Article pubs.acs.org/IECR
Separation of Olefin/Paraffin Gas Mixtures Using Ceramic Hollow Fiber Membrane Contactors Rami Faiz,† Marcos Fallanza,‡ Inmaculada Ortiz,‡ and K. Li*,† †
Department of Chemical Engineering, Imperial College London, South Kensington Campus, London SW7 2AZ, U.K. Advanced Separation Processes Research Group, Department of Chemical Engineering and Inorganic Chemistry, University of Cantabria, 39005 Santander, Cantabria, Spain
‡
ABSTRACT: Previous studies on gas/vapor separations using polymeric membrane contactors have always been accompanied with a membrane stability issue: either a wetting phenomenon or a membrane degradation concern because of extensive contact with the solvent. To possibly overcome these conventional challenges and provide extended stability, the use of a ceramic hollow fiber membrane contactor is proposed in this study where propylene/propane separation using silver nitrate as a carrier was investigated for a continuous period of 6 months. Alumina hollow fibers with asymmetric structures consisting of a spongelike outer layer and a fingerlike inner layer were successfully fabricated, modified, characterized, and finally examined for propylene/ propane separation. The modification step using silane solutions was critical in improving the membrane hydrophobicity to ensure that no wetting occurred during contact with the solvent. Initial studies on the separation performance confirmed that the membrane module was able to operate in a nonwetting mode, where the observed overall mass-transfer coefficient was the highest. Moreover, the ceramic membrane module proved to be stable throughout continuous experiments up to a period of 2 months, where no decline in the performance was observed. However, beyond this investigation period, deposition of silver on the membrane surface started to appear significantly where the membrane’s appearance becomes dark. This silver deposition seemed to diminish the membrane’s hydrophobicity as it continued to accumulate on the membrane surface, where a slight decline in the separation performance was finally observed by the end of the 6-month period. By exposing the advantages of ceramic membranes over their polymeric counterparts, a novel regeneration method was demonstrated where the membrane module undergoes treatment with strong nitric acid to remove silver deposits, followed by remodification of the membranes with silane solutions to restore its hydrophobicity. The performance of the membrane modules was regained completely after regeneration. Thus, this technology can be performed for an extended period without the need to replace the membranes whenever a drop in the performance is observed due to deformation of the membranes, an issue commonly found with polymeric materials.
1. INTRODUCTION Olefin/paraffin separation is considered to be one of the most important petrochemical processes, but with a heavy energy penalty. Membranes for the olefin/paraffin separation have been extensively studied for their possible replacement in the existing separation technique.1,2 The significance of both olefins and paraffins relies on their utilization as raw materials and building blocks for many chemical products. For instance, paraffins such as propane are commonly used as fuels for engines and residential central heating, while propylene is a raw material for a wide variety of products including polypropylene, which is used in packaging and other important applications such as the manufacture of piping systems. However, light hydrocarbons are usually produced together as byproducts from natural gas and petroleum refining processes. As a consequence, separation of these compounds is of high importance. Unfortunately, the separation process is considered to be very difficult because of the similar physical and chemical properties of olefins and paraffins such as molecular size, volatility, and solubility.2 The separation process is primarily carried out by cryogenic distillation in a single- or double-column process consisting of 150−200 trays. Because of the fact that distillation relies on the difference in volatility between olefins and paraffins to separate © XXXX American Chemical Society
the complex gas mixtures, the separation is considered to be a highly heat-integrated process with typical ethylene/propylene refrigeration systems used for low-temperature cooling.3 These systems are expensive to build and operate and are only economically attractive for streams containing high quantities of olefins such as those from large refinery catalytic crackers. Nevertheless, the energy and capital input of the cryogenic distillation process is enormous, and possible replacements of the process have been studied extensively.1−4 In the past few decades, the use of membranes for olefin/ paraffin separation has been given considerable attention by many researchers1,2,5 and has eventually emerged as one of the fastest-growing technologies because of its low operating cost compared to others. It is, therefore, not surprising that there are a large great number of articles in the literature that have focused on olefin/paraffin separation using several different membrane techniques. Earlier research was focused on the use of various polymeric materials in which olefins were more permeable to the corresponding paraffins because of their Received: December 27, 2012 Revised: May 11, 2013 Accepted: May 16, 2013
