Ammonia Absorption Refrigeration Unit Provides Environmentally Friendly Profits for an Oil Refinery

The 27,000 barrel-per-day Denver refinery of Ultramar Diamond Shamrock (UDS) suffered from low profitability and low energy efficiency. One indicator of these problems was the large flare that burned almost continuously. Many refineries, like the UDS refinery, have waste gases that are routed to a fuel header. When the amount of waste gas exceeds the requirements of the refinery furnaces and boilers, the excess is flared. A small fraction of the waste gas propane (C₃) or heavier-about 4% by volume. If it can be recovered, the refinery can sell the C₃+ as either liquefied petroleum gas (LPG) or gasoline. But, at the low partial pressure present in the fuel header, the C₃+ will not condense at ambient temperature and thus cannot be recovered. Calculations show that about half of the C₃+ can be condensed by chilling one of the waste gas streams to -20°F (-29°C).

SOLUTION

The refinery convened a team of experts to identify the best solution. The team considered a number of approaches before deciding on the waste-heat-powered absorption refrigeration unit (ARU). The gaseous effluent from the reformer proved to be an ideal target for applying the refrigeration. Chilling that stream to –20°F at nominal refinery summer operating conditions recovers 200 barrels per day of liquid C₃+, which is more than enough to extinguish the flare.

Solving the problem with a waste-heat-powered ARU also requires a suitable source of waste heat. A good source was found at the reformer. The cooling curve showed that the required 6 million Btu/hr (1758 kW) of waste heat could be extracted from the target stream by cooling it from 290°F to 260°F.

The ARU is integrated into the refinery process and uses enhanced, highly compact heat and mass exchangers.

RESULTS

The ARU is recovering 200 barrels per day of LPG and has recovered as much as 315 barrels-per-day. The unit also has achieved the design chilling temperature of –20°F. It is always operated to achieve the lowest possible temperature and routinely achieves –50°F during the winter.

It is now very rare that any gas is released from the refinery fuel header into the flare header. The average LPG recovery is at least 50% more than what was formerly flared, so it is usually necessary to import natural gas to make up the deficit. The recovered liquid falls 60% within the gasoline fraction and 40% in the LPG fraction. Thus, it may be considered that the newly imported natural gas is being converted into gasoline and LPG at effectively 100% thermal efficiency, with excellent economic value added.

An integral part of the project is the associated reduction in regulated emissions. Recovering salable hydrocarbon products from the refinery fuel gas system substantially reduces the amount of flared fuel gas. This not only saves money and reduces wasted energy, but also lowers emissions from the refinery flare. Additionally, reducing the flared fuel gas minimizes the size of the flare; large flares from refineries are often a source of complaint among neighboring communities.

The Association of Energy Engineers designated this project "Environmental Project of the Year" for 1998.

The ARU reduced emissions at the plant by: 9.91 tons/yr CO; 1.82 tons/yr NOx; 0.07 tons/yr PM10; 1.5t/yr VOC and; 7,741 t/yr CO2.

APPLICATIONS

This technology is widely applicable, not only to refineries, but to many industries. The waste-heat powered absorption refrigeration unit can be successfully used in mainstream industrial refrigeration applications, such as cold storage warehouses and frozen food processing.

BENEFITS