TRANSPORTING. SOLIDS BY PIPE LINE
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H. S. ELLIS P. J. REDBERGER
L. H. BOLT
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Small scale experiments 1
indicate that this concept a
i j economica/b feasible The first commerczal lines wjlzprobabh carry solid slugs but
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or many applications slurries have inherent disadvantages, particularly when the solid is contaminated by the liquid or when separation of the solid and liquid is expensive. Further, difficulties are encountered for abrasive solids or where attrition is a Droblem. Pumping power required increases rapidly with increases in either particle sue or density, as shown in the previous article. 8 An answer to some of these problems may be transportation of solids in the form of long trains of capsules or slugs. Invatigations to date have been on laboratory scales only, but several advantages are indicated. For example, except for slurries having very fine particle sizes, power requirements are less. Also, power and pumping costs compared to those of oil alone actually decrease as density of the capsule product increases. Commercial transport will probably begin with solid spheres or cylinders, but common carrier limes, which more than one product possibly using oil a~ IMY the liquid, can be expected in the future. The word capsule as used here refers to a container for solids which need protection from attrition or 1 1 contamination by the &ansporting liauid. Also, for . . convenience the term is applied to all shaped solids of any density which are transportable in pipe lines, whether hollow or solid, cylindrical or spherical, rigid or semirigid. The concept of transporiig solids enclosed in a capsule originated during experimental work on two phase liquid flow (2). If water and oil of the same density flow through a horizontal pipe line, there are stable conditions where spherical or elongated bubbles of oil in water or water in oil are formed (Figure 1). This observation led to further investigation (5) which showed that when the water was in turbulent flow, presence of oil bubbles oi slugs actually reduced the pressure gradient at Reynolds numbers up to 15,000; at higher Reynolds numbers little or no increase oc-
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V O L 5 5 NO. 9 S E P T E M B E R 1 9 6 3 29
The concept suggested by bubbles of water in oil or oil in water was curred. This gave rise to the suggestion that the bubbles could be capsules containing solids, provided the total density was the same as that of water. Then the suggestion was extended to include capsules which were heavier than water, and also slugs or ingots which at most would need a thin covering film for protection and added strength. Materials suitable for encapsulation are wheat or chemicals, and those suitable for making into slugs or ingots are sulfur, hriquetted coal, or minerals. Iron can be transported as spheres (5). The theoretical investigation of equal-density capsules was continued by analyzing the flow of a long cylinder moving concentrically in a pipe line and capsule velocities and pressure gradients were predicted for either laminar or turbulent flow in the pipe and in the annulus between the cylinder and the pipe wall (7). Later, a digital computer was used to predict velocity profiles and volumetric flow rates for the laminar flow of a Newtonian fluid in a circular pipe containing a fixed eccentric core (6). Experimental Plan
In conjunction with these theoretical studies. an experimental program was undertaken (3). A 35 foot long horizontal experimental pipe line of transparent plastic was built, having an inside diameter of 11/, inches. It was equipped with a capsule injection system similar to that used for inserting scrapers into pipe lines. Rotameters measured the flow of liquid, and the capsules were timed over a 10-foot test section by meansof a photoelectric cell at each end of the section. Pressure losses were measured over a 20-foot length of pipe line with a differential pressure cell, and the pressure drop across this section was recorded on a chart; thus the pressure gradient can be calculated. Using water and two oils of different viscosities, capsule and liquid velocity measurements have been made on a large number of cylindrical capsules of various end
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shapes, diameters, lengths, and densities. Velocity has also been measured, using spheres of various diameters. In order to obtain information on the effect of scale-up, a 60 foot long transparent plastic loop, 8/g inch in diameter, is being used to record similar velocity and pressure gradient measurements. This line is fitted with an adjustable inclined section so that information can be obtained on the effects of gradients. However, most of the investigation was carried out in the l'/.-inch line. The capsule bodies are made of threaded plastic tube, and to obtain the desired density and fore-and-aft balance the capsules can be weighted with lead shot or even mercury secured with wadding. The capsules are of five diameters from '/? to l l / b inches, corresponding to approximately 0.4 to 0.9 of the pipe diameter, and in three lengths; thus, effects of capsule diameter, length, and length-diameter ratio can be determined. Accordingly, 15 body sizes of hollow plastic cylinders are available. Equipped with two flat ends, they measure 2,4, and 7 inches over-all length for all diameters. Flat, concave, ellipsoidal, and hemispherical ends can be fitted in any configuration. Hollow plastic spheres of the same five diameters are also used. These are made in two halves which can he screwed together and are weighted in the same way as the cylinders. In addition to the loaded hollow plastic capsules, slugs 2nd spheres were made from solid plastic and aluminum, with densitiesof 1.19 and 2.84 grams per ce., respectively. These capsules were fabricated in one piece and were of the same five diameters. The solid cylinders have either two flat ends or one Aat and one ellipsoidal end, and are of three different lengths for each of the two types of end shapes. Plastic and aluminum are therefore both represented by 30 different sizes and shapes of cylindrical slugs and five diameters of solid spheres. Three to six similar capsules were made of each size and shape so that the effects of individual variations can he averaged.