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increasing the temperature.17 However, one of the major disadvantages of this technique is membrane wetting and the limited thermal and chemical stability of most polymeric membranes when operated with corrosive solvents,18−20 which in return limits the application of this process in industrial scales where harsh conditions are commonly employed. To avoid the above-mentioned problems and provide a possible solution, the use of ceramic hollow fiber membranes for gas/vapor separation is proposed.21 In this manner, the ceramic hollow fibers provide a more stable system where wetting and changes in the membrane properties can be avoided because of the robust nature and chemical stability of the ceramic materials. The ceramic membrane contactor system in this study is examined for propylene/propane separation using an aqueous silver nitrate solution as the absorbent solvent. A complete description of the ceramic hollow fiber membrane preparation, modification, and characterization is presented within. It is interesting that, even though the longterm performance of membrane contactors is quite questionable, most reported studies in the literature were carried out for a few days22 or weeks.19 In this work, the long-term stability of the ceramic membrane module is investigated up to a period of 6 months. Because the performance is expected to decline over time, a unique technique is developed and implemented to regenerate the ceramic membrane module for further operation without the need to actually replace the membranes.
preferential diffusion based on differences in their molecular size. However, the use of such membranes would not be encouraged on a commercial scale because of their low permeation rates and separation factors.2 While research is steadily moving from polymeric to inorganic membranes,6 studies have been focused on the use of facilitated transport membrane and membrane contactor techniques for gas/vapor separation, which has shown great success in laboratory experiments and could be potential technologies to succeed in large-scale applications.7,8 A comprehensive review on various membrane-based facilitated transport/chemical absorption techniques for olefin/paraffin separation was recently published.1 These techniques include immobilized liquid membranes, flowing liquid membranes, membrane contactors, membrane electrolytes, and ionic liquid membranes. The advantages, shortcomings, and long-term stability of each specific membrane technology were discussed in great detail. It is of particular interest to employ hollow fiber membrane contactors for olefin/paraffin separation with a suitable carrier. This process imposes several practical advantages including high surface area per unit contactor volume, which contributes significantly to the reduction of the unit size. In addition, operating drawbacks occurring in conventional absorption columns such as flooding, loading, weeping, or foaming do not take place in hollow fiber membrane contactors because of independent control of the gas and liquid flow rates.9 Owing to these advantages, the use of hollow fiber membrane contactors has been studied extensively by various authors. Nymeijer et al.10,11 investigated composite hollow fiber membrane contactors for ethylene/ethane separation while using a silver nitrate solution as the absorbent solvent. Teramoto et al.12 developed a novel hollow fiber membrane contactor technique for ethylene/ethane separation where both the gas and liquid phases flow on the same side of the membrane contactor. They named this technique a “bulk flow liquid membrane”. Although this technique is quite different from a regular hollow fiber membrane contactor process where the gas and liquid flow in different compartments, the system proposed by Teramoto et al.12 has its advantages and disadvantages. For instance, high performance is achieved because of convective transport through the membrane compared with only diffusion forces achieved in a normal membrane contactor system. On the other hand, the product gas stream contains water vapor and requires additional purification equipment. On the other hand, the use of ionic liquids as an alternative absorption medium for propylene/propane separation in membrane contactors has shown some great interest recently, as demonstrated by Ortiz et al.13,14 and Fallanza et al.15,16 Industrial firms such as BP Amoco also studied the membrane-based contactors for olefin separation extensively using polypropylene hollow fiber membranes and silver nitrate as a carrier up to a pilot-plant stage.8 It was found that, although the process was technically sound, the operating cost of the process was too expensive mainly because of the short life span of the membranes caused by limited thermal and chemical stability when extensively used with silver nitrate. Hollow fiber membrane contactors seem to be the most effective and efficient method for gas/vapor separation applications using a carrier because they provide a high membrane surface area for mass transfer as well as a continuous gas−liquid contact for facilitated transport.1 In addition, regeneration of the solvent could be carried out by simple engineering methods such as decreasing the pressure or
