Langmuir 1996,11, 4143-4152
4143
Effect of Synthesis Parameters on the Properties of Calcium Phosphonate Precipitates F. Henry Browning and H. Scott Fogler" Department of Chemical Engineering, The University of Michigan, Ann Arbor, Michigan 48109-2136 Received April 7, 1995. I n Final Form: July 10, 1995@ Phosphonates are commonly used in industry for scale prevention in a wide variety of water systems. In many of these applications, the phosphonates are able to react with divalent cations such as calcium to form stable divalent cation-phosphonate precipitates. The focus of this paper is to identify and study how different precipitating factors will affect the resulting properties of calcium-phosphonate precipitates and to show how the formation of different precipitates can be used to enhance scale treatments in oil field applications. The phosphonateused in this study was 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP). The three precipitating conditionsunder study were (1)the pH of the precipitating solution, (2)the calcium/ HEDP molar ratio of the precipitating solution, and (3) the degree of supersaturation of the precipitating solution. The results in this paper showed that while the degree of supersaturation had a minimal effect on the resulting precipitate properties, the coupled effects of pH and calcium/HEDP molar ratio had a significant effect on theresultingprecipitate properties. At the extreme conditions,two distinct precipitates were synthesized: one comprised of fibrous spindles having a calcium/HEDP molar ratio of 1:l and the other comprised of spherical particles having a calcium/HEDP molar ratio of 2:l. Finally, micromodel experiments were carried out to test the performance of these two distinct precipitates with respect to phosphonate treatments in petroleum production systems. The results showed that the slow dissolution of the spherical particles and the morphology of the fibrous spindles were highly advantageous in slowing the phosphonate release from a porous medium, ensuring successful phosphonate treatments in oil field applications.
Introduction The precipitation of undesirable solids, commonly referred to as scale, is a major problem in many different facets of industrial and biological processes. Some of the more pronounced scaling problems occur in places such as cooling water towers, biological systems involving brines, and petroleum production systems. If this scaling problem is left untreated, some of the problems that could result include equipment foulingldamage andlor a loss in process efficiency. Hence, it is of vital importance to control this scaling problem. One of the most effective ways to combat the problem of scale is to inject threshold scale inhibitors into these problem areas to prevent or slow the crystal nucleation and growth of scale. Some of the most common threshold scale inhibitors used come from the family of inhibitors called phosphonates.2 The structures and physical properties oftwo well-knownphosphonates are shown in Figure l.3,4Some of the advantages that these phosphonates offer include the following. 1. Phosphonates are threshold scale inhibitors that have been shown to inhibit scale a t low concentrations (ppm). 2. Phosphonates are nontoxic chemicals stable over a wide range of temperatures and pH values. Hence, they will be able to inhibit scale in many systems a t different conditions. 3. Phosphonates have been shown to be effective at preventing many different types of scale, making them very flexible in various scaling system^.^,^
* To whom correspondence should be addressed. Abstract published in Advance ACS Abstracts, September 15, 1995. (1)Kan, A. T.; Oddo, J. E.; Tomson, M. B. Langmuir 1994,10,1450. (2) Vetter, 0. J. JPT 1972,997. (3) Dequest 2010Phosphonate. Monsanto PublicationNo. 9024,1986. (4)Dequest 2060 and 2066 OrganophosphorusCompounds. Monsanto Publication No. WT-8601, 1986. ( 5 ) Nancollas, G. H. Proceedings - at NACE Corrosion '87, San Francisco, CA, 1987. @
4. The concentration ofphosphonates in fluids is easily determined by oxidation and colorimetric techniques. Consequently, it is easy to determine when more inhibitor needs to be placed in a specific application.6 In many instances, phosphonates are placed into highsalinity waters where they can potentially precipitate with divalent cations such as calcium and magnesium. This phenomenon often occurs in oil field applications when phosphonates are injected into the subsurface and are left to interact with calcium-containing formation waters. This precipitation mechanism is, in fact, often advantageous in oil field treatments since the precipitation/ dissolution processes can enhance the amount of phosphonate placed during a treatment as well as the release characteristics of the phosphonates into the produced fluids during production. These advantages of phosphonate precipitation lead to enhanced treatment lifetimes, which is often how the success of a treatment is j ~ d g e d . ~ Phosphonate precipitation, however, can also be disadvantageous, notably in systems where equipment fouling can occur as a result of precipitation. Hence, it is important to have a firm understanding of the phosphonate precipitation process and to know the conditions under which phosphonate precipitation will occur. Given the importance of phosphonates and their precipitates discussed above, it is surprising that relatively little research has been done in the area of phosphonate precipitation chemistry and the factors that affect the properties of resulting divalent cation-phosphonate precipitates. Other researchers have recently studied and modeled the precipitation reaction of calcium with DTPMP (a phosphonate with five phosphate In this research, a model was developed to predict the stability constants and chemical compositionsof different calciumDTPMP solution complexes and precipitates a t different (6) Phosphonates. Hach Chemical Co. Publication, 1990. (7) Shuler, P. J. (ChevronPetroleum Technology Company). Personal communication, 1992. (8) Tomson, M. B.; Kan, A. T.; Oddo, J. E. Langmuir 1994,IO, 1442.
0743-746319512411-4143$09.00/0 0 1995 American Chemical Society
4144 Langmuir, Vol. 11, No. 10, 1995
OH I
Browning and Fogler
'i" PH
HO-~-~--$-OH
0
0
3
1-Hydroxyethylidene-1, 1-diphosphonic Acid (HEDP)
Diethylenetriaminepenta(methylenephosphonicAcid) (DTPMP)
Molecular Weight: 206 Color: Colorless to pale yellow In solutlon Speclflc Gravlty In Solutlon(BO% wt. HEDP): 1.45 Crystalllzationpt.: e 273 K
Molecular Welght: 573 Color: Colorless In rolutlon Spoclflc Gravlty In SolutIon(50%wt. DTPMP): 1.4 Crystalllzcltlonpt.: e 253 K
Acidity constants in 0.5 M tetramethylammonium chloride at 298 K:
Acldity constants In 0.1 M KCI at 298 K:
HEDPHEDP- + H+ HEDP-HEDP-? + H+ HEDP-2HEDP-3 + H+ HEDPSHEDP-4 + H+
pK14 pK2=2.54 pK3c6.97 pK4=11.41
pK14 pK2
