Formation and prevention of calcite
scale at Dåvamyran
Måns Kjellander
Degree Project in Engineering Chemistry, 30 ECTS
Report passed: September 2015
Supervisors:
Tomas Hedlund, Umeå University
Jarkko Kuismin, Umeå Energi
Åsa Benckert, Umeå Energi
Abstract
Calcium carbonate, in the form of calcite, has for a long time plagued a pump-and tank system at
the municipal power plant of Dåvamyran in the Swedish city of Umeå. The scaling causes problems
that leads to lower efficiency, economical losses and practical inconveniences and must be
mitigated. Cause and prevention of the calcite scaling were the primary goals of this thesis. Data
concerning the chemical content of the processes were taken from three representative locations
and analyzed using ICP-AES , ICP-MS and a custom TIC method. WinSGW, a chemical
equilibrium calculation program, was used to investigate and illustrate the equilibrium chemistry
present in the systems. Precipitation tests and dissolution kinetics were examined off-line using a
combination of actual process water and modeling with fabricated solutions. Results indicated that
the primary affected areas are below the precipitation limit for calcite but the existing coating of
calcite in tanks and pumps can be explained with the help of physical parameters, foremost the
effect of incomplete mixing and pressure gradients. The recommended course of action was to
induce a complete mix in affected areas, and to a lesser extent exchange limewater to sodium
hydroxide in a specific process step. Future explorations could include the practical
implementations of additives and surface treatment of affected areas.
Table of Content
1 Introduction ......................................................................................................................................... 1
2 General procedure ............................................................................................................................... 2
2.1 Setup of the mixing tanks ................................................................................................................................................ 3
2.2 Locations affected by calcite scaling ............................................................................................................................... 4
3 Theory ................................................................................................................................................. 6
3.1 Effect of temperature ....................................................................................................................................................... 6
3.2 Effect of pH ...................................................................................................................................................................... 7
3.3 Effect of Pressure ............................................................................................................................................................. 8
3.4 Presence of metal ions ..................................................................................................................................................... 8
3.5 Incomplete mixing ........................................................................................................................................................... 9
3.6 Removal of scale............................................................................................................................................................... 9
3.7 Scale prevention methods.............................................................................................................................................. 10
4 Materials and methods ....................................................................................................................... 11
4.1 Testing sites and sample extraction method ................................................................................................................. 11
4.2 Temperature and pH-measurement ............................................................................................................................. 13
4.3 Precipitation tests .......................................................................................................................................................... 13
4.4 Kinetics experiment ....................................................................................................................................................... 13
4.5 Methods used by ALcontrol........................................................................................................................................... 14
5 Results ................................................................................................................................................ 15
5.1 Equilibrium calculation in WinSGW ............................................................................................................................. 18
5.2 Precipitation tests .......................................................................................................................................................... 19
5.3 Kinetics Experiment ...................................................................................................................................................... 19
6 Discussion ......................................................................................................................................... 20
7 Conclusions ........................................................................................................................................22
8 References ..........................................................................................................................................23
Appendix .............................................................................................................................................. 24
1 Introduction
Umeå Energi (UE) is a corporate group owned by the municipality of Umeå to offer services in
energy solutions, electricity and communication in the fields of television and internet. The
company has 340 employees and has a turnover of 1.6 billion SEK in 2014. The municipal powerplant Dåva Kraftvärmeverk was initiated in 2000 and ten years later in 2010, another power-plant
by the name of Dåva 2 was built. UE is structured to contain five major business departments,
ranging from electricity grid to renewable energy production. As a company, Umeå Energi has
visions of carbon neutrality by 2018 and have many project dedicated to this development. 1 This
project falls under the department of Energilösningar, which roughly translates to "Energy
solutions" which is located at Dåva district heating plant outside of Umeå.
Dåvamyran is a municipal plant that supply the city of Umeå with district heating and electricity. It
consists of two plants, Dåva 1 and Dåva 2. The former operates by incinerating household garbage
and withdrawing the latent energy content to produce electricity and thermal power. Dåva 2
extracts energy from various bio-fuels using fluid bed combustion and yield the same products as
Dåva 1. The flue gas from these two plants are lead together in a last cleaning step before the
process water is channeled in a long pipe-line to the municipal water treatment plant. A problem
that arose when Dåva 2 was implemented in 2009 was that calcite scale, a dense but brittle solid,
begun to slowly form in the shared water-cleaning system, primarily the pumps and pipes
containing the mixed process water. The calcite scaling has lead to loss in efficiency in the pump
system, economical losses and tedious work to replace and repair affected parts.
The aim of this thesis is to investigate the chemistry behind precipitation of calcite in the last
process step at Dåvamyran, a municipal plant that supplies Umeå with district heating and
electricity. Important factors in the chemical process that leads up to the formation of scales are
explored using data acquisition from three primary points of interest in the industrial plant and
analysis using equilibrium chemistry. This thesis also includes inspection of possible solutions to
the prevalent problems.
