Equilibrium calculations for fatty acid calcium soaps … calculations for fatty acid calcium soaps...

Equilibrium calculations for fatty acid calcium soaps in pulp

washing

Marianne Björklund Jansson and Rickard Wadsborn

2005

According to Innventia Confidentiality Policy this report is public since 2011-02-04

Equilibrium calculations for fatty acid calcium soaps in pulp

washing

Marianne Björklund Jansson and Rickard Wadsborn

Report no.: STFI-Packforsk 140 | December 2005

Cluster: Chemical pulp, fibre line

Distribution restricted to: AGA, AssiDomän Cartonboard, Billerud, Borregaard, Eka Chemicals, Holmen Iggesund Paperboard, Kemira, Korsnäs, Metsä-Botnia, Mondi

Packaging, M-real, Peterson & Søn, Stora Enso, Södra Cell, Voith

a report from STFI-Packforsk

According to STFI-Packforsk's Confidentiality Policy this report is assigned category 2

Equilibrium calculations STFI-Packforsk report no. 140

Acknowledgements This work has been performed within the Cluster research program of Chemical pulp - fibre line, in the period 2004-2005.




Table of contents Page

1 Summary .....................................................................................................1

2 Background.................................................................................................2

3 The reactions of calcium during pulping and washing ...........................3 3.1 Sources of calcium .......................................................................................3 3.2 Sources of carbonate ...................................................................................3 3.3 Calcium bound to fibres and lignin ...............................................................4 3.4 Calcium soaps of fatty and resin acids .........................................................4

4 Experimental ...............................................................................................8 4.1 Equilibrium calculations ................................................................................8 4.1.1 The calculation program ............................................................................................8 4.1.2 Components ..............................................................................................................8 4.1.3 Equilibrium constants.................................................................................................8 4.2 Chemical analysis of mill samples................................................................9

5 Results.......................................................................................................10 5.1 Composition of mill samples.......................................................................10 5.2 Equilibrium calculations ..............................................................................12 5.2.1 Guide to the diagrams .............................................................................................12 5.2.2 Influence of the solubility product of the calcium soaps. .........................................13 5.2.3 Influence of the carbonate concentration on the formation of calcium soaps .........17 5.2.4 Influence of the pulp consistency ............................................................................19 5.2.5 Calcium binding capacity of carboxyl groups in the fibres.......................................21 5.2.6 Influence of calcium concentration ..........................................................................22 5.2.7 Case “bleached pulp”...............................................................................................23

6 Discussion and conclusions ...................................................................26

7 References ................................................................................................28

8 Appendix A. Discription of mill samples ...............................................31




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1 Summary The influence of some parameters on the precipitation of calcium soaps during the conditions of pulp washing and bleaching have been investigated, using a process simulation program extended with a tool that handles chemical equilibrium calculations. The aim of the calculations was to identify parameters where a small change in concentration will influence the formation of calcium soaps. The calculated phase diagrams illustrate how the precipitation of different calcium soaps is affected by e.g. calcium and carbonate concentrations, and pH.

For the concentrations of the ingoing chemical components, data from three mills, all producing hardwood kraft, was used. The mill samples, pulps and filtrates, were taken from the washing stage after the oxygen stage and from the last washing stage in the bleaching line.

A marked influence on the solubility product, pKs, for the different calcium soaps is seen. Especially the formation of calcium soaps from the most prevailing, unsaturated fatty acids, e.g. linoleic acid, seems extensively to be depending on the concentrations of ingoing components, e.g. calcium and carbonate, and the pH. Resin acids, with a pKs for the calcium soaps around 9, do not form calcium soaps at any of the tested conditions.



Equilibrium calculations STFI-Packforsk report no. 140 2 (32)

2 Background Wood resin (pitch) often gives problems in the pulp production line e.g. by clogging wires and screens. Further, pitch deposits formed on equipment during pulp screening and bleaching often are the main source for dirt specks in the bleached pulp. The deposits often consist of a combination of calcium soaps of fatty acids and calcium carbonate mixed with other resin components and fibres (Ollandt 1979, Dorris et al, 1983). The formation of calcium soaps will impair the washing of wood resin, both because the formed calcium soaps are water insoluble and therefore will stick to the fibres and also since the capacity for solubilization of the neutral pitch components will decline when the concentration of soluble sodium soaps is decreased. The sodium soaps of fatty and resin acids work as a detergent in the pulp washing and are able to solubilize the water-insoluble neutral components, e.g. sterols, in the micelles formed.

Besides the concentrations of calcium and fatty acids the formation of calcium soaps depends on a number of parameters. Among the most important are: pH, temperature, ionic strength and the concentration of inorganic ions, such as carbonate and metal ions, and the concentration of dissolved lignin. Further, both the amount of the wood resin itself and its composition will have a major influence on the deposition of calcium soaps, due to the complicated solubility behaviour of the resin components. Birch extractives normally are more problematic depending on the higher percentage of neutral compounds. Further birch resin contains a higher proportion of long chain, saturated fatty acids, forming more insoluble calcium soaps, compared to spruce and pine.

In this report the influence of some of these parameters on the precipitation of calcium soaps have been investigated, by calculations using a process simulation program extended with a tool that handles chemical equilibrium calculations (Berggren et al 2003, Gu 2004 a and b). The calculated phase diagrams illustrate how the precipitation of different calcium soaps is affected by e.g. calcium and carbonate concentrations, and pH. However, some effects such as solubilization and dispersion of resin components may not be accounted for by the conducted simulations, and these effects were not considered.

The aim of the calculations was to identify parameters where a small change in concentration will influence the formation of calcium soaps. Since the system is very complex and it is not possible to take account for all factors, it has in this work not been our intention to transfer the calculation directly to a specific mill situation.




