18

Le  Floch  -  What  are  Dispersants?

Photo-oxydation

Air
Water

Evaporation  Oil  slick

Volatilization

Dissolution  Dispersion

Sedimentation

Emulsification

Figure  4.3:  Oil  weathering  processes.
The  weathering  of  oil  at  sea  results  in  an  increasing  viscosity,  and  as  a  consequence  it  will
reduce  the  efficacy  of  chemical  dispersion.  However,  the  speed  of  these  phenomena  depends ­
  directly  on  the  environmental  conditions  in  the  zone,  i.e.  the  same  oil  may  remain  dispersible ­
  depending  on  the  sea  state  and  on  the  temperature,  as  temperature  greatly  affects
the  oil  viscosity.  Therefore,  oil,  which  is  fluid  in  tropical  climates  and  thus  potentially  dispersible, ­
  may  become  highly  viscous  in  arctic  regions  and  thus  be  non-dispersible.
Therefore,  after  a  certain  time,  floating  oil  will  become  non-dispersible  due  to  weathering
processes.  In  operational  terms,  this  is  reflected  by  a  “window  of  opportunity”  for  dispersant
use,  i.e.  strategically  speaking  the  decision  whether  or  not  to  disperse  an  oil  slick  has  to  be
taken  quickly.
Chemical  composition  of  oil
Even  though  information  on  the  viscosity  of  the  oil  is  an  important  parameter  for  the  assessment ­
  of  its  dispersibility,  this  information  alone  is  not  sufficient.  On  two  oil  types  with  similar
viscosity,  dispersants  may  show  very  different  efficacy  depending  on  the  chemical  composition ­
  of  the  oil  (Mukherjee  et  al.,  2011).  It  is  commonly  accepted  that  the  higher  the  ratio  of
polar  compounds  in  the  oil,  the  more  the  oil  is  dispersible,  as  long  as  the  viscosity  does  not
exceed  500  cSt.  By  contrast,  paraffin  type  oils  are  regarded  as  having  a  low  dispersion  potential. ­

Experimental  research
There  is  no  simple  rule  on  how  to  evaluate  the  efficacy  of  dispersants  with  regard  to  a  given
oil  type  and  given  environmental  conditions.  Therefore,  Cedre  has  developed  an  experimental ­
  test  chamber  in  order  to  conduct  efficacy  tests  in  controlled  conditions.  This  tool
(Flume  Tank,  Figure  4.4)  can  be  used  to  implement  controlled  weathering  of  an  oil  under
specific  conditions  (Table  4.3)  and  to  subsequently  assess  its  dispersibility  over  time  in  order
to  derive  the  “window  of  opportunity”  (Figure  4.5).