A Comparative Study of Phosphonate and Phosphorus-Free Antiscalant Efficiency by Static and Dynamic Methods. Do We Have Reliable Tools For An Adequate Reagent Selection?-Juniper Publishers
Juniper Publishers
Abstract
A relative ability of four industrial samples of
phosphorus-free polymers (polyaspartate, PASP; polyepoxysuccinate, PESA;
polyacrylic acid sodium salt, PAAS; copolymer of maleic and acrylic
acid, MA-AA) and of two phosphonates (aminotris(methylenephosphonic
acid), ATMP; 1-hydroxyethane-1,1-bis(phosphonic acid), HEDP) to inhibit
calcium carbonate precipitation at a dosage 10 mg•dm-3 is tested in
static experiments following the NACE Standard TM0374-2007 and in a
dynamic modeŠ¾fevaporation plant for Caspian Sea water imitate. The
reagent efficiency ranking following NACE Standard gives evidently a
preference to phosphonates over polymers: ATMP~HEDP>PESA (400-1,500
Da) ~PASP (1,000–5,000 Da)>PAAS (3,000–5,000 Da) ~MA-AA. At the same
time the kinetic tests exhibit the better efficiency of PESA and MA-AA:
PESA>MA-AA>PAAS~HEDP>ATMP~PASP. Therefore, a lot of work is
still needed to elaborate a system of laboratory tests for antiscalants
in order to provide reliable assessment and selection on their way from
laboratory to industry.
Keywords: Scale inhibition; Calcium carbonate; Polymers; Phosphonates; NACEIntroduction
Scale formation in the oil and gas industry,
evaporation plants, reverse osmosis desalination processes, steam
generators, boilers, cooling water towers and pipes is a serious
problem, causing significant plugging of wells, pipe-lines, membranes,
and increasing the production expenses [1,2]. A widely used technique
for controlling scale deposition is an application of chemical
inhibitors [1-4]. Commonly used commercial antiscalants are represented
by organophosphonates and numerous modifications of polyacrylates (PA).
Among these the organophosphonates are dominating recently at the World
market [5]. At the same time phosphorus-based inhibitors are hardly
biodegradable and persist for many years after their disposal, which
leads to eutrofication problems. Phosphorus discharges are therefore
regulated in many countries worldwide, and permissible limits are
constantly decreasing.
Increasing environmental concerns and discharge
limitations have forced the scale-inhibitor chemistry to move toward
“green antiscalants” that are readily biodegradable and have minimal
environmental impact. Intensive efforts are applied recently to develop
the “green” alternatives to organophosphonates and nonbiodegradable
polyacrylates [1-4]. Among these novel inhibitors, such chemicals as
polymaleates (MA), polyaspartates (PASP), polyepoxysuccinates (PESA), as
well as their various derivatives, including co-polymers with PA are
the most promising. It is important to note, that the new antiscalants
should have acceptable levels of performance at a cost-effective dose
rate. This requirement raises a problem of reliable tests, which permit a
correct “old red” and “novel green” inhibitors efficiency comparison
[6]. Indeed, most of the data published on CaCO3 (CaSO4) deposition are
studied under hardly comparable conditions, e.g., different CaCO3
supersaturationindex, brine composition, temperature, pH, measurement
technique, etc. This leads to the quite opposite opinions on the
relative Antiscalant’s efficacy, reported by different research groups
for one and the same set of reagents proposed for one and the same scale
(see [1,7,8] and references there).
Specific Technology Group (STG) 31 on Oil and Gas
Production-Corrosion and Scale Inhibition has elaborated
a procedure of static laboratory antiscalants screening-
NACE Standard TM0374-2007 [9]. These test methods are
recommended only for ranking the performance of different
scale inhibitors under laboratory conditions set by these
methods. They are not intended to provide actual field treating
rates. Surely, the scale inhibitor concentration required for a
field application is likely to be different from that determined
under these laboratory conditions. However, it is assumed,
that for a particular set of reagents the ranking would be the
same, and an inhibitor evaluation prior to final scale inhibitor
selection is valid for the field use as well. Although particularly
the NACE Standard is not very common, a lot of researchers use
recently very similar approaches [1,7,8].
Present paper is therefore focused on the assessment of
relative antiscalants efficacy against CaCO3 scale formation
predicted following NACE protocol, and their ranking found by
kinetic experiments run under conditions close to those used
at evaporation plants.
Materials and Methods
Polymer based industrialantiscalants, polyasparticacid
sodium salt (PASP, 1000–5000 Da), copolymer of maleic
and acrylic acid (MA-AA), polyepoxysuccinic acid (PESA,
400–1500 Da), and sodium salt of polyacrylic acid (PAAS,
3000–5000 Da), have been kindly supplied by Shandong Taihe
Water Treatment Technologies Co., Ltd., while industrial
solid posphonates aminotris (methylenephosphonic acid),
ATMP and 1-hydroxyethane-1,1-bis (phosphonic acid), HEDP
have been supplied by a manufacturer OAO “Khimprom,”
Novocheboksarsk, Russia.
