1、IMPLEMENTATION OF BEST PRACTICES FOR WATER WASHES IN HYDROPROCESSING UNITS R.J. Horvath Shell Global Solutions (US) Inc. P.O. Box 1380 Houston, TX 77251-1380 K.R. Lewis Shell Global Solutions International BV Badhuisweg 3 Amsterdam, 1031-CM, The Netherlands ABSTRACT This paper describes a systematic
2、 approach that is used to diagnose and mitigate corrosion problems in hydroprocessing unit reactor effluent air cooler systems. The methodology is based on an extensive evaluation of hydroprocessing water wash best practices and addresses both design and operational issues by using state-of-the-art
3、process engineering and corrosion prediction models. The goal is to develop cost-effective and robust solutions to reliability problems in reactor effluent air coolers and associated piping. The paper provides examples where this knowledge and methodology have been applied effectively to a variety o
4、f hydroprocessing units around the world. Keywords: ammonium bisulfide, NH4HS, water wash, reactor effluent air cooler, REAC, hydroprocessing unit, best practices, reliability, risk INTRODUCTION Despite decades of operating hydrocrackers and hydrotreaters, many of these hydroprocessing units continu
5、e to experience corrosion and fouling in the reactor effluent air cooler (REAC) and associated piping systems. Ammonium salts, especially ammonium bisulfide (NH4HS), have typically caused this corrosion and fouling. However, ammonium halide salts such as ammonium chloride (NH4C1) have also caused si
6、milar problems in some units. Wash water is typically injected upstream of the REAC to prevent fouling by solid ammonium salts and Linda Goldberg - Invoice INV-943138-V3N4L4, downloaded on 7/9/2015 1:29PM - Single-user license only, copying/networking prohibited.dilute the concentrations of the salt
7、s dissolved in the sour water to an acceptable level consistent with the corrosion resistance of the materials of construction of the REAC system. In many cases, inadequacies in the amount of wash water injected or non-uniform distribution of the wash water to the various air cooler banks has caused
8、 corrosion of carbon steel materials and subsequent fouling by the iron sulfide corrosion products. In turn, local accumulations of iron sulfide deposits have produced further maldistribution of flow and localized increases in corrosion rates. Mitigating corrosion and fouling problems and improving
9、the reliability of REAC systems in hydrocrackers and hydrotreaters is an ongoing challenge. Business-driven changes in unit operation to increase profitability (e.g., increased throughput, lower quality feeds, higher activity/severity catalysts) or to meet more demanding finished product specificati
10、ons (e.g., reduced sulfur levels required for pollution control) are very prevalent and usually tend to increase the likelihood of fouling and corrosion in these units. All too often the approach that has been used to cope with these changes is to rely on experiential rules of thumb to limit ammoniu
11、m bisulfide corrosion and more frequent and extensive inspection. Unfortunately, the industry record of lost production and major incidents has demonstrated that this approach has not been adequate in many instances. Best practices for water washing to mitigate corrosion and fouling in the REAC syst
12、ems of hydroprocessing units have been developed from Shell and other industry technology and experience. In addition, new quantitative engineering test data on the effect of many process parameters on ammonium bisulfide corrosion rates has allowed the integrity operating window for a given REAC sys
13、tem to be established with much more confidence. These best practices and ammonium bisulfide corrosion data are being applied to mitigate the risks of corrosion and fouling in existing hydroprocessing unit REAC systems and for new construction projects. For existing units, a structured multi-discipl
14、ine approach has been used to identify significant deviations (gaps) from these best practices and reach agreement on an optimized set of strategies to mitigate the integrity risk to an acceptable level. These strategies include a clearly defined integrity operating window, system design, materials
15、of construction, and inspection. An overview of tfiis approach is presented. Three case histories are discussed where this structured multi-disciplined approach has led to operational improvements and in one case a major material upgrade to improve the overall reliability and reduce reliance on insp
16、ection. DEVELOPMENT OF BEST PRACTICES Within the last few years a major effort has been made to develop and document best practices for water washing to mitigate corrosion and fouling in hydroprocessing units. This has been a multi-discipline activity involving process engineers and corrosion, mater
17、ials and inspection specialists. Considerable effort was made to review technology within the Shell advised refineries and within the refining industry as a whole to develop recommended best practices that could be applied to a wide variety of unit types, unit configurations, and wash water injectio
