Ind. Eng. Chem. Process Des. Dev. 1981, 20, 443-445
Table I. Temperature and Water Pressure at the Liquidus in the CaC0,-Ca(OH),-CaO System CaO, CaCO,, pressure, mol % temp, "C MPa mol % 4.05 9.50 777 6.4 15.00 6.1 4.06 7 56 3.22 7 24 20.00 7.0 4.54 20.00 4.2 739 20.00 4.96 7 53 3.2 5.25 20.00 770 2.6 4.47 24.50 3.6 726 3.16 30.00 6.4 677 3.86 30.00 688 4.5 4.24 30.00 3.3 696 4.56 30.00 2.2 702 4.07 35.00 674 2.8 3.00 40.00 5.9 64 1 3.28 40.00 5.0 650 3.52 40.00 4.2 651 3.78 659 40.00 2.9 4.02 44.60 1.7 676 3.49 6 76 45.00 3.4 2.54 50.00 718 6.6 2.82 50.00 725 5.5 2.98 50.00 4.8 740 3.19 3.8 50.00 749 2.92 55.00 788 4.3 2.52 60.00 788 5.7 2.25 64.50 789 7.0
(OH), as shown by the reaction in eq 2. This phenomenon is reflected in the data presented in Figure 3 in that those melts prepared at the highest steam pressures have the lowest CaO contents. The CaO content of the melt must also increase with Ca(OH), level as shown by the reaction presented in eq
443
2, and the data presented in'the ternary phase diagram also illustrate this effect. For example, at a water pressure of 5.0 MPa, the CaO content of the melt rises from 2.72 mol % when the Ca(OH), level is 40 mol % to 4.42 mol % at a Ca(OH), level at 80.0%. The fact that melts retain carbonate ion in this system allows the temperature and pressure required for fusion to be established readily. Because retention occurs, the carbonate content of the melt must follow a line of constant carbonate content on the ternary phase diagram (Figure 3). The conditions under which fusion occurs are specified by the intersection of this line with the isobar or isotherm that describes the melting process. With regard to the CaC03-Ca(OH), binary system, the data obtained in the present study indicate that the eutectic is approximately 40 mol % CaC03 as reported by previous investigators. However, data could not be obtained in the binary system because decomposition of Ca(OH)z occurs to a significant extent even at steam pressures as high as 7.0 MPa. Literature Cited Curran, G. P.; Gorin, E. "Phase 11-Bench Scale Research on CGS Process", 1968, NTIS Publ. No. PB 184 719.
Gittens, J.; Tuttle, 0. F. Am. J . Sci. 1964, 282, 66. Wyllie, P. J.; Raynor, E. J. Am. Mineral. 1965, 50, 2077. Wyllie, P. J.; Tuttle, 0. F. J . Petrol. 1960, I , 1.
Received for review October 29, 1979 Accepted December 30, 1980 The authors wish to acknowledge the financial support provided under subcontract from the Consolidation Coal Company, prime contractor with the Office of Coal Research.
Liquidus Temperatures in the CaC0,-Ca(OH),-CaO CaC0,-CaS0,-Cas Ternary Systems. '2
and
M. C. Fuerstenau,' C. M. Shen, and B. R. Palmer Department of Metallurgical Engineering, South Dakota School of Mines and Technology, Rapid Cky, South Dakota 5770 I
Liquidus temperature is presented as a function of composition for the CaC03-CaS0,-CaS system. Melts form most readily in this instance near the CaCO3-CaSO, eutectic. The eutectic composition if 42 mol % CaSO,, and the eutectic temperature is 1010 O C .