1
2 General procedure
Dåva 1.
Dåva 1 is the older of the two power plants at Dåvamyran and it operates by incineration of waste.
Due to the nature of the fuel, the mix is ultimately inhomogeneous (although steps are taken by the
process operator to maintain a fitting fuel mix for the prevalent conditions). The flue gas is cleaned
using various methods but most important for this thesis is a pH-regulation using limewater, which
is very rich in calcium. Limestone is also added earlier in the process as another source for calcium
ions. Dåva 1 is also called Panna 8 or P8, which it will be referred to.
Dåva 2.
The plant Dåva 2, which will be referred to as P9, is a highly stable and efficient power plant. It
extract energy from biomass using a fluid-bed combustion process that consumes a known fuelmixture. The composition of this mixture can be changed to meet the demands of the district
heating system, as example, a small amount of oil can be added into the fuel mix to increase the
efficiency of the combustion in response to a particularly cold winters day. P9 contains a few
cleaning steps to rid the flue gas from harmful substances. Steps that are important in the scope of
this thesis is a pH-increasing step where sodium hydroxide is used. Dåva 2 is mainly in use during
the winter season and turned off during summer, when P8 stands for all energy production.
Mix
The flow from P8 and P9 are mixed in the two tanks adjacent and linked to the affected pumpsystem. These tanks have an outer diameter of around 1,6m and during nominal conditions have a
water pillar of 2,2m. The condensate is drawn from the tanks by a corresponding pump with an
optimal flow of 55m3h-1. A more detailed picture of the pump setup and overall construction can be
found in the appendix. The flow proportions from the two plants are roughly 20% from P8 and 80%
from P9 during the winter season.
2
2.1 Setup of the mixing tanks
The condensate process water from P8 and P9 are mixed in a mutual tank before being carried
away by a pumping system towards Strömpilen pump station. It is at this location that the problem
of calcite scaling is prevalent. The two tanks shown in figure 7 keep the same volume and have
roughly the same composition in terms of pH, temperature and species concentration, according to
constructors.
Figure 1-a. and 1-b. The left mixing tank (a) and right tank (b) with condensate flow from P9 (1) and P8 (2).
The mixing tank and the pipes are of the same material: glass reinforced plastic (GRP). It is used
due to its high corrosion resistance, high thermal insulation.2
3
2.2 Locations affected by calcite scaling
The primary location where the precipitation of calcite is prevalent are the two pump impellers and
adjacent tubes tasked with moving the mixed process water away from Dåvamyran.
Figure 2. Effected pump wheel and tubing
The two pumps have a flow of 55m3/h under optimal conditions and are designed to withstand up
to 12 bar of pressure. Formation of calcite have an adverse effect on these qualities due to the
smaller inner diameter and the obstruction of the pump impellers. The tubes have been opened
numerous times and clear signs of calcite scaling are observed.
There are no easily accessible records of how often the pipes have been investigated, but according
to a few operating technicians it varies from months to years since the construction of Dåva 2.
4
Figure 3. Tube affected by calcite scale
5
3 Theory
Calcite is the most stable morphological setting of calcium carbonate and is formed by the following
reaction:
𝐶𝑎𝐶𝑂3 (𝑠) ⇌ 𝐶𝑎2+ + 𝐶𝑂32−
Where the solubility product, Ksp is defined as:
{𝐶𝑎2+ } ∗ {𝐶𝑂32− }
𝐾𝑠𝑝 =
{𝐶𝑎𝐶𝑂3 (𝑠)}
(1)
(2)
From the solubility product constant the presence of precipitation can be determined using the
product of the two ions actual concentration.
𝑄 = [𝐶𝑎2+ ] ∗ [𝐶𝑂32− ]
(3)
If the product Q produces a smaller value than the solubility product constant, there should be no
precipitation occurring in the solution. In the other case where the product is larger than the
constant, precipitation should occur. It is worth noting that information regarding the rate of
precipitation cannot be determined using this equation. Information regarding the speed of
dissolution and formation can only be explained by kinetic tests and thermodynamic modeling.
The value for the solubility product varies with temperature and has been found empirically to vary
according to:3
− log(𝐾𝑠𝑝 ) = +171,9065 + 0,077993 ∗ 𝑇 −
2839,319
− 71,595 ∗ log(𝑇)
𝑇
(4)
Carbonate is primarily introduced into the system by the carbonic acid system, which includes the
solution of carbon dioxide in water and the formation of carbonic acid, bicarbonate and carbonate.