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3 The reactions of calcium during pulping and washing

3.1 Sources of calcium The main part of the calcium coming into the pulp mill originates from the wood. Calcium is, after potassium, the most common non-process element in wood and bark. Typical concentrations in wood and bark are given in table 1. As seen, the calcium intake can be increased significantly with poor debarking. Another source of calcium is the white liquor. However, with a proper white liquor clarification this amount may be about 50 mg/kg pulp, which is less than 5 % of the amount entering the mill with the wood. During periods with problems with the white liquor clarification the amount of calcium coming with the white liquor may be substantial. Values up to 1900 mg/kg pulp have been reported (Douek and Allen, 1980b).

Table 1. Calcium and magnesium in Swedish pine, spruce and birch wood and bark, (Magnusson et al 1980).

Wood Bark Ca

mg/kg Mg

mg/kg Ca

mg/kg Mg

mg/kg Pine 620 170 5210 650 Spruce 770 95 4990 640 Birch 680 190 3020 490 The form in which calcium is present in the wood is not totally clear. Analysis across the cross-section of a pine tree (Fossum et al 1972) shows that the concentration of calcium is highest in the cambium. Further it is somewhat higher in the heartwood than in the sapwood. It is also somewhat increased in parenchyma cells and resin canals. However, resin and fatty acids are not believed to exist in the form of calcium soaps in the wood where the pH is slightly acidic.

3.2 Sources of carbonate During the cook a major part of the calcium in the wood will dissolve in the cooking liquor. A part will also be precipitated as calcium carbonate (Hartler and Libert, 1973). Carbonate ions are present in the white liquor and are also formed during the kraft cook, mainly from decarboxylation of acids in the hemicellulose. According to Hartler and Libert (1973) the concentration of carbonate in cooks with carbonate-free white liquor may amount to about 20 mmol/l at the end of the cook. The carbonate content in industrial white liquor is about 200 to 300 mmol/l. Hartler and Libert also determined the calcium concentration in the liquor during the cook. The calcium concentration was highest when carbonate-free white liquor was used and amounted then to about 1.5 mmol/l, which




corresponds to 60 mg/l. If all the calcium in the wood had been dissolved the concentration would have been about 5 times as high. At the end of the cook the concentration of calcium decreased to about 20 mg/l. Hartler and Libert drew the conclusion that the decrease was due to precipitation of calcium carbonate. Calcium hydroxide is normally not precipitated during kraft cooking.

Calculating with a logarithmic apparent solubility product of calcium carbonate of -7.3, the maximum concentration of dissolved calcium in a liquor holding 20 mmol/l carbonate, would only be 0.1 mg/l. The reason for the much higher concentrations actually observed is that supersaturation of the liquor is necessary before calcium carbonate precipitates.

3.3 Calcium bound to fibres and lignin Calcium and other divalent ions are bound much harder to the fibres than monovalent ions such as sodium and potassium. This, in combination with the formation of CaCO3(s), transfers a substantial part of the calcium ions present in the cook with the pulp into the pulp bleaching. After the oxygen stage the amounts of calcium in the three birch pulps used in this project were around 1500 mg/kg. Similar values have been reported in the literature. The calcium is most probably present in the pulp as CaCO3(s) (Wadsborn et al 2005) or bound to the acidic groups of the fibres. Other parts of the calcium may be in the form of calcium soaps of fatty acids, which are adhered to the fibres.

The precipitation of calcium soaps is also affected by the lignin concentration. This was demonstrated by Douek and Allen (1980a) in a model experiment showing that the precipitation of calcium oleate did not take place in the presence of Indulin, a commercial kraft lignin. Whether this was an effect of the binding of calcium ions to the lignin, or by a dispersion effect on formed calcium soaps is not known. However, some phenolic structures such as cathecols are known to be strong complexing agents for metals (Westervelt et al 1982; Werner et al 1998).

3.4 Calcium soaps of fatty and resin acids Calcium soaps of fatty acids often constitute a major part of the wood resin deposits in the washing and screening departments and a part of the fatty acids from the wood is probably transported with the pulp to the bleaching stages in the form of calcium soaps. Resin acids or resin acid soaps are in several investigations reported not to be present in such deposits (Ollandt 1979, Dorris et al 1983). This difference is probably due to the difference in solubility product for the soaps of resin acids as compared to fatty acids.

The pKs-values of the calcium soaps are of course critical in the calculations. A small literature check on the variations of the solubility with ionic strength and temperature has been done, but only limited data, especially for the temperature dependence, have been found. Examples of solubilities of some calcium soaps are given in table 2. As seen, the solubility increases with temperature and ionic strength, also illustrated in figure 1. The




5 (32) solubility of the soaps is thus higher during the cook and decreases when the temperature is decreased and the ionic strength lowered during the pulp washing.

One article from 2001 gives the pKs for calcium oleate at pH 9 and 67°C to 11.6 (Beneventi et al), which is lower than the values in figure 1. However, the wood resin contains several different fatty acids, both with more and less soluble soaps compared to oleic acid.

Also seen in table 2 is that the solubility of the calcium soaps decreases with increased length of the fatty acid carbon chain and increases by the number of double bonds in the chain. Long chain, saturated fatty acids can therefore be expected to comprise a higher percentage of the deposited material than their percentage in the wood resin. Since the unsaturated acids, linoleic, linolenic and oleic acids, are dominating in the wood resin these may still constitute a substantial part of the deposits. The major fatty and resin acids presents in pine, spruce and birch are shown in table 3.

Table 2. Solubility product, pKs, for some calcium soaps at two ionic strengths and two temperatures.

