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Jul 26, 2012 - CaC. CO. 2. (1). Although research on the reaction of coke and CaO can be found in the literatures, some of the reports are inconsisten...
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CaC2 Production from Pulverized Coke and CaO at Low TemperaturesReaction Mechanisms Guodong Li, Qingya Liu, and Zhenyu Liu* State Key Laboratory of Chemical Resource Engineering, Beijing University of Chemical Technology, Beijing 100029, People's Republic of China ABSTRACT: CaC2 production with pulverized feeds was proven viable at temperatures about 400 °C lower than that required by the current electric arc technology (2100−2200 °C), which led to development of an autothermal process. For the purpose of reactor design, the reaction mechanism of coke (C) with CaO at temperatures up to 1750 °C is studied in a thermogravimetric analyzer (TGA) coupled with a mass spectrometer (MS). Results indicate that the reaction of C with CaO generally proceeds in three stages, starting at 1460 °C for CaC2 formation, followed by reaction of CaC2 with CaO and by decomposition of CaC2. The second stage is observable only at the complete consumption of C while the third is controlled by surface evaporation of Ca. Reaction behavior at the surface of coke and CaO particles is proposed, including the three-stage reactions and the role of CaC2−CaO eutectics, that promotes CaO diffusion and CaC2 production.

1. INTRODUCTION Calcium carbide (CaC2) is an important coal-derived platform chemical for the production of many commodity chemicals, such as vinyl chloride, vinylacetic acid, and 1, 4-butyl glycol. With an annual production of more than 15 million tons in 2009, CaC2 is produced entirely in industry via reaction of coke with CaO, Reaction 1, in a moving-bed reactor with electric arc heating, as done in the past century. This technology requires large granular feeds (generally 5− 30 mm in size) to allow unrestricted release of the side-product CO and consequently high reaction temperatures (about 2200 °C) and long reaction time (1−2 h) due to the poor contact between the solid reactants.1,2 The use of electric arc heating consumes a large amount of electricity (∼4000 kW·h per ton of pure CaC22), complicates reactor design, and limits the reactor size to less than 70 kilotons per year. All these made the technology high-energy intensity and high cost. Research has been carried out worldwide in the past decades to develop new CaC2 production technologies of high thermal efficiency and large reactor throughput,2−4 including those employing autothermal heating via oxygen-combustion of coke. Most of the research, however, was on laboratory scales, with a few aborted at pilot scale. The major obstacle may be attributed to the use of large size feeds. Recently, we reported that CaC2 can be produced at lower temperatures using fine feeds, 1750 °C and less than 0.1 mm in particle size, for example, and hence, we proposed a novel autothermal method for its production.5,6 However, detailed information on the reaction and other reactions associated with it is not clear, which hampers development of the reactor. 3C + CaO → CaC2 + CO

(2)

Ca + 2C → CaC2

(3)

Wang et al. studied the reaction of coke and CaO with particle sizes smaller than 0.147 mm using a thermogravimetric analyzer (TGA) coupled with a nondispersive infrared radiation.10 They reported the occurrence of Reaction 2 in a temperature range of 950−1450 °C, which was lower than that reported by Tagawa and Sugawara,7 but they concluded that CaC2 was formed directly through Reaction 1 at temperatures higher than 1450 °C; that is, CaC2 formation is independent of Reaction 2. Rai et al. studied pressure change during reaction of fine graphite and CaO at C/ CaO ratios of 0.3−6.0 in a temperature range of 1150−1250 °C, and they reported formation of CO and Ca vapor without formation of CaC2.11 However, Knapp and Ruschewitz found that the Ca element may quickly react with C to form CaC2 at 800 °C,12 which is obviously contradictive with the results of Wang et al.10 and Rai et al.11 Mu and Hard studied the reaction behavior of fine Ca(OH)2 with various fine carbon sources at temperatures up to 1870 °C in a TGA system.4 Two mass loss DTG peaks were observed at a C/CaO molar ratio of about 2 when the carbon source was a bituminous coal. The first peak started at 1400 °C and ended in 30 min at 1800 °C, where the second peak appeared and was maintained for an additional 30 min. However, a single DTG peak starting at 1350 °C was observed at a C/CaO ratio of 4 when the carbon source was a petroleum coke. The authors attributed all these peaks to the formation of CaC2, presumably through Reaction 1 without further analysis. We recently studied the reaction of CaO with various carbon sources in a TGA coupled with a mass spectrometer (MS).5 We assumed occurrence of Reaction 1 and found the reaction being

