Innovative approach

Oily wastewater is co-produced during oil and gas production. Its management presents considerable challenges and costs to operators and disposal of this wastewater can be problematic in environmental terms due to its highly saline nature. Oily wastewaters constitute a major environmental problem in many industries. Metal, textile, automotive, petrochemical and aeronautical industries are affected by this problem. Conventional treatment of process effluents typically involves a combination of physical, chemical and biological processes. Produced water is oily wastewater that is co-produced during oil and gas production. It is basically water trapped in underground formations that is brought to the surface along with oil or gas. It is by far the largest volume by-product or waste stream associated with oil and gas production. Management of produced water presents considerable challenges and costs to operators and disposal of produced water can be problematic in environmental terms due to its highly saline nature. In addition to formation water, produced water from gas operations also includes condensed water and has higher contents of low molecular-weight aromatic hydrocarbons such as benzene, toluene, ethylbenzene and xylene (BTEX) than those from oil operations; hence they are relatively more toxic than produced waters from oil production. Studies indicate that the produced waters discharged from gas/condensate platforms are about 10 times more toxic than the produced waters discharged from oil platforms. Standard oily-wastewater remediation relied for decades on API 650 for oily-wastewater separation (OWS) treatment using gravimetric lagoon separation, then reprocessing the recovered floatable oil portion and using holding-pond clarification of the wastewater portion before ‘land-farming’ discharge, with led to substantial groundwater and air pollution. OWS certainly can’t be expected to meet the more stringent requirements of modern environmental regulations, or be deployed for remote sites as a package treatment plant option. Oily wastewaters can be generally separated into oil and aqueous phases by gravity separation, using either the API separator, or a parallel-plate separator. The surface oil can then be skimmed off by various devices. Air flotation can also be used for the more difficult separations, where better performance or rapid recovery is required. Chemical additives may be used in air flotation to improve separation. Ultra-filtration is an important technology employed to clean up the wastewater to make it suitable for discharge into a municipal sewer and provide an oily concentrate rich enough to support combustion. Various new configurations of separation technology have expanded oily-wastewater treatment options; everything from hydro-cyclones to coalescing plate filters, dissolved air flotation and even the use of ultra-filtration to separate and concentrate the individual waste streams. While these methods offer good process response through a wide range of flows, and can meet typical 100 mg/l total hydrocarbon cleanup regulations, they are incapable of meeting proposed European environmental protection legislation, and also risk noncompliance with the ATEX Directive for processes operating in explosive environments. Moreover, none of these filtration methods offer the capability of treating the produced wastewater for heavy metals, COD, nitrogen and phosphorus removal without more advanced treatment processes, such as chemical precipitation, air stripping, chemical oxidation, or activated carbon adsorption. Again, these advanced processes generally cannot be deployed for remote sites as a package treatment plant option and all produce a toxic concentrate or sludge requiring further treatment or disposal as special waste. The water de-oiling plants
From 2000 to 2006 ABB has studied and built in North Africa seven water de-oiling plants in three different locations for the a primary oil and gas company Construction of these plants has been performed in partnership with SARPI (joint-venture ABB/Sonatrach). After handover, the plants were operated and maintained locally until 2007 when, because of a change in policy, it was decided to outsource the related services. ABB and SARPI were selected to perform both operation and “full service” activities for a period of 5 years on 4 of the water de-oiling plants in Gassi Touil and Hassi R’Mel. Process technology
Schematically it is possible to distinguish three separate treatment cycles:
a) Water treatment cycle – Water coming from existing oil & gas production plant, which contains hydrocarbons and solid particles in suspension, is collected in a storage tank. The water is passed through a corrugated plate interceptor (CPI) and then to a flocculation unit, where specific chemicals are added. Water is transferred to a flotation unit and the cleaned water is passed through a filter unit before underground injection.
b) Oil treatment cycle – Floating hydrocarbons on the surface inside the storage tank and the CPI are recovered by oil skimmers and collected in a recovered oil tank before being sent to the Client oil production unit.
