Article pubs.acs.org/IECR
Hollow Fibers Structured Packings in Olefin/Paraffin Distillation: Apparatus Scale-Up and Long-Term Stability Dali Yang,* Loan Le, and Ronald Martinez Divisions of Materials Science Technology and Chemistry, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, United States
Malcolm Morrison Morrison Consultants, El Rito, New Mexico 87530, United States S Supporting Information *
ABSTRACT: Following the conceptual demonstration of high separation efficiency and column capacity obtained in olefin/ paraffin distillation using hollow fiber structured packings (HFSPs) in a bench scale (J. Membr. Sci. 2006, 2007, and 2010), we scaled-up this process with a 10-fold increase in the internal flow rate and a 3-fold increase in the module length. We confirmed that the HFSPs technology gives high separation efficiency and column capacity in iso-/n-butane distillation for 18 months. We systematically investigated the effects of packing density, concentration of light component, reflux ratio, and module age on the separation efficiency and operating stability. Comprehensive characterizations using scanning electron microscopy (SEM), Brunauer−Emmett−Teller (BET), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and dynamic mechanical analysis (DMA) were carried out to probe the changes in the morphological, thermal, and mechanical properties of polypropylene (PP) hollow fibers over the aging process. The results suggest that after a long-term exposure to light hydrocarbon environments at ≤70 °C the morphological and mechanical properties of the PP polymer do not degrade significantly in a propane/propylene and iso-/n-butane environment.
1. INTRODUCTION Separation technologies crosscut all manufacturing industries and consume ∼4500 trillion Btu/yr (TBtu/yr), or about 22% of all in-plant energy use in the United States.1,2 Among different separation technologies, distillation stands out as the highest energy-intensive process yet is the most effective to achieve necessary purification among complicated mixed matrices.2−4 Most distillation in refining is thermally driven (heat of vaporization) with low energy and separation efficiency.5,6 Significant difficulties with distillation are confronted due to small differences in relative volatility between separated compounds that are often less than 2 over a wide operational range.7 The largest opportunities for energy reduction are offered by replacing the high-energy consumption distillation separation with more energy efficient structured packings, such as the proposed hollow fiber structured packings (HFSPs).8−14 Since 2003, we have explored the use of nonselective microporous HFSPs for the olefin/paraffin distillation. Compared to conventional packing materials, the HFSPs give high separation efficiency and capacity.10,15,16 The enhancements in both separation efficiency and capacity are due to the large specific area of the hollow fibers, which enables liquid and vapor to contact much more intimately without dispersion and excessive pressure drop. In the distillation column using HFSPs, the fibers are assembled into a shell-and-tube heat-exchanger configuration.9,10,17 Although this configuration is similar to that used in the well-known membrane distillation (MD) apparatus,18 the requirements and working principles for the membrane in the © XXXX American Chemical Society
HFSPs are different from those in the MD. The MD membranes should be porous, not wetted by the process liquids, and should not permit condensation to occur inside the pores. However, the nonwetted requirement limits the usage of commonly available hydrophobic membranes in the MD applications.5,6,18−23 Furthermore, the MD membrane (except for vacuum membrane distillation) should have a low thermal conductivity so that a high temperature gradient (to enable the necessary partial pressure gradient) can be built across the membrane wall. Therefore, the MD processes are operated under nonisothermal conditions. The HFSPs technology uses wetted hollow fibers for the following reasons: (1) to reduce the energy consumptioncan run distillation under an isothermal condition; (2) to enhance the operating stabilitysince the pores are filled with liquid, for the vapor phase to get into the lumen side, its pressure must not only be higher to overcome the gravity force acting on the liquid but also break the surface tension of the liquid acting on the pores. Therefore, the wetted wall will have better tolerance to the fluctuation of the vapor pressure; and (3) to maximize the economic feasibility because a wide variety of hydrophobic hollow fibers are commercially available. These fibers have been in mass production for decades and are quality-reliable and readily available with a low cost. Furthermore, when the liquid Received: January 11, 2013 Revised: April 2, 2013 Accepted: June 7, 2013
