STABILIZED POLYMER DRAG REDUCING AGENT SLURRIES

This invention relates to processes for producing polymeric drag reducing agents, and more particularly to processes for producing stabilized slurries of polymeric drag reducing agents.

The use of polyalpha-olefins or copolymers thereof to reduce the drag of a hydrocarbon flowing through a conduit, and hence the energy requirements for such fluid hydrocarbon transportation, is well known. These drag reducing agents, or DRAs, have taken various forms in the past, including slurries of ground polymers to form free-flowing and pumpable mixtures in liquid media.

In general, the DRA polymer may be obtained via solution polymerization of an alpha olefin monomer, or a mixture of olefinic monomers, or from bulk polymerization (i.e., without solvent) of such monomer(s). The DRA polymer may then be subsequently made into particulate form by cutting, chopping, granulating, and/or grinding, at cryogenic or ambient temperatures. Alternatively, it may be precipitated from solution by addition of a non-solvent component. Mixtures of polymer solids from both sources may be used.

A problem that has been frequently addressed in the art, however, is that there is a natural tendency for such slurries, containing the particulate polymer DRA, to settle over time, or to separate or agglomerate such that the slurries no longer maintain a free-flowing and pumpable nature. A generalized description of how this problem has been addressed is that the slurry of polymer DRA and liquid carrier also frequently includes a partitioning agent, a wetting agent, and/or a rheology modifier. These three components, which are frequently all included, are referred to generally as "suspension aids". The purpose of the partitioning agent is to physically hold the polymer DRA particle surfaces apart. The purpose of the wetting agent is to wet the polymer DRA surface, and the purpose of the rheology modifier is to increase the viscosity of the liquid carrier to slow down polymer DRA particle settling or rising. In some cases a single ingredient may serve multiple purposes within the suspension aid package.

The liquid carrier is, in some embodiments, a non-solvent for the polymer DRA and may vary widely. Selections for this component may include both aqueous and non-aqueous liquids, including, for example, water and aqueous solutions of various pH and ionic strength; alcohols and fatty alcohols; glycols and diols; glycol ethers; glycol esters; mixtures of these; and the like.

The wetting agent is virtually always included in such formulations. Without a wetting agent the liquid carrier would quickly drain away from a non- wetted polymer surface. This would result in a highly separated slurry. Fatty acid waxes have been used as wetting agents, as well as commercially available surfactants such as SPAN™, TWEEN™, BRIJ™, and MYRIJ™. These surfactants, which are generally sorbitan esters, ethoxylated sσrbitan esters, ethoxylated fatty acids and ethoxylated fatty alcohols, are available from Uniqema.

Inclusion of a partitioning agent is also generally desirable in these slurries. This is because polymer DRA's are often in the form of soft, tacky particles that will agglomerate, or "cold flow", when their unaltered surfaces come into contact with one another. Again, fatty acid waxes are often used as partitioning agents, as well as polyolefin homopolymers and copolymers of various densities; oxidized polyethylene; polystyrene and copolymers; carbon black and graphites; precipitated and fumed silicas; natural and synthetic clays, and organo-clays; aluminum oxides; talc; boric acid; polyanhydride polymers; sterically hindered alkyl phenol oxidants; magnesium, calcium and barium phosphates, sulfates, carbonates and oxides; mixtures thereof; and the like.

A rheology modifying agent may, in some embodiments, also be added to minimize settling of the polymer DRA slurry. By adding the modifying agent to the liquid carrier, settling or rising of the DRA polymer may be hindered or prevented. Some rheology modifiers, such as modified cellulosics and natural gums, may be heated to effect complete dissolution and may also function as wetting agents. However, these have no particulate properties that would render them capable of also serving as partitioning aids. A further disadvantage to this approach is that the higher viscosity automatically reduces the flowability properties of the polymer DRA slurry.

Another practice employed to stabilize polymer DRA slurries is to match the suspended particle density to the liquid carrier density, by using a combination of carrier components in appropriate proportions. The drawback to this practice is that there are relatively few available selections for carriers that are economical, have the desired densities, and also exhibit desirable hydrophobic or hydrophilic properties.

In view of the above, there is still a need in the art to discover ways to produce stabilized polymer DRA slurries that are convenient and economical and which do not suffer from the drawbacks listed hereinabove.

An object of the invention is to provide a method for producing a polymer drag reducing agent slurry that can be readily shipped and stored for extended periods of time.

