1. Application background of titanium in refining and chemical industry
Due to its excellent corrosion resistance, high specific strength and good processing performance, titanium materials have been widely used in chlor-alkali, salt production, urea, pesticides, synthetic fibers, organic synthesis, pharmaceuticals since the 1980s. , hydrometallurgy and other chemical process industries [1-7]. However, due to the long service life of titanium equipment, even as long as 30 years, it is not easy to be damaged and updated. Therefore, to expand the application of titanium in the civil industry, only efforts can be made to develop and explore new application fields. We believe that the application of titanium in the refining and chemical industry has a lot of room for development. Titanium equipment in the domestic refining and chemical industry is still rarely used, while it has been used in the refining and chemical industry as early as 40 years ago. Due to the shortage of oil sources, Japan mostly imports from the Middle East and has a long history of processing high-sulfur crude oil, so it has accumulated rich experience in material selection and anti-corrosion. Since 1955, Japan has used titanium materials to solve the environmental corrosion problem of atmospheric and vacuum low temperature H2S-HC1-H2O.
Europe and the United States also applied titanium heat exchangers to oil refineries as early as 1960. At first, it was mainly to solve the corrosion of seawater, brackish water and polluted water. Later, in order to prevent the damage of low-quality crude oil and corrosive oil and gas to equipment, it was applied from titanium tube heat exchangers to plate heat exchangers and other equipment. Since Daqing oil with low sulfur, acid and salt content was the first in China, the corrosion problem of oil refining equipment is not prominent, and carbon steel can be used to solve the problem. However, with the rapid development of my country's economy for more than 20 years, the demand for oil has risen rapidly, and the growth rate of domestic crude oil cannot keep up with the demand. Therefore, the import of high-sulfur crude oil from abroad, especially from the Middle East, has increased year by year, which has brought about the technology of improving the quality of oil refining equipment. Renovated to meet the anti-corrosion requirements for refining high-sulfur crude oil in the Middle East. With the increase of production volume of domestic crude oil, the content of sulfur, acid and salt also increases. The diversification of oil sources makes the problem of equipment corrosion more prominent and improves the material selection standards. Like Japan and the United States, the selection of titanium materials to make heat exchange or condensation cooling equipment with severe corrosion in some key parts should be put on the agenda. This is not only a production need, but also a need for the promotion and application of titanium to increase economic and social benefits.
2. Analysis of titanium corrosive medium and corrosive environment for refining equipment
From the perspective of mechanical properties, the operating temperature of industrial pure titanium equipment should not be higher than 230 °C, titanium alloy equipment should not be higher than 300 °C, and composite equipment can be used up to 350 "C. Therefore, only the low temperature suitable for titanium applications in refining equipment will be discussed below. (<230℃) light oil corrosion environment:
H2S—HCl—H2O (atmospheric and vacuum tower overhead condensation cooling system)
H2S—HCN—H2O (catalytic cracking absorption and desorption system)
H2S—CO2—H2O (desulfurization regeneration tower overhead condensation system)
H2S-CO2-RNH2-H2O (desulfurization solvent regeneration bottom system)
Corrosive environments such as H2S—NH3—H2O (acid water stripping cooling system) and industrial cooling water are not discussed. Corrosive environments such as high temperature H2S, H2, H2S+H2 and naphthenic acid are not discussed. Because the high temperature heavy oil corrosion environment can be solved by Cr-Mo steel, CrB, 18-8 steel, etc., titanium materials are generally not suitable.