𝐶𝑂2 (𝑎𝑞) + 𝐻2 𝑂(𝑙) ⇌ 𝐻2 𝐶𝑂3 (𝑙) 𝑝𝐾𝑎 = 2,77
(5)
𝐻2 𝐶𝑂3 + 𝐻2 𝑂(𝑙) ⇌ 𝐻𝐶𝑂3− + 𝐻 + 𝑝𝐾𝑎 = 6,37
(6)
𝐻𝐶𝑂3− + 𝐻2 𝑂(𝑙) ⇌ 𝐶𝑂32− + 𝐻 +
(7)
𝑝𝐾𝑎 = 10,25
3.1 Effect of temperature
Unlike most equilibrium systems containing the possibility of precipitation, the extent of
precipitation in the calcium carbonate system decreases with decreasing temperature. I.e. the
solubility of calcite increases with lower temperature, rendering the Ks larger. This effect can be
explained by the solubility of carbon dioxide, that in turn effects the solubility of calcite. The
following equations illustrates the solubility of carbon dioxide in water.
𝐶𝑂2 (𝑔) ⇌ 𝐶𝑂2 (𝑎𝑞)
(8)
𝐶𝑂2 (𝑎𝑞) + 𝐻2 𝑂(𝑙) ⇌ 𝐻2 𝐶𝑂3 (𝑎𝑞)
(9)
It is known that the solubility of carbon dioxide decreases with increasing temperature and in the
scenario where the temperature is lowered the concentration of carbonic acid will increase. This
leads to a decrease in pH that, in turn, causes the extent of precipitation to be lowered.
6
3.2 Effect of pH
The effect of pH mainly contributes to the composition of the different carbonates in the carbonic
acid system. This system is well known and is illustrated below in the case of a homogeneous
system.
Figure 4. Illustrating the pH-dependence of the inorganic carbon forms
Increasing pH leads to an increase in carbonate concentration, which inflates the probability of
calcium carbonate precipitation. As such, it is desirable to lower the pH to minimize this
concentration in order to avoid scaling. The fractional concentration of the three kinds of inorganic
carbon forms is detailed as follows:
Figure 5. Relative distribution of inorganic carbon forms at varying pH
In the case of Dåvamyran, where the pH ranges from around 6.5 to 9, the primary forms in the
system is bicarbonate, and to a lesser extend carbonic acid.
7
The pH also depends upon the presence of carbon dioxide, which forms a heterogeneous pHdiagram with the following appearance:
Figure 6. pH dependence of carbonic species with the influence of carbon dioxide
In the homogenous case the same trend can be observed, namely that the bicarbonate form
dominates in the pH-range of 6-9.
3.3 Effect of Pressure
Scale formation can be induced or at least favored by changes in pressure.4 In itself, an increase in
pressure has very little effect on the solubility of calcite. It has been shown in literature that a shift
in pressure will increase the rate of precipitation and circumvent otherwise undersaturated
solution. The rate of calcite precipitation depends on this shift in physical conditions, to which
extent is highly depended on unique parameters present in the process. 5
In the case of Dåvamyran, most of the calcite scaling occurs near and on the pump impellers. In this
area the chemical parameters such as Total Inorganic Carbon, TIC, [Ca2+] etc. does not qualify it as
being supersaturated, which further emphasizes the importance of the physical aspects. The effect
of pressure drop can be observed on-site where precipitation occurs at the ridges of fused tubes and
is less prevalent in straight pipes with homogeneous surfaces.
During the event of a pressure change, carbon dioxide is forced out of water solution, which causes
pH to rise according to reaction (5)-(7). As previously explained, calcite solubility drastically
increases with increasing pH, which creates favorable conditions for scaling.
3.4 Presence of metal ions
Analysis of the process water indicates that there are relatively large concentrations of Ba2+ and
Sr2+ present in P8, and to a lesser extent the mix due to dilution from P9. These two species can
form the very insoluble carbonate compounds, BaCO3 and SrCO3. Strontium carbonate has similar
temperature-dependence as calcite, i.e. lower solubility with increasing temperature. Barium
carbonate works the other way around, as is most common for solids. 6
8
3.5 Incomplete mixing
Equilibrium chemistry assumes that solutions are completely mixed, which is not always the case in
industrial settings. Solutions that otherwise would not yield a product can do so when mixing is
incomplete due to local concentration gradients. Say a compound is introduced to another, there is
a clear gradient where the concentration is higher than the bulk, which is precisely where the
solution is introduced into the other. The equilibrium criteria for precipitation can be met and give
yield to a solid in the higher concentration area. In many cases this effect is negligible but when the
dissolution of the precipitation is slow, the effect becomes relevant. Solids precipitated in such a
way can be accumulated in the mixing tank and other suitable locations, for example ridges and
jagged surfaces . Particles formed in this way can act as nucleation spots where more solids can
grow and create layers, which makes the chances of dissolution lower and scaling a fact. 7
3.6 Removal of scale
There are many techniques for removal of scale, which can roughly be divided into chemical
techniques and physical methods. Many of the techniques present in literature are optimized and
developed for usage in the oil extraction industry, but are applicable in the domain of industrial
precipitation.
