Continuous Production of Activated Carbon from Distilled Spirit Lees

Article pubs.acs.org/IECR

Continuous Production of Activated Carbon from Distilled Spirit Lees: Process Design and Semi-Industrial Pilot Yin Wang,†,‡ Qiang Li,§ Siyu Wang,∥ Bin Yi,⊥ Jun Yang,⊥ Juan Yang,† and Guangwen Xu*,† †

State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, P.O. Box 353, Beijing 100190, China ‡ Institute of Urban Environment, Chinese Academy of Sciences, Xiamen 361021, China § CNOOC New Energy Investment Company, Ltd., Beijing 100016, China ∥ School of Materials Science and Metallurgy, Northeastern University, Wenhua Road, Shenyang 110819, China ⊥ Luzhoulaojiao Group Company Ltd., Luzhou 646000, China ABSTRACT: This study is devoted to developing a continuous activated carbon (AC) production process integrating drying, carbonization, and physical activation without external heat input. The massive byproduct in the distilled spirit industry, distilled spirit lees (SL), was used as the raw material in this study. The kinetic behaviors in each step, including drying, carbonization, and activation, were first investigated via laboratory tests. The results show that the whole AC production process can be completed in 30 min, including most of the time for drying, several minutes for carbonization, and several seconds for activation. On the basis of these laboratory results, an integrated process for continuous production of AC was proposed. The mass and heat balance calculation demonstrated a good balance for the developed process technology, and a pilot plant treating 2000 kg of SL/ h was in turn built and commissioned to run autothermally and continuously. This demonstrated thus the technology for application to granular feedstock such as SL, although the produced AC from SL had surface areas of only about 191 m2/g and relatively low adsorption values, including 610−630 mg/g for iodine and 20−30 mg/g for methylene blue, due to the too short activation time in the pilot activator.

1. INTRODUCTION It is known that activated carbon (AC) is a kind of porous carbon material with a high specific surface area and large adsorption capacity, and it is widely used in many fields, including various industrial purifications and decolorizations, wastewater treatment, catalyst preparation, and so on.1,2 Great efforts have been devoted to upgrading the technology process and optimization of experimental conditions for preparing activated carbon in the laboratory.3−8 Most of the acquired results, however, have not been put into practice on an enlarged scale. Thus, the study of AC production technologies at the pilot, even industrial, scale is highly necessary in view of developing commercialized new technologies. The production method of AC can grossly be divided into two types, chemical activation and physical activation. Although chemical activation can obtain AC with better performance, physical activation is commonly used in industry due to its environmental friendliness and also its product adaptability to various special applications such as for the food and drink industries. The process of physical activation includes generally the steps of drying, carbonization, and activation, and external heat input is usually required. On the other hand, the simultaneously generated tar and waste gas actually represent wasted energy and also pollutants of the process. The commonly used furnaces for physical activation are the socalled multihearth furnace, SLEP furnace, and rotary kiln furnace,5 which produce AC in a batchwise manner. The batchtype production line has disadvantages of low productivity, unstable product quality, high labor intensity, and high energy © 2013 American Chemical Society

consumption. Therefore, it is highly necessary to develop economically continuous AC production technology that can make full use of the wasted energy and does not require external heat input. Theoretically, AC can be prepared from any carbonaceous material. In addition to the conventional materials of coal and wood, biomass waste from agriculture and industrial processes may provide a kind of cheap raw material for AC production. In recent years, some agricultural wastes, including rice husk,9,10 rice straw,11 corn cob,12,13 and cotton stalk,14 and also some industrial biomass wastes such as herb residues,15 bagasse,16 peanut hull,17 and nut shells18,19 have been tested as materials for AC production. Our previous study has demonstrated that the distilled spirit lees (SL) of the brewage industry can also be used to produce AC with acceptable quality.20 In China, more than 20 million tons of SL is produced each year, and currently most of it was simply combusted or disposed to the open air. This not only wastes the biomass resource but also causes environmental pollution.21 The conversion of SL into AC would provide a value-added utilization of the waste, while it also protects against environmental pollution caused by direct disposal. In our previous study, the process parameters for preparing AC from SL by steam activation have been optimized in the Received: Revised: Accepted: Published: 6761

December 16, 2012 March 25, 2013 April 28, 2013 April 28, 2013 dx.doi.org/10.1021/ie303489v | Ind. Eng. Chem. Res. 2013, 52, 6761−6769

Industrial & Engineering Chemistry Research

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laboratory.20 This study is devoted to developing an autothermal continuous AC production process integrating drying, carbonization, and activation free of external heat input. We investigated first the kinetics involved in each of the three steps and in turn designed and tested a newly developed AC production process. Heat balance calculation regarding the process demonstrated a good self-supply of the processing energy. A pilot plant treating 2000 kg/h of granular feedstock, here the distilled SL, was built and commissioned to run autothermally and continuously. Some performance parameters are presented and show essentially the technology feasibility of the developed process for producing AC continuously with high efficiency (no external energy input) from granular biomass wastes such as SL, sawdust, and rice husk.

