PROCEEDINGS OF ECOS 2012 - THE 25TH INTERNATIONAL CONFERENCE ON

EFFICIENCY, COST, OPTIMIZATION, SIMULATION AND ENVIRONMENTAL IMPACT OF ENERGY SYSTEMS

June 26-29, 2012, Perugia, Italy

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Production of Mg(OH)2 for CO2 emissions removal applications: parametric and process

evaluation

Experience Nduagua*

, Inês Romãoa,b

, Ron Zevenhovena

a Åbo Akademi University, Åbo/Turku, Finland, enduagu@abo.fi. CA

b University of Coimbra, Coimbra, Portugal

Abstract:

Technological processes that accelerate natural and geochemical weathering of abundantly available Mg-silicate minerals have the potential for large-scale, safe and permanent storage of CO2. One of these CO2 sequestration routes involves as a first step the production of reactive Mg(OH)2 from Mg-silicates using recoverable ammonium sulfate (AS) salt. This route avoids the very slow kinetics of carbonating magnesium silicates. A recently identified Mg(OH)2 production process involves a closed loop, staged process of Mg extraction followed by Mg(OH)2 precipitation and reagent (AS) recovery. This process has been applied to different Mg-silicate (serpentinite and olivine rocks in particular) minerals from worldwide locations, having varying physical and chemical properties. Experimental results showed some dependence of Mg extraction and mass of the Mg(OH)2 product on the reaction parameters: mass ratio of Mg-silicate mineral (S) to AS salt reacted, reaction temperature (T) and time (t). This paper statistically evaluates the contribution of these effects and their interactions using a 2n-1 factorial experimental design. Both Mg(OH)2 production and carbonation were simulated using Aspen Plus® software while process heat integration was done by pinch analysis. Process energy evaluation, on an exergy basis, gives 3.88 GJ of energy requirement for 1t-CO2 sequestered (for Finnish serpentinite). This value is ~ 0.5 GJ/t-CO2 (10 % points) less than the energy requirement of the process in a previous model. The results of this analysis would be beneficial for optimization and pilot scale studies of this process.

Keywords:

Mg-silicates, Magnesium hydroxide, CO2 mineralization, Process evaluation.

1. Introduction

Weathering of alkaline silicate rocks plays a significant role in absorbing atmospheric CO2 [1].

Alkaline and alkaline-earth silicate mineral deposits are abundant and larger than fossil

resources[2]. A resource of this magnitude, over 300,000 Gt of Mg-based silicate minerals[3]

provides significant amounts of base ions for the natural process of neutralizing atmospheric CO2

emissions. However, natural weathering has very slow kinetics and occurs on geological

(multimillion-year) timescales [4]. So, it becomes foolhardy to rely on natural weathering in

reducing or stabilizing atmospheric CO2 emissions. The goal of meeting both current and future

energy demands in a “carbon neutral” manner has therefore spurred research that aims at

accelerating the kinetics of the reaction of mineral silicates and CO2. This geochemical option of

carbon (dioxide) capture and storage is known as CO2 mineralization or mineral carbonation.

The direct carbonation chemistry of Mg silicates is exothermic, and potentially allows for a process

with a zero or negative overall energy input [5]. Mg silicates, for example, serpentine and olivine

which are abundantly available (with a combined capacity of ~ 200,000 Gt[3]) reacts with CO2

according to (1) and (2)[6].

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Mg2SiO4(s) + 2CO2(g) →2MgCO3(s) + SiO2(s),

∆H (298 K)= - 69...-109 kJ/mol CO2 (1)

Mg3Si2O5(OH)4 + 3CO2 (g) →3MgCO3(s) + 2SiO2(s) + 2H2O(l),

∆H (298 K) = -46… -64 kJ/mol CO2 (2)

Direct gas/solid carbonation of Mg-silicates appears simple but suffers from slow chemical kinetics

and poor energy economy even at elevated temperatures and pressures. Surprisingly, most of the

routes presented in the literature do not take benefit of the exothermic nature of the overall mineral

carbonation chemistry. A staged process of CO2 mineralization via production of Mg(OH)2

followed by gas/solid carbonation is the major focus of the mineralization research at Åbo Akademi

University (hereafter ÅA), Finland. This route allows for a good process heat integration utilizing

the exothermic heat produced from Mg(OH)2 carbonation to drive the upstream Mg(OH)2 process.

Mg(OH)2 produced in the first step can be used to capture and store CO2 via the following ways:

i. carbonation using a high temperature pressurized fluidized bed (FB) reactor (480-600 oC, <50

bars)[7, 8]. Recent developments[9, 10] involve applying CO2 mineralization to flue (or other

CO2-containing) gases directly. This would eliminate the very expensive and CO2 capture step

from the CCS process chain.

ii. direct aqueous reaction with CO2 from air at near ambient temperature and pressure

conditions[11].

iii. application in oceans (and water bodies) to capture atmospheric CO2 as well as to reduce

alkalinity of oceans[3, 12].

