324 Heat Exchangers

High-pressure heat exchanger efficiency can be significantly increased and downtime reduced with CFCCs. Heat exchangers made of tough CFCCs survive thermal shock, operate at higher temperatures longer, and resist fouling erosion and corrosion. Where reactions are conducted in the exchanger, higher operating temperatures lead to faster reactions, less residence time, and improved efficiency. Many processes use heat exchangers to capture heat from exhaust streams to preheat inlet streams. In one example, in a CFCC natural-gas preheater, compared to metals, the overall efficiency improved from 35% to a new efficiency of 47%. The metal heat exchanger was limited to 816°C (1500°F) or less. It required cooling the gas stream and reheating it to 1260°C (2300°F) downstream. The use of CFCCs eliminated cooling the gas upstream of the exchanger and the reheating step. This saved 33% of the thermal loading. Reduced fuel consumption reduced cost and lowered emissions (Fig. 3.9).

In another example a heat exchanger preheats a stream of combustables prior to incineration. This facility incinerates a wide variety of waste, both solid and liquid, except polychlorinated biphenyls (PCBs) and dioxin, from 80 locations. The flue gas typically contains HCl, water vapor, oxides of carbon, sulfur, and nitrogen. The ash is comprised of oxides of aluminum, calcium, iron, sodium, potassium, and silicon along with small amounts of heavy metals. A combination of solid and liquid waste was burned at a rate of 1360-1810 kg/h (3000-4000 lb/h). CFCC heat exchanger tubes were exposed to inlet air and flue gas temperatures of 425-980°C (800-1800°F). After 6 months operation the strength of CFCC heat exchanger tubes did not change. This test was the first successful demonstration of a high-temperature CFCC heat exchanger in a highly corrosive environment under actual industrial conditions.

FIGURE 3.9 Individual tubes on left are assembled into heat exchanger on right. Thermal stability and nonbrittle nature of CFCCs make them candidate components of high-temperature or corrosive environment heat exchangers.

Since CFCCs withstand higher temperatures than the previously used metal, the downstream incinerator burns the incoming stream more completely with less noxious emissions, reduced energy consumption, reduced operating cost, and reduced landfill.

High-pressure heat exchangers are used as the reaction vessel in a new process to form ethylene (Fig. 3.10). This new method will dramatically improve this process. The thermal stability and corrosion resistance of a CFCC heat exchanger will improve reformer efficiency. This is particularly important because ethy-lene production requires more energy than any other organic chemical process. Steam cracking, the process in place for 40 years, was optimized long ago. The new process, called reforming, will improve efficiency and reduce energy consumption. Materials of construction must withstand methanol, hydrogen, and ammonia. As an intermediate step, CFCCs are being evaluated to improve the conventional steam cracking process that is used today to form ethylene and other hydrocarbons.

In conventional steam cracking systems, the feedstock is mixed with steam and passed at high-temperature and pressure through metal tubes in a direct-fired furnace heat exchanger. The process is constrained by the metal alloys used. By replacing those alloys with CFCC, higher temperature and pressure can be achieved that will significantly improve ethylene yields. Boosting process temperatures to 980°C (1800°F) from the current maximum of 900°C (1650°F) will increase the yield from 27 to 37%, an increase of 36%.

Continuous fiber-reinforced ceramic composites resist high-temperature corrosive reforming by-products: methanol, hydrogen, and ammonia. Coking, a process-retarding carbon deposition catalyzed by the metals normally used, is a problem. Steam, normally mixed with the feedstock, is added, in part, to reduce coking. The use of CFCCs minimizes coking and is expected to allow the process

Reformer with CFCC Tube

FIGURE 3.10 CFCC thermal stability and toughness improve reformer yield.

to run 50% longer before downtime for maintenance. Run length is expected to increase from 60 to 90 days in the case of ethane as the feedstock. Steam use is also reduced.

Overall, the combination of increased yield, greater run lengths, reduced feedstock, and steam and energy use is expected to increase ethylene production capacity by 10%. Similar results are expected for reformers making cleaner gasoline. Participants in this endeavor include Stone & Webster Engineering Corporation and the CFCC suppliers.

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