Chemical tower equipment: 【One question per day】April 29-30, 2011

1. Top of tower temperature: Control of the top of tower temperature is achieved by adjusting the amount of reflux oil at the tower top. 2. Top of tower pressure. 3. Control of the liquid level in the vapor side of the tower. 4. Material balance between the inlet and outlet of the distillation column. I. Introduction: Zhongzai possesses comprehensive plant solutions as well as extensive experience in project implementation within the petrochemical industry. Currently, the SunytTech series of control systems are widely used in various mainstream units across the oil refining and petrochemical industries, such as atmospheric and vacuum distillation, catalytic cracking, hydrorefining, delayed coking, solvent deasphalting, gas separation, various hydrogen production processes, sulfur recovery, PVC production, phthalic anhydride production, aniline production, and more. They hold a leading market share in China’s oil refining and petrochemical sectors, and many large and medium-sized petrochemical enterprises in the country have adopted the control systems and solutions provided by Zhongzi to manage their oil refining and petrochemical production processes. II. Introduction to the Process Flow The atmospheric and vacuum distillation unit is a fundamental facility in oil refining plants; it represents the first stage in crude oil processing and plays a very important role in oil refining. Its processing method involves using heating and distillation, along with repeated processes of condensation and vaporization, to separate crude oil into oils or semi-finished products with different boiling points, thereby carrying out the separation of crude oil. The main separation products include: reforming feedstock, gasoline components, aviation kerosene, diesel, materials for further processing (lubricating oils, catalytic cracking feedstock, etc.), and residue (for reforming and coking, as well as asphalt production). In an atmospheric pressure tower, crude oil is distilled to enable thorough heat and mass exchange between the gas and liquid phases. With top reflux and bottom stripping provided, product gasoline with a lower boiling point is distilled from the top of the tower, while heavier oils with a higher boiling point are distilled from the bottom. Side products such as kerosene, diesel, heavy diesel, and wax oil are obtained by drawing from the middle section of the tower. The heavy oil components remaining after atmospheric distillation have large molecular weights and are prone to decomposition at high temperatures. To separate the various high-boiling-point lubricant components present in atmospheric-pressure heavy oil, vacuum distillation is employed using a vacuum tower. The heated atmospheric heavy oil is fractionated under negative pressure, allowing the high-boiling-point components to be distilled out sequentially at corresponding temperatures to be used as lubricant material. Vacuum distillation in atmospheric and vacuum distillation units often employs various technical measures such as crude oil transfer lines, large tower diameters, and high-efficiency structured packing (GEMPAK). Achieve pressure-reduction operations with low furnace temperature, high vacuum, narrow fraction range, and light color to improve the quality of lubricant base materials. III. Control Scheme 3.1 Key Controls of the Unit: Atmospheric and vacuum distillation units are typically controlled using conventional single-loop control methods, supplemented by a small number of more complex control strategies such as cascade control, uniform control, and switching control. 1. Electrodesalination section: differential pressure control of the desalination tank, fixed-value control of water injection flow, and fixed-value control of drainage flow. 2. Initial distillation section ★ Top of tower temperature control: The top of tower temperature is controlled by adjusting the amount of reflux oil at the tower top, and the reflux flow rate is automatically recorded to monitor any changes in reflux. ★ Bottom liquid level control: A differential pressure level gauge is used at the bottom of the initial distillation tower, along with indoor indication and audio-visual alarms, to prevent overfilling of the tower or pump cavitation at the bottom. ★ Top pressure control: To ensure the efficient fractionation in the distillation tower, a pressure transmitter is generally installed at the top of the tower, with monitoring and recording carried out in a control room. ★ Level and interface control of the reflux tank: An automatic level regulator is installed on the reflux tank to control the flow rate of the oil coming from the vapor overhead, thereby ensuring an adequate amount of reflux ； At the same time, an interface regulator is used to maintain a constant oil-water interface (the control valve is installed on the drain pipe). ★ Control: To reduce the load on the atmospheric pressure furnace and increase the processing capacity, a vapor-side column was added next to the initial distillation tower. The liquid level in the vaporization side column must be automatically controlled (a control valve is installed at the distillation outlet of the initial distillation column), and a flow regulator is provided to control the flow rate entering the atmospheric pressure column. 3. Atmospheric pressure section – Key controls: ★ Feed flow control for the heating furnace: To maintain the outlet temperature of the atmospheric pressure heating furnace, flow regulators are installed on each of the four feed lines leading to the furnace, in order to regulate the flow rate of material into it. ★ Control of the outlet temperature of the heating furnace: The outlet temperature of the atmospheric-pressure heating furnace is maintained constant by adjusting the amounts of fuel oil and fuel gas (the fuel gas can be switched between low-pressure gas fuel, self-produced gas fuel, and high-pressure gas fuel). ★ Control of the outlet temperature at the top of the atmospheric pressure tower: The temperature at the outlet of the top of the atmospheric pressure tower is maintained constant by adjusting the reflux flow rate to the tower ； If the constant line is used in the process of producing aviation kerosene, the temperature of this constant line can be employed to control the reflux flow rate, and this reflux flow rate can be recorded. ★ Level and interface control of the reflux tank: To ensure an appropriate amount of reflux oil at the top of the atmospheric tower, a level regulator is installed at the upper part of the reflux tank to adjust the amount of gasoline that leaves the unit at the top of the atmospheric tower. Below it is equipped with an interface regulator to maintain a certain oil-water interface (the control valve is located on the drain line). ★ Pressure regulation in atmospheric pressure towers: Generally, pressure transmitters are installed, and the readings are recorded indoors for use as a reference in controlling product quality ； A pressure regulator can also be installed in the reflux tank to control the amount of gas released. ★ Regulation of the bottom liquid level in the atmospheric pressure tower: To ensure a stable bottom liquid level in the tower, thereby preventing overfilling and ensuring that the feed pump of the vacuum heating furnace does not experience vacuum conditions, a differential pressure type level transmitter is installed at the tower bottom. There is also a system for recording the liquid level along with audible and visual alarms, which facilitate the analysis of any incidents that may occur. To save energy, some units are equipped with frequency converters at the bottom of the tower; the liquid level at the bottom of the tower is controlled by switching between control valves and frequency converters. ★ Regulation of stripper level and flow: To control the quality of the products from the side streams, automatic level controllers are installed in the strippers of each side stream in the atmospheric tower, in order to regulate the amount of oil flowing from the distillation outlet of the atmospheric tower to the side stream strippers. There are flow regulators at the outlets of the finished products from each side line; the control valves are installed on the outlet pipelines of each fraction extraction pump. 4. Pressure reduction section – Key controls: ★ Regulation of the temperature at the top of the pressure reduction tower: To maintain a constant temperature at the top of the tower and prevent oil and gas loss, a temperature regulator is installed on the outlet pipeline at the top of the tower to adjust the amount of reflux oil. ★ Top product flow regulation: To increase the yield of light oil, a flow regulator is installed on the pipeline leading to the light oil outlet at the top of the tower. The reduction line is also equipped with a temperature regulator to control the amount of return oil. A flow regulator is also installed on the pipeline leading out from the reduction unit. ★ Side line reflux flow regulation: To remove heat from the vacuum distillation column and ensure the quality of the side line products, reflux from the second and third side lines is provided. At the same time, to ensure a constant reflux flow, a flow regulator is installed on the reflux oil pipeline after it enters the tower. ★ Regulation of the bottom liquid level in the vacuum tower: To maintain a stable bottom liquid level, a bottom liquid level regulator is installed to control the amount of oil discharged from the residue outlet of the vacuum tower. To save energy, some units are equipped with frequency converters at the bottom of the tower; the liquid level at the bottom of the tower is controlled by switching between control valves and frequency converters. ★ Vacuum level recording at the top of the vacuum distillation tower: To ensure the effective fractionation in the vacuum distillation tower, a vacuum pressure transmitter is installed at the top of the tower to record its vacuum level. 3.2 Advanced Control Solutions: Ziji is one of the few leading manufacturers in China capable of providing and implementing advanced and optimized control systems for petrochemical production units. For atmospheric and vacuum distillation, it offers control strategies such as atmospheric pressure optimization, vacuum pressure optimization, and balance control for the vacuum furnace branches. ★ Optimization at normal and reduced pressure aims primarily to maintain stable operation within the constraints of the process, while maximizing the yield of products under the condition that all product specifications are met. ★ The principle of furnace branch balancing is to achieve similar outlet temperatures in the branches by adjusting the distribution of the feed volume to the heating furnace among these branches ； In fact, the uniform distribution of flow is sacrificed to achieve temperature uniformity. The goal of optimal control is to achieve uniformity in both flow rate and temperature. The optimization control strategies for mainstream units such as atmospheric and vacuum distillation, catalytic cracking, PTA, and hydrorefining in China are on par with or even exceed international advanced levels, enabling the country to compete with leading foreign companies. To date, there are already dozens of successful applications that provide relevant experience. As petrochemical companies experience an increasing demand for advanced control systems, they will surely gain a stronger competitive advantage.

