Recover Low Level Heat, Part II

Venting of and managing excess low-pressure flash steam is a common problem in many process facilities. If condensate recovery is critical, typically, design engineers either choose to vent the low-pressure steam into the atmosphere or install air-fin condensers. In some plants, process engineers have added thermocompressors to utilize the vented low-pressure steam.

Part one of this series, www.ChemicalProcessing.com/articles/2011/recover-low-level-heat-pt-1.html, highlights several innovative concepts for recovering such heat, including installing heat pipes to preheat the combustion air and blowdown heat recovery systems to recover vented flash steam. These are not the only options to improve critical process cooling areas. Plant engineers may want to consider using the flash steam in an absorption chiller to produce chilled water.

It may seem funny to say heat can create cooling, but that is perfectly true with the help of absorption chillers. The most interesting characteristic of an absorption chiller is its ability to recover very low-grade heat, such as hot water.

For example, at a Wyoming refinery, processing capacity was limited at the fluid catalytic cracking (FCC) unit due to the main air blower's performance — especially during the summer months. A condensing steam turbine supplied with cooling-tower water drove the air blower. In the same refinery, excess low-pressure steam was continuously vented to atmosphere from a 15-psig steam header.

We recommended installing an absorption chiller to recover rejected low level heat to produce the chilled water, which was then used to reduce the temperature of the cooling water supplied to the turbine condenser. This low level heat recovery application improved the process unit's performance during summer months.

Because chilled water generated from waste heat absorption chillers can enhance cooling, this concept applies to many processes where cooling towers can't lower the temperatures of process streams to required levels. One such application is the light-ends recovery from vapor streams of any fractionation column or from compressed process gas streams. If process cooling isn't critical, you definitely can substitute chilled water at the air conditioning units of the control rooms.

Installing Organic Rankine Cycle (ORC) power generating units is another innovative approach to recover low level heat. Such a unit utilizes a low boiling organic fluid as the working medium; thus, it can recover heat at much lower temperatures than practical for conventional waste-heat recovery systems. An ORC unit can use hot water as its energy source and is well established at many geothermal energy plants where hot water is naturally abundant.

In addition, because the ORC unit uses low boiling organic fluid, there's no condensation or erosion in the turbine blades compared to conventional Rankine Cycle turbines. ORC turbines are available for commercial power generation at sizes ranging from 50kW to 5MW ranges. For smaller-capacity power generating units, such as diesel generator sets, adding an ORC turbine package could generate 12–15% additional electricity for the same amount of fuel firing.

In modern dry process cement plants, even after five stages of feed preheating cyclones, the stack gases are sent into the atmosphere at temperatures of ~570°F.  The vented air from their clinker coolers also leaves at ~400°F. Because the existing heat recovery systems are limited, and no additional heat sinks are available in the cement manufacturing process, power generation from wasted low level heat is the only option. In such cases, installing an ORC power plant could be the best choice. The rejected low level heat at the kiln exhaust and cooler vent together could generate about 20% of the plant's total electricity needs.

As energy prices continue to rise and new energy efficiency standards take hold, both conventional and innovative options for low level heat recovery may gain importance.