Another object of the invention is to provide a method for producing a polymer drag reducing agent slurry that is relatively stable against settling, separation and/or agglomeration.

In carrying out these and other objects of the invention, there is provided, in one aspect, a method for producing a stabilized polymer drag reducing agent slurry comprising combining a fatty acid wax and a liquid carrier to form a dispersion; pre- treating the dispersion by heating to partially dissolve the fatty acid wax in the liquid carrier; and adding a particulate polymer drag reducing agent thereto, to form a stabilized polymer drag reducing agent slurry.

In another aspect the invention provides a stabilized polymer drag reducing agent slurry produced by combining a fatty acid wax and a liquid carrier to form a dispersion; pre-treating the dispersion by heating to partially dissolve the fatty acid wax in the liquid carrier; and adding a particulate polymer drag reducing agent thereto, to form a stabilized polymer drag reducing agent slurry.

The inventive polymer DRA slurry is relatively stabilized against settling, separation and agglomeration. In general, the invention includes both a method of preparing a stabilized polymer DRA slurry, and the stabilized slurry prepared thereby. Its key feature is first combining the fatty acid wax and the liquid carrier and heating them together sufficiently to partially dissolve the wax in the liquid carrier. This pre-treatment imparts superior resistance to settling, separation and agglomeration when the polymer DRA is then added to form the slurry. It is both economical and convenient to practice. While art-known stabilizations include use of fatty acid waxes, none has discerned or suggested the improvement attained when the fatty acid wax and liquid carrier are first heated together, and when the wax is partially dissolved in the liquid carrier. This fatty acid wax may be selected from the group consisting of fatty acids, fatty acid salts, fatty acid esters, and fatty acid amides. In one non-limiting embodiment, these include Groups 1 , 2, 12, and 13 (IUPAC standard notation) metal salts of stearic acid; stearic acid amides; and stearic acid esters. Such may include, for example, magnesium stearate, calcium stearate, zinc stearate, aluminum stearate, sodium stearate, and potassium stearate, as well as ethylene

bis-stearamide, stearamide, ethylene glycol monostearate, ethylene glycol distearate, propylene glycol monostearate, propylene glycol distearate, glycerol stearate, glycerol distearate, glycerol tristearate, and diethylene glycol distearate and stearic anhydride. "Stearic acid" and "stearates", as used herein, refer, respectively, to fatty acids and fatty acid derivatives containing a range of carbon chain lengths, provided that the compound meets the definition of a wax, i.e., is solid at ambient temperature. They may be either saturated or unsaturated compounds. In some non-limiting embodiments chain lengths from about C10 to about C24 may be selected. In other non-limiting embodiments a carbon chain length of about C18 may be effectively selected.

The fatty acid waxes may be used in the invention in neat form, from commercial or in-house sources, for example, or may be formed in situ in the liquid carrier, for example, by mixing stearic acid and amines or a metal hydroxide therein, followed by heating. Mixtures of the various wax types may also be used. The liquid carrier may be any that are typically used for preparing polymer

DRA slurries, but are generally non-solvents for the polymer DRA. Such may be selected from both aqueous and non-aqueous liquids, according to economy, convenience, and, desirably, the nature of the stream in which the polymer DRA slurry will be ultimately used for drag reduction. Water itself, as well as aqueous solutions having adjusted pH and/or ionic strengths or other desired modifications may, in some non-limiting embodiments, be advantageously employed. In other non-limiting embodiments alcohols and fatty alcohols, glycols and diols, glycol ethers, glycol esters, or mixtures of these may provide the desired liquid carrier.

A key to the invention is first combining the fatty acid wax and the liquid carrier to form a dispersion (without presence of the polymer DRA). This dispersion is then pre-treated by heating sufficiently to partially dissolve the fatty acid wax in the liquid carrier. This pre-treatment is unique, because the fatty acid wax functions both as a partitioning agent in the suspending liquid carrier, in its non-dissolved form, and as a wetting agent and rheology modifier, in its dissolved form.

