Several process heating applications exist in most chemical plants. Fired heaters usually handle high-temperature heating requirements, while steam or hot oil take care of medium- and low-temperature heating needs. Plants designed during low-cost energy periods or those scaled-up from previous designs stand to benefit from alternative process heating methods.
For example, in a paraffin process plant, the raffinate feed was heated from 250°F to 390°F, using about 3,000 lb/hr of 350-psig steam. In this same unit, four fired heaters were in continuous service, releasing stack gases to atmosphere at temperatures ranging between 580°F and 868°F. Space limitations and large-scale structural modifications prevented attempts to install air preheaters. Hence, we recommended installing a raffinate preheating coil just above the convection of one of the heaters, and the idea was accepted immediately. The existing steam-heated raffinate exchanger was left in place for temporary use in the future. Such alternative heating methods utilizing waste heat streams generally have attractive payback periods.
Tuning the burners of fired heaters and boilers provides another opportunity to save energy . When multiple process units are located in the same site, some fired heaters in one plant are well tuned and well maintained, while others operate with very high excess air levels. The first and foremost activity is to tune the burners of a boiler or fired heater and develop the equipment's load versus excess air characteristics. Of the 200+ energy assessments I've completed, I've always found at least one fired heater or boiler that required burner tuning. This is the simplest of all energy savings opportunities that can be implemented without any capital requirement.
Identifying and fixing the root cause of the problem will likely offer additional spin-off benefits. In one of the process units in the same plant, engineers attempted to minimize the flash steam venting from a condensate receiver — or at least recover it for other use. Analysis revealed that medium-pressure steam intentionally was supplied to the condensate receiver to increase the suction head of a condensate pump that failed frequently. Further investigation disclosed the pump discharged into the boiler feed water supply line to the steam generators. The pump's discharge was modified and re-routed to the deaerator inlet line. Once the condensate pump's discharge pressure was reduced, the steam supply to the condensate tank stopped and the flash steam venting reduced significantly. Fixing the root cause of this energy waste problem not only decreased flash steam venting, but also increased the condensate pump's life cycle and performance.
Optimizing energy consumption in steam-powered vacuum systems can also generate savings. In one process plant, three-stage steam jet ejectors, with condensers between stages, provided the desired level of vacuum in the distillation columns. Though steam jet ejectors are more suitable than liquid ring vacuum pumps to maintain near-absolute vacuum, they're comparatively less efficient in the mild vacuum ranges. Because the required vacuum level is always achieved in stages, it's best to design with liquid ring vacuum pumps in the low-vacuum ranges and add steam jet ejectors in tandem for high-vacuum stages. In this plant, all steam jet ejector vacuum systems were modified to a combination vacuum system with water ring vacuum pump and steam jet ejector. This helped achieve energy savings of more than $200,000 annually.
Preheating the cold make-up water to the deaerators or steam boilers can also help save energy. Only some plants utilize this opportunity, mostly by recovering heat from the boiler blowdown water or wasted heat from stack gas. Many engineers consider only heat rejected from processes or stacks as a good preheating source. That isn't necessarily true — low-level heat rejected from equipment like air compressors and chillers can also be used to preheat cold make-up water.
In one chemical plant I visited in western Tennessee, my team suggested preheating the cold make-up water by cooling the plant's air compressors. The cold make-up water quantity and the air-compressor cooling water flow quantities were well-matched. Implementing this suggestion also helped eliminate the plant's air compressor cooling tower, which had posed chronic problems to its maintenance staff.
Energy-saving opportunities are endless, and by trimming down energy losses, plants can gain a lot more.
VEN V. VENKATESAN is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@putman.net