Shell companies have been producing and supplying styrene monomer (SM) to customers around the globe for more than 40 years. The Shell Group is the largest producer of SM, with an integrated global manufacturing base and supply network, and a leading position in the merchant market.
The Group has accumulated an extensive amount of data and knowledge around the behaviour of the product while it is transported or held in storage tanks. It has utilised this experience to play a leading role in industry initiatives that are aimed at raising standards of product stewardship.
The common industry practice of using third party terminals and depots in different parts of the world has brought to light an issue relating to the conditions under which SM is kept in storage tanks for extended periods.
"We have observed an isolated but increasing number of examples across the industry where product held in tanks had begun to polymerise, due to one or more factors relating to how the product was kept," explains Hidde Goïnga, Global Business Manager for SM.
"It was important for us to address this issue, both as a responsible producer and a key player in the industry. It led us to look more closely at the underlying factors that influence SM polymerisation during storage, and the potential implications for quality and safety.
"We have used this work to develop checklists to appraise our own facilities as well as those we contract from third parties, but because this is an industry-wide issue we have also made the information available to our co-producers and customers through industry groups.
"If the right attention is not paid to the equipment, processes and monitoring procedures for storing SM, situations may arise where at best the product quality degenerates or, in worst cases they may pose a serious safety concern."
Styrene is a very reactive material, which is what makes it one of the most versatile chemical intermediates. Due to its chemical structure, under certain conditions it can start to polymerise (form solid polystyrene polymer) during storage.
A small amount of polymer build-up is normal and has no negative impact, but if the process is not controlled it can snowball. "The polymerisation reaction generates heat which if not removed or dissipated, could cause the styrene temperature to rise to a level at which the reaction becomes rapid and self-sustaining, resulting in a runaway polymerisation," explains Michiel Bovendeert, SM Process Technologist.
"This involves the release of large quantities of heat together with volumetric expansion which, in a contained environment, may be violent."
Polymerisation may be initiated by high ambient temperatures, lack of inhibitor and/or oxygen in the tank, or contact between the product and contaminants such as rust - or a combination of these factors.
"To maintain the monomer content for quality as well as safety reasons the polymerisation reaction is usually retarded by adding inhibitors," explains Susanne Trebert-Haeberlin, Global Product Steward for SM. "But in addition we know that oxygen levels in the tanks also need to be maintained at certain levels for the inhibitor to function properly.
"A method for circulation is also required in order to ensure that when inhibitor is added, it is properly mixed with the product. A homogeneous mix will give a more representative sample for detecting polymer levels while regular circulation may also help to contain temperature rises." At normal ambient temperatures styrene polymerises slowly, but at higher temperatures the reaction can be rapid.
"If tanks regularly experience high ambient temperatures the lining and ancillaries should be designed and constructed with appropriate materials - stainless or carbon steel - and may need some form of insulation or cooling," says Trebert- Haeberlin. "Effective cleaning procedures should also be in place to minimise sources of contamination."
SM storage tanks should also be monitored for any changes in the condition of the product. "Regular inspections of the tanks should be carried out, including tests to measure levels of polymer, inhibitor, oxygen, contaminants and temperature. A critical change to these parameters means that operational controls may need to be initiated," says Trebert-Haeberlin.
As a result of the input from Shell HSSE and Technology experts, a new set of guidelines for the storage of SM was developed. "We have used these guidelines to carry out an internal review of all our storage facilities and those that we contract from third parties around the world," says Hidde Goïnga.
"We've prepared and implemented training for operators at these facilities, as well as making a number of small changes to equipment or procedures. In a few isolated cases we have withdrawn from third party storage facilities where they could not be brought up to our minimum standards."
He says the learnings have also been shared across the industry. "Although they are guidelines, not formal standards, this is a cross-industry issue and so we had a responsibility to make the information available to customers and other producers."
As a result, the findings have been incorporated into CEFIC's official health, safety and environmental standards for SM, which it makes available to the industry through the Styrene Sector Group. "We know this was becoming a significant issue across the industry and that these new guidelines have been well received."
Styrene monomer content
CEFIC styrene monomer HSE guidelines - opens in new window
