Application of steel-plastic composite pipe fittings in naphthenic acid plants

The high corrosion rate of pipe fittings in naphthenic acid plants has always been a hot and difficult issue in anti-corrosion of domestic refinery equipment. Due to corrosion, the unit cannot run smoothly. In August 1999, steel-plastic composite pipe fittings were adopted, which reduced the corrosion rate of the device and put the production of the device into a virtuous cycle.

1. Medium situation and process principle of naphthenic acid unit 1.1. Medium situation of naphthenic acid unit The medium of naphthenic acid unit is mainly naphthenic acid, acidic water and remaining alkali residue with incomplete reaction; the main components of alkali residue are NaOH, NaHCO. With neutral oil and a small amount of sulfide sodium salt and phenol sodium salt, as shown in Table 1. Table 1 Types and composition (mass fraction) of alkali residue Type NaOH neutral oil sulfide sodium salt crude phenol liquid hydrocarbon alkali residue 3～5 2 O, 2～0.4 0.1～0.5 Chang 1, 2, Third line alkali residue 3 O. 3～3 0.1～1.2 0.1～0.3 Steam alkali residue 4, 7～8 2～4 2, 0 6～231.

2. Process Principle The naphthenic acid plant of the fifth joint workshop of Luoyang Branch mainly processes the alkali residues from the first, second and third lines of the atmospheric and vacuum units, the steam-driven alkali residues and the liquid hydrocarbon alkali residues from the demercaptan unit. The device consists of two sections: acidification and sedimentation. The alkali residues from the upstream devices are neutralized with 98% concentrated sulfuric acid to generate compound acidic water containing dilute HSO, naphthenic acid and incompletely reacted alkali residues with a temperature of about 100°C. This section is called the acidification section. The reaction equation is as follows: 2Na0H+ H2SO4-Na2SO4+ H2O2RCO0Na+ H2 SO4-Na2 SO + 2RCOOH. The settling section is the acidified mixed solution. After standing and settling, it is divided into two layers. The upper layer is crude phenol and naphthenic acid, and the lower layer is acidic. Figure 1 Acidification process flow Unexpected 2 Corrosion mechanism and material selection difficulties of naphthenic acid equipment 2.1 Corrosion mechanism Corrosion of naphthenic acid equipment is first manifested in chemical corrosion represented by dilute sulfuric acid, high-temperature sulfide and naphthenic acid: dilute sulfuric acid It will cause great corrosion to metal at any temperature, and the reaction will produce iron; and the corrosion of high-temperature sulfide is essentially based on H. S-based active sulfur corrosion. In naphthenic acid medium, although the sulfide content is not high, the Hzs decomposed by sulfides (mercaptans, disulfides) at 50 to 100 oC is extremely corrosive, and H s reacts with Fe to form FeS; in addition, the HS Alkanoic acid also reacts with iron at this temperature to form iron naphthenate; in addition, naphthenic acid also reacts with FeS to perform cyclic corrosion. The reaction equation is as follows: 3H2SO4+ 2Fe— F02(SO3)3+ 3H2OFe+ H 2S— FeS+ H 2Fe+ S— FeS2RCOOF+ Fe— Fe(RCoO)2+ H22RCooH + FeS— Fe(RCOo)2+ H2 Although the corrosion resistance of S18-8 steel is good, outside the weld of the steel pipe, due to the metal The phase structure is not completely consistent, and the presence of impurity elements or precipitated phases objectively causes potential differences between elements, leading to intergranular corrosion caused by selective dissolution at grain boundaries. At the same time, due to the intermittent production of the device, which is essentially the effect of the temperature difference (normal temperature ~ 100°C), the acid water pipe fittings are subjected to greater tensile stress, which is concentrated in the welds with lower strength, together with the welding residual stress. Formation of stress corrosion. In addition, the erosion of pipe walls by solid particles in the medium also aggravates the corrosion of pipe fittings.

2.2 Difficulties in material selection (1) Corrosion caused by the coexistence of highly corrosive media and solid particles. Media containing dilute sulfuric acid, alkali slag and naphthenic acid are extremely corrosive to metal pipe fittings. Carbon steel pipe fittings and pipe fittings selected before 1991 The average lifespan of the welds of the 18-8 steel pipe fittings selected in 1992 and 1993 was less than one month before causing large-scale corrosion and leakage. In addition, solid particles have a strong scouring effect on the inner wall of pipe fittings, which restricts the use of sheet metal pipe fittings. From the dismantled 18-8 steel pipe fittings, it was found that the inner surface became uneven in less than three months. Therefore, the inner surface of the pipe fittings used in this occasion must have a certain degree of corrosion resistance, as well as a certain degree of plasticity and wear resistance to ensure its service life.

(2) The medium composition is complex and the temperature is high, which accelerates the corrosion of pipe fittings. The medium composition of the naphthenic acid plant is quite complex. In addition to acidic water, alkali residue and naphthenic acid, with the refining of high-sulfur crude oil, there are also It was found that there is still a certain amount of C1; therefore, so far, there is no effective method for analyzing the medium components of the naphthenic acid plant, which brings great difficulties to the selection of materials for the naphthenic acid plant equipment; and the acidified The medium temperature is higher, making the medium more corrosive to pipe fittings; in normal production, the corrosion of pipe fittings in the acidification section is much greater than the corrosion of pipe fittings in the settling section, which is affected by high temperature. (3) Environmental temperature difference The macronaphthenic acid plant is an intermittent production, so the pressure-feeding medium is also intermittently pressure-feeding, which causes the temperature of the pipe fittings to be unstable and the temperature difference to be large; especially in winter, the temperature difference is extremely obvious. When the pressure feeding medium stops, the pipe fitting temperature is 0~10℃, and when the medium feeding pressure is stopped, the pipe fitting temperature is 50~80℃. Large temperature differences can easily cause expansion and contraction deformation of pipe fittings, causing damage to the pipe fittings. In 1995 and 1997, the acid-corrosion-resistant FRP pipe fittings and steel-enamel composite pipe fittings that were successively selected experienced rapid breakage and de-ceramics, making the FRP and enamel ineffective in anti-corrosion due to the influence of temperature differences.