18、n designs. Best practices and guidelines that can be applied to the design of new units and for maintaining the desired reliability of existing operating units have been developed. Linda Goldberg - Invoice INV-943138-V3N4L4, downloaded on 7/9/2015 1:29PM - Single-user license only, copying/networkin
19、g prohibited.Process Modeling Hot hydroprocessing reactor effluent streams typically contain various ammonium salt precursors such as ammonia (NH3), hydrogen sulfide (H2S), hydrogen chloride (HC1), and sometimes hydrogen fluoride (HF) that are present as gases. Solid ammonium salt deposits can form
20、directly from these gases as the temperature of the reactor effluent stream falls below the salt deposition temperature. Wash water injection cools the reactor effluent stream, usually creating a three-phase mixture of vapor, hydrocarbon liquid, and sour water. Process simulation models are often us
21、ed to calculate heat and material balances, stream and phase compositions, and phase flow rates and properties. These results are then used to determine the fraction of injected wash water remaining as liquid, the aqueous dew point and salt deposition temperatures, line velocities and flow regimes,
22、etc. It is important to select appropriate tools when simulating hydrocracker and hydrotreater processes. When simulating water wash systems, consideration must be given to the rigor of the simulation methods. These sour water systems involve ionic equilibria, which most simulation methods only appr
23、oximate. For many applications these approximations are adequate, and use of a less-rigorous non-ionic thermodynamic model is acceptable. However, some applications require the use of a rigorous ionic thermodynamic simulation method. Criteria have been established for selecting the appropriate simul
24、ation method. The process simulation model can then be used to predict the values of key process parameters under various operating scenarios. Integrity Operating Window Reliability of the REAC system is highly dependent upon establishing the appropriate integrity operating window and maintaining th
25、e operation within that window. The integrity operating window is a defined set of process parameter limits that are established to maintain control of the corrosion and fouling in the system. These process parameter limits are generally based on the design of the system, existing materials of const
26、ruction, and an assessment of integrity risk associated with sustained operation of the process within this window versus outside this window. Operation outside this established integrity operating window should require management of change (MOC) considerations. Each unit will have a specific set of
27、 key process parameters with specified limiting values for each of those parameters to properly define the integrity operating window. Key process parameter limits that are often specified include feed rates (which may relate to stream velocities and flow regimes), concentration of corrosive contami
28、nants in the feed streams (e.g., sulfur, nitrogen, chloride, fluoride), flow rate of wash water injected (total rate and rate to each nozzle in multiple injection systems), quality of wash water (e.g., oxygen content, solids, pH), composition of the sour water (e.g., NH4HS concentration), etc. Proce
29、ss Monitoring. Routine monitoring is needed to assure that the key process parameters are maintained within their specified limits. Operation outside of these limits may result in accelerated corrosion rates and loss of containment. A sound process monitoring program involves taking the samples at t
30、he correct locations, using safe and proper sampling and Linda Goldberg - Invoice INV-943138-V3N4L4, downloaded on 7/9/2015 1:29PM - Single-user license only, copying/networking prohibited.sample-handling procedures to preserve the imegrity of the sample until it is analyzed. These samples must then
31、 be analyzed by the proper methods in a timely fashion. In these complex sour water solutions, interferences by other solution contaminants can invalidate desired process parameter data obtained by some test methods. Monitoring frequency of each process parameter should be established such that oper
32、ations can make a timely response to maintain operation of the unit within the integrity operating window. Corrosion Rate Prediction. Until recently the understanding of ammonium bisulfide corrosion has been somewhat limited due to the lack of basic corrosion data for various materials of constructi
33、on and service conditions, especially at higher flow velocities. The industry generally relied on experientially based rules of thumb to establish guidelines on NH4HS concentration and velocity limits. As these guidelines proved to be inadequate in a number of applications, a specially designed labo
