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Solvent de-ashing from heavy product of brown coal liquefaction using toluene:
Fuel Processing Technology 56 Ž1998. 229–241
Solvent de-ashing from heavy product of brown coal liquefaction using toluene: 2. concentration and separation of ash with a continuous de-ashing system Osamu Okuma
a,)
, Kaoru Masuda b, Noriyuki Okuyama c , Tatsuo Hirano c
a
Chemical and EnÕironmental Technology Laboratory, Kobe Steel, 1-chome 5-5, Takatsukadai, Nishi-ku, Kobe 651-2271, Japan b Analysis Research Section, Kobelco Research Institute Inc., 1-chome 5-5, Takatsukadai, Nishi-ku, Kobe 651-2271, Japan c Takasago Research Laboratory, Nippon Brown Coal Liquefaction, cr o Takasago Works of Kobe Steel, 2-chome 3-1, Shinhama, Arai-cho, Takasago, Hyogo 676-8678, Japan Received 8 September 1997; revised 19 February 1998; accepted 19 February 1998
Abstract The brown coal liquefaction ŽBCL. process is a two-stage liquefaction Žhydrogenation. process developed for Victorian brown coal in Australia. The BCL process has a solvent de-ashing step to remove the ash and heavy preasphaltenes from the heavy liquefaction product Žvacuum residue. derived from the coal in primary hydrogenation and named CLB Žcoal liquid bottom.. This solvent de-ashing step uses toluene or coal-derived naphtha as a de-ashing solvent ŽDAS.. After dissolving the CLB into the solvent ŽCLBrsolvent ratio, 1r8–1r4, wrw. under high temperature Ž200– 2908C. and high pressure Ž4–5 MPa., insoluble solid particles which consist of ash and heavy preasphaltenes are settled by gravity and separated from the solution as an ash-concentrated slurry. The ash-concentrated slurry and the de-ashed solution are withdrawn from the settler as an underflow and overflow, respectively. The de-ashed heavy product is recovered from the solution by eliminating the solvent and is further hydrogenated in secondary hydrogenation. The authors have reported on the solubility of CLB in toluene and the settling velocity Ž V . of the boundary of ash content in the settler under de-ashing conditions. This paper discusses the effects of de-ashing conditions on ash concentration in the settler bottom and the operating conditions of a continuous de-ashing system. The ash content in underflow Ž C UF , kgrkg or wt.%. at the settler bottom was
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Corresponding author. Fax: q81 78 992 5547
0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 8 . 0 0 0 5 7 - 5
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found to increase with temperature and to decrease with the rate Žflux. of downward flow Ž underflow . . The m axim um C U F , Z, is expressed by the equation: Z s BCLBŽFLr0.35.y0.32 ŽTr523. 4.26 , where BCLB , FL and T are the characteristic parameters of organic CLB Žkgrkg or wt.%., flux of underflow in the settler Žkgrm2 s. and temperature ŽK., respectively. BCLB is also expressed by using the analytical results of organic insolubles in the CLB under de-ashing conditions. Finally, stable operating conditions of a continuous de-ashing system are confirmed to be determined as the following qualifications: < Vu < - < V <, WUF ) WSArC UF and Z ) C UF , where < Vu <, < V <, WSA and WUF are the upward velocity of the solution in the settler Žmmrs., settling velocity of the ash boundary Žmmrs. in the settler, flow rate of ash in the feed slurry Žkgrh. and flow rate of underflow Žkgrh., respectively. Under these qualified conditions, the 50 trd pilot plant constructed in Australia was operated under stable conditions for 3700 h using toluene as a DAS. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Solvent de-ashing; Brown coal liquefaction; Toluene; Coal liquid bottom
1. Introduction All direct coal liquefaction processes must include a solid–liquid separation step, which is usually called a de-ashing step, because the liquid derived from coal contains mineral matter Žash. originating from the coal w1x. Therefore, many de-ashing processes have been developed for coal liquefaction based on the techniques of distillation, filtration, centrifugation, settling Žsedimentation., and so on w2–9x. The brown coal liquefaction ŽBCL. process developed by Nippon Brown Coal Liquefaction ŽNBCL. is a two-stage liquefaction process for Victorian brown coal in Australia w10,11x. It consists of four unit steps: dewatering, primary hydrogenation, de-ashing and secondary hydrogenation. In the BCL process, NBCL has developed a solvent de-ashing step to remove the ash and heavy preasphaltenes from the heavy liquefaction product Žvacuum residue. derived from the coal in the primary hydrogenation, which is called CLB Žcoal liquid bottom, BP ) 4208C. w12x. This step is expected to have high de-ashing efficiency and is easy to apply to a large-scale plant. The solvent de-ashing processes such as the anti-solvent de-ashing by C.-E. Lummus w6x and critical solvent de-ashing by Kerr McGee w7x are well-known. However, the de-ashing conditions and efficiency of these processes depend on the liquefaction conditions and kinds of liquefied coal w1–11x. The de-ashing step in the BCL process is required to have a very high de-ashing performance because the de-ashed heavy product ŽBP ) 4208C. is further hydrogenated over Ni–MorAl 2 O 3 catalyst using fixed bed reactors in the secondary hydrogenation w13–15x. Therefore, ash contained in the de-ashed heavy product should be less than 3000 ppm for long-term stable operation Žca. 8000 h. of the secondary hydrogenation plant. In this de-ashing step, toluene or naphtha obtained in the primary hydrogenation is used as a de-ashing solvent ŽDAS. w12,16x. Fig. 1 shows a simplified flow diagram of the de-ashing step in the BCL process. The molten CLB is mixed and dissolved into the DAS 4–8 times under high temperature Ž200–2908C. and high pressure Ž5 MPa.. Then, the mixture is fed to the settler as a slurry, and insoluble solid particles consisting of ash and heavy preasphaltenes are separated from the
O. Okuma et al.r Fuel Processing Technology 56 (1998) 229–241
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Fig. 1. Simplified flow diagram of de-ashing step in BCL process. CLB: Heavy liquefaction product Žcoal liquid bottom, BP) 4208C.. DAO: De-ashed heavy liquefaction product ŽBP) 4208C..
solution as an ash-concentrated slurry of downward flow Žunderflow., and the de-ashed solution is recovered as an upward flow Žoverflow. w12,17x. In a previous paper w17x, the authors reported that ash and heavy preasphaltenes can be separated from CLB as insoluble solid particles by solvent de-ashing, which dissolves the CLB and settles the insolubles by gravity. The ash content in the recovered heavy product was less than 3000 ppm in order to be useful as a feedstock for the secondary hydrogenation. As the solid particles form the boundary of ash content in the settler, its settling velocity Ž V . is estimated from the de-ashing conditions such as temperature, and the content and properties of CLB in the feed slurry. For de-ashing with a continuous system, the upward linear velocity of the solution must be determined to keep the ash-boundary in the settler during operation. In addition, almost all of the ash in the feed slurry must be withdrawn as the underflow from the settler bottom. Therefore, it is necessary to know the ash concentration in the settler for predicting stable operating conditions of the de-ashing system, although the ash contents in both feed slurry and underflow are required to be as high as possible to increase the process efficiency. This investigation was carried out to clarify the effects of de-ashing conditions on the ash concentration in the settler bottom with a continuous de-ashing system using toluene as a DAS, confirming the conditions for stable operation of the plant which provides de-ashed heavy product with low ash content enough to feed the secondary hydrogenation.