(1)

Although research on the reaction of coke and CaO can be found in the literatures, some of the reports are inconsistent and even contradictory. Tagawa and Sugawara reported that Reaction 1 proceeded via a two-step mechanism (Reactions 2 and 3) in a temperature range of 1600−1800 °C.7 However, thermodynamic calculation reported by Yu et al. showed that Reaction 2 is unfavorable at temperatures below 2000 °C.8 The two-step mechanism was also questioned by other researchers.9 © 2012 American Chemical Society

CaO + C → Ca + CO

Received: Revised: Accepted: Published: 10742

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2.3. Analysis of the Product. Ca and CaC2 in the solid products were qualitatively analyzed using MS by detecting the gaseous product(s) generated upon contacting the solid with water at ambient temperature. Formation of H2 indicates the presence of Ca in the solid while the formation of C2H2 indicates the presence of CaC2.

controlled by mass transfer of CaO toward the carbon sources. The initial reaction temperature decreased with a decrease in particle size, and the state of the product was related to the C/CaO ratio in the starting materials. Besides the research on the CaC2 formation, CaC2 was reported to be unstable at temperatures of its production. It may react with CaO to generate Ca and CO via Reaction 49,13 or decompose into Ca and C via Reaction 5.7 CaC2 + 2CaO → 3Ca + 2CO

(4)

CaC2 → Ca + 2C

(5)

3. RESULTS AND DISCUSSION 3.1. Reaction Behaviors of Coke and CaO. The reaction behaviors of coke and CaO are analyzed based on the TG/ DTG and CO release data. The results shown in Figure 1,

It is clear that the temperature needed by Reaction 2 and the role of this reaction in the production of CaC2 are in debate, and the reactions involved in the CaC2 production are complex. It may be possible that a few of the earlier findings are not accurate, but most of them are correct and the differences in the reports may be attributed to differences in reaction temperature, the feed C/CaO ratio, and/or carbon source. Hence, systematic studies are needed to reveal the reaction mechanism in the production of CaC2 to facilitate design of novel reactors. In this paper, the reactions of C and CaO are studied quantitatively in a high temperature TGA coupled online with a MS.

2. EXPERIMENTAL SECTION 2.1. Preparation of Raw Materials. Analytical grade CaO was grounded and sieved to 0.125−0.154 mm and then treated at 900 °C for 3 h in N2 to decompose residual Ca(OH)2 and CaCO3 into CaO, which yields a CaO purity of 98.3%. A coal derived coke was subjected to a de-ashing treatment in a HF + HCl solution at 60 °C for 24 h followed by washing with deionized water and then to a thermal treatment in N2 at 900 °C for 3 h. The coke obtained was grounded and sieved to 0.125−0.154 mm. The Proximate and Ultimate analyses indicate that the ash content of the de-ashed coke is 3.68 wt % and the C content is 88.15 wt %. 2.2. Reaction of Coke with CaO. The reaction was carried out in a TGA (Setsys Evolution 24, Setaram) coupled with a MS (Omnistar 200, Balzers). The amount of coke and CaO used are shown in Table 1. The feeds were loaded into a

Figure 1. TG/DTG and MS−CO data during reaction of de-ashed coke and CaO at a C/CaO ratio of 2.2.