1. If mtn and oil of tha s w dnrriryflolu together, stable conditior~~ occw w h m sphmicd or elrmgolcd bdblts of watcr in oil or oil in wolnfmn. This concept was exknddd ' 0 hollow capsules
OIL SLUG IN A WATER STREAM
TYPICAL HOLLOW CAPSULE 30
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
extended to hollow capsules Three liquids of different physical properties have been used in order to determine the effect of viscosity on the performance of the capsules, and also in order to reproduce laminar, turbulent, or transitional motion of the liquid in the free pipe over a wide range of liquid velocities. The three liquids used are water and two oils (kinematic viscosities, about 45 and 7 cs. at 77O F.). Expnimenlal Rtnulh
One of the most interesting findings was that large diameter capsules and slugs nearly always move faster than the average liquid velocity in the free stream. Because of the velocity prolile of the liquid in the pipe, the average liquid velocity near the capsules or slugs is significantly greater than that of the whole liquid. This remains true even when the slug is sliding along the bottom of the pipe, providing its diameter is greater than half that of the pipe. In Figure 2, velocities of spheres and oil become the same at an average oil velocity of less than foot per second and cylinders and oil at about 1 foot per second. At high oil velocities both spheres and cylinders move about 10% faster than the average oil velocity. Similar results are obtained using the lighter oil or water. As expected, capsules move more slowly relative to the average liquid velocity as capsule diameter decreases. Increasing the capsule density has surprisingly little effect on its velocity relative to the liquid, especially when the capsule is smooth. For example, a plastic capsule loaded to a density of 7.7 grams per cc. travels only about 3% slower than an aluminum bar (2.84 grams per cc.). Even an iron bar travels at the average oil velocity when the latter reaches about 2 feet per second and 5% faster at higher oil velocities. Exact information on effect of gradients is not yet available, although cylinders having a specific gravity of 1.2 have surmounted gradients of 45' in the %-inch pipe line with little loss of velocity. In any case heavy
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materials would probably be transported as spheres, since the frictional forces are less. Photographic studies of plastic and aluminum spheres having a sphere-pipe diameter ratio of 0.89 showed that the spheres rolled in water with little slippage at low and medium velocities. In a horizontal pipe the behavior of steel spheres is similar to that of aluminum spheres. Therefore, commercial possibilities of spherical capsules of materials such as compacted iron powder are interesting, although effect of increasing the capsule and pipe diameters is yet to be tested. Pressure gradient measurements are being made on slugs or spheres singly and in a train, using as the transporting medium 7-cs. oil. The range of volumetric concentrations which can be covered is small, because the apparatus is deigned for single capsules and slugs. Nevertheless, over this limited range the percentage increase of pressure gradient due to the slugs and spheres is proportional to their volumetric concentration at any given oil velocity and falls rapidly with increase of velocity. In Figure 3, although the percentage increase of prepsure gradient falls with increase of oil velocity, the absolute pressure gradient naturally increased. This
Figura 3. Parclntaga inncm in pressure graditnt u prsbationnl
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NO. 9 S E P T E M B E R 1 9 6 3 31
result is similar to that obtained with slurries where, for a given concentration, the fractional increase of pressure gradient (zm - iw)/imdecreases as velocity increases, but i, itself increases. Steel spheres can be transported with less percentage increase of pressure gradient than aluminum slugs at the same concentration and oil velocity. Figure 3 shows that aluminum slugs cause about 30% greater pressure gradient increase than the steel spheres under the same conditions. For example, at an oil velocity of 4.04 feet per second and a concentration of 6%, the aluminum slugs cause a pressure gradient increase of about 5oyO, but the steel spheres only 37yo',. However, because of differences in shape, the maximum linear concentration
Fipre 4. Effectof spec& gravitv
continue to be independent of pipe diameter. In Figure 3, the minimum horsepower per mile curve for a slurry of particle specific gravity 2.65 has been reproduced from Figure 2 in the previous article on slurries, and arrows have been added to show the estimated minimum horsepower per mile for aluminum slugs and spheres giving the same throughput as the slurry. Figure 5 indicates that the slugs are more economical to pump than the slurry when particle sizes are above about 50 microns. Steel spheres have the advantage over comparable slurries having particle sizes of about 10 microns or more. One great advantage of slug transport over slurry transport, especially at high specific gravities, became clear during the calculations; that is, slugs and spheres may be transported at virtually