Application of synthetic lubricating oil in processing and hydrocarbon presses
Synthetic lubricants are used in positive displacement devices, lubricant processing and hydrocarbon gas compressors are controversial. Many of these applications require synthetic products to limit toxic catalysts, reduce gas solubility and thereby lead to loss of viscosity and limit reactions with gases.
The lubrication requirements of rotary screws, reciprocating engines and rotary vane compressors in hydrocarbon and gas processing applications are under review. The physical and chemical properties of synthetic lubricants are critical to the compressor lubrication. The influence of gas in synthetic lubricating oil under compressor operating conditions is also important. In many applications, synthetic lubricants can extend oil change intervals, reduce fuel consumption, increase efficiency and extend compressor life.
Hydrocarbon and process gas compressor lubricants
Historically, hydrocarbon, chemical processing and inert industrial gas compressors have used traditional mineral oil-based lubricants. However, mineral oil cannot meet the needs of many modern rotary compressor designs and other complex applications. A typical gas processing application is at an outflow temperature of 80-1100C and a pressure of 7-70barg. Mineral oil may be thermally degraded under these conditions. Their instability causes the vapor phase of the lubricating oil in the compressor to leave a downward flow. In many applications, there are chemically active gases that may cause chemical degradation of mineral oil. In some applications, it may happen that the mineral oil is severely diluted by the compressed gas. This dilution will reduce the viscosity of the lubricating oil, often causing insufficient lubrication somewhere. The corrosion protection of the system is another aspect where mineral oils often perform poorly. Synthetic and semi-synthetic lubricants meet the needs of these complex applications. They provide many advantages over traditional mineral oils. Some of these include excellent chemical inertness, high viscosity index, low pour point, good hydrolytic stability and complete demulsibility. These products also provide excellent natural lubricity, excellent thermal stability, resistance to dilution by hydrocarbons, low volatility and compatibility with elastic materials and metals. Since not every type of fluid has all the advantages, it is very important to understand the physical and chemical properties of each synthetic and semi-synthetic base fluid. It is also important to understand the requirements of the compressor and its application to select the appropriate fluid with the best performance. The semi-synthetic lubricants discussed in this article are high-viscosity index oils that have been strictly hydrogenated. The two-stage hydrotreating process produces a high-quality base oil. The raw materials are hydrogenated under high pressure and high temperature, followed by fractional distillation, dewaxing and second-stage hydrogenation treatment. After becomes a high molecular weight isotonic oil. The viscosity index is in the range of 105-120.
Rotary vane compressor! Each type of compressor has different requirements for lubricating oil. The lubricating oil function of the rotary vane compressor is to lubricate the blades that slide in and out during the compression process. Lubricating oil is also used as a sealant between the blade and the frame, making gas compression possible. Usually ISO68-150 products meet the viscosity requirements of rotary vane compressors.
Reciprocating compressors provide a large discharge pressure capacity ranging from 1 bar g to 1000 bar g (4). Oil-lubricated cylinders, crankcase parts, coils, pistons, valves and loading rods of reciprocating compressors. The crankcase components include cross head bearings, cross joints, cross head guides and crank pins. Recent refrigeration applications have shown that ISO15 lubricants with an operating viscosity of less than 10 cSt can provide suitable lubrication. However, depending on gas molecular weight and flow pressure operation, processing and classic use of hydrocarbon gas reciprocating compressors are ISO68-680 products. ) In most reciprocating compressors, a fluid is used as a lubricant for all parts. Smaller reciprocating compressors use splash lubricant. Larger devices usually use an oil pump system to lubricate the upper crankcase components. Some large equipment uses two different lubricants, one for the cylinder and the other for other parts that require lubrication. Since cylinder lubricating oil must coexist with gas, it must be compatible with the downward flow process. Cylinder lubricating oil can be designed to provide lubrication for special gas or operating conditions.
Filled screw compressors usually use compressed hydrocarbons and process gases, and the flow pressure ranges from 1-25 bar g(5). They have many advantages, including improved compression efficiency, low outflow temperature, high reliability and less maintenance due to simple mechanical construction. Spiral gas compressors must have several functions. They lubricate the bearings, provide adequate sealing between the screw and the frame, remove heat during compression, flush out any particles in the compressor, and protect the system from corrosion. The lower viscosity limit is 10-20cSt at the oil supply temperature to the bearing and 5cSt at the outflow condition to ensure proper sealing. The viscosity of the upper lubricating oil depends on the ability to provide enough lubricating oil for the bearing. The typical upper viscosity limit is 30-100 cSt. Usually ISO68-220 lubricants meet the viscosity requirements of screw compressors. The exact viscosity grade depends on the operating conditions and air flow composition. .
