Written by: Amish Shah
In the chemical and associated industries, there are a lot of raw materials that are either in short supply or available only in limited amounts. Either of these situations will hike up the cost and post usage, the pressure on landfills will also increase, causing an environmental problem as well. It is to avoid such situations that several industries are looking at sustainable methods of operation and recycling emerges as one of the core factors.
Solid catalytic materials have a very important role to play in the oil refining industry and while there are several types of catalysts that are used, the most commonly used ones are hydro-processing catalysts, fluid catalytic cracking catalysts and reforming catalysts. When the processing is going on, the catalyst becomes contaminated – impurities could come in the form of coke, sulfur, vanadium and nickel, which is present in crude oil feed. Over time, the catalyst loses its functionality and becomes “deactivated”. While diesel hydrodesulfurization catalysts tend to have a life span of about 3-4 years, fluid catalytic cracking or FCC catalysts are lost to the atmosphere on a daily basis, which means that it needs to be off loaded either every fortnight or every month, depending on the activity.
Once the catalysts have completed their life cycle, there is a need to withdraw them from the process, because at this point of time, they are considered ‘spent’. And because they have become contaminated, they are considered hazardous waste too. Studies have revealed that somewhere near 150,000 to 170,000 tons of hydro-processing catalysts are generated each year, all over the world. The reason for such large volumes is mainly because there is an ever increasing demand for ultra-low sulfur transportation fuels and these catalysts are an essential part. With high demand, there is an over utilisation of the catalysts, reducing the life cycle much faster. For fluid catalytic cracking or FCC catalysts, the number will be much higher, because the life cycle is much shorter.
The traditional method of spent catalyst disposal is dumping the same into landfills, which is not environmentally safe, because these will not decompose and there is a high chance of soil and ground water contamination. In pursuit of more sustainable methods, there have been developments in the form of regeneration of catalysts, recovery of some or even all the components in the material or reutilisation of spent catalysts for other processes.
Rather than sending all spent catalyst into landfill, there are several attempts being made by companies to either reuse, recycle or extract remainder materials from the same. So, for instance, the residue from atmospheric distillation might be subjected to an additional round of distillation and the resultant lubricant fractions are extremely valuable. The lighter hydrocarbons that are received as by-products in several refinery units quite often finds new use – by being converted into high octane fractions by catalytic alkylation and polymerization.
In order to even consider spent catalyst procurement, the first point to be taken under consideration is whether trying to recycle or reuse will be the most feasible approach. For instance, if a spent catalyst contains only aluminium, silicon or iron, it can be disposed off quite easily without the need for any special precautions, because there are not deemed hazardous. However, in hydro-processing, the spent catalysts mainly contain vanadium and molybdenum and these need to meet several criteria, before they can be disposed, which is why recycling or reusing them is the better solution.
There are two major methods used for the recovery of metals or the separation of various components of the spent catalyst:
The regeneration of spent catalyst can be considered when the metal contamination is extremely low – the heating rates and gases need to be controlled at all times. However, during regeneration, some of the active metal might react with the supporting elements, which is why complete and absolute recovery of fresh catalysts is not possible. Now, although the regenerated catalyst can be used for the same process it was spent during, it can also be used for processes, where the degree of catalytic activity is much lower. Spent catalysts find full-fledged reuse in industries such as cement manufacturing, construction and asphalt paving; but major production of spent catalyst remains from residue fluid catalytic cracking.
Based on the concept of a fluidized bed reactor (a type of reactor device that can efficiently carry out several multiphase chemical reactions), the ebullated bed reactor utilises the process of ebullition, or in simple terms, bubbling to ensure that there is a proper distribution of reactants and catalysts. By using a three phase reactor, namely liquid, vapour and catalyst, this device can execute exothermic reactions with ease. Basically, the bed reactor will continuously and effectively mix the liquid and the catalyst particles, while ensuring the maintenance of proper temperature and ensuring that pressure drops are stabilised. Through this process, the NiMo catalysts are able to pick vanadium as well as nickel from the residues.
The hazardous nature and the strict environmental laws have been forcing several companies to develop processes that minimize catalyst waste. Thankfully, Rubamin has been not only reducing the waste, but has also developed state of the art procedures that allows for effective and efficient recycling of the spent catalyst, trying to be one of the most sustainable chemical companies, in India.