2. EXPERIMENTAL SECTION The raw distilled SL from Luzhou Laojiao Corp. of China was used, which had a moisture content of 60 wt.%. The shape of the SL varied, but most was similar to rice husk and had an average size of 3 mm (width) × 7 mm (length). For the laboratory carbonization test, the raw SL was dried in an oven at 110 °C for 24 h to remove its moisture in advance. The dried SL was in turn carbonized at 450 °C for 1 h in nitrogen, and the resulting carbonized SL or carbonized carbon (CC) was in turn used as the material for the activation tests. The specific surface area (SBET) of the resulting AC material was determined with nitrogen adsorption in a gas sorption analyzer (ASAP 2020, Micromeritics. Norcross, GA). The iodine and methylene blue (MB) absorption values for the AC were measured according to the China national standards GB/ T 12496.8-1999 and GB/T 12496.10-1999, respectively. According to the IUPAC classification, the borderline between micropore and mesopore is 2 nm. The methylene blue molecule has a minimal molecular cross-section size of about 0.8 nm, and it can enter pores above 1.3 nm; that is, the methylene blue molecules mainly enter mesopores and some large micropores. The iodine molecule is about 0.27 nm in size, thus allowing its penetration into narrower micropores below 1 nm. Consequently, iodine adsorption quantifies the numbers of micropores in the analyzed AC, while methylene blue adsorption provides a quantitative measure of the mesopore numbers in the material. 2.1. Kinetic Studies. Figure 1 shows the experimental setup used for studying the kinetics of biomass waste drying, carbonization, and activation. In studying the drying kinetics, a nitrogen stream of 4 L/min was introduced into the bottom of the quartz tube and the gas stream was preheated to a preset temperature by passing it through the alumina ball layer in the bottom of the tube reactor. After the furnace and nitrogen temperatures were stable, 10.0 g of wet SL placed in a basket in advance was quickly inserted into the hot gas flow of the reactor at the position shown in Figure 1 to start the drying process of SL. For a preset time of drying, the dried SL was taken out and weighed immediately. The dewatering ratio, X (%), was calculated by X=

MCraw − MCdry MCraw

× 100

Figure 1. Experimental setup used in studying the kinetics of biomass waste drying, carbonization, and activation.

flow temperature reached the desired temperature, 5.0 g of dried SL, which was placed also in a basket in advance, was quickly inserted into the hot N2 stream of the reactor to initiate pyrolysis. After an expected period of reaction, the sample was taken out and weighed immediately. The yield of CC, Y (wt %), was determined by mpyrolyzed Y= × 100 mdry (2) where mdry and mpyrolyzed represent the mass of dried SL and obtained CC, respctively. Steam was used as the activation agent in measuring the activation kinetics. The reactor was first heated to a preselected temperature, while the whole system was meanwhile purged with N2 to remove oxygen. Then water was introduced into the reactor at 4.5 g/min and evaporated into steam in the alumina ball bed. After the system temperature was stable, 5 g of CC placed in a basket in advance was quickly inserted into the steam stream of the reactor to activate the CC for 40−130 s. While the feed of steam and nitrogen from the reactor bottom was stopped, the basket was lifted to the expanding area of the reactor and cooled in a nitrogen flow supplied from the side of the reactor. Finally, the obtained AC was collected and vacuumdried for subsequent characterization. 2.2. Pyrolysis Characterization. The batchwise testing apparatus in Figure 1 is unable to measure the product distribution in carbonization or pyrolysis. To understand the pyrolysis characteristics of SL, carbonization of SL was performed in a laboratory fluidized bed pyrolysis facility shown in Figure 2, which consisted mainly of a fluidized bed reactor of 30 mm inner diameter, several tar absorbers (with acetone) immersed in an ice−water bath, and a silicon gel gas drier before gas sampling for composition analysis. A given rate flow of N2 was injected into the reactor to purge air from the system, and it also served as the fluidizing gas during carbonization (or pyrolysis). For carbonization tests at different temperatures, the nitrogen flow rate was adjusted to ensure similarly full fluidization. The test was started with heating the reactor to the desired temperature in the reaction atmosphere depicted above. Then a given amount of dried SL was instantaneously dropped into the bed to be fluidized and meanwhile pyrolyzed. The produced tar

