Article pubs.acs.org/IECR
R23/R22 Separation and Recovery Using the DIST-PSA Hybrid System Qiang Fu, Yan Zhou, Yuanhui Shen, Haiyu Yan, Dongdong Li, Yingjie Qin, and Donghui Zhang* Collaborative Innovation Center of Chemical Science and Engineering, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China S Supporting Information *
ABSTRACT: Trifluoromethane (R23), a ubiquitous byproduct of chlorodifluoromethane (R22) synthesis, is one of the most potential greenhouse gases. The encouraged approach is thermal oxidation to process the vent gas by the clean development mechanism (CDM). Though such an approach could avoid the adverse environmental impacts, it will result in a great economic loss that includes not only the incineration cost but also the waste resource of R23. R23 is valuable as a refrigerant, fire suppression agent, and plasma etchant (purity: 99.999 mol %). In this work, the distillation-pressure swing adsorption (DIST-PSA) process was employed to concentrate R23 from 88% to 99.999% in the vent gas economically. Dynamic breakthrough experiments were performed to predict the adsorbent separation performance. Results indicated the coconut activated carbon (SAC-1) is appropriate material for R23 and R22 separation. The SAC-1 has a high separation factor value for R22 and R23 under the experimental conditions. Afterward, the DIST-PSA hybrid system was studied by process simulation and their energy consumption was analyzed. According to the comprehensive analysis, the best performance is obtained with a feed composition of 88% R23/12% R22 for the PSA unit. Process simulation predicts R23 recovery, purity, and energy consumption, respectively, of 69.52%, 99.9993% and 99.59 kJ·kg−1 R23. Overall, our studies have revealed that coupling a PSA unit with distillation is a feasible and promising technique to separate highpurity R23 efficiently and economically in the R22 synthesis industries.
1. INTRODUCTION Chlorodifluoromethane (R22) is widely used as a propellant and refrigerant in air-conditioning applications, as well as a common versatile intermediate in organofluorine compounds synthesis.1,2 The commonly preparation method of R22 is the reaction between chloroform with HF. An unavoidable and ubiquitous byproduct of this reaction is trifluoromethane (R23), which has a very high global warming potential.3−5 According to the CDM project, the greenhouse effect of R23 in the atmosphere is 11 700 times higher than the same weight of CO2.6 In general, when 1 tonne of R22 is produced, there is usually 30−40 kg of byproduct for most facilities.7 According to statistical data, there would be at least 30 kt·a−1 of R23 synthesized by 2050.8 Thermal oxidation has been encouraged to be used as the method to reduce R23 emissions by the CDM which the United Nations Framework Convention on Climate Change outlined.9,10 However, this approach results in a great economic loss because of its high incineration cost and the loss of R23. Besides, as an important refrigerant and fire suppression agent, R23 with a purity of 99.999% is widely used as a plasma etchant.11−14 In this regard, it is desirable to make a further separation for recycling R23 in the vent gas. Currently, multistage condensation is the most common way to concentrate R23 from R22.15,16 Under the usual operation condition of −40 °C and 0.55 MPa, R23 can only be enriched to © XXXX American Chemical Society
about 86−88% by bulk. Consequently, fractional distillation is required to generate higher purity R23. However, R23 has a boiling point of −82.1 °C, which requires energy-intensive cryogenic refrigeration for converting the gas stream into a liquid phase in the tower top condenser. To avoid the cryogenic refrigeration and enhance the energy efficiency, pressure swing adsorption is an alternate technique. Pressure swing adsorption or vacuum pressure swing adsorption is the most desirable method for R23 separation because of its low cost in energy consumption, no hazardous byproducts, and highly efficient automatic operation.17−22 Since its first practical application in the late 1950s, PSA and PSA have been used in many separation processes, but from our experience, there has no research reported on R22/R23 separation using the PSA technology. To develop a clean and energy efficient system to generate purity of 99.999% R23 from R22 synthesis vent gas, coupling PSA with a distillation hybrid system is adopted in this study. Hereinto, adsorbent selection is of vital importance. According to the literatures, the activated carbon has a higher adsorption capacity for R23 adsorption comparing with other materiReceived: Revised: Accepted: Published: A
September 22, 2016 November 24, 2016 December 16, 2016 December 16, 2016 DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 1. Breakthrough curves of R23 and R22 for different mixture gas pressures on SAC-1.
