Article pubs.acs.org/IECR
Thermodynamic Analysis of Low-Rank-Coal-Based Oxygen-Thermal Acetylene Manufacturing Process System Jing Guo† and Danxing Zheng*,† †
Beijing University of Chemical Technology, Beijing 100029, China S Supporting Information *
ABSTRACT: In this paper, the low-rank-coal-based oxygen-thermal method acetylene manufacturing process is established and simulated. Through the novel graphic analysis tool EFGD (Exergy-flow Framework Grassman Diagram), the energy supply and demand, the energy utilization, and energy consumption distribution are therefore obtained. Results show that the carbide furnace unit is the largest exergy loss unit, and its internal exergy loss accounts for 57.52% of the total internal exergy loss, reducing carbon consumption in a carbide furnace and reusing the off-gas will do better to improve the energy consumption of the whole system. Moreover, to further investigate the thermodynamic mechanism of energy coupling and energy conversion as well as the cause of high energy consumption, the energy configuration with ΔG-T and the α-H-ε diagram analysis is established. It reveals that the carbon combustion reaction in oxygen for CO production plays a major role in promoting the reaction for this process. carbide is required to be high quality cokes,12 which are produced by coking with high-rank coal but not low-rank coal with larger reserves. If the large energy consumption problems of the low-rank-coal-based acetylene route are solved, to replace the traditional petroleum-ethylene route, the diversification of feedstock utilization will be realized, basic organic synthesis monomer manufacturing will get ride of its dependency on petroleum resources which will help a great deal to step out of the predicament of the tight energy supply. At present, the existing processes of low-rank coal to coke include ENCOAl’s LFC (Liquid From Coal) technique, which has processed 247,000 tons of crude coal and obtained 115,000 tons of semicoke during the trial run from 1992 to 1997 and has realized the conversion from low-rank coal to high-rank coke with higher carbon content. Based on the technique of LFC, DaTang Huayin Electric Power Co., Ltd. developed LCC (Low-rank Coal Conversion) technology, in which PMC (Process Middle Coke) is obtained after the upgrading of lignite and the carbon content of coke products can be promoted to 87.3%. In order to reduce the energy consumption in the calcium carbide production process, another way is to add oxygen to burn part of the coke so as to replace electric consumption and heat the conversion reaction simultaneously. This is an oxygen-thermal process of coke to calcium carbide.7 A typical oxygen-thermal method process such as the partial combustion shaft furnace oxygen-thermal process of BASF7 used coke with 88% carbon content as feedstock and obtained 100 tons of 80.5% carbide per day. A filled type shaft furnace oxygen-thermal process was mentioned in a Japan Patent JP61178412,13 and also a fluidized bed oxygen-thermal calcium carbide production process with powder feeding was presented
1. INTRODUCTION In recent years, the demand of world energy and chemical products in the world has been growing rapidly, and the energy supply has become increasingly tense. Among all the mineral resources, coal has the largest reserve in the world. A proven available coal reserve is 984 billion tons, which is supposed to supply us for about 190 years,1 and especially the lignite (a kind of low-rank coal) an has abundant reserve which is able to supply the world for 227 years.2 Because of the lower price and the larger reserve of coal than petroleum and natural gas, replacing petroleum and natural gas with coal to manufacture other chemicals and feedstock has attracted much attention recently.3 According to the existing process route of coal based chemicals and feedstock,2 there are two generally kinds of process routes to produce low-carbon hydrocarbons from coal. One is the gasification route4 to produce methanol, which is then transferred to olefins by the MTO (Methanol-To-Olefin) technique.5,6 The other one is the acetylene route to produce acetylene from water and calcium carbide which is a product from the reaction of coal and lime.7 In a traditional coal-coke-calcium carbide-acetylene route, the coke reacts with quick-lime at 1600−2000 °C to produce calcium carbide and off-gas, which contains carbon monoxide,8 and then the calcium carbide reacts with plenty of water in a wet acetylene generator to produce acetylene.9 The advantage of this process is the much easier production of acetylene from water and calcium carbide than in the gasification route. However, since 1970s, a downward trend has occurred in this process route because of its high consumption of electric energy during the production of calcium carbide (about 3000− 3500 kWh/t 80% calcium carbide) at high temperature.10 The wet acetylene process needs to consume a large amount of water (about 7−9 t water/t calcium carbide), which makes it difficult to dispose the lime hydrate with plenty of water after the reaction.11,26 The feedstock for the production of calcium © 2012 American Chemical Society
Received: Revised: Accepted: Published: 13414
July 25, 2012 September 6, 2012 September 21, 2012 September 22, 2012 dx.doi.org/10.1021/ie301986q | Ind. Eng. Chem. Res. 2012, 51, 13414−13422
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Article
Figure 1. Low-rank-coal-based oxygen-thermal acetylene manufacturing process: U1-calcium carbide production unit; U2-gas−solid separation unit; U3-calcium carbide preparation unit; U4-acetylene production unit; U5-lime hydrate recycling unit.
