Exergy Analysis and Retrofitting of Natural Gas-based Acetylene Process

Chinese Journal of Chemical Engineering, 16(5) 812ü818 (2008)

Exergy Analysis and Retrofitting of Natural Gas-based Acetylene Process* WANG Zhifang (ฆᄝֺ) and ZHENG Danxing (ᄂӥ໲)**

School of Chemical Engineering, Beijing University of Chemical Technology, Beijing 100029, China Abstract This article presents an acetylene production process by partial oxidation/combustion of natural gas. The thermodynamic performance and exergy analysis in the process are investigated using the flow-sheeting program Aspen Plus. The results indicate that the most important destruction of exergy is found to occur in the reactor and water quenching scrubber, amounting to 8.23% and 10.39%, respectively, of the entire system. Based on the results of thermodynamic and exergy analysis, the acetylene reactor has been retrofitted. The improvement ratios of molar O2 to CH4 and molar CO to CH4 are 0.65 and 0.20, respectively. An improvement of the acetylene production system is proposed. Adopting the improvement operation conditions and using oil to realize the reaction heat recovery, the feedstock of natural gas is reduced by 9.88% and the exergy loss in the retrofitting process is decreased by 19.71% compared to the original process. Keywords acetylene, exergy analysis, natural gas, partial oxidation, hydrogen

1

INTRODUCTION

With the considerable decrease in worldwide oil reserves, it can be expected that natural gas will play an increasingly important role in energy and chemical supplies in the future. Partial oxidation/combustion of natural gas for producing acetylene is one commercial single process step of natural gas conversion to chemicals. Acetylene is produced by natural gas in a burner at a temperature of about 1700 K and at a very short reaction time. The disadvantage of methane pyrolysis is that the high reaction temperature requires an intensive energy input and a high operating cost (including reactor materials, heat transfer, and heat recovery) [1, 2]. It is a highly energy consuming process, because of the raw material used and the process route adopted. Hence, it is essential to understand the causes of its high energy consumption to design a retrofitting process. Exergy analysis is a method of thermodynamic analysis commonly used in the investigated processes, with the aim of calculating the second thermodynamic law efficiency of the process [37]. The thermodynamic performance of a process is best evaluated using exergy analysis [810]. The greater the exergy destroyed, the more energy input must be provided to run the process. Therefore, the partial oxidation of a natural gas-based process for making acetylene is one of the most attractive opportunities for the application of second thermodynamic law-based methods in process engineering. The objective of this research was to conduct a thermodynamic analysis of the process of acetylene production by partial oxidation of natural gas. For this purpose, the process was simulated with Aspen Plus, which is widely used for flow sheet simulation in the process industry [1114]. Based on the simulation results, the thermodynamic analysis of the process has been performed. Then a retrofitting process can be proposed by developing the irreversibility or lost work.

2

PROCESS DESCRIPTION

Amongst the various partial oxidation processes for making acetylene, the BASF process has been the most widely adopted [15]. A flow sheet of the BASF process [15] is schematically shown in Fig. 1. It consists of three units: the partial oxidation/combustion (POC) reaction section, the absorption section, and the stripping section. The process produces acetylene by burning feedstock with a supply of oxygen to provide the energy for the endothermic reaction [16]. Oxygen and natural gas, preheated in direct-fired heaters H01 and H02, respectively to 920 K, are charged to the burner reactor R101. The burner reactor consists of a mixing zone at the top, a reaction zone, and a quenching zone. The gas mixture burns and causes part of the methane to crack to acetylene. Principal reactions during partial oxidation of natural gas in the burner reactor are: CH 4  O 2  o CO  H 2 O  H 2

'H 298K

278 kJ·mol

ˉ1

(1)

CH 4  2O2  o CO2  2H 2 O 'H 298K

886 kJ·mol

ˉ1

(2)

CO  H 2 O  o CO2  H 2 'H 298K

42 kJ·mol

ˉ1

(3)

2CH 4  o C2 H 2 + 3H 2 'H 298K

377 kJ·mol

ˉ1

(4)