als.23−25 So we have chosen the most promising material, the coconut activated carbon (SAC-1), as the adsorbent. Dynamic column breakthrough experiments were then conducted to measure the specific adsorption quantity of R22 and R23 on SAC-1 and use the curving fitting results to get the Langmuir equation coefficient values of these two gases. On this basis, the practical R23 purification system was developed by coupling the PSA unit with distillation. A three-bed PSA system, including a 12-step process was considered in the design. The influence of a feed composition, pressure of vacuum evacuation step and purge gas flow rate on the performance was investigated by simulation of an industrial scale PSA system. Moreover, the optimal parameters were determined regarding to the purity and recovery efficiency. The simulation results showed that the hybrid system of PSA and the distillation designed can concentrate R23 with 99.9993% purity and 69.52% recovery efficiently under the optimal operating conditions. The energy consumption is only 99.59 kJ·kg−1 of R23.
2. ADSORBENT SEPARATION PERFORMANCE PREDICTION 2.1. Chemicals and Adsorbent. All gases used in this work stated the flowing fractional purities: He 99.999%, R22 99.9%, and R23 99.999%. The adsorbent was the self-made coconut activated carbon (SAC-1). The measurements of BET surface area, pore size, and micropore volume were performed by nitrogen adsorption at 77 K using an automatic sorptometer BELSORP-max (MicrotracBEL Japan, Inc.). The BET surface area was 1103 m2·g−1. The average pore size was 1.712 nm. The activated carbon was heated at 423 K for 12 h before packing into each column. Before each experiment, the column was flushed several times with 99.999% He at 600 kPa (abs) and was depressurized to 8 kPa (abs) for 10 min. The estimated uncertainty of the adsorption measurements due to the gas purity was assumed to be negligible. 2.2. Breakthrough Experiments. Breakthrough experiments were performed in a single stainless steel column with 0.46 m height and 0.025 m i.d. The column was filled with 65.813 g of SAC-1. Before beginning each experiment, the B
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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because the dynamic adsorption capacity of R23 and R22 on the SAC-1 increased with a further increase in partial pressure. 2.4. Separation Performance Analysis. The separation factor is the key factor in selecting the optimum sorbent for a particular gas separation application.21 Separation factors of SAC-1 for R22/R23 can be calculated from eq 2, and the results are shown in Table 1 Simulation studies in the literature have revealed that for an economic separation process, the minimum acceptable separation factor for the desired component is about 3. Although a separation factor beyond 4 may not be that helpful for the separation process comparing with that of 3.22 The Table 1 shows that SAC-1 has a higher selectivity for R22 and R23 under the pressure of experimental measurement at 298 K. Therefore, SAC-1 would be an appropriate adsorbent for R22/R23 separation.
column was pressurized to the adsorption pressure several times with 99.999% He. A gas mixture consisting of 35.06% R23/5.07% R22/59.87% He v/v/v was fed at a flow rate of 0.4 L·STP·min−1 into the bottom of the column. Analysis of the column effluent gas was accomplished by the gas chromatograph (Beijing Ruili’s SP2100A type). The breakthrough experiments were performed at 298 K and under five different pressures of the mixture gas: 0.31, 0.52, 0.74, 1.0, and 1.25 MPa. The R23 and R22 saturation capacity in the column was calculated according to eq 1.26 SCR 23 =
Qxi1 (tw1 − 22.4mj
∫0
SCR 22 =
Qxi2 (tw2 − 22.4mj
∫0
t w1
ct1 dt ) c01
t w2
ct2 dt ) c02
(1)
α=