2.1 Brackish water, brackish water and sewage
Refinery production is inseparable from cooling water. Titanium has excellent corrosion resistance to corrosive cooling water including seawater, brackish water and sewage. This is why titanium was first used in refineries. Many refineries are close to the source of fresh water. In order to prevent and reduce the corrosion of carbon steel, water treatment must be carried out. However, for some fresh water, the life of the steel pipe is not long despite strict water treatment, and more corrosion-resistant pipes must be used. In order to save water, the refinery needs to reuse the production sewage, but the sewage is highly corrosive, so titanium materials should be used to reduce water treatment as much as possible. In foreign countries, due to the relevant laws to protect the environment, it is impossible to use the traditional water treatment methods containing chromium and phosphorus. Therefore, the use of seawater or salt water as cooling water is actually the first choice for preventing and eliminating corrosion on the water side of the cooler. Saving fresh water is important because for a typical refinery, 80% of the water is recycled and the remaining 20% is lost to evaporation, air cooling and equipment leakage. For example, a refinery that processes an average of 100,000 barrels of crude oil per day loses more than 300 gallons of water per day. Such a large demand for fresh water is generally difficult to achieve. Therefore, in order to expand the scale of oil refining, a refinery should be established in the coastal area, which not only facilitates the import of crude oil, but also can use titanium seawater coolers to save fresh water as much as possible. It is said that about half of the cooling water in the heat exchangers of Japanese refining and chemical companies uses seawater.
Gr2Ti has almost no corrosion to seawater and can be used up to 113℃～121℃. In most cases, even in tight crevices, there is no need to worry about corrosion; however, above 113°C to 121°C, Ticodel2 is recommended to prevent possible crevice corrosion under chloride deposits and in potential crevices. Ticodel2 can be used in seawater at temperatures up to 260°C.
Another example is that almost all the heat exchangers in the Amuay refinery in the United States use salt water as the cooling medium. Due to the frequent corrosion and leakage of copper alloys, more than 60 sets of equipment use titanium tubes instead of copper tubes, which are not only resistant to salt water corrosion, but also resistant to H2S. Corrosion of materials, mainly used in power station condensers, compressor lubricating oil coolers, acid water coolers, potassium carbonate-CO2 or MEA regeneration tower top coolers, etc. The water speed of the cooler is (0.9～3.6)m/s. When the water speed is the upper limit, the copper alloy pipe end will be eroded. To prevent erosion, it is more economical to use titanium sleeve protection than to replace the titanium tube as a whole.
2.2 Hydrogen sulfide
Sulfur is present in crude oil, mostly as a compound, and a small amount as H2S. It cannot be removed in the oil field, but in the refinery, through high temperature heating, atmospheric pressure and vacuum distillation of crude oil, some sulfides become H2S, and H2S is also formed through hydroprocessing and certain catalytic reactions. Titanium is particularly resistant to sulfidation and pitting corrosion of higher, humid H2S in refinery coolers and is also immune to sulfide stress corrosion cracking (SSCC). Over the past 30 years, due to the processing of high-sulfur crude oil, the oil and gas in the overhead condensation system of the refinery contains high concentrations of H2S, and titanium shows excellent corrosion resistance. In a refinery that processes crude oil containing 3% to 5% sulfur, titanium is used to completely solve the corrosion of the condensed gas at the top of the tower. Replacing the copper-nickel alloy tube bundle with titanium eliminates scaling and eliminates cleaning requirements. But in hot H2S/Cl-containing oil and gas, when connected to certain active metal galvanic couples, titanium will experience hydrogen absorption and possible hydrogen embrittlement.
To prevent this, avoid joining titanium with carbon steel in an H2S/Cl environment above 77°C. Materials compatible with titanium include copper, copper-nickel, and stainless alloys (but remain passive).
2.3 Sulphur dioxide
Sulfur dioxide is formed during the dissimilation of sulfated olefins. Titanium resists sulfidation corrosion caused by sulfur gas and SO2 combined with condensed water to form sulfuric acid. Titanium has been used for over 10 years in wet sch reboiler SO2 removal and has shown fairly good performance.
2.4 Carbon dioxide
Carbon dioxide is present in crude oil and natural gas or dissolved in wash water and stripping water. For example, the corrosion of wet CO2 in surface condensers used in the treatment of amine-containing feedwaters has become a nuisance in the use of traditional materials. On the other hand, titanium has quite good corrosion resistance to dry CO2 or wet CO2. For example, in the gas separation part of the surface condenser, the general material will cause corrosion of steam condensate, but titanium has quite good corrosion resistance.