When attempting to remove scale with chemicals it is important to use the most economically
feasible technique. These methods are advantageous when the scale is inaccessible and when the
damage to the equipment is to be minimized. Amongst the primary chemical techniques is the use
of a chelating agent, which functions by binding to the metal ions in the scale, which causes it to
break down. In the case of calcite, which readily dissolves at very low pH, hydrochloric acid can be
deployed to dissolve the scale by the following reaction. It goes without saying that the equipment
affected by scale need to be resistant to corrosion when employing this method.
2𝐻𝐶𝑙(𝑎𝑞) + 𝐶𝑎𝐶𝑂3 (𝑠) → 𝐶𝑎𝐶𝑙2 (𝑎𝑞) + 𝐶𝑂2 + 𝐻2 𝑂
(10)
Calcium chloride is highly soluble in water which imposes the risk of calcite being reformed if the
treatment is not properly deployed. 8 Other chelating agents include EDTA, citric acid, acetic acid
and other weak acids in the case of calcite. There also exist custom made agents designed to remove
specific scales in oil-extraction settings. How exactly the chelating agent is introduced into the
process is highly dependent on the construct itself. In the case of Dåvamyran, the pipes near the
water purifying house must be cleaned and channeling the waste further down the line must be
avoided. Due to this, some form of bypass must be used to avoid causing environmental damage on
the nearby area.
Another way to remove scale is to use mechanical methods, which are highly useful in cases where
the affected areas are accessible by physical means. An important aspect to keep in mind when
using mechanical methods is the potential damage on the affected areas, methods such as scale
removal with explosives poses a risk to damage and can possibly destroy unwanted material if used
incorrectly. Other methods include the usage of abrasive water jetting that can contains solid
particles, making it grind down scale and regretfully the effected tubing if not properly deployed.
These sorts of methods are naturally more useful when used in industries that have resistant
equipment. In general, there are numerous customized systems to mechanically remove scale but
few universal methods.
9
3.7 Scale prevention methods
In most cases it is more economically sound to prevent the formation of scale instead of removing it
when the effects become detrimental to the process. There are a multitude of methods for the
prevention of scale, ranging from physical changes to the process to chemical inhibition by
additives.
When attempting to prevent formation of scale it is important to consider the design of the affected
area. Parameters such as unnecessary amount of porous material or a large amount of
pressure/temperature gradients can cause the kinetics of the scale formation to occur more rapidly.
Physical effects depend highly on which scale is prevalent in the process and should be adjusted
accordingly.
A simpler way to mitigate the effect of scale is to implement a dilution system in which water is
introduced in the process flow to lower the saturation of scale-forming material. A dilution setup
implementation can range in difficulty to construct depending on the industry and process in
question, as well is the amount of diluent needed to achieve the desirable goals.4
Chemical additives act by preventing the growth of the scale in various ways, either by selectively
binding to specific scale-forming compounds or disrupting the scale growth. Metal ions such as
Mg2+ has shown to have a disruptive effect on calcite growth by binding to the crystal surface, which
renders the crystal growth rate significantly lower.9 Similar investigations has been made on other
metal ions that have the ability for the same sort of inhibition, such as Cu2+.10 Chemical inhibition
can also be carried out in a myriad of other ways, in the case of calcite it has been shown that
natural acids have a significant effect on calcite growth restriction. A prevalent example is malic
acid that has shown to have good inhibitory effects on calcite growth by advocating the formation of
vaterite and crystalline structures instead. The picture below shows an example of this observed
effect11
Figure 7. Calcite inhibition by an addition of 5ppm malic acid (right)
10
4 Materials and methods
In order to determine the conditions at the different system sites, samples were taken and sent
away in two batches to ALcontrol Umeå, where inductively coupled plasma atomic emission
spectroscopy (ICP-AES), -mass spectrometry (ICP-MS) and TIC content determination using a
custom method not included in the standard procedures was carried out. The samples were stored
in a dark cupboard at room temperature until delivered to ALcontrol.
4.1 Testing sites and sample extraction method
The samples used in the experiments and analyses were taken from three separate sites
corresponding to P8, P9 and the mix. The mix was drawn from the last point available near the
affected pumps using a simple bypass already in place. Samples from P8 and P9 were taken as late
in the respective processes as possible before entering the mix. All samples were collected in plastic
containers and firmly sealed to prevent the TIC-concentration from deviating too far from the real
value and stored in room temperature.
Figure 8. Testing site for process water P8
11
Figure 9. Testing site for process water P9, the circle indicate the bypass valve used
Figure 10. Testing site for the mix process water, the circles indicate the bypasses used depending on which pump is active
12
4.2 Temperature and pH-measurement
pH and temperature measured at the sampling sites was done with a Multi 3420 by WTW. During
the testing periods it was calibrated roughly twice a month using pH 7 and 10 Hamilton DC buffers.