(Energy-flow Framework Diagram) for the exergy analysis.18 EFD describes the utilization of energy based on the concept of node and flow. In addition, it describes the energy transfer and exergy loss between internal systems and utilities. Although a great deal of work has been done, the previous designs did not consider the following crucial factors for the low-rank-coal-based oxygen-thermal acetylene manufacturing process. First, there is little public reporting about the modeling research of this process. Second, exergy analysis has not been performed so far in the low-rank-coal-based oxygen-thermal acetylene manufacturing process. Finally, it is indispensable to figure out exergy losses in the process and the solutions for a more energy-saving process. Moreover, deep knowledge about energy matching of key units, mechanism of process coupling, and causes of exergy loss deep knowledge have not been obtained yet. In this work, a low-rank-coal-based oxygen-thermal acetylene manufacturing process system is established and simulated in which PMC refined from low-rank coal is used as the coke feeding. The energy analysis and evaluation are carried out, and the relationship between energy supply and demand in internal processes as well as the relationship between the processes and utilities are investigated. This study shows the causes of high energy consumption and situation of energy conversion as well as thermodynamic mechanism of energy coupling through thermodynamic analysis of the system. Finally, the ways to improve the process are presented.
in US2011/0123428A1 with the reaction temperature at 1750− 2000 °C.14 Compared with the wet acetylene process, the dry acetylene process has less water consumption and a more convenient treatment of its lime hydrate than the wet acetylene process. For example, Hoechst developed a dry acetylene process using a vertical dry generator. The product acetylene was dehydrated after passing through two scrubbing towers and one dip seal with the gas production of up to 3750 m3/h.15 Because the dry acetylene process produces lime hydrate with lower water content than the wet process, it is easy to convert lime hydrate to quick-lime through calcinations for recycling calcium feedstock of carbide furnace. The patent US439178616 presented the coal-calcium carbide-acetylene process and this process, which also included the lime hydrate recycling process. The traditional oxygen-thermal method has many problems such as high consumption of coke and oxygen, large quantities of unused off-gas (3.5t off-gas/t carbide).7 However, as a new technique, the research and development of an oxygen-thermal method have still attracted a great deal of attention of many researchers for a long time. In order to achieve energy-utilization efficiency, a variety of ways are proposed to analyze reasons and positions of energy loss in a system so as to improve the system. The graphical analysis method is featured among all the methods. For instance, during the research of a combined cycle system, Badami performed energy and exergy analyses with the Sankey graphical analysis method and the Grassman graphical analysis method, respectively.17 A feature of both graphical analyses is that it is easy to show the input and output relations of the energy balance and give a quantitative description of the energy flow as well as a complete concept of the system energy flow structure. Besides, based on these two graphical analysis methods, Zheng proposed another graphic tool named EFD
2. PROCESS DESCRIPTION As is shown in Figure 1, the entire system of a low-rank-coalbased oxygen-thermal acetylene manufacturing process studied in this paper can be divided as an acetylene manufacturing process system and an auxiliary system for utilities. There are five units in the acetylene manufacturing process system: 13415