34、ratory test apparatus was used to generate corrosion data for ammonium bisulfide solutions over a wide range of concentrations and velocities. Ionic modeling was used to produce desired solution properties at actual test conditions. The test program included typical materials of construction current
35、ly used in, or potential candidates for, REAC systems. This program was very successful, and resulted in significantly increased understanding of the effect of concentration and velocity on ammonium bisulfide corrosion. These data on the effect of concentration and velocity were incorporated into an
36、 expanded program to further study ammonium bisulfide corrosion in refinery sour waters, which has been administered as a joint industry program (JIP) known as the Sour Water JIP . The impact of other system variables including hydrogen sulfide partial pressure, temperature, chloride concentration,
37、hydrocarbon/sour water mixture ratio, and presence of inhibitors have been evaluated. A key deliverable of the Sour Water JIP is the development of a software tool that can predict the ammonium bisulfide corrosion rate at actual service conditions based on the quantitative test data generated in thi
38、s program. The learnings from the Sour Water JIP have led to a much better understanding of ammonium bisulfide corrosion in REAC systems. They have led to improved materials selection for the REAC and associated piping, and have also been invaluable for better defining the integrity operating window
39、 for existing REAC systems. The case histories reported below have made use of these learnings. Inspection Considerations Equipment and piping in the REAC system is subject to various types of corrosion damage associated with the ammonium salts or HC1. Wall thinning can be somewhat uniform, but in m
40、ost cases the corrosion is very localized. In these multi-phase REAC systems, the pattern of thinning can vary widely from system to system, and even within the same system. This is usually related to the flow regime present. The localized corrosion can take the form of pitting or underdeposit attac
41、k in stagnant or low flow regions (often associated with ammonium chloride) or erosion-corrosion that produces grooving or washouts that usually occur in regions of high flow turbulence or impingement (often associated with ammonium bisulfide). In addition to wall thinning, other forms of degradatio
42、n may also occur in the REAC systems, depending on the susceptibility of the various materials of construction used. For steels, this includes hydrogen blistering, sulfide stress cracking (SSC), hydrogen-induced cracking Linda Goldberg - Invoice INV-943138-V3N4L4, downloaded on 7/9/2015 1:29PM - Sin
43、gle-user license only, copying/networking prohibited.(HIC), and stress-oriented hydrogen-induced cracking (SOHIC). For austenitic stainless steels and alloys, this includes chloride stress corrosion cracking and polythionic acid stress corrosion cracking. Periodic inspection should be performed to m
44、onitor the REAC system for the presence and rate of deterioration resulting from the various forms of degradation that may occur. It must be recognized that thinning may be, and usually is, very localized. Therefore, inspection methods and coverage that can provide a high confidence of detecting suc
45、h localized damage should be chosen. Special attention should be given to inspection of locations subject to high velocity and turbulence or impingement such as changes in flow direction (e.g., tees, elbows and valves). Areas of stagnant or low flow (e.g., dead legs) that may be subject to pitting o
46、r underdeposit attack should also be given special attention. REAC header boxes are typically very difficult to inspect properly due to geometric considerations, and thus may require that special ultrasonic testing procedures be utilized. REAC system equipment and piping should be inspected in accor
47、dance with appropriate and applicable inspection codes, and condition-based and risk-based criteria. Thermographic inspection and local temperature measurements have been shown to be useful tools that can provide an indication of maldistribution of wash water. IMPLEMENTATION OF BEST PRACTICES These
48、best practices are now being used as the technical basis to conduct systematic reviews of the water wash systems in existing hydrocrackers and hydrotreaters. They are also being used as key input to the basis for design of new water wash systems for major construction projects or revamps of existing
49、 hydrocracker and hydrotreater units. Water Wash Review The water wash review is a thorough review of hydrocracker and hydrotreater water wash and integrity management practices conducted in a multi-discipline workshop. The risks of fouling and corrosion problems are assessed, with a primary focus on ammonium salts. Recommendations are developed to mitigate the significant risks identified. The review team usually consists of hydroprocessing and corrosion/materials specialists from corporate staff, and the unit process engineer, operations specialist, pressure equipment inspector and/or corro