obtained at a feed C/CaO ratio of 2.2, as an example, indicate that the reaction starts at about 1460 °C (70.9 min) (defined as the temperature where the mass loss rate reaches 0.25 mg min−1) and then proceeds in three stages. In the first stage, the mass loss is fast and accompanied with fast CO release. In the second stage, the mass loss is slow and accompanied with slow CO release. In the third stage, the mass loss is constant and the slowest among the three stages and accompanied with no CO release. Figure 2 shows the effect of C/CaO ratio on the reaction behaviors. For all the runs, the reaction starts at 1460 °C, as observed by Wang et al.,10 indicating that the feed ratio does not influence the reaction initiation. However, the subsequent reaction behaviors vary with the C/CaO ratio. At C/CaO ratios of less than the stoichiometric ratio 3, i.e. CaO is in excess, three mass loss stages are observed as defined in Figure 1. At a C/CaO ratio of 3.6, i.e. C is in excess, two mass loss stages are observed. The trend is that, with an increase in C/CaO ratio, the mass loss and CO release increase in the first stage but decrease in the second stage; at a C/CaO ratio of 3.6, the second stage disappears, resulting in the observation of only the first and the third stages. If the third stage is ignored due to the small mass loss rate, the number of DTG peaks observed in Figure 2 agrees with that reported by Mu and Hard using different carbon sources at different C/CaO ratios.4 This suggests that the different reaction behaviors reported by Mu

Table 1. Amounts of De-ashed Coke and CaO Used in the Experiments of Different C/CaO Ratiosa

a

C/CaO molar ratio

de-ashed coke (mg)

CaO (mg)

1.8 2.2 2.8 3.6

52.6 64.7 80.1 103.9

121.0 121.2 120.6 121.5

CaO purity is 98.3%, and C content in the coke is 88.15%.

tungsten crucible of 10 mm in inside diameter and 10 mm in height and heated in the TGA in a flow of Ar at 50 mL min−1 from ambient temperature to 1750 °C at a rate of 20 °C min−1 and then maintained at 1750 °C for 60 min. The TG and MS signals were simultaneously recorded by a computer. To eliminate the buoyancy effect on TG data during temperature raise, the weight change of the empty crucible was deducted from the data of the reactions. To eliminate error in the CO data caused by baseline change in the MS signal, the CO signal (m/e = 28) was divided by the Ar signal (m/e = 40) obtained at the same time and the ratio was set to zero at time zero. 10743

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Table 2. Mass Loss and the Amount of CaC2 Formed at Different C/CaO Molar Ratios in the First Stage in Figure 2 mass loss (mg) theoretical C/CaO ratio 1.8 2.2 2.8 3.6

experimentala

CaO + 3C → CaC2 + CO↑b

CaO + C → Ca↑ + CO↑c

CaC2 formed (mg)

36.7 45.2 55.1 60.8

36.1 44.3 54.9 59.7

144.0 144.3 143.5 144.6

82.4 101.3 125.5 136.0

a

Obtained from the TG curve shown in Figure 2. bCalculated with the C amount at C/CaO < 3 and the CaO amount for a C/CaO ratio of 3.6. cCalculated with the CaO amount because C is in excess.

To verify this, the behaviors of CaC2 and CaC2 + CaO are studied under the conditions of Figure 2. The amounts of CaC2 used (a reagent with a purity of 74% and CaO as the main impurity) in these two experiments were 154.5 and 156.8 mg, respectively, and the amount of CaO added was 95.1 mg. The results in Figure 3 indicate that the mass losses in these Figure 2. Effect of C/CaO molar ratio on the reaction behaviors of de-ashed coke and CaO.

