4 and parts of alkaline carbonate by weight.

In order to be sure that the proper treatment has been accomplished the sodium hydroxide is added until the pH of the slurry has been adjusted to between 9 and 10. At pH below 9 all the bicarbonate will not have been reacted; and at pH above 10 there is danger of getting into the silica leach range. Then the hardness is determined and an amount of the alkaline carbonate added as required to precipitate the remaining calcium as calcium carbonate, about twice the stoichiometric equivalent usually being used.

The alkaline hydroxide and carbonate may be added to the slurry in any desired way upstream of the vaporizing zone in a heater tube, but it is simple and otherwise advantageous to introduce these materials directly into the slurry in a supply tank. This may be accomplished by continuously or periodically adding to the slurry either the dry compounds, or concentrated water solutions of,

the compounds. Also, the compounds may be mixed together dry or in water solution for addition to the slurry, or they may be added individually.

The ammonium compounds have a "distinct advantage over the sodium compounds which tend to attack and fliix the refractory lining in a synthesis gas generator Ammonium compounds, however, do not reduce the hardmess to as great an extent as sodium compounds. For example, the addition of 53 pounds of ammonium hydroxide along with 21 pounds of ammonium carbonate to 6000 gallons of by weight coal slurry reduces the hardness from about 600 p.p.m. only to 8 p.p.m. The amount of ammonium hydroxide can bereduced to as little'as 11 pounds per 6000 gallons while still lowering the hardness to the same value or less.

Ammonium hydroxide by itself also has a similar beneficial effect.

Example I 6600 gallons of a coal-water slurry containing 46%- by weight of '40mesh bituminous coal particles was made up, with water containing 600 p.p.m. total hardness expressedas CaCO Upon adding 0.1 gram of Na CO and 0.1 gram of NaOH per pint of slurry the hardness was reduced to 1 ppm. This slurry was pumped at 1457 lbs./hr. and 1032 p.s.i. into a long coil of heated iron pipe wherein the water was vaporized and a dispersion of coal particles'in steam was formed at a temperature of 639 F. The dispersion was then passed into a mullite lined gas generator wherein the coal was burned to produce CO and H the temperature in the generator. being maintained between 2050 and 2070 F.

Upon shutting down the unit, no sign of scale was found in the heater tube. There was no pressure build up after 42 hours, whereas ordinarily pressure build up.

occurs in 3-12 hours without the additives, due to the accumulation of insoluble scale.

About 3 times as much Na CO alone would be required to accomplish a similar result, thus introducing so.

much sodium as to damage the generator lining.

Example II 0.1 pound of NadH and 1.0 pound of Na CO per ton- I passed at 4 tons per hour and 1100 p.s.i. into a long coil of heated iron pipe wherein the water was vaporizedv through a pair of opposed nozzles to grind thetalc by.

j impact.

No pressure build up was observed in hours whereas ordinarily pressure build-up occurs in 2-20 hours.

. Example 111 Fiftytons of a coal-water slurry containing 50% by weight of minus 200 mesh coal are prepared ina large tank. Upon analysis the water is found to have a total hardness of 600Qp.p.m. expressed as calcium carbonate,

and a temporary hardness of p.p.m. expressed as calicur ipbicarbonate.

To this slurry are added 21 pounds .of ammonium carbonate and 53 pounds of amonium hydroxide, after which the total hardness is only 10 p.p.m. as the result of the precipitation of most of .the hardness-formingf materials in the slurry tank.

9 departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

We claim:

1. In a process for treating particles of a solid material which include ingredients tending to form a detrimental scale in a heating coil, said process comprising forming a flowable mixture of particles of said solid material in vaporizable liquid, passing said mixture into and through an elongated tubular heating zone, and heating said mixture during passage through said heating zone to vaporize said liquid and form therein a flowing dispersion of solid particles in vapor; the improvement which comprises, introducing into said flowable mixture a small quantity of treating agents comprising both water soluble alkaline carbonate and alkaline hyroxide to reduce scale formation in said heating coil, said water soluble alkaline carbonate and alkaline hydroxide being present in aggregate in an amount between /2 and 20 pounds per ton of liquid in said flowable mixture, 1 part of said alkaline hydroxide being present for between 0.4 and 1 parts of said alkaline carbonate by weight.

2. In a process in accordance with claim 1, said treating agents comprising sodium carbonate and sodium hydroxide.

3. In a process in accordance with claim 1, said treating agents comprising ammonium carbonate and ammonium hydroxide.

4. In a process in accordance with claim 1, said solid material being selected from the group consisting of coal and talc.

5. In a process inaccordance with claim 1, said solid material being talc, and said process including grinding said talc by passing said flowing dispersion in high velocity turbulent flow.

ass sts 6. In a process in accordance with claim 1, said solid material being carbonaceous material, and said process also comprising feeding said dispersion into a gasification zone, and oxidizing ingredients of said carbonaceous material therein to produce synthesis gas.

7. In a process in accordance with claim 1, said liquid being water.

8. In a process for treating articles of a solid material which comprise ingredients, including calcium ion, tending to form a detrimental scale in a heating coil, said process comprising forming a fiowable mixture of particles of said solid material in vaporizable liquid, passing said mixture into and through an elongated tubular heating zone, and heating said mixture during passage through said heating zone to vaporize said liquid and form therein a 'fiowing dispersion of sol-id particles in vapor; the improvement which comprises, introducing into said flowable mixture a substantial amount of alkaline hydroxide efiective to adjust the pH of said fiowable mixture to between 9 and 10, and then introducing into said flowable mixture a substantial amount of alkaline carbonate elfective to precipitate remaining calcium from solution as calcium carbonate.

References Cited in the file of this patent UNITED STATES PATENTS 913,034 McMurn'ie Feb. 23, 1909 1,547,111 Finn July 21, 1925 1,689,036 Leiss Oct. 23, 1928 2,534,284 Magill Dec. 19, 1950 2,735,787 Eastman et al. Feb. 21, 1956 2,787,326 Hughes Apr. 2, 1957

Claims (1)

1. IN A PROCESS FOR TREATING PARTICLES OF A SOLID MATERIAL WHICH INCLUDE INGREDIENTS TENDING TO FORM A DETRIMENTAL SCALE IN A HEATING COIL, SAID PROCESS COMPRISING FORMING A FLOWABLE MIXTURE OF PARTICLES OF SAID SOLID MATERIAL IN VAPORIZABLE LIQUID, PASSING SAID MIXTURE INTO AND THROUGH AN ELONGATED TUBULAR HEATING ZONE, AND HEATING SAID MIXTURE DURING PASSAGE THROUGH SAID HEATING ZONE TO VAPORIZE SAID LIQUID AND FORM THEREIN A FLOWING DISPERSION OF SOLID PARTICLES IN VAPOR, THE IMPROVEMENT WHICH COMPRISES, INTRODUCING INTO SAID FLOWABLE MIXTURE A SMALL QUANTITY OF TREATING AGENTS COMPRISING BOTH WATER SOLUBLE ALKALINE CARBONATE AND ALKALINE HYROXIDE TO REDUCE SCALE FORMATION IN SAID HEATING COIL, SAID WATER SOLUBLE ALKALINE CARBONATE AND ALKALINE HYDROXIDE BEING PRESENT IN AGGREGATE IN AN AMOUNT BETWEEN 1/2 AND 20 POUNDS PER TON OF LIQUID IN SAID FLOWABLE MIXTURE, 1 PART OF SAID ALKALINE HYDROXIDE BEING PRESENT FOR BETWEEN 0.4 AND 10 PARTS OF SAID ALKALINE CARBONATE BY WEIGHT.