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the data are on an ash-free basis. C, H, O, N, and S mean carbon, hydrogen, oxygen, nitrogen, and sulfur in the PMC, respectively. Moreover some basic inputs needed in the simulation are given in Supporting Information Table S2.7,15,16,24−26 All of the feedstock inputs are at 25.00 °C and 0.10 MPa. 3.2. Methodology. 3.2.1. Energy and Exergy Calculation. Energy analysis and exergy analysis are widely used approaches for analyzing and improving chemical and thermal processes. Combining exergy and energy analyses for a complicated process provides a comprehensive understanding of the energy utilization status and exergy loss status, which is useful for further promoting the process design. For calculating energy, exergy, and other properties, the Peng−Robinson equation of state (PR EOS) is used as the global property method.27−29 Neglecting the changes of kinetic and potential energy between inlet and outlet, the energy balance and exergy balance for the system considered are given by eqs 1 and 2
calcium carbide production unit, gas−solid separation unit, calcium carbide preparation unit, acetylene production unit, and lime hydrate recycling unit. Calcium Carbide Production Unit..7,15,19 PMC, which is obtained from low-rank coal, is ground and shifted in an oxygen-thermal carbide furnace through conduit 1. Next, quicklime and oxygen rich gas are introduced into the oxygenthermal carbide furnace of U1 through conduits 2 and 3, respectively. Calcium carbide, which is produced from the bottom of carbide furnace through conduit 7, enters into U3. All technological parameters are referenced to the oxygenthermal carbide process of BASF Corporation.7,15,19 Gas−Solid Separation Unit.15 The calcium carbide production process simultaneously produces off-gas on the top of the oxygen-thermal carbide furnace. The off-gas in conduit 4 at 600 °C,16 chiefly CO, is freed from dust in a separator and then CO and dust leave the system through conduits 5 and 6, respectively. Thus, the off-gas originating from gas−solid separation unit U2 can be used as fuel gas or feedstock for demand of other processes. Calcium Carbide Preparation Unit.16 From conduits 7 to 9, the calcium carbide is cooled and crushed by cooler and crusher. This is a pretreatment process of acetylene production for producing calcium carbide. In unit U3, calcium carbide has a lower temperature, and its particle size is ground to meet the requirement of dry acetylene generator. Acetylene Production Unit..7,15,20,21 The carbide and water through conduit 9 and 10 are fed to acetylene production process U4. Carbide and water form crude acetylene and lime hydrates through generator. The crude acetylene in conduit 11 is fed to the purification unit. To produce the purer acetylene, the water vapor in crude acetylene is removed by water from conduit 10. Then, purer acetylene is produced in conduit 13, and the excess water is discharged through conduit 14. Partial water from purification is reused through conduit 12 by generator. One part of the lime hydrates traveling through conduit 16 is introduced into the lime hydrate recycling system, and the other part of lime hydrates which in conduit 15 is the byproduct of the system. All the technological parameters are referred to from the dry acetylene production process of the Hoechst Corporation.7,15,20,21 Lime Hydrate Recycling Unit.16 Lime hydrate from conduit 16 is introduced into drier with water. Meanwhile, the lime hydrate is freed from residual moisture through conduit 17. Then dry lime hydrate undergoes dehydration to calcium oxide in calciner. The calcined lime is discharged through conduit 20 and simultaneously as lime feedstock to fulfill the requirement of the oxygen-thermal furnace. The water from drier and calciner is discharged in conduits 18 and 19. All the technological parameters are referred to from US4391786.16 Utilities. The utilities of the entire low-rank-coal-based oxygen-thermal acetylene manufacturing process include electricity supply, heat supply, and cooling water. Crusher, separator, and acetylene production units need electricity to drive. Drier and calciner can be heated by heat supply.22 It is necessary to cool the calcium carbide from furnace before crushing, which usually uses the cooling water to drop the temperature of calcium carbide.7