(1)

where MCraw and MCdry represent the total moisture contents in the orginal wet SL and the dried SL, respectively. The test procedure for the carbonization kinetics was similar to that for the drying kinetics shown above. When the nitrogen 6762

dx.doi.org/10.1021/ie303489v | Ind. Eng. Chem. Res. 2013, 52, 6761−6769


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Figure 2. Laboratory setup used for testing SL pyrolysis.

was collected by the tar absorbers, and the cleaned and dried pyrolysis gas was sampled using an air bag. A micro gas chromatograph (Agilent GC3000, Santa Clara, CA) was used to measure the concentrations of H2, CH4, CO, and CO2 of the pyrolysis gas. The tar liquid collected in the absorbers was quantified via the method described in the literature.22

3. RESULTS AND DISCUSSION 3.1. Laboratory Data Acquisition. Due to its high moisture content, the raw SL has to be dried before treatment. To avoid carbonization of SL and the large release of COD at temperatures above 250 °C,23 the drying temperature was kept below 200 °C. A nitrogen flow of 4 L/min was adopted to fluidize SL particles and also enhance the drying (dewatering) process. Figure 3 shows the characteristic data of drying for wet

Figure 4. Carbonization behaviors of dried SL under different temperatures.

efficiency for activated carbon production. This would also lead to a higher CC yield as a result of the lowered gas-phase product release in the shorter time, and it is surely beneficial to the final yield of AC. Similar to carbonization, the literature-reported time of activation is usually 30 min to several hours.6,16,19 Figure 5 presents our results on the effects of activation time and activation temperature on iodine and methylene blue values of the resulting ACs prepared by fluidized bed activation using steam as the activation agent. At 900 °C, both iodine and methylene blue adsorption capacities increased with prolonged activation time, indicating gradual development in both micropores and mesopores. At temperatures of 950 °C and above, the reaction between steam and carbon becomes gradually too violent to open pores and to convert micropores to mesopores and macropores. This caused the iodine value to decrease and the methylene blue value to increase at 950 °C and both to decrease at a rather high temperature of 1000 °C with increasing activation time. These results show that the quick activation would lead to a high quality of produced AC and also a high production efficiency due to the shortened activation time. At times shorter than 40 s in Figure 5 (including the particle heating time), an activation temperature of 1000 °C may lead to higher adsorption values for both iodine and methylene blue, for example, 336 and 140 mg/g for the SL-derived AC at 40 s. Thus, appropriately high activation temperatures, such as 900−1000 °C, are needed. The higher the temperature, the shorter the activation time. The quality of the AC described above is comparable to that of sample SA-800 prepared by activation at 800 °C for 1 h reported in our previous study (581 mg/g for iodine and 94 mg/g for methylene blue).20 Thus, it is fully possible to obtain

Figure 3. Drying characteristics of wet SL in a gas flow of 195 °C.

SL in a hot gas stream of 195 °C. In 10 min only 53.3% of the water was removed, and for a deep drying of raw SL at this temperature the drying time needs to be over 30 min. This drying condition represents well the conditions present in the rotary oven drier, and it will be the basis for the design of the drier shown later in the pilot test. The existing literature studies report16,19,24 that the carbonization process usually takes a long time, for example, 1 h or even longer. Figure 4 shows the CC yield varying with the carbonization time at different temperatures for SL measured by the apparatus in Figure 1. When the material was carbonized at 450 °C for 300 s, the yield became steady at about 39.0%,20 indicating that for SL the carbonization time should be within 300 s. Raising the carbonization temperature can further shorten the carbonization time, which was about 90 s at 650 °C. Not as the literature reports, the carbonization of SL can thus be implemented in a much shorter time to raise the overall 6763



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increased. The percentage of pyrolysis water was about 25 wt % (dry base) at the tested temperatures. Figure 7 compares the compositions of the dry pyrolysis gases obtained at the three tested temperatures corresponding

Figure 7. Composition and lower heating value (LHV) of pyrolysis gas under different carbonization temperatures.

