Control of Vapour Inventory

There are a number of ways to control the vapour inventory or column pressure. Even though one of the five flows indicated in Figure 2 could be used for pressure control, only the use of W and S gives sufficient power of control. If S were used for pressure control, a step increase in S would result (via higher pressure and top temperature) in an increase in the vapour flow in the top. Consequently, the concentration of the less volatile components in the top would increase, resulting a higher top temperature and consequently a higher vapour flow in the top. This positive feedback can sometimes be so strong that the pressure finally attains a lower value (inverse response). Therefore, the most common method of pressure control is through manipulation of the coolant flow, W.

If a water-cooled condenser is used, the water flow rate is manipulated to control pressure: if an air-cooled condenser is used, the fan speed is generally changed. The attractiveness of this pressure control option is that the condensed liquid is at its bubble point and is not subcooled, as may be the case with other pressure control options.

If there are incondensable gases in the system, pressure could also be controlled through manipulation of a bleed valve, through which the gases are bled from the column. The bleed valve is often installed on top of the reflux drum.

It should be emphasized that the pressure has a large degree of self-regulation. If the pressure in a column increases, the temperature will also increase and because of this the heat transfer in the condenser will increase; consequently the pressure will decrease again to a certain extent.

The general recommendation for pressure control is:

1. if the condenser performs partial condensation, control the column pressure by manipulating the vapour flow leaving the column;

2. if there is no net vapour flow from the column, the next preferred option is to control pressure by manipulating condenser duty, e.g. by changing the coolant flow.

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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