Kinetic tests have been run operating with a model
evaporation plant bench-scale facility. A freshly prepared
imitate of the Caspian Sea water ([Ca2+] 0.35 g•dm−3, 0.0088
mol•dm−3; [HCO3-] 0.22g•dm−3, 0.0037 mol•dm−3; NaCl 8.57 g•dm−3, 0.15 mol•dm−3; pH 8.5-9.8) was kept boiling at 85 °Š”
under reduced pressure (59kPa) for a period of 240 minutes.
A reflux condenser provided the total liquid phase volume
constancy. Periodically the boiling brine was sampled and
analyzed for [Ca2+] content by EDTA titration procedure.
Then the induction precipitation time (Ļind, min) and a half
deposition time (Ļ1/2) have been calculated. Here Ļ1/2 is denoted
as a moment, when (Š” – Š”f )/(Š”0 – Š” f ) = 0,5; where Š”- current
calcium concentration; Š”f – final calcium concentration found
from a blank experiment, Š”0– initial calcium concentration. All
experiments were run in duplicates. The difference between
duplicate run results was less than 10%.
Following the NACE Standard TM0374-2007 [9] two
synthetic brines was prepared with distilled water: calciumcontaining
brine (12.15 g•dm−3 CaCl2•2H2O; 3 .68 g •dm−3
MgCl2•6H2O; 33.0 g•dm−3 NaCl) and bicarbonate-containing
brine (7.36 g•dm−3 NaHCO3; 33.0 g•dm−3NaCl) saturated by
CO2. Being mixed at 1:1 volume ratio, these brines give a
supersaturated calcium carbonate solution: 6.07 g•dm−3(6,070
ppm) CaCl2•2H2O, 1.84 g•dm−3(1,840 ppm) MgCl2•6H2O, 3.68
g•dm−3 (3,680 ppm) NaHCO3and 33.0 g•dm−3(33,000 ppm) NaCl.
The ionic strength of this solution provided mostly by NaCl by
the end of the precipitation process wasaround 0.71mol•dm-3.
Supersaturated solution of calcium carbonate with a calculated
amount of inhibitor (10 g•dm-3) was then kept for 24 h at 71°C,
cooled and analyzed for residual calcium content by EDTA
titration. The pH of the solutions at 25°C was about 7. All
experiments were run in duplicates. The difference between
duplicate run results was less than 5%.The performance of
the tested compounds as calcium carbonate antiscalants was
calculated as inhibition percentage (I, %):
I,% = 100 ×([Ca]exp-[Ca]final)/([Ca]init-[Ca]final). Where: [Ca]
exp: concentration of calcium in the filtrate in the presence of
an inhibitor at 24 hours; [Ca]final: concentration of calcium in
the filtrate in the absence of an inhibitor at 24 hours; [Ca]init :
concentration of calcium at the beginning of the experiment.
All experimental results are presented in a Table.
Results and Discussion
The reagent efficiency ranking following NACE Standard
gives evidently a preference to phosphonates over polymers:
ATMP ~ HEDP > PESA (400-1,500 Da) ~ PASP (1,000-5,000 Da)
>PAAS (3,000-5,000 Da) ~MA-AA. Thus, among the studied set
of reagents ATMP and HEDP should be expected to become a
matter of choice for carbonate scaling. However, an attempt
to implement these reagents to evaporation plants gives a
sufficiently different ranking. According to Ļind: PESA~MAAA>
PASP>PAAS>HEDP~ATMP
The Ļ1/2 datareveals in turn another sequence: PESA>MAAA>
PAAS~HEDP>ATMP~PASP.
In any case PESA and MA-AA look more preferable than
phosphonates for evaporation plants. Actually Ļ1/2 seems to be
a more adequate indicator than Ļind. The latter characterizes
only the initial nucleation step, while the former is responsible
for both: initial nucleation and further crystal growth kinetics.
Anyhow the data given above demonstrates clearly that a lot of
work is still needed to elaborate a system of laboratory tests
for antiscalants in order to provide reliable assessment and
selection on their way from laboratory to industry.
Conclusion
A comparison of static and dynamic laboratory testaments
of scale inhibitors indicates for one and the same set of
reagents rather conflicting results. Static test gives preference
to the phosphonates ATMP and HEDP, while the dynamic one -
to polymers PESA and MA-AA. Thus a lot of work is still needed
to elaborate some conventional methods for the reliable,
particular case-focused reagent efficiency prediction.
Acknowledgement
The authors would like to thank the Russian Foundation
for Basic Research (Project No. 17-08-00061) and partly the
Ministry of Education and Science of the Russian Federation.
Conflict of Interest
For a present study no any economic interest or any conflict
of interest exists.
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