c) Mud treatment cycle – Flocs developed inside the CPI and the flocculation unit are sent to the flotation unit. Flocs grow until they become mud, which is recovered by the scraper inside the flotation unit and sent to the thickening machine. Mud collected at the bottom of storage tank, flotation unit and flocculation unit are also sent to the thickening machine. Water coming from the existing oil & gas production plant contains hydrocarbons and solid particles in suspension. These impurities are eliminated by using physical methods (density difference; settling; filtering; spin-drier) and by adding specific chemical products. The treated water is transferred to an external basin where it evaporates or comes inside filtering unit before to be injected under-ground. The fifth and last phase is solid thickening and drying where collected solids such as mud from the storage tank, from the CPI and from flocculation unit are agglomerated and centrifuged. The centrifuged mud is stored in an external area. Tackling water treatment in these specific cases has been particularly challenging. A novel stoicheiometric formulation of traditional products achieved surprisingly good results and gave indications about the best directions to be followed. Additional experiments resulted in the design and realization of a skid-mounted device, which is able to automatically process and prepare the additive in the optimal doses, starting from raw materials available also in developing regions. Also the filtration phase has been improved because water entering into the filters is mixed with the same additive. This way the traditional mechanical filtration is enhanced by chemical filtration where sand grains become coated with the chemical additives. Finally the process is flexible enough to allow further tailoring to the specific plant features and/or needs. Advantages of the innovative implementation
The innovative approach and equipment are currently patent-pending and present the additional advantage to be relatively cheap and potentially able to significantly remove heavy metals through an ionic exchange-like procedure. The water treatment process has been selected because of the advantages inherent in its implementation. In fact the proposed approach:
_ can be adapted to treat oily water with high salinity
_ is not dependent on the pH of the wastewater
_ independent on the temperature of the wastewater
_ has full flexibility of flow (0% to 100% of max inlet water flow)
_ improves energy efficiency by minimizing the number of pumps through the use of gravity flow. Additionally it is characterized by a reduced footprint (it is included into an indoor area just 35 meters wide and 80 meters long), it allows to be easily managed by local operators, and it utilizes chemicals which can be produced on site starting from easily available, cheap base ingredients, a feature which is highly advantageous in desert areas like the ones involved in these specific projects. It should be finally noted that, because of its design, it can be built on skids (in factories or workshops) and then hauled to site for final installation and commissioning. Operations and results
Gassi Touil and Hassi R’Mel plants were commissioned after revamping in the first quarter of 2009. For each water de-oiling plant, two teams have been set-up in order to assure continuous and efficient operation. Maintenance activities are based on a detailed maintenance plan which has been prepared, starting from equipment maintenance manuals and analysis of equipment criticality, in order to ensure plant availability and reliability and to guarantee full compliance to health, safety and environmental requirements. After plant start-up and first operational experience, laboratory analysis provided consistently excellent result. The hydrocarbon content and suspended solid concentration values in the outlet water are respectively 7 and 55 times smaller than the Client contract specifications. The plant uses standard equipment (pumps, motors, air compressors, etc.) and instruments (indicators, transmitters, etc) that don’t need specific know-how or experience for operators. Operation of the water de-oiling plants is relatively simple and the process is controlled and regulated by a DCS located in the control room. The plant lay-out and hydraulic profile are designed to maximize the use of gravity flow and to reduce the number of pumps. Electrical and instrument components are ATEX. The plant is protected by a fire fighting system (water/ foam/CO2). Possible extensions and conclusions
The oily-water treatment strategy and implementation described above is proving a remarkable success in a number of respects. 1. First it has reached and exceeded performance targets in terms of quality of the treated and released waters. 2. Secondly this performance has been obtained in reasonably short period of time and with a clever and careful procedure and has proved to be sustainable over time. 3. Last but not least, this design is energy efficient, allowing the operator to minimize operating costs. Because of its inherent features, the described is suitable for the treatment of highly saline wastewaters, making it a fit for the treatment of wastewater from Oil & Gas production plants. However the methodology promises to be easily and successfully extended to water treatment units in such diverse environments as oil refineries and pulp and paper plants, not to mention the potentially large market of oil production from oil sands. Authors: Marco Apicella, Nunzio Bonavita, Paolo Capelli (ABB SpA), Raimondo Cianfruglia (Oil and Gas Services Srl)