OCR Text |
Show pressurized conditions, the energy requirements of low effective permeability membranes will be about 3 times that of membranes with similar oxygen to nitrogen selectivity which have an effective permeability compatible with vacuum operation. The A/G Technology Corporation hollow fiber membranes offer a preferred combination of sufficiently high selectivity for oxygen to nitrogen and high oxygen effective permeability. Thus, oxygen concentrations of up to 40% can be achieved in single-stage, vacuum mode systems. To produce 35% OEA, the following comparative energy requirements are estimated: Operating Power Company Mode (Khw/ton) -------------- --------------- ------------- VOP Vacuum or Cannot Pressure achieve 35% OEA Dow (6) Pressure 540 A/G Technology Vacuum 215 Assuming a power cost of $0.05/Kwh, the Dow power requirement would resul t in a cost of $27/ton prior to consideration of operating labor, maintenance, depreciation and capital amortization. In comparison to a projected $28/ton total operating cost for an A/G Technology membrane system with a capacity of 10 ton/day free available oxygen at a 35% concentration (7). Thus, a balance between membrane oxygen/ nitrogen selectivity and oxygen effective permeability is required for an energy-efficient (e. g., vacuum mode), economical OEA system capable of 35% or higher oxygen concentrations. In reviewing the remaining key parameters affecting membrane system OEA economics, only the A/G Technology hollow fiber membrane system will be considered. This will permit comparisons independent of membrane intrinsic properties. MODE OF OPERATION The selection of the mode of operation for the membrane OEA system is, based upon the above discussion, straightforward. However, a comparative assessment of the same membrane type may more clearly show the advantages of the vacuum mode of operation. Such a comparison is shown in relative terms in Figure 3 for A/G Technology hollow fiber membranes producing 35% OEA (7, 8). The small capital cost advantage to the pressurized mode which results from lower membrane area requirements is overshadowed by the Significant increase in energy which brings the overall operating costs for a pressurized mode system to a level about 60% higher than a comparable vacuum mode system. Once the vacuum mode of operation is selected, an operating parameter option and a key factor in the overall economics of the system is the temperature of the feed air. For this analysis, a comparison was made of the effect of two temperatures: 25 and 50 C, on membrane system energy 182 3~----------------------------------; Q) ~ IV > Q) > 2 ...... --... § 1 ...... ......,.~'!"'t ~ 'Ix1JX..1}) 'X)} 'X) Energy Operating Cost Capital Cost IXm Vacuum c::J Pressure Figure 3. Effect of Operating Mode on OEA Energy Requirements, Operating Costs, and Capi tal Costs. consumption, operating costs and capital costs. The results of this comparison are presented on a relative basis in Figure 4. Although a 20% increase in energy occurs at the higher temperature, the overall economic effect is a reduction in both the operating cost and the capital cost of the membrane system by 10 to 20%. These reductions are primarily associated with lower membrane area requirements resulting from higher membrane productivity at increased temperature. It is of note that a decrease in membrane selectivity occurs as process temperature is raised. However, over the narrow temperature range of interest, the decrease in membrane selectivity observed was negligible (9). OEA CONCENTRATION The preferred level of oxygen enrichment for industrial combustion processes is not entirely defined, however it is generally regarded to be between 35 and 40%. Beyond this range, the benefits from increased oxygen concentration diminish. Furthermore, for retrofit operations, the installed furnace must be capable of tolerating the higher combustion temperature which sets an upper limit on allowable oxygen concentrations. As depicted in Figure 5, under the preferred operating conditions for each concentration, it requires about 25% more energy to produce 40% oxygen versus 35% OEA. To balance this energy increase, about 25% less membrane area is required at the higher concentration, primarily due to the higher compression ratio under which the system is operated. The vacuum pump size increases by about 50% on a volumetric basis, increasing the capital cost of the vacuum pump. Overall, however, the operating costs and capital costs are essentially the same for both 35% and 40% OEA. |