any velocity sufficient to give the throughput, whereas most slurries have a relatively high critical velocity. Since the horsepower is approximately proportional to the (velocity of the liquid) 3, great economies can be realized by reducing the liquid velocity and using high \. olumetric concentrations of capsules up to about 70% (80 to 90% by weight). Such concentrations are not attainable for transport of slurries. The investigation was carried out in a smooth, smalldiameter horizontal line and much more remains to be done before performance of capsules, slugs, or spheres in a commercial-sized line can be predicted. I n particular, pressure gradients due to capsules in various sizes of commercial lines must be measured, both on horizontal and inclined sections. In addition, a number of technical problems require solution, such as methods of capsule injection and of by-passing pumps. However, responsible engineers in the industry believe that these problems can be solved once the economic possibilities of capsule and slug transport are established. Preliminary Economic Study
of spheres in a line is only "3 that of cylinders. For equal volumetric throughputs, therefore, velocity of spheres must be 1 1 / 2 times greater. This reduces the advantage of the spheres, but still leaies them in a favorable position, especially for high density material. Increasing density of the slugs increases the percentage pressure gradient increment approximately in proportion to the density, though it should be borne in mind that the surfaces of the slugs are different (Figure 4). Further studies are in progress to determine the effect of the surface and diameter of the slugs and the viscosity and density of the liquid. Pressure gradients due to solid slugs in large pipe lines cannot be estimated even with the certainty possible for slurries. But based on pumping power required, an attempt has been made to determine at what slurry particle sizes slug transport is likely to be more attractive than slurry transport. In scaling up from the laboratory line to a 4- or 6-inch line, a number of assumptions must be made, notably that the percentage increase of pressure gradient due to the slugs continues to be proportional to the volumetric concentration of the slugs at high concentrations, and that it will 32
INDUSTRIAL AND ENGINEERING C H E M I S T R Y
A preliminary economic evaluation, based on substantially equal-density capsules formed by a film or shell surrounding a cargo susceptible to contamination ( 4 ) ,points out that the principal cost is for the pipe and its installation. Then, because extra care would be needed in finishing the pipe interior so that snagging does not occur, a 20% surcharge for installation is suggested. Such extra care would be needed mainly for encapsulated products such as wheat or chemicals where the encapsulating material might be plastic, but also the surcharge could cover the cost of larger radius bends or of avoiding steep gradients. The report points out that more equipment would be needed for capsule transport because a by-pass system for the capsules must be provided at each pumping station and also a local and regimal control system, should more than one type of capsule be transported. The extra cost to cover the additional equipment is estimated at about loo'% of the cost of the pumping stations. Taking 10% of the pipe line capital costs to relate to pumping stations, and placing 10070 surcharge on these and 20% on the remainder, the capital cost of a capsule pipe line would be about 25 to 3Oy0more than that of
Fipre 5. Aluminum slugs W E more economical to pump than rlurricr when r l u q particle sizes arc obovs obout 50 microns. Steel rphcres arc more economical mhcn porlicle r i m arc about IO microns or mow. Velocity and concentration (in parentheses) ore rhown for tach point. For additional data, ste table below:
AI cnprulos AI sphncr Stccl rphncs
2.85
3
62
7.5
a n equivalent oil pipe line. However, because the capsule diameter would be not more than 90% of the pipe diameter, and the line would never be completely full longitudinally, the capsules would only fill about 60 to 75% of the pipe volume. Thus, if no credit is taken for the liquid pumped, the capital cost of a capsule lime might be twice that of an oil line. For equal-density capsules, operating costs would be about 200'% more than for an equivalent oil line. Careful monitoring of the capsule line is needed, and if different types of capsules are transported a complex data processing system may be required. Since thii report (4) was issued, pressure measurements have been taken on trains of heavier-than-liquid dugs and spheres, and costs of transporting heavier-thanliquid dugs and spheres has been compared with those for oil alone. However, this preliminary economic review assumes also that laboratory results may be scaled up very considerably, particularly that the pressure gradient increase over that of the supporting liquid alone is independent of pipe diameter. For capsules heavier than oil, pumping and capital costs cannot be compared AUTHORS H. S. Ellis i s a research engineer; P. J . Redbcrgn, a mathematician; and L. H . Bolt, a research mgimer with the Research Council of Alberta, Edmonton, Alberta, Cannda.