A
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relative volatility α is defined as α = (y*/x)/[(1 − y*)/(1 − x)] or αx y* = 1 + (α − 1)x (2)
flows downward and also creeps through the pores from the lumen side to the outer wall, convection transfer may potentially reduce the mass transfer resistance in the liquid side and even in the membrane wall. Of course, we understand that we are taking the risk that liquid can potentially flood the shell side and reduce the contact area for the mass transfer. We reason that when vapor and liquid counter-flow in different channels the vapor pressure is larger than that of the liquid pressure in most portions of the column (or module) where the liquid leakage should not be the issue. The most vulnerable section is the top portion of the module where the liquid pressure can be larger than the vapor pressure. However, if the pore size of the membrane is small enough and the fibers are well separated, the hollow fiber (HF) module should be able to tolerate some leakage. In the previous studies, we focused on the conceptual demonstration of the HFSPs technology.10,15,24 Several commercially available hollow fibers, such as polypropylene (PP), polysulfone, and polyvinylidene fluoride (PVDF), were evaluated. We concluded that Celgard PP hollow fibers possessed desired properties (small inner diameter, thin wall, and small pore size) for the HFSPs application. Although the stability of PP is not as good as PVDF and polyimide,25,26 PP fibers do have acceptable thermal stability and mechanical integrity in propane/propylene and iso-/n-butane environments. However, all experiments were performed in a small apparatus with a max internal flow rate less than 25 g/min and a module length less than 35 cm. A larger-scale experiment was deemed necessary to obtain better steady-state operating data and to confirm that high separation efficiency and capacity can be preserved for a long-term operation. In this work, we scaled our apparatus with a 10-fold increase in the internal flow rate and a ∼3-fold increase in the module length. We systematically investigated the effects of packing density, concentration of light component, reflux ratio, and module age on the separation efficiency and operating stability. Simultaneously, we also studied the long-term stability (morphological, thermal, and mechanical properties) of PP fibers in the light hydrocarbon environments (e.g., propane/propylene, iso-/n-butane, pentane, hexane/cyclohexane, benzene, and xylenes).
Since Raoult’s assumption applies in the iso-/n-butane binary system,7 α = pA0/pB0 (pA0 and pB0 are the saturated vapor pressure of iso- and n-butanes). When the pressure ranges from 30 to 135 psia, the value of α ranges from 1.5 to 1.35. At a steady state, G and L are constant. The rate equation (eq 2) can be integrated to give l=
0
y1
dy = HTU·NTU y* − y
(3)
with uG (cm/s) being the vapor velocity in the shell side and KG (cm/s) the overall mass transfer coefficient based on the gas side concentration as a driving force. For the most efficient conventional structured packings, the specific area is typically less than 10 cm2/cm3. However, the specific area of HFSPs can be easily larger than 30 cm2/cm3, which is the key factor to the reduced HTU. NTU is the number of transfer units and an intrinsic property of a pair of chemicals to be separated. A large NTU means a difficult separation. In the HFSPs, the mass transfer encounters three resistances in series as the more volatile compound moves from the liquid to vapor side.6,17,26 If we assume the diffusion process dominates the mass transfer process, the overall mass transfer coefficient KG is 1 1 H do H do = + + KG kG kM dlm kL d i
(5)
