научная статья по теме MEMBRANE GAS SEPARATION PROGRESSES FOR PROCESS INTENSIFICATION STRATEGY IN THE PETROCHEMICAL INDUSTRY Химическая технология. Химическая промышленность

Текст научной статьи на тему «MEMBRANE GAS SEPARATION PROGRESSES FOR PROCESS INTENSIFICATION STRATEGY IN THE PETROCHEMICAL INDUSTRY»

HE&TEXHMH3, 2010, moM 50, № 4, c. 284-295

MEMBRANE GAS SEPARATION PROGRESSES FOR PROCESS INTENSIFICATION STRATEGY IN THE PETROCHEMICAL INDUSTRY

© 2010 P. Bernardo1, E. Drioli1' 2

1 National Research Council — Institute for Membrane Technology (ITM-CNR) Via P. Bucci, c/o University of Calabria, cubo 17/C, I-87036Rende CS, Italy 2 Department of Chemical Engineering and Materials, University of Calabria, Via P. Bucci, cubo 44/A, I-87036Rende CS, Italy Received February 15, 2010

The focus of this paper is on the progresses in membrane gas separation technology applied in the oil refining and petrochemical sector. Industrial applications, research trends on new materials and technical solutions, challenges and possible applications will be discussed. Other membrane operations will be briefly addressed, owing to their increasing number of installed systems in the refinery/petrochemical industry. This paper outlines how implementation of membrane technology in refineries and in the petrochemical industry result in Process Intensification (e.g., reduced footprint, better material utilization, reduced energy, reduced utilities and waste).

INTRODUCTION

In the last few decades, membrane engineering has introduced significant innovation in processes and products, giving new opportunities in the design, rationalisation and optimisation of innovative productions. Membrane processes have several advantages than many other conventional separation techniques (e.g., distillation, extraction, absorption and adsorption). They are in particular compact and easy to scale-up, fully automated and with no moving parts; they do not require energy-intensive phase changes or potentially expensive adsorbents and/or difficult to handle solvents. Successful applications are already in the desalination: a reverse osmosis system is over 10 fold more efficient in energy consumption than the thermal approach for seawater desalination [1].

This innovative technology represents a viable option in order to implement the Process Intensification strategy, decreasing equipment size, raw materials and energy utilization, waste generation. Moreover, significant progresses have been done in the recent years in developing other unit operations such as membrane reactors and membrane contactors, covering practically all the units requested in process engineering. Petrochemical Industry might significantly benefit from these innovative technologies due to the necessity of meeting stringent environmental standards, to control production cost and final products quality.

The industrial interest in membrane technology is not limited to the separation of gas mixtures, but various membrane operations are already introduced in the petrochemical industry such as reverse osmosis [2, 3], per-vaporation [4], organic solvent nanofiltration [5, 6], membrane contactors [7], membrane reactors [8, 9, 10]. The applications covered are: wastewater treatment and

water recycling, enhanced oil recovery (e.g., produced water treatment), separations of organic liquids, oil to gas conversion.

The integration ofmembrane separation systems with membrane reactors and membrane contactors (such as membrane distillation, membrane strippers and scrubbers, etc.) might in principle offer the opportunity of redesigning industrial operations largely based on membrane units. The integration of different membrane systems (MF, GS, MCs) for separation has been proposed in an ethylene production cycle by steam cracking for a plant installed in Europe, producing 800.000 ton/yr of ethylene (the number one largest volume base-petrochemical) and consuming ca. 30 GJ/ton ethylene [11]. The membrane systems considered showed exergy reduction with respect to the conventional separation methods [11]. This redesign strategy would allow to upgrade refineries for clean-fuels production and other important petrochemical productions, improving the materials utilization by a separation/recycle approach, reducing at the same time chemicals, energy/utilities consumption and waste streams production.

1. MEMBRANE GAS SEPARATION

IN THE PETROCHEMICAL FIELD

Membrane-based gas separation (GS) is a relatively well-established unit operation in the petrochemical industry, with different applications (Table 1) which represent, however, only a small fraction of the potential applications [12].