(25 °C) (50 °C) (67 °C) Acid (no of carbons:double bonds)

I = 0.1 I = 1.0 I = 0.1 pH 9.0

Lauric (12:0) 11.9a) 11.3 a) 10.7 a) Myristic (14:0) 14.5 a) 13.9 a) 13.5 a) Palmitic (16:0) 17.1 a) 16.2 a) Stearic (18:0) 19.7 b1) 14.8 c)

Lignoceric (24:0) 27.1 b2) Oleic (18:1) 14.9 a) 13.5 a) 11.6 c)

Linoleic (18:2) 14.2 a) 12.8 a) 10.1 c)

Linolenic (18:3) 9.9 c)

Abietic 9.0 d) a) Irani and Callis; b1) Al Attar and Beck; b2) Calculated from Al Attar and Beck c) Beneventi et al; d) Calculated from Pohle




Solubility product of Ca(Ol)2

10

11

12

13

14

15

16

17

0 0,2 0,4 0,6 0,8 1 1,2

Ionic strength, mol/l

pKs

25C50C60C80CT=25C, Exp

Figure 1. Influence of temperature and ionic strength on the solubility product of calcium oleate, based on experimental data from Irani and Callis. The temperature dependence is based on data for calcium myristate (-15.8 kcal/mol).




7 (32) Table 3. Example of fatty and resin acid composition in pine (Pinus sylvestris), spruce (Picea abies) and birch (Betula pendula). Data for pine and spruce from Holmbom 1980.

Fatty acid Pine wood (% of total fatty

acids)

Spruce wood (% of total fatty

acids)

Birch wood 1) (% of total fatty

acids) Saturated:

Palmitic (16:0) 17:0ai* Stearic (18:0) Lignoceric (24:0) Other saturated**

1.0 0.8

<0.2 <0.2 0.9

3.6 3.0 0.6

<0.2 2.8

9.2 n.d. 4.9 8.1 5.3

Unsaturated: 11-18:1 Oleic 9-18:1) Linoleic (9,12-18:2) Linolenic (9,12,15-18:3) Pinolenic (5,9,12-18:3) 20:1 5,11,14-20:3 Other unsaturated**

0.5

35.3 40.5 0.8

10.6 <0.2 4.6 3.5

2.0

25.0 36.4 0.9

14.9 <0.2 3.4 4.6

n.a. 5.6

59.0 1.3 0

4.7 n.d. n.d.

Other minor < 0.2 % each 1.5 2.8 1.9 Sum 100 100 100 Resin acids Pine wood

(% of total resin acids)

Spruce wood (% of total resin

acids)

Birch wood

Pimaric Sandaracopimaric Isopimaric Levopimaric Palustric Abietic Neoabietic Dehydroabietic Other minor

8.1 1.6 3.5

30.0 15.1 15.8 11.1 14.4 0.4

6.2 6.4

13.3 16.2 13.5 11.2 10.2 22.6 0.4

Resin acids not present

- - - - - - -

Sum 100 100 - * a.i. (ante-iso) = branched at the carbon atom n-2. ** < 1.0% each 1) Compilation of data from different sources




4 Experimental

4.1 Equilibrium calculations

4.1.1 The calculation program Calculations testing the potential for formation of calcium soaps in model systems at different conditions have been performed using the process simulation program WinGEMS (Pacific Simulation) extended with a tool developed to calculate the distribution of metals in the fibre line at equilibrium (Gu et al 2004a and b, Berggren et al 2003). The tool, called MeteQ, is based on SOLGASWATER (Eriksson 1979) and uses any chemical equilibrium model predefined by the user. The formation constants of the compounds are adjusted for variations in temperature and ionic strength according to the Vant-Hoff and Davies equations, respectively. Details of the implementation and the validation of the tool may be found elsewhere (Gu et al 2004a and b Berggren et al 2003).

4.1.2 Components Most of the calculations have been done using the following components: H+ (= pH), Ca2+, CO3

2-, resin components such as resin acids (RA) and fatty acids (FA) with different pKs-values for formation of calcium soaps. The carboxyl groups in the fibres have also been included. The fatty acids have been divided in two groups, FA1 and FA2 with different pKs as described in 5.2.1 below.

4.1.3 Equilibrium constants The following equilibrium reactions have been included:

Inorganic compounds H2O → OH- + H+

H+ + CO32-→ HCO3

-

2 H+ + CO32-→ H2CO3 (aq) dissolved carbonic acid

Ca2+ + H2O → CaOH+

Ca2+ + CO32- → CaCO3 (s) precipitated calcium carbonate

Ca2+ + 2 OH- → Ca(OH)2 (s) precipitated calcium hydroxide

Organic compounds H+ + FA- → HOl (aq) dissolved undissociated acid

Ca2+ + 2 FA-→ CaOl (s) calcium soap

H+ + FA- → HOl (s) precipitated undissociated acid

Corresponding reactions for resin acids (RA) Ca2+ + 2 pulp-COOH → (pulp-COO)2Ca Ca bound to fibres




9 (32)

The equilibrium constants have been adjusted to a temperature of 70 C . Sodium has not been used as a component, but the equilibrium constants have been adjusted as close as possible to the estimated ionic strength in the different cases.

For water, carbonic acid, calcium hydroxide, calcium carbonate and carboxyl groups in the pulp the constants estimated previously at STFI-Packforsk (Ulmgren 2003) have been used.

For the fatty acids calcium soaps pKs-values from 12 to 20 has been used. For the resin acids a pKs-value of 9.0 has been used.

4.2 Chemical analysis of mill samples TOC was determined according to SS 028199.

Carbonate was determined as total inorganic carbon according to SCAN-N 32

Calcium Pulp samples were ashed at 575 °C and dissolved in hydrochloric acid. The filtrates were wet digested in a microwave oven with nitric acid and peroxide. Calcium was quantified by ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry)

Extractives The pulp samples were extracted with acetone after acidification with acetic acid, according to SCAN CM 49.

The filtrate samples were acidified to pH 3 and extracted with petroleum ether after addition of acetone and methanol, according to (McMahon, 1980).

The silylated extracts were analysed by GC-MS (gas chromatography-mass spectrometry) for quantification of individual compounds.

The sum of unsaturated fatty acids has been used for component FA1 (unsaturated fatty acids). The main acids were palmitoleic, oleic, linoleic, and linolenic acid.

The sum of saturated fatty acids has been used for component FA2 (saturated fatty acids). The main acids were palmitic, stearic, arachinic, and lignoceric acid.

