Some Insights Into the Tube-Blocking-Test Method To Evaluate the Efficiency of Mineral Scale Inhibitors

Abstract

Abstract The Dynamic Tube Blocking Test (TBT) method is used to evaluate the efficiency of chemical inhibitors to prevent the formation and deposition of mineral scales such as calcium carbonate, as well as calcium, barium and strontium sulfates. The TBT method aims at (1) determining the Minimum Inhibitor Concentration (MIC) required to prevent the formation of scale, and (2) performing comparative tests with different inhibitors in the same conditions. Inhibitor efficiency is measured by the ratio of the time needed to block the tube in the presence of inhibitor divided by the time needed to block the tube without inhibitor, i.e. blank time. Conditions for the blank test are adjusted to induce a significant precipitation in a reasonable time frame. Main adjusting parameters are solution degree of saturation of scaling species and flow rate. Other parameters such as capillary length and diameter can be chosen arbitrarily. The purpose of this study is to check the accuracy of the TBT method and to compare the MIC values obtained in various experimental conditions. Three types of inhibitors were used, namely, phosphonates, polyacrylates, polyaspartates and a carboxymethylated polysaccharide. Scales were calcium carbonate, calcium sulfate and barium sulfate. The results show that the TBT method is very sensitive to the effect of flow rate and capillary dimension.A careful control and monitoring of the temperature is also required.The TBT method is shown to give information on the physical mechanisms acting on the scale inhibition process. Introduction Inhibitor performance to prevent scale formation is evaluated in terms of Minimum Inhibitor Concentration (MIC). Laboratory test protocols use the Jar Test and the Tube Blocking Test (TBT). The conventional Jar Test is performed routinely in the industry and the tests procedures are described in the NACE standards N° TM 0197–97[1] and TM0374–2001.[2]Such a test aims at ranking the various products in static conditions and is generally completed with TBT which is more appropriate for inhibitor evaluation in dynamic conditions. However, on the contrary of Jar Tests, there is no standard method for the TBT. [3,4] Results obtained from dynamic measurements often give rise to a different ranking than tests performed in static conditions. The discrepancies are explained in terms of residence times which are much longer in the static method (a few hours) than in the dynamic method (a few minutes).[5] Moreover, depending on their chemistry, some inhibitors are efficient to prevent crystal growth while others are more efficient to prevent nucleation. [6,7,8]Bothmechanisms are time dependent. A test in static condition may favor products which prevent crystal growth while nucleation inhibitors are more efficient when tested in dynamic conditions. Tests results can thus be misinterpreted if the most relevant mechanism of inhibition is not considered. Despite some efforts to rationalize the TBT, there is still a lack of methodology for selecting the best inhibitor treatment by this technique.[9]In the TBT method, the differential pressure across a thin steel tube is monitored as a function of time while the two components of the scaling brine are injected at the inlet of the tube. Inhibitor efficiency is measured by comparing the time needed to block the tube in the presence of inhibitor to the time needed to block the tube without inhibitor, i.e. blank time. The main experimental parameters in this method are the capillary geometry, length and diameter, and the flow rate.[10]