Corrosion Identification And Management In Crude Oil Refineries
The refining of crude oil into usable merchandise is a fancy process with many alternatives for corrosion to gain a foothold. Understanding the typical atmospheric distillation course of stream in a crude oil refinery is a priceless software to assist determine corrosion and implement controls in these locations where corrosion is more likely to happen.
Upstream, Midstream and Downstream Sectors of the Oil and Gasoline Trade
The oil and gasoline industry is classified into downstream, midstream and downstream sectors. The upstream sector involves the exploration and manufacturing of oil and fuel, the midstream includes the transportation of the produced oil and gasoline, and the downstream sector covers the processing and marketing.
Understanding Oil: Sweet and Sour, Heavy and Mild
Crude oil is called “sour” when it accommodates unacceptable amounts of sulfur compounds, while “sweet crude oil” is the alternative. Crude oil is termed “heavy” when it’s made up of longer hydrocarbon chain compounds, and “gentle” when it comprises shorter chained hydrocarbons (light diesel to methane). Sometimes, crude oils are made up of paraffins (alkanes), naphthenes (cycloalkanes), aromatic hydrocarbons and asphalts.
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An overview of Crude Oil Refining
Crude oil refining is the key chemical engineering strategy of the downstream sector as a result of it entails the production of gaseous and refined liquid merchandise for the end person. The commonest merchandise are methane, liquefied petroleum gas (LPG), gasoline, naphtha, aviation kerosene, diesel, gas oil and heavy residue. Determine 1 shows a typical atmospheric distillation course of move in a crude oil refinery.
Determine 1. Process circulation diagram of a typical atmospheric distillation process in a crude oil refinery.
The crude oil from the tank farm (prime-left in Figure 1) is first slightly heated earlier than sending it to a desalter to take away most of the salt content. Downstream of the desalter is the crude oil preheating practice where the crude oil is preheated prior to coming into the fired heater. The crude oil is usually preheated with the distillates from the distillation column because this promotes heat integration. Gas oil or surplus gasoline fuel from the refinery is usually used to provide the heat mandatory for the fired heater. The fired heater raises the crude oil temperature to the atmospheric distillation temperature of 350°C to 400°C (662°F to 752°F). The 2 section crude oil is sent to the bottom of the distillation column the place completely different fractions distill alongside the column top based on the differential boiling level. The trays in the distillation column permit the distillates to be collected and drawn off, and then sent to either a storage tank or additional processing (e.g. hydrotreating). The heavy residue, that are hydrocarbons with greater than 25 carbon atoms drops to the bottom of the column and is distributed to the vacuum distillation unit. The lightest hydrocarbons with just one to four carbon atoms leave the top of the distillation column together with water vapor and impurities. The vapor stream is condensed to recuperate the water vapor, which is then refluxed back to the column to boost the distillation and separation course of.
Crude Oil Corrosion Sources
Crude oil by itself is not corrosive, however the reservoir supply and method of manufacturing might include any of several impurities, reminiscent petroleum refinery turnaround you of water, salts, sulfur compounds, oxygen compounds and carbon dioxide (CO2). (Be taught extra within the 6 Corrosive Components That can be Present in Crude Oil.)
Salts and Water Content
The commonest salts seen in crude oils are sodium chloride (NaCl), calcium chloride (CaCl2) and magnesium chloride (MgCl2). However, the salt focus of crude oils depends closely on the kind and source of crude oil. Additionally, crude oil incorporates some free water which is often removed by a floor separator.
The presence of those salts and water in the crude oil results in the formation of hydrochloric acid often known as salts hydrolysis according to Eqs. (1) to (4).
Eq. 1: CaCl2 + H2O ↔ CaO + 2HCl
Eq. 2: CaCl2 + 2H2O ↔ Ca(OH)2 + 2HCl
Eq. 3: MgCl2 + H2O ↔ MgO + 2HCl
Eq. Four: MgCl2 + 2H2O ↔ Mg(OH)2 + 2HCl
The salts hydrolysis normally occurs at 120°C and 210°C (248°F and 410°F) for MgCl2 and CaCl2 respectively and is more prevalent in regions of high temperature (from the preheating exchanger to the distillation column).