2. EXPERIMENTAL SECTION 2.1. Ceramic Hollow Fiber Fabrication. 2.1.1. Materials. Aluminum oxide (Al2O3) powders of 1 μm (α, 99.9% metal basis, surface area 6−8 m2/g) was purchased from AlfaAesar and was used as the membrane material in this work. Poly(ether sulfone) (PESF; Radal A300, Ameco Performance, USA), dimethyl sulfoxide (DMSO; HPLC grade, Rathbone), and Arlacel P135 [poly(ethylene glycol) 30-dipolyhydroxystearate, Uniqema] were used as a binder, a solvent, and an additive, respectively. Tap and deionized water were used as the external and internal coagulants, respectively. 2.1.2. Method. Ceramic hollow fiber membranes were fabricated using a dry−wet spinning/sintering technique.23 More details about the fabrication procedures and protocols can be found elsewhere.24,25 It should be noted that most of the previous studies utilized N-methylpyrrolidone (NMP) as the solvent, whereas in this work, DMSO was used instead. However, the same protocols and procedures were followed accordingly. First, Arlacel P135 was dissolved in the solvent (DMSO), where the 1 μm particle sizes of Al2O3 powders (63 wt %) was added afterward. The dispersion was rolled with 20 mm agate milling balls for 48 h, and milling was continued for a further 48 h after the addition of PESF (6.3 wt %). The suspension was then transferred and degassed under vacuum for a few hours to ensure that no bubbles were trapped in the suspension before spinning. The presence of bubbles usually results in defects and holes on the membrane surface upon fabrication, and therefore it is desired to obtain a homogeneous and smooth suspension to obtain defect-free fibers. Immediately after degassing, the spinning suspension was transferred to a Harvard stainless steel syringe pump (Harvard PHD 22/2000 Hpsi), where it was extruded through a tube-in-orifice spinneret (3 mm o.d. and 1.2 mm i.d.) into an external water bath with an air gap of 15 cm. The extrusion rates of the suspension and internal coagulant were 7 and 12 mL/min, respectively. The fiber precursors were left in the external coagulation bath B
dx.doi.org/10.1021/ie400870n | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX
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2.3.3. Breakthrough Pressure. For breakthrough pressure measurements, a relatively short hollow fiber was assembled in polyester clear tube using epoxy resin. The inlet pressure of the module was monitored with a pressure gauge, where the solvent was flown through the lumen side using a gear pump. The flow at the outlet of the module was stopped by installing a needle valve at the exit. The inlet liquid flow rate was gradually increased, while the pressure was monitored in the range of 1− 3.5 bar. The breakthrough pressure of the asymmetric hollow fibers was determined to be the pressure at which the first drop of solvent appears on the outer surface of the hollow fiber. 2.3.4. Permeation Tests. Gas permeation tests were carried out on the original and modified ceramic hollow fibers to check whether any significant reduction in the permeation properties has occurred. One hollow fiber was sealed in a stainless steel tube with 22 cm length, where one end was completely sealed with epoxy resin while the other was open for gas-flow measurement. Argon (supplied by BOC) was pressurized through the shell of the module, and the gas permeation flux was measured using a bubble flowmeter from the lumen outlet. The gas permeation data were collected for operating pressures in the range of ΔP = 0.3−1.7 bar (gauge). 2.4. Propylene/Propane Separation. 2.4.1. Materials. Pure propylene (99.5% purity) and grade propane (>99% purity) were supplied by BOC. The silver salt used in this work was silver nitrate (>99.999% purity) supplied by Sigma-Aldrich. All chemicals were used as received without further modifications. 2.4.2. Membrane Contactor Setup. The experimental setup of the membrane contactor for propylene/propane separation using a silver nitrate solution is illustrated in Figure 1. The
