CHEMICAL CLEANING OF EQUIPMENT AND PARTS OF DESALINATION PLANTS FOULED BY OIL SPILLS1 P.C.Mayan Kutty and Abdul Ghani I. Dalvi
ABSTRACT
Chemical contamination of seawater feed by oil spills is a major threat to the safe operation of desalination plants. Tar or residue which constitute about 55- 60% of oil content if cam’ed over to plant may deposit on the various parts of the plants adversely affecting product quality and equipment performance. Therefore, one should be able to clean the affected parts of the plant effectively and rapidly to avoid unscheduled plant shut down. This report describes the studies cam’ed out to find an effective solvent for easy and safe cleaning of plant equipment contaminated by oil spills. Laboratory scale
(ii)
Carry over of these toxic chemical to product water thereby making the product water unpotable.
(iii)
Endangering the biological life in seawater.
(iv)
Oil tar and residues, if carried over to plant, will coat on vital equipments such as pumps, travelling screen filters, chlorine generating cells, deaerator, tubes, demister, flash chambers, and piping etc. in MSF plants, micron filters and multimedia filters in reverse osmosis (RO) plants.
Most of the organic substances other than those of simple structure and low boiling point when pyrolyzed yield dark , generally viscous liquid termed as tar and pitch. When the product is liquid of fairly low viscosity at ordinary temperature it is regarded as tar and if it is very viscous, semisolid it is called pitch. The distillation of crude petroleum yield a pitch-like residue termed as bitumen or asphalt. Largest source of tar and pitch is the pyrolysis or carbonization of coal. The terms tar and pitch are synonymous with coal tar.
2.
EXPERIMENTAL Chemicals
Four commercial grade solvents viz (i) Hexane (2) Xylene (3) l,l,l trichloroethane and (4) Carbon tetrachloride were selected for the study. 2.2
Experimental
Methods
The solubility of tar in the selected solvent and its dissolution kinetics were determined as follows: 2.2.1
Kinetic studies
In 200 ml beaker 100 ml of the solvent was taken and beaker is placed on magnetic stirrer. The solvent was stirred at slow and constant speed using a Teflon stirring bar. In an aluminium boat 20 gm of tar was weighed and the
2.2.2 Solubility Studies
The determination of solubilities of tar in the solvents were determined as follows: 10 ml of the chemical was taken in a 50 ml preweighed beaker placed on a magnetic stirrer. Small aluminium planchette (preweighed) was taken and 5 gm of air dried tar was weighed into it. Whole planchette along with tar was placed in beaker containing the solvent and stirred. After about 1 to 1.5 hr of stirring and no more tar was getting dissolved, the solution was decanted. Planchette was removed from beaker and kept for drying at room temperature. Planchette was then weighed along with the remainder of tar in it. Solubility of tar in the solvent was calculated from the difference in weights. Correction was also applied if any undissolved tar was found to stick to the beaker by weighing the air dried beaker after the experiment. 3.0
RESULTS AND DISCUSSION
Composition of coke-oven tars varies according to their origin see Table 1.
gm/litre which is minimum among these solvents. Other two chemicals, xylene and trichloroethane, show intermediate solubilities. These results suggest that carbon tetrachloride is the most suitable solvent from solubility point of view among these 4 chemical agents. 3.3
Spectral Studies
Spectral scans carried out in the UV-visible regions using blank solvents indicate the absence of absorption peaks in all four solvents. However, transparency in this region of the spectrum (i.e. 190-600 nm) disappeared when coal tar is dissolved in these solvents as shown in Figure.1 for tar in hexane. A broad absorption peak develops around 280 to 320 nm in all these solvents on tar dissolution. This broad peak is characteristic of coal tar composition. Figure.2-5 show the absorption spectra taken in different solvents containing tar. These figures also show the spectra taken after different time intervals indicating increasing solubilities with contact time as described in para 2.2.2. Figure.6 shows absorptions measured as peak heights at 280 nm in different solvent as a function of contact time. It is seen that trichloroethane reaches
the lowest cost at the rate of 5.5 SR/Kg. Cost of xylene and trichloroethane are slightly higher but comparable to that of CCl4 However, great difference in solubility and slightly lower cost of CCl4 again indicate its superiority over other solvents. 3.5
Health Aspects and Toxicity
Toxicity data of these solvents are given in Table V. It can be seen that all the solvents have adverse and chronic effects on human. Hexane is a suspected neurotoxic whereas xylene damages liver and kidney if inhaled. Trichloroethane is known to affect nervous system and CCl4 is carcinogenic. Lowest published lethal concentration for inhalation of xylene is 6125 ppm/l2 hr. and for trichloroethane is 27 gm/m3/l0 min. However, for CCl4 it is 1000 ppm for inhalation and 43 mg/kg of human for oral dose. Maximum contamination level (MCL) in drinking water is lowest for carbon tetrachloride (0.005 ppm) and 0.7 ppm for trichloroethane. Xylene has MCL value of 10 ppm which is the highest. From toxicity data it looks that all solvent show toxicity of varying degree thereby rendering them unsuitable. Still in spite of adverse health aspect these solvents can be used for cleaning purposes, as l,l,l trich-
with a combustible solvent and burn in chemical incinerator with after burner and scrubber. 4.
CONCLUSION
The results of this study show that out of four chemicals tested hexane and xylene are unsuitable for decontamination of equipment fouled with tar. Both trichloroethane and carbontetrachloride are suitable cleaning agents. Considering several aspects such as solubility, kinetics of dissolution, cost etc carbon tetrachloride appears superior to trichloroethane. Further studies will be carried out on the MSF and RO pilot plants using the two most suitable solvents during the oil carry over tests scheduled on these plants. REFERENCES
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