Investigating barium sulfate scale and how additives influence crystallisation
Barium sulfate is an undesired product in some industrial processes, most notably in off-shore oil production where the changes in temperature, the mixing of aquifer and sea water, combined with the changes in pressure, conspire to precipitate barium sulfate both in the pores of the formation rocks as well as in the pipes used to collect the oil. It is normal practise to add precipitation inhibitors in order to prevent this precipitation and make the process viable. It is not surprising then, that there is interest from both a fundamental and from an industrial view to determine how these additives work and thereby to derive better, more environmentally friendly, additives.
Above: Aged (12 months) barite crystals grown in the presence of EDTP
Above: Barite precipitated in the presence of EDTP and calcium cations
Above: Barium sulfate crystals precipitated in the presence of HEDTMP and zinc cations
Above: Barium sulfate particles formed in EDTA and NaCl
Above: AFM image of the (001) surface after dissolution at pH 2
Work within the NRI in this area is a collaborative, multi-pronged approach dedicated to understanding the mechanisms at play at the molecular level. In this way, a thorough understanding of the interactions occurring at the molecular as well as bulk level can be obtained. As an overview of what we do, below are some projects that have been part of the work on understanding barite scale.
Effect of cations
We have undertaken to investigate the effect of cations since most precipitation does not occur in ‘pure’ water. We have found in the case of calcium for instance that the effect is not just one of increased ionic strength.
F. Jones, A. Oliveira, G. M. Parkinson, A. L. Rohl, A. Stanley, T. Upson, "The effect of calcium ions on the precipitation of barium sulphate 1: calcium ions in the absence of organic additive", J. Crystal Growth, 262, 572-580(2004).
When cations and organic additives are present it is found that the inhibitive strength of the organic additive is dependant on the ‘free’ organic.
E. Barouda, K. D. Demadis, S. R. Freeman, F. Jones, M. I. Ogden "Barium Sulfate Crystallization in the Presence of Variable Chain Length Aminomethylenetetraphosphonates and Cations (Na+ or Zn2+)", (submitted)
Effect of phosphonates
The most commonly used precipitation inhibitors are phosphonate based. A series of phosphonates have been investigated from both a computational perspective, as well as experimentally. It has been found that the speciation of the phosphonate is important, as is the number of phosphonate groups.
F. Jones, A. Stanley, A. Oliveira, A. L. Rohl, M. M. Reyhani, G. M. Parkinson, M. I. Ogden, "The role of phosphonate speciation on the inhibition of barium sulfate precipitation"J. Crystal Growth, 249, 584-593(2003).
It has also been found, not surprisingly, that for barite the carboxylate group is not as effective as the phosphonate functional group for inhibition.
F. Jones, J. Clegg, A. Oliveira, A. L. Rohl, M. I. Ogden, G. M. Parkinson, A. M. Fogg, M. M. Reyhani, "Anomalous behaviour within a systematic series of barium sulfate growth modifiers", CrystEngComm, 40 (2001) 1-3.
Interestingly, the computational studies were able to predict those faces of barite most likely to be affected by the presence of the phosphonate and carboxylate additives.
F. Jones, W. R. Richmond, A. L. Rohl (2006) "Molecular modelling of phosphonate molecules onto barium sulfate terraced surfaces"J. Phys. Chem. B, 2006110 7414.
F. Jones, A. L. Rohl (2004), "Empirical molecular modelling of crystal growth modifiers", presented at the Pacific Rim Conference in NanoScience, Broome, Western Australia, Sept 7-12, 2004, published in special edition of Molecular Simulations, 31(6-7), pp393, 2005.
Molecular Dynamics
Recently, we have undertaken molecular dynamics investigations into the effect of water on the crystallization kinetics of barium sulfate. It has been found that water adsorbs very strongly onto the (001) and (210) faces in particular and leads to a kinetic barrier to 2D nucleation. This kinetic barrier can then explain why the (001) face is the dominant face at low supersaturations despite it not having the lowest surface energy from thermodynamic considerations.
S. Piana, F. Jones, J. D. Gale, “Assisted desolvation as a key kinetic step for crystal growth” J. Am. Chem. Soc., 128(41) 13568-13574