2. Experimental 2.1. Samples of heaÕy liquefaction product (CLB) The CLBs used were heavy products ŽBP ) 4208C. recovered by vacuum distillation from the liquefaction products derived from Victorian brown coal ŽYallourn. in a pilot
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Table 1 Liquefaction conditions and properties of CLBs ŽBP- 4208C. CLB
Liquefaction conditions Temperature Ž8C.
PA PB PC PD PE
435 450 450 450 450
Pressure ŽMPa.
15 15 15 20 15
Properties of CLB BTMrRa
Yes Yes Yes Yes Yes
Ash Žwt.%.
14.2 13.2 13.1 12.5 14.6
Solvent extraction Žwt.%. HS
HI-BS
BI-PS
Pb
19.7 23.6 21.1 26.0 25.1
30.6 43.9 42.0 43.7 39.5
30.7 16.2 18.3 17.1 16.2
19.0 16.3 18.6 13.2 19.2
a
Feed solvent containing bottom fraction ŽCLB, BP) 4208C.. Pyridine insolubles containing ash. Other liquefaction conditions in the primary hydrogenation step of the pilot plant Ž50 ton-dry coalrday.: nominal reaction time: 1.0 h, solventrdaf-coal ratio: 2.5 Žwrw.. cat. 3.0 wt.% on daf-coal as Fe, SrFe Žatomic ratio.: 1.2. b
plant of 50 ton-dry coalrday in Australia w18x. Typical Yallourn coal ŽC: 67.0, H: 4.7, N: 0.6, S: 0.2 and Odiff.: 27.5 wt.% on daf basis. contained 11.6 wt.% moisture and 1.4 wt.% ash after drying. The liquefaction conditions were: temperature: 430–4508C, pressure: 15–20 MPa and nominal residence time: 1.0 h. A catalyst Žiron oxide and sulfur. of 3.0 wt.% was added to daf coal as Fe, and the SrFe atomic ratio was 1.2. Table 1 shows the liquefaction conditions and the properties of the representative CLBs. 2.2. De-ashing with a continuous system Fig. 2 shows a continuous system used for the de-ashing experiments ŽDA-PDU.. This system has two types of settlers shown in Fig. 3. The settler-A and settler-B were used for changing the linear velocity and the residence time of overflow and underflow.
Fig. 2. Flow diagram of a continuous de-ashing system Žprocess development unit, DA-PDU.. Ž1. De-ashing solvent ŽDAS. tank, Ž2. DAS preheater, Ž3. CLB melter, Ž4. dissolver, Ž5. settler, Ž6. overflow receivers, Ž7. underflow receivers.
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Fig. 3. Types of settlers used for de-ashing with DA-PDU. Settler-A: Inner diameter: 50 mm, height: 1850 mm, volume: 0.0037 m3. Settler-B: Inner diameter: 150 mm, height: 1800 mm, volume: 0.0307 m3.
The experimental procedure is as follows. At first, only toluene of a desired flow rate was fed from a DAS tank Ž1. through a DAS preheater Ž2. into a dissolver Ž4. and settler Ž5. which was heated up to a desired temperature. Then, a CLB from a melter Ž3. at ca. 2008C was fed into Ž4. with a desired flow rate. The CLB was mixed with toluene in Ž4.,
Fig. 4. Ash distribution in the settler and its change during operation without withdrawing underflow. De-ashing with settler-B Žsee Fig. 3. for CLB-PB. Conditions: CLBrtoluene ratio: 1r3 Žwrw., flow rate of feed slurry: 20.0 kgrh, temperature: 2508C, overflowrunderflow ratio: 1r0 Žwrw.. Keys: operating period, I 6 h, q 9 h, ` 12 h, = 17 h.
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and dissolved under agitation. The mixture Žthe feed slurry. fed to Ž5. was withdrawn from the top and bottom of Ž5. to each receiver Ž6. and Ž7. as overflow and underflow, respectively. Two receivers were used in turn for overflow or underflow. The temperature in Ž4. was controlled to be the same as that in Ž5. at a range of 230–2908C. The pressure of vessels Ž4. – Ž7. was maintained at 5 MPa to suppress the vaporization of toluene. CLBrtoluene ratio of the feed slurry was changed from 1r4 to 1r8 Žwrw., and the range of the slurry feed rate and overflowrunderflow ratio was 10–30 lrh and 9r1–4r1 Žwrw., respectively. Both the overflow and underflow sampled at 0.5 h intervals were distilled to remove toluene, and the ash content in the recovered heavy products was analysed. Based on these analytical results, the de-ashing performance was estimated. In addition, the distribution and concentration of ash in the settler under de-ashing conditions without withdrawal of underflow were measured using settler-B by analysing the samples obtained from the sampling spots shown in Fig. 4. 3. Results and discussion 3.1. Ash distribution and settling in the settler Fig. 4 shows the change in ash distribution in settler-B for CLB-PB Žsee Table 1. during the operation of DA-PDU without withdrawal of underflow. The de-ashing conditions of this experiment were: temperature: 2508C, CLBrtoluene ratio: 1r3 Žwrw. and slurry feed rate: 20.0 kgrh. This result shows that the boundary of ash content forms in the settler of DA-PDU, as well as in the settler of a batch de-ashing system w17x. The ash content in the slurry at the lower part of the settler gradually increased along with the operating period by the accumulation of the fed ash, resulting in raising the boundary, although it decreased at the upper part. These results indicate that, for de-ashing with a continuous system, it is necessary to maintain the ash boundary in the settler by controlling the withdrawal of the upper and lower parts from the top and bottom of the settler as overflow and underflow, respectively. 3.2. De-ashing performance Fig. 5a and b show the change in the ash contents in overflow and underflow for CLB-PA Žsee Table 1. during the operation of the DA-PDU with settler-A ŽRun-A1 and Run-A2, respectively. at 2508C, with a slurry feed rate: 12.5 kgrh and overflowrunderflow ratio: 1r4 Žwrw.. The CLBrtoluene ratios in the feed slurry for Run-A1 and Run-A2 were 1r7 and 1r5 Žwrw., respectively. The height of a boundary in the settler of a continuous system depends on the upward velocity of overflow Ž Vu . and the settling velocity of the ash boundary Ž V .. Therefore, < Vu < must be smaller than < V < to maintain the boundary at a lower position in the settler because the boundary is raised under the condition of < Vu < ) < V <. The < Vu < for both Run-A1 and Run-A2 was fixed at 2.17 mmrs, because the < V < of CLB-PA estimated by using Eq. Ž1. w17x are 10.65 and 8.20 mmrs, respectively, V s A CLB Ž CSA 2.5 .
y0 .91
Ž Tr523.
9.5
Ž 1.
