Corrosion monitoring solution for HF alkylation units

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1 Corrosion monitoring solution for HF alkylation units 1. Overview of the alkylation process Alkylation units are a key part of product upgrading in modern refineries, important to maximising profitability and the achievement of critical gasoline quality specifications, such as controlling aromatics content, octane number and vapour pressure. The process converts an olefin feedstock, mostly butene, normally produced by the catalytic cracker with iso-butane, which is either imported, recovered from other refinery units or produced by a dedicated n-butane isomerisation process. The reaction product is a branched-chain molecule, isooctane, normally referred to as alkylate, which has a high octane number for production of premium quality gasoline grades. Commercial processes use catalysts like hydrogen fluoride (HF) to enable the alkylation reactions to occur at ambient temperature. The mixed alkylation unit feed is first pre-treated to remove contaminants like water and sulphur. The treated feed is mixed with recycle iso-butane and then contacted with hydrofluoric acid as a catalyst in the reaction section at C ( F). The reactor product is separated by phase in a settler drum, and acid is recycled back to the reactor. A small slipstream of acid is sent to the acid regenerator to help the removal of acid soluble oil (ASO) and water. The hydrocarbon phase from the settler passes through fractionation columns, which separate the product into propane, n-butane and alkylate products, recycle iso-butane and hydrofluoric acid. The n-butane and propane are posttreated to remove any organic fluorides that may have been formed as well as any trace amounts of hydrofluoric acid that may be present. HF alkylation unit configurations vary according to the individual design by the licensors, UOP and heritage Phillips66. Corrosion, and therefore monitoring locations, can differ according the unit designs, but general corrosion issues apply to all of these designs. 2. Overview of corrosion issues within HF alkylation units The simplified process flow diagram in Figure 1 highlights the locations with high corrosion risk in a HF alkylation unit. Most equipment is constructed from carbon steel. Upon contact with highly concentrated acid, a stable iron fluoride layer is formed inside the equipment - which provides adequate protection for the carbon steel against further corrosion by HF acid. But upon contact 1 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

2 with an acid/water mix, an unstable and non-protective hydrate scale is formed, which swells and can become detached from the equipment. Figure 1: HF alkylation process flow diagram showing high risk corrosion areas Corrosion in HF alkylation units depends strongly on a complex and sensitive system of operational parameters to handle the hydrofluoric acid catalyst safely and cost-effectively. The combination of temperature, phase change, HF concentration and locations with low- or no-flow conditions makes some equipment and pipework particularly vulnerable to attack. Corrosion is often aggressive, short-term and transient, driven by process upsets, like excursions of temperature or water content, or changes in feedstock quality - this makes corrosion difficult to track using traditional methods such as manual ultrasound that are carried out periodically. Key corrosion drivers within the HF alkylation unit include: a. Water content in circulating acid although both the iso-butane and the butane olefin feeds are dried, water content in the circulating acid can build up. Figure 2 shows the detrimental effect of increased water content in the HF acid on the corrosion rate of carbon steel. The water content in the circulating acid is normally kept below 2%, which results in generally low corrosion rates in the main reactor/settler circuit operating in liquid phase. But more diluted acid can be formed under the following conditions: Upon phase change: if a 98% HF / 2% water vapour mixture is condensed, the first droplet will contain about 30% water. An example of these conditions is condensation of warm acid vapour in a cool dead-leg. Figure 2 below shows that this concentration corresponds to the maximum corrosion rate. By extraction: for example, in entrained acid droplets from the settler. If free acid (containing 2% water) is contacted with HF sub-saturated hydrocarbons, the HF will be extracted into the hydrocarbon phase. Hence, the water content of the remaining free acid phase will increase. 2 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

