Fertilizer International 502 May-June 2021
31 May 2021
Sulphuric acid projects and technology
SULPHURIC ACID CASE STUDIES
Sulphuric acid projects and technology
Developments in sulphuric acid technology and engineering know-how are highlighted by recent project case studies from DuPont Clean Technologies, Metso Outotec and thyssenkrupp Uhde.
DUPONT CLEAN TECHNOLOGIES
AZFC Unit 6 revamp project
By 2015, the Unit 6 sulphur burning plant operated by Abu Zaabal Fertilizer and Chemical Company (AZFC) had been in service for 31 years. Unsurprisingly, three decades of operation at the company’s fertilizer production complex in Egypt’s Qulubia Governorate had taken their toll. Due to a host of problems, including corrosion, sulphate build-up and gas leaks, the unit was experiencing daily downtimes, multiple shutdowns and significant losses in production.
AZFC had originally considered shutting the plant down when its new Unit 7 was commissioned in 2009. But rising demand for phosphoric acid meant the company needed the extra capacity. The time had clearly come for a complete revamp. AZFC decided to act and selected DuPont Clean Technologies (DuPont) as a partner to revamp the acid plant and re-design the acid towers.
The AZFC production complex, owned by Polyserve Group, was first commissioned in 1984 and includes two sulphuric acid production plants – Unit 6 and Unit 7. Unit 6 had been AZFC’s workhorse. In its time, 18 million tonnes of single superphosphate (SSP) fertilizer had been manufactured from more than six million tonnes of sulphuric acid yielded by the unit. Unit 6 had also produced over 600 million KW of clean electricity over its lifetime, allowing AZFC to cut CO2 emissions by 11,000 tonnes.
The decision to revamp Unit 6 was not an easy one to make, as Dr Eng Sherif El-Gabaly, chairman of AZFC, explains: “Unit 6 produces around 30 percent of AZFC’s sulphuric acid. We therefore wanted to keep the shutdown to a maximum of 12 months. We needed a reliable and experienced partner who could oversee the project and support us with any technical issues. Given the experience, references and success DuPont had with similar projects, AZFC chose DuPont.”
As well as resolving productivity issues, the overhaul of Unit 6 would enable AZFC to meet newly introduced Egyptian emissions regulations. These cut permissible SO2 emission limits from 1,500 mg/m3 to 450 mg/m3 for new plants and to 800 mg/ m3 for existing plants.
Before the revamp
The list of challenges facing AZFC was long. The original brick-lined drying and absorption towers were in very poor shape with visible signs of deterioration. In particular, acid leaking from the bottom of the vessels and at the outlet nozzles of the acid tower was causing sulphate build-up and severe corrosion.
Sulphate build-up on the tube sheet of the mist eliminators in the inter-pass absorption tower (IPAT) was also triggering shutdowns and causing corrosion. Additionally, sections of the old brick tower lining were regularly coming loose and falling into the acid cooler, leading to further leakage from cooler pipes. The furnace baffles had also fallen over. This suggested that the gas was not mixing sufficiently, and that sulphur was entering downstream equipment and causing corrosion.
AZFC was not only concerned about the acid towers at Unit 6. Severe corrosion on cast iron piping, notably on the elbow of the IPAT, had created a number of holes. Because the pipes were very heavy, installed at height and difficult to access, repairs were problematic and led to extended downtimes.
This was not all, either, as the following issues also needed to be put right:
- A significant plume from the stack
- Gas leaks from the shell of the cold heat exchanger
- Missing refractories in the waste heat boiler
- Corrosion on the tube sheet bundle and expansion joint of the boiler jug valve
- Unsafe demineralised water tanks due to corrosion.
Sometimes, the plant was shut down three to four times per day to attend to these myriad problems. Controlling the old plant had thus become extremely difficult and unsafe.