to Figure 6. Plotted are the major components CO (35−55 vol %), CO2 (10−45 vol %), H2 (∼10 vol %), and CH4 (∼10 vol %). Raising the temperature obviously increased the productions of CO and CH4 but decreased that of CO2. The H2 content in the gas was about 12 vol %. These caused the heating value of the pyrolysis gas (right axis) to increase with increasing temperature from about 10 to 18 MJ/(N m3). Thus, the pyrolysis gas can be a fuel in the AC production process, as will be shown herein in the pilot facility where the gas from carbonization is combusted directly in the downstream of the carbonizer to generate the hot flue gas needed for SL drying. 3.2. Pilot Process Design. The above results show that the entire process of powder AC production can be completed in about 30 min, including most of the time for drying, several minutes (2−5 min) for carbonization, and about 10 s for activation. On the basis of this information, a pilot-scale AC production process for granular feedstock such as SL, which integrates drying, carbonization, and activation without external heat input, was developed. The rotary drier, cyclone carbonizer, and pneumatic riser activator were used to match the reaction time for drying, carbonization, and activation, respectively. The previously determined time requirements for drying and activation are directly used to estimate the necessary (although not optimal) lengths and diameters of the rotary oven drier and riser activator (for using granular CC), respectively. For carbonization, we noted that the particle residence time in the cyclone should be shorter than the detected carbonization time of 2−5 min. Thus, the design adopted a large storage area beneath the cyclone carbonizer (see Figure 8) to increase the reaction time. Figure 8 shows the schematic diagram of the designed AC production process. In practical production, raw SL with a high moisture content is continuously fed into the rotary drier and dried by the flue gas generated in firebox 1 (1). The dried SL is then separated by the first cyclone (6) and proportionally divided into two parts. The first part is burned in firebox 2 (9), and its resulting high-temperature flue gas, mixed with some unburned carbon, is subsequently delivered into the cyclone reactor (10) to carbonize another part of the dried SL. With carbonization, the resultant CC and volatile gas (including tar and noncondensable pyrolysis gas) are separated in the cyclonic reactor simultaneously. The volatile gas is in turn sent to firebox 1 as fuel to produce high-temperature flue gas for drying by burning. The generated CC is stored in a hopper (11) just

Figure 5. Effects of the activation time and temperature on (a) iodine and (b) methylene blue values of the resulting ACs.

AC efficiently from SL via fluidized bed activation. Heat transfer calculation further showed that in the above-tested activation the heating of the carbon (char) sample to the activation temperature would take a relatively long time, for example, about 32 s for heating to about 1000 °C. Thus, the actual reaction time between steam and carbon for developing pores at a temperature of 1000 °C would be very quick and on the order of about 10 s. This would be the cause especially when the gas−solid interaction is more extensive, such as in a transport bed in comparison with a fluidized bed. In the carbonization process, the raw material has to be cracked into char, tar, and pyrolysis gas. To understand the product ratio, which is the basis for the mass and heat balance calculation in the process of pilot plant design shown later, the pyrolysis characteristics of SL were investigated in the fluidized bed reactor shown in Figure 2. Figure 6 shows the effect of temperature on the pyrolysis product distribution. With increasing temperature from 450 to 650 °C, the char (30−40 wt %, dry base) and tar (∼10 wt %) productions gradually decreased, whereas the gas production (20−30 wt %)

Figure 6. Product distribution varying with the carbonization temperature. 6764



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Figure 8. Schematic process diagram of the pilot plant for continuous AC production with granular material such as SL.

3.3. Process Evaluation. To demonstrate the operational feasibility of the designed production process shown above, its mass and heat balances were calculated by assuming a treatment capacity of 2000 kg/h of SL. The major parameters involved in the calculation are given in Table 1. The

