Some questions I’ve compiled on clay towers used for benzene hydrogenation

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Chapter 1 Process Technical Specifications 1.1 General Overview of the Plant 1.1.1 Introduction to the Plant I The clay treatment plant was originally built in 1964 and came online in 1965; after several expansions and technical upgrades by 1989, its processing capacity reached 120,000 tons per year. In July 1993, three 70 m2 machines with a single filtration plate were upgraded from manual to 100 m2 automatic plug-flow machines. In 1996, four steam pumps were replaced with mud pumps. In July 1999, the system was upgraded from an open-flow type to a closed-flow plate press; in August 1999, the control of the equipment was changed from conventional instruments to centralized DCS control. After several upgrades, the maximum processing capacity of the equipment reached 150,000 tons per year. The energy consumption of the device has decreased significantly, the workload on operators has been greatly reduced, the working environment has improved markedly, and the level of automation continues to rise. 1.1.2 Process principle: With the rapid development of hydrogenation-based refining, the use of clay refining units has declined. However, hydrogenated essential oils suffer from poor light stability and an increased freezing point. Due to advantages such as lower investment costs and better light stability of essential oils, clay refining units still remain highly effective. 1.1.2.1 Composition of bentonite 1.1.2.1.1 Bentonite can be divided into natural and activated types; activated bentonite has a strong decolorizing ability, and its main component is aluminum silicate (Al2O3SiO2), with the remainder being compounds of iron, magnesium, and calcium. 1.1.2.1.2 Activated clay is produced from bentonite found in clay deposits; the main mineral component of bentonite is montmorillonite, which is formed by the weathering of volcanic ash. The bentonite used to manufacture activated clay is high-quality bentonite containing more than 85% montmorillonite. Bentonite is referred to as sodium bentonite when sodium ions account for more than 50% of the total exchangeable cations in it. When calcium ions account for over 60%, it is called calcium bentonite. 1.1.2.1.3 Montmorillonite consists of two layers of silicon-oxygen tetrahedra, with an aluminum-oxygen layer in between that is an OH octahedron layer. According to isomorphous substitution, a certain amount of exchangeable cations in the interlayer layers are acidified and then replaced by hydrogen ions. Thereby turning the bentonite into activated clay with multiple structures, a large specific surface area, and good adsorption properties. 1.1.2.1.4 Activated clay can be produced by dry and wet methods. The wet method involves reacting bentonite with dilute acid, followed by washing and drying to obtain the desired product ； The dry method involves mixing bentonite with acid to form a shape, aging it, then calcining and crushing it. 1.1.2.2 Properties of bentonite 1.1.2.2.1 Activity 1.1.2.2.1.1 It is an indicator for assessing the adsorption capacity of bentonite for polar substances, expressed in milliliters of 0.1 mol/l NaOH solution consumed per 100 g of the sample during neutralization. 1.1.2.2.1.2 White clay particles consist of numerous very small pores (a few millimeters in size) and channels, thereby creating a large internal surface area for adsorbing polar substances. High-quality white clay can have a pore surface area of over 4.50 m2 per gram. 1.1.2.2.1.3 Activity is related to the chemical composition, particle size, moisture content of the clay, and whether the surface pores are clean. The decolorization rate (using the kerosene-asphalt method) is generally greater than 90%. 1.1.2.2.2 Particle size: It indicates the degree of fragmentation of the kaolin, expressed as the percentage of kaolin that passes through a 75μm sieve. It is generally advisable for the passing rate to be between 80% and 95%; if the particle size is too large, filtration becomes difficult and processing losses increase ； It has a large particle size, a small specific surface area, and low adsorption capacity. 1.1.2.2.3 Moisture: 1.1.2.2.3.1 refers to the weight percentage of water contained in the kaolin. Generally, kaolins with a water content of 10–14% have better adsorption capacity. Excessive drying reduces activity; generally, the drying temperature should not exceed 400°C. 1.1.2.2.3.2 During high-temperature contact refining, the water in the pores of the clay evaporates; at this point its adsorption capacity is at its highest, and the water vapor that evaporates from the clay also plays a role in stirring. It increases the opportunities for contact, thereby enabling better mixing of the oil with the clay. However, if the clay contains too much moisture, this moisture can cause emulsification, leading to an increase in pressure within the furnace tubes and the formation of large amounts of foam in the evaporation tower after the oil is discharged, which may result in tower flooding accidents. 1.1.2.3 Principles of clay-based refining 1.1.2.3.1 Even after undergoing a series of refining processes, lubricating oils still contain trace amounts of gums, asphaltenes, naphthenic acids, sulfonic acids, residual inorganic salts, solvents, moisture, and mechanical impurities. They have a significant impact on the colority, antioxidant properties, stability, and anti-emulsification capacity of oils. Since most of them are polar substances, bentonite has a strong adsorption capacity for them, but a relatively weaker adsorption capacity for the ideal components. 1.1.2.3.2 Resins, asphaltenes, oxygen, and sulfides are easily adsorbed. The order of adsorption of various hydrocarbons by clay is: asphaltenes and resins > aromatics > naphthenes > alkanes. The more rings present in aromatic and cycloalkane compounds, the easier they are to adsorb. 