In spite of the abundance and global spread of Mg-silicate minerals (which are raw materials for

Mg(OH)2 production) and the potential applications of Mg(OH)2 in removing CO2 emissions, very

few studies aim at producing Mg(OH)2 for this purpose. This paper intends to bridge this gap by

studying various factors affecting Mg extraction and Mg(OH)2 production from olivine and

serpentinite rocks. These mineral-containing rocks tested possess different chemical and physical

characteristics: elemental compositions (Table A1 in the Appendix section), porosity and

hardness[6].

2. Experimental

2.1 Mg-silicate rocks preparation and characterization

The mineral rocks tested in this study are Finnish serpentinite (Finnish serp.) from the Hitura nickel

mine owned by Finn Nickel Oy; Australian serpentinite (N.S. Wales serp.) from the Great

Serpentinite Belt of New South Wales; serpentinite from the Varena region of Southeast Lithuania

(Lithuania serp.); serpentinites from Bragança, northeast Portugal (Bragança serp, Donai serp., 7

Fontes serp.); olivine from Åheim (Åheim olivine), Norway and olivine minerals from Finland

(labeled Satakunta olivine, Vammala-1, Vammala-2, Suomusjärvi-1 and Suomusjärvi-2). Details of

composition of these minerals can be found in the Appendix section (Table 1A). The composition

of the rocks were in most cases determined from the results of a combination of two of the

following analytical tools: X-ray fluorescence (XRF), X-Ray diffraction (XRD) or inductively

coupled plasma optical emission spectrometry (ICP-OES). Aside the high Mg content, most of the

minerals also contain significant amounts of iron compounds and silica (SiO2). The form in which

iron (FeO, Fe2O3 or Fe3O4?) exists in these rocks is yet to be completely ascertained. For example,

there is conflicting information on the form that iron appears in the Finnish serpentinite rock used.

Teir et al.[13] reported an XRD analysis which shows that iron is present in serpentinite as

magnetite (Fe3O4), constituting 14 wt.% of this serpentinite. On the other hand, Rinne [14] reported

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an XRD analysis showing that a combination of FeO and Fe2O3 compounds (which of course could

be summed up to be Fe3O4) is present in the same rock sample.

2.2. Method for producing Mg(OH)2 from Mg-silicate minerals

This section describes the method for producing Mg(OH)2 from Mg-silicate minerals, a procedure

that has been previously reported in literature[6, 15, 16]. The process route of producing Mg(OH)2

involves a staged, closed loop process of Mg extraction using recoverable AS salt. The process

schematic is presented in Fig.1.

Figure 1. Schematic of process route for the production of Mg(OH)2 from Mg-silicate minerals.

After Nduagu et al.[17].

Mg is extracted from the reaction of Mg-silicate rocks and AS salt at 270-550 oC (depending on the

rock type) in an oven/reactor. Information on the ranges of reaction parameters tested is presented

in Table 1. The reaction in the oven produces MgSO4, SiO2, water vapor and recoverable gaseous

NH3. For details of the reactions and thermodynamics of the (Mg/Fe/Ca) extraction, refer to the

Appendix section (Table A2 and Fig. A1). Mg/Fe/Ca sulfates obtained from the extraction reaction

are leached in water at room temperature and pressure conditions. The elemental amounts of Mg, Fe

and Ca and other metals extracted were determined by ICP-OES analysis.

Increasing the pH of (Mg/Fe/Ca) sulfates-rich solution (using the recovered ammonia) results in the

precipitation of hydroxides or oxy-hydroxides. Of major interest are Fe, which is precipitated as

goethite (FeOOH) and Mg, precipitated as Mg(OH)2[16]. At pH ranges of 8–9 and 11–12 Fe and

Mg respectively precipitate out of the solution. FeOOH by-product could be a useful raw material in

the iron- and steelmaking industry. Due to the high concentration of iron compounds in these

minerals (in different oxide forms), iron oxide by-product stream may be a useful raw material for

the iron- and steel-making industry[17-19]. In Finland, for instance, the iron and steel sector is the

largest point-source CO2 producer. Thus, integrating steel industry’s CO2 emissions with

mineralization is crucial and would result in emissions reduction, and in the replacement of raw

materials (iron ore) using the iron oxide by-products. However, we have shown earlier[17] that the

processing of Fe together with Mg in a CO2 sequestration process comes with a significant energy

penalty. The results showed that the contribution of iron to the energy requirement of CO2

sequestration increases by 70%, 30% and 16% points for rocks containing Fe as Fe3O4, Fe2O3 and

FeO compounds respectively as compared to an iron-free rock.

After filtering precipitated Fe/Mg/Ca (oxy)hydroxides from the solution, AS salt is then recovered

via crystallization. The following crystallization techniques may suffice: evaporative, mechanical

vapor recompression (MVR) or anti-solvents (especially alcohols)[17]. This study focuses on MVR.

The Mg(OH)2 thus produced from Mg-silicate mineral rocks is then used to sequester CO2 in the

form of thermodynamically stable magnesium carbonates.

3. Parametric evaluation by 2n-1

experimental design This section studied the extent to which the reaction parameters affect Mg extraction, and in

extension their effects on Mg(OH)2 production. These parameters include elemental Mg to Fe ratio

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(Mg/Fe) of the mineral rock, Mg-silicate to AS mass ratio (S/AS), reaction temperature (T), time (t),

and the interaction of these effects.