2.5 Hydrogen chloride
Crude oil generally contains a small amount of brine, which is difficult to remove in oil fields and can only be removed by desalination in refineries, but it cannot be completely removed. Hydrogen chloride is formed when thermal distillation, hydrotreating, and certain catalytic reactions break down chlorides in salts. Titanium will corrode in hot and humid hydrogen chloride and pH less than 1.5, but it is corrosion-resistant if oxidative inhibitors such as Fe3+, Cu2+, Ni2+ or HNO3 are present in the process medium. The amount of HCl in the oil and gas in the overhead system is generally controlled by deep desalination and injection of ammonia or amines to protect carbon steel. However, due to the organic chlorine brought by the oil recovery process, deep desalination cannot be removed, and HCl will be generated during the heating process of the oil, which requires the use of corrosion-resistant alloys such as titanium.
2.6 Ammonia
Ammonia is decomposed from various organic nitrogen compounds in crude oil or added to the process feed for the purpose of neutralizing acid. Titanium is resistant to ammonia up to 149°C and can remain passivated if sufficient water is present at the same time. Titanium works well in crude distillation column overhead condensers and sour water ammonia stripper condensers. Titanium hardly corrodes in concentrated boiling ammonium hydroxide (up to 70%).
2.7 Ammonium chloride
Ammonium chloride is formed when ammonia reacts with hydrogen chloride and is deposited as a solid in the equipment, which is one of the reasons for the corrosion of the top of the distillation column and its condensation cooling system. The deposition of ammonium chloride will cause crevice corrosion on industrial pure titanium when the temperature is higher than 93 °C, and the ammonium chloride scaling can be removed by washing with water, which should become one of the routine operation processes of the refinery. Ticodel2 alloy is recommended for tube coolers when deposition is unavoidable and the temperature exceeds 93°C. Ticodel2 alloy resists crevice corrosion over 176.5°C ammonium chloride deposition.
2.8 Oxygen
Although oxygen is rarely present in most processes, it is introduced as air in many feedstocks. If it exists in crude oil first, or enters through leakage of negative pressure equipment, it can also be dissolved in oil by contacting air with steam or water during stripping and washing. Oxygen generally promotes further passivation of titanium and helps maintain its protective oxide film. Different corrosive solutions, aerated ones are less corrosive than non-aerated ones.
2.9 Hydrogen
Titanium is generally suitable for use at temperatures as high as 315°C, moderate hydrogen partial pressure and the presence of moisture, however, it may lead to hydrogen embrittlement under certain circumstances. If the surface oxide film is scratched and absorbs more than (800~900)×10-4% hydrogen, according to experience and laboratory data, water or other passivating agents can promote the oxidation of titanium surface and reduce the possibility of hydrogen absorption. Surface contamination, especially titanium into the surface oxide film. The best way to remove iron pollution on titanium surface is to use 35vo1% HNO3+5vo1% HF solution at room temperature for (3~5)min pickling; anodization and thermal oxidation also show that it is beneficial to the formation of surface oxide film, and is often recommended for temporary use. Pre-treatment of hydrogen-titanium equipment. Tests and operating experience have proved that the above method is feasible in all cases where hydrogen embrittlement will occur, but anhydrous conditions should be avoided. If 2% or more moisture is generally effective to avoid hydrogen absorption. Titanium is not recommended for pure hydrogen environments.
3. Application analysis of titanium in refining equipment
Foreign refineries first used titanium tubes as coolers. Due to the corrosive oil and gas on the shell side and the polluted seawater on the tube side, the commonly used traditional metals are not suitable. With the development of refining processes and cost reduction, titanium is used not only in seawater corrosion areas, but also in process heat exchangers that are not cooled by seawater. The titanium application of related devices is described as follows.