and Hard under different conditions resulted from the difference in C/CaO ratio instead of that in the carbon source. It is important to note that the absence of CO release and mass loss at temperatures lower than 1460 °C indicates the absence of either Reaction 1 or Reaction 2 in the temperature range. This is different from the result of occurrence of Reaction 2 at temperatures of 950−1450 °C reported by Wang et al.10 but agrees with the thermodynamic calculation reported by Yu et al.8 3.2. Analysis of Reactions Involved in the Three Stages. To understand the detailed reactions occurring in the three stages, the experimental and theoretical mass losses obtained in each of the stages were analyzed quantitatively. It should be noted that although the XRD and chemical analyses performed in our previous work showed that the solids obtained in the first stage are mainly CaC2, indicating occurrence of Reaction 1,5 the simultaneous mass loss and CO release in the first stage may also result from Reaction 2. Therefore, the theoretical mass losses from these two reactions were calculated, assuming that the mass loss in Reaction 1 is from CO release while that in Reaction 2 is from CO release and Ca evaporation, since the reaction temperatures are higher than the boiling point of Ca (1484 °C). Table 2 shows the mass losses in the first stage at different C/CaO molar ratios. It is clear that the experimental mass loss at each of the C/CaO ratios is very close to the mass loss calculated by Reaction 1 but far smaller than those calculated by Reaction 2. This indicates that the first stage is dominated by Reaction 1, i.e. direct CaC2 formation, with little occurrence of Reaction 2. As a result, the amounts of CaC2 formed in this stage can be estimated by Reaction 1, as shown in the last column in Table 2, which is used for the analysis of the second stage. Since CaC2 is formed in the first stage with yields close to the theoretical prediction and the appearance of the second stage occurs only at C/CaO ratios of less than 3, i.e. with extra amounts of CaO, the reactions in the second stage should happen between CaC2 and CaO as shown in Reaction 4.9

Figure 3. Behaviors of CaC2 and CaC2 + CaO under the conditions of Figure 2: (a) TG; (b) DTG and MS−CO (note: CaC2 is a reagent with 74% purity and CaO as the main impurity).

experiments start at about 1520 °C (74 min) and are accompanied with CO release, indicating initiation of Reaction 4. The mass loss rate (DTG value) increases with an increase in temperature up to 1750 °C (86 min), the maximum temperature of the experiments. With a further increase in reaction time at 1750 °C, the mass loss and CO release in the case of CaC2 are much smaller than that in the case of CaC2 + CaO due to quick exhaustion of residual CaO in CaC2. It is interesting to note that the mass loss rates in the case of CaC2 + CaO 10744

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at 1750 °C are not far from that of the second stage in Figure 2, which may also support Reaction 4 in the second stage. It is likely that both CO and Ca (bp 1484 °C) produced in Reaction 4 are released to the gas phase. Under this assumption the mass losses in the second stage in Figure 2 are estimated theoretically and shown in Table 3. It is clear that the estimated Table 3. Mass Loss and the Amount of CaC2 Consumed at Different C/CaO Molar Ratios in the Second Stage in Figure 2

Figure 4. Appearance of the products from the experiments in Figure 2: (a) C/CaO = 1.8; (b) C/CaO = 2.2; (c) C/CaO = 2.8.

amount of carbon increases with an increase in reaction time in the third stage, 9.3, 24.2, and 44.7 min for C/CaO ratio of 1.8, 2.2 and 2.8, respectively (note: the reaction time of the third stage is obtained from Figure 2). 3.3. Reaction Pathway of CaC2 Formation. The above analyses confirm formation of CaC2 in the first stage, but it is still not clear whether its formation involves the intermediate Ca, as shown in Reaction 3. To understand the mechanism, a series of experiments were performed and shown in Table 5.

mass loss (mg) C/ CaO ratio

CaO at beginning of the second stage (mg)

experimental

1.8 2.2 2.8 3.6

46.8 30.4 8.7 0

69.4 45.9 13.1 0

a

theoreticalb 2CaO + CaC2 → 3Ca↑ + 2CO↑

CaC2 consumed (mg)

72.9 47.3 13.2 0

26.5 17.2 4.8 0

a Obtained from the TG curve shown in Figure 2. bCalculated with the CaO amount at the beginning of the second stage.

