Factors affecting performance of crude oil wax-control additives
John S. Manka
Lab studies define application procedures that can reduce effectiveness of pour-point depressant chemicals on crude oils and what can be done about it Crude oil producers have used pour-point depressants (PPDs) with great success for several decades. However, chemical wax-control packages can be plagued by crude-oil specificity, large package treating rates, and waxy components that can be hard to apply.
This article discusses what factors affect the performance of crude oil wax-control additives and what contributes to crude-oil “specificity,” as will be defined. This study investigated effects of the wax-control additives’ polymer backbone, pendant chains and molecular weight on cold-flow performance. The impact of solvents used, the extent of the packages’ dilution and the effect of proper additive dosing is addressed.
Concluding comments summarize the study, noting in particular that effective mixing of the additive into the crude oil has a great effect on PPD performance. And specificity can be overcome by using mixtures of depressants to be able to treat a broad range of crude-oil wax distributions.
INTRODUCTION
Handling and transporting crude oil represents a major challenge for producers. Transporting crude from its source to the refinery can involve subsea umbilicals, pipelines, intermediate storage tanks and transportation vessels, all of which depend on the crude remaining liquid in the various environments and transportation conditions.
Oil producers use several methods to ensure a liquid product and an uninterrupted crude flow. These methods include heating, dilution with lighter stocks, and the preferred method, “additizing” with PPDs.
Pour-point depressants have been used with great success for several decades. However, they can be plagued by crude-oil specificity, large package treating rates, and waxy components that can be hard to apply. [1] This article discusses what factors affect the performance of crude wax-control additives and what contributes to crude-oil specificity.
Crude oil, pour-point depressants (PPDs). The pour point of crude oil is the lowest temperature at which crude movement is observed. In this test, after preliminary heating, the crude is cooled at a specific rate and examined at intervals for movement. The lowest temperature at which movement of the specimen is observed is recorded as the pour point.
When the crude reaches this point, the sample is not frozen solid. What actually happens is that paraffins in the crude crystallize and form a matrix of wax crystals. The wax-crystal matrix holds the bulk of the liquid portion of the crude within it. By trapping the liquid portion within the matrix, the crystals prevent the liquid in the crude from flowing, and the sample no longer moves. Anything that disrupts formation, or properties of, the wax-crystal matrix, such as PPDs, will affect the pour point.
Wax-control additives (WCAs), which include crude-oil PPDs, are polymers with pendant hydrocarbon chains, that interact with paraffins in the crude and thus inhibit the formation of large wax crystal matrices, Fig. 1. The interaction retards crystal formation/growth, alters the paraffin’s heat of crystallization and, subsequently, depresses the crude’s pour point while affecting crystal size/shape. [2]
Examples of the types of chemistries used as crude PPDs include: ethylene-vinyl-acetate- copolymers, vinyl-acetate-olefin copolymers, alkyl-esters of styrene-maleic-anhydride copolymers, alkyl-esters of unsaturated-carboxylic acids, polyalkylacrylates, polyalkylmethacrylates, alkyl phenols, and alpha-olefin copolymers.
Crude oil specificity. WCAs for crude oils are plagued by “specificity.” This is exhibited when a cold-flow pack age will work only for a specific crude. Even slight changes in the crude’s wax composition can reduce or cause a total loss of performance.
Specificity arises from the interaction between pendant chains of the WCAs and waxes present in the crude. Waxes in the crude are specific to the crude but can, and do, change over time. If the crude’s characteristics change, i.e., the wax distribution changes, the control additive may no longer be well matched, and pour-point performance suffers. If the crude change is slight, an increase in the WCA’s treat rate often restores the performance. But if the crude’s change is more significant, change to a new WCA package may be needed to restore pour-point depression.
Experimental procedures. A modified ASTM D97-96A Standard Test Method for Pour Point of Petroleum Products was used to determine pour points of the crude oils tested. In this test, after preliminary heating, the sample is cooled at a specified rate and examined at intervals of 1[degrees]C for flow characteristics (in the unmodified test, 3[degrees]C intervals are used). The lowest temperature at which specimen movement is observed is recorded as the pour point. Repeatability of this measurement is less than 2.52[degrees]C, and reproducibility is less than 6.59[degrees]C.
