Holistic approaches to equipment inspection and maintenance is crucial to ensuring plant reliability and avoiding unplanned shutdown. In parallel, technology advancement in materials of construction and specialised equipment can offer operators opportunities to improve reliability and performance efficiency. This session will examine options for improving the effectiveness of plant equipment, and showcase the benefits of new approaches to equipment and construction.
Features and benefits of Wet Electrostatic Precipitators
Industrial-grade sulphuric acid, still the most widely used industrial chemical in the world, continues to be sourced primarily as a nondiscretionary byproduct from the roasting, smelting and refining of nonferrous metals (70%), and from natural gas processing, electric power generation and spent acid regeneration. These industries are usually heavy emitters of particulates, sulphur and nitrogen oxide gases, and sulphuric acid mists, among other pollutants. They are also subject to increasingly strict environmental regulations.
When concentrations of sulphur dioxide from these operations exceed five to seven percent of exhaust-gas volumes, a common and cost-effective solution is the incorporation of a downstream sulphuric acid manufacturing plant. Owners of these facilities can capitalise on the high industrial market value of purified sulphuric acid, while achieving greater operating efficiencies and easier regulatory compliance.
However, an efficient sulphuric acid manufacturing process requires the maximum possible removal from input gas streams of fine particulates, acid mists, condensable organic compounds and other contaminants. This high level of gas-cleaning efficiency is necessary to prevent poisoning of the catalysts and fouling or plugging of the catalyst beds. An optically pure input gas is essential for avoiding the formation of a “black” or contaminated acid end-product. Proper gas cleaning also helps protect sensitive acid plant components against corrosion damage, thus lowering long-term expenditures for maintenance and equipment replacement. Ultimately, cleaner gas streams facilitate the production of stronger concentrations of sulphuric acid, suitable for a wider range of end-uses.
Modern gas cleaning and emission control strategies continue to evolve in response to the increasing complexity, toxicity and corrosiveness of industrial exhaust and process gases--not to mention increasingly stringent environmental regulations. Sulphuric acid plant owners have employed several gas cleaning techniques, including wet or dry scrubbers, cyclones and fabric filters. These systems can capture larger particulates, but are usually energy-inefficient or impractical to use on fine particulates, acid mists, oily residues, or condensed organic compounds. Many plant operators thus find that the best solution for removing these contaminants is a modern adaptation of a traditional technology: the electrostatic precipitator, or ESP.
The basic ESP design features an array of discharge electrodes, such as negatively charged wires or rods, able to generate a strong corona field. Each electrode in the array is surrounded by a positively charged or grounded collection surface, traditionally the interior of a square or cylindrical tube. In operation, the source gas is passed through the array, as the ionising electrodes induce a negative charge in even the most minute, submicron-size particles. These are instantly propelled toward the interior collection surfaces, where they adhere as the cleaned gas is passed through.
This simple, basic design concept offers several distinct, inherent advantages, the primary one being the ability to capture fine, submicron-size particulates and acid-mist droplets, whose minimal mass enables them to escape through scrubbers and other mechanical equipment. Certain precipitators can even achieve collection efficiencies approaching 100 percent.
Another advantage inherent to precipitators in general is the extremely low pressure drop experienced as gases pass through the system—as low as 1 - 2 kPa. With virtually no mechanical impedance or obstruction of target pollutant particles traveling through the ESP, gas velocities, and thus operating efficiency, can be extremely high. This enables plant designers to use smaller-scale, less costly equipment and still achieve superior collection efficiencies compared to other systems.
For engineers and plant designers in the field of gas cleaning, several operational goals stand out:
• achieving the highest level of particle collection efficiency;
• cleaning greater volumes of source gases, with faster throughput speeds; and,
• achieving the greatest reductions in costs related to capital investment, operating cost, energy consumption, equipment maintenance, and long-term equipment replacement.
To help sulphuric acid plants and other industries achieve these goals, the engineering staff at Beltran Technologies, Inc. in New York have been researching and developing a specific type of advanced, innovative precipitator technology which offers proven superior performance and efficiency: the Wet Electrostatic Precipitator, or WESP. However, WESPs can vary greatly in design, materials, gas flow rates and durability, as well as collection efficiency. It is thus important for engineers to recognise the key differences among these various systems.
Primarily targeted at capturing submicron-scale particulate matter, saturated sulphuric or other acid aerosols and condensable organic chemicals, the Beltran WESP utilizes aqueous flushing nozzles to clean the collection surfaces. The captured particles are cleansed from the collection surfaces by recirculating water sprays; residues, including aqueous sulphuric acid, are extracted for further use or disposal. The cleaned gas is ducted to downstream equipment or to the stack, depending on the application. A well-designed and correctly operated WESP can clean complex gaseous emissions of particulates and acid mists down to submicron scale (PM 2.5) with up to 99.9% efficiency, and very low energy drain—far superior to other equipment.