beneath the cyclone to further prolong the volatile release time for the CC and also to provide a continuous feed of CC into the pneumatic riser activator (12). Air is introduced into the riser reactor from its bottom as both a fluidizing gas and combustion agent. In this way, a small part of CC and also the combustible gas generated in the activation are burned in the reactor to maintain the high temperature (∼1000 °C) needed for CC activation. For activation, steam is introduced from the side of the pneumatic riser via the steam feeding ports set along the riser. The hightemperature activation products, including AC and gas, first go through the heat recovery boiler (13) to generate steam needed for the activation process and then pass through a series of heat exchangers (14) to cool the AC. Finally, the produced AC is collected through two cyclones in series (the two lower cyclones). In this AC production process, all the steps, including drying, carbonization, activation, and their integrated combustion for hot gas and heat exchange for making steam are implemented in series and continuously in one system without external heat input. The combustion of the carbonization-generated combustible gas provided the heat needed for drying, while the carbonization relied on burning a small part of the dried raw material (here SL) during its initiation. The activationrequired steam was generated in cooling the activation products, both gas and AC (13 and 14). All the conditions and parameters of the process are adjustable. The drying time can be controlled by changing the rotation rate of the rotary drier. Downstream of the rotary drier, long conveying tubes (4) were set to allow the hot gas from the rotary oven to dry the material further during its transport. This can reduce the moisture content of the resulting SL to below 20 wt %. Meanwhile, the carbonization temperature can be controlled by adjusting the ratio of the dried SL sent for burning (9) and carbonization (10). The activation temperature and activation time in the riser (12) can be adjusted by regulating the air flow into the activator.

Table 1. Parameters Used for the Mass and Heat Balance Calculation Shown in Figure 9 parameter

unit

value

treatment capacity of SL water content of raw SL water content of dried SL flue gas temperature into the drier flue gas temperature out of the drier temperature of raw SL heat utilization efficiency of the drier temperature of the dried SL carbonization temperature activation temperature yield of AC (assumed against CC) steam temperature from the boiler of the activator heating value of tar heating value of the pyrolysis gas

kg/h wt % wt % °C °C °C % °C °C °C kg/kg of CC °C MJ/kg MJ/(N m3)

2000 60 15 1000 130 20 60 130 450 900 0.5 200 17.1 10.0

carbonization and activation temperatures were set at their lower end of the above-determined applicable temperatures, which were 450 and 900 °C, respectively. The pyrolysis product distribution was based on the preceding results from laboratory pyrolysis tests (Figures 6 and 7). The values of many involved parameters and further the estimated results are summarized in Table 1 and the datasheet of Figure 9. These include, for example, the specific heat capacities of conversion products such as tar and pyrolysis gas, the reaction heat of fuel devolatilization, the latent heat of water vaporization, the energy with the various streams, and many others. 6765



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Figure 9. Mass and heat balances of the AC production process shown in Figure 8 for a treatment capacity of 2000 kg/h of SL (E is the input or output energy per hour).

As shown in Figure 9, the 2000 kg/h of wet SL is equivalent to 953 kg/h of dried SL with a water content of 15%. By burning about 13.75% of the dried SL, the carbonization leads to 250 kg/h of CC. The pyrolysis gas and tar produced in carbonization are combusted to generate high-temperature flue gas with 8230 MJ of heat. Only its latent part (6183 MJ) is used to dry the wet SL, and the remaining part (2047 MJ) is exhausted directly with the flue gas. Most of the heat input into the drier is finally exhausted with the effluent gas of the drier, which contains the steam from evaporating the water in the feedstock as well. In activation, the produced CC (with an energy of 4227 MJ) is partly combusted to generate 1190 MJ of heat (including 10% heat loss) to keep the activation reactor at 900 °C and provide the reaction heat required by the activation reactions. In the waste heat boiler of the activator, about 300 kg/h of saturated steam at 200 °C (838 MJ) is generated by recovering the sensible heat with the effluent hot gas as well as the AC from the activation reactor. The steam is in turn sent to the activator as the activation agent. Finally, 125 kg/h of AC (included an energy of 2090 MJ) is produced, while a part of the heat is disposed with the effluent gas of the activator and also with the cooling water of its heat exchanger. For an easy understanding of the energy as well as mass flows in the process, Figure 10 presents the energy balance data in a concise plot. The input energy is finally split into four major streams, the produced AC, effluent gas of the drier, effluent gas of the activator, and directly exhausted gas in the combustor for pyrolysis gas products (including tar). The exhausted energy from the off-gas of the drier comes from the combustion of the carbonization gas, while the steam required for activation is from using the high-temperature sensible heat of the exhausted gas of the activator. This calculation of the mass and heat balances clarified that, for the SL feedstock with a moisture content of about 60 wt %, the developed production process for AC can be operated autothermally without any external heat input.

Figure 10. Energy footprint in the designed process evaluated with the balance calculation (E is the input or output energy per hour).