In addition to these reactions, there are also a few side reactions that produce a little coke and other higher alkyne (containing butadiyne, methylacetylene, propadiene, and so on). The stream leaving the reaction zone, at 1773 K, is rapidly quenched with water. The gas then goes to a water scrubber C103, where a large

Received 2007-12-03, accepted 2008-05-20. * Supported by the National Natural Science Foundation of China (90210032, 50576001). ** To whom correspondence should be addressed. E-mail: [email protected]

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Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008

Figure 1 Flow sheet of typical acetylene synthesis loop

stream of water further cools it to 310 K. The gas from the scrubber enters the absorption section. The acetylene recovery method is essentially an absorption by N-methylpyrrolidone (NMP) under pressure. The C2H2 can be recovered via a series of operations of absorption and resolving. In pretreating column A201, the gas comes into contact with a stream of NMP. According to the solubility and partial pressure in the NMP solution, butadiyne and other higher alkynes are absorbed first. The cracked gas, after being compressed to 1.01 MPa in three stages CP203, is absorbed in an absorber A204 with a cooling stream of NMP. In the absorber, the C2H2 and the components, whose solubility is higher than that of C2H2, are absorbed entirely. But all the NMP insoluble H2, CO, and so on escape at the top of the absorber to produce the by-products of synthesized gas. The NMP solution enters the stripper S205 after the pressure has been reduced. The product of the acetylene leaves at an intermediate point in S205. The stream from the bottom of S205 enters the stripping section and is heated in the exchanger H04 and heater H03. It is degassed in D301. The released acetylene gas from D301 goes to S205 as a stripping agent, and NMP from the degasser is cooled in H06 and recycled for absorption. At the point in D301 where the concentration of higher acetylenes is the highest, a stream is sent to the vacuum stripper. The vapor from S302 is scrubbed in S303 to get more acetylene by-product. 3 METHODOLOGY OF THERMODYNAMIC ANALYSIS

The second-law efficiency of a process expressed in terms of the exergy function is

Kex

H out H in

(5)

where, Hin is the exergy input to the system and Hout is its output. The total exergy balance in the process satisfies the relationship

H out

H in  I int

(6)

in which Iint is the exergy destroyed because of thermodynamic irreversibilities; it is generally referred to as the internal losses. In this study, the exergy efficiency is defined as

Kex

H out,useful

(7)

 ¦ H w  H fs  H q in

where, ™Hw is the total work consumed by the cycle containing compressors and pump, Hfs is the exergy of the steady flow stream, Hq is the exergy of heat and Hout,useful is its useful output exergy. The exergy of useful output in a real process is compared to the exergy leaving this process, expressed as:

H out,useful İ H out

(8)

Disregarding the kinetic and potential exergy terms, the basic exergy expression for a steady flow stream is defined as:

H T , p, x ª¬ H T , p  H 0 T0 , p 0 º¼  T0 ª¬ S T , p  S0 T0 , p 0 º¼

(9)

The environmental parameters are assumed as: p0 101.325 kPa and T0 298.15 K [17]. The exergy of a stream (specified temperature T, pressure p, and composition x) can be expressed as [18, 19]: H fs T , p, x

¦ xiH i0 T0 , p0  ¦ xi ^ª¬ H i T , p  H i0 T0 , p0 ¼º 

`

T0 ¬ª Si T , p  Si0 T0 , p 0 ¼º  RT0 ¦ xi ln

fˆi fi0



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Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008

ª w § fˆ · º § T · RT ¨1  0 ¸ ¦ xi « ln ¨ i0 ¸ » ¨ ¸ T ¹ © ¬« w ln T © f i ¹ ¼» p , x

(10)

where, H i0 T0 , p 0 denotes the standard exergy of any species i [18, 19], and fi 0 is the standard fugacity of each chemical species i. Hi, Si, and fˆi are the enthalpy, entropy, and fugacity of the stream at any state. According to the chemical characteristics and consistent conditions of the object, Hi, Si, and fˆi can be estimated by adopting the appropriate equation of state for streams in the system. The exergy of work and electricity is

