US3548846A - Pipeline transportation of waxy oils - Google Patents

Description

137-013. OR 3548846 5R United States Patent Joseph C. Allen Bellaire, Tex.

Sept. 20, 1968 Dec. 22, 1970 Texaco Inc.

New York, N.Y.

a corporation of Delaware [72] Inventor [21 Appl. No. [22] Filed [45] Patented [73] Assignee [54] PIPELINE TRANSPORTATION OF WAXY OILS 5 Claims, 1 Drawing Fig.

[52] U.S. (I 137/13, 62/55; 103/6; 137/210 [51] Int. Cl ,Fl6d 1/16 [50] Field ol'Search 137/13,

210; 103/6; 62/55 port [56] References Cited UNITED STATES PATENTS 2,110,905 3/1938 Chase 137/13 2,958,205 11/1960 Mc Conkey l37/l3X 3,256,705 6/ l 966 Dimentberg 62/55 Primary Examiner-Robert G. Nilson Attorneys-K. E. Kavanagh, Thomas H. Whaley and Lewis I-I.

Phelps, Jr.

PIPELINE TRANSPORTATION OF WAXY OILS The present invention relates to the pipeline transportation of waxy oils and particularly waxy crude oils having a pour point substantially above temperatures normally encountered in the pipeline. The energy necessary to drive such a product thru a conduit is obviously quite substantial, particularly in those sections where temperatures are near or below the pour point.

Various means have been proposed to obviate viate this difficulty as, for example, dilution of the petroleum by wax-free fractions; namely, by simply increasing the proportion of waxfree fluids. So also it has been proposed to introduce gas into the flowing waxy hydrocarbon to form therein a myriad of minute gaseous Myles which are said to tend to separate the waxy particles a'fili prevent their agglomeration into a continuous wax structure. The presence of the minute gaseous particles moreover not only inhibits formation of a rigid waxy structure but is also said to facilitate flow of the mixed solid and liquid phases.

In accordance with the present invention it has been found that particular advantages are realized by the addition to the waxy oil of a relatively light hydrocarbon solvent of the class consisting of butane, propane and--mixtures thereof in an amount sufficient to substantially completely dissolve the wax but insufficient to establish a vapor pressure as great as the pressures prevailing in the pipeline conduit at the operating temperatures thereof.

In short, these light hydrocarbons, which may exist as gases under normal temperatures and pressures, are in fact employed in the pipeline exclusively under conditions of temperature, pressure and dissolution at which they exist as a liquid. In other words, in solution in the waxy oil, the combined vapor pressure of the solvent-oil solution is such that only a liquid phase is present. Therefore, for the purpose of preventing wax formation in pipelines, the solvent exists, not as a gas, but as a liquid under the prevailing conditions.

The solvent moreover, instead of acting as a mere diluent, in accordance with my observations, also converts the solid wax into a dilute liquid phase. Moreover its solvent capacity for wax, in common with heavier hydrocarbons, appears to be dependent upon the molal concentration rather than molecular weight of the solvent. As a result, substantially smaller volumetric proportions of the present solvent are ample to effect dissolution of wax equivalent to that of a heavier molecular weight hydrocarbon solvent. For example, as contrasted with a naphtha or gas oil fraction the volume of butane solvent required to effect equivalent dissolution of the wax, is ordinarily at least 50 percent less on a volumetric basis. For propane the volume requirement maybe even less.

Of greater importance, however, is thefact that by obviating the rigid waxy structure the viscosity of the oil is reduced by an even greater proportion, as for example 90 percent and more.

The present invention has an advantage over supersaturation of the oil with a gas in that the proposed and intended separation of gas bubble'sf'as a separate gaseous phase, introduces instability inwthe system requiring increased pipe lining facilities and conduit sizes. This is particularly so at points within the conduit where the pipeline pressure approaches a minimum, particularly at relatively high temperatures. At such points a substantial separation of the gas phase occurs, requiring substantially larger conduits.