directly with those of oil alone because the weight throughputs are different. For weight throughputs to be the same, oil would either have to be pumped at a much higher velocity or flow in a much larger pipe. The most useful basis for comparison seems to be power requirements per unit weight of salable material. The transporting liquid in a capsule pipe line could br either water or a liquid such as crude oil for which a significant transportation credit could be obtained. In the present survey we shall not include the latter possibility in order more clearly to assess the cost of transporting the capsules themselves. Cylindrical capsules having a specific gravity of about 1.2, moving at 5 feet per second, and at a volumetric concentration of 70% seem to need about 3.3 times the power required by 7-cs. oil alone moving at the Sam? velocity in the same diameter pipe. The capsules have a density of 0.70 X 1.2 = 0.84 gram per cc. of pipe which is approximately the density of oil, so that the weight throughputs of the cargo in the oil and oil-capsule systems would be the same, but the power required by the capsule system would be 3.3 times as great, as indicated above. A similar calculation for aluminum cylinders (specific gravity 2.84) shows that they would need about 5l/1 times the power for oil alone, but because the cylinders have a density of 2.4 grams per cc. of pipe compared with 0.84 for the oil, the power for the same weight throughput as the oil would only be 2.3 times as great as for 7-cs. oil. Thus, power to pump the same weight throughput of capsules as of oil decreases as capsule density increases. However, the pressure losses and power required for heavy cylindrical capsules become very large and it would be advisable to use spherical capsules for materials having specific gravities from 2.5 to 3. Because the capital costs of pumping stations may be taken as proportional to the power developed, it appears that pumping station capital costs would also be increased by 130 to 230% when cylinders having a specific gravity 1.2 to 2.8 are transported instead of oil alone. If a surcharge of 100% is added to the oil pumping station capital costs to cover by-pass and control systems for the capsules, a total surcharge of 230 to 330% would be added to the 18% (most recent figure) of the pipe line costs relating to pumping stations. Retaining the suggested 20% surcharge for the pipe itself, a capital cost increase of 60 to 80% results. Assuming that the liquid pumped earns no credit, pumping costs would be 130 to 230% above those for the oil. Considering now the transport of spheres, if the laboratory measurements can be scaled up directly, aluminum spheres (specific gravity, 2.84) moving at 5 feet per second in a concentration of 40% would need 2.6 times the power required for oil moving at the same velocity. Assuming that relative density of the two systems is about 1.35, the spheres would need about twice the power for the same weight throughput. Steel spheres appear to need little more power than the aluminum spheres at 5 feet per second, but at 40% concentration they have a density of 3.76 times that of the V O L 5 5 NO. 9 SEPTEMBER 1 9 6 3 33
oil. A calculation similar to that for aluminum spheres shows that the steel spheres would need only about 75y0 of the power required to pump the same weight throughput of oil alone. 4dding the 100yo surcharge to cover the injection and by-pass systems, as before, and the 20% on the pipe itself, the capital costs of the pipe line system for the spheres would show a 30 to 55%1, increase over those for oil alone for the same weight throughput, depending on the density of the spheres. Again no credit has been claimed for the liquid transporting the spheres. T h e pumping costs would range from twice the oil pumping costs in the case of the aluminum spheres down to 75% of these costs for the steel spheres, both for the same weight of cargo. The total operating costs would depend a good deal on the amount of overseeing necessary, being a minimum if all the capsules were of the same material and were being transported to the same destination. E S T I M A T E D CAPITAL AND P U M P I N G COSTS O F T R A N S P O R T I N G CAPSULES COMPARED T O THOSE FOR 7-CS. O I L ALONE
(Equal weight throughputs)
Spec. Grav .