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Fig. 5. Change in ash contents in overflow and underflow withdrawn from the settler during operation. Ža. Run-A1 with settler-A Žsee Fig. 3. for CLB-PA. De-ashing conditions: CLBrtoluene ratio: 1r7 Žwrw., flow rate of feed slurry: 12.5 kgrh, temperature: 2508C, overflowrunderflow ratio: 4r1 Žwrw., upward velocity of overflow, Vu : 2.17 mmrs. Žb. Run-A2 with settler-A for CLB-PA. De-ashing conditions: CLBrtoluene ratio: 1r5 Žwrw., other conditions are the same with Run-A1.
where A CLB is the characteristic parameter of organic components in CLB Žthe A CLB for CLB-PA was 7.80 mmrs., CSA is the ash content in the feed slurry Žwt.%. and T is the temperature ŽK.. During the operation of Run-A1, shown in Fig. 5a, the ash content in the overflow was kept almost constant at a very low level Žca. 0.018 wt.%. for 14 h. As this ash content corresponded to 1600 ppm of ash in de-ashed heavy product, it was suitable for a feedstock to the secondary hydrogenation. In the latter period of the operation, the ash content in the underflow saturated at ca. 8 wt.%, and the amount of withdrawn ash in the underflow became almost equal to that in the feed slurry. These results mean that the ash boundary was maintained in the settler, and that the ash content in the upper part of the settler became very low during the operation of Run-A1. On the other hand, for Run-A2 shown in Fig. 5b, the ash content in the overflow raised sharply at 11–12 h after starting the operation, although it was also very low until that time. During this operation, the amount of withdrawn ash as underflow was always smaller than that in the feed slurry. This indicates that, for Run-A2, the ash content or flow rate of the underflow was too small to withdraw all of the fed ash into the settler. Consequently, a part of the fed ash accumulated in the settler, and the ash boundary gradually raised during the operation of Run-A2. A flow rate of the ash in the feed slurry must be smaller than that defined by the product of the ash content and flow rate of underflow in order to withdraw all of the ash contained in the feed slurry. Therefore, the following Eq. Ž2. is introduced: WUF ) WSA C UF Ž 2. where WSA is the flow rate of ash in a feed slurry Žkgrh., WUF is the flow rate of underflow Žkgrh. and C UF is the ash content in underflow Žkgrkg.. Eq. Ž2. explains the results for Run-A1 and Run-A2, and reveals that the ash concentration in the settler bottom must be estimated to fix the de-ashing conditions for stable operation of a continuous system.
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3.3. Ash concentration at the settler bottom The ash concentration in the settler bottom is considered to be affected by the same factors as those affecting the settling velocity of the ash boundary Ž V . discussed in a previous paper w17x, because they depend on the same repulsive force between the insoluble particles including the ash w19x. Therefore, the effects of slurry residence time, temperature and ash content in the feed slurry on the ash concentration at the settler bottom were examined by using the DA-PDU. The flow rate Žflux. of underflow, which is determined by the slurry feed rate and overflowrunderflow ratio, is also an important factor affecting the ash concentration, because it determines residence time of the underflow and the height of the ash boundary in the settler described earlier. Fig. 6 shows the effects of flow rate Žunderflow flux. on the ash content in the underflow for CLB-PB at 2508C. For Run-A3 Ž`. and Run-A4 Ž^. with settler-A, the underflow fluxes were 0.214 and 0.397 kgrm2 s, respectively. For Run-B1 Žv . and Run-B2 Ž'. with settler-B, they were 0.0172 and 0.0555, respectively. For all Runs, the ash content in the underflow increased with the operating period and was saturated in the latter period of the operation. During these operations, the amount of withdrawn ash as underflow was smaller than that in the feed slurry, and a part of the fed ash, except for one in the underflow, flowed out from the top of the settler as overflow. This means that the boundary of the ash content raised along with the operating period, and the region of very low ash content in the upper part of the settler disappeared during the operation, as described earlier ŽFig. 5b.. The results shown in Fig. 4 also suggest that the accumulated ash at the settler bottom was compressed by its weight w20x. Therefore, the
Fig. 6. Effect of flow rate of underflow Žunderflow flux. on ash content in underflow. De-ashing for CLB-PB at 2508C. ` Run-A3 with settler-A Žsee Fig. 3.. Conditions: CLBrtoluene ratio: 1r3 Žwrw., flow rate of feed slurry: 7.5 kgrh, overflowrunderflow ratio: 4r1 Žwrw., underflow flux: 0.214 kgrm2 s. ^ Run-A4 with settler-A. Conditions: CLBrtoluene ratio: 1r3, flow rate of feed slurry: 12.5, overflowrunderflow ratio: 4r1, underflow flux: 0.397. v Run-B1 with settler-B Žsee Fig. 3.. Conditions: CLBrtoluene ratio: 1r3, flow rate of feed slurry: 11.1, overflowrunderflow ratio: 9r1, underflow flux 0.0172. ' Run-B2 with settler-B. Conditions: CLBrtoluene ratio: 1r2.5, flow rate of feed slurry: 17.5, overflowrunderflow ratio: 4r1, underflow flux: 0.0555.
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Fig. 7. Effect of flow rate of underflow Žunderflow flux. on the maximum ash content in underflow Ž Z .. De-ashing for CLB-PB at 2508C with settler-A and settler-B Žsee Fig. 3.. Conditions: See Fig. 6.
saturated ash content in the underflow for each operation is found to be at its maximum under specific de-ashing conditions. Fig. 7 summarizes the relationship between the underflow flux and the maximum ash content Ž Z, wrw or wt.%. in the underflow obtained from the saturated values of Runs shown in Fig. 6. This result shows that the ash in the slurry at the settler bottom was concentrated with a decrease in the underflow flux, resulting in an increase in Z. Fig. 8 shows the effects of temperature on Z for CLB-PB with settler-A under the conditions of CLBrtoluene ratio: 1r4, flow rate of feed slurry: 12.5 kgrh and underflow flux: 21.0 kgrm2 s. This result also indicates that Z increases along with the de-ashing temperature. Based on the results shown in Figs. 7
Fig. 8. Effect of temperature on the maximum ash content in underflow Ž Z .. De-ashing for CLB-PB with settler-A Žsee Fig. 3.. Conditions: CLBrtoluene ratio: 1r3 Žwrw., flow rate of feed slurry: 12.5 kgrh, overflowrunderflow ratio: 4r1 Žwrw., underflow flux: 0.397 kgrm2 s.
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and 8, assuming that the parameters are independent of each other, the following Eq. Ž3. is introduced to estimate Z: a
b
Z s BCLB Ž FL . Ž T . s BCLB Ž FLr0.35 .
y0.32
Ž Tr523.
4.26
Ž 3.
where BCLB is the characteristic parameter of organic components of CLB except ash content Žkgrkg or wt.%., FL is the flux of downward flow Žunderflow flux. in the settler Žkgrm2 s. and T is the temperature ŽK.. Here, a and b were determined for CLB-PB using the slopes of lines shown in Figs. 7 and 8, respectively. Table 2 shows the BCLB values of the CLBs determined by the DA-PDU operations. 3.4. Properties of insoluble matter and characteristic parameter of CLB (BC L B ) BCLB in Eq. Ž3., which represents the effects of the properties of organic components in CLB on the ash concentration at the settler bottom, is determined as a constant for each CLB by the de-ashing experiment with DA-PDU as well as A CLB expressing its effects on the settling velocity of the ash boundary Ž V . w17x. However, it is very troublesome to determine the BCLB for each CLB by the operations of the DA-PDU. The A CLB has been found to be expressed by using the analytical results of CLB and the insoluble organic matter with tetrahydrofuran ŽTHF-I. at its boiling point of 668C, which closely corresponds to the insolubles Žsolid particles. under the de-ashing conditions w17x. Therefore, the relationship between BCLB and A CLB was examined to express the BCLB by the A CLB calculated by using the following Eq. Ž4.: A CLB s 5.3 Ž ar1.7 .