Furthermore it will be apparent that the intermingling or solution of two miscible fluids contemplated by the present invention under conditions at which they are both liquids, obviates the problems and limitations involved in dissolving a gaseous phase in the liquid oil. In short there is no real problem of obtaining a true solution or of maintaining the two liquids in mutual, intimate, single phase mixture.

In short therefore, the present invention contemplates conducting a waxy crude at temperatures substantially below its normal pour point in the presence of a solvent of the class consisting of butane and propane, and mixtures thereof, thru a pipeline, the solvent being present in a substantial proportion, at least above 10 volume percent and sufficient to maintain the wax in substantially complete solution while operating the pipe line or conduit in a range of temperatures and pressures sufficient to prevent formation of a gas phase under the flowing conditions of said line.

Another important aspect of the present invention provides for facilitating the function of the booster or pumping stations in raising the pressure of the hydrocarbon liquid herein involved. As is known, the vapor pressure of a flowing petroleum may be limited by pipeline specifications which are usually determined by the fact that passage of the flowing product thru pumping stations, with a substantial boost in pressure, may involve cavitation or other pumping difficulties.

In accordance with one aspect of the invention I therefore propose to largely separate and remove the solvent prior to introduction of the contents of the pipeline into the booster station. This may be done by reducing the pressure on the flow of oil to a level below that of the booster station inlet, permitting the solvent to gasify and separate and then reinjecting it into the pressurized contents of the pipeline downstream of the booster station.

Separation of solvent from the oil may be effected simply by a substantial decrease in pressure, assisted, if desired, by heating, followed by induction of the resulting degasified oil into the booster and reinjection of the solvent into the pressurized oil.

One illustrative embodiment of the present invention will be hereinafter described more in detail in the following example, with the assistance of the attached drawing wherein the sole FIG. shows more or less diagrammatically, the flow system at the booster stations.

The example in question concerns transportation of Amal- Nafoora Crpde Oil which is a product of Libya with a pour point of 70 F. The present process contemplates transportation of crude thru a pipeline operating at a booster pump discharge in the range of from about 400 to 1,250 p.s.i.g. and at temperatures in the range of from about to 130 F. The pressure in the pipeline falls to a minimum value at the inlet to the booster stations which is in the range of about 30 to 70 p.s.i.g. or an average of 50 p.s.i.g. The maximum line temperature at this point is about 70 F. with a minimum above 35 F. Intermediate temperatures vary with areas and with the seasons but remain above 35 F. at the ultimate minimum.

Referring to the present drawing, the crude is picked up as at 10 by pump 12 and discharged at, for example, 1,000 p.s.i.g. pressure into the pipeline 14. Butane is injected via line 16 at a rate carefully controlled in the amount of 14 volume percent of the total crude butane mixture. Upon the basis of various correlations which I have developed or which have been reported in the literature, the properties of the resulting crude-solvent mixture are approximately as follows:

Solvent content, vol. percent, 14; Pour Point, F., 35; Viscosity, C.P. at F. 1.81.

By contrast, if a gas oil diluent in the proportion of 29 volumetric percent is substituted for butane the pour point is approximately the same but the viscosity of the resulting mixture amounts to 7.15 cp at 100 F. Manifestly the energy saving in the operation of the pipeline reflects this advantage.

At the inlet to the succeeding booster pump 18 the line leads upwardly a substantial distance as at 20 into surge tank 22. An inlet orifice 24 (or choke) to the tank may be provided to prevent premature breakout of gas bubbles in line 20. A pressure substantially less than the normal induction pressure of booster pump 18 is maintained by line 26 and pump 28. As a result, therefore, the pressure within the surge tank 22 may be maintained as, for example, at around atmospheric, such that the butane or propane fraction tends to vaporize and pass off thru pump 28.

As previously indicated, heating may also be used to facilitate the separation or stabilization operation.

Thereafter stabilized oilflows downwardly thru line 30 a sufficient distance to provide the required induction head to i thepump 18.

As previously indicated, the.pipeline or conduit per se,

. therefore operates throughout ranges of pressure and ternperatures well above the vapor pressure of the liquid, whereas the booster stations handle a liquid product sufficiently free of volatile fractions and thus avoid difficulties which might otherwise be associated with the pressure boosting mechanism.