The revamp
As part of the revamp project, DuPont was asked to:
- Re-design and install new acid towers and reduce overall SO2 emissions
- Improve converter performance
- Provide site supervision during the installation of MECS® equipment.
The new high-efficiency acid towers included modern UniFlo® acid distributors in corrosion-resistant MECS ® ZeCor-Z ® alloy as well as MECS® Brink® mist eliminators.
DuPont also introduced new MECS® GEAR® catalyst for increased conversion and supplied hard-wearing MECSZeCor-Z ® ® acid piping for the acid towers and the drying tower mesh pad.
As well as guaranteeing a sulphuric acid production rate of 615 t/d, the aim was to also achieve SO2 emission targets of less than 800 mg/Nm3 , as well as SO3 absorption and SO3/acid vapour emission control levels of less than 35 mg/Nm3 .
The revamp began in late 2015 with the dismantling of the old acid towers and repairs to the foundations. The new carbon steel tower vessels were fabricated off-site by local engineering company ASF–EL Sewedy Industries Group and transferred to Abu Zaabal in three sections – where they were then welded, lifted into place, and erected in a simple operation.
Highly corrosion-resistant MECS® ZeCor®-Z alloy was used for pipes, all elbows, bends, fittings and spool pieces. Their installation was carried by local engineering company IEMSA construction, whose welders had been qualified by DuPont.
Mr Abd El Hafeiz recalls: “Throughout the project, DuPont not only provided the process design and engineering for the acid towers, but also supported the revamp in an advisory role, and assisted us with the start-up of the plant.”
During the revamp, most of the cast iron internals for the first pass converter, such as support grids and columns, were also replaced, and new woven wire screens installed for all passes. When the brick lining had been restored, the converter was then loaded with the new MECS® GEAR® catalyst. The revamp further included:
- Repair of the main blower and almost all rotating equipment
- Fitting of seven new gas valves
- A new gas duct for the IPAT
- Re-tubing of the cold inter-pass heat exchanger
- A pristine demineralised water plant
- The creation of a modern control room.
Success
The entire multi-million project was managed in a very short delivery time by AZFC’s Ayman Abd El Hafeiz and Hassan Hussein, the coordinating manager, project department. The revitalised Unit 6 successfully started up on the 15th November 2016 after a shorter than anticipated shutdown. The full revamp was concluded on budget, on time and without a single incident or injury.
Since start-up, Unit 6 has operated to performance guarantees, producing up to 640 t/d of sulphuric acid. The revamped unit had its first cold shut down for maintenance in December 2019, following more than three years’ operation. For its budget of $6.5 million, AZFC now has a highly effective plant at between one-fifth to one-tenth of the cost of a new plant, but with the same life expectancy.
Unit 6 revamp achievements
- The plant’s average sulphuric acid production capacity increased from 480 t/d to 640 t/d
- Stack SO2 emissions decreased from 2,000 mg/Am3 to < 600 mg/Am3
- Plant availability improved with reduced downtime
- Downtime average pre-revamp (average over 2 years) = 138 days per year
- Downtime average after revamp = 3.9 days per year
- Improved availability has eliminated LE 59 million per annum ($3.8 million p.a.) in lost sulphuric acid sales over that period
- Loss in production cut from 41.7 percent in 2014 to 0.87 percent in 2019.
- A step change in plant reliability
- The revamped plant ran for more than three years before the first cold shutdown, compared to shutdowns every 4-6 months previously
- No injuries or accidents during the entire revamp project
- A sparklingly clean and pristine plant with even roses now cultivated on site.
METSO OUTOTEC
Mazidagi project, Turkey
Cengiz Holding acquired Eti Bakır from the Turkish government in 2004 with a clear strategic aim: to build an industrial complex in the Mazidagi district of Mardin, Turkey – one that combined fertilizer production with metals processing at a single location. This plant was designed to create a high level of energy self-sufficiency by efficiently capturing energy from the various unit processes on site. The Mazidagi project concept, by successfully delivering a truly integrated production complex, has set a new benchmark for future plant designs.