It is important to point out the nature of the test data (statistical details are presented in Table 1).

The initial batch of tests were done using mostly Finnish serpentinite between 2008 and 2009, and

were reported in [15] and [16]. The aim at that time was to prove that Mg(OH)2 can be produced

from Finnish serpentinite, and efficiently too. After this, efforts were channeled towards applying

the method to different Mg-silicate rocks from worldwide locations[6, 20-22].

Table 1. Statistical overview of the parameter values tested in 82 experiments.

Parameters Minimum Maximum Median Average Standard Deviation

Mg/Fe (kg/kg) 0.31 5.90 2.16 2.81 1.32

S/AS (kg/kg) 0.40 4.0 0.67 0.85 0.6

T (oC) 270 550 475 457 63

t (min) 10 120 22 32 27

Clearly, earlier tests did not focus on identifying reaction trends as experiments were performed at

varying reaction conditions chosen almost at random - targeting to cover a broad range of each

parameter. However, after testing a range of values of each of the factors, it now becomes necessary

to identify which parameters have the most significant effects on Mg extraction and Mg(OH)2

production. More so, parameter cross-correlation effects would be determined as well. A better

understanding of these effects and their interactions is essential for optimization of Mg(OH)2

production from Mg-silicate minerals for the purposes of fixing CO2 as carbonate(s).

Due to the range of values parameters considered (Table 1) the choice of a reasonable reference

point was important in order to design a two-level fractional factorial design. We used a reference

level “0” condition to classify each factor according to levels: high (+1) or low (-1) (in Table 2).

The first “0” level was chosen to reflect the median value of the parameters while a second “0”

level was chosen at values of the parameters at near optimal conditions. The response parameter

(dependent variable) in this analysis is % Mg extraction (% Mg ext). The % Mg extraction is the

amount of Mg (grams) extracted from the Mg-silicate rock divided by the total amount of Mg

(grams) present in the Mg-silicate, expressed as percentage. The motivation for focusing on the

parametric analysis of Mg extraction is the fact that the amount of Mg(OH)2 produced from the

process strongly correlates with values for Mg extraction[16].

Table 2. Reference level “0” conditions for evaluation of factors and their interactions.

Parameters Mg/Fe (kg/kg)

A

S/AS (kg/kg)

B

T (oC)

C

t (min)

D

Levels high

(+1)

low

(-1)

high

(+1)

low

(-1)

high

(+1)

low

(-1)

high

(+1)

low

(-1)

Condition Iᵝ > 2.16 ≤ 2.16 ≤ 1 > 1 ≥ 480 < 480 > 25 ≤ 25

Condition IIᵝ > 2.16 ≤ 2.16 ≤ 0.67 > 0.67 ≥ 440 < 440 > 60 ≤ 60

ᵝ Condition I reflects the median of the data. ᵝ Condition II is chosen at near optimal

experimental conditions. “+1” and “-1” are the high and low levels respectively.

3.1 Fractional factorial design

Fractional factorial design (2n-1

, where n represents the number of parameters) enables the analysis

of only a subset of treatment combinations, while still obtaining a meaningful result that is

statistically representative of the entire data set. In this analysis n=4 (A, B, C and D in Table 3) and

the objective function is Y which represents % Mg extraction. The fractional factorial design is

constructed by partitioning the runs into two blocks; one block, which is a contrast of the other, is

completely sacrificed [23]. Instead of using a full 2n design with 16 design points, the 2

n-1 design

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with only 8 design points was chosen at points ABCD=I (1, ab, ac, ad, bc, bd, ad and abcd). Design

points having ABCD=-I (a, b, c, d, abc, abd and bcd) which are considered as complementary to the

points with ABCD=I were excluded in the 2n-1

factorial design (as illustrated in Table 3). At this

stage, the third and fourth order interaction effects of the parameters (ABC, ABD, ACD, BCD and

ABCD) were also neglected in order to avoid ambiguity.

Table 3. 24-1

factorial design.

Treatment

Effects and interactions

Observation

Y Mg/Fe

(A)

S/AS

(B)

T

(C)

t

(D=BCD)

AB

(=CD)

AC

(=BD)

AD

(=BC)

ABCD

(=I)

1 − − − − + + + + ---

ab + + − − + − − + ---

ac + − + − − + − + ---

ad + − − + − − + + ---

bc − + + − − − + + ---

bd − + − + − + − + ---

cd − − + + + − − + ---

abcd + + + + + + + + ---

The estimated effect (see (3)) of each design factor is represented mathematically as the average at

the high level (+) of the factor minus the average at the low level (-) of the factor.

Effect=Contrast/(n’2

n-1) (3)

Where n and n’ are the number of factors and replicates respectively, and Contrast is the sum of the

values of each factor at its high level minus the sum of the values of the same factor at its low level.

The significance of any parameter or the interaction of parameters was determined at 95 % (α=0.05)

confidence level. This value is determined by using a student t-test to obtain t-values and assessing

that with the probability (p value) associated with the test statistic. MINITAB®

statistical software

[24] was used to analyze the data from experimental tests using the 2n-1

(and a 2-level) factorial

design.