Under the particular conditions of the present example the degasification of the stream prior to the booster stations takes place at approximately atmospheric pressure. This enables the product to meet a particularly severe pipeline specification as, for example, a maximum vapor pressure limit of 12.5 p.s.i.g. at the booster station.

In the pipeline, per se, the operating pressures, as above indicated, are substantially abovethe pressure of a solvent-containing mixture. For example, my observations and correlations indicate the following vapor pressures of the Amal- Nafoora crude mixed-with l4 percent added butane and contrasted as well with a mixture containing 12 percentadded propane.

Amal-Nafoora Crude with indicated quantities of added solvent:

Vapor pressure, p.s.i.a.

percent 32 F. 50 F. 100 F.

From-the foregoing it is apparent that the butane solvent easily meets the foregoing specification. The propane, while .-requiring a somewhat relaxed solvent specification, is obvipart of the butane. Moreover it is to be noted that the consumer demand for butane and propane fractions usually tends to be at a maximum in the cold months when the specifications can more readily be relaxed.

it is important to note, in accordance with theforegoing invention, that the ease of flow of the liquid thru the conduit is reflected by a contrast between the relative viscosity of the butane-crude mixture of my above example and that of the same crude diluted with a greater volumetric proportion of gas oil (as established bymy correlations and expressed in centipoises at 100 F as follows:

Referring'to the drawing, the separatiomdrum or surge tanlt 22 .operates at any pressure below that of the inlet to the booster station to effect a desired vapor separation. As also previously intimated, this may be effected in one respect by elevating the drum 22 suitably upon a frame or support 32. As a result, the How of stabilized oil to the booster 18 occurs under a pressure head determined by the elevation of the conduit 30. j

As previously indicated the tankneednot-necessarily be elevated'if the pressure therein is suitably controlled by the suction frompump 38.

Referring o the ma or ob ective of the present invention,

namely the use of a specific effective solvent in an amount sufficient tosubstantially-completely dissolve the wax within the operating range of temperatures in the pipe or conduit, it is to be recalled that we are here dealing withpressures and temperatures which vary within certain limits but nevertheless are so selected, in light of the volumetric concentration of the solvent, that the presence of free gas in the line is specifically avoided at all times.

At the same time it is, as indicated, desirable to effect substantially complete solution of the wax and for this reason, in highly waxy crudes for example, to maintain the volumetric concentration of solvent up to approximately this limit.

For this reason the term in an amount sufficient to substantially completely dissolve the wax or any paraphrase therefore as usedherein means either complete dissolution of the wax to form a single liquid phase or the presence of only minor and relatively insignificant amounts of solid wax particles, in

any event, below that amount present at or in the vicinity of the pour point. Since the conditions which favor maximum solid wax formation exist only outside the extreme minimum of the operating temperature range it is apparent therefore that throughout the vast bulk of the operation the wax is usually totally in a highly fluid, low-viscosity, liquid phase.

Iclaim:

1. The method of transporting a hydrocarbon oil thru a pipeline conduit, which oil contains asubstantial, flow impairing proportion of undissolved wax at pipeline operating temperatures, which comprises adding to said oil in said pipeline a substantial quantity of a wax solvent of the class consisting of butane and propane and mixtures thereof, sufficient to effect substantially complete dissolution of the solid wax under the temperatures and pressures prevailing in the pipeline and maintaining the pipeline pressure at every point inv said pipeline substantially above the vapor pressure of the solventhydrocarbon oil mixture at the operating temperature therein so that the pipeline operates gas-free with the solvent solely in theliquid phase. 5

2. The method as defined in claim 1 wherein the solvent consists of butane.

3. The method as defined in claim 1 wherein the solvent consists of butane and propane.

4. Themethod as defined in claim 1 wherein the solvent is present in a proportion greater than 10 percent by volume.

5. The method as defined in claim 1 which includes passing said pipeline product thru a series of booster stations, degasifying said oil to a substantial predetermined extent prior to the inlet to each said booster station and reinjecting said gaseous effluent into the oil downstream of said booster means.

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