Heat Engines

A Carnot heat engine is useful in determining the theoretical maximum heat efficiency of an ideal, reversible distillation. The heat engine analogy consists of the simple column arrangement depicted in Figure 1. Heat is added at the bottom of the tower at a high level of temperature and then removed at the top of the tower at a lower temperature. This degradation in heat provides the work to separate the feed into the products. If we consider a distillation column as a heat engine, then the theoretical minimum work required for the heat engine is:

C^condenser(1 ( Ta/Tcondenser))

The availability change of the streams in and out of the column is equal to this heat work minus a lost work term to account for thermodynamic inefficiency, or:

-^products feeds

lost

To account for real processes, a lost work term is included to account for irreversible changes within the column. This extra work is required to overcome pressure, temperature and composition driving forces. There are a number of commercial algorithms for determining each driving force contribution to lost

work. These lost work contributions are generally classified as:

1. Momentum loss due to pressure drop.

2. Heat transfer loss from temperature difference.

3. Mixing losses due to composition differences.

As the lost work term increases, the reboiler and condenser loads must increase in order to supply the same amount of useful work. As the driving forces approach zero, the lost work approaches zero and less energy is wasted.

For a particular column design, the availability of the feed and products and the theoretical minimum work are fixed. The lost work energy is solved by difference. The actual heat duty required for the distillation column will be the sum of the theoretical minimum plus the lost work, or (Wheat + Wlost). By comparing values of this sum, the designer has a yardstick in which to rate different column designs. For existing columns, a simple calculation of the total lost work will quickly focus as approach towards the most potentially beneficial areas. For example, if the lost work is less than 5% of the total heat duty, then this is probably within the design margin of the equipment and not worth pursuing.

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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