where Q is the volumetric flow rate of the mixed stream at the bed entrance (L at STP·min−1), xi1 and xi2 are the mole fractions of R23 and R22, respectively, in the gas mixture at the bed entrance, mj is the initial dry weight of the adsorbent (kg), c01 and c02 are the R23 and R22 concentrations, respectively, in the mixed stream at the entrance of adsorber, ct1 and ct2 are the contents of R23 and R22, respectively, in the gaseous effluent stream at time t, tw1 and tw2 are the time (min) when the R23 and R22 concentrations in the gaseous effluent stream become c01 and c02, respectively. 2.3. Breakthrough Curves and Saturation Capacity. Breakthrough of R22 and R23 from the adsorption column was investigated at a fixed total flow rate of 0.4 L·STP·min−1 with 35.06% R23/5.07% R22/59.87% He v/v/v and an inlet gas temperature of 298 K. In this study, the aim was to obtain saturation capacity of R23 and R22 on SAC-1. Therefore, the breakthrough saturation point was defined as the time when the effluent gas stream consisted of R23 and R22 becomes c01 and c02, respectively. The results of breakthrough experiments and simulation predictions are shown in Figure 1. By the comparison between the experimental and simulated results, it can be found that the experimental data points are evenly distributed on both sides of the simulated curve, and the experimental results agree well with the simulation results. As shown in Figure 1, the measured breakthrough saturation points of R23 were 470, 580, 778, 935, and 1118 s, respectively, for the R23 partial pressure of 114, 201, 262, 349, and 437 kPa. Meanwhile, the measured breakthrough saturation points of R22 were 6891, 7570, 7682, 7973, and 8180 s, respectively, for the R22 partial pressure of 16, 29, 38, 51, and 63 kPa. At the breakthrough saturation points, the R23 and R22 loading onto the SAC-1 was estimated from eq 1 to be shown in Table 1. Results from the breakthrough experiments suggested a higher partial pressure to be beneficial to separate R23/R22 efficiently,
Pt (MPa)
α
Pi (kPa)
SC (mol·kg )
Pi (kPa)
SC (mol·kg−1)
0.31 0.52 0.74 1.0 1.25
8.28 8.68 8.88 11.17 11.29
16 29 38 51 63
5.21 7.78 10.61 14.06 16.26
114 201 262 349 437
0.59 0.91 1.12 1.18 1.35
23
(2)
3. PROCESS DEVELOPMENT 3.1. Hybrid System Design and Description. The fundamental structure of the hybrid system is shown in Figure 2. According to the actual plant of a company in China, the crude entering distillation tower is composed of R22 and R23. R22 is exported at the bottom of tower as qualified product, and R23 is concentrated at the tower top and then sent to the adsorption unit. Then, R23 is further purified as high-purity product in the adsorption unit. The discharged R22 is enriched in the adsorption bed and returns to distillation column. The stream parameters around the distillation equipment are listed in Table 2. Its bottom product and the head vent gas flow rate would change in a certain extent. To perform R23 purification with high degree of separation (purity: 99.999 mol %) and energy efficiency, it is necessary to introduce a proper and practical pressure swing adsorption system. Commonly speaking, the more columns used in the process, the higher product recovery would get. However, the higher operating cost and equipment cost associated with compressors and the more complex structure would be incurred. Hence, the adsorption unit studied in this work contained three beds, which would realize constant feed while saving money for investment. Besides, sorption steps have been adjusted properly to maximize process efficiency and keep the capital cost relatively low. 3.2. Adsorption System Design and Description. In our study, a three-bed and 12-step PSA system is employed for R23 upgrading to 99.999% from a mixture of R22/R23 (86−88 mol % R23) that produced at the top of the distillation tower. Figure 3 depicts a graphical overview of the PSA process system and the cyclic schedule is defined in Table 3. During the adsorption (AD) step, the high-pressure feed gas was introduced to adsorption bed from the feed buffer and R23 is obtained as the light product. Steps like equalization− depressurization (ED) and equalization−repressurization (ER) were successively performed by the equalizer buffer and the adsorber, which would help increasing product recovery as well as saving energy by adjusting pressure automatically and appropriately. Adsorbent regeneration and R22-rich product
R23 −1
22
SCR 23 /yR
where SCR22 and SCR23 are saturation capacity of the R22 and R23 on the adsorbent SAC-1 and yR22 and yR23 are the corresponding mole fractions of the R23 and R22 in the gas phase.
Table 1. Sorption Capacity and Separation Factors of R22 and R23 on SAC-1 at 298 K R22
SCR 22 /yR
C
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Figure 2. Schematic diagram of the distillation−adsorption hybrid system.