The ASTM D 445 Standard Test for Kinematic Viscosity of Transparent and Opaque Liquids was used to mea sure viscosity of the diluted pour-point packages.
The gel-permeation chromatography, molecular-weight determinations were done using a Waters 2690 unit equipped with a Waters 410 refractometer. Molecular-weight determination was made on the analytical sample, and the values reported are given relative to polystyrene standards. The relative calibration curve was derived from a set of 12 narrow polystyrene standards (PSS ReadyCal) injected in duplicate.
The crude-oil PPDs used in this study (PPD A through H) included both experimental and commercially available packages. The solvents were commercial grade, and were used as received.
The crudes employed were chosen because they represented a variety of different geographical areas of North America, Europe and Asia.
FACTORS AFFECTING PERFORMANCE OF WCA
Crude-oil PPDs are polymers made from discrete monomers. The polymers have three variable characteristics that may affect their performance: polymer backbone, length of pendant chains and polymer molecular weight. The backbone and pendant-chain length can be changed by using different monomers. Polymer molecular weight can be changed by adjusting reaction conditions, amount of initiator used, etc.
Effect of polymer backbone.
The polymer backbone is not believed to play a large role in pour-point depression performance of “comb-type” polymers. It is believed that the backbone only provides a structure from which the important pendant chains are suspended. [3] This is not necessarily the case in ethhylene-vinyl-acetate copolymer PPDs, so these copolymers will not be considered here.
To investigate effect of the polymer backbone, PPDs with the same pendant chain lengths, but different backbones were tested. Every effort was made to keep molecular weight of the polymers the same (vida infra). Table 1 shows performance testing of the PPDs with different polymer backbones.
Data shows that the polymer backbone has a slight but statistically significant effect on performance of the WCA.
Effect of pendant-chain length.
The interaction between WCAs and paraffin in the crude is crucial, and the additives work best when they are matched to the paraffin distribution in the crude.
Fig. 2 shows the effect on performance that qualitatively matching the PPD’s pendant-chain length to the wax in the crude has on the additized crude’s pour point. The various PPDs were dosed into the crude at equal active chemical.
As the average carbon number of the pendant chain on the PPD increases, pour point of the additized crude drops until it reaches a minimum, and then increases again. The minimum in the data shows that PPD E’s average pendant-chain length is most closely matched with the paraffin distribution in the crude and the greatest pour-point depression results.
Effect of polymer molecular weight. Molecular weight of the wax-control polymer may affect interaction of the polymer with the paraffins. A very short, low-molecular-weight polymer may not have the molecular volume to disrupt the paraffin crystals as it co-crystallizes within the paraffin matrix. A very long, high-molecular weight polymer may be so large that it interacts with itself instead of the crude-oil paraffins, or the polymer’s solubility in the crude may be limited and actually initiate paraffin crystallization, and thus raise the crude’s pour point.
Table 2 shows the effect of molecular weight on pour point performance of a family of depressants. The backbone and side-chain lengths are the same for each polymer, only the molecular weight varies.
The data shows that, over the molecular-weight range tested, neither weight-average molecular weight, number average molecular weight nor peak molecular weight affects pour-point performance of the WCAs. The monomer has no pour-point depression activity, as one would expect. Although the monomer may interact with the paraffins, its “molecular volume” is apparently too small to disrupt paraffin crystal formation.
Effect of solvent/dilution on WCAs. Undiluted PPDs are waxy materials that are often solids at ambient temperature. To pump these products in the field, they usually need to be drastically diluted with solvent. Therefore, solvents comprise a very large portion of these finished formulations to make a product that can be handled.
Much research has gone into developing and testing new and existing polymers, additives and formulations for this application. However, few studies have examined the role that solvent plays on performance of the cold-flow polymer. [3,4]
Effect of solvent. It is generally accepted in polymer science that solvents have a large effect on physical proper ties of polymers. It is known that solvent influences the effective hydrodynamic specific volume of the polymer. This property is indicative of the degree that a polymer interacts with the solvent, and is a measure of how coiled or uncoiled the polymer is in that solvent. The degree of polymer coiling may be important because a solvent that causes the polymer to be highly coiled and interact predominantly with itself may be less likely to interact with paraffins of the crude. And it may have less pour-point performance than the same polymer dissolved in a solvent in which the polymer is well solvated, uncoiled and readily accessible.