However, the energy efficiency of this production process, i.e., 17.03% for the AC product (i.e., 2090/12272), is not high. This is characteristic of any biocarbon production process, especially for the high-moisture feedstock. In the developed process, the combustible gas from carbonization is burned to supply the heat needed for fuel drying, and the sensible heat of the off-gas from activation of CC is used to produce steam, the activation agent. In the process there is still some heat wasted with the effluent gas flows. This indicates that the energy efficiency can be further improved by, for example, reducing the ratio of dried SL sent for burning and also the air into the activator (decreasing the combusted CC in the activator). The heat carried with the effluent gas flows can also be used for other purposes, such as making hot water and preheating air. 3.4. Pilot Plant Test. A pilot plant with a raw SL treatment capacity of 2000 kg/h was built according to the preceding design and evaluation. It was commissioned for successful continuous running in 2011. The startup procedure is as follows. Dry rice husk was first continuously fed into the bottom of the pneumatic riser and combusted with air to preheat the activation reactor. When this combustion and also the temperature in the activator were steady, rice husk was 6766



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activator fluctuated in a small range, and the pressure fluctuation was ±0.2 kPa. Table 2 gives the typical ranges of

further sent to firebox 1 to generate the high-temperature flue gas required for the rotary drier. Once the drier was ready to accept feedstock, raw SL was continuously fed into the drier (gradually up to 1500 kg/h). The dried SL went through the carbonizer without carbonization and was fed directly into the activation reactor to replace the rice husk. When steam was steadily produced and reached pressures over 0.5 MPa in the boiler of the activator, it was imported into the activation reactor to form the activation atmosphere. Via the startup operation shown above, the SL drier and also the activation reactor would finally run stably. In turn, the cyclone carbonizer was started by combusting part of the dried SL in firebox 2 to produce CC for the activator. Its generated pyrolysis gas and tar were accordingly forwarded to firebox 1 to gradually replace the rice husk burned there. The produced CC was subsequently forwarded to the pneumatic riser to initiate the activation reactions in the reactor. At last, the pilot plant was run autothermally and continuously with the only feed of wet SL into the rotary drier without any external heat input. The rice husk fed into the activator and firebox 1 (for startup) was finally all topped. Figure 11 shows the temperature and pressure variations in a typical startup and its successive steady operation of the plant.

Table 2. Ranges of the Major Running Parameters for the Pilot Plant Operation parameter

unit

feeding rate of raw SL temperature at the exit of the drier temperature in the carbonizer temperature of the activator bottom middle top temperature of steam generated in the activator temperature at the exit of the activator boiler temperature at the exit of the heat exchanger pressure fluctuation in the activator

kg/h °C °C °C

parameter range 1500 150−180 500−600 850−900

°C °C °C kPa

1100−1200 1100−1200 160−180 500−530 300−350 ±0.20

the major running parameters after the pilot plant achieved stable operation. The actual temperatures for carbonization and activation were slightly higher than the prediction in the heat and mass balance analysis, thus showing the plausibility of the process design. The AC produced through the pilot plant is characterized in Table 3. In comparison with the AC prepared in the laboratory Table 3. Comparison of ACs from the Pilot Plant and Laboratory Tests parameter iodine adsorption value (mg/g) MB adsorption value (mg/g) specific surface area (m2/g) ash content (%)

AC from pilot plant

AC in Figure 5 at 1000 °C and 40 s

AC from ref20

610−630

336

581

20−30

140

94

191

372

70−75

43.2

under optimal conditions20 and that obtained by fast activation at 1000 °C for 40 s (Figure 5), the AC produced by the pilot plant shows a comparable iodine adsorption value but a lower methylene blue value and specific surface area. This can be attributed to the following reasons. First, in the production process a part of the dried SL and CC are combusted to ensure the autothermal running of the plant, which has to result in a lower fixed carbon content in the produced AC and thus a lower specific surface area. As shown in Table 3, the ash content of the AC produced by the pilot plant was much higher than that of the AC prepared in the laboratory. Second, the activation was somehow at a high temperature of 1100 °C, and the reaction time was roughly a few seconds. The reaction was possibly too quick (residence time being too short) to allow the substantial formation of meso- and macropores so that the produced AC product had lower methylene blue adsorption but relatively high iodine adsorption. In addition, the high ash content of up to 75 wt % in the product might contribute to the measured high iodine adsorption. In comparison with the inert activation atmosphere used in the laboratory, the air atmosphere adopted for the pilot plant may negatively affect the AC quality too. Consequently, optimization of the process conditions in the pilot plant is needed.

Figure 11. Temperature and pressure variations in a typical startup and steady operation period (t1, starting activator; t2, starting drier; t3, steam feed; t4, starting carbonizer).