The sum of individual resin acids has been used for the component RA (resin acids).




5 Results

5.1 Composition of mill samples In order to acquire relevant indata for the simulations, samples from three different mills (A, B and C) all producing bleached hardwood pulp were collected in the fall 2003. Both pulps and filtrates were sampled at three different positions: after cooking, after oxygen delignification and from fully bleached pulp. The content of extractives in the different samples are shown in table 4. Mill A has a higher extract content after cooking, mainly depending on the higher amount of tall oil added to the cooking, 1.6 % compared to 1.3 % in mill B and no addition in mill C. The degree of washing accomplished in the sampling point “after oxygen stage” may also be different. Both the sodium content and the pH in the oxygen stage filtrate are much higher in mill A compared to mill B and C.

Table 4. Extract content in the mill samples. Pulp samples were extracted with acetone after acidification. The filtrates were extracted with PAM (petroleum ether/acetone/methanol) after acidification After cook After oxygen Bleached pulp Pulp, g/kg Filtrate, g/l Pulp, g/kg Filtrate, g/l Pulp, g/kg Filtrate, g/l Mill A 78.5 2.1 6.8 2.9 0.7 0.01 Mill B 38 3.9 3.4 0.32 1.8 0.06 Mill C 32.3 1.7 8.8 0.97 2.7 0.006

In this report only the positions “after oxygen” and “bleached pulp” have been used for the simulations. Some data describing the mill processes and the samples analysed are given in Appendix A. The complete analytical results are reported in (Björklund Jansson, 2005).

The major part of the simulations has been performed for the case “oxygen stage” and the pulp consistency 2 %. The concentrations used in these calculations are given in table 5. Concentrations for the case “bleached pulp” and the pulp consistency 25 % are given in Appendix A.




11 (32)

Table 5. Conditions and concentrations used for the case “after oxygen stage”, pulp consistency 2 %.

Mill A Mill B Mill C I, M 0.5 0.2 0.2 T, °C 70 70 70 Ca2+, mM 1.7 0.76 1.1 CO3 mM 189 61 66 FA1, mM 1.7 0,50 1.2 RA, mM 2.6 0.65 0.05 FA2, mM 0.82 0,27 0.6 pH 11.6 10.7 10.9 pulp*, mM 2.5 2.5 2.5

The concentrations of carbonate and the pH-values in the filtrates from the three mills are illustrated in figure 2. Mill A uses addition of carbon dioxide in the washing line. This is most probably the reason for the substantially higher concentrations of carbonate ions in the filtrates from mill A. Increased amounts of carbonate ions has earlier in laboratory experiments been shown to improve the washing with regard to removal of extractives (Back and Björklund Jansson, 1987). In this case, the carbon dioxide addition is used mainly in order to improve the dewatering effect on the washers.

The higher pH in the filtrate from the bleached pulp in mill B compared to mill A and C is caused by the differences in bleaching sequence, peroxide stage was used in mill B compared to chlorine dioxide in mill A and mill C.

0

50

100

150

200

250

300

350

400

450

Filtrate pressedfrom pulp after

cooking

Filtrate afteroxygen stage

Filtrate from lastwashing stage(bleached pulp)

mm

ol/l

0

2

4

6

8

10

12

14

pH

Carbonate mill A

Carbonate mill B

Carbonate mill C

pH mill A

pH mill B

pH mill C

Figure 2. pH, and carbonate concentration in the mill filtrates.




5.2 Equilibrium calculations

5.2.1 Guide to the diagrams

In order to make estimations of the distribution of metal ions at equilibrium, the number of components in the system has to be defined and all the possible reactions of each component described, using the equilibrium constants for the reactions during the present conditions. Furthermore, the total concentrations for all the components, except the one that is used as a master variable, have to be fixed. In these calculations we have mainly used pH as the master variable, but in some diagrams also the carbonate concentration or the calcium concentration has been used as the master variable. The pH was fixed in these cases.

The components and equations used in this work are given in the experimental part (See 4.1).

In most simulations, three different acidic components have been used.

• The first group (FA1) represents the unsaturated fatty acids, such as oleic, linoleic and linolenic acids. The solubility product for the formation of calcium soaps for these acids was varied between 11 and 15, in order to exemplify the influence of the different pKs-values.

• The second group (FA2) represents the saturated fatty acids, which form more insoluble calcium soaps. In most calculations the pKs-value for this component was set to 19.7. However, as seen in table 2, the pKs for some acids included in this group is much higher.

• The third group represents the resin acids (RA) and for this component a pKs for formation of calcium soaps of 9.0 was used. (Value from Pohle, 9.6, adjusted to higher ionic strength.)

In the calculations two different sets of conditions were used, representing concentrations in the oxygen stage and in the bleached pulp. The concentrations from the analysis of the samples from the three mills (A, B and C) were used as input data for the calculations.

Each given combination of input data in the form of concentrations will result in a set of graphs, one for each component, showing the different forms in which this component is present at different pH:s, if pH is the master variable.

The results are plotted as “fraction diagrams”. An example is given in figure 3, showing the equilibrium diagrams from a simulation where the concentrations from the oxygen stage in mill C have been used as input. As seen the fatty acid in the group FA1, here with a pKs of 12, is partly present in the form of calcium soaps and partly as a free anions.

Further, the fraction diagram for calcium in this example shows that calcium, at pH:s relevant to the pulp washing, is present both in the form of calcium carbonate and bound to FA1 and FA2. At a pH above 13,3 calcium hydroxide is formed and the calcium soaps




13 (32) of FA1 and the calcium carbonate are dissolved. However, the calcium soaps of FA2 are still present.