Table 2. Key Parameters of Distillation in Actual 75 kt·a−1 R22 Synthesis Plant parameters
middle feed
bottom output
head output
pressure (MPa) temperature (°C) flow rate (kg·h−1) R22 (mol %) R23 (mol %)
0.65 −25 9700 95 5.0
0.6 8 9320−9326 99.0 1.0
0.55 −41 374−380 12−14 86−88
(1) The gas phase behaves as an ideal gas. (2) No radial variation occurs in the gas concentration, temperature, and pressure. (3) The pressure drop along the bed is calculated by the Ergun equation. (4) Thermal equilibrium exists between the gas and solid phases. (5) The porosities of bed and adsorbent particles are uniform along the bed. (6) The adsorption rate is approximated by the linear driving force (LDF) approximation model with a single lumped mass transfer coefficient. (7) The extended Langmuir isotherm model is used to describe the adsorption behaviors. 4.1. Mass Balance. The component and overall mass balance equations based on the ideal gas assumption are given in eqs 3 and 4, respectively.28
obtainment are achieved simultaneously in both the blowdown (BD) and vacuum (VU) or purge (PU). Finally, pressurization with the light product (PR) step enabled the adsorption bed pressurize back to feed pressure which make it ready for a new cycle.
4. PSA MODEL Models of PSA process simulation were established with the commercial software, i.e., Aspen Adsorption. To establish simplified and generic mathematical models considering the physical complexities of an actual PSA process, the following assumptions are taken into account during model specification:27
−
∂(vgci) ∂c ⎞ ∂c ∂⎛ + (εb + (1 − εb)εp) i ⎜εbDax i ⎟ + ∂z ∂t ∂z ⎝ ∂z ⎠ + ρp (1 − εb)
∂qi ∂t
=0
(3)
Figure 3. Overview of the PSA process. D
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 3. Schedule of the PSA Processa time/s Bed1 Bed2 Bed3
50
50 AD↑ ER2↓ ED2↑
VU/PU↓ ED1↑
50
250
ER1↓ BD↓
PR↑ VU↓
50 ED1↑ VU/PU↓
50 ED2↑ AD↑ ER2↓
50 BD↓
250 VU↓
ER1↓
PR↑
50 VU/PU↓ ED1↑
50 ER2↓ ED2↑ AD↑
50 ER1↓ BD↓
250 PR↑ VU↓
a
AD, adsorption; ED, equalization depressurization; BD, counter-current blowdown; VU, vacuum desorption; VU/PU, vacuum and purge; ER, equalization repressurization; PR, pressurization with light component; ↑, co-current from the bottom to the top of column, which is in accordance with the feed direction; ↓, counter-current from the top to the bottom of column, which is contrary to the feed direction.
∂(vgc) ∂z
+ (εb + (1 − εb)εp)
Table 4. Extended Langmuir Model Fitting Parameters and LDF Model Parameters at 298 K
∂q ∂c + ρp (1 − εb) =0 ∂t ∂t (4)
The axial dispersion coefficient is calculated from eqs 5 and 6. νgR p
Dax = 0.73Dm +
0.01013T Dm =
εbD ⎞ ⎛ εb⎜1 + 9.49 2ν Rm ⎟ ⎝ g p⎠
(
1.75
P(Dv,A
1/3
+
1 MA
+
1 MB
(5)