With the Mazidagi production complex now fully operational, Eti Bakir has fulfilled its ambition to increase the contribution domestic production makes to Turkish fertilizer demand – a strategically critical objective for a country whose large agricultural sector has (to date) relied on large volumes of fertilizer imports. Crucially, it has been the additional revenues from metals recovery that have made the Mazidagi project economical.
Metso Outotec was chosen as the key technology partner for the Mazidagi project as its technology portfolio was able to cover a large proportion of key plant sections for the industrial complex (Figure 2, sections in dark green). In this case study, we focus on the technology applied in the interconnected pyrite roasting, off-gas cleaning and sulphuric acid production section (Figure 2, highlighted in yellow).
The Mazidagi production complex
The Mazidagi complex, which takes its names from its location in the Mazidagi district of Mardin province in Turkey, was built at an investment cost of approximately $1.1 billion, making it the highest budgeted project in Eastern Turkey. Construction began in the first-quarter of 2015 and was completed in the first-quarter of 2019, with the project entering commissioning after 50 months and 20 million person-hours.
The complex incorporates six key production units (Figure 1):
- Pyrite roaster/off-gas/sulphuric acid plant
- Phosphate beneficiation plant
- Phosphoric acid plant
- Ammonia plant
- Fertilizer plant
- Hydrometallurgical plant.
Additionally, a relatively large number of auxiliary utilities are also required to efficiently operate this integrated facility, namely:
- Demineralised water production plant
- Chemical water treatment plant
- Water cooling plant
- Condensate purification plant
- Steam turbine and gas engine
- Auxiliary boiler
- Natural gas distribution stations
- Air compressor plant
- Pyrite and ash transport units
- Fire reservoir and distribution system
- Main water tank and water wells
- Switchyard
- Packaging facility.
The Mazidagi complex consumes 550,000 tons of locally-extracted phosphate rock and processes this to produce 750,000 tonnes of fertilizer annually. This includes 200,000 tonnes of diammonium phosphate (DAP) and 550,000 tons of NP products. Being the only fertilizer plant in the region, Mazidagi meets the entire fertilizer requirements of the surrounding Harran Plain. The plant’s overall share of the Turkish phosphate fertilizer market will be 20 percent initially.
Roasting plant
In specific cases, pyrite roasting provides a viable alternative to sulphur burning as a source for sulphuric acid production, especially when pyrite is available from local sources. This is especially true for a landlocked location such as Mazidagi, where the logistics associated with supplying sulphur to the plant are particularly challenging.
The standard processing technology for pyrite is roasting in a fluidised bed reactor, as has been used on a commercial scale since the 1950s. While the principles of roasting remain unchanged, process control technology for roasting plants has advanced and improved greatly. In the past, environmental and safety standards were often the driving forces behind improvements in equipment and plant design. Today, however, the emphasis has shifted to plant efficiency and optimisation – as delivered via Metso Outotec’s advanced process control philosophy. In recent times, responsibility for plant control and operation, traditionally the domain of owner/operators, has also shifted to process technology companies due to their ability to offer both integrated digital tools and specific process know-how.
Within the Mazidagi complex, the roasting plant fulfils two fundamental process objectives:
- Firstly, to provide an SO2 off-gas source for sulphuric acid production
- Secondly, to produce a calcine capable of being processed in the downstream hydrometallurgical complex.
These dual process requirements meant there was a need to monitor and control roaster plant operations to ensure that – based on the composition of the concentrate, the operating temperature and the atmospheric conditions – roaster product quality remained constant.
Temperature control at this roasting plant was a key factor, given that different types of metals were being recovered, and was therefore best handled by an optimisation system. Excessively high roasting temperatures, for example, could potentially lock-up sub-microscopic particles in the calcine, while too low temperatures might negatively affect overall plant performance. Good process control was therefore of the utmost importance.