where kG, kM, and kL (cm/s) are the individual mass transfer coefficients in vapor, membrane, and liquid, and do, di, and dlm (cm) are the outside, inside, and logarithmic mean diameters of the fiber. The partition coefficient H is equal to mρG/ρL, where m is the slope of the equilibrium line of the distillation system and ρG and ρL (g/cm3) are the vapor and liquid concentration of more volatile species at the vapor and liquid interface. The H value of the iso-/n-butane mixture is between 0.015 and 0.035. For vapor flowing parallel to the fibers, its mass transfer coefficient can be calculated using32 ⎛ d 2u ⎞0.93⎛ ν ⎞0.33 k Gdh = 1.25⎜ h G ⎟ ⎜ G ⎟ DG ⎝ lνG ⎠ ⎝ DG ⎠
(6)
where νG (cm/s) is the kinematic viscosity of the vapor phase; DG (cm2/s) is the diffusion coefficient of iso-butane vapor, and dh (cm), as defined in eq 7, is the hydraulic diameter in the shell side
dy = K ya(y* − y) dz a = nπdo
∫y
where HTU (cm) is the height of transfer units defined as u G HTU = = G K ya K Ga (4)
2. DATA ANALYSIS METHODS Detailed data analysis methods were given in our previous papers10,24 and related literature.27,28 Here, we will only give a brief description of them. 2.1. Mass Balance and Mass Transfer Equation. Total reflux operation is commonly used to evaluate new packing materials and here to test the HFSPs. Along the column length direction, from the mass balance on the vapor side we find G
G K ya
(1)
where G (mol/cm2 s) is the molar vapor flux in the column, z (cm) the distance from the bottom of the column, Ky (mol/cm2 s) the overall gas-phase mass transfer coefficient along the cross section, y* the vapor equilibrium mole fraction corresponding to liquid composition x, a (cm2/cm3) the interfacial area per unit volume29−31 or specific area, n the area density of the packed fibers, and (do) (cm) the outer diameter of the fiber. For a binary system, under thermodynamic equilibrium, the
d Rh = h = 4
(πDTi )2 4
−n
(πdo)2 4
πDTi − nπdo
(7)
where Rh is the mean hydraulic radius and DTi is the inner diameter of the column. For a porous membrane, the mass transfer coefficient (km) in the membrane wall is equal to the diffusion coefficient DL (cm2/s) of iso-butane liquid times the porosity, divided by the B
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Figure 1. Conceptual illustration of HFSPs module used as a distillation column. Liquid flows downward in the lumen side, while vapor flows upward in the shell side of the module (process fluid wets the pores in the fiber wall). An inset illustrates the appearance of hollow fiber strings and a mesh.
tortuosity and membrane thickness (δ) (cm).33,34 For a Graetz number (Gz) much larger than 10,35 the following equation is valid to predict the mass transfer coefficient in the lumen side36 ⎛ d 2u ⎞1/3 kLd i d 2u = 1.26⎜ i L ⎟ , when Gz = i L > 10 DL DL l ⎝ lDL ⎠
lbf s2). The F-factor (FG in Pa0.5) is also used to describe the column capacity.7 It is calculated using the vapor velocity (uG) times the square root of the vapor densities (ρG) FG = uG ρG
2.3. Pressure Drop and Friction Coefficient. Due to friction, liquid flowing downward inside the fibers experiences pressure loss (ΔPL). With the measured ΔPL, the friction factor ( f L) of the liquid phase can be estimated using eq 12
(8)
where uL is the liquid velocity. Here, di = 0.024 cm, uL < 20 cm/ s, l = 90.6 cm, and DL = (6−9) × 10−5 cm2/s; so, Gz is around 10. The diffusivities of iso-butane (A) in gas and liquid phases are calculated using the Chapman−Enskog equation36 and the Reddy−Doraiswamy equation,37 respectively. 2.2. Vapor and Liquid Load. Conventionally, both vapor and liquid fluxes are calculated using the cross-sectional area of a distillation tower.7 Under the total reflux, the flow parameter equals to the density ratio of vapor to liquid phase (ρG/ρL). Conversely, since the vapor and liquid flow in their own channels in the HFSPs, as illustrated in Figure 1, their fluxes should be calculated using their own cross-sectional area. Thus, the flow parameter in the HFSPs depends not only on the ρG/ ρL but also on the ratio of their cross-sectional areas (AG/AL) 0.5 0.5 ⎛ A ⎞⎛ ρ ′ ⎞ ⎛ L ′ ⎞⎛ ρ ′ ⎞ Flow parameter = ⎜ ⎟⎜⎜ G ⎟⎟ = ⎜ G ⎟⎜⎜ G ⎟⎟ ⎝ G ′ ⎠⎝ ρ ′ ⎠ ⎝ AL ⎠⎝ ρL ′ ⎠ L
⎛ ΔP ⎞ d fL = ⎜ L ⎟ i 2 ⎝ l ⎠ 2ρL uL
G′2 Fψη0.2 ρG ′ρL ′gC
(12)
For a laminar flow in a long tube, an empirical equation (eq 13) is commonly used to estimate the friction factor of the flow fL =
16 , ReL < 2.1 × 103 ReL dLuLρL
ReL =
μL
(13)
where ReL is Reynolds number in the liquid phase. Similarly, with the measured ΔPG, the friction factor (f G) of the vapor phase can be estimated using eq 14