Mill C, oxygen stage (2 %)FA1 pKs 12

0,0

0,2

0,4

0,6

0,8

1,0

4 5 6 7 8 9 10 11 12 13 14pH

Frac

tion

FA1

Ca-soap

Undissolved acid

Free acid anion

Mill C, oxygen stage (2 %)

FA1 pKs 12

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14pH

Frac

tion

Ca

Ca(FA2)2

Ca2+

CaCO3

Ca(FA1)2

Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Capulp2+

CaOH-

Ca(OH)2

Mill C, oxygen stage (2 %)FA1 pKs 12

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14pH

Frac

tion

Ca

Ca(FA2)2

Ca2+

CaCO3

Ca(FA1)2

Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Capulp2+

CaOH-

Ca(OH)2

Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Capulp2+

CaOH-

Ca(OH)2

Figure 3. Equilibrium diagrams illustrating the different forms of one group of fatty acids (FA1) with a pKs of 12 (top) and the different forms of Ca (bottom) as a function of pH in mill C. See also figure 4 and 5 showing the fraction diagrams for RA (resin acids) and FA2.

5.2.2 Influence of the solubility product of the calcium soaps.

The most critical input value in the calculations is of course the solubility product (Ks) of the fatty acid calcium soaps. Most values in the literature are given at room temperature and low ionic strength. However some information about the influence of temperature and ionic strength can be found in the literature, see table 2 above. From these values an extrapolation has been made with the result that the pKs of calcium oleate decreases from 14.9 at I = 0.1 M and 25 °C to about 12.2 at I = 1.0 and 70 °C. Similarly the pKs of




linoleic acid, which is one of the most abundant fatty acids in wood, decreases from 14.2 to 11.5.

For this reason the calculations have been performed for different pKs-values, from 12 to 15 for the unsaturated acids (= FA1) and at 19.7 as an example of a saturated acid (FA2).

From the results it can be seen that acids with a pKs of 9, as used for the resin acids, or below, do not form calcium soaps under any of the conditions used in the calculations. This is in agreement with the experience from mill situations, where resin acids normally are missing in deposits from the washing, screening and bleaching areas. A typical diagram of the resin acids is shown in figure 4, taken from the same simulation conditions as in figure 3. The undissociated resin acids, present at lower pH:s, have a higher solubility than the fatty acids and are therefore also present in the form of dissolved, undissociated acid.

Mill C, oxygen stage (2 %)Resin acid pKs 9

0,0

0,2

0,4

0,6

0,8

1,0

4 5 6 7 8 9 10 11 12 13 14pH

Frac

tion

RA

Undissolved acid

Free acid anion

Dissolved undissociated acid

Figure 4. Typical example of equilibrium diagram for the resin acids. Same simulation as in figure 3 and 5. Further, the group FA2 (= fatty acids with a pKs of 19.7 or higher) were in the calculations present only in the form of calcium soaps, regardless of the pH and the carbonate concentration. A typical example is shown in figure 5. The fatty acids in group FA2 are only present as calcium soaps at all pH:s above 5.8. Below pH 5.8 the fatty acids FA2 are present in the undissociated form, with a very low solubility in water.




15 (32)

Mill C, oxygen stage (2 %)FA2 pKs 19.7

0,0

0,2

0,4

0,6

0,8

1,0

4 5 6 7 8 9 10 11 12 13 14pH

Frac

tion

FA2

Ca-soapUndissolvedacid

Figure 5. Typical example of equilibrium diagram for the saturated fatty acids (FA2). Same simulation as in figure 3 and 4. For the group FA1, (= the most prevailing acids, such as the unsaturated C18 acids, oleic, linoleic and linolenic) the results show a more complicated picture. Some results, using the concentrations in the oxygen stages in mill A and B, are shown in figure 6. The figure illustrates the pH-interval where fatty acids soaps are formed using pKs-values between 12 and 13. For the acids in this group probably variations in the conditions during washing will influence the precipitation of the soaps.




Mill A O2 (2 %)

0,0

0,2

0,4

0,6

0,8

1,0

4 5 6 7 8 9 10 11 12 13 14pH

Frac

tion

pKs of 13

pKs of 12

pKs of 12.5

pKs of 19.7

Mill A O2 (2 %)

0,0

0,2

0,4

0,6

0,8

1,0

4 5 6 7 8 9 10 11 12 13 14pH

Frac

tion

pKs of 12

Actual carbonate conc.189 mM

Lower carbonate conc.90 mM

Mill B O2 (2 %)

0,0

0,2

0,4

0,6

0,8

1,0

4 5 6 7 8 9 10 11 12 13 14pH

Frac

tion

pKs of 13

pKs of 12

pKs of 12.5

pKs of 19.7

Figure 6. Partial equilibrium diagrams illustrating the pH-intervals for formation of calcium soaps in mill A and mill B for fatty acids with four different pKs-values. The ordinate gives the fraction of the fatty acid forming calcium soap. For mill A also a comparison with a lowered carbonate concentration 90 instead of 189 mM, is shown. The carbonate concentration in mill B was 61 mM.




17 (32) 5.2.3 Influence of the carbonate concentration on the formation of calcium

soaps

For the unsaturated acids in group FA1 there is also in some cases a marked influence of the carbonate concentration. One example is shown in figure 7. In this case the pH has been set to the value measured in each mill (Table 5) and the different forms of FA1 (unsaturated fatty acids) was estimated as functions of the carbonate concentration. As seen the formation of calcium soaps was markedly affected by the carbonate concentration.

The carbonate concentrations in figure 7 should be compared to the concentrations of carbonate measured after the oxygen stage in mill A, using an addition of carbon dioxide in the washing line, 193 mM, and the carbonate concentrations in mill B and C, in both cases about 65 mM.




Mill A, oxygen stage (2 and 25 %)FA1 pKs 12

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 50 100 150 200 250 300

Carbonate, mM

Frac

tion

FA1

Free acid anion

Calcium soap

25 % 2 %

25 % 2 %

Mill B, oxygen stage (2 and 25 %) FA1 pKs 12

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 50 100 150 200 250 300

Carbonate, mM

Frac

tion

FA1

Free acid anion

Calcium soap

Mill C, oxygen stage (2 and 25 %)

FA1 pKs 12

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 50 100 150 200 250 300

Carbonate, mM

Frac

tion

25 %2 %

Free acid anion

Calcium soap

25 %

2 %

Figure 7. Influence of carbonate in mill A, B and C. Fatty acid pKs = 12, Pulp conc. 2 and 25 % Less influence of carbonate in mill B compared to mill A and C.