3.2. Mg extraction: parametric effects and interactions

3.2.1 Effect of Mg/Fe ratio of rock types

Thirteen different Mg-silicate minerals (nine serpentinite and four olivine rocks) were studied in a

total of eighty-four tests performed at varying reaction conditions. The results showed a huge

difference in reactivity of serpentinites and olivines using the method applied in this study. Based

on maximum extraction values obtained so far for each rock type, serpentinite is about 5x as

reactive as olivine (see Fig.2). This confirms previous results for two olivine-containing rocks (from

Åheim, Norway and Satakunta, Finland) samples tested and found not to be suitable for Mg

extraction[6]. It was observed that the olivine rocks tested had a harder texture, smaller internal

Brunauer, Emmett and Teller (BET) surface area as well as pore volume. The range of % Mg

extraction between the maximum (Max.) and minimum (Min.) in Fig.2 is due to results obtained at

wide range of reaction conditions.

At varying reaction conditions the factors and interactions that have a significant effect on the

extent of Mg extraction were obtained (see Table 4). While keeping constant some parameters and

varying others, the effects of changing levels of each parameter was determined. This sensitivity

analysis was performed in order to determine if the parameters are important or not. If any

parameter was found to contribute significantly to Mg extraction, it was important to determine the

levels to which that factor is significant.

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Figure 2. Effect of Mg/Fe ratio of the Mg-silicate rocks on % Mg extraction. The figure on the left

shows results from both serpentinite and olivine rocks while the one on the right is for only

serpentinite rocks.

Table 4. Sensitivity analysis for the factors affecting Mg extraction by varying the conditions.

# Selected conditions

Factors Interactions

Mg/Fe (g/g)

A

S/AS (g/g)

B

T (oC)

C

T (min)

D

AB

AC

AD

R2

1 A>2.16 g/g, B≤1 g/g,

C>480 oC, D>60 min

√ 27%

2 A>2.16 g/g, B≤1 g/g,

C>440 oC, D>60 min

√ 31%

3 A>2.16 g/g, B≤0.67 g/g,

C>480 oC, D>60 min

22%

4 A>2.16 g/g, B≤0.67 g/g,

C>440 oC, D>60 min

25%

5 A>2.16 g/g, B≤1 g/g,

C>480oC, D>25 min

√ √β √ 47%

6 A>2.16 g/g, B≤1 g/g,

C>440oC, D>25 min

√ √ √ 52%

7 A>2.16 g/g, B≤0.67 g/g,

C>480oC, D>25 min

√ √ √ 45%

8 A>2.16 g/g, B≤0.67 g/g,

C>440oC, D>25 min

√ √ √ 50%

√ and √β represent positive and negative effect of the factors/interactions respectively. R

2 is the

regression coefficient obtained for each condition.

It is obvious from Fig. 2 that serpentinite rocks with a Mg/Fe ratio ≥ 2.16 show an exceptionally

(>2x) higher % Mg extraction than others. This was the reason why Mg/Fe ratio level was

benchmarked at > 2.16 (Table 4) in the sensitivity analysis. Given the information deductible from

Fig. 2, it was more interesting to understand the effects and interaction effects of the more reactive

minerals with Mg/Fe ratio > 2.16.

Our goal is to obtain a process condition that allows us to design a Mg extraction reactor that

operates at optimal reaction conditions. The reactor should be able to process different types of

serpentinite minerals with varying Mg/Fe ratios, minimal amounts of AS salt reagent (slightly less

than 1 g/g), temperatures < 440 oC and reaction time ≤ 60 min. The combination of parameters in

Table 4 which mostly suits this goal is condition 6 which also has the highest regression coefficient

(R2=52%). It is arguable that the R

2 value obtained is not sufficient enough to describe a process;

however, it is not surprising that a system as complicated as the one simulated here would give a

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comparatively low R2. The results reported here contain tests performed on the reaction of solids

(solid state reaction) with multivariate parameters. Solid state reactions are less predictable than

those involving other states/phases. We assume that not all the factors contributing to Mg extraction

have been identified and studied. Some other factors like particle size difference between the

reacting Mg-silicate mineral and AS salt, heat and mass transfer, geometry and size of the reactants

and their containers may affect solid/solid reactions. These are the main subjects of ongoing

investigation as we embark on the next stage - pilot scale development.

3.2.2 Effect of amount of reagents (S/AS ratio)

By varying S/AS ratio of the tests between ≤0.67 g/g and ≤1 g/g, its effect on the extent of Mg

extraction was evaluated. For conditions 5-8 (Table 4), at 95 % (α = 0.05) significance level, S/AS

ratio has a significant positive effect on Mg extraction. The results obtained show that an increase in

S/AS salt ratio above both the level of 0.67 g/g or 1 g/g does not significantly affect Mg extraction

beyond a reaction time of 60 min (see conditions 1 to 4 in Table). In other words, a change in the

amount of AS salt reagent levels is more important when the reaction time is less than 60 min.

Increasing S/AS salt from low (-1) to high (+1) levels results in a 10% point increase in Mg

extraction. The effects of S/AS ratio, those of the parameters and their interactions can be visualized

from Fig.3 which is plotted for condition 6.