3.1 Crude oil distillation
Titanium tube is used in the condensation cooling system at the top of the crude distillation tower, which can prevent chloride and sulfide corrosion, that is, prevent HCl-H2S-H2O environmental corrosion. In crude oil distillation, high boiling heavy oil is heated and fractionated to obtain a series of light oil products. But during distillation, heating above 121°C causes chlorides to form Hl, and above 260°C causes organic sulfides to form H2S. In this way, after reflux and separation, light hydrocarbons, water vapor, H2S and HCl are accumulated at the top of the column and condensed (see Figure 1). It has been confirmed by the on-site coupon test that the corrosiveness of titanium in the heat exchanger and condenser at the top of the tower is almost zero. Titanium tubes were first used in crude distillation overhead condensers in 1960 and have been reported to be doing well since then. The combination of titanium tube bundles and tube sheets has been used for many years not only in the United States but also in the United Kingdom, including heat exchangers for heating crude oil and final product condensers for cooling with sewage, with gas temperatures exceeding 149°C. In the above applications, titanium is used to replace Monel, CuNi alloys, aluminum brass and carbon steel. In some tower overhead condensers, when the gas temperature exceeds 121 ℃ ~ 149 ℃, it is found that there is corrosion under scale. If washing with water cannot eliminate salt scaling, it is recommended to use Ticodel2 tube instead of Gr2 titanium. Shiro Sato, a Japanese industry insider, believes that titanium will not undergo general corrosion in the condensation cooling system of atmospheric and vacuum distillation units, and is not affected by the sulfur content in crude oil and the pH adjustment.
3.2 Hydrodesulfurization
In the hydrodesulfurization process, when the temperature reaches 343 °C, the raw material is passed through the hydrogen to undergo a catalytic reaction, and the organic sulfur compound is cracked to form H2S. The effluent of the hydrogenation reactor is cooled and the residual hydrogen is separated from the product vapor, and the gas phase product This includes H2S, HCl, NH3, and water vapor, which is usually separated from the product vapor by distillation (Figure 2). The titanium tube can be used in the top condenser of the reactor to prevent chloride and sulfide corrosion, and can also be used in the effluent cooler of the desulfurization tower. The inlet temperature is 204°C and the pressure is 4.92MPa. Some units can also be used for brackish water and mixed water cooling with an outlet temperature of 49°C. Titanium can replace naval brass in these practical applications.
Basic hydrodesulfurization process
In 1972, Getty Oil Company of the United States used a large number of titanium tubes as heat exchangers (see Table 1 for details). Its corrosion resistance is not a problem at all. Among them, the use of titanium tubes in a fluid with a partial pressure of H2 of 652mmHg and a temperature of 160"(2 is remarkable.
Getty Oil Company Delaware Refinery Titanium Tube Usage Experience H2 Amount and Temperature in Shell Side Fluid
3.3 Acid Gas Removal
One of the most common methods for removing acid gases (H2S, CO2) from refinery process gases is absorption with ethanolamine solutions. Its representative system is the application of monoethanolamine (MEA) or diethanolamine (DEA), which can absorb H2S and CO2, and its rich solution is regenerated by heating to remove acid gas (see Figure 3). This is a H2S-CO2-RNH2-H2O corrosive environment, in which the reboiler is usually more severely corroded than other equipment in the system because of its high temperature. Titanium tubes can be used satisfactorily as MEA reboilers so far. On the MEA system inspection, the H2S top condenser titanium tube has no signs of corrosion for 6 years. Titanium can be used in the equipment if the existing material has a short service life in the lean-rich heat exchanger.
Typical Acid Gas Removal System
3.4 Solvent extraction
Titanium tubular heat exchangers are advantageous for most extraction processes. Because solvent extraction is recirculated, it can cause the build-up of corrosive compounds that can cause severe corrosion to common materials. In general, the process uses solvent and product effluents mixed, passed through a contactor, and then passed through a separation column to separate one or more product effluents from the feed/solvent mixture by flashing or stripping, while flashing The evaporated solvent vapor is condensed and, more importantly, regenerated for reuse (see Figure 4). Although there is no corrosive content such as H2S in the incoming solvent, it is oxidized and decomposed to form acidic substances during the recycling process, which can produce strong corrosion; in addition, the accumulation of trace H2S or other pollutants in the recycling process can also achieve corrosiveness Level. The main cause of corrosion in this process is the cooler. Freezers and regeneration equipment. Titanium has been used for many years in condensers in propane dewaxers and in wet SO2 reboilers in SO2 extraction systems.