∑ (mH )fs, i +∑ Q j + ∑ Ws, k = 0 i
j
⎡
∑ (mε)fs, i +∑ ⎢⎣∫ i
Q
j
(1)
k
⎛ T0 ⎞ ⎤ ⎜1 − ⎟δQ ⎥ + ⎝ T ⎠ ⎦j
∑ Ws,k − mI = 0 k
(2)
where ∑i(mH)fs,i and ∑i(mε)fs,i are total enthalpy and total input and exergy output of the system. Subscript fs means this term can calculate any flowing stream. Q, Ws, and I denotes heat, work, and exergy loss, respectively. I is composed of internal exergy loss and external exergy loss. Internal exergy loss is caused by process irreversibility, and the external exergy loss includes the exergy output of cooling water and waste discharge. The enthalpy of multispecies fluid can be represented by H(T , p , x̲ ) =
∑ xi⌊Hi(T0 , pθ ) + ∫
T
T0
+
∫p
CpdT
p θ
[V − T (∂V /∂T )p ]dp⌋
− RT 2 ∑ xi[∂ln(fi ̂ /f iθ )/∂T ]p , x
(3)
where f ̂ and f θi stand for fugacity and standard fugacity of species i, respectively, which can be calculated by PR EOS. The volume V can also be calculated by PR EOS. For the heat capacity Cp, the equation which was developed by Barin30 is used to calculate its value. Hi(T0,pθ) is the standard enthalpy of the pure species i.31,32 The exergy of the multispecies fluid can be calculated by32 ε(T , p , x̲ ) =
∑ xiεiθ(T0 , pθ ) + ∑ xi{[Hi(T , p) − Hiθ(T0 , pθ )] − T0[Si(T , p) − Siθ(T0 , pθ )]} + RT0 ∑ xi ln(fi ̂ /f iθ ) + RT (1 − T0/T )
∑ xi[∂ln(fi ̂ /f iθ )/∂ln T ]p,x
3. SYSTEM SIMULATION AND MODELING 3.1. System Input Conditions. The coke used in this study is PMC. The proximate and ultimate analyses of PMC are all listed in Supporting Information Table S1.23 In the table, all
(4)
where the standard entropy of pure species i, Sθi (T0,pθ), can be obtained by a handbook.33 The heat capacity Cp can be used to calculate both Hi(T,p) and Si(T,p). εθi (T0,pθ) is the standard 13416
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Figure 2. Energy-flow diagram MW.
Figure 3. Exergy-flow framework Grassman diagram MW.
exergy of pure species i. Based on Kameyama’s method,34 εθi (T0,pθ) can be obtained. 3.2.2. Chemical Equilibrium Reactor. The entire reactor is simulated at total Gibbs energy minimization of multiphase chemical equilibrium. When the temperature and pressure are fixed at a chemical equilibrium state, the Gibbs energy of reaction reaches zero.35,36 With the constraint condition of element conservation, the composition of output stream can be calculated. Some assumptions are made in this simulation work as follows: (1) Solid molten state is not considered in the process simulation. For example, the oxygen-thermal carbide furnace only includes solid phase and gas phase. (2) The ash including some metal oxide such as SiO2, MgO, Fe2O3, etc. is assumed as inert. (3) If the component which only contains H, N, or S is neglected for their small quantities, carbide furnace, acetylene generator, and calciner can be respectively simplified as C−O−Ca, C−H−O-Ca, and Ca−O−H reaction systems, respectively. According to the independent reaction selection rule, the independent reactions of three reactors including carbide furnace, acetylene generator, and calciner are described in Supporting Information Table S3. For the following thermodynamic property analysis, the independent reactions and their changes of the standard properties are also listed in Supporting Information Table S3.33
3.3. Results of the Simulation. To validate the approaches, a comparison of simulated results is listed in Supporting Information Table S4. It can be seen that the relative deviation of the simulation values and literature values is less than 3%. The maximum deviation appears in the amount of off-gas is 2.42%. There is little difference between the simulation results and literature data15 in the range of permissible errors. Therefore, the selected models and parameters are reliable when applying to the low-rank-coalbased oxygen-thermal acetylene manufacturing process.