For the Mazidagi project, Metso Outotec applied state-of-the-art digitalisation tools for process monitoring, control and advisory activities – to ensure the best available support for commissioning of the plant. This was achieved by installing proprietary Roaster Optimizer technology at the roaster plant.
The Roaster Optimizer, configured with Metso Outotec’s advanced process control (ACT) platform, functions independently from the plant’s distributed control system (DCS) (see Figure 2), and DCS functionality therefore remains untouched. Instead, the role of the Roaster Optimizer is to read process values from the DCS, use these to calculate an optimised solution for plant operation, and then send back optimised values – such as feed rate, airflow or water addition rate – to the DCS.
Gas cleaning and sulphuric acid plants
The gas cleaning plant plays a central role in the smooth and efficient operation of a metallurgical sulphuric acid complex.
Nowadays, with most prime global deposits depleted, metal producers are increasingly having to process complex ores and concentrates. This often involves the dual challenge of extracting less valuable metals at higher production costs. In many cases, complex ores and concentrates are also associated with potentially polluting contaminants (e.g. arsenic and mercury) whose concentration tends to increase as the desired metal content reduces.
Given the challenging nature of complex ores, as highlighted above, the design of the gas cleaning plant clearly needs to consider all eventualities regarding off-gas impurities. For the Mazidagi project, the purity of the sulphuric acid required for downstream fertilizer production was another key consideration.
Dry/hot gas cleaning is the first gas cleaning step downstream of the roasting process. This involves removing any solid dust emitted by the process with the highest possible efficiency – generally by employing hot electrostatic precipitators (Hot-ESP).
The dedusted off-gas then enters the wet gas cleaning plant where the remaining impurities are removed using Metso Outotec’s Otovent scrubber and packed gas cooling tower. This traditional and well-proven configuration is coupled with primary and second stage Editube wet electrostatic precipitators (Wet-ESP). Mercury is the main volatile generated by the roaster. This is removed via Metso Outotec’s B-N mercury removal system, which remains the benchmark technology for the industry.
Once it has been processed in the wet gas cleaning section, the off-gas is then suitable for further handling by a conventional sulphuric acid plant (see main photo).
Summary
Cengiz’s $1.1 billion investment in the Mazidagi metal recovery and integrated fertilizer project provided Metso Outotec with the opportunity to showcase process technologies that encompass the complete value chain at one production complex.
For the roasting plant-gas cleaning plant-sulphuric acid plant section at the Mazidagi complex (Figure 2), Metso Outotec’s successfully implemented proven process technologies that demonstrated the following advantages:
- Adaptable to specific process conditions – being adjustable with respect to the complex mineralogical composition of the pyrite ore and the fine particle-size distribution of the feed material.
- Integration of a state-of-the-art ACT optimiser – complementary to the plant’s control system.
- Installation of a gas cleaning system – capable of conditioning the off-gas to produce sulphuric acid quality suitable for use in the downstream fertilizer complex.
- Installation of a sulphuric acid plant with a low pressure (LP) steam system – to complement the steam produced from the roaster plant.
After Mazidagi’s successful start-up, the operators expressed their satisfaction with the smooth operability of the plant. The project detailed in this case study demonstrates Metso Outotec’s proven abilities in developing and improving roasting, gas cleaning and sulphuric acid technologies with every new plant design.
THYSSENKRUPP UHDE
Small scale sulphuric acid plants – availability first
Small-scale sulphuric acid plants offer an environmentally, technically and commercially feasible way of converting sulphur-containing off-gases (acid gases) into a valuable product. The sulphuric acid produced in these plants can be used directly as a feed material to produce marketable fertilizers, such as ammonium sulphate or potassium bisulphate.