In a “good” solvent, the polymer maximizes its interaction with the solvent; and it is uncoiled and has a large radius of gyration, and thus a large effective hydrodynamic specific volume. In a “poor” solvent, the polymer minimizes inter action with the solvent, leading to a coiled conformation which exhibits a small radius of gyration and, thus, a small effective hydrodynamic specific volume. The concept of radius of gyration and effective hydrodynamic specific volume is shown schematically in Fig. 4.
Package viscosity is used to determine which solvents are “good” and “poor” for the polymers. Good polymer solvents exhibit a higher package viscosity than poor solvents. Good solvents cause the polymer to be fully expanded, maximizing its interaction with the solvent. This expansion causes the polymer/solvent complex to act like a very large molecule with an effective increase in viscosity. In an analogous manner, “bad” solvents cause the polymer to act as a small molecule, with a corresponding, lower apparent viscosity.
To investigate effect of the solvent used on the wax-control package per formance, PPDs C and D were dissolved in various solvents, and their package viscosity and PPD performances were assessed.
Table 3 shows viscosity of the polymers at equal-volume “actives” in various solvents. The data shows that viscosity of the solvent-polymer package varies greatly with solvent. Solvents that exhibit high package viscosity are good solvents for this particular polymer and those with low package viscosity are poor, for this particular polymer.
Based on viscosity data in Table 3, pour-point packages using heptane, No. 2 diesel fuel and methylene chloride were chosen, additized into crude oil, and their pour points were determined. Heptane, diesel fuel and methylene chloride were selected because they represent poor, intermediate and good sol vents for these particular polymers.
Results in Table 4 show that, within repeatability of the ASTM D97-96A standard test method, regardless of whether a good, intermediate or poor solvent for the polymer is used, wax control performance is the same.
The solvent has no effect on pour point performance because the solvation from the solvent used in the package is immediately lost upon addition to the crude. Upon addition, the wax-control polymer is solvated exclusively by the crude. Subsequently, identity of the sol vent used in the package is not important to ultimate pour-point performance.
Effect of polymer dilution. It is generally accepted in polymer chemistry that polymer concentration in a solvent has a large effect on physical properties of the polymer. [5]
The polymer concentration in the sol vent will affect interaction extent of the polymer with itself. At high concentrations, the polymer may interact with other polymer molecules and become entangled. This entanglement may impact accessibility of the polymer to the paraffin in the crude and, thus, may impact performance of the cold-flow modifier package. At low concentrations, the polymer is fully solvated and should not interact with other polymer molecules, and it should be very accessible to the paraffins in the crude oil.
Table 5 shows the effect that dilution has on the performance of WCAs in a Gulf of Mexico crude. PPD G and PPD H were diluted with xylene. The effect of the xylene is also shown.
Within repeatability of the ASTM D97-96A test method, extent of dilution of a wax-control product has no effect on package performance. Upon its addition, the wax-control polymer is “infinitely dilute” at the typical treating rates for pour-point-depression performance of these polymers in crude oils. Infinitely dilute means that one polymer has no interaction with, nor any effect on, another polymer in the solution. Subsequently, the extent of dilution of the depressant package is not important to ultimate pour-point performance.
Effect of mixing. People have reported that dilute, cold-flow packages actually had better performance than concentrated packages, even when added to crude at equal polymer actives. These results led some to believe that dilution was beneficial 6 In the previous section, it was showed that dilution had no effect on performance.
To explain why some labs have seen this apparent dilution benefit, the mixing of the cold flow polymer into the crude was studied to see if more efficient mixing was the cause of the enhanced performance of diluted polymers.
For this investigation, the extent of mixing was varied, and the effect on pour-point performance was noted. To investigate the effect of mixing, three samples of the crude oil used in Table 4 were used with 200 ppm of PPD C. For Sample 1, PPD C was added to the crude via micropipette, and the crude was not mixed. Sample 2 was blended the same way, but gently mixed by hand for 5 sec. Sample 3 was blended the same way, but was mechanically shaken for 5 min. ASTM D97-96A pour points were then run on these three “additized” samples.
