Online cleaning and decontamination of a butadiene unit
Chemical treatment by closed-loop recirculation reduced cleaning time for a butadiene unit turnaround from a typical 20 days to 24 hours
Dino Polverini and Cosimo Cucinelli, Polimeri Europa
Marcello Ferrara, ITW Technologies
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Extractive distillation of butadiene is accompanied by a characteristic fouling issue. The material laid down in trays is hard and polymeric, typically formed in vapour spaces, and is commonly identified as popcorn polymer. It is difficult to remove from equipment surfaces, inflammable and can cause serious damage to internals, so its removal is a priority during unit turnarounds.
Most butadiene is a by-product of ethylene production from steam crackers. The C4 stream isolated from the steam cracking process is fed to butadiene extraction units, where butadiene is separated from the other C4s by extractive distillation. The amount of C4s produced in steam cracking depends on the composition of the feed to the cracking unit. Heavier feeds yield higher amounts of C4s and butadiene than lighter feeds. Crackers using light feeds typically produce low quantities of C4s and do not feature butadiene extraction units. Butadiene capacity is determined by ethylene cracker operating rates, the type of feed being cracked and the availability of butadiene extraction capacity. Butadiene is typically isolated from other C4s produced in steam cracking by extraction into a polar aprotic solvent such as acetonitrile, from which it is then stripped by distillation.
Polimeri Europa, part of the Eni Group, has 18 production sites in Europe, along with four research facilities. The company’s Brindisi, Italy, site includes a 147 000 tonne/year butadiene unit, which uses acetonitrile as the extractant solvent. The extractive distillation section of this unit comprises four columns: a first and a second extractor (containing 90 trays each), a stripper (also containing 90 trays) for stripping butadiene and acetylenic com-pounds from the solvent, and a solvent regenerator. The unit undergoes turnaround every five years.
During the latest turnaround, to improve operations, the company wanted to find ways of reducing the time it takes to achieve safe entry conditions and of modifying the polymer to make it easier to remove. During previous turnarounds, operators encountered a bottleneck during the removal of the polymer inside the extractive distillation section. This problem is inherent to the butadiene unit and is encountered, to a greater or lesser extent, worldwide.
So-called popcorn polymer is a well-recognised operational problem inside butadiene units in particular and more generally throughout the petrochemical industry. This polymer is sticky and hard to remove, to such an extent at Polimeri Europa that the trays could not be cleaned on-site and instead had to be dismounted and mechanically cleaned off-site.
This operation was particularly difficult and time consuming because the first and second extractor, as well as the stripper, are each 93m high. The dismounted trays had to be cleaned at ground level by mechanical scraping, as hydroblasting would have had little effect. This was made more difficult by the adhesive nature of the polymer. After cleaning, the trays were remounted and restored to operation. Together, these difficulties added up to a lengthy cleaning operation that had a huge impact on plant downtime. Based on historical cleaning data from the site, 20 days of mechanical cleaning was scheduled for a typical turnaround.
Ensuring safe entry conditions for operators was also problematic. Conventional gas-freeing methods were not sufficient. Extra precautions had to be taken and operators needed to wear breathing apparatus when they were working inside the columns. Once the polymer was eventually removed by mechanical means, its noxious odour added to the problems. To try and solve some or all of these problems, Polimeri Europa decided to â€¨field trial two novel technologies â€¨for modifying polymers and degassing/decontamination.
ITW has developed and patented a technology for online cleaning productions units, which has a number of advantages over conventional manual cleaning or hydroblasting, including:
• A closed-loop operation, with no need to open units during cleaning
• A safe and environmentally friendly operation
• No waste generation
• No emissions.
The technology has already been applied successfully to many refinery units to remove heavy asphaltenes/paraffins and coke-like deposits. A production unit can be Online Cleaned in 24 hours on an oil-to-oil basis, and a crude distillation unit can be Online Cleaned, spec-to-spec, in about 30 hours. If cleaning is performed just prior to turnaround, a subsequent ITW Improved Degassing/Decontamin-ation step can be applied to improve turnaround operations and reduce downtime.
One characteristic of ITW’s Online Cleaning process is that no waste is generated and washing fluids can be fully reused and/or reprocessed. Moreover, because it is a closed- loop operation, no airborne emissions are generated.
To address the removal of polymeric compounds from petrochemical units, particularly popcorn-type polymer, ITW successfully set up a five-year research programme to develop a chemical that could modify the polymeric deposits to make them much easier to remove.
The novel polymer modifier is completely safe for use inside a unit as it does not contain any:
• Compounds of P, B, S, As, Bi, Si or Pb
• Compounds that might be harmful to plant metallurgy
• Carcinogenic compounds
• Compounds that, at operational dosages, might interfere with biological waste treatment processes.
According to laboratory tests, this chemical treatment is able to modify butadiene polymers from a hard-to-remove, sticky substance into a sand-like material that is much easier to remove.
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