A small-scale sulphuric acid plant will produce approximately 10-200 t/d of acid. In the design of large-scale acid plants, heat recovery is the focus, as this significantly impacts on plant profitability. Small-scale sulphuric acid plants, in contrast, require a plant design that provides maximum availability, trouble-free operation and minimal maintenance. This allows operators to focus on their core product – the production of coke or pulp, for example.
In this article, we highlight an alternative process design for small-scale sulphuric acid plants, along with recommendations for specific equipment. This draws on the long-term experience gained from running a small-scale sulphuric acid plant. This reference plant has been on-stream and operating trouble-free for almost 40 years. Under continuous operation, its catalyst service time, without screening, is higher than 15 years.
General approach
For acid plants, process and mechanical design should focus on known criteria which affect plant availability, for example:
- Corrosion allowances, type of alloys and fouling allowances – as these have a direct impact on the service time and maintenance type of each equipment or pipe.
- Equipment manufacturers – some have better availability of spare parts and some have longer service time.
- Mechanical design – particularly the handling of the cold spots, as these can induce acid condensation and ultimately result in corrosion.
Given that the above criteria are the bread and butter of plant design, even more can be achieved at process level.
Alternative process design
Conventional sulphuric acid plants consist of combustion, conversion and absorption sections. The small-scale alternative process design (Figure 1) also includes an additional post-combustion gas cleaning step. Gas cleaning can be carried out dry or wet, although wet cleaning is most efficient.
In the alternative sulphuric acid process, acid gas and air are combusted in a chamber to form SO2. The resulting heat of combustion is recovered by the medium pressure (MP) boiler. Downstream of combustion chamber, the gas enters the condensation tower to remove water vapour and wash out any dust. The cleaned gas is then completely dried in the drying tower (using sulphuric acid) before entering the converter where it is reacted with oxygen over a catalyst to form SO3 . The final acid is produced in the final absorption tower downstream of the converter. Typically, the off-gas leaving the final absorption tower will not require additional treatment.
This alternative process design offers clear advantages in terms of availability, compared to plants without gas cleaning, as has been demonstrated by the long-term operational history of a reference plant. The plant’s operators report that no catalyst screening or change in catalyst has been necessary in 15 years of operation. For this plant design, therefore, the availability of the converter section is six times higher than for a conventional system.
Advantageously, the alternative process design also avoids specific or proprietary equipment or material design. This allows generic repairs or replacements to be arranged with appropriate local service providers and manufacturers.
A small drawback of the alternative process is the reduced potential for heat recovery, compared to a conventional process. This is because recovered heat is mainly required to preheat dry gas before it enters the converter, while in conventional processes it can be recovered for steam production. Despite this, in small capacity plants, the heat recovery potential is negligible, and more than offset by the equipment savings and the value gained from higher availability and lower maintenance costs.
Equipment maintenance
Pumps and acid coolers – similar to the catalyst in a conventional plant – do require frequent maintenance in the alternative process design. This can be compensated for by adopting equipment redundancy (duplication) to ensure full availability. Indeed, ‘1+1’ equipment redundancy has been successfully applied as common practice at the reference plant, i.e. while one item is in operation a replacement item is always held in stock.
Additionally, the combustion burner process unit requires regular maintenance due to the corrosion associated with acid gas combustion. The presence of corrosive as well as solidifying components, like ammonia with hydrogen sulphide and naphthalene, respectively, are a particular challenge. Nevertheless, the burner developed for the reference plant by Uhde and its partners shows no requirement for wear parts after 10 years in operation. This burner features a special mechanical design as well as its own dedicated control system.
Summary
Process design should always match up to long-term project requirements. In many ways, therefore, selecting the optimum sulphuric acid process for a specific project is similar to the everyday choice between a bicycle, car or truck: as different types of sulphuric acid process serve different purposes and fulfil different expectations regarding efficiency and availability. For projects focused on high availability and low maintenance costs, thyssenkrupp Uhde’s alternative sulphuric acid process design offers key advantages.