4.3 Precipitation tests
Process water drawn from testing site P8 and P9 was mixed in different proportions to find if
calcite precipitation could be replicated. In order to find out which factors are most important a full
factorial design with the factors mixing proportion and temperature were established with three
center-points. The center points corresponded to the average measured parameters. The mixing
proportions were based on the average flow rates at P8 and P9 and shifted to accommodate a full
factorial design, as was the temperature factor.
Exp No
1
2
P8
6,5
8,5
P9
31,5
31,5
Temperature
29,5
29,5
Total Volume
38
40
P8:P9
4,8
3,7
3
4
5
6
7
8
6,5
8,5
6,5
8,5
6,5
8,5
33,5
33,5
31,5
31,5
33,5
33,5
29,5
29,5
35,5
35,5
35,5
35,5
40
42
38
40
40
42
5,2
3,9
4,8
3,7
5,2
3,9
9
10
11
7,5
7,5
7,5
32,5
32,5
32,5
32,5
32,5
32,5
40
40
40
4,3
4,3
4,3
Table 1. Calcite precipitation full factorial design
The stock solutions of process water were measured with a turbidimeter to allow eventual
precipitation determination to be relative. Samples were prepared according to the full factorial
design, transferred into 50ml falcon tubes and immediately placed in a water-bath with the correct
temperature. The samples were left at room temperature for about 24 hours and their
Nephelometric Turbidity Unit, NTU, were measured sporadically and at the end of their run.
4.4 Kinetics experiment
The kinetics for the dissolution of calcium carbonate in various solutions was investigated. These
experiments were carried out by introducing a small amount of wet calcium carbonate, containing
no more than a few droplets of water, into a solute and measuring the time required for complete
dissolution. Due to the calcium carbonate being wet, the amount is considered slightly less than
indicated by the weighing. Keeping the calcium carbonate wet is necessary to avoid crystal
formation that can misrepresent the kinetics for the reaction. Calcium carbonate (5.35 mg) was
mixed into 400ml distilled water and 4,6mg into 400ml process water taken from the mixing site at
Dåvamyran. The two containers were left to stand in room temperature and kept under magnetic
stirring. At regular intervals, visual inspections were made to determine if the solids had undergone
dissolution.
13
4.5 Methods used by ALcontrol
Most of the analyses used in the scope of this thesis are carried out by ALcontrol, Umeå. Their
primary method of determining the concentration of ions, metals etc. is by ICP-AES and ICP-MS.
The ICP-AES functions by initiating a plasma field fueled by argon gas and induced by a radio
frequency generator. The sample is transformed into an aerosol using a nebulizer and particles of
the correct size are passed through the plasma field. As the temperature is around 7000 Kelvin the
atoms becomes atomized and ionized that can be detected and quantified. Atomic emission created
by this transition is captured using a monochromator that can identify the emission to belong to
specific atoms.12
For screening of a larger array of atoms and the ability to detect trace metal concentrations, ICPMS was used. As the case of ICP-AES, the sample is ionized using a plasma field. The ionized atoms
are passed through a vacuum and separated according to their atomic weight using methods such
as quadrupole magnets. The ionized atoms then strike into the detector surface, which creates an
electronic signal that can be measured using an electron-multiplier. A common problem with ICPMS is that atoms can be confounded with combinations of atoms with same molecular weight (for
example in the determination of element75 can get interference from 40Ar + 35Cl).
14
5 Results
Data of temperature and pH was collected frequently in the beginning and later on at every
instance of sample extraction using a handheld pH-electrode with inbuilt temperature
measurement.
Temperature at testing sites
45
Temperature [°C]
40
35
P8
30
P9
25
Mix
20
15
1
2
3
4
5
6 7 8 9 10 11 12 13 14
Sample number
Figure 11. Temperature data from the three testing sites
pH
pH at testing sites
8
7.8
7.6
7.4
7.2
7
6.8
6.6
6.4
6.2
6
P8
P9
Mix
1
2
3
4
5
6 7 8 9 10 11 12 13 14
Sample number
Figure 12. pH data from the three testing sites
Ca2+, TIC, Sr and Ba were the primary parameters investigated by ALcontrol. The testing periods
were five days each, one in February and another in March
15
Calcium
9000
8000
7000
mg/L
6000
5000
P8
4000
P9
3000
Mix
2000
1000
0
4
5
6
7
8
9
Sample number
10
11
12
13
Figure 13. Calcium content in P8, P9 and Mix process waters. Data from ALcontrol
Total Dissolved Inorganic Carbon (TIC)
35
30
25
mg/l
20
P8
P9
15
Mix
10
5
0
4
5
6
7
8
9
Sample number
10
11
12
13
Figure 14. TIC content in P8, P9 and Mix process waters. Data from ALcontrol
The sudden dip in values at calcium sample 4 and 5, as well as pH/temperature sample7 and 8, are
due to P8 being turned off at these dates.