6 .6
Ž br0.6 .
y4.5
Ž 4.
where a is the ratio of content of organic THF-I to the ash content in CLB Žwrw., and b is HrC atomic ratio of organic THF-I. Fig. 9 shows the relationship between the A CLB calculated by Eq. Ž4. and the BCLB determined by the operations of the DA-PDU for the CLBs shown in Table 2. Since they
Fig. 9. Relationship between A CL B estimated by Eq. Ž4. and BCLB determined by experiments with DA-PDU. The used CLBs are shown in Table 1.
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Table 2 The characteristic parameter of organic components of CLB Ž BCL B . in Eq. Ž3. CLB
PA
PB
PC
PD
PE
BCL B Žwt.%.
10.9
8.0
5.9
9.3
7.0
have a good correlation, the BCLB is found to be empirically expressed by the A CLB as follows: d
BCLB s c Ž A CLB . s 3.08 Ž A CLB .
0.619
Ž 5.
Fig. 10 shows the correlation of the Z between the experimental values and the calculated ones by using Eqs. Ž3. – Ž5.. This good correlation Ž r s 0.978. indicates that these equations are useful to determine the maximum ash content in underflow Ž Z . under specific de-ashing conditions. In addition, these results also indicate that the same interaction among the insoluble solid particles including ash, such as surface charge w19x, plays an important role in their agglomeration, settling and concentration under de-ashing conditions. 3.5. Prediction of the operating conditions of a de-ashing plant The present and previous investigations w17x confirm that the conditions in the settler of a solvent de-ashing plant can be predicted by calculating the V and Z using the analytical results of CLB when the slurry feed rate, overflowrunderflow ratio and temperature are fixed. The de-ashing conditions for stable operation of the plant are represented by the following equations as described earlier. < Vu < - < V < , and WUF ) WSA C UF To operate the de-ashing plant efficiently, the CLBrtoluene ratio should be as high as possible to raise the processing capacity of the plant. In addition, the flow rate of underflow is desired to be as small as possible to increase the efficiency of recovery of
Fig. 10. Correlation of maximum ash concentrations in underflow Ž Z . estimated by Eqs. Ž3. – Ž5. to ones determined by experiments with DA-PDU. The used CLBs are shown in Table 1.
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de-ashed heavy product and DAS Žtoluene.. Therefore, the conditions of C UF s Z is most advantageous for the operation of a de-ashing plant because smaller W UF provides higher de-ashing efficiency. Under the conditions qualified by these equations, the 50-ton-dry coalrday pilot plant constructed in Australia was successfully operated for 3700 h w18x. Based on this operation, the solvent de-ashing process is confirmed to be excellent for removing the ash from the heavy liquefaction product ŽCLB. derived from Victorian brown coal. 4. Conclusion Solvent de-ashing from heavy liquefaction product ŽCLB. derived from Victorian brown coal using toluene was investigated with a continuous system from the view point of the ash concentration in the settler bottom to clarify the stable operating conditions of the plant. From this investigation, the following results were obtained. Ž1. Insoluble solid particles including ash in toluene solution formed the boundary of ash content in the settler of a continuous de-ashing system, as well as in a batch de-ashing system. Since the ash content of the heavy products recovered from the overflow of the settler was less than 3000 ppm, the de-ashing performance with the continuous system was enough to prepare a feed stock for secondary hydrogenation with fixed bed reactors. Ž2. The settled ash Žsolid particles. was concentrated at the settler bottom. The ash content in the underflow withdrawn from the settler bottom increased with increasing temperature and with decreasing flow rate Žflux. of underflow. According to these results, the equation expressing the maximum ash content in the underflow Ž Z, kgrkg or wt.%. is introduced by using the characteristic parameter of the organic components of CLB, the flux of downward stream Žunderflow. in the settler and the temperature. Ž3. The effect of properties of organic CLB on the ash content in the underflow is found to be expressed by the BCLB which correlated well with the A CLB expressing its effect on the settling velocity of the ash boundary. Ž4. The present and previous investigations confirm the equations qualifying stable operating conditions of a de-ashing plant using toluene which depend on the properties of CLB produced in primary hydrogenation. These equations can determine the flow rate of a feed slurry, the upward velocity of solution Žoverflow. and the flow rate of underflow in the settler by using the analytical results of CLB such as ash and THF-I contents. Under the conditions predicted by these equations, the 50-ton-dry coalrday pilot plant constructed in Australia was successfully operated for 3700 h, and the solvent de-ashing process is confirmed to be excellent for removing the ash from the heavy liquefaction product derived from Victorian brown coal. Acknowledgements This investigation was carried out through the financial support of New Energy and Industrial Technology Development Organization ŽNEDO.. The authors thank NEDO for their permission to publish this paper.
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References w1x w2x w3x w4x w5x w6x w7x w8x w9x w10x w11x w12x w13x w14x w15x w16x w17x w18x w19x w20x
W.F. Leu, F.M. Tiller, Powder Technol. 40 Ž1984. 65. J.A. Kleinpeter, D.C. Jones, P.J. Dudt, F.P. Burke, Ind. Eng. Chem. Process Des. Dev. 18 Ž1979. 535. E. Gorin, C.J. Kulik, H.E. Lebowitz, Ind. Eng. Chem. Process Des. Dev. 16 Ž1977. 95. E. Gorin, C.J. Kulik, H.E. Lebowitz, Ind. Eng. Chem. Process Des. Dev. 16 Ž1977. 102. N.G. Moll, G.J. Quarderer, Chem. Eng. Prog., November 1979, 46. M. Peluso, D.F. Ogren, Chem. Eng. Prog., June 1979, 41. R.M. Adams, A.H. Knebel, D.E. Rhodes, Chem. Eng. Prog., June 1979, 44. I. Romey, Fuel 61 Ž1982. 988. J.W. Clarke, T.D. Rantell, Fuel 65 Ž1986. 295. K. Terunuma, Y. Nakako, T. Matsumura, Proceedings of World Cong. III of Chem. Eng., Tokyo, 1 Ž1986. 73. O. Okuma, K. Saito, A. Kawashima, K. Okazaki, Y. Nakako, Fuel Processing Technol. 14 Ž1986. 23. O. Okuma, K. Masuda, N. Okuyama, T. Hirano, T. Matsumura, Proceedings of 1993 International Conference on Coal Science, Banff, 1 Ž1992. 295. S. Nagae, Y. Mito, O. Okuma, K. Saito, T. Matsumura, Proceedings of 1989 International Conference on Coal Science, NEDO, Tokyo, 2 Ž1989. 931. Y. Mito, T. Takahashi, O. Okuma, K. Saito, T. Matsumura, Y. Nakako, Proceedings of 1983 International Conference on Coal Science, Pittsburgh Ž1983. 774. T. Kaneko, Y. Kageyama, T. Kimura, S. Kawai, Proceedings of 1989 International Conference on Coal Science, NEDO, Tokyo, 2 Ž1989. 919. O. Okuma, K. Masuda, N. Okuyama, T. Hirano, Nippon Enerugi Gakkai-Shi 75 Ž1996. 257, J. Jpn. Inst. Energy. O. Okuma, K. Masuda, N. Okuyama, T. Hirano, Fuel Processing Technol. 51 Ž1997. 177. Y. Nakako, T. Ohzawa, H. Narita, Proceedings of 1991 International Conference on Coal Science, Univ. of Newcastle-upon-Tyne Ž1991. 652. B.R. Rodgers, AIChE Symp. Ser. 76 Ž192. Ž1980. 68. K.R. Valdyanathan, J.D. Henry Jr., F.H. Verhoff, Ind. Eng. Chem. Fundam. 20 Ž1981. 165.