The presence of sulfur in coals undergoing gasification can cause a number of operating problems in the COz Acceptor Process related to the formation of Cas. This sulfide is formed through the reaction of H2S present in the gasification products with CaO in the acceptor, i.e. CaO(s) + H,S(g) + CaS(s) + H20(g) (1) Difficulties can occur when a portion of the CaS oxidizes subsequently to CaS04, that is
*
(2) CaS(s) + 2OZ(g) CaS04(s) because melts can form in the CaC03-CaS04-CaS ternary system causing acceptor particles to bond through surface 0196-4305/81/1120-0443$01.25/0
cohesive forces. As a result acceptor particles agglomerate causing collapse of the fluidized bed in the CaO regenerator, terminating operation of the gasification process. If this problem is to be avoided, it is imperative that the behavior of the CaC03-CaS04-CaS system at elevated temperatures be known. In this view this phase of the study was undertaken to investigate the liquidus temperatures in this ternary system. Experimental Materials and Techniques Reagent grade anhydrous materials were utilized in this study. Prior to use, CaS and CaC03were dried for 4 h at 105 "C, and CaS04 was dehydrated at 230 "C for 4 h. Following this treatment, CaC03, CaS04, and CaS were stored in a water-free CO, atmosphere. 0 1981 American Chemical Society
444
Ind. Eng. Chem. Process Des. Dev., Vol. 20, No. 3, 1981 I zoo
Table I. Liquidus Temperatures in the CaC0,-CaSO,-Cas System Cas, mol % CaSO,, mol % temp, "C 1105 30.00 0 1072 35.00 0 1084 33.25 5.00 1095 10.00 31.50 1101 30.80 12.00 1110 29.75 15.00 1038 3 8.00 0 1057 5.00 36.10 1062 3 5.34 7.00 1067 34.20 10.00 1082 32.30 15.00 1018 40.50 0 1032 5.00 38.48 1040 36.45 10.00 1091 20.00 32.40 1010 0 4 2.00 1028 39.90 5.00 1035 31.80 10.00 1040 35.70 15.00 1059 20.00 33.60 1029 0 4 5.00 1041 5.00 42.75 1053 40.50 10.00 1058 12.uo 39.60 1069 38.25 15.00 1054 0 50.00 1070 47.50 5.00 1078 46.50 7.00 1091 10.00 45.00 1070 0 55.00 1101 51.50 7.00 1118 10.00 49.50 1077 0 58.00 1102 55.10 5.00 1085 0 60.00 1089 0 61.70 1100 0 65.00
I I50
V p-
\ 1100
W
3
5 W
p
1050
W
IO00
950 20
40
30
50
60
70
1
Mol % '2404
Figure I. Liquidus and solidus temperatures in the CaS04-CaC03 binary system.
Experiments were initiated in the CaC03-CaS04-CaS ternary system by mixing a predetermined quantity of each of these materials in a C02-filled glove box. A platinum crucible was packed with 14 g of this mixture, and the crucible was placed in the stainless steel reactor described previously. The cover was placed on this vessel, and after removal from the glove box, the cover and body were joined by electric arc welding with a Type RA 330 stainless steel rod. After sealing was effected, the vessel was heated to 260 O C and was pumped down to a pressure of 7.0 KPa to remove any air or water vapor that may have entered the reactor during welding. The system was back-filled to a pressure of 1.5 MPa with C02 and was again evacuated to a pressure of 7.0 KPa to further remove any gaseous contaminations. The process was repeated three additional times. Following this treatment, the C02 pressure was raised to 1.5 MPa, and the reactor was heated in a vertically mounted tube furnace for 1 h. The sample temperature and COz pressure increased to 1150 "C and 3.4 MPa, respectively, under these conditions. The system was held at 1150 "C for 30 min, after which time power was removed from the furnace and a cooling curve for the system was
generated. After cooling, the stainless steel vessel was opened on a lathe, and the materials in the platinum crucible were examined. The platinum crucible was subsequently cleaned in an aqueous solution containing 50% HC1 by volume to remove the solidified melt. Experimental Results The first phase of this investigation involved measurement of the liquidus and eutectic temperatures in the CaC03-CaS04binary system. The results of this work are cds
-
20 Coco3
30
40
50 Mol % CaS0,
Figure 2. Liquidus temperatures in the CaC03-CaS04-CaS ternary system.
-
70
60
caso,
Ind. Eng. Chem. Process Des. Dev. 1981, 20, 445-450
presented in Figure 1. The eutectic temperature is 1010 "C, and the eutectic occurs at a composition of 42 mol % CaS04. The 90% confidence interval for the solidus temperature is f2.1 "C. Melt formation in the CaC03-CaS and CaS04-CaS binary systems was also investigated. Fusion was not observed for temperatures up to 1150 "C. The liquidus temperatures which were measured in the ternary system are shown on the diagram presented in Figure 2, and the data are also tabulated in Table I. As can be noted, a large region occurs in which melts can be formed at relatively low temperatures. Material with a molar CaS04/CaC03ratio equal to that of the binary eutectic can be melted most readily in the ternary system. Melts can be formed in this instance for Cas levels of up to 21 mol % at a temperature of 1100 "C. The region of fusion encompasses a relatively large portion of the ternary system. In the absence of Cas, this area extends from 31 to 65 mol % CaS04at a temperature of 1100 "C. At a Cas level of 10 mol % at 1100 "C, fusion is observed for melts which contain 31 to 46 mol % CaSO4. The retention of C02 in these melts was checked for a system containing 41 mol % CaC03 and 59 mol % CaSO1. These materials were fused at a temperature of 1159 "C. Upon analysis of this melt for CaC03, it was found that less than 0.15 mol % of the C02present initially could not be accounted for after fusion and solidification. Discussion of Results Of the three binary systems which can form from the materials involved in this study, the CaC03-CaS04system exhibits the lowest liquidus temperatures which are shown in the data presented in Figure 1. A eutectic is observed in this instance at 42 mol 90 cas04 which exhibits a fusion temperature of 1010 "C.