3 By formation of liquid water: a free water phase is formed in the product defluorinators when exceeding the water solubility concentration in the propane or n-butane product. Traces of HF result in a low ph and a corrosive water phase in the product rundowns. Figure 2: Corrosion rates of aqueous HF as function of HF content in water at ambient temperature b. Poor acid settler performance - this condition leads to HF acid entrainment into downstream fractionation columns and a free acid phase can appear in the overheads as a result. c. Temperature a higher operating temperature accelerates the corrosion of carbon steel. Above C ( F), Monel (Alloy 400) is commonly used. Important areas of the unit requiring close monitoring for corrosion include: i. Acid regeneration section - a critical part of the unit as it enables control of the acid quality by separating ASO and water. The bottom of the acid regenerator has to handle a corrosive mixture of hydrofluoric acid, water and ASO at high temperatures of 150 C (300 F) or more. The acid vaporisers are equally vulnerable equipment parts due to the phase change that occurs there. The overhead line is also exposed to condensing HF vapours. ii. Main fractionator and depropaniser feed lines - these operate at higher temperatures (which can exceed 65 C/150 F), and in case of acid entrainment from the upstream settler, dilute acid will be formed by extraction. iii. Downstream defluorinators - water is formed in the defluorinators by the main reaction. Depending on the concentration of organic fluorides in the defluorinators feed and the product rundown temperature, free water can be formed downstream of the rundown cooler. Traces of HF will lower the ph of the water phase and create corrosive conditions. Defluorinator bed channelling and HF breakthrough can occur when spent defluorinator treating beds are not replaced frequently enough. iv. Dead-leg corrosion - acid vapours can condense in dead-legs and form dilute acid, which creates a non-stable, swollen hydrated iron fluoride scale. This results in loss of wall thickness and internal fouling. Dead-legs can be minimized by careful planning of pipe runs, but normally cannot be completely avoided - for example at the inlets of relief valves, see Figure 3. 3 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

4 Figure 3: Internal corrosion and fouling of a relief valve inlet line v. High level of residual elements in carbon steel piping the content of key residual elements (Cu, Ni, Cr) in carbon steel has an upper limit for HF alkylation units service. When carbon steel pipe components with high and low content of residual elements are welded together, severe corrosion can occur. Figure 4 shows an example of severe corrosion from this mechanism. Figure 4: CS pipe showing preferential corrosion of the section with a high residual element content 3. Commercial impact of HF alkylation unit shutdowns Alkylation units add significant economic value to the refinery operation and enable production of premium gasoline grades. The average gross upgrading margin of an alkylation unit in the USA in the period was estimated at 37 $/bbl alkylate in summer and 15 $/bbl alkylate in winter (332 and 135$/ton alkylate) [Yee & Lippe 2013]. The annual gross upgrading margin for an alkylate production capacity of 10,000 BPD would therefore be of the order of $95 million. In addition to this margin loss, the following issues will be caused by an unplanned shutdown: Delays to gasoline shipments to customers - with a knock-on effect from the potential loss of market share and/or customer goodwill; Distressed sales of butane or iso-butane due to storage and containment constraints; 4 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

5 A greater risk, however, would result from an accidental leak of light hydrocarbons or of hydrofluoric acid to atmosphere, resulting in: Potential health and safety impact of HF release on the refinery staff and on the local community; Potential for explosion from light hydrocarbon leakage; Damage to the corporate brand image within the local/national market. 4. Permasense sensors build confidence in the safe and reliable operation of HF alkylation units Permasense continuous wall thickness measurement sensors are ideally suited to monitor corrosion in the highest risk areas of alkylation units. The non-intrusive sensors are quick and easy to install and communicate wirelessly with a central gateway which delivers the data direct to the desk of the integrity or operations engineer, avoiding the need to enter the unit to take or collect measurements. The monitoring data enables refiners to reliably determine if corrosion is taking place in the high risk areas, helping to enable rigorous management of the unit integrity. This is especially valuable in understanding the correlation between corrosion rates and changes in feedstock and process conditions, particularly due to short term upsets, helps avoid HF acid attack throughout the alkylation unit, minimises the risk of leaks and enables better forecasting of equipment retirement. Permasense systems support the optimisation of corrosion prevention and mitigation strategies, as well as delivering data to enable justification or validation of metallurgy upgrade decisions to HF acid resistant alloys, such as Monel (Alloy 400). 5 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