Hw

(11)

ws

The Hq is used to express the exergy of heat. The Hq is calculated by

Hq

§

T ·

³q ¨©1  T0 ¸¹ Gq

(12)

The exergy analysis is determined according to Eqs. (7), (10), (11), and (12). 4

RESULTS

To reveal the locations of exergy loss, the overall process has been divided into three sections containing specific process steps: the reaction section, the absorption section and the stripping section, as indicated in Fig. 1. The exergy flow rates of all streams were calculated by the flow sheet simulator Aspen Plus. For these exergy calculations, a series of operating parameters for the process of acetylene partial oxidation of natural gas are presented in Table 1 according to the published report [15, 20]. The acetylene burner was modelled as a combination of three units. First, a mixer M01 was used to mix the natural gas stream with the oxygen stream; second, a yield reactor model was used to find the composition of synthetic gas from the reactor of partial oxidation of natural gas to make acetylene. Finally, a cooler C102 was used for the gas quenching process. For the compressors, the models of isentropic compression were used. For the condensers and heaters, the standard available models were applied. The absorber column for acetylene separation was simulated using a multi-component distillation and absorption model. In addition, the Peng-Robinson equation of state on the Boston-Mathias mixing rule, applicable for hydrocarbon and light gas mixtures, was selected for the reactor and heaters, and the Peng-Robinson equation of state on the Wong-Sandler mixing rule that is applicable to the mixture of polar and nonpolar compounds was selected for C2H2 absorbing separaTable 1

4.02

5.53

90.51

5

DISCUSSION

5.1

Analysis of reactor performance

Based on the same feedstock temperature and pressure of the POC reactor, the effects of different molar oxygen to methane ratios (O/M) and molar carbon monoxide to methane ratios (n) in the feedstock on the performance of the POC reactor was investigated. Considering the non-equilibrium due to reaction kinetics, the effects were modelled using the chemical approach to equilibrium temperature, the

Basic process parameters of studied acetylene synthesis

Raw material for 1 ton acetylene production Reaction Conversion Conversion temperature/K Methane/t Oxygen/t of methane of oxygen /% /% 1770.15

tion by NMP. The thermodynamic properties shown in Eq. (10) were respectively estimated by the above models. In this study, the feedstock is considered to be ˉ 6.572 kg·s 1 of natural gas at 298.1 K and 0.5 MPa. The feedstock contained 93.8% methane, 3.9% ethane, 1.1% propane, 0.1% butane, and 1.0% carbon dioxide. ˉ The O2 consumption was equal to 8.310 kg·s 1. The pressure of the absorber was 1.01 MPa, and the pressure of the stripper and degasser was 0.13 MPa and 0.10 MPa respectively. The exergy losses of an acetylene process according to Fig. 1 were calculated by Eqs. (10)(12). The results of the exergy analysis will be mainly presented and discussed on the basis of the exergy loss per unit product yield and acetylene yield. The advantage of presenting exergy analysis results in absolute values is that the importance and cost of the determined exergy losses can easily be judged, since absolute exergy losses can be readily converted into economic terms, such as primary fuel consumption in the form of natural gas equivalents (1 m3 natural gas §3.9 MJex) [21], a term process engineers are familiar with. Table 2 shows the overall exergy balances of the main utilities for the studied acetylene production process. The table summarizes the input exergy, output exergy, and exergy loss figures of the process configurations on an absolute basis per unit of acetylene. The second-law efficiency of the process was calculated from Eq. (7). The exergy efficiency of the acetylene production process was 77.09% for acetylene, synthesized gas and higher alkynes products. A detailed exergy loss distribution in the overall process is also shown in Table 2. It is clear that the irreversibility of the acetylene synthesis reactor, as well as the cooling of the gas mixture (quenching column and scrubber) were the major sources of exergy loss. They amounted to 20434.12 MJ and 25787.88 MJ per ton of C2H2, respectively, accounting for around 18.62% of the input exergy in this process, that is, 76.71% of the total exergy losses. The external exergy loss subsystem was the wastewater stream leaving the cooler and scrubber, accounting for 1.46% of the input exergy.

