NOTES
Feasibility of Oil Slick Removal from Seawater Using Power Lasers Elmar Laisk Visiting Fellow, Physics, at Macquarie University, Australia
A high-power-cw COz laser was used to investigate, in the laboratory, the conditions for the evaporation and decomposition and burning of crude oil from water surface. The range of activation energies for decomposition of crude oil (80% of it) on water surface is estimated to be from 1-10 kJ.g-’, and requires about 25 times higher beam intensities (at least 200 W.cm-2, but preferably > 1kW.cm-2), than that for evaporation. On the basis of these data, the performance of a projected shipborne 100-500-kW laser head, powered by either a conventional generator or atomic reactor, may become competitive with Filter-belt (“Spill Spoiler”) or Foam-belt separators, if the overall efficiency of the laser exceeds 25%. It also became evident that pulsed operation will offer distinct advantages for beam control and efficiency.
It is estimated that oil is produced now at a rate of 2 X gal per year (I).Less than 1ppm of it (about 1.9 x IO9gal) is spilled on water from various sources, causing ever-increasing pollution problems. The relative contributions from these sources are ( 2 , 3 ) :transportation and terminal operation, 35%; river and urban runoff, 31%; atmospheric fallout and natural seeps, 20%; coastal refineries and industrial waste, 13%;offshore oil production, 1%. The current techniques of oil slick removal are ( 4 , 5 ) : skimming it off the surface using “Filter Belt” (MartinMarietta), “Foam Belt” (Br. Petroleum Trading Ltd.), floats, suction devices and separators, all of which are claimed to recover 90% of oil trapped in containment booms; burning and decomposing the oil; adsorbing and absorbing it into straw, plastic foam chips, and so forth; converting it into gel1 which is much easier to collect; sinking it by spraying with suitable chemicals; emulsifying and dispersing it (6);consuming it with various bacteria; photooxidizing it using photosensitizing agents, or employing several other less practical methods. The most important factors in using any of these techniques are their economy, speed, environmental safety, and convenience. Although economy may not be decisive in environmental emergency situations, speed of removal is. In this sense a feasibility study of oil slick removal by means of ir-power lasers was undertaken on a limited scale in this laboratory, with preliminary results reported here in order to assess the merits of a proposed shipborne laser head. Physical Properties of Oil Slicks
The crude oil consists mainly of liquids of the paraffin series C5H12 to C&34 and solids from C17H36 to C45H92, with small amounts of other compounds depending on the origin of the oil ( 5 ) . It is known that after 12 h of the initial oil spill at sea about 18%, by weight, almost all light components of crude oil (Kuwait) have evaporated ( 5 ) .In 24 h this loss reaches 20%, and thereafter the residue disperses only slowly. The residue consists mainly of hydrocarbons containing C14 to C45 ( 5 ) .The heat of evaporation of 80% of this residue is typically from 0.5-1 kJ.g-’ (0.5-1 X lo9 J.ton-l). The activation energy for decomposition of the lighter fraction of t h e residue is given elsewhere (7, 8), but the given value becomes lower in the 814
Environmental Science & Technology
presence of oxygen. Its effective value, accounting for losses .in water and scattering, is estimated in our experiments to be 1-10 kJ.g-l ( 2 - 10 X lo9 J.ton-’), varying widely with the effective absorption of laser beam energy in the oil layer, in dependence of its thickness. One m3 of crude oil (Kuwait) spreads in 10 min, typically ( 5 )over an area of about 2 X lo3 m2, varying in thickness from 0.1 to > 1mm. After 24 h the thickness is typically from 1-2 wm, which corresponds to 1200-2400 1. km-2, averaging in weight 2 tons.km-2. However, in contaminated water, the oil film thickness may stay, in the central area, between 1and 3 mm. Thus, the laser evaporation technique was tested on oil films between these limits in conjunction with several known collection techniques, such as the electrostatic precipitation and separation by suction filtering. T o evaporate a 1-mm thick oil film on water, a laser beam (COz) intensity of 40 W.cm-2, a t least, but preferably >lo0 W.cm-2 is necessary. Consequently, for a proposed laser scanning head of 5 X 0.05 m2 working area (at 10-m distance from the exit), a total beam power from 100-250 kW is required, depending on the “sweeping” speed. E x p e r i m e n t a l Data
The laboratory experiments initially involved a 125-W-cw COz laser of 1-m cavity, operating at multimode between 20 and 25% efficiency. These experiments did not account for wind, waves, or seawater contamination. First, it was difficult to avoid the co-evaporation of a lot of water when evaporating a thin film of oil. Although water vapor exhibits a wide transmission “window” for wavelengths from 8-13 wm to enable the beam to penetrate the cloud, nevertheless the energy loss due to large heat of evaporation of water (2.3 kJ.g-l) will be prohibitive-unless the laser beam is switched on and off by an oil-and-water-vapor-sensingdevice. The already established ir, microwave and UV fluorescence oil-sensing techniques can be adapted for this purpose (9-1 4 ) . Also, it must be considered, that water vapor behaves as a saturable absorber for high-energy COz laser pulses ( 1 5 ) .Of course, the evaporation of oil assumes an effective collection and separation, either by electrostatic or suction techniques, both of which were attempted qualitatively only. On the other hand, for the decomposition and burning of heavy oil on water surface, a laser beam intensity > 2 0 0 . W . ~ m -and ~ energy from 2-10 kJ.g-’ is required, both varying with the thickness and composition of oil film. Evidently, oil on water in a thickness less than 3 mm would not burn steadily ( 5 ) ,but decomposes in miniexplosions. Conclusions
If we assume the price of shipborne electricity, tentatively, at $4.00 for 1000 kWh and a 25% laser efficiency, the energy cost of evaporating oil from a water surface would amount to $2.2 to $18 per ton, not accounting for overhead. A 100-kW laser head can be accommodated on a relatively small (
