Green retarder technology for the â€¨styrene industry
Increased environmental and safety awareness in the styrene industry has led to the development of more acceptable alternatives to toxic nitrophenolic retarders
Lisheng Xu, Javier Florencio, Vincent Lewis and Christopher Morrison, Nalco
Ana Guzman, Carmen Monfort and Ana Olivares, Repsol QuÃmica Tarragona
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Styrene is an important petrochemical product that is used as a starting material for a variety of polymer products, polystyrene in particular. There are two main industrial routes for the production of styrene; namely, the EB/SM route and the PO/SM (sometimes referred to as the SM/PO) route. The EB/SM route produces styrene through the dehydrogenation of ethylbenzene. The PO/SM route furnishes styrene through the oxidation of ethylbenzene and subsequent addition of propylene to co-produce propylene oxide and styrene, after dehydration of the intermediate alcohol. Ethylbenzene, used in both cases, is produced through an alkylation reaction between benzene and ethylene. As of 2009, total global styrene capacity was about 30 million t/y, of which about 80% was produced through the EB/SM process and 20% through the PO/SM process.
Regardless of specific licensed technologies for styrene production, crude styrene produced in the reaction section needs to be purified to yield polymer-grade styrene monomer. This is typically accomplished through a series of distillation towers designed to separate styrene from unreacted ethylbenzene as well as other reaction byproducts.
Polymer control in a styrene unit
Styrene is a very reactive monomer. If not properly inhibited, it will polymerise rapidly via a free radical mechanism at elevated temperatures typically encountered in the styrene purification process. The polymerisation reaction is self-initiating; no initiators such as peroxides are required to initiate the reaction process. The amount of polymer formed in a specific production unit is a function not only of the process temperature, but also of the residence time and styrene monomer concentration in the distillation towers. Higher process temperatures result in increased rates of polymerisation. Furthermore, increased residence times result in a greater amount of polymer forming.
Proper polymer control in the purification section of a styrene plant is important for two main reasons. First, styrene monomer that is polymerised is an economic loss for the producer, as it can no longer be sold as styrene monomer. Second, if polymerisation goes unchecked, high polymer levels in styrene monomer will result in an increase in stream viscosity. While polystyrene is soluble in styrene monomer, very high levels of polystyrene can make the stream overly viscous, causing difficulties in pumping the material through the process. Occasionally, high levels of divinylbenzene (DVB) can also result in the formation of insoluble crosslinked polymer that will deposit inside the tower.
Technologies to control polymer
There are two categories of compounds that are commonly used for polymer control. The first category is a slow-reacting compound, typically known as a retarder. Today, the most commonly used retarders in the industry are â€¨nitrophenolic-based products, and in particular DNBP (2,4-dinitro-6-sec-butylphenol). The second category is much faster reacting and is sometimes referred to as a true inhibitor. The true inhibitors available on the market are mostly proprietary formulations that offer a faster polymer inhibition rate than DNBP. Figure 1 shows the difference between a retarder and an inhibitor. In the graph, uninhibited styrene has an inherent polymerisation rate, as represented by the line marked as untreated. A retarder would slow the polymerisation rate and is represented by the red line with reduced slope. A true inhibitor is faster reacting than a retarder; therefore, the polymerisation rate when an inhibitor is used would be less than a retarder, as represented by the orange line. However, due to its rapid rate of reaction, inhibitor is consumed much faster than a typical retarder. Once consumed, polymerisation resumes at the rate of an uninhibited system, as represented by the second half of the orange line.
Optimal approach to polymer inhibition
The optimal approach to polymer control in a styrene unit is to use a combination of inhibitor and retarder. This ensures polymer control during both normal operations and emergency shutdowns. During normal plant operations, a faster reacting true inhibitor provides superior polymer control versus a retarder such as DNBP. In fact, the faster the reaction rate of an inhibitor, the more effective it is in reducing polymer formation. Nalco offers Prism inhibitors that are several orders of magnitude faster than DNBP for polymer control in a styrene unit. Some styrene producers only use DNBP for polymer control, but with less satisfactory results. However, when emergency shutdowns occur, a retarder is essential in protecting the unit from excessive polymer build-up. When a plant loses power, it loses, in most cases, its ability to pump the hot process stream out of the distillation towers. It also loses in most cases its ability to inject polymer inhibitor and/or retarder. Due to tower insulation, the hot process fluid will remain at elevated temperatures for an extended period of time. This long residence time can cause excessive polymer build-up if not properly controlled. In severe cases, high polymer levels eventually lead to solidification and can turn the distillation tower contents into a solid block of polymer — a devastating outcome for any facility. A fast-reacting inhibitor, while excellent for controlling polymer during normal operations, can be quickly consumed during an emergency shutdown, thus losing its ability to protect the tower. On the other hand, a retarder — owing to its slower reaction rate — will remain effective for longer and offer prolonged protection for the tower. The ratio between inhibitor and retarder should be considered carefully to result in optimal cost performance while ensuring safety during an emergency shutdown.
A summary of the relative polymerisation rates of various inhibitors and retarders is shown in Table 1.
As an effective retarder, DNBP has been the product of choice for the styrene industry. It is a reliable and economic means of protecting the tower during emergency shutdowns. But DNBP is also highly toxic. It causes reproductive and developmental system damages in mammals. It is classified by European Regulations as a CMR (carcinogenic, mutagenic and reproductive toxin substances) because it is a reproductive toxin category 2 and 3, a chemical that will be subject to further reviews for its future use. Potential exposure to DNBP poses severe health risks. It is also soluble in water at levels that can be highly toxic and therefore a potential hazard in the environment. Initially sold as a herbicide, its use in agriculture has long been banned in the US and many other countries. While the risk of exposure to DNBP in an industrial setting is certainly lower than in an agricultural setting, handling DNBP is by no means safe, since it has a very low LD50. Industrial accidents do occur, and operators and the environment in the immediate vicinity of the plant are always at risk as long as DNBP is utilised. In recent years, the potential risks of using DNBP in a styrene plant have been highlighted in many countries.
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