Due to the closed-loop design of the system, synthetic products are particularly suitable for screw compressors (Figure 1). The lubricating oil and compressed gas enter the separator. The separated oil passes through an oil cooler and flows back into the compressor. The degradation of the lubricant in this process can lead to compressor problems such as bearing failure, insufficient sealing or corrosion. In many applications, the use of synthetic compressor lubricants can result in effective hydrocarbon compression and gas production (7).
Application of synthetic lubricating oil in processing and hydrocarbon gas presses
The choice of suitable viscosity in hydrocarbon and process gas compressors is critical. The dilution of lubricating oil by gas is the main problem. Dilution viscosity vs. temperature curve (Figure 2) Dilution can cause a direct decrease in viscosity. Lubricating oil that is above the compressor's viscosity limit without dilution can be reduced to below the limit by dilution. This condition often leads to an increase in compressor wear and/or failure. It can also be a side effect of compressor performance.
The information needed to predict the dilution of lubricating oil includes compressor model, operating conditions and complete gas composition, that is, the amount of each gas component present. The balance calculation is based on the vapor pressure of the gas composition and the potential lubricant dilution provided by Raoult's law. The effect of dilution on lubricant viscosity can be determined by mixing the lubricant with a known amount of hydrocarbons. The relationship between viscosity and temperature can also be determined by these diluted mixtures. Dilution level, operating temperature and dilution effect all affect the actual operating viscosity of the lubricant. If necessary, increasing the operating temperature can help minimize the dilution of the compressed gas.
Equipment model is also a factor in the selection of lubricants. In some applications, there are a lot of pollutants in the gas stream, including water, hydrogen sulfide (acid gas), asphaltene or other trace pollutants. Other applications are particularly sensitive to the operation of the oil and the chemistry of the lubricant/additives. The dilution of these factors and viscosity can be used to determine the best lubricant.
Hydrocarbon gas applications
Hydrocarbon gas applications include natural gas, refining gas, biogas, gas turbochargers, refrigeration plants, steam recovery boilers, refinery exhaust and production gas applications. The difference in gas composition mainly depends on the type of application equipment. In addition, the application site can affect the gas composition of many applications (Table 1). It is very important that lubricants meet the viscosity requirements of compressor manufacturers for all hydrocarbon applications under dilution conditions.
There are many other considerations that can affect the choice and performance of lubricants. Gas containing moisture can cause system corrosion. Operating the compressor at a temperature higher than the dew point of the gas stream can minimize it. The lubricating oil formulation contains rust inhibitors to help protect metal parts in contact with water. The application of acid gases requires special anti-corrosion additives to protect the system from corrosion due to the presence of hydrogen sulfide. Volatile additives can be vaporized under operating conditions to accomplish this task. They wrap the metal parts of the system so that they do not come into direct contact with the lubricant. The selection of materials for acid gas systems is important because some yellow metals corrode significantly when exposed to gases containing hydrogen sulfide.
Light natural gas, distilled gas
Properly formulated semi-synthetic lubricants are suitable for natural gas and acid gas applications when the dilution is not serious. Semi-synthetic lubricating oil can also be used for refining gas consisting essentially of methane, carbon dioxide and trace hydrogen sulfide. Semi-synthetic lubricants have a higher viscosity index than mineral oils, so they meet the compressor viscosity requirements over a wide temperature range. The low volatility of these materials can minimize carryover.
Natural gas applications containing asphaltenes must use a PAO or semi-synthetic oil. These lubricants dissolve the asphaltenes and keep them in a dissolved state. Poly(alkylene) glycols (PAGs) cannot dissolve asphaltenes, and their use may cause blockage of oil filters and oil pipes.
Heavy natural gas, refinery exhaust gas, steam recovery boiler gas
Some natural gas, refinery exhaust and steam recovery boiler gas produce higher molecular weight hydrocarbon gases. This can lead to an increase in the dilution level of the lubricant. These applications require appropriately formulated poly(alkylene) glycols (PAGs). There are several different types of PAGs, which are classified by the monomer from which they are produced (Figure 3) (9). Polyethylene and polypropylene glycol copolymers limit the dilution of hydrocarbons. Relying on the proportion of ethylene oxide polymerized with propylene oxide, lubricating oil with a dilution limit of 10-20% can be produced. Above this level, the lubricating oil becomes saturated.
Some polyethylene and polypropylene glycol copolymers have unique miscibility. They are less soluble in water at high temperatures than at low temperatures (Figure 4). This reverse miscibility may have advantages in corrosion protection in compressor applications where moisture is present. Polyethylene and polypropylene glycol copolymer are soluble in condensed water at room temperature. This prevents the corrosive environment caused by free water pooling in the system during downtime. Once the compressor restarts and reaches normal operating temperature, the lubricating oil will become less soluble in water. This water then vaporizes and coexists with the effluent gas in the compressor.