16
Strontium
5000
4500
4000
3500
µg/L
3000
2500
P8
2000
Mix
1500
1000
500
0
5
6
7
8
9
10
Sample number
11
12
13
Figure 15. Strontium content in P8 and Mix. Data from ALcontrol
Barium
400
350
300
µg/L
250
200
P8
150
Mix
100
50
0
5
6
7
8
9
10
Sample number
11
Figure 16. Barium content in P8 and Mix. Data from ALcontrol
17
12
13
5.1 Equilibrium calculation in WinSGW
Data collected from the three testing sites were used in the chemical equilibrium calculations
program WinSGW, with the purpose to illustrate the precipitation of calcite. Values used in the
calculations are illustrated in table 2. The parameters are varied to see how the formation of calcite
responds.
P8
P9
MIX
Ca2+ [mM]
152
0,0437
19,7
TIC [mM]
0,088
0,315
0,235
pH
7,2
6,9
6,8
T [°C]
38,75
31,87
31,66
Table 2. Average values from testing sites used in equilibrium calculations
To illustrate the formation of calcite, the activity of the solid is used as measurement of possible
precipitation. Per definition, the activity of a present solid is 1, which means in this setting that
precipitation is occurring. Activity values lower than 1 means that the solid presence is
insufficiently large to have a major chemical role in the equilibrium, which translates to negligible
calcite precipitation.
The results from investigating the different parameters explained in the theory section in the
equilibrium chemistry calculations are summarized into two predominance diagrams.
Below is a predominance diagram that shows which is the most prevalent species under certain
conditions.
Figure 17. Predominance diagram for mixing site. The dot represents the observed value
18
The lines represent the highest observed values at each location. In the diagram, the red circle
indicates that the precipitation limit is reached in the theoretical event where the flows from P8 and
P9 are mixed, this would correspond to the state in the two tanks (figure 1-a and 1-b).
Figure 18. Predominance diagram
Figure 17-19 indicates that no precipitation occurs according to the collected data. Figure 20 and 21
illustrates that the overall effect of varying the parameters points to the collected data being very
near the limit of precipitation.
5.2 Precipitation tests
The stock solutions of P8 and P9 had NTU-values of 0.4 and 0, respectively. No sample gave any
significant amount of precipitation.
Exp No
P8
P9
1
2
3
4
5
6
7
8
9
10
11
NTU-value
0,4
0
0,3
0,2
0,2
0,2
0,3
0,2
0,3
0,2
0,2
0,2
0,2
Table 3. Precipitation test NTU-values
5.3 Kinetics Experiment
The kinetics for the two solutions consisting of calcium carbonate in distilled water and mixing
water from the date 3/19 indicated that the kinetics of dissolution is in the order of days. A portion
of the solids were dissolved into the respective liquids but a significant amount was left in the
distilled water sample even after 48 hours, during which another 100ml water had been added. The
sample containing water from the mixing site was observed over 24 hours and indicated the same
trend .
19
6 Discussion
Considering the vast quantities of limewater introduced in P8 that have a direct effect on the
calcium ion concentration and in the end the formation of calcite, it is easy to eliminate the
problem, or at least mitigate it, by substituting the pH-adjustor with another. A suitable candidate
is sodium hydroxide, which is a bit more expensive but would drastically lower the calcium
concentration. Furthermore, it has been observed that limewater causes other problems in
Dåvamyran, such as uncontrollable flooding into various cleaning steps and equipment damage
that can be avoided if replaced with sodium hydroxide.13 Exchanging limewater with another pHadjustor would most likely not only spare potential process disturbances but also solve the problem
of precipitation due to lower calcium concentrations.
According to the chemical equilibrium calculations the mixing water does not reach the
precipitation limit for calcite, nor does it do so at the other testing locations. This is further
illustrated by the precipitation tests that show that no significant amount of solids are formed with
a simple mixing of the process waters. These observations combined with the results from the
equilibrium chemistry calculations, lead to the idea that the scaling was process unique and that
physical parameters were of great importance. Since equilibrium chemistry assumes complete
mixing, it is sometimes insufficient to fully explain what happens in processes like these. For these
reasons, process unique and specific parameters were investigated.
The setup in the water house indicates that the mixture between the process waters from P8 and P9
is incomplete and may contain concentration gradients that can form calcite. The kinetics of
dissolution were investigated and found to be within the order of days, which results in the
possibility of accumulation occurring within the two tanks. Accumulation is a huge problem since it
can lead to layer deposition on the walls, which makes the dissolution of calcite highly improbable
and the dissolution kinetics significantly slower. According to operating technicians the tanks were
investigated last year and was found to contain 4-5mm calcite along the inner walls.14
As such, there is a high probability that the incomplete mixing leads to precipitation of calcite that
does not dissolute immediately and accumulates in the tanks. The presence of calcite may serves as
seed crystals in further process steps, primarily the pump system.