Figure 3.Main effects and interaction for Mg extraction under Condition I

3.2.3 Effect of reaction temperature and time

The effect of reaction temperature is not important under most of the conditions evaluated, but

shows negative dependence on Mg extraction above 480 oC (condition 4 in Table 4).

Figure 4. Effect of temperature (left) and time (right) on Mg extraction

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Figure 4 shows that an increase in temperature results in a reduction in % Mg extraction is already

at 440 oC (i.e. within the 401-450

oC temperature range). For the selected reaction condition 6, the

effect due to temperature is almost flat (see Fig.3). However, reaction time has a significant effect

on magnesium extraction; an increase in reaction time from low (-1) to high (+1) levels leads to a

15 percent points’ increase in magnesium extraction. More so, reaction time significantly affects

Mg extraction at all the conditions modeled except when t > 60 min and S/AS ≤0.67 g/g (conditions

3 and 4). Besides, this effect of reaction time on Mg ext seems not straightforward from Fig.4; more

investigation is needed.

3.2.4 Interaction effects

Under the conditions modeled, the interaction effects of Mg/Fe-S/AS ratios and T-t are significant at

95 % significance level. The interaction effects presented in Fig.3 show that increasing the reaction

time from high (+1) to low (-1) (above 25 min) levels significantly increases (by 30 % point) the

value for Mg extraction if the reaction temperature are kept below 480 o

C. Above this temperature,

no increase in Mg extraction is possible, presumably due to thermal decomposition of AS above at

high temperatures leading to the formation of sulfur trioxide gas, which could alter the

thermodynamics [16]. On the other hand, increasing S/AS ratio levels from high (+1) to low (-1) (≤1

g/g) at both high (+1) and low (-1) levels Mg/Fe leads to a significant increase in % Mg ext. But, the

% Mg ext values obtained with high (+1) level of Mg/Fe (>2.16 g/g) are higher. This confirms

previous results which showed that rocks with high Mg/Fe ratios respond better to Mg extraction

than those with low Mg/Fe ratios[6].

4. Process evaluation using exergy and pinch analysis

4.1 Process simulation

The Mg(OH)2 production, AS recovery and Mg(OH)2 carbonation were modeled using Aspen

Plus® software. The process flow diagram is presented in Fig.5. Pinch analysis was done using

Aspen Energy Analyzer®.

4.1.1 Mg, Fe and Ca extraction

The base property method used for this simulation is the ELECTRTL method. The solid state

reaction of serpentinite and AS salt was simulated using a stoichiometric reactor (REACTOR) with

the extraction equations and thermodynamics specified as presented in the Appendix section ((R1),

(R3) and (R5) in Table A2). The serpentinite feed has its composition simulated after the Finnish

serpentinite which contains ~83 %-wt Mg3Si2O5(OH)4, ~14 %-wt Fe2O3 and ~1 %-wt CaSiO3. The

AS feed (AS-1) is a product from the MVR section, where AS salt is crystallized. The specified

conversion of this reactor is 100% – meaning that serpentine and AS feed react completely to form

products. This assumption is based on the best case scenario of the extraction reaction which is the

aim of an ongoing optimization study. However, all the scenarios have previously been explored

using life cycle analysis (LCA)[25].

4.1.2 Dissolution of extraction products

The product stream from the reactor (PRDTS) was separated in a solid/gas separator (SEP-1) into a

solid stream (SOLIDS-1) and a gas stream (GASES) before cooling. The dissolution of the solid

products was modeled using a stoichiometric reactor (CONVTR) and an RGibbs reactor

(DISSOLUT) respectively. At the CONVTR the solid compounds were converted to aqueous

compounds before dissociating into anions and cations at the DISSOLUT. The DISSOLUT

simulated the dissolution reactions of MgSO4, FeSO4, Fe2(SO4)3 and CaSO4 in water streams at 40

°C by calculating both the phase and chemical equilibrium based on Gibbs free energy

minimization. The water stream (DISS-H2O) used for dissolution is made up of the following: a

recycled water stream (MVR-H2O) from the MVR section, a water stream (WATER) recovered from

the separation of the GASES stream into H2O and NH3 gas. After dissolution, the mixture is

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separated by filtration into a solid stream (SIO2), containing mainly silica and a liquid stream (DIS-

PRDT) of mainly Fe- and Mg-sulfate compounds.

4.1.3 Precipitation of FeOOH and Mg(OH)2

The stoichiometric reactors, PREP-1 and PREP-2 were used for precipitation of FeOOH and

Mg(OH)2 respectively, and the following reactions and thermodynamics (4) - (5) specified:

Table 5. Chemical reactions and thermodynamics of the precipitation stage

# Precipitation reactions ∆Hr (T=313K)

4 Fe2(SO4)3(s)+6NH3(g)+4H2O(l)→2FeOOH(s)+3(NH4)2SO4(aq) -720 kJ/mol Fe

5 MgSO4(s)+2NH3(g)+2H2O(l) →Mg(OH)2(s)+(NH4)2SO4(aq) -85 kJ/mol Mg

The pH of Fe- and Mg-rich solution stream (DIS-PRDT) was increased (using the recovered NH3

gas from the flash separator SEP-4) in stages of 8–9 and 11–12 to precipitate hydroxides of iron and

magnesium respectively. Ca(OH)2 precipitates together with Fe in the first precipitation stage.

