Measurement and Simulation of Low Vapor Pressure Blends of NGL and Stabilized Crude

Abstract

Due to operational constraints natural gas liquids (NGL) are sometimes blended with stabilized crude. This option is considered when there is a temporary surplus of NGL and for better NGL management. The NGL blend ratios are generally very small and conservative and are not meant to impact the crude quality. The blends have to meet rigorous vapor pressure specifications. A systematic study was conducted to determine the vapor pressures of different NGL/crude oil blends. The evaluation considered various factors that impact the vapor pressure of the blends, including the NGL/crude oil ratios, degree of crude stabilization, temperature, and the retention time in the pipeline and/or in the crude tanks. An important aspect of the study was to determine the "foaming tendency" and volume changes during blending. Vapor pressure curves were determined for different blends of stabilized crude with NGL as a function of temperature. The data included both the true vapor pressure and Reid vapor pressure of the blends. The experimental data were supplemented with equation of state simulations. Low vapor pressure data of blends are scarce, and the paper discusses the physical properties of NGL/crude oil blends. Introduction The gas plants occasionally reject NGL into stabilized crude streams to effectively manage the produced NGLs. This could be due to excess NGL inventories, process limitations or a temporary lack of customers for NGL. A small amount of NGL can be rejected into the stabilized crude streams instead of flaring it. A simplified schematic of the possible NGL rejections into various crude streams is shown in Fig. 1. The main rejections include C3+, C4 and C5 products into the Arab Light (AL) crude oil. At present, the rejection limits are based on empirical data and past experiences. Such limitations may be resulting in product and revenue losses. Considerable revenues gains can be achieved if the rejection rates can be safely maximized without impacting the crude quality. The availability of maximum safe rejection limits at various plants will also provide better NGL management and flexibility in operations. This paper provides the results of a comprehensive study that was conducted to provide the maximum safe rejection limits of various NGL into stabilized crude streams. The crudes included Arab Light (AL), Arab Extra Light (AXL) and Arab Medium (AM). The NGLs included C3+ NGL, C4 and C5 products, and pure NG. The results take into consideration all factors that impact NGL rejection into the crude, including crude stabilization depth, temperatures, and the retention time in the pipeline and/or in the crude tanks. The paper also provides comprehensive vapor pressure data of different NGL+crude blends, the "foaming tendency" and volume changes of the blends. The procedures described in this paper provide a framework for low-pressure PVT experimentation and can be used for determining the vapor pressures of crudes and low volatility hydrocarbon products. Experimental Details The systems and variables investigated in this study included:Crude Oils: Stabilized AL, AXL and AM crudes.NGLs: C3+ NGL, C4 and C5 products, Pure NG.Temperatures: 60–150°F.Blending Ratios: up to 5% (vol) rejection.Stabilization Depths: Shallow and Deep. The crude oil samples were collected from the crude stabilization plants, and the NGLs were collected from different gas plants. The samples were collected in pressurized cylinders and were not allowed to get contaminated with air. The fluid compositions of the samples were analyzed by gas chromatography, and the results are presented in Table 1 (crude oils) and Table 2 (NGLs). Three types of experiments were conducted with the samples and their blends:Low pressure vapor-liquid equilibrium (VLE) data including the true vapor pressure (TVP) and Reid Vapor Pressure (RVP).Blend densities for selected blends.Foaming tendency of the crude during NGL rejection.