4.2. Momentum Balance. The pressure drop is estimated via the Ergun equation, given by eq 7.
(7)
i=1
i=1
+ vgρg
∑ Cpg,i ∂T ∂z
i=1
+
∂T ∂t N
+ (1 − εb)ρp ∑ i=1
∂qi ∂t
ΔHi
2h(T − Tw ) ∂P ∂ 2T − (εb + (1 − εb)εp) − kg 2 = 0 ∂t Rb ∂z (8)
4.4. Mass Transfer Rate. The linear driving force (LDF) is expressed by eq 9:30 ∂qi ∂t
= kLDF, i(qi* − qi)
(9)
The single lumped mass transfer coefficient is calculated according to eq 10:31,32 kLDF,i =
ΩcDc,i rc
2
=
(12)
5. RESULTS AND DISCUSSION To find the optimum operating variables for the DIST-PSA hybrid system, several simulations about the PSA unit (employing the cycle sequence described in Table 3) were carried out to understand the effect of different operating variables (feed composition, evacuation degree and purge/ product ratio) on the process performance. PSA system parameters were set up according to the relational streams. The adsorption pressures of the PSA process are fixed at 0.5 MPa. The feed temperature and flow rate are 298 K and 5.18 kmol·h−1, respectively. The pressure profiles obtained at the outlet of column under cycle steady state (CSS) are presented in Figure 5. The highest
3Dc,i rc 2
(11)
4.6. Performance Indicators for the PSA Process. To evaluate the stand or fall of the PSA process, performance indicators, namely, target product purity and recovery and energy consumption, are widely employed. In this study, the PSA process performance was evaluated by the corresponding indicators, which are defined in Table 5. In this work, the relevant physical characteristics of activated carbon and the adsorption column are listed in Table 6. Adsorption processes are operated in a cyclic manner. Each cycle is described by a series of single or multiple sequential steps. The pressure drop of vent head gas from the distillation tower was assigned with 0.05 MPa before entering into the adsorption bed. Those gas streams recycled into PSA unit were heated to 25 °C by heat exchangers (heat transfer area, 11m2), which were ignored in Figure 2 of section 3.1. After adsorption process R22-rich product obtainment by adsorbent regeneration was then compressed (compressor shaft power, 3.2 kW) to 0.6 MPa uniformly, and recycled into the distillation tower. The vacuum pump shaft power used in Figure 3 of section 3.2 is 2.5 kW.
N
N
1 + ∑ biPi
⎛ ΔHi ⎞ ⎟ bi = b0 exp⎜ − ⎝ RT ⎠
((εb + (1 − εb)εp) ∑ ci(Cpg, i − R ) + (1 − εb)ρp Cps + (1 − εb)ρp ∑ qi(Cpg, i − R ))
qm,ibiPi
where the adsorption affinity is calculated as follows:
4.3. Energy Balance.29 The energy balance is
N
R23 3.468 2.23 −19.51 0.0801
qi* =
(6)
(1 − εb)Mρg 150μ(1 − εb)2 ∂P − = vg + 1.75 |vg|vg 3 2 ∂z εb (2R pψ ) 2R pψεb3
R22 3.302 95.21 −29.76 0.020
SAC-1; the corresponding fitting parameters are listed in Table 4. The extended Langmuir model is presented in eqs 11 and 12.
)
Dv,B1/3)
parameters b (MPa−1) qm (mol·kg−1) ΔH (kJ·mol−1) kLDF (s−1)
(10)
By fitting the experimental results of breakthrough curves to the model of Klinkenberg,33 we estimate the LDF model parameters for R22 and R23 and they are presented in Table 4. 4.5. Equilibrium Isotherm. Figure 4 displays the adsorption isotherms of R22 and R23 obtained from the breakthrough experimental data for adsorption of the pure components on SAC-1. Langmuir equation coefficient values were the curving fitting results of their adsorption quantity on E
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 4. Adsorption equilibrium isotherms of R22 and R23 on SAC-1.
Table 5. Performance Indicators of the PSA Process parameters
correlation
∫0
R23 purity (%)
tcycle
Fproduct,R23yproduct,R dt 23
t
∫0 cycle Fproduct dt ∫0
R23 recovery (%)
tcycle
(13)
Fproduct,R23yproduct,R dt 23
∫0
tcycle
Ffeed,R23yfeed,R dt
(14)
23
tcycle VinPoutγ [(Pout /Pin)1 − (1/ γ ) − 1] η(γ − 1) 0 tcycle Vproduct,R23yproduct,R dt 0 23
∫
PSA unit energy consumption (kWh·N·m−3 feed)
dt
∫
t
−3
recycle gas compressor energy consumption (kWh·N·m
∫0 BD+ VU+ PU
feed)
(15a)
Pout,recycleVin,recycleγ ⎡⎛ Pout,recycle ⎞ ηp(γ − 1)
⎟ ⎢⎜ ⎢⎣⎝ Pin,recycle ⎠
γ − 1/ γ
⎤ − 1⎥ dt ⎥⎦
t
∫0 cycle Vproduct,R23yproduct,R23 dt
(15b)
(Hin,feed − Hout,feed)Fout,feed
crude R23 heat exchanger energy consumption (kWh·N·m−3 feed)
t
∫0 cycle Vproduct,R23yproduct,R23 dt
(15c)
Table 6. Physical Characteristics of the Adsorption Column and Adsorbent parameters
value
Hb (m) Db (m) εp εb ρp (kg·m−3) ρb (kg·m−3) rp (m) Ψ Cps (J·kg−1·K−1) ks (W·m−1·K−1) kg (W·m−1·K−1) hf (W·m−2·K−1)
2.5 0.5 0.35 0.33 863 525 0.0012 0.83 709 16.0 0.452 219.0
Figure 5. Pressure history profiles of three columns for one cycle at the cyclic steady state.
