Membrane separation of olefins from FCCfcc naphtha and gases for production of reformulated gasoline
The new EU directives will require reduction of olefins in gasoline from 30% to 12% to reduce the environmental impact of transportation fuels. The only available technologies today for olefin reduction in FCC-derived gasoline require new investments, with high energy consumption and operating costs in a refinery. Furthermore their environmental impact is significant since they require large quantities of pollutant materials, such as catalysts. This project aimed to achieve the desirable olefin separation with reduced energy requirements, no need for hydrogen and catalysts, and no reduction in gasoline quality (RON). The objectives of this project were to develop: -A membrane system for more energy efficient and more profitable olefin separation. -A system for naphtha production with predetermined amounts of olefins, without RON (Research Octane Number) losses. -Olefins recovery for further exploitation. -Software for membrane process simulation and scale-up. -The techno-economic evaluation of the process. The methodology to accomplish these goals included first the selection of the most suitable polymer material for olefin paraffin separation. Seven membrane materials have been selected and flat sheet membranes were prepared and characterised. Taking into account the permeability, selectivity, price and availability, a polyimide (Matrimid 5218) was selected as the best material to use for developing hollow fibre membranes. Several configurations of hollow fibre modules were prepared and tested for polymer plasticity , and finally a module with polymer housing and heat treated fibres of sufficient chemical strength, was adopted for pilot experiments. For the pilot membrane gas separation and pervaporation testing two pilot units were designed and constructed, based on the vacuum time-lag method. The Hellenic Aspropyrgos Refinery performed six commercial tests for the production of samples at various flow rates, with two catalysts, covering the most extreme cases for FCC naphtha and gas compositions. These samples were used in the gas separation and pervaporation pilot tests. The membrane pilot experimental conditions were varied in the range of Dp = 1-2 bar, stage cut q = 0.1-0.4, and temperatures 20-70 degrees Celsius. Initial tests with untreated fibres showed a decline of membrane separation performance with time, possibly due to plasticity or condensation phenomena in the polymer. With the thermally heated fibres, steady state permeation was achieved and working temperatures up to 70 degrees Celsius were possible. This operating temperature ensured high permeation rates and absence of condensation effects inside the membrane. Results show a significant increase of olefins concentration in the permeate stream especially at low stage cuts. Selection of the optimal stage cut depends on the objective goal of the application. For the production of olefin rich streams a small stage cut is needed, while for the recovery of LPG isoparaffins higher stage cuts are preferable. Increase of the feed pressure results in improved olefin purity and recovery, while at the same time, the paraffin purity and recovery in the residue also increase. For liquid phase olefin/paraffin separations, a pervaporation module was prepared and tested using four light naphtha samples, two from the FCC pilot unit and two from the HAR refinery. From group type analysis, the membrane seems to be selective in olefins and unsaturated naphthenes. However, the results showed that the membrane is much more selective in hydrocarbons with small number of carbon atoms, and this could be an indication that steady state conditions may have not been reached. Experimental and pilot tests results were used to develop and validate a 1D and a 3D model and simulation code for hollow fibre membrane separations. A 1D simulation code for a single hollow fibre separation was completed by CPERI, describing the gas separation and the pervaporation of olefins and paraffins in the gas and liquid state, respectively. The 1D model results compare well with experimental data on gas separation of olefins and paraffins in the pilot membrane unit. Also, a 3D code was developed by FLUENT, describing the flow, pressure and concentration profiles inside the hollow fibre module. The obtained results compare also well with the experimental and the respective results of 1D code. Using data from experimental and model results, a technoeconomic evaluation of three alternative processes for olefin separation was undertaken: -Hydrogenation. -Undercut Distillation. -Membrane Separation. At present membrane technology has the highest Pay Out Time (POT) of all other technologies. However, with minor improvement of operational conditions, the POT can decrease considerably (about 100%). Therefore the proposed membrane technology has high potential to become economically competitive to the conventional techniques. Furthermore, its attractiveness for olefin paraffin separation from FCC naphtha is strengthened by its environmental friendliness since it does not require, pollutant materials, such as catalysts.