Prevention of Refinery Tower Plugging by Residual Oil Gellant Chemicals in Crude: Pilot Plant Evaluation of Alternative Oil Gellants

Abstract

Abstract An earlier paper(1) described refinery plugging caused by volatile phosphorus components originating from phosphate ester oil gellants. Also documented were two successful field trials of new phosphonate ester oil gellants that were shown to address this problem. A second associated paper(2) presented results of additional field testing of phosphonate ester gellants directed at the optimization of cost and performance. In addition to the phosphonate ester systems previously tested, a new modified phosphate ester system has become available. The modified phosphate ester also reduces volatile phosphorus according to ASTM D86 distillation testing. However, several questions require further investigation. One of these is the comparative ability of phosphonate and modified phosphate esters to control volatile phosphorus and related tower fouling under more representative conditions. The D86 distillation procedure used to evaluate volatile phosphorus To date has a 250 °C endpoint. The actual bottom temperature in the towers is approximately 340 °C. Also, fluids typically have 2 to 4 minutes residence time in the tower bottom. However, the pot temperature in the D86 distillation is not specified as part of the procedure and may exceed 340 °C in some instances, even given that the endpoint measured in the distillate is 250 °C. Based on observed smoking of the oil samples in some distillations, the oil in the distillation flask may be cracking. Consequently, results may be inaccurate and difficult to reproduce. In addition, esters may decompose at elevated temperatures through a hydrolysis mechanism if water is present. Steam is injected into the bottom of the towers, serving as a source of water. However, the D86 test methodology does not include the addition of an aqueous phase. Another continuing area of concern has been organic halide formation under distillation tower conditions. Although no organic halides were detected in the flowback of the two initial field trials reported in an earlier paper(1), further testing under more realistic conditions is required. It has been hypothesized that on the larger scale of an actual distillation tower, water hydrolysis may occur. This could result in localized areas of acidity and causticity. In the presence of acid at elevated temperatures, halogenation reactions could occur if halogen ions, such as chloride ions, are present. Any carryover of salts not removed in the de-salters, could serve as this source of halide ions. To address these questions in the most meaningful way possible, full-scale pilot plant testing was conducted over several days with flowback captured after actual fracturing treatments (see Figure 1). Fouling of distillation tower trays were measured, as well as fouling of the packing material. In addition, any changes in the operating parameters such as rate, temperature or pressure during each test were noted because these could also be indicative of fouling. Three full-scale pilot evaluations were conducted using actual flowback fluids from fracturing treatments conducted with three different oil gellants:Convention phosphate ester currently in use. The purpose of this test is to serve as a blank for comparison and for validation of test methods. We expect tower fouling with this product. If no fouling occurs, the test method is invalid.