Table 5. Analysis of the Solid Products Obtained at Different Reaction Conditions

values are very close to the TG data under the same conditions, indicating dominance of Reaction 4 in this stage. The slight difference between the experimental data and the estimation may be attributed to incomplete release of Ca under the conditions as well as to experimental errors. The above analysis suggests that the third stage in Figure 2 starts at exhaustion of CaO by Reaction 4 and that the reaction in this stage should only be the decomposition of CaC2, Reaction 5. Based on the CaC2 formed in the first stage (Table 2) and CaC2 consumed in the second stage (Table 3), the amounts of CaC2 at the beginning of the third stage can be estimated to be 55.7, 83.9, 120.5, and 136.0 mg for C/CaO ratios of 1.8, 2.2, 2.8, and 3.6, respectively. It is interesting to note that, with the large differences in CaC2 amount, the mass loss rates of the third stage for all the runs are the same, about 0.19 mg min−1, which is also consistent with the case of CaC2 itself after 127 min in Figure 3, as residual CaO in CaC2 was exhausted. This indicates that the decomposition rates of CaC2 in these experiments, in terms of mole per volume per time or change in mole % of CaC2 per time, are significantly different, as shown in Table 4, which does not agree with the zero-order

mass loss rate (mg min−1)

CaC2 at beg. of 3rd stage (mg)

init react. rate (% min−1)

1.8 2.2 2.8 3.6

0.19 0.19 0.19 0.19

55.7 83.9 120.5 136.0

0.34 0.23 0.16 0.14

run

Ca

1 2 3 4 5 6

2.12 2.12 2.12

CaO

2.11 2.12 2.12

graphite

reaction tempa (°C)

mass loss (%)

Ca

CaC2

8.58 8.58 8.60 8.58 4.73

1270b 1000 1600 1000 1600c 1750 for 12.5 mind

50.1 0 0 0 0.5 19.9

√ √ √ × × √

× √ √ × √ √

At a heating rate of 20 °C min−1 from 20 °C to the target temperature. bMass loss of element Ca started at about 860 °C. c Corresponding to the first stage in Figure 2. dCorresponding to the second stage in Figure 2. a

The TG data of pure Ca (Run 1) indicates its evaporation starting at about 860 °C (boiling point 1484 °C). In the presence of graphite, Ca does not evaporate but reacts with graphite to form CaC2 (Runs 2 and 3) at temperatures as low as 1000 °C. This is confirmed by the release of C2H2 when the

Table 4. CaC2 Decomposition Data at Different C/CaO Molar Ratios in the Third Stage C/CaO ratio

solid product

feed (mmol)

reaction kinetics reported by Tagawa and Sugawara.7 The constant mass loss rate in all the cases regardless of the amounts of CaC2 in the sample crucible suggests that Reaction 5 is limited by the surface evaporation rate of Ca because the vapor pressure of C is very low under the conditions. This further indicates that Reaction 5 is reversible in the bulk but nonreversible on the surface due to evaporation of Ca. This is supported by the appearance of the products shown in Figure 4, where deposition of carbon on the surface is obvious and the

Figure 5. CaC2 yields with increasing reaction time for the experiments in Figure 2. 10745

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Figure 6. Schematic diagram of the reaction behavior for CaC2 production from de-ashed coke and CaO under the conditions of Figure 2.

direct reaction of coke and CaO to form CaC2 (CaO + 3C → CaC2 + CO) starting at 1460 °C. The second stage is reaction of CaC2 with CaO (CaC2 + 2CaO → 3Ca + 2CO) starting at 1520 °C, which is observable only when coke is exhausted. The third stage is decomposition of CaC2 into Ca and C (CaC2 → Ca + 2C), which is controlled by surface evaporation of Ca. The reduction of CaO by coke to form Ca does not occur at temperatures lower than 1460 °C. The reaction behavior occurring at the surface of coke and CaO particles is more complex than the superficial feature observed. It may involve formation CaC2−CaO eutectics at temperatures higher than 1660 °C that allow the CaC2 formed at the coke/CaO interphase to transform a fraction of CaO into the liquid state and consequently promote CaO diffusion and CaC 2 production.

