Corrosion monitoring solution for sour water stripping units

Transcription

1 Corrosion monitoring solution for sour water stripping units 1. Overview of the sour water stripping process Many process units in a refinery, such as fluid catalytic crackers, delayed cokers, and hydrotreaters generate significant quantities of sour water. These streams normally have a high hydrogen sulphide (H 2 S) and ammonia (NH 3 ) content at a level that prevents them being discharged to the environment without further treatment. These compounds are present as ammonium bisulphide (NH 4 HS) within the sour water stream, but in an aqueous solution, this salt is hydrolysed to form free H 2 S and NH 3 which are volatile. The rate of hydrolysis is higher with increasing temperature, allowing the gaseous H 2 S and NH 3 to be removed by the application of heat in a sour water stripper tower. Depending on the environmental limits, a single stage or two stage sour water stripping arrangement may be utilised. Increasing sour crude processing also often results in a rise in crude nitrogen content, which is a precursor to the production cyanides, like HCN. Cyanides can also create corrosion issues in the sour water system. Produced in the downstream conversion units (such as the FCC or delayed coker), cyanide compounds concentrate into the water phase of the main fractionator overhead. Free cyanides can be deposited in the wet gas stream, causing hydrogen blistering and can destabilise any passivation (iron sulphide) layer causing it to flake off as free iron sulphide, resulting in plugging and fouling downstream. Sour water feed is collected in a sour water flash drum, which operates near ambient conditions, to facilitate the efficient separation of any gases and the removal of any liquid hydrocarbons by decanting. The sour water is introduced to the top section of the sour water stripper column after heating in the feed/effluent exchanger. In the stripping section of the sour water stripper column, the sour water is contacted counter-currently with steam (from the reboiler) to liberate the H 2 S and NH 3. The sour gases from the stripper column overheads are cooled with air or cooling water, so that the majority of the water vapour is condensed. The separated sour condensate is reintroduced into the stripper column as reflux to enable the tower top temperature to be controlled in the C ( F) range, in order to prevent ammonium salt formation and to minimise the water content of the gases being fed to the downstream sulphur recovery unit. The sour gas is routed to the sulphur recovery unit, which enables elemental sulphur to be recovered from the H 2 S and the stripped water (from the bottom of the sour water stripper) is cooled further before being discharged to the waste water system or recycled to the crude unit desalter(s) as wash water. 1 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

2 2. Overview of corrosion issues within sour water stripping units The simplified process flow diagram in Figure 1 gives a general overview of corrosion and fouling in a sour water stripper: Figure 1: Sour water stripper process flow diagram showing high risk corrosion areas Sour water stripper tower corrosion and fouling (partly from corrosion by-products like iron sulphide) are common operational problems that compromise asset integrity. The tower and overhead sections are exposed to high levels of H 2 S and ammonia and can experience high rates of ammonium bisulphide corrosion. Corrosion risks can be compounded by high levels of cyanides from upstream units that concentrate in the overheads, as highlighted previously. 3. Commercial impact of sour water stripper unit shutdowns Sour water strippers are one of the forgotten workhorses of the refinery in many instances, capital cost constraints in the original refinery construction, from refinery expansion projects or from clean fuels projects, have resulted in a lack of spare capacity or redundancy in the sour water stripping facilities. Many refineries operate with sour water storage facilities to enable the sour water stripper to be shut down for maintenance for a period of time. However, increasing severity of operation of hydrotreating units driven by ever-lower sulphur specifications of finished gasoline, jet fuel and diesel has increased the pressure on the sour water treatment system, and the quantity of H 2 S and NH 3 increased as a result. In some cases, the original facilities are being operated at significantly higher processing rates than the original design, as a means of conserving capital from upgrading projects. 2 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

3 In turn, this has limited the flexibility that the refinery has to shut down the sour water stripper for repairs in the event of a corrosion-induced leak, as the increased water load reduces the time that any surplus sour water can be stored. Once storage for sour water is exhausted by an unplanned sour water stripper shutdown, the facility would be forced to take more drastic action to minimise incremental sour water production, including: a. Reduced throughput on key units to lower sour water production and enable sour water volumes to be managed within the available storage. b. Reduced stripping steam to key fractionation towers, resulting in worsened fractionation and the loss of high value product (e.g.- diesel) into low value product (e.g.- fuel oil). c. Risk of violation of effluent water quality limits, resulting in greater scrutiny of refinery operations by the local and/or national regulators. The commercial impact of a sour water stripper outage on a given refinery will depend on the specific configuration. However, on an assumption that an outage would constrain total refinery throughput by an average of just 10% for a 5 day period while repairs were effected, and using a representative refining margin of $7/bbl, the impact on a refinery of 200,000 bpd capacity would be $0.7 Million plus the costs of the repair. A replacement bundle for the stripper overhead exchanger, which is at the highest risk of corrosion would be of the order of $ Million. It could therefore be anticipated that an unplanned 5 day shutdown of the sour water stripper could cost in excess of $1 Million in lost production, or 0.2% of the refinery s gross margin. 4. Permasense sensors build confidence of safe and reliable sour water stripper operations Permasense continuous wall thickness measurement sensors are ideally suited to monitor corrosion in the highest risk areas of sour water stripper towers. The sensors 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 manage the unit integrity. This is especially valuable in understanding the correlation between corrosion rates and changes in feedstock and process conditions, minimising the risk of leaks and enabling better forecasting of equipment retirement. Permasense systems support the optimisation of corrosion prevention and mitigation strategies, as well as delivering data to enable justification of metallurgy upgrade decisions to corrosion resistant alloys. 3 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