19 (32) 5.2.4 Influence of the pulp consistency

Since the calculations in WinGEMS have been conducted for a set sample mass, e g per kg wet pulp sample, it is needed to know the total concentration of each component in that sample. In these calculations the total concentrations have been set using the values from the analysis of samples from three different mills (A, B and C). Since some of the components are mainly bound to the fibres, and other components are dissolved in the liquor, the amount of a specified component will depend on the pulp consistency. Here we have used two fibre concentrations, 2 % and 25 %. For soluble components, such as e.g. pH and sodium, the pulp consistency will not affect the concentrations significantly. However, for components that are present mainly in different insoluble forms, e g calcium ions and saturated fatty acids (FA2), and of course the carboxyl groups in the fibres, the pulp consistency chosen for the calculations will alter the total amount of that component.

As an example, the different forms of two of the components, calcium and carbonate,- in the position “oxygen stage in mill A” are shown in figures 8 and 9 for a pulp consistency of 2 % and 25 % respectively.

The concentration of carbonate in all mill filtrates in the position “oxygen stage” is in all three mills much higher than the calcium concentration. For this reason the major part of carbonate is present in free form, as illustrated in figure 9. At the higher pulp consistency also carbonate in the form of calcium carbonate is present. The situation is similar in mill B and mill C.




Mill A, oxygen stage (2 %)

FA1 pKs 12

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14pH

Frac

tion

Ca Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Capulp2+

CaOH-

Ca(OH)2

Ca(FA2)2

Ca2+

CaCO3

Ca(FA1)2

Mill A, oxygen stage (2 %)FA1 pKs 12

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14pH

Frac

tion

Ca Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Capulp2+

CaOH-

Ca(OH)2

Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Capulp2+

CaOH-

Ca(OH)2

Ca(FA2)2

Ca2+

CaCO3

Ca(FA1)2


0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14

pH

Frac

tion

Ca Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Ca-pulpCaOH-

Ca(OH)2

Ca2+

CaCO3

Ca(FA1)2

Ca(FA2)2

Ca-pulpCaOH-

Ca(OH)2Ca(FA2)2

CaCO3Ca-pulp

Ca2+

Figure 8. Equilibrium diagrams illustrating the different forms of calcium as a function of pH in mill A at two different pulp consistencies, 2 (above) and 25 % (below).




21 (32)

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14

pH

Frac

tion

carb

onat

e


CO32-

HCO3-H2CO3

CaCO3

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14

pH

Frac

tion

carb

onat

e


CO32-

HCO3-H2CO3

CaCO3

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14pH

Frac

tion

carb

onat

e


CO32-HCO3

-H2CO3

CaCO3

0

0,2

0,4

0,6

0,8

1

2 4 6 8 10 12 14pH

Frac

tion

carb

onat

e


CO32-HCO3

-H2CO3

CaCO3

Figure 9. Equilibrium diagrams illustrating the different forms of carbonate as a function of pH in mill A at two different pulp consistencies, 2 (above) and 25 % (below).

5.2.5 Calcium binding capacity of carboxyl groups in the fibres

The carboxylic groups in the fibres, have been used as one component in the calculations, since they may compete for the calcium ions. An amount of 126 mmol of carboxylic groups per kg of pulp has been assumed (Athley and Ulmgren, 2001). Figure 10 illustrates the binding of calcium to these carboxylic groups in mill A and mill B in the position “oxygen stage” at two different pulp consistencies. As seen, the carboxylic groups are undissociated at low pH:s and in free form at high pH:s. At pH:s around 3 to 8 a part of the carboxylic groups are associated to calcium ions. The pH area for binding of calcium to the fibres is somewhat larger at the higher pulp consistency of 25 %.




However, since most of these calculations have been made for pH:s above 8 the presence of the fibres do not have any major influence on the precipitation of the calcium soaps of fatty acids.

Mill A and B, oxygen stage (2%)

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8 10

pH

Frac

tion

pulp

12

Pulp-HpulpCaPulpPulp-HpulpCaPulp

Mill A and B, oxygen stage (25 %)

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8 10

pH

Frac

tion

12

Pulp-HpulpCaPulpPulp-HpulpCaPulp

Figure 10. The different forms of the carboxyl groups in the fibres in the position “oxygen stage” in mill A and B. Above at a pulp consistency of 2 %; below at a pulp consistency of 25 %. Mill A (open symbols) and mill B (filled symbols).

5.2.6 Influence of calcium concentration One simulation has been done with the calcium concentration as a master variable. The concentrations of ”mill B oxygen stage” have been used, that is a pH of 10.7 and a carbonate concentration of 61 mM. The results are shown for FA2 (pKs = 19.7) in figure 11 and for FA1 in figure 12. In figure 12 also a simulation with a three-fold increase in the concentration of the FA1 component is shown. The increased fatty acid concentration gives a formation of calcium soaps.




23 (32)

Mill B, Oxygen stage (2 %)

FA2 pKs 19.7

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1 2 3

Ca (mM)

Frac

tion

FA2

Free acid anion

Ca-soap

Figure 11 Influence of the calcium concentration in mill B Fatty acid pKs = 19.7, Pulp conc. 2 %. (pH 10.7, carbonate 61mM)

Mill B, Oxygen stage (2 %)FA1 pKs 12

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1 2 3

Ca (mM)

Frac

tion

FA1

Free acid anion

Ca-soap

Mill B, Oxygen stage (2 %)FA1 pKs 12

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

1

0 1 2 3 4

Ca (mM)

Frac

tion

FA1

Free acid anion

Ca-soap

Figure 12 Influence of the calcium concentration in mill B Fatty acid pKs = 12.0. Left: Actual concentration of FA1. Right: FA1 increased three times to 1.5 mM. The calcium concentration in mill B was in the oxygen stage 0.8 mM.