Initial tests indicated that a significant amount of strontium is present in the P8 process water. This
was unknown to UE and it is unclear where it comes from but speculations indicate that it may
come from hospital waste. The presence of barium is previously known and regulated. Both
strontium and barium have the ability to create insoluble carbonates, not unlike that of calcium
carbonate. The impact of the potential strontium carbonate and barium carbonate formation was
speculated upon but was ultimately neglected due to physical parameters being of such importance,
more so than exactly which composition the scale have. Future investigations can be employed to
trace these two elements in the chemical process, especially strontium since its high concentration
was unknown prior to this project.
The choice of inhibition chemical is highly dependent on the scale in question but another very
important aspect is the environmental effects of the chemical. To this end, natural polycarboxylic
acids such as malic acid or fulvic acid are good candidates due to their almost non-existent
detrimental effect on the ecosystem. As it stands, no experiments regarding chemical inhibitors
were explored due to the suspected physical parameter that primarily afflicted the system. Future
investigations may explore where such an additive should be introduced in the process and in
which volumes, should it be found to be an attractive method of calcite precipitation mitigation.
20
As mentioned in the theory, lower pH directly correlates to lower carbonate concentration, which
leads to lower extent of calcite precipitation. Due to this, it is advantageous to allow the process to
operate at lowest available or allowed pH. More often than not, P9 operates around the lowest pH
according to stated rules at Dåvamyran, which is 6,5. Given the somewhat fluctuating pH, it could
be useful to implement a pH detection method in the mixing tanks to gain insight into the process
operating conditions.
Another possibility is to somehow decrease the large calcium concentration in P8. According to the
equilibrium chemistry it would lessen the amount of calcite precipitation. There is a problem with
this line of thinking, however, which is the stoichimetric requirements of the additive. To efficiently
bind calcium into complexes one need to have an additive with the ability to bind at least 1-1 in
calcium proportion for it to be efficient, the best case scenario would be using an additive with the
ability to bind 2 calcium ions per molecule. Creating these complexes at 1-1 stoichimetric
proportions would require a staggering amount of additive for a significant decrease in calcium
concentration. The task does not become easier knowing that most calcium complexes are very
weak and likely to break down into calcium. Overall, the idea to lessen to problem by creating
calcium complex binders will result in cumbersome volumes and cost of additives.
This thesis has investigated the calcite scaling during the winter period, where P8 and P9 are both
in full effect. Beyond the winter setting there are two other major scenarios that occur at
Dåvamyran, namely the summer setting and the transition period between the two. During the
transition setting, P9 is operating at lower effect and is shut off in the summer period whilst P8
remain at more or less same operative effect. According to the empirical findings, the problems
stemming from calcite scaling is most severe during the winter season. As such, the theory and
conclusions presented are applicable in the other two settings with the reservation that the scaling
is less prominent due to more dependency on P8.
21
7 Conclusions
According to the equilibrium chemistry, calcite precipitation does not take place at the designated
locations under the theoretical assumptions of the model. When combined with physical factors
such as incomplete mixing of the process water and effect of pumps, the cause of precipitation can
be explained. Equilibrium chemistry states that the mixing water is slightly below the limit of
precipitation, which is theoretically adjusted due to incomplete mixing of flow from P8 and P9 that
allows the precipitation of calcite. The rate of dissolution for calcite is in the order of days, which
leads to a high probability of accumulation in the condensate tanks. Colloids of calcite formed in
this manner is likely to influence the precipitation rate in later process steps and manifest itself in
areas such as the pump impeller and uneven surfaces of pipes. The pump and impeller areas are
observed to be the primary affected areas, given years of empiric study.
The importance of the physical parameters have been established by experimental tests of calcite
dissolution, precipitation test using actual process water and equilibrium calculations. All
investigated chemical parameters indicate that the prevalent concentrations are below the limit of
precipitation outside the affected environment at Dåvamyran.
Many procedures can be made to lessen the rate of calcite but the most recommended is to favor a
stronger mixing of the two process flows. As it stands now, the pipes from the two plants are
separated and mixed no later than the tank under very slow conditions. Fusing together the pipes to
induce a turbulent flow could be a feasible procedure, as would introducing more rapid mixing
conditions in the tanks themselves.
Other actions can be taken to mitigate the extent of scaling, such as chemical additives of natural
polyprotic acids. Exactly how these additives would function in the Dåvamyran setting has not been
explored but is very likely to work given further investigation. Methods such as substituting
limewater in P8 could also be an attractive choice due to overall lower calcium concentration. The
implications of changing to sodium hydroxide has not been fully investigated but a shift could
lessen many other process disturbances that has been observed.