Solvent de-ashing from heavy product of brown coal liquefaction using toluene: 2. concentration and separation of ash with a continuous de-ashing system Osamu Okuma
a,)
, Kaoru Masuda b, Noriyuki Okuyama c , Tatsuo Hirano c
a
Chemical and EnÕironmental Technology Laboratory, Kobe Steel, 1-chome 5-5, Takatsukadai, Nishi-ku, Kobe 651-2271, Japan b Analysis Research Section, Kobelco Research Institute Inc., 1-chome 5-5, Takatsukadai, Nishi-ku, Kobe 651-2271, Japan c Takasago Research Laboratory, Nippon Brown Coal Liquefaction, cr o Takasago Works of Kobe Steel, 2-chome 3-1, Shinhama, Arai-cho, Takasago, Hyogo 676-8678, Japan Received 8 September 1997; revised 19 February 1998; accepted 19 February 1998
Abstract The brown coal liquefaction ŽBCL. process is a two-stage liquefaction Žhydrogenation. process developed for Victorian brown coal in Australia. The BCL process has a solvent de-ashing step to remove the ash and heavy preasphaltenes from the heavy liquefaction product Žvacuum residue. derived from the coal in primary hydrogenation and named CLB Žcoal liquid bottom.. This solvent de-ashing step uses toluene or coal-derived naphtha as a de-ashing solvent ŽDAS.. After dissolving the CLB into the solvent ŽCLBrsolvent ratio, 1r8–1r4, wrw. under high temperature Ž200– 2908C. and high pressure Ž4–5 MPa., insoluble solid particles which consist of ash and heavy preasphaltenes are settled by gravity and separated from the solution as an ash-concentrated slurry. The ash-concentrated slurry and the de-ashed solution are withdrawn from the settler as an underflow and overflow, respectively. The de-ashed heavy product is recovered from the solution by eliminating the solvent and is further hydrogenated in secondary hydrogenation. The authors have reported on the solubility of CLB in toluene and the settling velocity Ž V . of the boundary of ash content in the settler under de-ashing conditions. This paper discusses the effects of de-ashing conditions on ash concentration in the settler bottom and the operating conditions of a continuous de-ashing system. The ash content in underflow Ž C UF , kgrkg or wt.%. at the settler bottom was
)
Corresponding author. Fax: q81 78 992 5547
0378-3820r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 7 8 - 3 8 2 0 Ž 9 8 . 0 0 0 5 7 - 5
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found to increase with temperature and to decrease with the rate Žflux. of downward flow Ž underflow . . The m axim um C U F , Z, is expressed by the equation: Z s BCLBŽFLr0.35.y0.32 ŽTr523. 4.26 , where BCLB , FL and T are the characteristic parameters of organic CLB Žkgrkg or wt.%., flux of underflow in the settler Žkgrm2 s. and temperature ŽK., respectively. BCLB is also expressed by using the analytical results of organic insolubles in the CLB under de-ashing conditions. Finally, stable operating conditions of a continuous de-ashing system are confirmed to be determined as the following qualifications: < Vu < - < V <, WUF ) WSArC UF and Z ) C UF , where < Vu <, < V <, WSA and WUF are the upward velocity of the solution in the settler Žmmrs., settling velocity of the ash boundary Žmmrs. in the settler, flow rate of ash in the feed slurry Žkgrh. and flow rate of underflow Žkgrh., respectively. Under these qualified conditions, the 50 trd pilot plant constructed in Australia was operated under stable conditions for 3700 h using toluene as a DAS. q 1998 Elsevier Science B.V. All rights reserved. Keywords: Solvent de-ashing; Brown coal liquefaction; Toluene; Coal liquid bottom
1. Introduction All direct coal liquefaction processes must include a solid–liquid separation step, which is usually called a de-ashing step, because the liquid derived from coal contains mineral matter Žash. originating from the coal w1x. Therefore, many de-ashing processes have been developed for coal liquefaction based on the techniques of distillation, filtration, centrifugation, settling Žsedimentation., and so on w2–9x. The brown coal liquefaction ŽBCL. process developed by Nippon Brown Coal Liquefaction ŽNBCL. is a two-stage liquefaction process for Victorian brown coal in Australia w10,11x. It consists of four unit steps: dewatering, primary hydrogenation, de-ashing and secondary hydrogenation. In the BCL process, NBCL has developed a solvent de-ashing step to remove the ash and heavy preasphaltenes from the heavy liquefaction product Žvacuum residue. derived from the coal in the primary hydrogenation, which is called CLB Žcoal liquid bottom, BP ) 4208C. w12x. This step is expected to have high de-ashing efficiency and is easy to apply to a large-scale plant. The solvent de-ashing processes such as the anti-solvent de-ashing by C.-E. Lummus w6x and critical solvent de-ashing by Kerr McGee w7x are well-known. However, the de-ashing conditions and efficiency of these processes depend on the liquefaction conditions and kinds of liquefied coal w1–11x. The de-ashing step in the BCL process is required to have a very high de-ashing performance because the de-ashed heavy product ŽBP ) 4208C. is further hydrogenated over Ni–MorAl 2 O 3 catalyst using fixed bed reactors in the secondary hydrogenation w13–15x. Therefore, ash contained in the de-ashed heavy product should be less than 3000 ppm for long-term stable operation Žca. 8000 h. of the secondary hydrogenation plant. In this de-ashing step, toluene or naphtha obtained in the primary hydrogenation is used as a de-ashing solvent ŽDAS. w12,16x. Fig. 1 shows a simplified flow diagram of the de-ashing step in the BCL process. The molten CLB is mixed and dissolved into the DAS 4–8 times under high temperature Ž200–2908C. and high pressure Ž5 MPa.. Then, the mixture is fed to the settler as a slurry, and insoluble solid particles consisting of ash and heavy preasphaltenes are separated from the
O. Okuma et al.r Fuel Processing Technology 56 (1998) 229–241
231
Fig. 1. Simplified flow diagram of de-ashing step in BCL process. CLB: Heavy liquefaction product Žcoal liquid bottom, BP) 4208C.. DAO: De-ashed heavy liquefaction product ŽBP) 4208C..
solution as an ash-concentrated slurry of downward flow Žunderflow., and the de-ashed solution is recovered as an upward flow Žoverflow. w12,17x. In a previous paper w17x, the authors reported that ash and heavy preasphaltenes can be separated from CLB as insoluble solid particles by solvent de-ashing, which dissolves the CLB and settles the insolubles by gravity. The ash content in the recovered heavy product was less than 3000 ppm in order to be useful as a feedstock for the secondary hydrogenation. As the solid particles form the boundary of ash content in the settler, its settling velocity Ž V . is estimated from the de-ashing conditions such as temperature, and the content and properties of CLB in the feed slurry. For de-ashing with a continuous system, the upward linear velocity of the solution must be determined to keep the ash-boundary in the settler during operation. In addition, almost all of the ash in the feed slurry must be withdrawn as the underflow from the settler bottom. Therefore, it is necessary to know the ash concentration in the settler for predicting stable operating conditions of the de-ashing system, although the ash contents in both feed slurry and underflow are required to be as high as possible to increase the process efficiency. This investigation was carried out to clarify the effects of de-ashing conditions on the ash concentration in the settler bottom with a continuous de-ashing system using toluene as a DAS, confirming the conditions for stable operation of the plant which provides de-ashed heavy product with low ash content enough to feed the secondary hydrogenation.