5.2.7 Case “bleached pulp”

For the ECF bleached pulp in mill A and C the pH is 4.4 and 6.7 respectively. At these pH:s also the carboxyl groups of the fibres will bind a part of the calcium ions, figure 13. As seen in figures 14 and 15 about 50 % of the calcium is bound to the pulp. The rest of the calcium is free calcium ions at lower pH:s, such as in the chlorine dioxide stages in mill A and C. In mill B that has TCF bleaching, and a pH of 10.7, a part of the calcium is still bound as calcium carbonate. At the conditions of the bleached pulp the FA2 are still bound as Calcium soaps, but the FA1 are in the calculations in the free form. The main




reason for this is that the concentration is very low. If the concentration of FA1 is increased about 30 times a part of the unsaturated acids will form calcium soaps.

Mill A, B and C Bleached pulp, 2%

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8 10pH

Frac

tion

pulp undissociateded

12

sodium form

calcium formC

A

B

C

AB

Figure 13. The form of the carboxylic groups in the fibres as a function of pH in the mills A, B and C. Conditions bleached pulp.

Mill B, Bleached pulp 2%

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8 10 12

pH

Frac

tion

calc

ium

CaCO3

Ca-pulp

Ca(FA2)2

Ca2+ CaOH-

Figure 14. Equilibrium diagram illustrating the different forms of Ca in the bleached pulp in mill B.




25 (32)

Mill A and C, Bleached pulp 2%

0

0,2

0,4

0,6

0,8

1

0 2 4 6 8 10 12

pH

Frac

tion

calc

ium

CaCO3 (mill C)

Ca-pulp

Ca(FA2)2

Ca2+

CaOH-

Figure 15. Equilibrium diagrams illustrating the different forms of Ca in mill A and C as a function of pH.

The reason for no formation of calcium carbonate in mill A even at higher pH-values and the low formation in mill C is an “artefact” depending on the input data. The input data for the bleached pulp is the carbonate content in the bleached pulp measured at the actual pH, which was 4.4, 6.7 and 10.5 in mill A, B and C respectively. For the simulations it might be better to use the carbonate content at a higher pH and this will be tested in future simulations.




6 Discussion and conclusions All the three mills used in the simulations produce hardwood kraft pulp. The extract content of the bleached pulp was 0.7 mg/kg in mill A, 1.8 mg/kg in mill B and 2.7 mg/kg in mill C. However, there are several differences between the mills that may affect the deresination of the pulp. E.g. addition of tall oil in the cooking was used in mill A (1.6 %) and mill B (1.3 %), but not in mill C. Further, mill A adds carbon dioxide after the oxygen stage, which increases the carbonate concentration in the filtrates. The results from analysis of the samples in the three mills and the washing efficiency are discussed also in (Björklund Jansson, 2005).

The aim of the simulations presented in this report has at this stage primarily been to identify parameters, where a small change in concentrations will influence the formation of calcium soaps, and not to simulate the degree of deresination. Formation of calcium soaps in pulp washing is detrimental both since the soaps themselves are very sticky and difficult to wash out and also because the formation of calcium soaps reduces the amount of soluble sodium soaps. The sodium soaps will act as a detergent in pulp washing by solubilizing otherwise insoluble wood extractives, such as e.g. sterols and steryl esters, in micelles. This is the reason for the positive effect of the tall oil addition.

A crucial value for the formation of calcium soaps is of course the solubility product, Ks, for the soaps. In the present study it was shown that saturated fatty acids, with a pKs of 19 or higher, will form calcium soaps during most of the tested conditions. On the other hand, the formation of calcium soaps from the most prevailing unsaturated fatty acids, e.g. linoleic acid, seems to be depending on the concentrations of the ingoing compounds, e.g. calcium and carbonate, and the pH. This means that the formation of these soaps may be possible to prevent by changing these variables. Resin acids with a pKs of the calcium soaps of around 9, do not form calcium soaps at any of the tested conditions. The effect of other cations, especially magnesium and manganese, would be possible to include in the simulations, but have not been included in this study.

At oxygen stage conditions a varying amount of carbonate seems to influence the formation of calcium soaps. Thus, the addition of carbon dioxide in mill A may be one of the factors contributing to the, for a birch pulp, very low extract content in the bleached pulp from this mill. The simulations also show that the formation of calcium soaps is more affected by the carbonate concentration at a higher pulp consistency.

The variation in the carbonate concentration has so far been tested only at the actual pH measured in the filtrates from the three mills. In future simulations it would be of interest to study the effect of an increase in carbonate concentration, on the formation of Ca-soaps, at different pH-values. Further, any formation of complexes between dissolved lignin and calcium ions has not been accounted for, due to the lack of complexing constants.

Another aspect that has so far not been addressed is the dynamics of the system. If calcium soaps are formed at some stage, how quickly will they dissolve when the




27 (32) conditions change as e.g. during the dilutions and concentrations done in each washing stage?