4 5. Permasense solution for high-risk locations for sour water stripper towers The diagram below shows an outline of a possible Permasense corrosion monitoring system for a sour water stripper tower: Figure 2: Outline monitoring locations for sour water stripping unit Potential monitoring locations are shown in Figure 2 above. A typical installation would comprise approximately monitoring locations, with 2-4 sensors per location. Permasense systems are designed to be robust to all industrial environments and, being certified intrinsically safe for installation in Zone 0 areas, are capable of monitoring all of the most critical locations. Temperature resistance - Sensors can be supplied with three waveguide lengths: 100mm, 300mm and 500mm. For many locations within sour water stripping units, 100mm waveguide sensors, providing temperature resistance up to 150 C (300 F) will be suitable. Higher temperature locations, such as near to the main fractionator reboiler, standard 300mm waveguide sensors will be required (with temperature resistance up to 600 C (1100 F). Figure 3: 100mm short waveguide sensor on clamp assembly 4 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

5 Metallurgies Waveguide-based 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. Sensor mounting methods - Stud mounting offers the greatest flexibility in choice of monitoring locations. Studs can be welded on to live piping and in hazardous areas, e.g. by employing friction stud welding. Figure 4: Stud mounted sensor installation When welding is not permissible or possible due to material restrictions, Permasense can supply clamps in a variety of diameters up to 40", and for towers and vessels, custom-built mounting saddles, which can enable magnetic attachment to vessel shells or fixed on using epoxy where nonmagnetic materials are present. Figure 5: Epoxy adhesive saddle for sensor mounting 5 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

6 Figure 6: Clamp assembly for sensor mounting Figure 7: Magnetic saddle assembly for sensor mounting 6. Introducing the Permasense ET210 sensor model Permasense has recently introduced the ET210 sensor for lower temperature applications (up to 120 C (250 F)) making it ideal for most areas of a sour water stripping unit. This sensor uses specially designed very low power EMAT technology which enables measurement of the metal thickness through external corrosion protection coatings, up to 1mm thick, without any damage. The very low power requirement enables EMAT-based technology to be permanently deployed in Zone 0 intrinsic safety rated areas of the plant for the first time. Each sensor is attached to the outside of the equipment via an integral magnet, which along with WirelessHART protocol compliant communications and battery power makes the installation very quick and convenient. A secondary plastic strap secures the sensor in place, as shown in Figure 8, and a steel lanyard is connected through the sensor body to provide total protection against falling. The measurement location requires light surface preparation prior to sensor mounting, such as wire brush and light emery paper. Hence, each sensor can be installed in just a few minutes. ET210 sensors can easily be mounted in circumferential arrays, as shown in Figure 9, to monitor localised corrosion attack mechanisms. 6 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

7 Figure 8: ET210 sensor. Non-intrusive, no paint removal required, magnetic mounting with lightweight securing strap. Figure 9: Circumferential array of ET210 sensors. 7. Example deployments 7.1. Case study A European refiner installed several sensors around their sour water stripper tower for general corrosion monitoring purposes. The focus was on the overhead condenser/overhead line, feed/effluent exchanger, tower bottoms line and reboiler outlet. In the course of routine monitoring, the customer observed very a high corrosion at the overhead condenser outlet (location 2600WP21 shown on the P&ID in Figure 10) which is fabricated from carbon steel. Figure 10: Extract of the customer s sour water stripper P&ID 7 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

8 Figure 11: Wall thickness trend at the overhead condenser outlet Initially (August to September in the chart shown in Figure 11) the corrosion rate was equivalent to 2.3 mm/year (91 mpy). However, testing on the unit (first half of September) showed that by altering the operating conditions (notably the reboiler duty) of the tower, it was possible to significantly reduce the corrosion in this location. Operating conditions were then returned to normal and the corrosion rate returned to the high rate observed earlier. From early October, the operating conditions were permanently changed and the corrosion effectively eliminated in this area of the tower. It is believed that the increased reboiler duty increased the steam rising up the stripper, with the result that more water was condensed in the overheads this served two purposes: it reduced the H 2 S and NH 3 partial pressure in the overhead system by dilution, and provided a washing of any Ammonium Bisulphide salts in the overhead condenser, thereby avoiding under-deposit corrosion. Conclusions: 1. The optimisation of the tower operating conditions (reboiler duty) to drive the more effective management of the integrity of the equipment was possible. 2. The lifetime of the overhead exchanger and line has been extended by many years, resulting in a significant net cost saving for the customer, from deferred equipment retirement and replacement costs. 3. This was only achieved because of the direct and rapid feedback of the effect of the process condition changes on the corrosion rate, as a result of having the wall thickness measurements provided by the Permasense sensors. 4. This case study also demonstrates the effectiveness of the Permasense data, in combination with process data, to provide a concrete basis for troubleshooting across the processtechnical/operations and integrity management/corrosion/inspection functions. 8 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK

9 Contact: For more information about Permasense continuous corrosion and erosion monitoring solutions, please contact us: Call us on: +44 (0) (Europe and MENA) (Americas) (Asia Pacific) 9 Permasense Ltd, Alexandra House, Newton Road, Manor Royal, Crawley, RH10 9TT, UK