pressure value is fixed in the AD step and the end of the PR step to get the maximal R22 adsorption efficiency and operation stability. On the contrary, the lowest pressure value appears at the end of VU step, which is introduced to regenerate the adsorbent thoroughly and enhance the R23 product purity greatly. The gas-phase temperature distribution is displayed in Figure 6, which is employed to analyze the thermal effect of the PSA process. It is easy to observe that the gas-phase temperature varies from a minimum value of 286 K to a maximum value of 317 K in one cycle under CSS. The
temperature distribution along the column displays a clear temperature gradient. 5.1. Feed Composition. The effect of the process performance was different with the feed composition, when the other parameters were constant. Results of product’s purity and recovery under different feed composition are shown in Figure 7. F
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 6. Temperature distribution with time and axial distance evolutions at the CSS.
Figure 8. Gas-phase (a) concentration profiles of R23 obtained along the bed at the end of AD step at CSS and solid-phase (b) concentration profiles of R23 obtained along the bed at the end of VU/PU step under different vacuum at CSS. Figure 7. R23 purity and R23 recovery under different feed composition.
comparing with other component feeds. Thus, we have settled it as the optimum feed composition. 5.2. Evacuation pressure. On the basis of the optimum feed composition (88% R23/12% R22), we have evaluated the effect of the evacuation pressure value on the process performances. In Figure 9a/b it can be observed that most of the R22 is removed in the vacuum step. It is easy to observe from Figure 9b that the lower the evacuation pressure value appearing at the end of VU step, the cleaner the adsorbent would regenerate. And a cleaner adsorbent would enhance the R23 product purity greatly after regenerating adsorbent thoroughly. The results presented in Figure 10 indicate that a lower evacuation degree value could lead to a higher purity of product with an acceptable recovery decrease. This situation is normal for this kind of cycles, but the energy consumption is quite high with the evacuation pressure reduction. And results showed the optimum vacuum pressure at 0.012 MPa (abs). 5.3. Purge/Product Ratio. The influence of the purge/feed ratio in the PSA process performance was also studied in our research. In Figure 11a,b it can be observed that the bigger the purge/product ratio value employed, the more thoroughly R22 is removed in the purge step. The results obtained can be observed in Figure 12. It was observed that when the purge/ product ratio was increased, there would be a rise in R23 purity and a little drop in recovery. For values of purge/product ratio
In Figure 7, when the R23 concentration in the feed rises from 86% to 88%, results showed that the R23 recovery increased more rapidly than purity. It is easy to observe that the R23 recovery reached its maximum value of 69.52% when the concentration of R23 in the feed is 88%. When the R23 concentration in the feed rises from 86% to 87%, the concentration of R23 in product is little increased. However, Figure 7 shows the R23 purity values just changed insidiously when its concentration is more than 87%. The gas-phase (a) concentration distribution profiles along the column at the end of AD step and solid-phase (b) concentration profiles of R23 obtained along the bed at the end of VU/PU step at the CSS with different feed composition (86−88% R23) are shown in Figure 8. It can be summarized from Figure 8 that the higher concentration of R23 in the feed (namely the high partial pressure value of R23 in the feed) makes the adsorbent regeneration more difficult. The R23 adsorption equilibrium will be reached easily on SAC-1 with a high concentration of R23 in the feed. The feed composition of 88% R23/12% R22 (v/v) could get a relatively higher purity R23 from the top of column with a handsome recovery G
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Figure 9. Gas-phase (a) concentration profiles of R22 obtained along the bed at the end of AD step at CSS and solid-phase (b) concentration profiles of R22 obtained along the bed at the end of VU/PU step under different evacuation pressures at CSS.
Figure 11. Gas-phase (a) concentration profiles of R22 obtained along the bed at the end of AD step at CSS and solid-phase (b) concentration profiles of R22 obtained along the bed at the end of VU/PU step under different purge/product ratio at CSS.