2. Experimental 2.1. Samples of heaÕy liquefaction product (CLB) The CLBs used were heavy products ŽBP ) 4208C. recovered by vacuum distillation from the liquefaction products derived from Victorian brown coal ŽYallourn. in a pilot
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Table 1 Liquefaction conditions and properties of CLBs ŽBP- 4208C. CLB
Liquefaction conditions Temperature Ž8C.
PA PB PC PD PE
435 450 450 450 450
Pressure ŽMPa.
15 15 15 20 15
Properties of CLB BTMrRa
Yes Yes Yes Yes Yes
Ash Žwt.%.
14.2 13.2 13.1 12.5 14.6
Solvent extraction Žwt.%. HS
HI-BS
BI-PS
Pb
19.7 23.6 21.1 26.0 25.1
30.6 43.9 42.0 43.7 39.5
30.7 16.2 18.3 17.1 16.2
19.0 16.3 18.6 13.2 19.2
a
Feed solvent containing bottom fraction ŽCLB, BP) 4208C.. Pyridine insolubles containing ash. Other liquefaction conditions in the primary hydrogenation step of the pilot plant Ž50 ton-dry coalrday.: nominal reaction time: 1.0 h, solventrdaf-coal ratio: 2.5 Žwrw.. cat. 3.0 wt.% on daf-coal as Fe, SrFe Žatomic ratio.: 1.2. b
plant of 50 ton-dry coalrday in Australia w18x. Typical Yallourn coal ŽC: 67.0, H: 4.7, N: 0.6, S: 0.2 and Odiff.: 27.5 wt.% on daf basis. contained 11.6 wt.% moisture and 1.4 wt.% ash after drying. The liquefaction conditions were: temperature: 430–4508C, pressure: 15–20 MPa and nominal residence time: 1.0 h. A catalyst Žiron oxide and sulfur. of 3.0 wt.% was added to daf coal as Fe, and the SrFe atomic ratio was 1.2. Table 1 shows the liquefaction conditions and the properties of the representative CLBs. 2.2. De-ashing with a continuous system Fig. 2 shows a continuous system used for the de-ashing experiments ŽDA-PDU.. This system has two types of settlers shown in Fig. 3. The settler-A and settler-B were used for changing the linear velocity and the residence time of overflow and underflow.
Fig. 2. Flow diagram of a continuous de-ashing system Žprocess development unit, DA-PDU.. Ž1. De-ashing solvent ŽDAS. tank, Ž2. DAS preheater, Ž3. CLB melter, Ž4. dissolver, Ž5. settler, Ž6. overflow receivers, Ž7. underflow receivers.
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Fig. 3. Types of settlers used for de-ashing with DA-PDU. Settler-A: Inner diameter: 50 mm, height: 1850 mm, volume: 0.0037 m3. Settler-B: Inner diameter: 150 mm, height: 1800 mm, volume: 0.0307 m3.
The experimental procedure is as follows. At first, only toluene of a desired flow rate was fed from a DAS tank Ž1. through a DAS preheater Ž2. into a dissolver Ž4. and settler Ž5. which was heated up to a desired temperature. Then, a CLB from a melter Ž3. at ca. 2008C was fed into Ž4. with a desired flow rate. The CLB was mixed with toluene in Ž4.,
Fig. 4. Ash distribution in the settler and its change during operation without withdrawing underflow. De-ashing with settler-B Žsee Fig. 3. for CLB-PB. Conditions: CLBrtoluene ratio: 1r3 Žwrw., flow rate of feed slurry: 20.0 kgrh, temperature: 2508C, overflowrunderflow ratio: 1r0 Žwrw.. Keys: operating period, I 6 h, q 9 h, ` 12 h, = 17 h.
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and dissolved under agitation. The mixture Žthe feed slurry. fed to Ž5. was withdrawn from the top and bottom of Ž5. to each receiver Ž6. and Ž7. as overflow and underflow, respectively. Two receivers were used in turn for overflow or underflow. The temperature in Ž4. was controlled to be the same as that in Ž5. at a range of 230–2908C. The pressure of vessels Ž4. – Ž7. was maintained at 5 MPa to suppress the vaporization of toluene. CLBrtoluene ratio of the feed slurry was changed from 1r4 to 1r8 Žwrw., and the range of the slurry feed rate and overflowrunderflow ratio was 10–30 lrh and 9r1–4r1 Žwrw., respectively. Both the overflow and underflow sampled at 0.5 h intervals were distilled to remove toluene, and the ash content in the recovered heavy products was analysed. Based on these analytical results, the de-ashing performance was estimated. In addition, the distribution and concentration of ash in the settler under de-ashing conditions without withdrawal of underflow were measured using settler-B by analysing the samples obtained from the sampling spots shown in Fig. 4. 3. Results and discussion 3.1. Ash distribution and settling in the settler Fig. 4 shows the change in ash distribution in settler-B for CLB-PB Žsee Table 1. during the operation of DA-PDU without withdrawal of underflow. The de-ashing conditions of this experiment were: temperature: 2508C, CLBrtoluene ratio: 1r3 Žwrw. and slurry feed rate: 20.0 kgrh. This result shows that the boundary of ash content forms in the settler of DA-PDU, as well as in the settler of a batch de-ashing system w17x. The ash content in the slurry at the lower part of the settler gradually increased along with the operating period by the accumulation of the fed ash, resulting in raising the boundary, although it decreased at the upper part. These results indicate that, for de-ashing with a continuous system, it is necessary to maintain the ash boundary in the settler by controlling the withdrawal of the upper and lower parts from the top and bottom of the settler as overflow and underflow, respectively. 3.2. De-ashing performance Fig. 5a and b show the change in the ash contents in overflow and underflow for CLB-PA Žsee Table 1. during the operation of the DA-PDU with settler-A ŽRun-A1 and Run-A2, respectively. at 2508C, with a slurry feed rate: 12.5 kgrh and overflowrunderflow ratio: 1r4 Žwrw.. The CLBrtoluene ratios in the feed slurry for Run-A1 and Run-A2 were 1r7 and 1r5 Žwrw., respectively. The height of a boundary in the settler of a continuous system depends on the upward velocity of overflow Ž Vu . and the settling velocity of the ash boundary Ž V .. Therefore, < Vu < must be smaller than < V < to maintain the boundary at a lower position in the settler because the boundary is raised under the condition of < Vu < ) < V <. The < Vu < for both Run-A1 and Run-A2 was fixed at 2.17 mmrs, because the < V < of CLB-PA estimated by using Eq. Ž1. w17x are 10.65 and 8.20 mmrs, respectively, V s A CLB Ž CSA 2.5 .
y0 .91
Ž Tr523.
9.5
Ž 1.