(4) The pipe fitting is long and has many low points. The pipe fitting is about 1200m long, has 47 elbows, is erected over a long distance, and has many low points. When the delivery of acidic media is stopped, acidic liquid accumulates at low points, causing leakage point corrosion. Judging from the corrosion of steel pipe fittings, the flange welds and recesses of the pipe fittings have the highest corrosion rate. The complexity of naphthenic acid media brings great difficulties to the selection of pipe fittings. In the 10 years from 1987 to 1997, carbon steel, stainless steel, fiberglass, enamel, plastic and other materials have been used. In addition to polyplastic pipe fittings, which are corrosion-resistant In addition to strong corrosion resistance, other materials cannot meet production requirements due to various defects. The usage of various materials is shown in Table 2. Although poly pipe fittings have strong corrosion resistance, they still have low strength, thermal deformation and poor aging resistance. and other defects; therefore, in order to solve this problem, we combined the characteristics of plastic pipes with good corrosion resistance but low strength with the properties of steel structures. In August 1998, we adopted steel-polymer composite pipe fittings, which achieved better results. . Table 2 Usage of pipe fittings made of various materials Temperature resistance value Usage Corrosion resistance Strength Aging resistance Plasticity (not greater than)/°C Carbon steel Poor Good 35O Strong Poor l8 8 Weld quality poor Good 85O Strong Poor Glass fiber reinforced plastic Good Poor 2 5O Stronger Bad Enamel Good Good Better 350 Strong Bad Plastic Good Good Bad 100 Bad Good 3 Theoretical basis and application of steel-plastic composite pipe fittings

3.1 Theoretical basis of steel-plastic composite pipe fittings There are many types of steel-plastic composite pipe fittings, among which the more typical and commonly used types are steel-fluoroplastic composite pipes, steel-polyylidene fluoride composite pipes, steel cluster composite pipes, etc. What they all have in common is that they are highly resistant to acid and alkali corrosion.

(1) In addition to molten metal, lithium, potassium, sodium, oxygen trichloride, and high-flow liquid fluorine, the steel-fluorine-plastic composite pipe can resist almost all chemical substances, including concentrated nitric acid and corrosion. It can be used for a long time at 230 ~250. Works at C temperature.

(2) Steel polyvinylidene fluoride composite pipe has good corrosion resistance to halogens, halogenated hydrocarbons, strong oxidants, boiling acids, alkali and other organic solvents, but it is resistant to concentrated hot sulfuric acid and nitric acid, and ketones above 9o°C. , grease, amines and high-temperature sulfonating agents are not corrosion-resistant. (3) Steel-polymer composite pipes are resistant to most organic and inorganic acids, alkalis, and salts, except concentrated nitric acid, acetic acid, and other strong oxidizing acids. They have good resistance to stress corrosion and can withstand nitric acid, acetic acid, and 36 Below, below 2O, dilute, various concentrations of carbonic acid, alkali and most salts, air, NH. , CO, CO, NO, HC1 and other gas corrosion. Table 3 shows the corrosion resistance test data of steel-polymer composite pipe fittings. Table 3 Corrosion resistance test data of steel-polymer composite pipe fittings Medium concentration, 6O. C 8O℃ 100 C 0～35 () O O Sulfuric acid 0～50 () O △Nitric acid 0～10 () O () 0～20 () △ ▲ 0～50 () O O times 0～15 () 0 O ( ) () O chlorine water 300×10-6 () () () Acetic acid 0～30 O () 0 Oxalic acid 0～100 0 () 0 Phosphoric acid 0～85 O () () Note: [ ] — — Excellent; △ — — Shangsi; ▲ Yibusi. 3.2 Application of steel-plastic composite pipe fittings in naphthenic acid plants. Various steel-plastic composite pipe fittings have good acid and alkali corrosion resistance. In particular, steel-polyfluoride composite pipes have the strongest corrosion resistance and have an extremely wide range of use. Known as the King of Steel and Plastic. However, due to its high cost (DN80mm, pipe is about 900 yuan per meter), which is more than three times that of steel-polymer composite pipe fittings (DN80mm pipe is about 250 yuan per meter), therefore, when using it, it is necessary to consider its use value , and its economic value must be considered. Therefore, in June 1998, a 15m steel-polymer composite pipe was tried in the worst medium section of the naphthenic acid plant, and it ran continuously for one year. When it was opened for inspection in July 1999, it was found that the inner surface of the steel-plastic composite pipe, in addition to the temperature Except for traces of local stretching and slight thinning, there is no corrosion phenomenon. Therefore, in August 1999, except for the steel-polyfluoride composite pipes used in the acidification section with the highest temperature, steel-polyfluoride composite pipes were used in other places. It has been running continuously to this day without corrosion and leakage, and the effect is good. 4. The importance of the promotion and application of steel-plastic composite pipe fittings in naphthenic acid plants. The toxicity of crude phenol in the medium of naphthenic acid plants and its volatility prevent people from inhaling it for a long time or excessively. This further affects the anti-corrosion and safety of the equipment. The aspects of production and environmental protection illustrate the practical significance of promoting the use of steel-plastic composite pipe fittings in naphthenic acid plants; although the types of steel-plastic composite pipe fittings are different and the costs vary greatly, different steel-plastic composite pipe fittings can be used in different parts. Tube,