Moreover, during the cooling down of the distillation column gentle ends, the hydrogen chloride fuel mixes with the steam condensate to form hydrochloric acid. This acid accelerates both basic corrosion and pitting corrosion. Corrosion associated to hydrochloric acid is difficult to regulate because the chlorine atom (with a really small measurement) easily penetrates the protecting layers of floor metals.
Subsequently, the acidity of the condensed water vapor (water condensate) in the overhead separator must be monitored frequently to determine if neutralization is required in addition to to vary the speed of alkaline injection. When this is established, natural neutralizers are injected at the vapor line (prime exit) of the atmospheric distillation column to cut back the focus of protons (H+). Probably the most often used neutralizers, which are alkaline in nature, are ammonia (NH3) and filming amines (RNH2), and they react with hydrogen chloride to form ammonium chloride (NH4Cl) and amine hydrochloride (RNH3Cl) salts in keeping with Eqs. (5) and (6).
Eq. 5: NH3(g) + HCl(g) → NH4Cl(s)
Eq. 6: RNH2(g) + HCl(g) → RNH3Cl(s)
The rate of neutralizer injection must be performed with precision to avoid a pH swing. It’s because the pH stability is necessary in direction of maintaining a corrosion free surroundings. The amine hydrochloride salts shouldn’t be allowed to accumulate for a very long time at the bottom of the overhead separator as a result of it could actually set off corrosion.
Crude oil and distilled fractions comprise certain amounts of sulfur compounds and they are principally bonded with carbon atoms, while the remaining are in the form of hydrogen sulfide (H2S) and elemental sulfur. Throughout atmospheric distillation, H2S exits with the light ends and water vapor from the top of the distillation column. This not only attacks the internals of the distillation column however corrodes the downstream piping that conveys the sunshine ends to the condenser and overhead separator. The corrosion additionally extends to light ends downstream processing gear. Additionally, a few of the H2S and different sulfur compounds additionally exit with liquid fractions such as gasoline, naphtha, aviation kerosene and diesel, causing extreme corrosion problems.
So as to attenuate corrosion points at the top section of the distillation column and in downstream tools, NH3 and/or filming amine is injected at the top exit (vapor line) of the distillation column. The reaction between NH3 and H2S results in the formation of ammonium bisulfide, whereas the H2S and filming amine response varieties amine bisulfate (See eqs. (7) and (8)). The bisulfate and bisulfide is withdrawn with the wastewater sent to the bitter water treatment section.
Eq. 7: NH3(g) + H2S(g) ↔ (NH4)SH(s)
Eq. Eight: RNH2(g) + H2S(g) ↔ RNH3HS
It will be significant to notice that the ammonium bisulfide and amine bisulfate also introduce corrosion problems within the bitter water remedy items. Nonetheless, these sulfides and sulfates are much less corrosive than H2S and their corrosion charges are significantly influenced by the fluid velocity.
As for the liquid fractions (gasoline, naphtha, petroleum refinery turnaround you aviation kerosene, diesel) containing H2S and different sulfur compounds, they are usually purified by a hydrodesulfurization course of. The opposite sulfur compounds embody but aren’t restricted to mercaptan (RSH), sulfide (RSR), disulfide (RSSR), and thiophene (C4H4S), where ‘R’ within the chemical components is a hydrocarbon compound. In the hydrotreating unit, pure H2 is used to desulfurize the liquid fractions as shown in Eqs. (9) to (12).
Eq. 9: 2RSH + H2 ↔ 2RH + 2H2S
Eq. 10: RSR + H2 ↔ 2R + H2S
Eq. Eleven: RSSR + H2 ↔ 2R + 2H2S
Eq. 12: C4H4S + H2 ↔ C4H4 + H2S
Single compounds containing both aromatics and sulfur (AS) are additionally present in crude oil distillates. The aromatic compound is ‘A’ while the sulfur compound is ‘S’. In such situations, hydrodesulfurization can be used to take away the sulfur compounds as shown in Eq. (Thirteen).