The time and temperature at which the dispersion is pre-treated will obviously vary according to the selection of liquid carrier(s) and fatty acid wax(es). However, heating above ambient temperature is necessary in order to accomplish the desired level of dissolution. Such level is, in one desirable and non-limiting embodiment, less than 99 percent by weight of the total fatty acid wax dispersed in

the liquid carrier, but at least about 0.5 percent by weight of the fatty acid wax. In another non-limiting embodiment, the level is less than about 10 percent by weight, and in still another non-limiting embodiment, the level is less than about 5 percent by weight, but in both embodiments it is at least about 1 percent by weight. For example, a proportion of ethylene bis-stearamide (EBS) may be combined with hexanol in a proportion of about 5.7 percent by weight of EBS. This dispersion may be heated, at about 13O ⁰ F, for about 4 hours. After cooling to ambient and filtering to remove any non-dissolved EBS, it is found that the filtrate containing about 0.25 percent by weight of EBS, from which it may be inferred that about 4 percent by weight of the original EBS solids are dissolved in the hexanol. Heating temperatures may, in some non-limiting embodiments, range from about 80 to about 200 ⁰ F, and in other non-limiting embodiments may range from about 120 to about 15O ⁰ F. Times may, in some embodiments, range from about 0.5 hour to about 10 hours, and in other non-limiting embodiments, from about 1 to about 5 hours. While time is not per se critical, provided that the appropriate level of dissolution is attained, it may be desirable in some cases to avoid heating for longer than necessary. This is because the dispersion may then tend to form a relatively immobile jelly-like material resembling grease. Achieving such a highly viscous state may be an impediment to forming a satisfactory slurry once the polymer DRA is added.

The proportional ratio of fatty acid wax to liquid carrier may, in some non- limiting embodiments, be from about 0.005:1 to about 0.2:1. In yet other non- limiting embodiments, the ratio may be from about 0.007:1 to about 0.19:1 , and in still other non-limiting embodiments, from about 0.01 :1 to about 0.2:1. In other non- limiting embodiments, the ratio of wax to DRA polymer may be from about 0.02:1 to about 0.48:1. In certain non-limiting embodiments, the wax concentration in the total slurry may range from about 0.1 to about 30 percent by weight, and in other non-limiting embodiments, it may range from about 0.5 to about 12 percent by weight, based on the weight of the slurry as a whole. Appropriate levels of dissolution may be confirmed both visually and by measurement of viscosity. For example, once a partial dissolution has taken place as a result of the heating, viscosity will increase. In certain non-limiting embodiments, this viscosity increase is at least about 100 percent, provided that the solution is not transparent to light, i.e., it must be either translucent or opaque. It is important to note that dissolution generally occurs at a temperature

significantly below the temperature at which melting of the fatty acid wax occurs, and in some embodiments at least about 70 degrees Fahrenheit below the melt temperature. In many cases, the difference between dissolution and melt temperatures will be much greater. Another measure of partial dissolution is the particle size of the fatty acid wax. Viscosity increase correlates to particle size, and thus, as the average particle size increases by, in some non-limiting embodiments, at least about 10 percent of their initial size, the viscosity will increase proportionately, in some embodiments to a level of 100 percent of the initial viscosity. Generally, the particle size increase may be graphically compared with heating time at a constant temperature and, as heating continues, particle size increases asymptotically. Without wishing to be bound by any theory, it is hypothesized that particle size increase is due to swelling and/or an Ostwald Ripening effect, in which the thermodynamics of the system favor growth of large particles at the expense of smaller ones during the simultaneous solvation and crystallization of the molecules.

Following the pre-treatment step, the dispersion may be cooled back to ambient temperature, or, in some embodiments, to any temperature that is at least about 20 degrees Fahrenheit lower than the heating temperature. Such cooling enables the amount of wax dissolved to remain relatively constant or to change only very slowly over a prolonged time period. Temperature control of the pre- treated material is not critical at this point, provided it is maintained significantly below the heating temperature, and in some non-limiting embodiments, at least about 20 degrees Fahrenheit below the heating temperature. The pre-treated dispersion is now ready for addition of at least the polymer

DRA to complete formation of a stabilized polymer DRA slurry. Polymer DRA's are, in some embodiments, ultra-high molecular weight poly alpha olefins which have been formed by polymerization of a selected alpha olefin monomer or combinations of alpha olefin monomers. This polymerization may be either a solution polymerization, wherein the polymer is precipitated from the solution via addition of a non-solvent component, or a bulk polymerization wherein no solvent is included.

