[pullquote]Many sites, including petrochemical plants and refineries, often have a lot of low-temperature heat (waste heat, low-pressure steam, etc.) available; in most cases, it is rejected in fin-fans, cooling towers, exhaust stacks, etc. While identifying and recognizing these sources of low-temperature heat is easy, sometimes economically justifying using such heat in process or utility areas is difficult. I will attempt to shed some light on one potential technology that can be used effectively to capture low-temperature heat (most often in the form of low-pressure steam).
Most of us who have had to deal with thermodynamics extensively realize that the variation in steam enthalpy (energy content) doesn’t significantly differ between pressure levels. For example, saturated steam enthalpy at 20, 200 and 600 psig is 1,167, 1,200 and 1,203 Btu/lb, respectively. The superheat in the steam at the different pressures will drive the enthalpy up but for all practical heating applications — excluding power generation — saturated steam is all that’s needed at the appropriate heating temperature.
So, if we have excess low-pressure steam currently being vented or sent to the fin-fans (or a cooling-tower water condenser) for condensation, investigating upgrading this steam to a higher pressure may be worthwhile. As I mentioned, the enthalpy isn’t that different, but a pressure upgrade also includes an inherent temperature upgrade and that’s where the second law of thermodynamics comes into play. Several methodologies exist to upgrade this low-pressure steam, including the use of a thermocompressor.
From a layman perspective, a thermocompressor is an extremely well-designed converging diverging nozzle that requires high-pressure steam to absorb (or suck in) low-pressure steam to create medium-pressure steam useful for process heating. The high-pressure steam is known as the “motive or live steam” and the low-pressure steam is known as “suction steam”. The medium-pressure steam is generally referred to as “discharge steam”. Apart from the specific pressures, the main design parameter is the mass ratio of the suction steam to motive steam. All thermocompressor designs start with identifying the amount of low-pressure waste steam available for recovery and then identifying the high-pressure motive steam that could be used to create the medium-pressure steam. Generally, for a given discharge pressure, the higher the pressure ratio between the motive steam and suction steam, the higher the mass ratio of the suction steam to motive steam (note the inverse relationship here). So, always try to use the highest pressure steam available in the plant as the motive steam!
Industry long has used thermocompressors, which are a time-tested technology. They have no moving parts, although control valves are used to regulate pressures if the steam requirement varies. Several applications exist in process plants for thermocompressors; the most common include:
1. Flash steam recovery from condensate tanks.
2. Multi-effect evaporators and other processes that sequentially remove water from products.
Thermocompressors also can be applied effectively in several other places. I came across one such application about six years ago when I was working on a steam system energy assessment at a refinery. We had a reboiler steam requirement at ~400 psig and the refinery was letting down high-pressure steam at ~800 psig through a pressure-reducing valve to provide steam to the reboiler. Interestingly, we had a 250-psig header in the vicinity. A thermocompressor fit perfectly well in that application; the refinery installed it, thus upgrading the 250-psig steam to 400-psig using the high-pressure (800-psig) steam. The energy and cost savings along with a higher overall energy efficiency came from:
1. a reduction in 800-psig steam generation (natural gas savings).
2. more power production from the existing 800/250 backpressure steam turbine generator (electrical power savings).
3. the elimination of steam flow through pressure-reducing/letdown stations.
In summary, review your plant’s steam system to identify vented and low-pressure steam that can be upgraded with thermocompressors. In addition, challenge yourself to think out-of-the-box to identify applications that integrate the process requirements and lead to significant energy and cost benefits!