99.12

Absorption Acetylene pressure yield Carbon Carbon by NMP Hydrogen Ethene /% dioxide monoxide /MPa

Product molar concentration (outlet reactor)/% Acetylene

Methane

7.91

4.42

3.93

54.73

26.06

0.30

31.92

1.01

Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008 Table 2

Exergy balances for the studied acetylene production process

Utility

Parameters

Exergy flows ˉ /MJ·t 1

Percent of input exergy/%

input

natural gas

231533.10

93.29

oxygen

1307.01

0.53

output

exergy loss

fuel

10143.23

4.09

power (pumps and compressor)

5214.02

2.10

total

248197.36

100.00

synthesis gas

132038.89

53.20

acetylene

55807.41

22.48

higher acetylene product

3492.93

1.41

total

191339.23

77.09

preheaters

2450.35

0.99

POC reactor

20434.12

8.23

quenching column and scrubber

25787.88

10.39

pretreating column

6.43

0.01

compressor

2219.44

0.89

absorber (containing pump)

600.31

0.24

stripper

725.56

0.29

degasser

230.25

0.09

exchanger H04

341.92

0.14

heater H03

20.34

0.01

heater H05

24.19

0.01

vacuum stripper

62.41

0.02

hot scrubber

134.85

0.05

NMP cooler H06

201.72

0.08

waste water stream

3616.77

1.46

total

56856.54

22.91

815

The effects of the n and O/M on the carbon monoxide, acetylene and hydrogen yields, namely, the moles of generated product per mole of methane, have also been investigated, as shown in Figs. 35. Fig. 3 shows that when n increases, the yield of carbon monoxide decreases. In this case, with an increase of O/M, the carbon monoxide yield increases. As shown in Figs. 4 and 5, the yields of C2H2 and H2 increase with the increment of n. The increase of CO is favorable for the reaction in Eq. (3), producing more hydrogen. However, it could prevent the reaction shown in Eq. (1) and cause the reaction in Eq. (4) to make more C2H2. So, the yields of acetylene and hydrogen increase with the increase of the CO. The lower the O/M is, the more notably the yields increase. In addition, Fig. 4 shows the impact of O/M on the C2H2 yield. When the value of n is higher than 0.57, the yield of C2H2 reduces with an increase of O/M; and when n is less than 0.57, the increasing trend of C2H2 yield is different at distinct values of O/M. The maximum yield of C2H2 will be reached at the O/M of 0.60 when n is between 0.42 and 0.57, and it will be gained

Figure 3 Effect of different n and O/M on generating carbon monoxide per mole methane O/M: Ƹ0.69;ƶ0.65;ƻ0.60;ͪ0.55

product gas composition being calculated at a temperature 30 K lower than the real temperature that the reacting mixture reaches. The reactor outlet temperature was calculated based on the chemical equilibrium on the adiabatic operation. The reactor outlet temperature as a function of n and O/M is presented in Fig. 2. When the O/M or n is increased, the temperature increases gradually. This is due to the oxidation of CH4 into H2O and CO2 or the water gas shift reaction of CO into CO2 and H2 by more O2 supply.

Figure 4 Effect of diverse n and O/M on generating acetylene per mole methane O/M: Ƹ0.69;ƶ0.65;ƻ0.60;ͪ0.55

Figure 2 The reactor outlet temperature for various n and O/M O/M: ƻ0.69;ƶ0.65;Ƹ0.60;ͪ0.55

Figure 5 Effect of different n and O/M on producing hydrogen per mole methane O/M: ƻ0.69;ƶ0.65;Ƹ0.60;ͪ0.55

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Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008

at the O/M of 0.63 when n is less than 0.42. So the yields of C2H2 and H2 may be improved by adding some carbon monoxide to the feedstock and increasing the value of n. Based on the thermodynamic equilibrium compositions obtained, an exergy analysis carried out for the reactor. Fig. 6 shows that when the O/M rises, the exergy loss in the reactor increases. This is because there are more CH4 reacting with O2 emitting a great deal of reaction heat and increasing the temperature difference of heat transfer between lower temperature reactants and higher temperature products. The changes of exergy loss are obvious under different values of n. When the n increases, the exergy loss of the reactor first decreases then increases. It is due to the increasing of the yields of C2H2 and H2 leading to the exergy loss reduced. However, with the continuative increasing of the n, more CO shift reaction Eq. (3) occurs and the increasing exergy loss caused by chemical reaction and heat transfer exceeds the decreasing exergy loss caused by the increment of C2H2 and H2 yields. So the exergy loss of reactor is increased sharply.