Next, the effects of mixing and polymer dilution were determined by making three samples of the same crude, blended with 800 ppm of a three-fold, diluted version of PPD C. This delivers the same amount of active pour-point polymer as in the mixing study in the previous paragraph. Again the extent of mixing was varied. In the preparation of Sample 4, the diluted PPD C was added to the crude via micropipette, and the blend was not mixed. Sample 5 was prepared the same way, but was mixed by hand for 5 sec. Sample 6 was mixed the same way but was mechanically shaken for 5 min. Pour points were then run on these three samples.
Table 6 shows the effect that mixing and dilution have on the performance of wax control additives in this crude. The data shows that the extent of mixing has a great effect on performance of the PPDs. Again, dilution has no effect on PPD, provided mixing is efficient.
OVERCOMING SPECIFICITY
Specificity arises from the interaction between the WCA’s pendant chains and the waxes present in the crude. Fig. 3 schematically shows how matching the control additive to the crude’s wax distribution leads to specificity.
WCAs work best when they are well matched to the wax distribution of the crude they are treating. [1] If the crude’s wax distribution changes, the control additive is no longer well matched, and pour-point performance suffers. It may even be necessary to use a different depressant. However, if the wax distribution changes yet again, the depressant may need to be changed again.
By using mixtures of WCAs, a broader range of wax distributions can be targeted, and specificity should often be overcome.
To investigate this proposed technique of addressing specificity by using a mixture of depressants, a “blended crude oil” was made by mixing equal proportions of two crudes, each only responsive to a particular pour-point depressant. The crude already discussed in Table 5 was used, which responds to PPD G and the crude described in Fig. 2, which responds to PPD E. The blended oil made from these crudes will have a different wax distribution than the individual crudes, and should not respond to individual pour-point depressants PPD G or PPD E, but should respond to a mixture of the two depressants.
Table 7 shows that the ASTM D97-96A pour point of the blended crude falls between the pour points of the two individual crudes, as one would expect. It is seen that neither PPD E nor PPD G have a significant effect on the blended-crude’s pour point. However, combining PPD E and PPD G did successfully depress the blended crude’s pour point. These results prove the concept of overcoming specificity using additive blends.
CONCLUSIONS
This article discussed what factors affect performance of crude oil wax-control additives. The data shows that:
1. Over the molecular weight range tested, neither weight-average molecular weight, number-average molecular weight nor peak molecular weight affects pour-point performance of the wax-control additives.
2. The polymer backbone has a slight but statistically significant effect on control additive performance.
3. The most important variable to wax-control performance is identity of the polymer’s pendant chains.
4. The interaction between wax-control additives and paraffin in the crude oil is crucial, and the additives work best when they are matched to the paraffin distribution in the crude.
5. The solvent used in the wax-control package has a great effect on the package viscosity, but has no effect on performance of the wax-control package.
6. Dilution also has no effect on wax-control package performance.
In summary, effective mixing of the additive into the crude oil has a great effect on performance of pour-point depressants. And specificity can be overcome by using mixtures of depressants to be able to treat a broad range of crude oil wax distributions.
ACKNOWLEDGMENT
The authors would like to thank Dr. Dennis M. Dishong for his contributions to this presentation. This article was prepared from paper SPE 67326, Factors affecting the performance of crude oil waxcontrol additives, presented by the authors at the SPE 2001 Production and Operations Symposium, Oklahoma City, Oklahoma, March 24-27, 2001.
Dr. John S. Manka, research manager for the Process Chemicals Group, The Lubrizol Corp., received a BS in chemistry from Canisius College, Buffalo, New York and a PhD in organic chemistry from the State University of New York, Buffalo. He has 10 years’ experience in additive research and formulation, including four years in the area of fuels, refinery and oilfield products.
Kim L. Ziegler is a laboratory technician in the Process Chemicals Group, The Lubrizol Corp. She has six years’ experience in additive research, in cluding four years in the area of fuels, refinery and oilfield products. Lubrizol is a worldwide supplier of performance chemicals for fuel lubricants and other specialty markets.