After the plant was started at time t1, the temperature in the drier and activation reactor increased gradually. With the sensible heat of the hot flue gas of the activator, the water in its boiler was gradually converted into steam which had a relatively steady temperature. When the temperatures of both the steam and activator reached their stable values at time point t3, the steam was introduced into the activation reactor, which in turn raised the pressure in the reactor. After the drier and activation reactor ran stably, the carbonizer was started at time point t4. This steeply increased the pressure of the carbonizer and gradually the carbonization temperature until the pilot plant finally ran steadily. The temperatures in the carbonizer and 6767



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(2) Dias, J.; Alvim-Ferraz, M.; Almeida, M.; Rivera-Utrilla, J.; Sánchez-Polo, M. Waste Materials for Activated Carbon Preparation and Its Use in Aqueous-Phase Treatment: A Review. J. Environ. Manage. 2007, 85, 833. (3) Ahmad, A.; Hameed, B.; Ahmad, A. Removal of Disperse Dye from Aqueous Solution Using Waste-Derived Activated Carbon: Optimization Study. J. Hazard. Mater. 2009, 170, 612. (4) Juan, Y.; Ke-Qiang, Q. Preparation of Activated Carbon by Chemical Activation under Vacuum. Environ. Sci. Technol. 2009, 43, 3385. (5) Lima, I. M.; McAloon, A.; Boateng, A. A. Activated Carbon from Broiler Litter: Process Description and Cost of Production. Biomass Bioenergy 2008, 32, 568. (6) Ioannidou, O.; Zabaniotou, A. Agricultural Residues as Precursors for Activated Carbon ProductionA Review. Renewable Sustainable Energy Rev. 2007, 11, 1966. (7) Amaya, A.; Medero, N.; Tancredi, N.; Silva, H.; Deiana, C. Activated Carbon Briquettes from Biomass Materials. Bioresour. Technol. 2007, 98, 1635. (8) Schroder, E.; Thomauske, K.; Weber, C.; Hornung, A.; Tumiatti, V. Experiments on the Generation of Activated Carbon from Biomass. J. Anal. Appl. Pyrolysis 2006, 79, 106. (9) Oliveira, L. C. A.; Pereira, E.; Guimaraes, I. R.; Vallone, A.; Pereira, M.; Mesquita, J. P.; Sapag, K. Preparation of Activated Carbons from Coffee Husks Utilizing FeCl3 and ZnCl2 as Activating Agents. J. Hazard. Mater. 2009, 165, 87. (10) Liou, T.; Wu, S. Characteristics of Microporous/Mesoporous Carbons Prepared from Rice Husk under Base- and Acid-Treated Conditions. J. Hazard. Mater. 2009, 171, 693. (11) Fierro, V.; Muñiz, G.; Basta, A.; El-Saied, H.; Celzard, A. Rice Straw as Precursor of Activated Carbons: Activation with OrthoPhosphoric Acid. J. Hazard. Mater. 2010, 181, 27. (12) Bagheri, N.; Abedi, J. Preparation of High Surface Area Activated Carbon from Corn by Chemical Activation Using Potassium Hydroxide. Chem. Eng. Res. Des. 2009, 87, 1059. (13) Cao, Q.; Xie, K. C.; Lv, Y. K.; Bao, W. R. Process Effects on Activated Carbon with Large Specific Surface Area from Corn Cob. Bioresour. Technol. 2006, 97, 110. (14) Li, K. Q.; Zheng, Z.; Huang, X. F.; Zhao, G. H.; Feng, J. W.; Zhang, J. B. Equilibrium, Kinetic and Thermodynamic Studies on the Adsorption of 2-Nitroaniline onto Activated Carbon Prepared from Cotton Stalk Fibre. J. Hazard. Mater. 2009, 166, 213. (15) Yang, J. A.; Qiu, K. Q. Development of High Surface Area Mesoporous Activated Carbons from Herb Residues. Chem. Eng. J. 2011, 167, 148. (16) Demiral, H.; Demiral, I.; Karabacakoglu, B.; Tumsek, F. Production of Activated Carbon from Olive Bagasse by Physical Activation. Chem. Eng. Res. Des. 2011, 89, 206. (17) Girgis, B. S.; Yunis, S. S.; Soliman, A. M. Characteristics of Activated Carbon from Peanut Hulls in Relation to Conditions of Preparation. Mater. Lett. 2002, 57, 164. (18) Yang, J. A.; Qiu, K. Q. Preparation of Activated Carbons from Walnut Shells via Vacuum Chemical Activation and Their Application for Methylene Blue Removal. Chem. Eng. J. 2010, 165, 209. (19) Gonzalez, J. F.; Roman, S.; Gonzalez-Garcia, C. M.; Nabais, J. M. V.; Ortiz, A. L. Porosity Development in Activated Carbons Prepared from Walnut Shells by Carbon Dioxide or Steam Activation. Ind. Eng. Chem. Res. 2009, 48, 7474. (20) Yang, J.; Yu, J.; Zhao, W.; Li, Q.; Wang, Y.; Xu, G. Upgrading Ash-Rich Activated Carbon from Distilled Spirit Lees. Ind. Eng. Chem. Res. 2012, 51, 6037. (21) Xu, G. W.; Ji, W. F.; Liu, Z.; Wan, Y. H.; Zhang, X. Y. Necessity and Technical Route of Value-Added Utilization of Biomass Process Residues in Light Industry. Chin. J. Process Eng. 2009, 9, 618. (22) Zeng, X.; Wang, Y.; Yu, J.; Wu, S.; Zhong, M.; Xu, S.; Xu, G. Coal Pyrolysis in a Fluidized Bed for Adapting to a Two-Stage Gasification Process. Energy Fuels 2011, 25, 1092. (23) Yao, C.; Dong, L.; Wang, Y.; Yu, J.; Li, Q.; Xu, G.; Gao, S.; Yi, B.; Yang, J. Fluidized Bed Pyrolysis of Distilled Spirits Lees for

