טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentLisitsin Marina
SubjectNovel Technique for Characterizing Scale Suppression
Effectiveness of Anti-Scalants on RO
Membranes
DepartmentDepartment of Chemical Engineering
Supervisors Professor Emeritus Raphael Semiat
Professor Emeritus David Hasson
Full Thesis textFull thesis text - English Version


Abstract

The objective of this research was to apply a novel technique enabling simultaneous dual characterization of the effect of a deposit on the membrane operation; viz. the membrane flux decline due deposit formation on the membrane and the increased frictional pressure drop generated in the flow passages by the scale deposit. The dual system could serve two objectives; from the fundamental aspect, the dual measurement of deposit effects provides an additional useful tool for the investigation of fouling and scaling mechanisms. On the practical side, the availability of a system enabling simultaneous measurement of flux decline and frictional pressure drop provides a technique for comparing the effectiveness of various anti-scalants used to control scaling and fouling processes.

Ø  An annular flow membrane module was constructed consisting of a tubular membrane held at the center by a stainless steel rod. A supersaturated calcium sulfate solution dosed with an anti-scalant flowed in the 1.25 mm annular gap. The membrane scaling was characterized by monitoring the flux decline and frictional pressure drop increase due to deposit formation. 

Ø  Flux decline in a scaling system is initially governed by surface coverage of the membrane. A surface coverage model was developed characterizing the scaling system by a single experimentally determined parameter   where km is mass transfer coefficient of calcium sulfate crystallization and N the density of membrane nucleation sites.

Ø  Experimental data measured over a range of supersaturation levels, anti-scalants concentrations and Re numbers all showed excellent fit to the surface coverage model.

Ø  Phenomena related to the initial formation of the scale layer were analyzed. Frictional data measurement on the basis of the fanning factor- Colebrook correlation. This analysis indicated that the frictional pressure drop was governed by the roughness height of forming crystallites.

Ø   The number of nucleation sites was found to increase with the increase in the supersaturation level of the scaling solution and with the concentration of the anti-scalant but to be rather insensitive to the type of anti-scalant.

Ø  Induction time data extracted from both permeate flux and frictional pressure drop measurements were used to compare the inhibition effectiveness of some friendly "green" anti-scalants with conventional anti-scalants. The "green" AS tested were found to provide inhibitory effectiveness comparable to that of conventional AS.

Ø The results of this investigation were accepted for presentation at the international desalination association (IDA) world congress 2015, San Diego, California, USA | August 30-September 4, 2015 (Appendix B).