Figure 6 Exergy destructions as a function of molar carbon dioxide-methane and oxygen-methane ratios in reactor O/M: ƻ0.69;ƶ0.65;Ƹ0.60;ͪ0.55

As shown in Fig. 6, a minimum of the exergy loss in the reactor can be reached when the n is about 0.20. Therefore, to reduce the exergy loss, a certain amount of CO can be added to the natural gas feedstock and the molar CO to CH4 ratio can be 0.20. Additionally, from Fig. 4, it can be seen when n is 0.20,

Figure 7

the maximum yield of C2H2 is 34.10% at the O/M of 0.65. So the improvement operating conditions of the POC reactor are at O/M of 0.65 and n of 0.20. 5.2

Process improvement

To improve the output of C2H2, decrease the heat loss and waste exhaust, and reduce the total exergy loss in the system, two retrofitting improvements are proposed as follows. (1) Retrofitting of operating parameters Adopting the method of membrane separation, the by-product of synthesized gas in the acetylene production process is sent to a set of membrane separation, and then the products of H2 and CO can be obtained. Some of the CO is cycled to stream 1, at the molar ratio of CO to CH4 n of 0.20 and the molar ratio of O2 to CH4 O/M of 0.65. (2) Retrofitting of the process flow structure By using oil to quench the gas mixture from the reaction zone instead of water, the reaction heat can be utilized to generate steam. The consumption of water can be reduced and the external exergy loss of wastewater can be avoided. The retrofitting process is shown in Fig. 7. The gas mixture leaving the reaction zone was rapidly quenched to 523 K by a circulating stream of oil; simultaneously a part of the oil was decomposed to benzene, toluene, and xylene (BTX) [15]. Then the gas, carrying oil, as well as its decomposition products, went to a quenching column and was cooled further. The oil from the bottom of the quenching column was passed through a waste heat boiler and returned to the reactor with fresh supplementary oil. In this way, the heat in the gas could be recovered. After leaving the quenching column, the gas entered a water scrubber, where a stream of water cooled it to 310 K so that the BTX was condensed. Then the gas from the scrubber was sent to the C2H2 absorption and stripping units to get C2H2 and the synthesized gas. CO was separated from the synthesized gas by-product and cycled to the natural gas (NG) feedstock. So the H2 product, a clean and efficient energy source [22, 23], was obtained.

The retrofitting process for making C2H2 based on natural gas

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Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008

During process simulation of the improved process, the models used in Aspen Plus were the same as the original acetylene production process. The results of the improved process are listed in Table 3. Based on the same size of C2H2 output, the new process can generate H2, a clean and efficient energy source, 86732.36 MJ per ton of C2H2. In addition, it also produces steam of 6184.33 MJ per ton of C2H2, coke of 3789.14 MJ per ton of C2H2 and BTX of 2091.72 MJ per ton of C2H2. The exergy losses of the reactor drop to 17280.35 MJ per ton of C2H2, the cooling of produced gas (quenching column and scrubber) falls to 18978.41 MJ per ton of C2H2, and the total exergy loss is reduced to 44032.08 MJ per ton of C2H2, a decline of 19.71% compared to the original process. Moreover, the consumption of natural gas feedstock in the new system is diminished by 9.88% in comparison to the original system. Table 3

Some variations of exergy analysis for the retrofitting process Exergy flows ˉ /MJ·t 1