Although the produced AC is not as good as that obtained in the laboratory, some methods can be applied to improve the AC quality. As demonstrated in our previous study,20 most of the ash in the produced AC can be removed by alkali treatment, thus greatly upgrading the AC product. Using low-ash and high-carbon feedstock other than SL, such as saw dust, would produce high-quality AC too. This has in fact been verified by running the pilot plant with saw dust (data not shown here for a different material). Overall, the pilot operation fully demonstrated that the developed process successfully integrated drying, carbonization, and activation for AC production and realized continuous and autothermal running without any external heat input.

4. CONCLUSIONS An integrated process was developed in this study to produce AC continuously using granular raw materials. By investigating the kinetic rate of its involved processing steps of drying, carbonization and activation, a pilot plant treating 2000 kg/h of SL was designed, evaluated, built, and commissioned for continuous running. Process evaluation in terms of heat and mass balance showed that the process can run autothermally and continuously even for SL with a moisture content of about 60 wt %. The continuous running of the pilot plant demonstrated the expected variation feature in process temperatures and pressures. The produced powder AC had iodine adsorption values of 610−630 mg/g, methylene blue adsorption values of 20−30 mg/g, and specific surface areas of about 191 m2/g. The causes for the low quality of the resulting AC were considered to be mainly the presence of O2 in the activation atmosphere and the possibly too short activation time in the pilot transport bed reactor, which led the AC to have a high ash content but fewer meso- and macropores. Ways to further improve the quality of the AC product were also discussed. Through these studies and analyses it was fully demonstrated that the developed process is technically feasible to treat granular biomass materials such as saw dust, rice husk, and SL to produce AC continuously without any external energy input into the process. In comparison with the existing various technologies for producing AC in a batchwise manner, this new technology would greatly improve the production efficiency and also reduce the cost.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +86-10-8254-4886. Fax: +86-10-8262-9912. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful for financial support from the National HighTech Research and Development Program of China (Grant 2012AA021401), the National Natural Science Foundation of China (Grants 21161140329 and 21006114), and the National Key Technology Development Program (Grant 2010BAC66B01).

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REFERENCES

(1) Dąbrowski, A.; Podkościelny, P.; Hubicki, Z.; Barczak, M. Adsorption of Phenolic Compounds by Activated CarbonA Critical Review. Chemosphere 2005, 58, 1049. 6768



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Adapting to Its Circulating Fluidized Bed Decoupling Combustion. Fuel Process. Technol. 2011, 92, 2312. (24) Bouchelta, C.; Medjram, M. S.; Bertrand, O.; Bellat, J. P. Preparation and Characterization of Activated Carbon from Date Stones by Physical Activation with Steam. J. Anal. Appl. Pyrolysis 2008, 82, 70.

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Continuous Production of Activated Carbon from Distilled Spirit Lees

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