overnight and then taken out to be straightened and dried for a few days. Finally, the precursors were calcined in air (CARBOLITE furnace) under high-temperature treatments. The temperature was increased from room temperature to 600 °C at a rate of 2 °C/min and held for 2 h and then to 1450 °C at a rate of 5 °C/min and held for 4 h. The temperature was then reduced to room temperature at a rate of 3 °C/min. 2.2. Ceramic Hollow Fiber Modification. Originally, the fabricated ceramic hollow fiber membranes possess a highly hydrophilic character because of the presence of hydroxyl (OH−) groups on the surface; however, the hydrophobicity of ceramic membranes can be promoted by surface modification, as previously shown.26 It was important to ensure complete surface modification of the hollow fibers before utilization of the membrane module for propylene/propane separation to avoid wetting with silver nitrate solutions. Although several methods and types of surface-modifying agents could be used to modify the surface of the ceramic hollow fiber membranes,26−28 fluoroalkylsilanes (FASs) were chosen in this study. FAS solutions are organosilanes that have hydrolyzable groups and other hydrophobic tails in their structures. The hydrolyzable groups are coupled with the hydroxyl groups (OH−) on the ceramic surface, forming a chemically bound hydrophobic layer. Several factors including the number of functional groups in the FAS structure, the length of the hydrophobic tails, grafting time, and grafting temperature play important roles in the surface grafting process.27,28 The specific type of silane solution used in this work was 1H,1H,2H,2Hperfluorooctylethoxysilane purchased from Sigma Aldrich. The sintered hollow fibers were immersed in 0.01 M FAS in a hexane solution at room temperature. The fibers were left in the solution for 2 h to allow the coupling reaction to occur. After immersion, the membranes were rinsed with hexane to remove any unreacted chemicals from the surface and finally dried at 100 °C for 12 h. 2.3. Hollow Fiber Membrane Characterization. The ceramic hollow fiber membranes were characterized by mercury intrusion, contact angle, gas permeation, scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDXS), and breakthrough pressure measurements. A comparison between the modified and original hollow fiber membranes was also carried out to examine the influence of the modification step on the membrane properties. 2.3.1. Membrane Morphology. The fibers were flexed at ambient temperature until a fracture occurred, the samples were then coated with gold particles under vacuum for 3 min at 20 mA (EMITECH model K550), and SEM images at varying magnifications were collected (JEOL JSM-5610 LV). In order to detect the influence of silver deposition on the membrane properties, EDXS analysis was used to identify the chemical composition and the elemental distribution of the membranes (EDS; INCA Energy by Oxford Instruments). 2.3.2. Contact Angle. Contact-angle measurements were carried out using the goniometry method.26 Although this method is commonly used to measure the contact angles of flatsheet surfaces, this technique can also be applicable because of the high outer diameter of the ceramic hollow fibers (≈2 mm). The contact angle was measured using a high-resolution camera and software to capture and analyze the contact angle resulting from the interface of the solvent with the outer surface of the hollow fiber membrane. The solvent used in the contact-angle measurements was 0.2 M AgNO3 similar to the solutions used in the experimental runs.
Figure 1. Membrane contactor experimental setup for propylene/ propane separation using 0.2 M AgNO3: (1) gas cylinders; (2) mass flow controllers; (3) needle valve; (4) pressure gauge; (5) membrane module; (6) pump; (7) rotameter; (8) solvent storage tank; (9) gas chromatograph.
membrane module was prepared using five hollow fibers with 22 cm length. The characteristics and dimensions of the membrane module used in this work are shown and summarized in Table 1. The feed gas mixture stream entering the membrane module was adjusted using mass flow controllers (Brooks Instrument MFC 5850) and flown through the lumen side, while a silver nitrate solution was flown through the shell side of the membrane module using a gear pump (Cole-Parmer model 75211-15). Other experiments were also carried out by interchanging the flow pattern, where the solvent and gas mixtures were flown through the lumen and shell sides, respectively. Figure 1 shows the experimental setup for the C
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was maintained at 1 bar, while the operating liquid pressure was set slightly higher at 1.2 bar to avoid gas bubbling. Because silver ions are very sensitive to light and air and can be easily reduced to Ag0, they were chosen to circulate the aqueous solution in the experiment to minimize exposure of the solution to its surroundings. However, to make sure that solvent saturation did not occur very frequently, a 2.5 L solution of the desired concentration was prepared and stored in a dark container, where it was utilized in the experimental runs. After each experiment, regeneration of the solvent (water−Ag+) was carried out by applying vacuum (