Reply 1# wu_pb Temperature, reflux ratio

High water content in crude oil, low bottom temperature of the tower, large reflux flow

Reply 1# wu_pb 1. Changes in the feed rate to the initial distillation tower and the amount withdrawn from the bottom of the initial distillation tower； 2. Changes in the properties of crude oil cause changes in the liquid level at the bottom of the tower ； 3. Changes in the feed temperature of the initial distillation tower ； 4. The levels of temperature and pressure at the top of the tower affect changes in the liquid level at the bottom of the tower.

1 Changes in the feed rate to the initial distillation tower and the amount pumped out by the pump at the bottom of this tower. 2 Fluctuations in the liquid level at the bottom of the tower caused by changes in the properties of the crude oil. 3 Changes in the feed temperature to the initial distillation tower. 4 Variations in the top pressure and temperature affecting the liquid level at the bottom of the tower

Reply 1# wu_pb: 1. Level gauge failure, 2. Influence of feed rate, 3. Influence of feed temperature, 4. Influence of top temperature and pressure, 5. Influence of bottom extraction volume

Reply 1# wu_pb Crude oil properties, bottom temperature of the tower

Reply 1# wu_pb 1. Stability of the heating steam 2. Continuity and stability of feed supply 3. Stability of reflux conditions

①Changes in the feed rate to the initial distillation tower and the discharge volume from the bottom pump； When adjusting the processing capacity or the liquid level at the bottom of the initial distillation tower, the flow rates in and out of the tower are not properly balanced. If the flow rate into the tower is low while the flow rate out of the tower is high, the liquid level will drop. ②Changes in the properties of crude oil cause fluctuations in the bottom liquid level of the tower. For example, as the properties of the crude oil become lighter, the liquid level at the bottom of the tower decreases ； The properties of the crude oil become heavier, and the liquid level at the bottom of the tower rises. ③Changes in the feed temperature of the initial distillation tower: a higher feed temperature leads to an increased vaporization rate of the feed, resulting in more product distillate at the top of the tower and a decrease in the liquid level at the bottom ； Conversely, the liquid level rises. ④The level of pressure and temperature at the top of the tower affects changes in the liquid level at the bottom of the tower. High top temperature and low pressure lead to an increased vaporization rate of the feed; the vaporized portion is removed from the distillation section, causing the liquid level at the bottom of the tower to drop ； Conversely, the liquid level rises.