Oil removal efficiency

Improved Disposal Water Treatment at Bahrain Oil Fields" by Mum el at. discusses solutions to environmental concerns at Awali field. As Awali field water cuts increased, the amount of produced water and oil carry-over increased. To avoid environmental damage, more efficient oil-removal equipment was necessary. A wide variety of equipment was evaluated and the vertical-tube coalescer was selected. A description of the unit is presented. Performance evaluations were conducted, and the units exhibited good oil-removal efficiencies... 

In Hydrocyclones A Solution to Produced Water Treatment, Meldrum presents the basic design principle of a dc-oilmg hydrocy leone. System design, early operational experiences, and test results on a full-scale application in the North Sea are discussed. Oil-removal efficiency was seen to rise with increasing reject ratio up to around 1%, producing acceptable outlet concentrations Early field test results on a tension leg platform in the North Sea are discussed. Preliminary data on a pumped system are also given. 

As measured in field tests, these units operate on a constant percent removal basis. Within normal ranges, oil removal efficiency is independent of inlet concentration or oil droplet diameter. 

For flotation of oil drops by bubbles with diameters from 0.2 to 0.7 mm. the surface chemistry of drop/drop interactions as it relates to liquid coalescence and droplet breakup governs the overall performance of flotation. As the rate of dispersed oil coalescence increases, the overall oil removal efficiency for the process increases. Thus, if process improvement is desired, one should concentrate on pretreatrnent of the emulsion to improve the oil s coalescing properties. These ohservalins are consistent with Leech el at.1 who found that the most important variables governing induced-air flotation were chemical treatment (type and dose) and the system residence lime. Smaller air bubbles also increased the removal rate in our experimental range however, bubble si2e is not independently variable in the field. 

Factors That Influence Performance. The oil-removal efficiency of the deoiling liquid/liquid hydrocyclooe is influenced by a number of factors, some of which currently arc not fully defined or understood. It is certain, however, that the inlet flow rale, reject rate, droplet size, oil concentration, differential specific gravity, and temperature play significant roles. This can be represented siraplisti-cally by the following approximation ... 

In most cases, Murchison and Hutton being no exception, very high oil-removal efficiencies can be achieved with a reject ratio of -1 %. However, while this holds true for the majority of cases, the required reject rate is obviously influenced by the level of inlet contamination and may be related to the volumetric throughput and centrifugal separation forces. At lower flow rates, lower specific-gravity differences, or with small droplet sizes, it is likely that the central core will be less dense and some of the smaller, slower-moving droplets would not be captured In these cases, an increase in the reject rate beyond the characteristic breakpoint previously mentioned is likely to be of more benefit... 

In summary, during normal operating conditions on Murchison, a reject ratio of 1 % offers the best performance in terms of oil-removal efficiency and minimizing the amount of fluid recycle. Should a process upset occur, causing levels of inlet contamination higher than 3.000 ppm. then it is necessary to increase the reject rale slightly. 

However, oil-removal efficiencies as high as 98% have consistently been obtained over a range of flow rates (Fig. 8). Tests have been earned out over a wide range of conditions with reject rates ranging from 0.32 to 16.0%. A reject ratio of — 1 % was sufficient to remove the bulk of the oil, but further improvements in efficiency could be obtained with reject ratios of up to 3%. Beyond this, no appreciable benefit was obtained. This is presented graphically in Fig. 2. 

Oil-removal efficiencies were typically in the 80 to 90% range, almost regardless of the reject and flow rate settings. This represents a 10 to 15% efficiency reduction over the primary separator test (Fig. 8). 

Oilseed Raw OII Content % Raw Protein Content % Oil Removal Efficiency % Extrusing Temperature °F Expelled Meal Oil Content % Meal Protein Content %... 

FIGURE 3.35. Improved oil removal efficiency of an SP Pack installed in a 12 -0 tank. 

FIGURE 3.36. Oil removal efficiencies of various size tanks. 

Field tests indicate that a properly designed unit with a suitahle chemical treatment program should have oil removal efficiency between 40% and 55% per active cell and an overall efficiency of about 90%. An excellently designed system might exhibit an efficiency as high as 95%, while a poorly designed, poorly operated unit, or difficult oil-water chemistry could easily degrade performance to as low as 80%. Equation (3.25) verifies the above efficiencies. For example, Equation (3.25) shows that a three-cell unit can be expected to have an overall efficiency of 87% while a four-cell unit can be expected to have an overall efficiency of 94%. The unit s actual efficiency will depend on many factors that cannot be controlled or predicted in laboratory or field tests. 

Graphs of the dispersed oil concentrations in the effluent water versus dispersed oil concentrations in the inlet feed stream are shown in Figure 3.45 for representative efficiencies achievable in a typical four-cell dispersed gas flotation unit. For inlet concentrations less than about 200 mg/1, the oil removal efficiency declines slightly. At low oil inlet concentrations, it becomes more difficult for the flotation unit to achieve intimate contact and interaction between the gas bubbles and dispersed oil droplets. As a result. Figure 3.45 may understate the effluent concentrations for influent oil concentrations less than 200 mg/1. 

Oil removal efficiency depends on chosing the correct chemical and dosage, (refer to Figure 3.46). 

Performance is chiefly influenced by the reject ratio and the pressure drop ratio (PDR). The reject ratio refers to the ratio of the reject fluid rate to the total inlet fluid rate. Typically, the optimum ratio is between 1 % and 3%. This ratio is also proportional to the PDR. Operation below the optimum reject ratio will result in low oil removal efficiencies. Operation above the optimum reject ratio does not impair oil removal efficiency, but it increases the amount of liquid that must be reticulated through the facility. The PDR refers to the ratio of the pressure difference between the inlets and reject outlets and the difference from the inlet to the water outlet. A PDR of between 1.4 and 2.0 is usually desired. Performance is also affected by inlet oil droplet size, concentration of inlet oil, differential specific gravity, and inlet temperature. Temperatures greater than 80 °F result in better operation. 

Differential specific gravity. At a constant temperature, the hydrocyclone oil removal efficiency increases as the salinity increases and/or the crude specific gravity decreases. As the specific gravity difference between water and oil increases, a greater driving force for oil removal in the hydrocyclone occurs. 

Rotational speed. High rotational speeds (between 1000 and 3000 rpm) generate higher centrifugal forces, which in turn yield better oil removal efficiencies at a given flow rate. 

The oil removal efficiency, which is a function of the drop size distribution, is between 50% and 75%. Dynamic units are more effective at removing small oil droplets (15 pm) from the water. 

Produced water treating equipment performance is commonly described in terms of its "oil removal efficiency." This efficiency considers only the removal of dispersed oil and neglects the dissolved oil content. For example, if the equipment removes half of the dispersed oil contained in the influent produced water, it is said to have a 50% oil removal efficiency. For a specific piece of equipment or an overall system, the oil removal efficiency can be calculated using the following equation ... 

The performance can be described by determining the inlet and outlet oil concentrations and the associated oil droplet size distributions at the equipment inlet and outlet. This information can then be used to define the oil removal efficiency for any given oil droplet size or range of droplet sizes. This concept is further discussed in Chapter 3. 

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