The polymer DRA is desirably added to the dispersion in a comminuted form, and in some non-limiting embodiments, in a relatively highly comminuted form. For example, the polymer DRA may be first granulated to relatively large

particulate form, followed by grinding to further reduce particle size. In some embodiments the particle size at the point of dispersion is desirably less than about 1 mm in diameter, and in other embodiments it is less than about 600 microns in diameter. Such small particle size helps, in itself, to maintain the suspension of the polymer DRA and also increases the rapidity of dispersion throughout the stream into which the polymer DRA slurry will be injected and wherein drag reduction is desired.

The polymer DRA may be added while the pre-treated dispersion including the liquid carrier and partially dissolved fatty acid wax is at any temperature wherein the partial dissolution of the fatty acid wax may be maintained. Such temperature is, in some non-limiting embodiments, higher than ambient temperature, but lower than the melting temperature for the fatty acid wax. Using such a closely-controlled temperature helps to avoid the possibility of precipitation of a significant portion of the dissolved fatty acid wax out of solution. While a stabilized polymer DRA slurry has now been formed, it is possible to include further components therein. Such additional component(s) may be added either before, concurrently with, or after addition of the particulate polymer DRA, but in all cases are added after the pre-treatment of the liquid carrier/fatty acid wax dispersion. Such additional components may include, for example, additional partitioning agents, wetting agents, and/or rheology modifiers that may in some cases be desirable to further enhance their imparted properties in a given slurry. Such enhancement may be desired according to all of the variables of a given system, including selection of each component of the slurry, the constituency and properties of the stream in which drag will be reduced, type of pumping equipment being used, desired flow rate, and the like. Materials known in the art to be useful for each of the types of additives may be used. For example, in certain non-limiting embodiments, one or more additional partitioning agents may be selected from the group consisting of fatty acid waxes (to be distinguished herein from the fatty acid waxes included in the pre-treatment of the dispersion including a fatty acid wax and a liquid carrier), polyolefin homopolymers and copolymers of various densities; oxidized polyethylene; polystyrene and copolymers; carbon black and graphites; precipitated and fumed silicas; natural and synthetic clays and organo-clays; aluminum oxides; talc; boric acid; polyanhydride polymers; sterically hindered alkyl phenol oxidants; magnesium, calcium and barium phosphates, sulfates, carbonates and oxides; mixtures thereof; and the like.

Additional wetting agents may, in some exemplary and non-limiting embodiments, be selected from the group consisting of fatty acid waxes, magnesium stearate, calcium stearate, stearamide, ethylene bis stearamide, nonyl phenol and nonyl phenol ethoxylates, and laureth carboxylic acid, as well as commercially available surfactants such as TWEEN™, SPAN™, BRIJ™, and MYRIJ™. These surfactants are available from Uniqema. Cationic and anionic surfactant types are of use also, such as, for example, cetyltrimethylammoniumbromide, sodium dodecyl sulfate, and sodium alkylbenzene sulfonic acid. Additional rheology modifiers may, in some exemplary and non-limiting embodiments, be selected from the group consisting of natural gums including, for example, gum arabic, xanthan gum, and guar gum, carrageenan, cellulosics such as hydroxypropyl methyl cellulose , carboxy methyl cellulose, hydroxypropyl cellulose, and hydroxyethyl cellulose, and natural clays. Relative proportions of all of the polymer DRA slurry constituents will, naturally, have an effect upon the final properties, including but not limited to stability to settling, separation and/or agglomeration, of the polymer DRA slurry. While a wide range of proportions may be employed according to the desirable properties of the final slurry, it has been found that, in certain embodiments, a ratio of polymer DRA to overall slurry ranging from about 10 to about 40 percent by weight is effective, while in other embodiments a ratio of polymer DRA to pre- treated dispersion may range from about 17 to about 26 percent by weight. Where additional partitioning agent is to be included it may be, in certain non- limiting embodiments, in the range of from about 2 to about 10 percent by weight, as compared to the overall slurry. Additional wetting agent may, in certain non- limiting embodiments, range from about 0.1 to about 2.0 percent by weight, as compared to the overall slurry, and additional rheology modifying agent may, in other non-limiting embodiments, range from about 0.01 to about 1.50 percent by weight. Once all constituents of the final slurry have been combined, and in some embodiments during such combination, appropriate mixing is desirable. Such may be carried out using any method and/or means known to those skilled in the art. The goal of mixing is desirable a relatively high level of homogenization, which serves to enhance consistency in the drag reducing performance of the product, and to reduce the occurrence of settling, separation and/or agglomeration later by

ensuring uniformity in the presence of each component such that partitioning and wetting actions are optimized. In some embodiments such mixing may be accomplished by use of a standard fixed blade agitator or high-shear impeller in a drum, tank or vessel for a time of from about 0.5 to about 4 hours at ambient temperatures.