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Fig. 5. Change in ash contents in overflow and underflow withdrawn from the settler during operation. Ža. Run-A1 with settler-A Žsee Fig. 3. for CLB-PA. De-ashing conditions: CLBrtoluene ratio: 1r7 Žwrw., flow rate of feed slurry: 12.5 kgrh, temperature: 2508C, overflowrunderflow ratio: 4r1 Žwrw., upward velocity of overflow, Vu : 2.17 mmrs. Žb. Run-A2 with settler-A for CLB-PA. De-ashing conditions: CLBrtoluene ratio: 1r5 Žwrw., other conditions are the same with Run-A1.
where A CLB is the characteristic parameter of organic components in CLB Žthe A CLB for CLB-PA was 7.80 mmrs., CSA is the ash content in the feed slurry Žwt.%. and T is the temperature ŽK.. During the operation of Run-A1, shown in Fig. 5a, the ash content in the overflow was kept almost constant at a very low level Žca. 0.018 wt.%. for 14 h. As this ash content corresponded to 1600 ppm of ash in de-ashed heavy product, it was suitable for a feedstock to the secondary hydrogenation. In the latter period of the operation, the ash content in the underflow saturated at ca. 8 wt.%, and the amount of withdrawn ash in the underflow became almost equal to that in the feed slurry. These results mean that the ash boundary was maintained in the settler, and that the ash content in the upper part of the settler became very low during the operation of Run-A1. On the other hand, for Run-A2 shown in Fig. 5b, the ash content in the overflow raised sharply at 11–12 h after starting the operation, although it was also very low until that time. During this operation, the amount of withdrawn ash as underflow was always smaller than that in the feed slurry. This indicates that, for Run-A2, the ash content or flow rate of the underflow was too small to withdraw all of the fed ash into the settler. Consequently, a part of the fed ash accumulated in the settler, and the ash boundary gradually raised during the operation of Run-A2. A flow rate of the ash in the feed slurry must be smaller than that defined by the product of the ash content and flow rate of underflow in order to withdraw all of the ash contained in the feed slurry. Therefore, the following Eq. Ž2. is introduced: WUF ) WSA C UF Ž 2. where WSA is the flow rate of ash in a feed slurry Žkgrh., WUF is the flow rate of underflow Žkgrh. and C UF is the ash content in underflow Žkgrkg.. Eq. Ž2. explains the results for Run-A1 and Run-A2, and reveals that the ash concentration in the settler bottom must be estimated to fix the de-ashing conditions for stable operation of a continuous system.
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3.3. Ash concentration at the settler bottom The ash concentration in the settler bottom is considered to be affected by the same factors as those affecting the settling velocity of the ash boundary Ž V . discussed in a previous paper w17x, because they depend on the same repulsive force between the insoluble particles including the ash w19x. Therefore, the effects of slurry residence time, temperature and ash content in the feed slurry on the ash concentration at the settler bottom were examined by using the DA-PDU. The flow rate Žflux. of underflow, which is determined by the slurry feed rate and overflowrunderflow ratio, is also an important factor affecting the ash concentration, because it determines residence time of the underflow and the height of the ash boundary in the settler described earlier. Fig. 6 shows the effects of flow rate Žunderflow flux. on the ash content in the underflow for CLB-PB at 2508C. For Run-A3 Ž`. and Run-A4 Ž^. with settler-A, the underflow fluxes were 0.214 and 0.397 kgrm2 s, respectively. For Run-B1 Žv . and Run-B2 Ž'. with settler-B, they were 0.0172 and 0.0555, respectively. For all Runs, the ash content in the underflow increased with the operating period and was saturated in the latter period of the operation. During these operations, the amount of withdrawn ash as underflow was smaller than that in the feed slurry, and a part of the fed ash, except for one in the underflow, flowed out from the top of the settler as overflow. This means that the boundary of the ash content raised along with the operating period, and the region of very low ash content in the upper part of the settler disappeared during the operation, as described earlier ŽFig. 5b.. The results shown in Fig. 4 also suggest that the accumulated ash at the settler bottom was compressed by its weight w20x. Therefore, the
Fig. 6. Effect of flow rate of underflow Žunderflow flux. on ash content in underflow. De-ashing for CLB-PB at 2508C. ` Run-A3 with settler-A Žsee Fig. 3.. Conditions: CLBrtoluene ratio: 1r3 Žwrw., flow rate of feed slurry: 7.5 kgrh, overflowrunderflow ratio: 4r1 Žwrw., underflow flux: 0.214 kgrm2 s. ^ Run-A4 with settler-A. Conditions: CLBrtoluene ratio: 1r3, flow rate of feed slurry: 12.5, overflowrunderflow ratio: 4r1, underflow flux: 0.397. v Run-B1 with settler-B Žsee Fig. 3.. Conditions: CLBrtoluene ratio: 1r3, flow rate of feed slurry: 11.1, overflowrunderflow ratio: 9r1, underflow flux 0.0172. ' Run-B2 with settler-B. Conditions: CLBrtoluene ratio: 1r2.5, flow rate of feed slurry: 17.5, overflowrunderflow ratio: 4r1, underflow flux: 0.0555.
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Fig. 7. Effect of flow rate of underflow Žunderflow flux. on the maximum ash content in underflow Ž Z .. De-ashing for CLB-PB at 2508C with settler-A and settler-B Žsee Fig. 3.. Conditions: See Fig. 6.
saturated ash content in the underflow for each operation is found to be at its maximum under specific de-ashing conditions. Fig. 7 summarizes the relationship between the underflow flux and the maximum ash content Ž Z, wrw or wt.%. in the underflow obtained from the saturated values of Runs shown in Fig. 6. This result shows that the ash in the slurry at the settler bottom was concentrated with a decrease in the underflow flux, resulting in an increase in Z. Fig. 8 shows the effects of temperature on Z for CLB-PB with settler-A under the conditions of CLBrtoluene ratio: 1r4, flow rate of feed slurry: 12.5 kgrh and underflow flux: 21.0 kgrm2 s. This result also indicates that Z increases along with the de-ashing temperature. Based on the results shown in Figs. 7
Fig. 8. Effect of temperature on the maximum ash content in underflow Ž Z .. De-ashing for CLB-PB with settler-A Žsee Fig. 3.. Conditions: CLBrtoluene ratio: 1r3 Žwrw., flow rate of feed slurry: 12.5 kgrh, overflowrunderflow ratio: 4r1 Žwrw., underflow flux: 0.397 kgrm2 s.
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and 8, assuming that the parameters are independent of each other, the following Eq. Ž3. is introduced to estimate Z: a
b
Z s BCLB Ž FL . Ž T . s BCLB Ž FLr0.35 .
y0.32
Ž Tr523.
4.26
Ž 3.
where BCLB is the characteristic parameter of organic components of CLB except ash content Žkgrkg or wt.%., FL is the flux of downward flow Žunderflow flux. in the settler Žkgrm2 s. and T is the temperature ŽK.. Here, a and b were determined for CLB-PB using the slopes of lines shown in Figs. 7 and 8, respectively. Table 2 shows the BCLB values of the CLBs determined by the DA-PDU operations. 3.4. Properties of insoluble matter and characteristic parameter of CLB (BC L B ) BCLB in Eq. Ž3., which represents the effects of the properties of organic components in CLB on the ash concentration at the settler bottom, is determined as a constant for each CLB by the de-ashing experiment with DA-PDU as well as A CLB expressing its effects on the settling velocity of the ash boundary Ž V . w17x. However, it is very troublesome to determine the BCLB for each CLB by the operations of the DA-PDU. The A CLB has been found to be expressed by using the analytical results of CLB and the insoluble organic matter with tetrahydrofuran ŽTHF-I. at its boiling point of 668C, which closely corresponds to the insolubles Žsolid particles. under the de-ashing conditions w17x. Therefore, the relationship between BCLB and A CLB was examined to express the BCLB by the A CLB calculated by using the following Eq. Ž4.: A CLB s 5.3 Ž ar1.7 .
