(ethylenediaminetetraacetic acid) for removalof inorganic deposits. In this case,
conductivitycould be usedto monitor and controlcaustic levelbut titration
wouldbe usedto monitorthe edta levelto ensurethat there is alwaysfree
sequestrantat the end of the cleaningprocess.
35.4 Laboratory/pilot-scalestudies
In additionto commercialscaleprocesses, monitoringhas a key rolein the
development of small-scaletechniquesthatallowthe fouling,cleaning and
disinfectioncharacteristics of newproductsor processes to be assessed.Such
monitoringsystemscan be usedto identify the key parameters that influence
foulingand cleaning and potentially offer the opportunity to optimise the
processbeforeimplementation at commercialscale.
Otherindustries havedevelopedportableunitsthat can be transported to the
processto be investigated,for example to measure the fouling characteristicsof
coolingwaters(Glenet al. 1999,Knudsenet al. 1999).Theycan thenbe
integratedwiththe processby running themin parallel as a side streamof the
mainflowand therefore ensuring that representative fluidsare beingused.Such
unitsare primarily concernedwithfouling ratherthancleaning. In the food
industry the installationof test unitsin parallelis not widelyused.
A numberof laboratoryor pilot-plant systems for the monitoringof fouling
and cleaninghavebeenreported.
35.4.1 Radial flowcell
Theimportance of fluid shear in foulingand cleaning has beengenerally
accepted and a radialflowcell (RFC)was developed(Fowler and Mackay 1980)
to investigate the effectsof shearstress on the formation and removalof
biofilms. The cell consisted of a pair of parallel discs, one of whichhad a central
holefrom which liquid passedradiallyoutwardsthoroughthe narrowgap
formedby the plates. As the liquidflowed outwards,its velocity and hencethe
shearstress on the walldecreased, providing an elegantmethodof generatinga
rangeof shearstresses for a givenflowrate.In principle, the criticalshearstress
for biofilm growthcouldbe calculated fromthe radiusat whicha biofilm could
be detected. Belowthis critical radius,the shearstresswas sufficiently highto
preventthe attachmentof the biofilm.Practicalproblems wereencounteredwith
maldistributionof liquidas it movedto the outeredgeof the plate,leading to
unevenfouling.
A heatedversion of the radialflowcell was developed(Fryeret al.1985),
whichpermitted the effectof surfaceshearstresson the initialstagesof fouling
to be measuredunderheat transferconditions.Heating oil was circulated across
the top and bottom plateswith a plategap of 0.56mm.It was foundthat for
mechanical reasons,maintainingsucha narrow flowpassage overthe whole
diameterrequired the use of relatively thick plates.Thesethickplatesincurred
Monitoringof fouling, cleaning and disinfection in closedprocessplant 581