4. DISCUSSION OF PROCESS ENERGY ANALYSIS DIAGRAMS 4.1. Energy Utilization and Energy Loss Distribution. Based on the results of simulated calculation, an energy-flow diagram of the low-rank-coal-based oxygen-thermal acetylene manufacturing process is shown in Figure 2. The width of the energy-flow in the diagram stands for the energy amount of stream. Although PMC feedstock contains 53.97MW accounting for 86.87% of the total energy input, 73.18% of PMC is burned and only 26.12% of PMC is used to produce calcium carbide. Through calcium carbide production unit U1, the offgas which contains a large amount of CO takes away a great deal of energy, which accounts for 68.45% energy input of U1. The calcium carbide owns the rest 31.55% energy input of U1. 13417
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Information Table S3, there are three independent reactions in carbide furnace: calcium carbide reaction R1 and carbon combustion reaction in oxygen R2 and R3. R1 is the target process for producing calcium carbide which is an endothermic reaction to accept external energy and which can be defined as the energy acceptor. R2 and R3 are the driven processes for producing calcium carbide which is an exothermic reaction to release energy to outside and which can be defined as the energy donor. If the flow rate of carbon input is 1.00 mol/s for carbide furnace, the carbon combusted in the practical reaction will be 0.73 mol/s, while 0.26 mol/s carbon will be involved in calcium carbide production with 0.01 mol/s excess carbon. Neglecting these excess carbons, the reactants necessary should be added according to the stoichiometric relation of the three reactions. Figure 4 conceptually illustrates the coupling
Through gas−solid separation unit U2, the off-gas contains much more energy than other products which is 37.81MW after dedusting, accounting for 61.05% of the energy output. After cooling and grinding through calcium carbide preparation unit U3, the carbide which contains CaC2 yielded by U1 is mixed with water in acetylene production unit U4 to obtain the main product acetylene gas. Acetylene gas contains 14.94MW and accounts for 24.13% of the total energy output. In U4, the byproduct lime hydrate accounts for 4.22% of the energy output. The electric input from utilities is 0.66MW, while the heat input is 1.56MW, accounting for 1.07% and 2.52% of the total input, respectively. If the product of this process system is acetylene, producing per ton acetylene will consume 7726.44 kgce. When the synthesis gas is produced by coal gasification37 and both off-gas and acetylene are the products of the system, considering that each ton of acetylene will produce 9.36 t of CO and simultaneously per ton CO will consume 771.91 kgce, the energy consumption of producing per ton acetylene is equivalent to 501.33 kgce. In this work, combined with an EFD (Energy-flow Framework Diagram) and a Grassman diagram, a novel graphic exergy analysis tool, EFGD (Exergy-flow Framework Grassman Diagram), is proposed. Figure 3 shows the exergy-flow framework diagram of the low-rank-coal-based oxygen-thermal acetylene manufacturing process system. Different from Figure 2, internal exergy loss bus and external exergy loss bus are added in Figure 3. The external exergy loss bus consists of cooling water and the discharged materials which are not products of the system. According to the simulated calculations, the exergy of feedstock fed into U1 is 52.08MW accounts for 96.16% of the total input exergy. Especially, the input coke contains higher exergy of 49.64MW, accounting for 94.37% of the total input exergy. Through U1, the off-gas with a large amount of CO takes away 57.30% of the exergy input of U1, but calcium carbide only has 32.28% of the exergy input of U1. After dedusting in U2, the off-gas contains 27.88MW exergy which accounts for 53.00% of the total output exergy, which is much higher than other products. The carbide including CaC2 from U1 is mixed with water and fed into U4, obtaining the main product acetylene gas. The exergy of acetylene gas is 12.66MW, accounting for 24.07% of the total output exergy. In U4, the byproduct lime hydrate accounts for 0.99% of the total output exergy. The whole system consumes electricity of 0.66MW from utilities, accounting for 1.25% of the total input exergy, while the heat exergy from utilities is 0.98MW and accounts for 1.86% of the total input exergy. The exergy loss of some units can be listed in the exergy loss bus of the diagram. The total exergy loss of the system is 17.95% of the total output exergy. The carbide furnace reaction unit has the maximum exergy loss, accounting for 57.52% of the internal exergy loss and 10.32% of the total output exergy. Detailed exergy loss analysis and improvements about carbide furnace will be discussed later in sections 4.2 and 4.3 of this paper. 4.2. Energy Analysis of Calcium Carbide with OxygenThermal Method Reaction Unit. 4.2.1. Energy Configuration of Carbide Furnace and Analysis of Thermodynamic Energy Quality. According to the results of simulation and energy analysis, U1 consumes the most energy and also accounts for the most exergy loss. As is shown in Supporting
Figure 4. Energy configuration of carbide furnace.