(9)
⎛ ΔP ⎞ R ⎛ ΔP ⎞ d fG = ⎜ G ⎟ 1 h 2 = ⎜ G ⎟ h 2 ⎝ l ⎠ ρ uG ⎝ l ⎠ 2ρG uG G
where L′ and G′ are the mass flux (lb/s ft2) and ρL′ and ρG′ are the density (lb/ft3) for liquid and vapor. Capacity factor is another commonly used term to measure the capacity of a column, which depends on the ratio of the kinetic energy in the vapor to the potential energy in the liquid and is defined as7 Capacity factor =
(11)
2
(14)
If the leakage of liquid is neglected and the vapor flow is stable, eq 15 can be used to estimate the friction factor in the vapor side fG =
(10)
where F is the packing factor (ft−1); ψ is the density ratio between water and process liquid; η is the viscosity (cP) of the process liquid; and gC is the gravitational constant (32.2 lbm ft/
16 , ReG < 2.1 × 103 ReG
ReGh = C
4R huGρG μG
=
dhuGρG μG
(15)
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Table 1. Summary of the Parameters of Six HF Modules (Active Length = 90.2 cm and Diameter of Module = 2.54 cm)a module label
HF-1
HF-2
HF-3
HF-4
HF-5
HF-6
number of fibers cross-section area for liquid AL (cm2) cross-section area for vapor AG (cm2) AG/AL specific area a (cm2/cm3) packing density (%) packing factor (cm−1)/(ft−1) hydraulic diameter dh (cm)
396 0.18 4.78 26.72 7.37 5.52 8.7/266 0.42
550 0.25 4.68 18.80 10.23 7.67 13.0/396 0.31
768 0.35 4.52 13.02 14.28 10.71 20.1/612 0.23
990 0.45 4.37 9.75 18.41 13.81 28.8/877 0.17
1320 0.60 4.13 6.92 24.55 18.41 45.2/1378 0.12
1980 0.90 3.67 4.09 36.83 27.62 97.1/2960 0.08
a
We had made one more module with packing density of 33.5%. Its performance was slightly better than HF-6. Since its module fabrication was different from these six modules, the detailed results of this module were not reported here.
However, when an appreciable amount of liquid leaks into the shell side where the hydrostatic pressure will be affected, eq 15 may not be valid. In the following discussion, the leak rate, defined in eq 16, will be used to determine the degree of the leakage Leak rate =
LFinlet − LFoutlet ·100% LFinlet
(16)
where LFinlet and LFoutlet are the measured mass flow rates (g/s) at the liquid inlet and outlet of the module, respectively.
3. EXPERIMENTAL SECTION 3.1. Hollow Fibers and Chemicals. PP hollow fibers (Celgard X30-240) were purchased from Celgard (North Carolina). The outer and inner diameters were 300 and 240 μm, respectively. The porosity was ∼40%. Pure iso-butane, pure n-butane, and iso-/n-butane mixtures (50%/50%) were purchased from Matheson Tri-Gas (Irving, TX). 3.2. Module Fabrication. The module design was similar to that used in the small apparatus.24 To construct a HF module, strings with 2−6 hollow fibers were individually strung through 15 plastic meshes. Each mesh contained ∼280 holes (an inset in Figure 1). These meshes were evenly spaced along a polycarbonate sleeve to minimize the contact among fiber strings. The bundle of the fibers was threaded inside the sleeve. The active length (l) and the internal diameter (DTi) of the modules were 90.4 and 2.54 cm, respectively. The fibers were potted using epoxy at both ends of the modules. Six modules were fabricated. Their parameters are summarized in Table 1. Among the six modules, the HF-6 module has the highest packing density. 3.3. Scaled-Up Distillation Apparatus. Back in 2003, we built a small apparatus.10 In 2009, we scaled up the apparatus, as shown in Figure 2. Its simplified process and instrumentation diagram is given in Figure 3. The detailed comparison between new and old apparatuses is given in Table S1 in the Supporting Information. With a set of catch tank and cooling systems, we were able to conduct the distillation under changeable reflux conditions. The large stock (up to 10 L) allowed a long experimental time without significantly altering the compositions in the reboiler. To ensure accurate and effective measurement, the sampling system was improved in the following ways: (1) the process fluid constantly flew through the sample loops; (2) the samples were collected at different locations simultaneously; and (3) the sample volumes were largely reduced. After collecting samples, a Micro-GC (Agilent micro GC 3000A) followed a predetermined sequence to analyze the samples in order. At a steady state, liquid and vapor samples were taken from four
Figure 2. Photograph of the HFSPs distillation apparatus in LANL.