Table 1. Major applications for membrane GS in refineries and petrochemical industry

Petrochemical Monomer recovery in polyolefin production (resin degassing vents/reactor purge) Monomer recovery in polyvinyl chloride production Ethylene recovery in ethylene oxide production Syngas H2/CO ratio adjustment for oxo-alcohol production, GTL, etc

Refining H2 recovery from hydrodesulphurization/hydrocracker purge streams H2 recovery from catalytic cracker off—gas H2 recovery from refinery fuel/flare gas H2 recovery from fluidized catalytically cracked (FCC) overhead gas H2 recovery from pressure swing adsorption (PSA) tail gas Catalytic reformer hydrogen upgrading LPG Recovery

Natural gas CO2 and N2 removal Fuel Gas conditioning NGL concentration, treating and recovery Natural gas treating to meet pipeline specifications CO2 for enhanced oil recovery and saleable gas from CO2 rich streams Production of sales gas from CO2 fractured wells Digester off-gas treating Landfill gas upgrading

1.1 Air Separation

Membrane performance for air separation has advanced during the past 15 years. Initially, the membranes were made by TPX (poly(4-metil-1-pentene)) and ethyl cellulose with O2/N2 selectivities up to 4, but more recently membranes with O2/N2 selectivity of 8—12 have been introduced, even with a reduced permeability, as is usual for polymeric materials.

1.1.1 Nitrogen Production in Oil-and-Gas Industry

Nitrogen is extensively used in the oil-and-gas industry for fire and explosion safety during transportation, trans-shipment and storage of hydrocarbons, as well as for testing, purging pipelines and vessels of explosive vapors. Nitrogen production from air by membrane technology, introduced in early 1980's, was a great success and today is the largest GS process in use. Membrane systems present high reliability, low dimensions and weight, minimum air pre—treatment. Thousands of compact on—site membrane systems generating nitrogen are installed in offshore and petrochemical industry, as PRISM® systems by Air Products [13].

1.1.2 Oxygen-Enriched Air

Another application of membrane GS technology is in the production ofoxygen-enriched air (OEA). Oxygen or OEA reduces energy consumption (fuel saving) by 25% to 60% and environmental emissions (such as NOx), lowering the ballast effect of inert nitrogen. As a

consequence, OEA combustion eliminates the need for a heat recovery system because ofthe large reduction in exhaust gas volume. Typically, membranes are used to produce OEA with concentrations in the range 30—50% [14]. Even at a modest enrichment, such as oxygen at 30% (an enrichment of only 9%), OEA will contain almost 40% less nitrogen per unit oxygen.

An interesting combination of membrane air separation and coal gasification process has been the task of a Project INCO-Copernicus (EU) [15]. High-ash and other low-quality (high moisture content) coals, available in huge quantities in Russia and in East Europe, were considered. The use of OEA as fluidisation medium allows to burn at least a part of fly ash before they leave the furnace, thus improving energy efficiency and ecological impacts of the process. The optimal content of O2 in the blower is in the range 27—33% vol. The heating value of syngas obtained in optimal conditions was in the range 3.5—4.7 MJ/m3 [16]. On the base of literature studies on the combustion of low quality carbon it has been considered a level of enrichment of O2 in the range 25—50%. In such range the membrane technologies are competitive with those of traditional type like the cryogenic distillation and the PSA [17]. A single membrane separation stage represents the configuration more economically convenient for enrichments in the range 25—45% [18]. The more energetically convenient arrangement is depicted in Fig. 1: air is fed at atmospheric pressure by means of a blower, while the permeate stream enriched in oxygen

O2 enriched air

Vacuum

Fig. 1. Scheme of a membrane system for producing OEA.

is recovered by a vacuum pump (with a flow rate lower than the feed stream side) [19].

Membrane systems producing OEA can be coupled with oxygen-enriched combustion and CO2 capture by membrane processes as suggested recently by Favre et al. [20]. Other important examples of using OEA are the industrial Claus sulfur process, steel production processes and medical breathing applications. Air Liquide installed an air separation unit with a capacity of 550 ton/day of oxygen to supply gaseous O2, nitrogen and argon to steelworks in Dalian (China) [21].

Apart from commercial polymeric membranes, dense inorganic membranes are permeated only by oxygen (or hydrogen) at high temperature (>700°C) [22, 23]. High-temperature air separation can be coupled with power generation systems. Commercial-scale ion transport membranes (ITM) modules by Air Products present a capacity of 5 ton/day of oxygen; this technology requires 35% less capital (much simpler flow sheet) and 35—60% less energy (less compression energy associated with oxygen separation) than cryogenic air separation [24].

1.2 Hydrogen Recovery in Refineries

Hydrogen is three times more valuable if recovered rather than if used as fuel [25]. Moreover, hydrogen recovery in a refinery (Tab. 1), represents a key strategy to meet the increased demand of hydrogen for hydrotreat-ing and hydrocracking. These processes produce a residual gas which contains a significant amount of hydrogen at pressure and, therefore, membranes provide an economical recovery and recycling method. The hydrogen concentration in refinery purges and off-gases is in the range 30—80

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