Figure 10. R23 purity and R23 recovery for different evacuation pressures.
Figure 12. R23 purity and R23 recovery for different purge/product ratios.
around 0.069, increasing the purge flow rate does not result in obvious variations in R23 purity anymore. Again, in Figure 12 it can be observed that for a fixed set of operating conditions, if the purge flow increased for a constant product stream, the decrease in R23 recovery was inacceptable. 5.4. Effects on Distillation System. According to the analysis above, the definite stream parameters boundary around
the PSA system were shown in Table 7 with R23 achieving 99.999% purity and the high recovery. After the identification of PSA unit optimum parameters, the influence of circulation on the distillation unit also needs to be analyzed for the hybrid system. The distillation product flow rates importing required PSA process recycle streams are compared in Figure 13. After coupling with PSA separation, it H
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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Industrial & Engineering Chemistry Research Table 7. Key Parameters of Input/Output Streams around the PSA System
a
parameters
feed (F)
recycle (Rec)
product (R23)
pressure (MPa) temperature (°C) R23 (mol %) R23 (mol %) R23 (mol %)
0.5 25 87 87.5 88
0.1 20 67.17 (2.05 kmol·h−1) 68.11 (2.03 kmol·h−1) 69.09 (2.01 kmol·h−1)
0.5 25 99.9991 99.9992 99.9993
The feed flow rate is 5.18 kmol·h−1.
Figure 15. Energy consumption for pure R23 in PSA system.
and the results are given in Figure 14. The energy load mainly depends on the total mass input to distillation, including the crude R22 and the recycle. Due to the proportion of recycle streams to the original feed input to distillation being about 1.8 mol %, the hybrid systems show similar energy duties with the original distillation. In other words, the DIST-PSA hybrid system can run as usual or just with a little adjustment. 5.5. System Energy Analysis. After increasing the pressure by the compressor and exchanging heat with the crude R23 at the distillation tower top, we found the discharged gas from bottom of the adsorption bed returns to the distillation column. The crude R23 was heated to 25 °C by heat exchangers using 90 °C hot water before entering into the adsorption bed. The capital energy consumption of the PSA system, i.e., the energy consumption of the crude R23 heat exchanger, recycle gas compressor, and vacuum pump, is estimated and listed in Table 8. As we can see, the increase in concentration of R23 in the feed would reduce crude R23 heat exchange energy consumption significantly. Nevertheless, the energy consumption of the recycle gas compressor and vacuum pump dropped slightly with the increased R23 concentration in the feed. The higher the R23 content from the distillation tower top, the lower the operation temperature of the column would be. The corresponding recycle gas into the distillation unit needs to be cooled to a lower temperature. The heat exchanged sufficiently between the crude R23 stream from the distillation tower top before entering the PSA unit and recycle gas. So the crude R23 heat exchanger energy consumption reduced with increasing R23 content from the distillation tower top. The total energy consumption of the PSA systems are calculated and given in Figure 15. The total energy consumption is about 99.59 kJ·kg−1 of R23 for the 88% R23/ 12% R22 (v/v) feed, 131.7 kJ·kg−1 of R23 for the 87.5% R23/ 12.5% R22 (v/v) feed, and 150.84 kJ·kg−1 of R23 for the 87% R23/13% R22 (v/v) feed, respectively. As expected, the 88%
Figure 13. Effects of PSA process recycle streams on distillation product flow rate.
Figure 14. Effects of PSA process recycle streams on distillation energy consumption.