Aqueous AS is formed at both precipitation stages (see (4) and (5)). Products of the precipitation

stages, FeOOH and Mg(OH)2 were separated by filtration while aqueous AS passes through a

converter (CONVTR-2) to the MVR section for crystallization before it is recycled. The role of the

CONVTR-2 was to combine anions and cations in stoichiometric amounts into aqueous compounds.

The application of the MVR crystallization method to this process has been reported earlier [17, 26];

however, this paper revisits the MVR crystallization application in the pinch analysis section

(section 4.2).

4.1.4 Mg(OH)2 carbonation

The reaction of CO2 and Mg(OH)2 is exothermic, and at suitable conditions forms

thermodynamically stable MgCO3 and superheated steam. Sequestration of CO2 using the gas/solid

route as being developed at ÅA [7, 8, 27] provides utilizable energy at high temperatures (480- 550 oC, ∆H ~ -59.5 kJ/mol Mg) and pressure conditions. Pressures can vary from 20 bars to 80 bars

depending on the concentration of CO2 – pure and concentrated or in flue gas stream [9, 10] . For

simulation purposes, it was assumed that the sequestration plant stores 1 ton/h CO2 (~ 8000 t/y).

As shown in Figs. 5 and 6, at high carbonation conversion (> 90%) the exothermic heat of

carbonation makes the temperature of outlet stream of the reactor hotter than those of the inlet

streams (according to (6)). This energy is at the same time sufficient enough to heat up the reactants

(Mg(OH)2 and CO2) and as well provide energy to the process (heat or power depending on what it

is designed to achieve). The carbonation section in Fig. 5 produces both heat and power while that

of Fig.6 produces only heat.

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Figure 5. Process flow diagram of Mg(OH)2 production and carbonation simulated using ASPEN PLUS software.

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Figure 6. Mg(OH)2 carbonation flow sheet producing utilizable heat.

, (T2> T1x, T1y) (6)

where z - MgCO3, s - H2O, x - Mg(OH)2, y - CO2, - heat of formation, ṅ - molar amount of

compound, Cp - specific heat capacity, T1- inlet temperature and T2 - outlet temperature. In this

case, the molar amounts associated with each term on the left side of (6) cancel out (since they are

equal). If the inlet temperatures of the reactants are same (i.e. T1x=T1y=T1), (6) reduces to (7).

, (T2> T1) (7)

It was assumed that CO2 was delivered at 20oC, 20 bars from stream CO2-IN. The CO2 pressure of

this stream looks optimistic; however, this value was based upon the fact that the CO2 capture and

purification unit would be located nearby the CO2 sequestration site. This in essence provides

compressor power savings that are required for CO2 compression to pipeline transport pressures of

~ 150 bars. The Mg(OH)2 product separated by filtration in SEP-3 was dried by heating to 150 oC

before entering the carbonation reactor (CARBONAT). The CO2 stream (CO2-IN) was heated to 520 oC before entering the CARBONAT. The exothermic nature of carbonation reaction led to a higher

temperature of the products than that of the reactants. This was beneficial since power and heat

were intended to be extracted from the steam and MgCO3 products using a turbine (TURBINE) and

heat exchangers respectively. More importantly, given the resulting temperature difference, the

outlet streams of the reactor can then be used to heat up the inlet streams.

4.2 Pinch analysis

Pinch analysis has become a useful energy targeting and design tool for thermal and chemical

processes and utilities[28]. This method enables the plotting of composite and grand composite

curves using temperature versus enthalpy axes[29]. These curves provide an insight on the process

heat availability and requirements.

These basic rules were followed while applying the pinch analysis[28]:

1. Separate the system into two independent sections – above and below the pinch, and do not

transfer heat across the pinch.

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2. Only cold utility is needed below the pinch. Heating of streams at the section below the

pinch incurs a heat penalty.

3. Only hot utility is needed above the pinch. Cooling of streams at the section above the pinch

incurs an energy penalty.

Table 6. Heating and cooling requirements of the process implemented using Pinch Analysis

Streams

Inlet T Outlet T Flow rate Enthalpy mCP oC

oC kg/h MJ/h MJ/

oC-h

Cold Streams

SERP-1 25 400 2529 909 2.4

AS-AQ1 Stream 1 40 107 10926 8413 125.6

Stream 2 107 115 10926 9381 1173

CO2-IN 20 520 1000 530 1.06

MG 40 150 1325 220 2

Hot streams

SOLIDS-1 400 40 4703 1603 4.45

GASES 400 25 1863 3559 9.49

MGCO3 532 40 1916 1160 2.36

STEAM - UT 246 40 520 1146 5.56

Figure 7. Working cycle (B) and Aspen process flow sheets (A&B) of mechanical vapor

recompression (MVR) crystallization of AS salt. Modified after Nduagu et. al[17].

