Pereira, Igor S. M.
(2013)
Microwave processing of oil contaminated drill cuttings.
PhD thesis, University of Nottingham.
Abstract
Easily accessible oil reserves are currently decreasing, leading to an increase in more complex offshore deep-sea drilling programs, which require increasingly greater depths to be drilled. Such wells are commonly drilled using oil based muds, which leads to the production of drilled rock fragments, drill cuttings, which are contaminated with the base oil present in the mud. It is a legal requirement to reduce oil content to below 1 wt% in order to dispose of these drill cuttings in the North Sea and microwave processing is suggested as a feasible method of achieving the desired oil removal. However, there are currently gaps in our understanding of the mechanisms behind, and variables affecting, the microwave treatment of oil contaminated drill cuttings. The work described in this thesis seeks to address some of these gaps in knowledge.
There were three main objectives for this thesis: (1) quantification, for the first time in the literature, of the main mechanisms driving oil and water removal during microwave processing of oil contaminated drill cuttings, (2) determination of key variables affecting performance during pilot scale continuous processing of oil contaminated drill cuttings and, for the first time, (3) treatment of drill cuttings with microwaves continuously at 896 MHz.
Bench scale experiments carried out in a single mode applicator were used to quantify the mechanisms involved in oil and water removal from drill cuttings. It was found that both vaporisation and entrainment mechanisms play a role in oil and water removal. Vaporisation was the main mechanism of water and oil removal, and typically accounted for >80-90% of the water and oil removed. For oil removal, vaporisation of the oil phase accounted for 70-100% of the overall removal. The absolute amount of water entrained and vaporised was found to increase with increasing energy input and power density. However, as a percentage of the overall amount removed, entrainment was found to increase with increasing energy input. This was mainly due to higher heating rates at higher energy inputs, leading to pressurised, high velocity steam, which increased liquid carry-over (entrainment).
Both the drill cuttings sample composition and applicator type were found to have an effect on the extent of entrainment/vaporisation. Samples consisting of a higher overall liquid content, tended to have a greater amount of surface liquid content. This led to a greater potential of carry over when steam generated internally left the sample. Increasing the power again led an increase in entrainment in this case.
Different applicators were found to impact the electric field strength and power density within the water phase of the sample. Oil removal in multimode applicators progressed mainly through vaporisation (steam distillation) until the water content was sufficiently low to generate steam at a velocity high enough to entrain liquid droplets. When treatment was changed to single mode operation, entrainment occurred at an earlier stage, probably due to higher electric field strengths and power densities. It was also noted that the vaporisation mechanism of oil was more efficient at higher field strengths and powers, which could again be attributed to superheating and higher velocity steam, which enabled better mixing and heat transfer.
Experiments were also run to determine the main variables affecting the performance of continuous processing of cuttings. Overall continuous processing showed a substantial improvement in the energy required, 150 kWh/t vs. >250 kWh/t, to reduce the oil content of a drill cuttings sample to 1 wt%. It was found that the initial water and oil content of the sample, as well as the sample particle size distribution, had the greatest effect on the efficiency of continuous processing.
The effect of initial water and oil content on residual oil content was investigated methodically for the first time for continuous microwave processing of oil contaminated drill cuttings. An increase in initial oil content was found to have a significant impact on the energy input required to treat the sample to 1 wt% oil content. As the oil content increased, the energy input required increased exponentially, mainly as a result of the change in the physical structure of the sample.
An increase in the water content led to an increase in energy input without any additional benefit to oil removal. However, as the water content was increased it was noticed that the theoretical energy input required to heat the entire sample approached the actual value measured for the energy input. This occurs as a result of the increasingly greater bulk dielectric properties of the sample as a result of higher levels of water content, which in turn leads to a higher efficiency in the conversion of microwave energy to heat in the sample.
The effect of particle size on oil content distribution and removal was investigated. Oil content was found to be substantially higher in particles of size <1.0 mm, with removal also being significantly higher in this particle size range. However, as the majority of the samples tested, >80%, consisted of particles >1.0 mm, this improved removal is diluted by the performance of the coarser particles. The improved removal in finer particles is likely to be due to larger surface area, reduced path length within the particles and potentially higher electric field strength.
Finally, samples processed continuously using a continuous microwave setup at 896 MHz showed improvements over both continuous microwave treatment at 2.45 GHz and bench scale setups. Increasing the f10wrate of the system at 896 MHz was also found to improve oil removal efficiency, which can be explained by the higher power requirements that would be required to maintain the energy inputs observed at the lower flowrate. Increasing the power leads to improved heating rates and thus increased removal rates through entrainment and vaporisation.
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