can be seen from the mass balance that the flow rate of the vent gas from the top of the distillation column was increased to 6.8 kmol·h−1; meanwhile, the bottom product would increase in small increments. Subsequently, the effects of PSA system recycle streams on the energy duties of the condenser and reboiler were studied Table 8. Energy Consumption Analysis of the PSA System
R23 (mol %) crude R23 heat exchanger energy consumption (MJ·h−1) recycle gas compressor energy consumption (MJ·h−1) vacuum pump energy consumption (MJ·h−1) I
87
87.5
88
16.57 10.32 6.16
12.69 10.22 6.14
5.85 10.13 6.12 DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
Article
Industrial & Engineering Chemistry Research
Db internal diameter of adsorbent layer, m Dci micropore diffusion coefficient of component i, m2·s−1 Dm molecular diffusion coefficient, m2·s−1 F gas molar flow rate, mol·s−1 Hb bed height, m hf film heat transfer coefficient between the gas the adsorbent particle, W·m−2·K−1 kg gas phase heat conductivity, W·m−1·K−1 kLDF,i LDF coefficient, s−1 ks adsorbent thermal conductivity, W·m−1·K−1 M molar weight of the gas, g·mol−1 P pressure, MPa Pt mixture gas pressure, MPa Q volumetric flow rate of the mixed stream at the bed entrance, L·min−1 qi adsorbed phase concentration of component i, mol·kg−1 qi* adsorbed phase concentration in equilibrium with bulk gas of component i, mol·kg−1 qm,i specific saturation adsorption capacity of component i, mol·kg−1 rc micropore radius, m rp adsorbent particle radius, m R ideal gas constant, J·mol−1·K−1 Rb bed radius, m Rp particle radius, m SCi saturation capacity of component i, mol·kg−1 t time, s T temperature, K Tw column wall temperature, K u0 superficial velocity, m·s−1 vg velocity of the bulk gas, m−1 V standard volume flow rate, L·min−1 yi molar fraction of component i, dimensionless z axial distance of the adsorption column, m
R23/12% R22 (v/v) feed possesses lower energy consumption in comparison with the other two composition feeds. From the energy analysis, it is evident that the 88% R23/12% R22 (v/v) feed is superior to the others.
6. CONCLUSIONS To purify and recover the R23 from the vent gas of an R22 synthesis plant, a PSA system was designed and coupled with the cryogenic distillation unit. On the basis of the adsorption separation critical parameters of R23 and R22, dynamic breakthrough experiments were performed and verified that SAC-1 has good performance in R23/R22 separation. The actual breakthrough tests with laboratory preparation gas mixture at 298 K had five different pressures of the mixture gas: 0.31, 0.52, 0.74, 1.0, and 1.25 MPa. The result shows that SAC-1 has a high separation factor value for R22 and R23 under the experimental conditions. The integration of the PSA unit and distillation column was studied by process simulation. To enhance the separation efficiency as well as reduce the total energy consumption, different operating variables were studied. A comprehensive analysis revealed that the best performance is obtained with the feed composition 88% R23/12% R22 for the PSA unit. With this simulation R23 recovery, purity, and energy consumption, respectively, were 69.52%, 99.9993%, and 99.59 kJ·kg−1 of R23. Our work shows that the hybrid system of PSA and distillation is a promising technique to process the vent gas in R22 synthesis, not only avoiding the adverse environment impacts but also increasing the profit.
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ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b03701. Comparison of breakthrough curves between experimental and Klinkenberg’s model fitting results for R22 and R23 (Figure S1) (PDF)
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Greek Symbols
AUTHOR INFORMATION
Corresponding Author
*D.-H. Zhang. Tel: +086-022-27892097. E-mail:
[email protected]. ORCID
Donghui Zhang: 0000-0003-4892-2461
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Notes
The authors declare no competing financial interest.
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NOMENCLATURE
α separation factor of adsorbent, dimensionless ρb bulk solid density of adsorbent, kg·m−3 ρg gas phase density, kg·m−3 ρp adsorbent density, kg·m−3 ΔHi isosteric heat of adsorption of component i, kJ·mol−1 μ bulk gas mixture viscosity, kg·m−1·s−1 ψ shape factor of the adsorbent particle, dimensionless Ωc LDF factor of the micropore, dimensionless γ ratio of specific heats (Cp/Cv), dimensionless εb bed porosity, dimensionless εp particle porosity, dimensionless
REFERENCES
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English Symbols
b0 pre-exponential factor, MPa−1 bi adsorption affinity, MPa−1 c total gas phase concentration, mol·m−3 c0i concentration of component i at the entrance of adsorber, mol·m−3 cti concentration of component i in the gaseous effluent stream at time t, mol·m−3 ci gas phase concentration of component i, mol·m−3 Cpg constant pressure specific heat of gas phase mixture, kJ· kg−1·K−1 Cps specific heat of the adsorbent, kJ·kg−1·K−1 Dax axial dispersion coefficient, m2·s−1 J
DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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DOI: 10.1021/acs.iecr.6b03701 Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