More rules were applied during designing of an optimal heat exchanger network as implemented

using the Aspen Plus model in Fig.5. These include:

1. For pinch matches, above the pinch the CPcold ≥ CPHot while below the pinch CPHot ≥ CPcold. CP

is a value calculated by dividing the enthalpy of a stream by the difference in temperatures of the

outlet and the inlet streams (see Table 6).

245-13

2. ∆T of 10 oC was the set minimum temperature difference.

3. Solid streams were not matched with solid stream as solid/solid heat exchange may be

problematic.

Fig 8. Hot and cold composite curves of the process shown in red and blue colors respectively

Two scenarios, with and without the MVR crystallization were compared. In the case of without

MRV, the AS-water stream (AS-AQ1 in Fig. 5, Fig.7A) was heated step-wisely, 40 oC 76

oC

115 oC. At 115

oC, all the water in the stream was evaporated with virtually no heat recovery. In

Fig. 7A the MVR was simulated with two crystallization vessels, allowing for an operation in two

different temperature regimes (107 and 115 oC) while Fig.7C used only one crystallizer. However,

compressing the water vapor stream from 1 to 2 bars (points 12 in Fig. 7) increases the enthalpy

as well as the temperature of the stream to a level it can transfer heat to saturated water at 100 oC.

The temperature-enthalpy plot (composite curve) of the process was first plotted (in Fig. 8A),

assuming that a complete evaporative crystallization of the AS salt was carried out. The upper and

lower pinch points are 40 oC and 50

oC. In this case, the latent heat added to evaporate water from

AS-water mixture would be lost as low grade heat. Most part of that heat is represented in Fig.8A as

QLv (~ 9.4 GJ/h). QH is the value of other low temperature heating (sensible heat) required. The

thick black arrow in the Fig. 8A pointing towards the cold composite curve (in blue color) gives an

insight to the temperature and enthalpy values of the hot utility required. The gap between the hot

and cold steams needs to be closed in order to optimize heat recovery. In order to achieve this, the

temperature and the enthalpy of the hot stream must be such that allows for a heat transfer to the

cold stream (saturated water at about 100 oC) while maintaining a minimum ∆T of 10

oC. Applying

MVR closes the gap by compressing low grade steam, consequently increasing its enthalpy and

temperature and forming superheated steam. The heat from the superheated steam is then used to

produce more saturated steam. This modification changes the pinch point from 40 - 50 oC to 400 -

410 o

C (Fig. 8) and reduces the hot utility requirements from 12290 MJ to 93 MJ. In achieving this,

however, a power penalty of 330 kWh/t-CO2 is incurred.

4.3 Exergy analysis

Exergy analysis was used to evaluate the process modeled based on the results from heat exchanger

network implemented through pinch analysis. At any specified surroundings temperature (here T0 =

15°C = 288 K), using exergy provides a standard basis for calculating the amount of valuable

energy[30] that can be extracted from a heat stream and comparing heat with power input

requirement P, for which the exergy Ex(P) = P. For example, ~ 9.1 Gt/t-CO2 heat requirement of the

A B

245-14

extraction process at 400 °C (~ 623 K) corresponds to an exergy equal to Ex(Q) = (1-T/T0)·Q = 9.1

– 3.9 GJ/t-CO2. (T/T0)·Q is the exergy destruction, ED.

Table 7. Energy (Q), exergy destruction (ED) and requirement (EQ) of the process in GJ/t-CO2

Q ED EQ

Mg(OH)2 production

Kiln 9.09 3.89 5.20

DISS 0.48 0.46 0.02

PREP1 1.10 1.06 0.04

PREP 2 -10.5 -9.70 -0.84

MVR Compressor 1.18

1.18

Sep-4 -0.89 -0.65 -0.24

Heat exchangers -2.40 -1.73 -0.67

Total -1.99 -6.67 4.68#

Mg(OH)2 Carbonation

Turbine -0.24

-0.24

Heat exchangers -1.78 -1.22 -0.55

Total -2.02 -1.22 -0.79

Net

3.88#

#These values are lowered by ~0.45 GJ/t-CO2 if the Fe form in mineral is assumed to be FeO

instead of the Fe2O3 used here.

The exergy destruction of a system, which is the measure of the amount by which the value of the

resource is consumed or degraded, is shown as (8) while the exergy flow is presented in (9);

(8)

(9)

where (S-S0) is the entropy change, T0 is the ambient temperature and (H-H0) the enthalpy change.

The results obtained here are compared with the results of a previous model [17] where no pinch

analysis was done.

The application of pinch analysis and the heat exchanger network as implemented in the Aspen Plus

model (Fig.5) resulted in a ~ 0.5 Gt/t-CO2 (~ 10% points) reduction in the exergy requirement of

producing Mg(OH)2. Mg(OH)2 carbonation unit provides ~ 17% points energy offset to the process.

When the Mg(OH)2 production and carbonation units are integrated, the process requirements of the

process becomes 3.88 Gt/t-CO2. This value becomes 3.4 GJ/t-CO2 (reducing by another ~0.5 GJ/t-

CO2) if the compound form of Fe in mineral is assumed to be FeO instead of the Fe2O3 used here.