LITERATURE CITED
(1.) Manka, J.S., J.S. Magyar and R. P. Smith, “A novel method to winterize traditional pour point depressants,” paper SPE 56571, 1999.
(2.) Manka, J. S. and T. M. Sopko, “The effect of cloud point depressants on diesel fuel properties,” paper 982575, Society of Automotive Engineers, 1998.
(3.) Beiny, D. H. M., J. W. Mullins and K. Lewtas, “Crystallization of N dotriacontane from hydrocarbon solution with polymeric additives,” Journal of Crystal Growth. Vol. 102. pp. 801-806, 1990.
(4.) Madsen, H. E. L. and J. C. S. Faraday, “Solubility of octacosane and hexatricosane in different alkane solvents,” Trans. I. Vol. 75, pp. 1254-1258, 1979.
(5.) Qian, J. W., et al., “Solvent effect on the action of ethylene-vinyl acetate copolymer pour point depressant in waxy solutions,” Journal of Applied Polymer Science, Vol. 60, pp. 1575-1578, 1996.
(6.) Xiong, C-X., “The structure and activity of polyalphaolefins as pour point depressants.” Lubrication Engineering, p. 196, March 1993.
Effect of polymer backbone on pour-point depressant performance
Polymer backbone, No. Pour point, [degrees]C
None 15
1 -1
2 -4
3 -3
4 -5
Effect of Molecular weight on pour-point
performance of 400 ppm Depressant E
Batch M number M weight M peak Pour point, [degrees]C
None — — — 24
Monomer E 489 489 489 24
1 56000 78700 86700 9
2 33000 42200 40300 11
3 64000 96200 96000 10
4 54000 81400 90000 9
5 61000 88900 92700 10
6 78000 133000 124000 11
7 62000 94000 94000 10
8 54000 75000 85000 9
9 34000 43000 41400 10
10 71000 128000 119000 12
Effect of solvent on 25% volume active pour-point depressant
Viscosity, cSt @
Solvent 25[degrees]c
PPD C PPD D
Kerosene 9.1 21.8
Heavy aromatic naptha 7.3 16.4
Petroleum naptha 4.9 16.0
Light aromatic pet naptha 7.0 10.8
Xylene 3.9 8.4
Heptane 2.0 5.5
Hexane 4.1 10.6
Methylene chloride 66.7 218.9
Methyl iso-butyl ketone 3.7 7.7
No. 2 diesel fuel 18.6 43.8
Effect of package dilution on pour-point depression
PPD % Actives Treat rate, ppm Pour point, [degrees]C
None — — 35
G 100 300 -8
50 600 -8
5 6,000 -6
None — — 35
H 100 300 1
50 600 1
10 3,000 0
None — — 35
Xylene 0 300 [*] 32
0 600 [*] 32
0 6,000 [*] 33
(*.)Of the neat solvent xylene.
Effect of solvent on pour-point depressant performance
Treat rate, ppm Solvent Pour point
[degrees]C
PPD C
— None 13
200 Heptane 1
200 Diesel 1
200 Methylene chloride 1
PPD D
— None 13
200 Heptane 7
200 Diesel 7
200 Methylene chloride 9
None
— None 13
200 [*] Heptane 11
200 [*] Diesel 13
200 [*] Methylene chloride 13
(*.)Of the neat solvent.
Effect of mixing on pour-point depressant performance
Sample No. PPD Treat rate, ppm Mixing Pour point, [degrees]C
— None — — 13
1 C 200 None 12
2 200 5 sec. 10
3 200 5 min. 0
— None — — [*] 13
4 1:3 C/ 800 None [*] 11
heptane
5 800 5 sec. [*] 10
6 800 5 min. [*] 1
(*.)Mixing and dilution.
Overcoming specificity by using multicomponent packages
Crude Treat rate, ppm PPD Pour point, [degrees]C
From Table 5 — None 35
From Fig. 2 — None 24
Blended — None 29
From Table 5 — None 35
300 G -8
300 E 34
From Fig. 2 — None 24
300 E 9
300 G 25
Blended — None 29
600 E 22
600 G 19
300/300 E/G 0
[Graph omitted]
[Graph omitted]
COPYRIGHT 2001 Gulf Publishing Co.
COPYRIGHT 2001 Gale Group
