Skimmer units

Density skimmers are a special type of bailer that are designed to remove LNAPL from the water surface in wells (Figure 7.4). Most of these circular skimmers are constructed with a hydrophobic (water-repelling) membrane around the intake area. When installed in a well (or other structure), such that the membrane is at or slightly below the water surface, the LNAPL seeps into the storage compartment. Periodically, the bailer is manually retrieved and LNAPL is recovered. Density skimmers are very useful at locations that have slow recovery and are in remote locations. A properly designed skimmer unit can remove LNAPL down to a thickness of /x in. (0.01 ft or 3 mm). 

Rope and belt skimmer units are quite efficient at removing thin layers of LNAPL from both open-trench wells and larger surface areas (ponds, or even manholes). Recent developments include development of belts that are sufficiently narrow to use in open wells. Both smooth flexible-tube and belt systems rely upon continuous rotation of a flexible closed-loop tube (or belt) constructed of an oleophilic material (Figure 7.16). The automatic unit draws the oil-covered tube (or belt) through scrapers and returns it to the surface to gather more oil. 

Solubility — the amount of a given substance (the solute) that dissolves in a unit volume of a liquid (the solvent). This property is of importance in the handling and recovery of spilled hazardous materials. Water-insoluble ehemicals are much easier to reeover from water than spills of water-soluble chemicals. Acetone, which is miscible/soluble in water in all proportions, is not readily reeoverable from water. In contrast, benzene, which is lighter than water and insoluble as well, can be readily trapped with a skimmer. For organie eompounds, solubility tends to deerease with inereasing moleeular weight and ehlorine content. 

Further considerations include skimmers and weir overflow rates. Skimmers should be provided on all units since even secondary effluents contain some floatable solids and grease. Overflow rates and sludge scraper design should conform to the requirements of other clarification units. 

Construction of a subsurface structure that penetrates the water table only a short distance can be an effective LNAPL retention technique. Most underflow structures function in the same way as a surface baffle in an oil water separator or a septic tank. The structure must be carefully installed perpendicular to groundwater flow, and have some arrangement to collect the free-phase LNAPL. Often, a simple French drain (constructed parallel to the retaining wall) leading to a recovery well (with skimmer) is effective. A schematic diagram of a hydraulic underflow structure with a skimming unit is presented in Figure 7.3. 

Suspended pneumatic skimmers recover LNAPL where sufficient product is accumulated to allow timed recovery. They are installed as single-pump recovery units and also as an NAPL recovery pump for two-pump recovery units. After the cycle time is adjusted for a semi-steady-state pumping situation, they are easily controlled. 

The advantage of either rope or belt skimmers is that they do not require significant operational attention. They can be left unattended (except for routine maintenance) for extended periods of time. Also, either unit can recover thin layers of floating product without sophisticated instrumentation. The primary disadvantage is that the recovery capacity is limited to a few hundred gallons per day per unit. Higher-viscosity oils attach more effectively to the oleophilic materials. Less-viscous products are recovered less effectively. 

The PetroTrap is available from the vendor at a cost of 885.00 per unit. The standard system includes a PetroTrap skimmer assembly (2- or 4-inch-diameter model), a 25-ft suspension hose, and a locking well cap (D17074L, p. 4). 

For the above estimate, total equipment costs for the SFC system was estimated at 35,540. This included the vertical fin coalescing unit, oleophilic ceramic granules, pumps, a pneumatic control system, and such ancillary equipment as a 2000-gal bulk tank with fittings, a skimmer, necessary switches, valves, piping, and wires (D11018P, p. 28). 

The skimmed oil Is collected and transferred to wet crude tank. Water from the SPI separator is discharged to induced air floatator. This unit is composed of four floatation cells. Each cell is equipped with a motor driven self-aerating rotor meonanistn. As the rotor spins, minute bubbles are generated ar.d oil and suspended solid particles attach to the gas bubbles as tney rise to the surface. Tne oil and suspended solids gather in a dense froth on the surface and are removed from the cell by skimmer paddles and collected in a scum tank. Then skum is pumped out of scum tank to the inlet of SPI separator by skum return pump. 

Precipltatora/fllterm. In the past, it was common to direct water to be treated through a bed of excelsior or a similar medium to aid coalescence of oil droplets (Fig. 6). However, the coalescing medium has a tendency to dog, and many of these devices in oil field service have had the medium removed. In such a case, these units act like vertical skimmers since oil droplets must flow counter-current to the downward flow of water through the area formerly occupied by the medium. 

By regulation, produced water must be treated (i r, passed through a skimmer tank, cualcscer or flotation unit) prior to disposal. Disposal piles are permitted to collect treated produced water, treated sand, liquids from drip pans and deck drains and as a final trap for hydrocarbon liquid in event of equipment upsets. 

Skimmer paddles are provided along both sides of the unit. Gas-tight inspection hatches are located above the skimmer paddles on each cell for inspection and maintenance. 

Vertical-Tube Coalescer This is the equipment finally selected to meet the design parameters listed in Table 2. The vertical-tube coalescer (VTC) unit equals the performance of a skimmer in less space or offers improved performance in the same space. The VTC tube packs provide up to five times more coalescing surface than a plate pack. The extra surface gives oil globules more area for coalescence Also, the vertical orientation of the tubes contributes to a more efficient separation. In a plate pack, the rising oil droplets must move perpendicular to the flow of influent. In a VTC pack, the oil droplets arc free to travel upward and to collect on the top surface of the tube bundles. 

The coalescer receives discharge from the skimmer tanks and a small quantity from the crude oil wash-tank drain The unit is capable of receiving produced water directly from FWKO drums in case skimmer tanks are out of service. The oily water mixture enters the unit in the inlet chamber where the reduced liquid velocity allows solids to settle to the bottom of the unit as sludge These solids are removed periodically The inlet divides the flow into two streams to direct it into two chambers within the unit. The two chambers maintain identical operating levels because of a large balancing hole in the partition wall. 

The skimmed and collimated cluster beam passes into the quadrupole mass spectrometer (Extrel, C-50), which has a mass range of 0-1200 amu with unit mass resolution. The mass spectrometer chamber is pumped by a turbomolecular pump (360Is-1). The pressure in the mass spectrometer chamber (P3), when the beam is in operation, is always less than 1 x 10 6 torr. This is necessary to ensure that the contributions from reactions of the cluster ions with the background gas are not significant. The distance of the nozzle from the ion source varies in the range 20.5-22.5 cm, depending on the nozzle to skimmer distance. 

Four-cell flotation unit 2280 L/h capacity at 25 C c/w one froth skimmer and disperser-impeller per cell ... 

Skimmers are available in a variety of forms, including independent units built into a vessel or containment device and units that operate in either a stationary or mobile (advancing) mode. Some skimmers have storage space for the recovered oil and some of these also have other equipment such as separators to treat the recovered oil. 

A skimmer s overall performance is usually determined by a combination of its recovery rate and the percentage of oil recovered. The recovery rate is the volume of oil recovered under specific conditions. It is measured as volume per unit of time,... 

Polyethylene — A polymer (substance composed of very large molecules that are multiples of simpler chemical units) of the alkene, ethylene. Polyethylene is highly resistant to chemicals, and has low water absorption and good insulating properties and can be manufactured in a number of forms. Polyethylene also has high oleophilic properties and has been used successfully as a sorbent for cleaning up oil spills. (See also Alkenes, Oleophilic surface skimmers, Synthetic organic sorbents.)... 

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