Petroleum refining is a highly energy-intensive process using both purchased energy (gas and electricity) and byproduct streams from the refining processes. Energy use accounts for approximately 50% of refining costs. As major energy suppliers, refineries must report and monitor self-consumption of energy as a percentage of their total crude intake for processing. A modern refinery is a highly complex but integrated system, separating as well transforming heavier hydrocarbons into saleable fuels and chemicals.
Improving energy efficiency in a refinery is very critical, as it directly relates to sending more products to the market. A refinery monitors its profitability by evaluating its refining margin. When more saleable products are sent out, the net refining margin increases. Blending, process heating, distillation, cracking, reforming, absorption, evaporation and cooling are typical processing operations involved in petroleum refineries.
Many improvement opportunities at petroleum refineries apply at other process plants as well. Typical among them are burner tuning at the fired heaters. While most large heaters, like crude-unit charge heaters, are monitored well, small process heaters are seldom monitored for optimum excess air levels. Many smaller fired heaters don’t have enough heat recovery features and release high-temperature flue gases directly to the atmosphere.
Recovering heat from the blowdown water of various boilers and waste-heat steam generators is another common way to save energy. In a large petroleum refinery in Louisiana, the continuous blowdown water from two fuel-fired boilers and two waste-heat boilers directly drained to the sewer. The refinery already was experiencing excess 50-psig steam generation, so it had no incentive to recover additional flash steam. Hence, we recommended recovering the waste heat from the blowdown water for better utilization of the excess low-pressure steam.
Because coker drums need large quantities of water for cutting and washing the coke, we also recommended routing the blowdown water to the coker for this purpose, saving fresh water and reducing the sewer load.
At the same refinery, the high-pressure (600- and 350-psig) and medium-pressure (150-psig) condensate collection lines from various steam users are routed to a common collection vessel. Flash steam venting from this tank was substantial. We recommended installing a new flash tank upstream to the existing condensate collection tank to handle the high pressure condensate. The level controller in the new flash tank would route the condensate to the existing condensate tank after separating the flash steam. This kind of flash steam recovery may suit other refineries, too.
Minimizing the use of low-pressure (LP) steam condensing is another possible way to cut energy costs. Excess LP steam is a common problem, so many refineries opt for air-fin type condensers to reject the heat in steam and recover the liquid condensate. By reviewing the steam users, it may be possible to replace some of the medium-pressure steam users with low-pressure steam or find some additional use for low-pressure steam. In this refinery, two air-fin condensers cooled the excess 29-psig steam and one air-fin condenser cooled the liquid condensate to minimize water hammer before it was sent to the secondary scrubber. Diverting excess steam to the 22-psig steam header and routing the liquid condensate to the scrubber without cooling helped stop the heat rejection at the condensers.
Process units in refineries often are widely spread out, demanding lengthy steam supply lines. Occasionally, the headers supplying steam to far away locations experience excessive heat and steam trap loss. The steam supply quality also deteriorates, especially to remote consumers.
In this refinery, the dock area and the tank farm were more than 1½ miles away. We recommended installing a small LP package boiler near the dock area with a water softener to feed the boiler. The new package boiler isolated the mile-long steam supply header. Similar opportunities also may apply at other refineries with widely spread out process units.
We’ll cover more energy efficiency measures for petroleum refineries in Part 2 next month.
VEN V. VENKATESAN is Chemical Processing's Energy Columnist. You can e-mail him at VVenkatesan@putman.net