The final slurry is, in some embodiments, a highly uniform polymer DRA slurry that is ready for shipment, storage and/or use for drag reduction in a variety of streams such as hydrocarbons, including, for example, crude oil, heating oils, liquefied natural gas, refined gasoline, and diesel fuel. It may be highly stable against settling, separation and/or agglomeration, even when stored for times exceeding six months and under a variety of conditions ranging from extreme cold (for example, as low as about -4O ⁰ F) to extreme heat (for example, as high as about 12O ⁰ F).

The following examples are included herein for illustrative purposes only, and are not intended to be, nor should they be construed as being, indicative in any way of the scope of the invention. Those skilled in the art will appreciate that many modifications may be made in the invention without departing from the spirit and scope, as defined in the appended claims, thereof. For example, the identity, nature and proportions of fatty acid wax, liquid carrier, polymer DRA, and other partitioning agents, wetting agents, and/or rheology modifying agents; times and temperatures of pre-treatment; equipment used to prepare any component or the slurry as a whole; and the like; may also be varied while remaining within the scope of the invention.

EXAMPLES Example 1

A 9.6 g quantity of calcium stearate was added to 150 g of 1-hexanol as a liquid carrier to form a dispersion. This dispersion was pre-treated by heating at 14O ⁰ F for about 4 hours. The result was a thick, milky-white suspension. This suspension was filtered to discover that less than about 1 percent by weight of the calcium stearate was present in a dissolved state. A 79.8 g quantity of this admixture was then combined with about 75 g of additional 1-hexanol, about 190.7 g of dipropylene glycol methyl ether, and 149.8 g of a precipitated poly alpha olefin polymer DRA material. The combination was then mixed with a dispersion-type mixer for about 15 minutes. The resulting polymer DRA slurry was stable toward

separation after sitting for several weeks, and exhibited a stable viscosity of 900- 1100 centipoise, as measured using temperature correction with a Brookfield DV- II+ viscometer using a "T-A" spindle at 20 rpm (helical path) at ambient temperature.

Example 2

About 26.4 g of ethylene bis-stearate was added to about 150 g of 1- hexanol and heated at about 14O ⁰ F for 4 hours. The result was a milky white suspension. About 88.2 g of this admixture was combined with about 75 g of additional 1-hexanol, 190.7 g of dipropylene glycol methyl ether, and 149.8 g of the same precipitated polymer DRA material as in Example 1. This combination was then mixed using a dispersion-type mixer for about 15 minutes. The resulting slurry was stable toward separation after sitting for several weeks and had a stable viscosity of 600-700 centipoise.

Example 3

A 4.1 g quantity of calcium stearate was added to about 250 g of 1-hexanol and heated at 132 ⁰ F for 1 hour while agitating with a Cowels mixer. The result was a thick, milky white suspension. This admixture was combined with 117.0 g dipropylene glycol methyl ether and 143.0 g of the same polymer DRA material as in previous examples, and then mixed with a dispersion-type mixer for about 15 minutes. The resulting slurry was stable toward separation after sitting for several weeks and had a stable viscosity of approximately 800 centipoise.

Example 4 (Comparative)

The procedures of Examples 1-3 were followed but the dispersion of the fatty acid wax (either calcium stearate or ethylene bis-stearate) and liquid carrier (1-hexanol) was not heated. It was found that the final polymer DRA slurries showed significant separation after several minutes.

Example 5 (Comparative)

A 4.5 g quantity of magnesium stearate was added to 208.7 g of 1-hexanol under ambient conditions (no heating). This admixture was then combined with 271.3 g of dipropylene glycol methyl ether and 212.0 g of the same precipitated polymer DRA materials as in previous examples. It was then mixed with a

dispersion-type mixer for about 15 minutes. The resulting slurry was found to show significant separation after sitting several hours.

Example 6 (Comparative)

A 154.8 g quantity of 1-hexanol was combined with about 190.7 g of dipropylene glycol methyl ether and 149.8 g of the same precipitated polymer DRA material as in previous examples. The admixture was then mixed using a dispersion-type mixer for about 15 minutes. The resulting slurry showed significant separation after sitting several minutes.