6 5. Permasense solution for high-risk locations for HF alkylation units The flow diagram below shows an outline of a Permasense corrosion monitoring system for a HF alkylation unit: Figure 5: Outline monitoring locations for HF alkylation unit Potential monitoring locations are along the entire hydrofluoric acid cycle around the reactor and the overheads system, where acid can condense and attack piping and other equipment. Other areas of special focus are the acid regeneration section and downstream post-treatment equipment. A typical installation would comprise approximately 30 monitoring locations, with 2-4 sensors per location. Permasense systems are designed to be robust in all industrial environments and are capable of monitoring all of the most critical locations, including the higher temperature areas of the alkylation unit where free hydrofluoric acid exists or other areas where phase changes could occur. The ET210 sensor: For lower temperature applications (up to 120 C (250 F)), such as those found in most alkylation unit areas, Permasense has recently introduced the ET210 sensor. This sensor uses specially designed low power EMAT technology to measure the metal thickness through external corrosion protection coatings. Each sensor is affixed on the outside of pipework or vessels via an integral magnet. A secondary plastic strap secures the sensor in place, as shown in figure 6. The measurement location therefore requires no preparation prior to sensor mounting, such as cutting and welding of special flanges as required for intrusive corrosion probes, or removal of external corrosion protection such as paint. Each sensor is installed in just a few minutes. ET210 sensors can easily be mounted in circumferential arrays, as shown in figure 7. 6 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

7 Figure 6: ET210 sensor. Non-intrusive, no paint removal required, magnetic mounting with lightweight securing strap. Figure 7: circumferential array of ET210 sensors. For higher temperature locations, Permasense supplies the WT sensor range which utilise our patented waveguide design enabling continuous operation on metalwork operating up to 600 C (1100 F)). For temperatures up to 200 C (400 F), WT sensors can be mounted onto clamps, as shown in figure 8. Metallurgies - Sensors can be mounted on a full range of materials including carbon and cast carbon steel, chrome steels (1% Cr (5130), P5, P9), Duplex, P265GH ( ), (316Ti), P295GH (17Mn4), Monel, HR120, Inconel, Incoloy and Hastelloy. Figure 8: WT sensors mounted using clamps for equipment operating under 200 C (400 F) 6. Case studies Case study #1 A US refiner was concerned by the variation of wall thickness measurements on the bottom of the Monel acid re-run tower, from manual ultrasound, which suggested very high corrosion rates were occurring. The tower base was inside a concrete skirt, with a temperature of 200 F inside the enclosure and 5' clearance height. Inspectors were making manual measurements very regularly inside this confined and difficult space in full chemical suits, with a significant inventory of HF acid overhead. The customer was planning to shutdown the unit to make a complete inspection of the 7 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

8 tower. However, Permasense manufactured two custom-built saddles to fit the geometry of the tower base, which were fixed to the tower using epoxy adhesive, as the customer was uncomfortable to weld studs. After curing, the sensors were fitted and the sensor data showed that, although there was some corrosion activity at the location, it was much less than was indicated by the earlier manual measurements. On the basis of this data, the customer built confidence to keep the unit on-stream, avoiding a very costly 10 day unplanned shutdown. Case study #2 A European refiner was concerned about corrosion at two key locations within the alkylation unit. This required very regular weekly manual ultrasound measurements by inspectors inside the unit battery limits. However, significant amounts of time were lost every time due to the need to put on, remove and decontaminate their protective chemical suits to enter the unit. Permasense sensors were installed at these locations to enable close monitoring from the engineer's office, enabling a less frequent manual inspection regime within the unit. 7. Specific benefits of Permasense monitoring for HF alkylation units Although Permasense continuous wall thickness measurements have demonstrated their value in numerous applications, the following are specific benefits for HF alkylation units: Corrosion in HF alkylation equipment is often accelerated by upset conditions or gradual changes in a temperature profile which can shift the acid dewpoint. Continuous monitoring will allow early detection of a change in wall thickness, and correlation of the corrosion rate to an upset or to changes in process conditions. Continuous wall thickness measurements at critical locations, coupled with internal visual equipment inspection by endoscope under a nitrogen atmosphere, have the potential to eliminate wet decontaminations, which could represent a major improvement for the integrity management of piping systems. Staff carrying out routine manual wall thickness measurement require HF specific safety induction, protective clothing and operations staff in standby. Health and safety risks can therefore be reduced by remote monitoring with data-to-desk. Contact: For more information about Permasense continuous corrosion and erosion monitoring solutions, please contact us: info@permasense.com Call us on: +44 (0) (Europe and MENA) (Americas) (Asia Pacific) 8 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK