Low Feed Pressure Research Articles

CO 2 and CH 4 permeation through T-type zeolite membranes: Effect of synthesis parameters and feed pressure

In this paper, effects of synthesis parameters (synthesis temperature and time) and feed pressure on CO 2 and CH 4 gas permeances through zeolite T membranes were investigated. Experiments were carried out at three levels of synthesis temperature: 100, 120 and 140 °C; synthesis time: 15, 30 and 50 h and two levels of feed pressure: 1 and 5 bar. The gas permeances versus synthesis temperature first decreased with increasing synthesis temperature from 100 up to 120 °C, then increased with further increasing the values of synthesis temperature. The same plots were acquired for gas permeances versus synthesis time. In the case of CO 2/CH 4 ideal selectivity, reverse trends were observed. This behavior was attributed to the dual effect of increasing synthesis temperature and time on gas permeance and ideal selectivity. More zeolites were deposited and larger crystals were formed at higher synthesis temperatures and times. Formation of larger crystals accelerated the rate of zeolite layer integration, which was responsible for gas separation, in one hand and reduced the density of deposited zeolite layer on the support, due to void increase, on the other hand. Gas permeances through the membranes were in the range of 10 −11 to 10 −6 (mol/m 2 s Pa). In terms of maximizing the CO 2/CH 4 ideal selectivity, medium synthesis temperature and time (120 °C and 30 h) and low feed pressure (1 bar) were selected. Maximum gas permeances were obtained at low levels of synthesis temperature and time (100 °C, 15 h) and high feed pressure (5 bar). Zeolite T membranes were found as good choices for the separation of CO 2 from CH 4.

High Recovery Cycles for Gas Separations by Pressure-Swing Adsorption

Cycling-zone pressure-swing adsorption (CZPSA) and combination PSA−CZPSA cycles have been developed to increase the high-pressure product recovery of the Skarstrom PSA cycle. CZPSA uses a low-pressure feed as purge, whereas the combination PSA−CZPSA cycles use both a low-pressure feed gas and a product gas as purge. Extensive parametric studies were simulated using ADSIM for the model system of dilute benzene in nitrogen on activated carbon to determine how process performance was affected by flow scheme, purge sequence, penetration distance, feed and purge pressures and velocities, and pressurization procedure. For example, a nitrogen product purity of yB,prod/yB,feed = 0.05 and a high-pressure nitrogen recovery of P/F1 = 0.46 were obtained using the Skarstrom cycle, whereas the CZPSA cycle achieved values of yB,prod/yB,feed = 0.18 and P/F1 = 0.58. Air drying with activated alumina and air separation with zeolite 13X were also modeled. The Skarstrom cycle produced dry air with a dew point of −71 °C and P...

Enriching PSA Cycle for the Production of Nitrogen from Air

A novel 2-bed 4-step enriching pressure vacuum swing adsorption (PVSA) cycle was developed for the production of N2 from air, using an X-type zeolite, with the separation being equilibrium-driven. The four steps were low-pressure feed, countercurrent heavy product pressurization, countercurrent high-pressure heavy reflux, and cocurrent depressurization. The enriched heavy and light products (N2 and O2, respectively) were withdrawn from the system during the feed and heavy reflux steps, with both products being produced at the high pressure. In a series of 20 PVSA experiments, which were performed at a constant high-to-low-pressure ratio (π = 4.1), the effects of the feed and heavy product flow rates on the periodic state process performance were studied in terms of the O2 impurity, N2 recovery, N2 productivity, and feed throughput. The observed trends were as expected, with higher N2 recoveries (and corresponding N2 productivities) and O2 impurities generally being obtained at higher feed throughputs and heavy product flow rates. With regard to performance, in terms of N2 purity, one run produced a relatively high-purity N2 product that contained 0.8 vol % O2 at a N2 recovery of 23.7%, N2 productivity of 7.0 L STP h-1 kg-1, and feed throughput of 38.2 L STP h-1 kg-1. In terms of N2 recovery, another run produced a relatively high N2 recovery of 69.4% at a N2 productivity of 38.5 L STP h-1 kg-1 and feed throughput of 60.6 L STP h-1 kg-1, with the N2 product containing 10.6 vol % O2. This run also resulted in the highest N2 productivity. The overall performance of this relatively simple 2-bed 4-step enriching PVSA cycle compared quite favorably with the performances of the more-familiar N2 PSA processes that are either equilibrium-driven or kinetically driven and based exclusively on the stripping PSA cycle concept, especially because π was reasonably small.

Performance of treated and untreated asymmetric polysulfone hollow fiber membrane in series and cascade module configurations for CO 2/CH 4 gas separation system

This study investigates the effects of one-, two- and three-stage membrane system configurations in series arrangement for the CO 2/CH 4 separation for both untreated and treated membranes. Asymmetric polysulfone hollow fiber membranes were fabricated from 33 wt.% of polysulfone polymer using dry/wet phase inversion process. The produced membranes were characterized by pure gas permeation experiments, Differential Scanning Calorimetry (DSC), Scanning Electron Microscopy (SEM), density measurement and Thermogravimetric Analysis (TGA). For both untreated and treated membranes, the pressure-normalized flux of CO 2 decreased with increasing of the membrane stages. In addition, the selectivities of asymmetric hollow fiber membrane showed a more constant trend with feed pressure. Treated membrane exhibited lower pressure-normalized flux than untreated membranes due to skin layer densification which increased the transport resistance, thus lead to the reduction in pressure-normalized fluxes. Among all the three configurations studied, two-stage membrane configuration showed the most constant trend in term of selectivity. However, three-stage cascade configuration produced the highest CO 2/CH 4 selectivity especially when tested at low feed pressure range. Effect of stage cut on feed pressure showed an increasing trend with increasing of CO 2 and CH 4 feed pressure for all configurations. This is due to the increase of the permeation driving force, which caused the passage of larger amounts of more permeable gas through the membrane. This study showed that, three-stage cascade configuration exhibited the smallest stage cut values thus produced higher purity of CO 2 in permeate stream.

Characterization of silicone rubber membrane materials at low temperature and low pressure conditions

This paper will describe the identification and characterization of synthetic membranes for the separation and purification of CO 2 from the Martian atmosphere for in situ resource utilization (ISRU) applications. An ISRU application is the production of propellant (methane) on the surface of Mars from CO 2. Candidate materials should have high selectivity for carbon dioxide over nitrogen and argon, and a glass transition temperature of −40 °C or less to remain in rubbery state at low temperature for high permeances. Membrane materials we identified include the rubbery polymers poly(dimethyl siloxane) (PDMS) and the copolymer poly(dimethyl, methylphenyl siloxane) (PMPS). Pure and mixed gas permeation experiments with CO 2, N 2 and Ar were performed with these membrane materials at below ambient temperatures. In experiments with commercially obtained PDMS membranes, the pure gas CO 2 permeability increases from 2645 to 2792 Barrers as the temperature decreases from 22 to −20 °C and differential pressure of 1033 mbar (15 psi). The CO 2/N 2 ideal separation factor increases from 10.8 to 19.5 over the same temperature range. However, in pure gas experiments at low pressures (50–500 mbar) for the PDMS membrane, the CO 2/N 2 ideal separation factor decreases as feed pressure decreases which is primarily due to decrease in CO 2 permeability with pressure. Pure gas permeation results with PMPS membranes also showed an increase in CO 2 permeability from 1450 to 1650 Barrers as the temperature decreases from 21 to −10 °C for differential feed pressure of 1378 mbar (20 psi). The CO 2/N 2 ideal separation factor increased from 12 to 27 over the same range of temperature. The ideal separation factors measured at lower feed pressures are closely related to the values at higher feed pressures for the PMPS membrane. This article includes transport data for simulated Martian atmosphere temperature and pressure conditions performed at Lockheed Martin, Denver. An interesting behavior in CO 2 permeance is observed at low feed pressures for both PDMS and PMPS membranes. The trend in CO 2 permeance with temperature changes with variation in feed pressure. A hypothesis is presented for this observation.

Rejection of Emerging Organic Micropollutants in Nanofiltration–Reverse Osmosis Membrane Applications

The rejection of emerging trace organics by a variety of commercial reverse osmosis (RO), nanofiltration (NF), and ultra-low-pressure RO (ULPRO) membranes was investigated using TFC-HR, NF-90, NF-200, TFC-SR2, and XLE spiral membrane elements (Koch Membrane Systems, Wilmington, Massachusetts) to simulate operational conditions for drinking-water treatment and wastewater reclamation. In general, the presence of effluent organic matter (EfOM) improved the rejection of ionic organics by tight NF and RO membranes, as compared to a type-II water matrix (adjusted by ionic strength and hardness), likely as a result of a decreased negatively charged membrane surface. Rejection of ionic pharmaceutical residues and pesticides exceeded 95% by NF-90, XLE, and TFC-HR membranes and was above 89% for the NF-200 membrane. Hydrophobic nonionic compounds, such as bromoform and chloroform, exhibited a high initial rejection, as a result of both hydrophobic-hydrophobic solute-membrane interactions and steric exclusion, but rejection decreased significantly after 10 hours of operation because of partitioning of solutes through the membranes. This resulted in a partial removal of disinfection byproducts by the RO membrane TFC-HR. In a type-II water matrix, the effect of increasing feed water recoveries on rejection of hydrophilic ionic and nonionic compounds was compound-dependent and not consistent for different membranes. The presence of EfOM, however, could neutralize the effect of hydrodynamic operating condition on rejection performance. The ULPRO and tight NF membranes were operated at lower feed pressure, as compared to the TFC-HR, and provided a product water quality similar to a conventional RO membrane, regarding trace organics of interest.

Strategy for optimal operation of a biomass-fired cogeneration power plant

Biomass-fired cogeneration not only is an environmentally friendly energy production, but also possesses high energy conversion efficiency. Generally, the wood product industry requires both heat and electricity. Combined heat and power generation cogeneration) using wood residue has a three-fold benefit: waste minimization, reduction of an energy-related production cost and additional income from selling the excess electricity to the utility. In reality, the process heat demand fluctuates according to the production activities in the factory. The fluctuation of process heat demand affects the cogeneration efficiency and the electricity output and, consequently, the financial return, since the prices of heat and electricity are different. A study by computer simulation to establish a guideline for optimum operation of a process heat fluctuating cogeneration power plant is presented. The power plant was designed for a sawmill and an adjacent plywood factory using wood wastes from these two processes. The maximum boiler thermal load is 81.9 MW while the electricity output is in the range 19-24 MW and the process heat 10-30 MW. Two modes of operation were studies, namely the full (boiler) load and the partial (boiler) load. In the full load operation, the power plant is operated at a maximum boiler thermal load, while the extracted steam is varied to meet the steam demand of the wood-drying kilns and the plywood production. The partial load operation was designed for the partially fuelled boiler to provide sufficient steam for the process and to generate electricity at a desired capacity ranging from the firmed contract of 19 MW to the turbine maximum capacity of 24 MW. It was found that the steam for process heat has an allowable extracting range, which is limited by the low pressure feed water heater. The optimum operation for both full and partial load occurs at the lower limit of the extracting steam. A guideline for optimum operation at various combinations of electricity output and steam demand is presented. The maximum achievable overall efficiency is 64.6 per cent when the power plant is operated at partial (boiler) load condition for 19 MW electricity and 30 MW process heat. The full load operation, although having lower overall efficiency, is preferable financially because of the higher unit price of electricity in comparison with heat.

Elevated temperature application of polymer hollow-fiber membranes

Three asymmetric hollow-fiber polymer membrane systems were studied for application in elevated temperature, low feed pressure systems: (1) a single component polyaramide, (2) a single component polyimide, and (3) a composite polyimide on a polyimide/polyetherimide blend support. Permeation driving force was increased for the 2.2 psig feed pressure by sweeping an inert gas along the downstream side of the membrane. Both cocurrent and countercurrent sweep flow patterns were examined with only minimal differences found. The polyaramide membrane was stable in the entire range of temperatures tested (23–220°C). After utilizing a silicone rubber post-treatment, the membrane exhibited a hydrogen permeance of approximately 300 GPU at 175°C with a hydrogen to n-butane selectivity of 700. The polyimide-containing membranes had superior room-temperature properties; however, the thin skins aged at elevated temperatures. This aging effect decreased the permeance of the membranes approximately 40% at 175°C and slightly increased the permselectivity; however, the effects of aging leveled out over 200–250 h at 175°C and the membrane properties became constant. At this level, the polyimide membranes exhibited approximately 400 GPU of hydrogen permeance with a 660 selectivity to n-butane.

Osmotic dehydration coupled reverse osmosis concentration: steady-state model and assessment

A steady-state analysis was made for evaluating the RO concentration of sugar solutions assisted by flowing a brine solution in counter-current mode through the permeate side. Calculations show that membrane area requirements for high solute concentrations could be reduced by combining RO with osmotic dehydration. The area requirements are sensitive to salt concentration polarization on the permeate side, particularly when low feed pressures are used. It appears feasible to design membranes/modules with low enough values of permeate side concentration polarization to enable effective feed solution concentration

Innovative design of nano- and ultrafiltration plants

The productivity of nanofiltration plants can be significantly improved by installing a reduced number of membrane elements serially in pressure vessels [1]. A reduced number of elements per pressure vessel results in a reduction of the hydraulic pressure losses, but more pressure vessels (with a shorter length) have to be applied. The water supply companies Friesland (WLF), Oost-Brabant (WOB) and the consultant DHV Water have developed a new pressure vessel and interconnector for nanofiltration and ultra low pressure reverse osmosis plants. This new pressure vessel in combination with the new interconnector allows optimization of the system hydraulics, without the need for extra pressure vessels. Compared to a traditional design with 6 elements serially placed in pressure vessels, an annual cost reduction of approximately 10–15% for membranes and pressure vessels can be achieved with the new design. In ultrafiltration plants, pressure vessels contribute to approximately 4% of the total construction costs. Since pressure vessels are principally not needed for pressure hold because of the low feed pressures applied, the pressure vessels can be replaced by the new interconnectors. Approximately 2% of the total construction costs can be saved in this way. Water supply company Friesland applies the new pressure vessel in an ultra low pressure reverse osmosis plant, that is started-up this year. Water supply company Oost-Brabant is testing the new interconnector for application in ultrafiltration plants.

Comparison of trickle-bed and upflow reactor performance at high pressure: Model predictions and experimental observations

Comparison of laboratory trickle-bed and up-flow reactors over a range of operating conditions, which cover both gas and liquid reactant limitations, has been investigated using hydrogenation of alpha-methylstyrene to cumene in a hexane solvent over 2.5% Pd on alumina extrudate catalyst as a test reaction. The results show that when the reaction is gas limited at low pressure and high liquid feed concentration, trickle bed reactor outperforms the upflow reactor. At high pressure and low liquid feed concentration, the reaction becomes liquid limited and upflow reactor performs better. It is concluded that the advantage of upflow or downflow depends on the reaction system type (i.e. whether the reaction is liquid or gas limited). A single criterion for identifying the limiting reactant is proposed which can explain most of the data reported in the literature on these reactors. Comparison of the experimental observations and the predictions of the reactor scale and pellet scale models available in the literature is presented.

A new concept to increase recovery from H2 PSA: processing different pressure feed streams in a single unit

A new process concept is outlined to utilize low and high pressure feed streams efficiently in a single pressure swing adsorption (PSA) process. The low pressure feed is processed first in the PSA unit to produce low pressure product. Following this, the high pressure stream is processed in the same unit. Low pressure generated in the first part of the process is used internally in the process to assist the production of high pressure product. Essentially 100% recovery of the high pressure product is possible. This is achieved in a single unit without the need to store gas product during the intermediate process steps. The process concept is demonstrated for pure hydrogen production. A process is simulated in which two streams, both containing 25% methane and 75% hydrogen at two different pressures, are supplied to a single PSA unit. The simulation shows that, using a five-bed process, one can produce 100% high pressure hydrogen at ultra-high purities (i.e. ppm levels of methane in the product). If the low pressure stream is not integrated, then the maximum high pressure H 2 recovery is only 85%.

Characterization of the permeation properties of CO2N2 gas mixtures in silicone rubber membranes

Abstract The separation characteristics of silicone rubber membranes are determined for CO 2 N 2 gas mixtures. The analysis is performed as a function of composition, flow rate and pressure of the feed gas. Results are presented in terms of the variation in component permeability and separation factor as a function of the above parameters. Component permeabilities are calculated using the complete mixing model. Data analysis over the studied pressure range shows that the permeability coefficient of pure CO 2 gas in silicone rubber is 15 times higher than that of pure N 2 gas. This behaviour is completely altered for a mixture of the gases, where the calculated separation factors at low feed pressures and low CO 2 mole fractions in the feed stream are two- to three-fold lower than the separation factors for the pure gases. At higher feed pressures and high CO 2 mole fractions in the feed stream, the above behaviour is reversed; the separation factors for the gas mixture are now higher than those for the pure gases. Comparison of the permeation characteristics of silicone rubber and cellulose acetate membranes for CO 2 N 2 gas mixtures shows similar ranges and values for the gas permeabilities and separation factors. However, much higher separation factors are obtained for the cellulose acetate membrane in the case of pure gas permeation.

Feed Purge Cycles in Pressure Swing Adsorption

Abstract Three new pressure swing adsorption cycles for the production of oxygen-enriched air at very high recoveries are developed. The cycles involve a purge step using low-pressure feed gas. Local equilibrium theory is used to predict purities and recoveries for air separation with 5A zeolite. Novel methods for drying the inlet air are developed. For low product purities these processes are predicted to have very high recoveries and adsorbent productivities. A simple new method for determining the location of a curved shock wave is illustrated. Other possible applications of PSA feed purge cycles are briefly discussed.

Heat treatment of pipes with thickened ends in an industion installation

In induction heating for hardening of pipes with thickened (upset) ends, preheating of these ends has to be carried out to 750–800°C immediately before the pipes are heated in the inductors of the main line. In heating prior to tempering the pipe ends also have to be preheated to 280–360°C. To eliminate nonuniformities of the temperature along the smooth part of the pipe caused by different wall thickness, it is recommended to envisage a zone of temperature equalization whose length may amount to as much as 30% of the length of the main heating line, depending on the required accuracy of heating. In the process of heating prior to hardening the pipes do not become bent. The pipes may become bent when they are not uniformly cooled in the sprayer on account of low water pressure or nonuniform feed of water over the circumference.

A comparison of some recycle permeators for gas separations

The performances of several different recycle permeators are compared by calculating the enrichment, extend of separation, and area requirements for different overall stage cuts and recycle ratios, using the separation of air as an example. Membrane modules investigated included a two-unit series type permeator with permeate recycle, a continuous membrane column with high-pressure feed, and two single-unit countercurrent permeators with permeate recycle (both high-pressure and low-pressure feed). The use of Rony's extent of separation in the analysis greatly aids in the visualization of limiting permeator performance at different recycle ratios. The composition of O 2 in the high-pressure product (reject) stream becomes zero at specific values of the stage cut which depend on recycle ratio for all but the low-pressure feed module. This would require an infinite permeator area, hence stage cuts greater than this limiting value are not possible. For the case of the low-pressure feed unit, the reject stream composition approaches a minimum finite value that depends on pressure ratio in the permeator. This limiting minimum composition would also be reached at specific values of stage cut that depend on recycle ratio and hence a stage cut greater than this limiting value cannot be obtained. Other permeator types (co-current, perfect mixed models) do not behave in this manner. It is concluded that the two-unit series type permeator gives the best separation and requires smaller membrane areas at high recycle ratios. Performance of the four permeators is not greatly different at lower recycle ratios.

Analysis of Power Plant Deaerator Storage Tank Instabilities

In some power plant deaerators storage tank steam is caused to flow over the surface of the water on its way to the condensing head. This steam flow, which may become very large under transient load operation, low pressure feed heater bypassing or isolation, may create water waves, a hydraulic gradient and possibly spray which carries over into the deaerator head via the steam transfer pipe to result in high differential pressure. This may lead to head flooding, surging discharge, tray stack damage, and also give rise to problems with associated components; for example vent condenser flooding. Steam transfer pipes may constitute a forcing system resulting in tank contents sloshing and severe vibration. These events in turn may precipitate an instability (the Kelvin-Helmholtz instability) whereby waves may grow to bridge the gap between water surface and tank crown to form a water-piston. A piston so formed is rapidly propelled along the tank to impact with a hammer-like blow causing serious internal and external damage and jeopardizing safety. The sequence of events finally precipitating the onset of the water piston is complex and renders an analytical approach which might establish design criteria difficult, for this reason simple modelling is suggested. This paper describes such studies using one-twelfth scale models of late generation 500-660 MW units, from which the parameters governing instability are evaluated.

Water Hammer Phenomena in Thermal Power Station Feed Water Systems

The phenomenon of water hammer in stagnant hot fluid systems has been manifested by the fracture of a number of cast iron valves and other components in power station low pressure feed water systems and other similar systems. A comprehensive investigation proved necessary into all aspects of this phenomenon. The investigation involved a survey of water hammer events, experimental work, data acquisition, pressure testing, calculation of forces and the determination of the strength of individual components. From this work has sprung a clear understanding of the sequence of events occurring when pressure is reduced on hot feed water and flash steam is produced which subsequently escapes or condenses and fluid columns recombine at speed and produce pressure pulses. Experimental and theoretical work has revealed the magnitude of such pressure pulses and also revealed that steel pressure parts will not fail when subjected to the forces arising from thermodynamically induced water hammer, because steel pipes limit transient hydraulic pressures by dilating and steel valves are sufficiently strong. The dynamic response of complex pipework system to stimulation by water hammer events requires further investigation, but practical experience indicates that failure of steel system components is unlikely from dynamic movements, although damage may be suffered by pipe hangers, seismic restraints etc.

A Preliminary Heat-Balance Computer Program Conforming to SNAME T&R Bulletin 3-11

This paper presents a FORTRAN IV batch-type computer program for the heat-balance calculation of a nonreheat marine steam plant. The program is in conformance with the commercial practice standards presented in SNAME Technical and Research Bulletin 3–11 "Marine Steam Power Plant Heat Balance Practices." The program treats the preliminary design heat-balance problem where feed heater outlet temperatures can be specified prior to the selection of specific heat exchanger characteristics. The system design includes a realistic auxiliary exhaust system utilizing auxiliary exhaust, bleed, augment, and unloading as available or necessary or both. The system must include a deaerating feed tank and may include up to two low-pressure feed heaters and up to three high-pressure feed heaters. The program is designed so that further extension and adaptation to special needs can be easily accomplished. Program design results in a minimization of input data and very efficient running times. A verification example is presented. User's Instructions, Programmer's Documentation, and a listing are included as Appendices.

Effects of Reaction Conditions on the Plasma Polymerization of Ethylene

Abstract Ehylene can be polymerized at low pressures in a radio-frequency glow discharge. The form of the resulting polymer may be a powder at low pressure (1 to 2 Torr) and low monomer feed rate (10 to 40 cc/min), a colorless film at low pressure and high feed rate (70 to 90 cc/min), or an oil at high pressure (4 to 5 Torr) and high feed rate. The powder and film forms of plasma-polymerized ethylene are insoluble in common organic solvents, indicating a highly cross-linked structure. The oily products, however, are soluble in acetone and xylene. Chemical evidence indicates that the oil is most likely composed of highly branched oligomers of ethylene. Mass spectrometric analysis of the gaseous effluents show that under film-forming conditions the only hydrocarbon species observable are those derived from ethylene. The powder- and oil-forming conditions, on the other hand, yielded oligomeric species. On the basis of this evidence, a mechanism for the plasma-polymerization of ethylene is proposed.