between the energy donor and the energy acceptor. The energy donor provides the necessary reaction energy of the energy acceptor. The reaction heat is ΔHr, and, at the same time, it provides the energy acceptor a driving force ΔGdri to make it possible to produce calcium carbide at the reaction temperature. ΔGdri makes an impossible reaction of the energy acceptor under normal circumstances become possible. When the temperature and pressure are fixed, the relationship among Gibbs free energy change, enthalpy change, and entropy change of the process are shown by ΔGT , p = ΔHT , p − T ΔST , p
(5)
where the values of ΔG, ΔH, and ΔS can be calculated from simulated calculation and standard thermodynamic data in Supporting Information Table S3. The ΔG-T diagram of the process of the carbide furnace reaction can be drawn according to the results of energy configuration and simulated calculation in Figure 4. As is shown in Figure 5, the vertical axis stands for
Figure 5. ΔG-T diagram of carbide furnace reaction. 13418
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ΔG, while the horizontal axis stands for reaction temperature. The length of the red arrow represents the value of ΔH, while the black arrow represents the value of ΔG. If M stands for any thermodynamic property, arrow-down denotes ΔM < 0 which means the system is outputting energy and arrow-up denotes ΔM > 0 which means the system is getting energy from outside. The upper line in Figure 5 represents the calcium carbide production reaction R1, and the lower lines represent the coke combustion reactions R2 and R3. The analysis of thermodynamic properties shown in Figure 5 reveals the mechanism of coupling between the energy acceptor and the energy donor during the process of carbide furnace reaction. At room temperature, the value of ΔG of R1 is greater than zero and declines with the increase of temperature. When the temperature is higher than the turning point, i.e. T* = 1840.94 °C, the value of ΔG is less than zero in the energy acceptor. However, the value of ΔG of R2 in the energy donor is always less than zero. The value of ΔG of R2 declines with the increase of temperature, while the value of ΔG of R3 changes little as the temperature changes. The mass of CO2 accounts for 0.98% of the off-gas. This demonstrates that the main reaction of the energy donor is R2, while R3 contributes little. The reactions of the energy donor (ΔG < 0) start first and will release heat (ΔHR2 < 0) when the energy donor and the energy acceptor are coupled at room temperature. The temperature of reactor increases because the energy donor keeps releasing heat to reach the reaction temperature of process, i.e. Tr = 2000 °C, which is higher than T*. The coupling process also provides a driving force to the energy acceptor, i.e. ΔGdri = −298.09 kJ/s. After getting the driving force, the value of ΔG of the energy acceptor declines as temperature increases. When temperature reaches Tr, higher than T*, the energy acceptor reacts spontaneously, i.e. ΔG = −40.98 kJ/s, and produces CaC2. The coupling of these processes realizes that the target process of the energy acceptor reacts spontaneously. It should be noted that when producing 1 mol product, the energy released from the energy donor is less than the energy absorbed by the energy acceptor (ΔHR2
