OCR Text |
Show efficiency of the air separation plants, the cost of electricity, the transportation distance and the volume of liquid oxygen. Since the flow requirement of combustion applications often varies widely, capacity utilization, storage and backup supply requirement must be considered in the realistic estimate of oxygen costs. For example, some industrial furnaces operate only 5 days a week which lower the capacity utilization factor to about 70%. Also, flow variation of 50 to 100% of the capacity is not uncommon . For relatively slow variation of flow pattern the oxygen supply systems can follow the use pattern and simply result in lower average utilization of the capacity without an extra penalty of higher unit power cost. For rapidly changing flow conditions, however, product storage as compressed gas is required to cope with the peaking conditions. The effects of these practical factors on the oxygen supply economics are compared for each supply method and shown in Table 4 . The bas e cas e economics are the s arne as those in Figure 9a at 20 TPD. "Pipeline" case assumes oxygen supply at 200 psig through an existing pipeline from the excess capacity of an existing cryogenic plant. Case 2 includes the cost of product compression to 15 psig for cryogenic and PSA and no compression for membrane. Case 3 shows the oxygen costs at 70% of capacity utilization. Case 4 assumes 12,000 SCF of product storage at 100 psig to meet the assumed peak flow requirement of 24,000 SCFH for three hours (i . e. 20% above the average flow rate of 20,000 SCFH). For membrane it is assumed to provide liquid for peak flow requirement rather than compressing the product for storage due to the higher cost of compression for the low purity product. Case 5 includes a liquid back-up system capable of supplying oxygen during equipment down periods, assumed to be 3%. Table 4 - Bffect of Use Pattern on Ozygen Cost for 20 TPD Plant (dollar per ton of equivalent pure ollJgen) Capacity Cryogenic Utilization Pipeline Liquid Plant PSA Relibrane (\) ($/ton) ($/Ton) ($/ton) ($/ton) ($/ton) 100 30-60 85-130 75-85 40-55 45-70 100 30-60 85-130 80-90 45-60 45-70 70 30-60 85-130 llO-130 55-70 55-80 70 30-60 85-130 120-140 65-80 8O-ll0 70 30-60 85-130 130-150 75-90 8O-ll0 Conditions Case 1. Steady state flow at 20.000 SCPH (20 TPD) equivalent pure OlIJgen. No product cOlllpression for cryogenic, PSA and llellbrane. (50 .il/Kwh power cost) . Case 2. Products cOlllpressed to 15 psig for cryogenic and PSA, no cOlllpression for llellbrane. case 3. Saae as Case 2 at 70\ capital utilization. Case 4. Saae as Case 3 and 12,000 sa of product stored at 100 psig for peat c:te.ands. Por aeabrane liquid storage ia used. case 5. Saae as Case 4 and liquid bact-up syste. during downtille (ass_d to be 3 percent.) 161 Significant increases in oxygen cost are shown in all cases except pipeline and liquid oxygen, both of which offer excellent flexibility for widely changing use patterns. Pipeline always offers the lowest cost oxygen due to the economy of scale . However, the number of the applications located near the existing oxygen pipelines is rather limited. Lower capital utilization (Case 3) results in greater penalty for cryogenic plants due to its higher capital cost. Product storage (Case 4) increases membrane oxygen cost more than PSA or cryogenic due to the lower oxygen purity. Case 4 may be consiered typical for most industrial furnaces. PSA offers the best economics for this example. It should be emphasized, however, that the optimum choice of oxygen supply system is site specific and the specific requirements of each application has to be evaluted carefully. Liquid oxygen has been used in many such applications for productivity increase due to its low capital requirement and flexibility in spite of higher unit cost. Since an on-site oxygen generation system usually requires a long term supply commitment with an oxygen supplier or a major capital investment by the user, the on-site system requires 15-20% cost advantage per unit amount of product over the short term liquid supply to be considered by the end user. ECONOMIC COMPARISON OF OXYGEN ENRICHED COMBUSTION AND HEAT RECOVERY Substantial differences exist in the equipment size and the scope of retrofit work required between the installation of a new oxygen enriched combustion system and that of a recuperator-hot air combustion system. Most recuperator retrofitting requires major revamping of the furnace, flue handling system and the air duct system. Limited space available around the furnace often makes it necessary to undertake significant structural modifications and results in a long furnace shut down period for the changeover. By contrast, the installation of a new oxygen combustion system is less complex, due to the small burner and pipe sizes (Ref. 3-5). Generally, little structural change is required and the complete changeover to a new system is often accomplished within a few days. These differences in equipment size and the installation work are reflected in the approximate ranges of total installed costs for recuperators and oxygen enriched combustion systems shown in Figure 10. Since the new firing rate after the retrofit is lower for the oxygen enriched combustion system as compared with the recuperator system, the total installed cost of the oxygen enriched combustion system is typically one half to one quarter of the corresponding recuperator system, including all systems for the safe handling of oxygen or oxygen enriched air. As seen in Figure 8, the net operat ing cost savings of an oxygen enriched combustion system is strongly dependent upon the oxygen to fuel cost ratio. Although fuel savings of oxygen |