22
8 References
http://www.umeaenergi.se/om-oss, 2015-04-21
http://www.technologystudent.com/joints/fibre1.html, 2015-04-21
3 Jean-Yves Gal et.al,Calcium carbonate solubility: a reappraisal of scale formation and
inhibition,1996
4 Mike Crabtree et. al. Fighting Scale, 1998
5 George E. King, Scale Basics, 2009
6 Robert J. Ferguson and Baron R. Ferguson, The Chemistry of Strontium and Barium Scales, 2010
7 Tomas Hedlund, supervisor, Umeå University
8 http://www.gcsescience.com/rc3-marble-chips-hydrochloric.htm
9 Yuping Zhang and Richard A. Dawe, Influence of Mg2+ on the kinetics of calcite precipitation and
calcite crystal morphology, 1999
10 Katrin I. Parsiegla, Joseph K. Katz, Calcite growth inhibition by copper(II) II. Effect of solution
composition, 2000
11 S. Muryanto et. al, Calcium carbonate scale formation in pipes: effect of flow rates, temperature,
and malic acid as additives on the mass and morphology of the scale, 2013
12 http://people.whitman.edu/~dunnivfm/FAASICPMS_Ebook/Downloads/CH3_FINAL.pdf,
2015-03-10
13 Roger Mäkitalo, operating technician, Umeå Energi
14 Lars Homlgren, operating technician, Umeå Energi
1
2
23
Table 4. Raw data from ALcontrol
*P8 not in service these dates
Method (Alcontrol)
2/11 Dåva 1 CA-H/TIC
2/11 Dåva 2 CA-H/TIC
2/11 Mix
CA-H/TIC
2/12 Dåva 1 CA-H/TIC/MOMF2
2/12 Dåva 2 CA-H/TIC/MOMF2
2/12 Mix
CA-H/TIC/MOMF2
2/13 Dåva 1 CA-H/TIC
2/13 Dåva 2 CA-H/TIC
2/13 Mix
CA-H/TIC
2/19 Dåva 1* CA-H/TIC
2/19 Dåva 2 CA-H/TIC
2/19 Mix*
CA-H/TIC
2/20 Dåva 1* CA-H/TIC
2/20 Dåva 2 CA-H/TIC
2/20 Mix*
CA-H/TIC
3/16 Dåva 1 CA-H/TIC/SR-L/BA-L
3/16 Dåva 2 CA-H/TIC
3/16 Mix
CA-H/TIC/SR-L/BA-L
3/17 Dåva 1 CA-H/TIC/SR-L/BA-L
3/17 Dåva 2 CA-H/TIC
3/17 Mix
CA-H/TIC/SR-L/BA-L
3/18 Dåva 1 CA-H/TIC/SR-L/BA-L
3/18 Dåva 2 CA-H/TIC
3/18 Mix
CA-H/TIC/SR-L/BA-L
3/19 Dåva 1 CA-H/TIC/SR-L/BA-L
3/19 Dåva 2 CA-H/TIC
3/19 Mix
CA-H/TIC/SR-L/BA-L
3/20 Dåva 1 CA-H/TIC/SR-L/BA-L
3/20 Dåva 2 CA-H/TIC
3/20 Mix
CA-H/TIC/SR-L/BA-L
[µg/l]
Ca2+ [mg/l] TIC [mg/l] Sr
Ba
Al
As
Be
6600
5,8
1,7
11
600
6,4
6500
6,5 3500 320 270
5,3 <0,5
0,55
14
2 <10 <20
0,3 <0,5
420
7,8
210
27
24
0,6 <0,5
7800
5,6
2,4
19
560
12
1000
6
1,6
26
190
19
620
6,5
0,95
19
120
15
8400
4,9 4500
4,9
3,5
21
1800
14
990
79
8300
4,1 4300 280
1,6
31
630
29
350
30
7100
5,2 3700 350
2,1
15
680
13
680
88
7900
4,9 4300 370
1
17
1600
12
830
84
6800
4,4 3700 370
2,1
19
1300
15
760
97
Cd
Co
Cu
Cr
Li
0,2 0,11 0,32 <0,5
9,9
22
<0,2 <0,02 <0,05 <0,5 <0,5 <5
0,7 0,04 <0,05 120
1,1 <5
Pb
Mo
Ni
Se
Ag
140
9,7
6,3
2 <0,1
13 <0,5 <0,5 <1
<0,1
20 0,71 <0,5 <1
<0,1
Mn
<1
<1
Ti
2
V
2,1
5,1
<0,1
0,51
0,13 <0,5
U
Zn
16
8
25
Appendix
Figure 19. Overview of water cleaning house at Dåvamyran. Note that piping for P9 is not included
Department of Chemistry
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