The simple elegance of this basic WESP concept makes them uniquely versatile over a broad range of industries, applications, operating conditions and gas chemistries. This adaptability is critical to metallurgical and petroleum refining operations, where source gases can be highly variable.
Taking advantage of the inherently low pressure drop of ESPs, Beltran’s staff devised a multistage system of ionizing rods bristling with star-shaped discharge points. These are encased within square or hexagonal tubes to maximize collection surface area and minimize overall space requirements. The unique electrode geometry generates a corona field 4 to 5 times more intense than that of conventional wet or dry ESPs resulting in greater particle migration velocity and adhesivity. The multistage charging configuration also avoids corona quenching due to high particle densities, and assures maximum corona field strength with a minimum of energy load.
A persistent challenge for traditional dry-operating precipitators is the re-entrainment of particles from the collection surfaces back into the gas stream due to the use of mechanical or acoustical rapping units. Beltran’s design uses a vertical flow and continuous aqueous flushing of WESPs to greatly minimize this problem. By eliminating the need for rappers, WESPs also reduces the higher cost and energy drain imposed by that equipment. For industries that generate oily or sticky residues, the aqueous flushing also prevents particle build-up, and overcomes electrostatic resistivity and back-sparking on the WESP collection surfaces.
Other critical features to look for in WESP equipment are sophisticated electronic controls linked to a close-coupled gas flow management system; these can optimize operating parameters such as gas velocity, saturation, temperature, corona intensity, etc., to achieve maximum efficiency.
A major threat to the cost-effectiveness of a WESP is corrosion of equipment caused by the harsh chemical components of treated gases. To prevent premature deterioration, critical surfaces are constructed with advanced protective materials such as fiber-reinforced plastics (FRP) or high nickel-chromium alloys. The high-voltage insulators are continuously purge-air cleaned to further reduce maintenance costs.
Improving the overall equipment effectiveness (OEE) in your sulphuric acid plant
When investing in (chemical) operations, return on investment (ROI) is one of the most important parameters to monitor for shareholders. To increase your internal rate of return, and to smoothen out your operations performance, maximising your operations effectiveness is key for all stakeholders. Industry standard “Overall Equipment Effectiveness” (OEE) or “Asset Utilization” (AU) can be applied to your sulphuric acid plant.
This abstract will determine 6 major actions to reduce the 6 major OEE losses in sulphuric acid plants.
OEE= Availability x Performance x Quality
Availability loss – Unplanned stops
Equipment failure is the main reason for unplanned stops. For all different types of assets conditioning monitoring standards (CMS) must be defined. These must be implemented in an adjustment or even a complete new maintenance strategy based on reliability centred maintenance (RCM). A good partner to implement, and also measure, this system on regular base in the plant is needed.
Availability loss – Planned stops
Even when RCM and CMS are implemented equipment’s will still fail. Key is to have the repair times planned, combined with other tasks (cleaning, other planned maintenance, quality inspections, …) and executed in the shortest possible window. Good planning and scheduling through Single-Minute Exchange of Die (SMED), a good shutdown methodology and an experienced sulphuric acid contractor are best success parameters to guarantee best results.
Performance loss – Small stops
Process plants need to be in tight control. When constructed they are assessed through strict safety reviews. Safety classifications are defined and trip values are set. Through good testing an calibration of the instrumentation and on the other hand training of the operators, a plant trip can significantly be reduced. Risk Based Inspection (RBI) criteria can reduce instrumentation trip an hence increase OEE.
Performance loss – Slow cycles
Even when your installation is monitored closely and risk are assessed, it is still possible that your unit operations (towers, convertors, sulphur burner, etc) are preventing you to reach maximum throughput. Internal inspections of your equipment during cold shutdowns can give you valuable information of the condition of your plant condition. No need to tell that these jobs in confined spaces, hot environment and still a lot of acid present creates dangerous conditions and are best performed by specialised a specialized company for sulphuric acid plant inspections.
Quality loss – Process defects
Incorrect equipment settings or performance, operator setting errors, bad quality control of raw materials, etc can lead to bad quality. Even when all effort was done to keep your plant up and running, your OEE can still be badly influenced by these mismatches. Make sure you have supportive management processes to maintain quality at all times.
Quality loss – reduced yield
Last but not least, well operation of your plant to maintain best conversion in your convertor, low pressure drop, little acid carry over, etc make sure your yields remain as high as possible. Good follow up will be necessary.