6 .6
Ž br0.6 .
y4.5
Ž 4.
where a is the ratio of content of organic THF-I to the ash content in CLB Žwrw., and b is HrC atomic ratio of organic THF-I. Fig. 9 shows the relationship between the A CLB calculated by Eq. Ž4. and the BCLB determined by the operations of the DA-PDU for the CLBs shown in Table 2. Since they
Fig. 9. Relationship between A CL B estimated by Eq. Ž4. and BCLB determined by experiments with DA-PDU. The used CLBs are shown in Table 1.
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Table 2 The characteristic parameter of organic components of CLB Ž BCL B . in Eq. Ž3. CLB
PA
PB
PC
PD
PE
BCL B Žwt.%.
10.9
8.0
5.9
9.3
7.0
have a good correlation, the BCLB is found to be empirically expressed by the A CLB as follows: d
BCLB s c Ž A CLB . s 3.08 Ž A CLB .
0.619
Ž 5.
Fig. 10 shows the correlation of the Z between the experimental values and the calculated ones by using Eqs. Ž3. – Ž5.. This good correlation Ž r s 0.978. indicates that these equations are useful to determine the maximum ash content in underflow Ž Z . under specific de-ashing conditions. In addition, these results also indicate that the same interaction among the insoluble solid particles including ash, such as surface charge w19x, plays an important role in their agglomeration, settling and concentration under de-ashing conditions. 3.5. Prediction of the operating conditions of a de-ashing plant The present and previous investigations w17x confirm that the conditions in the settler of a solvent de-ashing plant can be predicted by calculating the V and Z using the analytical results of CLB when the slurry feed rate, overflowrunderflow ratio and temperature are fixed. The de-ashing conditions for stable operation of the plant are represented by the following equations as described earlier. < Vu < - < V < , and WUF ) WSA C UF To operate the de-ashing plant efficiently, the CLBrtoluene ratio should be as high as possible to raise the processing capacity of the plant. In addition, the flow rate of underflow is desired to be as small as possible to increase the efficiency of recovery of
Fig. 10. Correlation of maximum ash concentrations in underflow Ž Z . estimated by Eqs. Ž3. – Ž5. to ones determined by experiments with DA-PDU. The used CLBs are shown in Table 1.
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de-ashed heavy product and DAS Žtoluene.. Therefore, the conditions of C UF s Z is most advantageous for the operation of a de-ashing plant because smaller W UF provides higher de-ashing efficiency. Under the conditions qualified by these equations, the 50-ton-dry coalrday pilot plant constructed in Australia was successfully operated for 3700 h w18x. Based on this operation, the solvent de-ashing process is confirmed to be excellent for removing the ash from the heavy liquefaction product ŽCLB. derived from Victorian brown coal. 4. Conclusion Solvent de-ashing from heavy liquefaction product ŽCLB. derived from Victorian brown coal using toluene was investigated with a continuous system from the view point of the ash concentration in the settler bottom to clarify the stable operating conditions of the plant. From this investigation, the following results were obtained. Ž1. Insoluble solid particles including ash in toluene solution formed the boundary of ash content in the settler of a continuous de-ashing system, as well as in a batch de-ashing system. Since the ash content of the heavy products recovered from the overflow of the settler was less than 3000 ppm, the de-ashing performance with the continuous system was enough to prepare a feed stock for secondary hydrogenation with fixed bed reactors. Ž2. The settled ash Žsolid particles. was concentrated at the settler bottom. The ash content in the underflow withdrawn from the settler bottom increased with increasing temperature and with decreasing flow rate Žflux. of underflow. According to these results, the equation expressing the maximum ash content in the underflow Ž Z, kgrkg or wt.%. is introduced by using the characteristic parameter of the organic components of CLB, the flux of downward stream Žunderflow. in the settler and the temperature. Ž3. The effect of properties of organic CLB on the ash content in the underflow is found to be expressed by the BCLB which correlated well with the A CLB expressing its effect on the settling velocity of the ash boundary. Ž4. The present and previous investigations confirm the equations qualifying stable operating conditions of a de-ashing plant using toluene which depend on the properties of CLB produced in primary hydrogenation. These equations can determine the flow rate of a feed slurry, the upward velocity of solution Žoverflow. and the flow rate of underflow in the settler by using the analytical results of CLB such as ash and THF-I contents. Under the conditions predicted by these equations, the 50-ton-dry coalrday pilot plant constructed in Australia was successfully operated for 3700 h, and the solvent de-ashing process is confirmed to be excellent for removing the ash from the heavy liquefaction product derived from Victorian brown coal. Acknowledgements This investigation was carried out through the financial support of New Energy and Industrial Technology Development Organization ŽNEDO.. The authors thank NEDO for their permission to publish this paper.
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W.F. Leu, F.M. Tiller, Powder Technol. 40 Ž1984. 65. J.A. Kleinpeter, D.C. Jones, P.J. Dudt, F.P. Burke, Ind. Eng. Chem. Process Des. Dev. 18 Ž1979. 535. E. Gorin, C.J. Kulik, H.E. Lebowitz, Ind. Eng. Chem. Process Des. Dev. 16 Ž1977. 95. E. Gorin, C.J. Kulik, H.E. Lebowitz, Ind. Eng. Chem. Process Des. Dev. 16 Ž1977. 102. N.G. Moll, G.J. Quarderer, Chem. Eng. Prog., November 1979, 46. M. Peluso, D.F. Ogren, Chem. Eng. Prog., June 1979, 41. R.M. Adams, A.H. Knebel, D.E. Rhodes, Chem. Eng. Prog., June 1979, 44. I. Romey, Fuel 61 Ž1982. 988. J.W. Clarke, T.D. Rantell, Fuel 65 Ž1986. 295. K. Terunuma, Y. Nakako, T. Matsumura, Proceedings of World Cong. III of Chem. Eng., Tokyo, 1 Ž1986. 73. O. Okuma, K. Saito, A. Kawashima, K. Okazaki, Y. Nakako, Fuel Processing Technol. 14 Ž1986. 23. O. Okuma, K. Masuda, N. Okuyama, T. Hirano, T. Matsumura, Proceedings of 1993 International Conference on Coal Science, Banff, 1 Ž1992. 295. S. Nagae, Y. Mito, O. Okuma, K. Saito, T. Matsumura, Proceedings of 1989 International Conference on Coal Science, NEDO, Tokyo, 2 Ž1989. 931. Y. Mito, T. Takahashi, O. Okuma, K. Saito, T. Matsumura, Y. Nakako, Proceedings of 1983 International Conference on Coal Science, Pittsburgh Ž1983. 774. T. Kaneko, Y. Kageyama, T. Kimura, S. Kawai, Proceedings of 1989 International Conference on Coal Science, NEDO, Tokyo, 2 Ž1989. 919. O. Okuma, K. Masuda, N. Okuyama, T. Hirano, Nippon Enerugi Gakkai-Shi 75 Ž1996. 257, J. Jpn. Inst. Energy. O. Okuma, K. Masuda, N. Okuyama, T. Hirano, Fuel Processing Technol. 51 Ž1997. 177. Y. Nakako, T. Ohzawa, H. Narita, Proceedings of 1991 International Conference on Coal Science, Univ. of Newcastle-upon-Tyne Ž1991. 652. B.R. Rodgers, AIChE Symp. Ser. 76 Ž192. Ž1980. 68. K.R. Valdyanathan, J.D. Henry Jr., F.H. Verhoff, Ind. Eng. Chem. Fundam. 20 Ž1981. 165.