Cy1ind e rs Spheres
1.20 2.84 2.84 7.80
60 55 30
+ 130 + 100 - 25
Considerable interest has been aroused in capsule transport of wheat or other grains, and the possibilities of expendable and reusable containers have been discussed ( 4 ) . Economically, expendable containers are more likely, especially if a salvage credit can be obtained. ,41so, a tough disposable film might be considered for the transport of chemicals and other products which would be contaminated by contact with the liquid. Large quantities of grain would be necessary to justify a capsule pipe line, but such quantities are in fact now being shipped. For instance about 12 million tons per year of grain is being moved out of the Western Plains of Canada, a figure comparable to the current oil tonnages being pipe-line& from a similar area, and justifying a similar sized pipe line. ,4 300-million bushel (about 9,000,000 tons) annual operation would appear to require a 30-inch line ( 4 ) . However, the immediate prospects appear to be belter for the transport of slugs or spheres of solid material where there is little or no cost for the protection of the material and where smaller tonnages appear more likely to be economical. LYhen large-scale data become available, it will be worthwhile to consider the possibilities of transporting sulfur in slug form, the sulfur being cast into cylinders. A IO-inch line would be adequate to convey an annual throughput of about 2 million tons. Sphere or slug transportation in some circumstances may also be economical for materials such as coal, phosphate, potash, metal ores, or refined metals. 34
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
Common Carriers
Once the technology has been mastered for injecting and retrieving capsules and for the by-passing pumping stations: the most likely first commercial lines would appear to be relatively small-scale. single-material slug or sphere lines. T h e transport of a number of different commodities in spherical, slug, or capsule form in a small line ~ o u l dappear to be a normal development. possibly with another commodity such as oil for the transporting liquid. Such common carriers would be in an economically preferred position. though the logistics of operation would be very complicated. As in the case of engineering technology associated with capsule transport. the logistic difficulties can surely be overcome once commercial-sized slug transport has been put on a secure footing. T h e achievement of the latter is a development rather than a research project and the scientific and preliminary economic investigations outlined here indicate that it is fully justified. Slurry vs. Capsule Transport
A choice between slurry and capsule transport cannot be made without further knowledge of the commercialscale performance of capsules, especially of pressure gradients and effects of physical gradients. Among conditions favoring the decision to employ capsules or slugs are a lower power requirement resulting from high density and large particle size of the product, nonContamination by the liquid, practically no liquidsolid separation expense, and the economic feasibility of small throughputs by transporting several products in the same line. LITERATURE CITED
(1) Charles, M. E., “Pipeline Flow of Capsules: Pt. 2, Theoretical Analysis of the Concentric Flow of Cylindrical Forms,” Can. J . Chem. Eng. 41, 46-51 (1963). (2) Charles, M. E., Govier, G . IV.. Hodgson, G. W., ”Horizontal Pipeline Flow of Equal Density Oil-Water Mixtures,” Ibid., 39, 27-36 (1961). (3) Ellis, H. S., “Pipeline Flow of Capsules, Pt. 3, Experimental Investigation of the Transport in Water of Single Cylindrical Capsules with Density Equal to That of IVater,” Ibid., in press. (4) Hodgson! G. W., Bolt, L. H., ”Pipeline Flow of Capsules: Potential Industrial t\pplication,” Eng. J . 45, 12, 25-30 (1962). (5) Hodgson, G. T.V., Charles, M. E., “Pipeline Flow of Capsules: Pt. I, The Concept,” Can. J . Chem. Eng. 41, 43-5 (1963). (6) Redberger, P. J., Charles, M. E., “Axial Laminar Flow in a Circular Pipe Containing a Fixed Eccentric Core,” I6id., 40, 70-5 (1962).
This i s the second and last of two articles on transporting solids by pipe line. The previous article, “Slurries-Basic Principles and Power Requirements,” appeared in the August issue of Industrial and Engineering Chemistry.