Item

input

natural gas oxygen

output

208654.11 90.00 1307.34

ˉ9.88

0.57

0 ˉ14.18

fuel

8704.55

4.38

oil

6407.21

2.76

power

5308.89

2.29

1.82

total

230382.10

100

ˉ7.18

acetylene

55807.01

24.07

0

steam

6184.33

2.67

coke

3789.14

1.63

BTX

2091.72

0.90

higher acetylene

3146.57

1.36

hydrogen

86732.36

37.41

carbon monoxide

28598.89

12.33

total exergy loss

Deviation Percent with the of input original exergy proc/% ess/%

186350.02 80.38

ˉ9.88

ˉ2.61

preheaters

2208.12

0.95

ˉ9.88

POC reactor quenching column and scrubber

17280.35

7.65

ˉ13.22

18978.41

8.62

pretreating absorber

5.01

compressors

2000.02

absorber

So, after the improvements have been made, the natural gas cost can be reduced. Additionally, for the same amount of C2H2, the system can generate H2 and the exergy loss of the new system is lessened drastically. 6

CONCLUSIONS

Exergy analysis of an acetylene production system from partial oxidation of natural gas has been investigated. The results indicated that the acetylene reactor and the cooling of produced gas (quenching column and scrubber) are the main exergy loss units in the entire system. Their exergy loss amounts to 20434.12 MJ per ton of C2H2 and 25787.88 MJ per ton of C2H2, respectively, accounting for 76.71% of the total exergy loss. Through thermodynamic equilibrium and exergy analyses, the acetylene reactor has been retrofitted. The retrofitting conditions for partial oxidation of natural gas has been found to be at a molar ratio of O2 to CH4 of 0.65 and a molar ratio of CO to CH4 of 0.20. Under these conditions, 34.10% moles of C2H2 are generated per mole CH4, and the exergy loss of the reactor is 17280.35 MJ per ton of C2H2. In addition, the exergy loss in the quenching column and scrubber resulting from the usage of cooling water accounts for a larger proportion. So an improvement of acetylene production process was proposed. The retrofitting process produces acetylene and hydrogen under improved conditions: a quenching oil auxiliary system to recover the reaction heat of product gas, membrane separation technology to separate H2, and cycling some CO to the feedstock to increase the C2H2 yield. For the same amount of C2H2, the system can also generate H2 of 86732.36 MJ per ton of C2H2 and steam of 6184.33 MJ per ton of C2H2. Comparing the improved process and the original process, the feedstock of natural gas has a drop of 9.88% and the exergy loss has a decline of 19.71%. NOMENCLATURE fi 0

standard fugacity of composition i, Pa

ˉ22.53

fˆi

fugacity of composition i, Pa

0

ˉ16.67

H 'H

enthalpy, kJ·kg 1 ˉ enthalpy change, kJ·kg 1

0.86

ˉ9.87

LHV 'H CH 4

CH4 lower heat value, Jxmol

540.51

0.23

ˉ9.83

stripper

650.03

0.28

ˉ10.47

degasser

204.15

0.09

ˉ10.87

exchanger H04

307.08

0.13

ˉ9.94

heater H03

18.22

0.01

ˉ10.00

heater H05

22.31

0.01

ˉ8.33

vacuum stripper

55.76

0.02

ˉ9.68

NMP cooler H06

182.01

0.08

ˉ9.90

hot scrubber

122.24

0.05

H2 separation

1457.88

0.63

total

44032.08

19.62

ˉ9.63

I p q R S 'S T x

H 'H

K

ˉ1

standard state

Subscripts fs

ˉ1

exergy loss, kJ·kg pressure, MPa ˉ heat quantity, kJ·kg 1 ˉ 3 gas constant, m ·Pa·mol 1 ˉ ˉ entropy, kJ·kg 1·K 1 ˉ ˉ entropy change, kJ·kg 1·K 1 temperature, K mole fraction ˉ exergy, MJ·t 1 ˉ exergy change, MJ·t 1 efficiency, %

Superscripts 0

ˉ19.71

ˉ

steady flow stream

818 in int out w

Chin. J. Chem. Eng., Vol. 16, No. 5, October 2008 input internal output work

12

13

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