product is contacted with water and agrees with that reported by Knapp and Ruschewitz.12 This indicates that the reaction of Ca with C is very fast and the formation of Ca should be the limiting step should it be the intermediate in CaC2 formation. CaO and graphite do not react at 1000 °C but form CaC2 at 1600 °C (Runs 4 and 5 in Table 5), which agrees with the observation in the first stage of Figure 2. The absence of Ca in the product of Run 5 suggests that the two-step mechanism for CaC2 formation (Reaction 2 followed by Reaction 3) reported by Tagawa and Sugawara7 is not likely. The only likelihood for this mechanism is the occurrence of Reaction 2 at 1600 °C and immediate completion of Reaction 3. This, however, indicates that the twostep mechanism is the same as Reaction 1. A reaction route that may play a notable role in CaC2 production may be the reaction of C with the Ca produced in Reaction 4 to form CaC2. This may be viewed as an “autocatalytic” effect to promote CaC2 production because consumption of one mole of CaC2 in Reaction 4 leads to formation of three moles of CaC2 via Reaction 3. This process may occur when a small amount of CaC2 is formed initially.14 To verify the role of this pathway in the CaC2 formation, the maximum CaC2 formation rate in the first stage and the maximum CaC2 consumption rate in the second stage are calculated. For the C/CaO ratio of 2.8 in Figure 2, the former is about 0.28 mmol min−1 while the latter is about 0.01 mmol min−1, indicating that the amount of CaC2 formed via the “autocatalytic” pathway is rather small, about 0.03 mmol min−1 (three times the CaC2 consumption rate by Reaction 4). Based on above analyses, the major reactions occurring in the experiments in Figure 2 can be elaborated in Figure 5, which shows the changes in CaC2 with respect to the amount of CaO in the feed. Clearly, Reaction 1 proceeds at a much faster rate than Reaction 4, and Reaction 4 is notable only at completion of Reaction 1 when coke is exhausted. For a high CaC2 yield, C/CaO molar ratios of more than 3 are needed and the reaction time should be controlled to avoid decomposition of CaC2. It should be noted that the above discussion, although in detail in time sequence, is still general and superficial because it views the reaction system in a homogeneous way. In fact, Reaction 1 starts at the contact points of coke and CaO particles when the temperature is 1460 °C or higher. The CaC2 produced may form a solid layer (the bp of CaC2 is about 2300 °C) between coke and CaO and hampers diffusion of CaO toward coke and consequently reduces the formation rate of CaC2. However, it has been reported that CaC2 and CaO may form a eutectic mixture at temperatures higher than 1660 °C that converts the solid CaC2 layer into a liquid mixture containing CaO.9,14 This certainly promotes the diffusion of CaO and then the formation of CaC2. The reaction mechanism shown in Figure 5, therefore, can be schematically incorporated in Figure 6 to elaborate the reaction behavior on the contact surface of coke and CaO particles.

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AUTHOR INFORMATION

Corresponding Author

*Tel/Fax: +86-10-64421073. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the Major State Basic Research Project (2011CB201306) and the National Natural Science Foundation of China (20976011 and 20821004) is acknowledged.

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REFERENCES

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4. CONCLUSIONS The reactions involved in CaC2 production from coke and CaO proceed through three distinct stages in general. The first stage is 10746

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(10) Wang, J.; Morishita, K.; Takarada, T. High-temperature interactions between coal char and mixtures of calcium oxide, quartz, and kaolinite. Energy Fuel 2001, 15, 1145−1152. (11) Rai, H. J.; Gregory, N. W. The time dependence of effusion cell steady-state pressures of carbon monoxide and calcium vapors generated by the interaction of calcium oxide and graphite. J. Phys. Chem. 1970, 74, 529−534. (12) Knapp, M.; Ruschewitz, U. Structural Phase Transitions in CaC2. Chem.Eur. J. 2001, 7, 874−880. (13) Wiik, K.; Raaness, O.; Olsen, S. E. Formation of calcium carbide from calcium vapor and carbon. Scand. J. Metall. 1984, 13, 3−6. (14) Lindberg, D.; Chartrand, P. Thermodynamic evaluation and optimization of the (Ca+C+O+S) system. J. Chem. Thermodyn. 2009, 41, 1111−1124.

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