Solvent extraction process diagram
3.5 Sour water stripping unit
Titanium tubes are extremely resistant to H2S-NH3-H2O environmental corrosion, and have been used in ammonia and hydrogen sulfide stripping tower overhead condensers. The hardness and tough oxide film of titanium can solve the erosion problem, while the use of softer aluminum can cause early tube damage. The use of titanium improves the operational stability of the sour water stripper. Individual instances of hydrogen embrittlement of titanium tubes have been reported in including MEA and sour water stripping overhead condensers. This is due to the high content of H2S medium and temperature higher than 77 ℃ environment, titanium and carbon steel or stainless steel contact
Contact causes galvanic corrosion. For example, ten air coolers in Amuay Refinery, the tube sheet is 316 and titanium tube expansion joint, in the corrosive environment containing H2S-NH3-H2O with a temperature of 122℃->82℃ and a flow rate of 6m/s, the galvanic corrosion of 316 steel is caused. Lead to hydrogen embrittlement of titanium nozzle. In order to eliminate this failure phenomenon, in the heat exchanger design, one is to use an all-titanium structure, and the other is to use alloys compatible with titanium, including copper-nickel alloys, Monel, Lncone1625 and Hastelloyc. A foreign company recommends the use of titanium tubes with aluminum fins for the stripping tower overhead air cooler in the acid water stripping unit with H2S>0.4%, NH3>0.15%, and CN->0.001% in a severely corrosive environment.
3.6 Catalytic Cracking
The sulfide in the raw oil produces H2S in the catalytic cracking, and some nitrogen compounds are also cracked, 10% to 15% are converted into ammonia, and 1% to 2% are converted into hydrogen cyanide, so that they can be absorbed and desorbed in the presence of water. The system constitutes a H2S-HCN-H2O corrosive environment. The temperature of this part is 40℃～50℃ and the pressure is 1.6MPa, which will cause uniform corrosion to carbon steel, hydrogen bubbling and SSCC, SSCC to austenitic stainless steel, and vulcanization and denickel corrosion of CuNi alloy. The U.S. Delaware Goldie Refinery uses titanium tube bundles in the fractionator condenser, secondary condenser, and debutanizer condenser of the catalytic cracking unit to replace traditional metals, and the effect is very good, and no corrosion phenomenon is found.
3.7 Other applications
Titanium has a successful record in most foreign refineries that process a large amount of oil. The best example is the use of titanium tube bundles in heat exchanger coolers. For example, the Exxon Bayway refinery has more than 40 successful application examples of titanium tubular heat exchangers. The refinery uses titanium in more than 10 different processes, including catalytic cracking, process water treatment, hydrorefining, desulfurization, fuel gas, unsaturated separation, hydrogen purification, reforming, tubular still and polymerization. According to Exxon's 1967 disclosure, titanium tubes expanded to Monel composite tube sheets showed good performance, and some all-titanium coolers were also used. The only corrosion problem is the under-scale corrosion of the heat exchanger when the temperature of the Gr2Ti tube exceeds 176.6°C on several treatment process sides. For this, the maximum temperature of the shell side reaches 148.9°C with seawater cooling or when the tube wall is on the process side containing chloride. Ticodel2 tube bundles are recommended when the temperature reaches 112.8"C, which can prevent corrosion under scale and avoid early failure. In addition, a special case of successful application of titanium in many refineries is the protection sleeve at the inlet end of the heat exchanger.
4. Conclusion
For heat exchangers used abroad in corrosive environments such as process media such as H, S, Cl, CN, CO2, NH3, RNH2 and seawater in the refining and chemical industry at low temperature (<230°C), titanium replaces Cu alloy, CuNi alloy, austenite Traditional materials such as stainless steel have been used for more than 40 years with outstanding results and are very popular, known as "dream metal". However, the application of titanium equipment in domestic refining and chemical enterprises is far from that of foreign countries, so there is a broad space for development. At present, domestic refineries mostly consider their high one-time investment, so less titanium materials are used. Should learn from the experience of the United States and Japan, from the comprehensive evaluation of life cycle and specific strength analysis, the selection of titanium should be very ideal. The current problem is that the publicity is not enough, and the market needs to provide titanium products that can compete with duplex stainless steel in terms of cost or price/specific gravity. If we can produce qualified welded titanium tubes, or use titanium tubes with a wall thickness of 0.7mm (equivalent to BWG20) or 0.9mm (equivalent to BWG22) titanium tubes in design, this will surely provide domestic refining and chemical enterprises with advanced technology. Tubes for heat exchangers and coolers.