sampling loops along the modules and one more loop for the reboiler composition. K-type thermocouples with accuracy ±0.1 °C and PX6133KG5V pressure transducers with accuracy ±0.2 psia (Omega Technologies Company) were used to measure temperature and pressure along the module, reboiler, catch tank, and condensers. Two PX771A differential pressure transducers (Omega Technologies Company) with accuracy ±0.1 psi were used to monitor the differential pressures in both phases. A LabView program was developed to control operation and sample analysis and recorded all operating parameters during the experimental course. The data collection rate was less than a few seconds. The whole system was well insulated to minimize the heat exchange between the surroundings and the apparatus. With all of the above improvements, we have conducted the experiments for more than three years with high reproducibility.38 More detailed hardware information about the new apparatus is given in the Supporting Information. 3.4. Experimental Method. Distillation experiments were conducted using iso-/n-butane mixtures. The concentration of iso-butane in the reboiler ranged from 50 to 80%. The temperature was 20−70 °C, and accordingly the pressure ranged from 30 to 135 psia (250 g/min, the time for reaching the steady state decreased from 2 to 0.5 h. Most experiments were conducted under total reflux. However, to investigate the effect of changeable reflux on the separation efficiency and operating stability, some experiments were conducted at the reflux ratio of 10−30. During the operation, the liquid in the condenser was controlled at the same level to ensure stable liquid flow rate. A standard set of tests was executed to evaluate the max capacity, operating stability, and separation efficiency of each module. A Matlab program was developed to calculate flow capacity, physical properties of process fluids at different phases, velocities, pressure drops, mass transfer coefficients, separation efficiency, etc. using the average data points over a period of 2− 4 h at the steady state. To evaluate the long-term stability, both
the HF-5 and HF-6 modules were tested for more than 9 and 18 months, respectively. 3.5. Aging Sample Preparation. A pressurized test station was constructed. As-received PP fibers were cut into many pieces of ∼5 cm length and loaded into the test station where the samples were exposed to the liquid iso-/n-butane mixture for different times. The exposed samples were analyzed over the aging process. 3.6. Characterizations. 3.6.1. TGA, DSC, and DMA. Thermogravimetric analysis (TGA) was conducted using a TA Instruments Q500 Thermogravimetric Analyzer. The samples were heated from 25 to 500 °C at a 10 °C/min heating rate under a nitrogen purge (10 mL/min). Differential scanning calorimetry (DSC) was conducted using a TA Instruments Q2000 Modulated Differential Scanning Calorimeter from −85 to 250 °C at a 10 °C/min heating rate under a nitrogen purge (50 mL/min) and was calibrated with indium and sapphire standards. Temperature was controlled using a refrigerated cooling accessory (RCA90). The samples were encapsulated in aluminum Tzero pans. Dynamic mechanical analysis (DMA) was conducted using a TA Instruments Q800 Dynamic Mechanical Analyzer and conducted at room temperature (20−22 °C).26 Oscillatory strain sweeps (beginning at 0.01% strain and continuing until breakage) were applied to each sample at a frequency of 1 Hz. E
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Figure 4. (a) HK plot and (b) BJH plot of PP fibers before and after exposure at room temperature for different times.
Figure 5. Effect of the oscillatory strain on the stress (a) and the dynamic moduli (b) of PP fibers before and after exposure at room temperature for different times (the oscillatory frequency = 1 Hz).
3.6.2. BET and SEM Analysis. Brunauer−Emmett−Teller (BET) characterization was conducted using an Autosorb Automated Gas Sorption System (model Autosorb-1, Quantachrome Co).26 Air-dried samples were degassed at 50 °C for >8 h prior to the measurement. Isothermal sorption was conducted at T = −195.65 °C (77.5 K) and at relative pressures (P/Po) ranging from 0.001 to 1.0. Po is the local atmosphere pressure. Volume−pressure data were reduced by Autosorb-1 software into BET surface area and micropore distribution. For scanning electron microscopy (SEM) characterization, the samples were coated with a thin layer of platinum (20 and