5. Conclusions This paper investigated the influence that reaction parameters has on the production of Mg(OH)2 by

analyzing the effects of Mg/Fe ratio, S/AS ratio, T and t on Mg extraction. Once produced Mg(OH)2

could be used to sequester carbon by direct reaction with flue gases or CO2 derived from power or

chemical plants. Notable among the results presented in this paper is the fact that olivine rocks are

5x less as reactive as their serpentinite counterparts. It was also obvious that serpentinite rocks with

Mg/Fe < 2.16 were less (>2x) reactive than others. This validates previous results which showed

that an increase in Mg/Fe ratio increases Mg extraction. Reaction time has a significant effect on

magnesium extraction as an increase in t above 25 minutes results in a 15 percent points’ increase in

Mg extraction, but this effect tends to diminish after 60 min. On the other hand, Mg extraction

shows a negative dependence on reaction temperature; T > 440 oC do not favor Mg extraction. This

245-15

appears to be due to thermal decomposition of ammonium sulfate leading to the formation of sulfur

oxide(s), which could alter the thermodynamics of the extraction reactions.

The application of pinch analysis and the heat exchanger network as implemented in an Aspen Plus

model resulted in a ~ 0.5 Gt/t-CO2 (~ 10% points) reduction in the exergy requirement of producing

Mg(OH)2. When the Mg(OH)2 production and carbonation units are integrated the process

requirements of the process becomes 3.88 Gt/t-CO2. Carbonating Mg(OH)2 in the carbonation unit

provides a ~17% points energy offset to the entire process. The overall energy requirement of the

process reduces by another ~0.5 GJ/t-CO2 if the compound form of Fe in mineral is assumed to be

FeO instead of the Fe2O3 used here.

Acknowledgements This work was supported by the Academy of Finland program “Sustainable Energy” (2008-2011).

Further support came from KH Renlund Foundation (2007-2009). Financial support from Åbo

Akademi University’s Graduate School for Chemical Engineering (GSCE) is also acknowledged.

Appendix

Apendix A - Tables

Table A1 . Composition of magnesium silicate minerals tested.

Elemental composition (%-wt) Mg/Fe

Rock type and locations Mg Fe Si Ca Al (kg/kg)

Serpentinite rocks

N.S. Wales serp. (Aus) 23.0 4.80 19.5 0.00 0.50 4.8

Donia serp. (Portugal) 22.0 5.01 19.4 0.18 0.88 4.4

7 Fontes serp. (Portugal) 23.3 5.77 19.5 0.09 1.02 4.0

Bragança serp. (Portugal) 21.6 5.70 19.6 0.00 0.60 3.8

Finnish serp. (Fin) 21.8 10.1 11.6 0.30 0.00 2.2

Lithuania serp. (Lit) 18.9 12.3 15.9 0.90 0.10 1.5

Olivine rocks

Åheim olivine (Nor) 29.6 5.00 19.5 0.10 2.80 5.9

Suomusjärvi-2 (Fin) 12.60 8.32 20.71 5.93 3.71 1.5

Vammala-2 (Fin) 16.88 12.87 18.37 1.00 0.69 1.3

Vammala-1 (Fin) 11.58 10.77 21.03 6.43 1.85 1.1

Suomusjärvi-1(Fin) 8.14 7.62 23.46 5.57 5.72 1.1

Satakunta olivine (Fin) 3.30 10.7 21.9 6.30 8.50 0.3

Reactions (A1)-(A5) represent the thermodynamics of reactions involving Mg3Si2O5(OH)4, Fe- and

Ca-based compounds; iron could be found as FeO/Fe2O3/Fe3O4 and calcium is present as

CaSiO3[16, 17]. It can be seen from the thermodynamic compositions of possible products of the

reactions (see also Fig.B1) that MgSO4 is the dominant solid product of the reaction.

245-16

Table A2. Extraction equations and thermodynamics

# Extraction reactions T (K)

∆G <0

∆Hr

T=873 K

(A1)

472 222

kJ/mol Mg

(A2)

447 167

kJ/mol Fe

(A3)

622 360

kJ/mol Fe

(A4)

614

818

kJ/mol Fe

(A5)

318 116

kJ/mol Ca

Appendix B – Figures

Figure B1. Thermodynamic compositions of the reaction of Finnish serpentinite rock and AS

salt[16].

Nomenclature ÅA Åbo Akademi University

AS Ammonium sulfate salt

E Exergy, J

FB Fluidized bed

G Gibbs free energy, J/mol

GJ Gigajoule

GJ/t-CO2 Gigajoule per ton CO2

H Enthalpy, J/mol

kWh/t-CO2 kilowatt hour per ton CO2

Mg/Fe Elemental Mg to Fe ratio of Mg-silicate rock

245-17

Mg-silicate Magnesium silicate mineral.

MVR Mechanical vapor recompression

P Pressure, atm

Q Heat, J

R&D Research and development

T Temperature, K

t time

S Magnesium silicate mineral

S/AS Mg-silicate to ammonium sulfate ratio

∆S Change in entropy, J/mol-K

W Work, J/s

Greek symbols

Δ difference

∑ sum

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