Operational Device and Procedure to Test the Initial Dissolution Rate of Chemicals After Ship Accidents: the Cedre Experimental Column

ABSTRACT

This paper describes an experimental procedure recently developed to simulate and study the behaviour of various chemicals in seawater during their transfer in a 5 m deep water column. Since chemical pollution can be harmful to the environment, anticipating the evolution of a chemical spill is important for authorities, so that they can act rapidly and efficiently after a spill. In the case of a sunken wreck containing chemicals with a lower density than seawater in its leaking tanks, it could be useful to estimate the quantity of chemicals that should be found at the sea surface. On the other hand, in the case of a spill occurring at the sea surface, the mass of chemicals liable to sink and accumulate on the seabed without being dissolved during sinking is an important piece of information required to choose the most suitable response strategy.

The solubility of a particular chemical is not always available from the literature. In particular, the solubility in seawater is seldom documented and, whenever it is, it is usually in the form of solubility limits. As important as this parameter may be, it is not the most significant one in situations in which the interface between the (sea) water and the chemical involved undergoes permanent renewal. A more operational parameter would be the initial dissolution rate.

The Cedre Experimental Column (C.E.C.) was designed for this purpose and a method was defined. The volume and dimensions of droplets of chemicals were studied in order to characterize the dissolution of different products throughout their journey through the water column. The dissolution rate can be expressed as a volume loss during a flow time or between two given depths. Experiments were performed on eight chemicals with different water solubility limits (8.7 to 290 g L-1) using video imagery to analyze the chemicals' behaviour. If the initial dissolution rate is dependent on solubility, it appears clearly that other physical parameters are also relevant to predict the behaviour of a product, for instance its density and viscosity. In addition to these intrinsic parameters, the dissolution kinetics of a product are also linked to its injection flow in the water column, determined, for example, by the size of the hole in the ship which will define the droplet speed.

The C.E.C is therefore an operational tool which can be used to study the behaviour of specific chemicals, either as part of a particular study or in the event of a spill, taking into account all the relevant factors involved. It provides accurate and detailed information on the product’s dissolution kinetics, enabling it to be used to assist in decision-making for emergency response.