Supercritical Water Oxidation Treatment Of Oil-based Drill Cuttings From Shale Gas Fields

The shale gas is type of unconventional natural gas.Oil-based fluids exhibit the best drilling performance and have been widely applied during the drilling process in the exploration and extraction of shale gas.While the oil-based drill cuttings(OBDC)are the most harmful drill wastes that contain various organic compounds and heavy metals.They have been updated in the last National Hazardous Waste Inventory of China in 2016(HW08).The current treatment techniques struggle to meet the increasingly treatment targets.In this study,supercritical water oxidation(SCWO)treatment of OBDC is carried out for the first time.The SCWO can degrade the organic compounds and stabilize the heavy metals in OBDC simultaneously.Moreover,on-site harmless treatment of OBDC can be achieved by developing skidded equipment as the compact structure of SCWO system.Two typical OBDCs are sampled from a vertical shale gas well(A)and a horizontal gas shale well(B),respectively.The based fluids are diesel for A and white oil for B,respectively.The SCWO treatment of OBDC are comprehensively investigated in a batch reactor,and a novel continuous reactor is developed for the treatment of OBDC.This study aims to exploit a new way for on-site harmless treatment of OBDC,and provide an optional reference for the industrialization of SCWO.The main contents of this paper are as follows:(1)The SCWO treatment of OBDC-A is carried out in a home-made batch reactor under the conditions of various oxygen coefficients(OC),temperatures(T)and reaction times(t).Preheating experiments indicate that most of the organic compounds in the initial OBDC sample are distributed within gaseous,oil,aqueous and solid phases,and the side reaction is not obvious when T<475?.All tested variables,i.e.,OC,T and t,positively affect the transformation of carbon compounds from the oil and solid phases to the aqueous phase and,ultimately,to CO2.Carbon monoxide is the primary stable intermediate.The SCWO reaction becomes more efficient when above the critical levels of OC=2.5,T=475? and t=3min.Analysis of the reaction pathways suggests both homogeneous and heterogeneous reactions exist in the reactor.The homogeneous reaction is a typical SCWO reaction that is governed by a free radical mechanism,and the heterogeneous reaction is dominated by mass transfer.The total organic carbon removal efficiency can reach up to 89.2%within 10 min at 500 ?.(2)A novel reactor is designed and manufactured to overcome the problems encountered in continuous supercritical water oxidation of semi-solid organic waste,such as OBDC.It is a Y-shaped reactor consisting of an inclined reaction section on the left and a vertical separation section on the right.There is no interference between these two zones.The organic waste from OBDC is degraded in the left reaction zone and the inorganic solid particles from OBDC are continually pushed forward in the inclined reaction section,and are completely separated and stored at the bottom of the vertical separation section.It is designed to minimize the counter-current flow that is prevalent in conventional reactor,and is easy to control the system and recover the reaction heat.A new equation for the residence time is derived based on the structure of the new reactor.(3)The performance of the reactor has been tested using municipal sewage sludge(MSS)as a representative waste.The effects of dry solid content(DS),type of oxidant,supercritical water flow rate and co-fuel concentration have been studied in details.There is no obvious difference for the SCWO reaction when use hydrogen peroxide,oxygen or air as the oxidant.Oxygen and air are optimized as the main oxidant and co-oxidant for our new reactor,respectively.The supercritical water flow rate is selected as the 2.5 time of the waste flow rate.The reactor plugging will happen when the supercritical water flow rate is too low.High supercritical water flow rate will dilute the waste and reduce the residual time.Increase of DS will lead to the increase of temperature in reaction zone.Adding isopropyl alcohol(IPA)as a co-fuel,with concentration of up to 12.5%,significantly increases the temperature of the reaction area and thereby improves treatment efficiency.The TOC of the effluents is as low as 16 mg/L and the CRE rises up to 99.94%.(4)Continuous SCWO treatment of OBDC-A and OBDC-B has been carried out using MSS with a DS of 12%as the diluents.The adding level of OBDC is range from 10%to 30%.Both the oil content and adding level significantly affect the temperature of the reaction area and thereby improves treatment efficiency.When the adding level is up to 30%,the reaction temperature,TOC in effluent water and CRE are 490?,351mg/L,98.44%for OBDC-A,and 444?,700mg/L,95.34%for OBDC-B,respectively.(5)The solid recovery performance suggests the solids from waste are continually pushed forward in the left inclined section and the flow regime can be considered as plug flow.Although the cumulative residual solid(ARS)in the reactor fluctuated in the range of 51-95 g,but not any accumulation during the experiments.In other words,the in-situ solid separation has been achieved in the new reactor.Moreover,the in-situ.separation performance is better during treatment of the waste mixture of OBDC and MSS due their different particle size.The solid recovery(SR)is range from 96%to 105%.(6)The heavy metals in both solid and liquid products have been detected.The concentration of all elements in effluent water are up to the primary standard of the Integrated wastewater discharge standard(GB8978-1996).However,more than the original contents of Fe,Cr,and Ni are found in the solid products due to the corrosion of the SCWO reactor,which is made of 316 stainless steel.Comparing to the results in batch reactor,the corrosion in the continuous reactor has been significantly minimized.This result suggests our new reactor has a good anti-corrosion performance.(7)The cost of a pilot scale plant with the treatment capacity of 3m3 OBDC(about 6.1 ton)per day has been estimated.The installed cost is 9.95 million CNY.The operating cost is 2928 CNY/t,and is hopeful to reduce to 1132 CNY/t,which is cheaper than the cost of conventional technology.As the good performance and environmental benefits of our new SCWO reactor,this reactor has potential for application in the harmless treatment of various hazardous organic waste.