445
Liquidus temperatures in the CaC03-CaS04-Cas system were also investigated, and the results obtained are shown in Figure 2. These data show that the fusion of materials is very pronounced near the CaC03-CaS04 binary eutectic and can occur for systems containing up to 21 mol % Cas at 1100 "C. Materials with a eutectic ratio of CaC03 to CaS04 in the ternary system exhibit only a slight increase in liquidus temperature with Cas addition until the level of this material reaches 20 mol %. These data indicate that problems with agglomeration in the C02Acceptor Process can be minimized by maintaining conditions in the system sufficiently reducing that oxidation of Cas to CaS04 does not occur as suggested previously by Fink et al. (1974). In this case the CaC03-CaS04binary system can be avoided, thus preventing the formation of the undesirable melts that are described in the present study. Of course, problems with fusion can also be avoided by the removal of sulfur from the gasification system. Under these conditions a lime-base acceptor would contain only CaO and CaC03. The eutectic temperature in the CaOCaC03 system is sufficiently high, 1230 "C (Wyllie and Tuttle, 1960) that fusion will not occur under the conditions involved in the C02 Acceptor Process. Literature Cited Fink, C. E.; Sudbury, J. D.; Curran, G. P. "COP Acceptor Gasificlation Process", presented at the 77th National Meeting AIChE, Pmsburgh, PA, 1974. Wyllie, P. J.; Tuttle, 0. F. J . Petrol. 1060, 7, I.
Received for review October 29, 1979 Accepted December 30,1980
The authors wish to acknowledge the financial suppofiprovided under subcontract from the Consolidation Coal Company, prime contractor with the Office of Coal Research.
Carbonation of Aqueous Sodium Chromate Syamalendu S. Bandyopadhyay, Asok K. Blswas," and Narayan C. Roy Chemical Engineering D e p a m " ,
Indian Institute of Technology, Kharagpur, India
The carbonation of aqueous sodium chromate, which may be a possible alternative to the conventional acidification route for the manufacture of sodium dichromate, has been carried out in two types of contactors-a bubble column as well as an agitated contactor. The reaction is found to be first order with respect to C02 pressure and second order with respect to chromate solution. The experimental results may be explained satisfactorily by assuming fast pseudo-first-order mechanism. If we follow Danckwerts' surface renewal model, the rate may be expressed as Ra = a[A'](DAk3[Bo]* k?)l'*. The reaction rate constant, k3, at 20 "C is 1.89 X lo6 [(~m)~/(g-moI)]~ s-' and the average activation energy is 17.63 kcal/mol.
+
The carbonation of aqueous sodium chromate may be a possible alternative to the conventional acidification route for the manufacture of sodium dichromate which is extensively used in leather, textile, and other industries. This process involves the absorption of carbon dioxide in aqueous sodium chromate solution. The overall reaction may be represented by the equation 2Na2Cr04(aq) + 2C02(g) + H20(1) = Na2Cr20,(aq) + 2NaHC03(aq) (1) The carbonation process produces sodium bicarbonate as 0196-4305/81/1120-0445$01.25/0
a byproduct which precipitates out of the system due to its low solubility. The bicarbonate on calcination gives soda ash and COPwhich may be used back in the process for the manufacture of sodium chromate from chromite ore. Besides this, sodium carbonate has a potential market. A judicious thought about the plant location and particularly the proximity of the plant to an industry producing excess C02,namely a cement factory or fertilizer factory, may find the carbonation process an attractive alternative to the acidification route (Shreve, 1956) for the production of sodium dichromate. 0 1981 American Chemical Society