Eq. Thirteen: AS + H2 ↔ A + H2S
Oxygen gearing hydrocarbons also exist each in crude oil and within the fractionated crude oil distillates. Naphthenic acids (RCOOH) are widespread oxygen-bearing hydrocarbons, the place R are primarily cyclopentane and cyclohexane derivatives. Most naphthenic acids have an atomic mass starting from one hundred twenty to more than seven hundred. Naphthenic acids are usually current in crude petroleum refinery turnaround you oil distillates that boil from 200°C to round 370°C (392°F to 698°F), and at temperatures above 370°C (392°F) and 400°C (752°F) the acids will thermally decompose.
The presence of naphthenic acids in crude oil is determined by the overall acid number (TAN) approach. In this method, the amount of potassium hydroxide (KOH) in milligrams that neutralizes one gram of crude oil quantifies the TAN. Crude oils are also said to be corrosive when their TAN is greater than 0.5. Based on TAN results, corrosion prevention techniques will likely be put in place.
Corrosion resulting from naphthenic acids follows the reaction proven in Eq. (14), producing naphthenates and hydrogen. This reaction progressively eats away the surface space of the metallic, therefore increasing in corrosion fee.
Eq. 14: Fe + 2RCOOH ↔ Fe(RCOO)2 + H2
In the course of the hydrotreating course of to remove sulfur compounds from crude oil distillates (Eqs. (9) to (12)), hydrogenation of naphthenic acids also happens (Eq. (15)). This hydrogenation response produces the corresponding hydrocarbon and water (steam).
Eq. 15: RCOOH + 3H2 ↔ RCH3 + 2H2O
Carbon Dioxide (CO2)
One other component that makes crude oil and distillates corrosive is the presence of CO2. The quantity of CO2 can differ from reservoir location to location, most particularly when CO2 enhanced oil recovery is applied. The dissolved CO2 within the crude oil leads to excessive corrosion especially in the presence of free water (Eq. (Sixteen)). This reduces the metallic thickness (most instances carbon steel) after a period of interplay with CO2. Apart from the concentration of CO2 dissolved in the crude oil, the stress of the crude oil also increases the corrosion fee. It is because the higher the crude oil strain, the higher the CO2 partial stress.
Eq. Sixteen: Fe(s) + CO2(aq) + H2O(l) ↔ FeCO3(s) + H2(g)
The formation of iron(II) carbonate (FeCO3) can precipitate out of the solution and kind a protecting layer on the metallic surface, hindering further corrosion. Nevertheless, a continuous buildup of the FeCO3 can cause a stress drop alongside the crude oil pipeline, which leads to further challenges. Moreover, the precipitated FeCO3 may be disturbed and washed away by the excessive stress crude oil, leading to further corrosion of the metallic floor. (Additional studying: An Intro to Pipeline Corrosion and Coatings.)
So as to combat such problems in crude oil pipelines, acceptable material choice (stainless steel) ought to be used. On the other hand, most of the CO2 with the crude oil exits the top of the atmospheric distillation column with the light ends. Due to this fact, CO2 seize utilizing reactive amine solvents are used to take away CO2 from the gas stream. In such a case, the amine solvent removes both CO2 and H2S from the fuel stream (acid fuel removing unit) before fractionating them into methane (demethanizer), ethane (deethanizer), propane (depropanizer) and butane (debutanizer).
The presence of sulfur compounds, oxygen compounds, salts and water leads to extreme corrosion problems in crude oil pipelines and downstream distillates equipment. Due to this fact, it’s crucial to cut back the concentration of these impurities.
The interaction between salts and water is a key contributor to corrosion in crude oil pipelines due to hydrochloric acid formation. Using ammonia and filming amines has the ability to scale back the related issues mostly at the top exit of the distillation column.
Sulfur compounds similar to H2S, mercaptans, etc. are additionally linked to severe corrosion points in crude oil refining items. Many of the H2S leaves with the vapor fraction of the crude oil, while some residual H2S and other sulfur compounds are current within the liquid distillates of crude oil. In the vapor line of the distillation column, ammonia (NH3) and filming amines are injected while H2 is used to remove all sulfur compounds from the liquid distillates (hydrodesulfurization). Any entrained H2S and CO2 leaving with the light ends from the highest of the overhead separator is eliminated in the acid gasoline elimination plant with reactive aqueous amine solutions. Hydrogenation reactions break down naphthenic acid into naphthenates and water, therefore altering the corrosion response route.
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