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This post was last edited by LQ198619 on 2017-5-22 at 17:23. Chapter 5: Basics of Gas Fractionation Units – Lecture 1. Overview of the Unit: With the development of the petrochemical industry, the comprehensive utilization of liquefied gas from refineries has received increasing attention. Fractions such as propylene, propane, isobutylene, and butadiene serve as raw materials for producing high-value chemical products. The gas separation unit in a refinery serves to desulfurize the liquefied gas coming from the catalytic unit, and through distillation to separate products such as dry gas, propylene, propane, and mixed C4. The gas separation unit consists of desulfurization and gas fractionation. l Desulfurization: It involves removing inorganic sulfur (H2S) and organic sulfur (thiols) from liquid hydrocarbons produced via catalysis, so that the sulfur content in the refined liquid hydrocarbons is within acceptable limits. l Gas fractionation: Using desulfurized liquid hydrocarbons as raw materials, they are distilled and separated into fractions such as propane, propylene, and mixed C4 hydrocarbons. II. Raw materials and products of gas separation units: Depending on the products, production capacity, and process requirements, and also to meet the production needs of downstream MTBE and polypropylene units, a three-tower process with relatively low investment and operating costs is generally adopted for the production of refined propylene, propane, and mixed C4, etc. Raw material for the gas separation unit: liquid hydrocarbons from catalytic cracking. Products of the gas separation unit: propylene with a purity of 99.6%, mixed C4, propane, and dry gas. III. Basic principles of the gas separation unit: Since the desulfurization process in the gas separation unit is similar to that used in catalytic units for liquefied gas desulfurization, the effects of sulfur compounds on petroleum processing and its products will not be discussed here. Nor will the corrosion of equipment during processing and storage, nor the impact on the stability of stored oil products and the resulting deterioration of those products, be addressed. The principle of gas separation: The gas separation unit uses liquefied gas as raw material, and through distillation, it produces high-purity propylene, propane, and mixed C4 fractions. In gas fractionation, distillation principles are applied in various columns. Under specific temperature and pressure conditions, the different volatilities of the components to be fractionated are utilized; that is, each component has a different boiling point at the same pressure. If the components to be separated have identical or similar boiling points under the same pressure, then conventional distillation methods cannot be used to separate them. The essence of the distillation process is that the unbalanced vapor-liquid phases come into contact with each other multiple times in reverse directions within the tower, undergoing partial vaporization and partial condensation on various occasions. This facilitates heat and mass transfer, resulting in an increasing concentration of light components in the vapor phase and an increasing concentration of heavy components in the liquid phase, thereby enabling thorough separation of the mixture. The most basic equipment for the gas fractionation process consists of a distillation column, a condenser, and a reboiler. Trays are installed inside the tower. Above the feed plate lies the distillation section, and the distillation process requires the following basic conditions: a. The relative volatility of the substances to be separated is not equal to 1. b. The gas and liquid phases must exist simultaneously and come into counter-current contact with each other. c. The two phases in contact with each other are not in equilibrium, that is, there are temperature differences and concentration differences. d. It has a certain number of trays. The most basic equipment for gas fractionation consists of a distillation column, a condenser, and a reboiler. The tower is equipped with trays; the section above the feed tray is the distillation section, while the section below it is the stripping section, and the distillation process takes place within the tower. The feed material enters the feed plate, where its vapor and liquid phases mix with the vapor and liquid phases inside the tower under the temperature and pressure conditions of the feed plate. The vapor phase rises to the distillation section, while the liquid phase flows downward to the stripping section. Through the distillation action of the trays, as one moves upward along the height of the tower, the concentration of the light components increases while the temperature decreases ； Moving downward along the tower height, the concentration of heavier components increases, as does the temperature. The tower top distillate is condensed in the tower top condenser; part of it is drawn off as the tower top product, while the rest is sent back to the top of the tower as tower top reflux. The bottom of the tower is heated by a reboiler, which warms the material at the bottom of the tower and causes some of the light components contained in it to evaporate, forming steam within the tower. A portion of the liquid phase at the bottom of the tower (or reboiler) is drawn off and sent back into the tower, while another portion is taken as the product. IV. Schematic diagram of the basic process of gas fractionation V. Typical process flow of the gas fractionation unit. Depropanization tower system: The liquefied petroleum gas feedstock coming from the autocatalytic cracking unit enters the feed buffer tank of this unit; it is then pumped by the depropanization tower feed pump to the feed heater, where it exchanges heat with circulating hot water at 120°C to reach a temperature of 80°C before entering the depropanization tower for the separation of C3 and C4 components. The C3 gas at the top of the tower is condensed by the depropanization tower condenser and then sent to the depropanization tower reflux tank. One portion of the liquid is pumped back by the depropanization column reflux pump, while another portion is sent to the deethanization column by the deethanization column feed pump as feed for that column. The C4 liquid at the bottom of the tower flows out by itself; after being cooled to 40°C in the bottom cooler, it is sent as feed to the MTBE unit. At the bottom of the tower, there is a depropanization reboiler. This reboiler uses circulating hot water at 120°C as the heating medium (it is also possible to use the oil from the top of the catalytic cracking unit’s distillation column as a heat source, with this heat source being recycled); this setup helps to maintain the temperature at the bottom of the tower at 110°C. The liquid flowing from the bottom of the tower enters the reboiler, where it partially vaporizes before returning to the space at the bottom of the depropanization unit. Deethanizer system: The feed to the deethanizer, coming from the deethanizer feed pump, is heated to 65°C by a feed heater before entering the deethanizer to remove components with molecular weights below C2. The gas at the top of the tower is condensed in the degasifier top condenser, and then enters the degasifier reflux tank; a small amount of ethane gas present at the top of the tank is discharged to the plant’s fuel gas network through a pressure control valve ； The liquid, as total reflux, is pumped back to the top of the tower by the deethanizer reflux pump. The liquid at the bottom of the tower serves as the feed for the propylene tower and is sent there under its own pressure. At the bottom of the tower, there is a deethanizer reboiler. This reboiler uses circulating hot water at 80°C as the heating medium to maintain the bottom temperature at 60°C. The reboiled liquid enters the reboiler from the bottom of the tower; after partial vaporization, it returns to the bottom section of the depropanizer. Propylene column system: Feed from the low-self-pressure deethanizer enters the propylene column for the separation of propylene and propane. The gas from the top of the propylene column is condensed in the condenser at the column’s top, and then flows into the reflux tank there. The liquid is pumped out by the reflux pump located at the top of the column; part of this liquid is used as reflux and sent back to the top of the column, while the other part is cooled in the propylene cooler before being exported as propylene product ； The bottom propane, as a product, is cooled by the propane cooler and then pumped out of the unit by the propane transfer pump. At the bottom of the tower, there is a deethanization reboiler. This reboiler uses circulating hot water at 80°C as the heating medium to maintain the temperature at the bottom of the tower at 55°C. The liquid coming from the bottom of the tower enters the reboiler, where it partially vaporizes before returning to the space at the bottom of the depropanation section. The propylene tower is used to separate propylene from propane, and since the relative volatility of propylene and propane is very high, a large number of trays are required for their separation. The purity requirement for propylene intended for polymerization is 99.6%. To separate propylene with a purity of 99.6% using a single tower, approximately 200 tray levels would be required, resulting in a tower height of around 130 meters, which is clearly unreasonable. Therefore, a dual-tower setup is employed to reduce the tower height.