THE INFLUENCE OF MECHANICAL EFFECTS ON DEGRADATION OF DRAG REDUCING AGENTS

        Mechanical  shear  degradation  hampers  the practical usage of polymers for turbulent drag reduction application. Mechanical degradation refers  to the chemical process  in which the  activation energy of polymer chain scission  is exceeded  by  mechanical  action  on  the  polymer  chain,  and   bond  rupture occurs.The mechanical degradation of high molecular weight Polyisobutylene  polymers  (Oppanol B)  was    studied    by  exposing  there dilute  solutions  to   high    mechanical    stirring  (1200, 1500 and 1800 rpm).Three Oppanol B, 150, 200 and 250   types of  molecular weight, 2.5,   4.1 and  5.9million  g/mole   were considered in present work. The shear viscosity of   Oppanol B stock solutions of concentrations,  0.5, 1.0 and 1.4w/v%was measured as function of exposure time.  It  has   been found  that the  extent of  the degradation is a function of  the molecular weight, stirring speed and   concentration. Therefore,   polymer chains  having  different  molecular  weights  show  different  time  dependent resistance by exposing there solutions  to mechanical stirring. It was observed, that the degradation efficiency of higher molecular weight  is larger than  that  of  lower  molecular  weight  in   a whole   polymer concentrations,and  stirring  speeds.  Thus,  the   highest   molecular  weight  Oppanol B  was more   susceptible  to   stirring   degradation  accompanying more   molecular w e i g h t  l o w e r i n g, The   drag–reduction   efficiency   which   has   been studied  for  dilute solutions  of  Polyisobutylene  (Oppanol B types)  with   the  three   molecular weight    mentioned    above, in a laboratory    circulation   turbulence   flow loop. The   time   dependence    drag−reduction    data    was  compared   with the  observation  of  shear degradation by  stirring  behavior. The  decrease  of drag  reduction  with  time  is  in   evident   to   the    mechanical   degradation observed by  shear  stirring  and  viscosity  decline  of polymeric additives. Xanthan Gum (XG)  have  been tested for its drag-reduction prformance,shear  stability   and   degradability.  0.5  and  1.0w/v%   solutions   were  also exposed  to  shear stirring  at  different  speeds (1200, 1500 and 1800rpm) and time(4hr). It  has  been   noticed   by   measuring   the  viscosity  changes, that XG  solutions    show    more    shear    stability   than   Oppanol B   polymers.The viscosity  lowering  was  noticeably  low  at exposure time up to 4hr. Xanthan gum  agent  was  tested  for  its  drag-reduction  effectiveness  at concentrations up 200ppm and  different  flow rates  in  turbulent  water  flow circulation   system. XG as a rigid polysaccharide  shows  relatively  poor drag-reducing agent. It requires much higher concentrations, 200ppm to cause an  expected  drag   reduction  about 19%,  compared   to  around   50ppm  for Oppanol B 250 additive operating at similar flowing conditions. The  time  dependence drag-reduction  effectiveness experiments indicate that  XG  additive  exhibit  more shear  stability than of  Oppanol B as flexible polymers in  turbulent  pipe flows. This observation is also in evidence to that noticed by shear stirring degradation results.