Some hydrocarbon gas presses use lubricating oil that is completely soluble in hydrocarbons will cause severe dilution, and polyethylene glycol can be used. Polyethylene glycol is completely insoluble in hydrocarbons. Experiments show that when lubricating oil is in contact with hydrocarbon gas up to 13790kPa, its viscosity is not lost. The lack of attractiveness between the hydrocarbon gas and the lubricating oil causes the gas in the oil separator to be better separated from the lubricating oil compared with other types of lubricating oil.
One way to improve the design of compressor systems using ethylene oxide containing PAGs is to remove all excess concentrated hydrocarbons from the tank. Concentration of hydrocarbons becomes the focus in applications containing gases with butane or higher molecular weight.
Concentration can also occur under conditions of pressure rise or temperature drop. A discharge valve inserted in the tank and running slightly above the filling pipe can discharge any concentrated hydrocarbons. A steam recovery boiler device collects liquid (n-)hexane production and provides a six-month payback period for a complete compressor system (3).
Compressing biogas and burning it for energy is another application of hydrocarbon gas. The most important consideration for biogas compressors is to protect the compressor system from corrosive trace pollutants (7). In response to these pollutants, PAO features outstanding chemical inertness. PAO also provides advantages such as high viscosity index and low vapor pressure. Low vapor pressure not only minimizes the amount of combined oil, but also minimizes air flow carryover.
Minimized carryover is a key requirement in gas compression applications that feed hydrocarbon gas turbines. The typical requirement of a gas turbine is a gas injection pressure of 1500-4500kPa. The gas compressor promotes the pressure of the gas from atmospheric pressure to injected pressure. The remaining lubricating oil can lead to the formation of carbon sludge, which can foul downstream devices or cause hot spots in the combustion chamber to cause burnout. Due to its feature of minimizing legacy, PAO is the lubricant of choice. The compressed hydrocarbon gas in these devices usually does not cause severe dilution.
The purity of the produced gas, the toxic catalysts derived from metals and other inorganic compounds that are left behind and found in lubricant additives, are important focus in some hydrocarbon production gas applications. The gas must be separated from the impurities in the lubricating oil in order to function properly in the production process. The carryover of lubricating oil must be minimized to reduce lubricating-related problems in downstream production equipment. The small amount of remnants reaching downstream must not contain any catalysts with side effects. The replacement cost of many catalysts is very high, and it is economical to replace the catalyst during shutdown. The basic raw materials or additives of lubricating oil used for gas production equipment must not contain any metals or other impurities, and no toxic catalysts are used in the production process. 4.5 Refrigeration equipment
The low-temperature physical properties and miscibility of lubricating oil and hydrocarbon gas refrigerants are important considerations for selecting suitable lubricating oils for refrigeration systems. A suitable separation device will minimize the amount of oil reaching downstream. Any lubricating oil that reaches the freezing surface of the system must not freeze in the piping system of the evaporator, otherwise it will cause the loss of system heat efficiency. The lubricating oil reaching downstream must have a lower pour point than the evaporator temperature or be miscible with the refrigerant at the evaporator temperature. If it is miscible, the lubricant/refrigerant mixture can be returned to the compressor in a single phase. Refrigeration equipment usually uses gas without impurities, so the amount of additives required in lubricating oil should be minimized.
Polypropylene glycol has a lower solubility in hydrocarbon gases than other types of lubricants. Low-concentration polypropylene glycol can be miscible with hydrocarbons at low temperatures (Figure 5). They are less diluted than hydrocarbon-based lubricants such as mineral oil and PAO. This dilution limitation in the compressor environment allows the lubricant to provide better sealing, thereby increasing the volumetric efficiency of the compressor. For this reason, they are often used in hydrocarbon gas refrigeration systems to improve performance.
Hydrocarbon refrigeration systems that require minimal carryover use polyethylene glycol lubricants. They are completely insoluble in hydrocarbons. Since there is no attraction between the lubricating oil and the gas, the lubricating oil/gas can be separated better than other lubricants. Polyethylene glycol will not mix with the liquid refrigerant in the evaporator. If the evaporator temperature is lower than the pour point of the lubricating oil, the lubricating oil will solidify. For this reason, typical low temperature applications do not use polyethylene glycol.
Hydrocarbon gas refrigeration systems with low dilution levels can use a PAO-based lubricant. PAO is completely miscible with hydrocarbon gas. Both PAG and PAO have extremely low volatility, so the gaseous residue of lubricating oil can be minimized.
Production gas compressor Inert gas production gas compressor can handle inert or reactive gases. Typical inert gases include hydrogen, helium, carbon dioxide and nitrogen. Except for carbon dioxide, most inert gases and most lubricants do not cause viscosity loss. Just like a hydrocarbon production gas compressor, the two focal points in an inert production gas compressor are the purity of the gas and the toxicity of the catalyst.
Most inert gas equipment uses PAO-based or PAG-based lubricants. The low volatility of these materials can minimize the impurities of gas and lubricating oil vapor. In addition, the low