1.1.2.4 Main factors affecting kaolin purification The main factors affecting kaolin purification include: the properties of the raw material and kaolin, the purification temperature, the contact time, and the degree of mixing between the oil and kaolin. During production: the heavier and more viscous the raw materials are, as well as the higher the requirements for product quality, the more stringent the operating conditions become. 1.1.2.4.1 Amount of clay used: 1.1.2.4.1.1 Once the properties of the raw materials and the clay are determined, the quality of the essential oil improves as the amount of clay used increases; however, beyond a certain level of clay usage, the quality no longer improves. 1.1.2.4.1.2 Excessive use of clay reduces the yield and increases costs; it also removes the natural antioxidants present in the oil, thereby decreasing its antioxidant stability ； It has a significant impact on operations, and it also causes considerable wear and tear on the equipment. 1.1.2.4.1.3 Depending on the type of refined oil and the quality requirements for that oil, the amount of clay added generally ranges from 3 to 7%. 1.1.2.4.2 Crude oils with different raw material properties have varying chemical compositions and levels of various non-ideal components. In actual production, it is necessary to appropriately adjust various operational parameters such as the amount of clay added and the refining temperature, based on the properties of the crude oil and the color of the distillate. 1.1.2.4.3 Refining temperature 1.1.2.4.3.1 The higher the temperature of the lubricating oil, the lower its viscosity, and the faster the movement of its molecules; under such conditions, the non-ideal components in the oil can be more readily adsorbed onto the clay. When the oil is heated to a temperature close to its flash point, it is on the verge of decomposing, and the temperature can no longer be increased; at this point, we consider that the adsorption capacity of the clay has reached its maximum. 1.1.2.4.3.2 The refining temperature during production should be selected between 180 and 320°C. (Slightly below the flash point of the oil being processed, but must not exceed 320°C; otherwise, the oil will decompose due to the catalytic effect of the clay.) 1.1.2.4.4 Contact time 1.1.2.4.4.1 Under high-temperature conditions, the contact time between oil and clay refers to the time that oil and clay remain inside the furnace tubes and evaporation towers. 1.1.2.4.4.2 Due to the short contact time, the surface of the clay cannot adequately adsorb the undesirable components in the oil onto itself; moreover, when the oil is dark in color, the contact time should generally be maintained between 20 and 40 minutes. In practice, this is achieved by controlling the liquid level in the tower. 1.1.2.4.5 Degree of mixing between oil and clay: In order for the clay to adsorb substances such as gums and asphaltenes, it is important that the clay and oil be thoroughly mixed. If a paste cannot be formed between them, and since the density of clay is higher than that of oil, uneven mixing will cause the clay to settle at the bottom; it will then fail to adsorb the impurities in the oil, resulting in a reduced amount of clay used and the oil not achieving the desired level of purification. 1.1.3 Process Flow Description 1.1.3.1 Mixing and Heating System Flow 1.1.3.1.1 Crude oil (from the dewaxing oil tank area, and from I and II furfural sources) → Tanks 502# and 503# → Fully mixed with clay added via a venturi tube and screw conveyor → Mixing tank. 1.1.3.1.2 Mixing tank → Pumps-7, 8 → Exchangers-1, 2, 3, 4 (inner tubes) → Feed to heating furnace-1. 1.1.3.2 Process of a filtration system 1.1.3.2.1 After being heated by Furnace-1, the material enters Tower-1 where it evaporates; superheated steam heated by Furnace-1 is blown into Tower-1. 1.1.3.2.2 Tower-1 bottom → Pumps-11, 12 → Exchangers-1, 2, 3, 4 (outer tube) → Cooler-2 → Plates-4, 5, 6 → Vessels-4, 5, 6. 1.1.3.2.3 Tank-4, 5, 6 → Pump-9, 10 → Tank-4, 5, 6 → Level control valve → Plate-1, 2, 3 → Tank-7 → Pump-1, 2 → Level control valve of Tank-7 → Cold-3 (shell) → Exit from the device after cooling. 1.1.3.3 Vacuum system 1.1.3.3.1 The gaseous substances at the top of Tower-1 enter Vessels-3A and B after being cooled by Cooler-1. 1.1.3.3.2 The tops of vessels 3A and B are evacuated using water ring vacuum pumps -5 and -6. 1.1.3.3.3 The water and trace solvents separated at the top of Tower-1 are vented through the bottoms of Vessel-3A and B. 1.1.3.4 Description of auxiliary system processes 1.1.3.4.1 Gas comes from the system → gas tank –11→ gas control valve→ various burners in the heating furnace. 1.1.3.4.2 Steam is released from the pipe rack of the new steam system; after entering the plant, it is distributed to various steam consumption points within the plant via the pipe rack at location Rong-6. 1.1.3.4.3 The waste steam comes from the lubricating oil plant; it is released behind the warehouse or next to the sawdust room, and then flows directly to various steam-consuming points in the facility. On the pipe rack at location -3, there is a cross-over switch that allows waste steam and fresh steam to interchange paths. 1.1.3.4.4 The fresh water flows, via a new water meter, partly to the cooler (Cold-2) and partly to the pump house to serve as cooling water for the pumps and for use in the bathhouse. 1.1.3.4.5 Air enters Container-1 from the system pipe rack, then goes to the pump and the plate actuator for cleaning; air and steam mix with each other in front of the 1# plate actuator on the upper floor. 1.1.3.4.6 The air for instruments comes from the system and enters directly at each point where such air is required; it mixes with air above the Rong-4 pipe rack. ……