7 References Athley K, Ulmgren P (2001) Acid-base properties of oxygen-delignified kraft pulps Nordic Pulp Paper Res 16:3, 195 Al Attar A and Beck W H (1970) Thermodynamic solubility products of long-chain normal fatty acids and their alkaline earth and lanthanum salts in water J Electroanal. Chem 27 p.59 Back, E.L and Björklund Jansson M: Harzkomponenten im Holz und Auswaschung derselben nach einer Sulfatkochung Wochenblatt für Papierfabrikation (1987) No 8, p.339 Berggren R, Lindgren K, Sarman S, Samuelsson Å Apparent solubility of sparingly soluble salts in the fiber line- validation of equilibrium calculations in WinGEMS STFI Report CHEM 106 (2003) Beneventi D, Carre B, Gandini A (2001) Precipitation and solubility of calcium soaps in basic aqueous media J Colloid Interface Sci 237, 142 Björklund Jansson M (2005) Birch extractives in kraft pulp washing STFI-Packforsk report no. 141 Douek M and Allen L (1980a) Calcium soap deposition in kraft mill brownstock systems Pulp Paper Canada 81:11 T318 Douek M and Allen L (1980b) The distribution of calcium in kraft mill brownstock systems Svensk Papperstid. 83:15, 425 Dorris GM, Douek M, Allen LH (1983) Analysis of metal soaps in kraft mill brownstock pitch deposits J Pulp Paper Sci 9:1,TR1 Eriksson G (1979) An algorithm for the computation of aqueous multicomponent, multiphase equilibria. Analytica Chimica Acta 112(1979), 375-382




29 (32) Fossum T, Hartler N, Libert J (1972) The inorganic content of wood Svensk Papperstid. 75(4)307 Gu Y, Edwards L Prediction of metals distribution in mill processes, Part 2: Fiber line metals profiles Tappi J 3(2004):2, 13-20 Gu Y, Malmberg B, Edwards L Prediction of metals distribution in mill processes, Part 1: Metals equilibrium model Tappi J 3(2004):1, 26-32 Hartler N, Libert J (1973) The behaviour of certain inorganic ions in the wood/white liquor system Svensk Papperstid. 76:12, 454 Holmbom, B (1978) Constituents of tall oil Dissertation, Åbo Academy Irani R R, Callis C F (1960) Metal complexing by phosphorus compounds. II. Solubilities of calcium soaps and linear carboxylic acids J. Physical Chem 64(1960)1741 Magnusson H, Mörk K, Warnqvist B (1980) Processfrämmande ämnen i sulfatmassafabriken SCAN-Forsk-rapport nr 224 McMahon D H (1980) Analysis of low levels of fatty and resin acids in kraft mill process streams Tappi 63:9, 101 Ollandt (1979) Harts i björksulfatamssaprocess Diplomarbete, Åbo akademi Pohle W D Solubilities of calcium soaps of gum rosin, rosin acids and fatty acids Oil&Soap 18(1941)244




Ulmgren P Solubility of slightly soluble compounds in bleach plant filtrates - a summary. STFI Report CHEM 88 (2003) Wadsborn R, Samuelsson Å, Rådeström R, Edebo A, Lendrup H, Hedlund-Björnwall T, Metal profiles in the bleach plant: modelling and mill validation International Pulp Bleaching Conference, Stockholm, 2005. Werner JA, Ragauskas AJ, Jiang JE (1998) Evaluation of the intrinsic metal binding capacity of kraft black liquor lignins Tappi Pulping Conf, Proceedings vol III, 1145 Westervelt HH, Frederick WJ, Malcolm EW, Easty DB (1982) New evidence concerning the role of black liquor organics in the calcium carbonate scale formation Tappi, 65:5, 179




31 (32) 8 Appendix A. Description of mill samples

Table A1. Process data for mill A, B and C.

Mill A Mill B Mill C Wood mix Birch ca 100 % Birch/aspen 90/10 Birch/aspen 70/30 Tall oil Carbon dioxide

1.6 % yes

1.3 % no

None no

Bleaching sequence ECF O-O-D-E-D-D

TCF (OO)-Q-OP-PAAQ-PO

ECF O-D-EOP-D-D

Kappa number: - after cooking - after oxygen - bleached pulp

17 11 0

16 9

4.5

19.9 11.1 0.3

Table A2. Sampling positions in the mills

Sampling position Pulp samples Filtrate samples 1. After cooking The pulp was taken from the blow

line and pressed warm to a dry content > 25 %.

The filtrate from the pressing of pulp 1

Mill A Mill B Mill C Pulp dry content 24.5 31.2 38.9 2. After oxygen Pulp from washing stage after

oxygen delignification Filtrate from press corresponding to pulp 2

Mill A Mill B Mill C Equipment Press Press Press Pulp consistency 30.8 27.1 21.7 3. Bleached pulp Pulp from last washing stage Filtrate from press/filter

corresponding to pulp 3 Mill A Mill B Mill C Equipment Filter Press Press Pulp consistency 10.9 21.1 27.1




Table A3. Concentrations used for simulations in position ”after oxygen stage” for two pulp concentrations.

Mill A Mill B Mill C

25 % 2 % 25 % 2 % 25 % 2 %

I, M 0.5 0.5 0.2 0.2 0.2 0.2

Ca2+, mM 9.2 1.7 8.5 0.8 9.6 1.1

CO3, mM 151 190 53.3 61.46 57 65

FA1, mM 1.4 1.7 0.4 0.5 1.1 1.2

RA, mM 0.6 2.6 0.02 0.7 0.0 0.1

FA2, mM 0.6 0.8 0.42 0.3 0.6 0.6

pH 11.6 11.6 10.7 10.7 10.9 10.9

pulp*, mM 31.5 2.5 31.5 2.5 31.5 2.5

Table A4. Concentrations used for simulations in position ”bleached pulp”

Mill A Mill B Mill C

25 % 2 % 25 % 2 % 25 % 2 %

I, M 0.01 0.01 0.01 0.01 0.01 0.01

Ca2+, mM 1.0 0.2 4.0 0.4 4.7 0.55

CO3, mM 0 0 6.4 4.5 3.8 0.5

FA1, mM 0.040 0.0032 0.035 0.003 0.010 0.0008

FA2, mM 0.040 0.0204 0.19 0.1012 0.119 0.0164

RA, mM 0.000 0.0000 0.000 0.0000 0 0

pH 4.4 4.4 10.7 10.7 6.7 6.7

pulp*, mM 31.5 2.5 31.5 2.5 31.5 2.5 * Carboxylic groups in the pulp


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Equilibrium calculations for fatty acid calcium soaps … calculations for fatty acid calcium soaps...

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