Let's see how we might do by splitting the vapor flow into two parts, and then condense them separately. This also requires two condensers, but simplifies the control because the volume of vapor is much larger than the condensed liquid reflux. You don't need a tiny, precision valve, and can take advantage of the fact that vapor will divide itself up in direct proportion to the area of available outlets. If one aperture has 9 times the area of the other, then about 90% of the vapor will flow through it to the reflux condenser, leaving about 10% to flow to the product condenser. The geometry of the pipes, the viscosity of the vapor, etc will all affect this ratio, but in the system we're designing, these effects are small in relation to the large quantities of vapor, and the errors are insignificant for practical purposes.
The simplest way to do this is by inserting a pipe in the side of the column head that has exactly 1/9 the cross sectional area of the column itself (assuming you want 90% reflux). This means that the diameter of side pipe must be 1/3 the diameter of column. All that you need to control the flow of vapor into this pipe is a simple valve that is either wide open or closed shut. Alternatively, you could use a larger diameter side arm and have complete control of the vapor passing through it by using a gate valve. Gate valves are cheap, commonly used for water supplies, and they provide a straight path for the vapor to flow through.
The big advantages of vapor management are:
• Excellent control may be achieved with a cheap gate valve, large movements producing finely graduated results.
• The reflux ratio is totally unaffected by variations in the cooling water supply
• The temperature of the water in the reflux condenser doesn't increase as the reflux ratio is decreased (as it does with cooling management).
• You have control over the temperature of the final product, which is delivered at the temperature of the cooling water initial supply.
There is one disadvantage: changes made to the tap setting take around ten seconds to be observed at the collection point as collected vapor takes about this time to traverse the product condenser. Anyone who lias steered Fig. 4-15 a large boat will find this a familiar situation!
A design for a variable reflux Vapor Management head unit using a gate valve is at Appendix 6. Control at the end of a run
It was noted earlier that towards the end of the ethanol run, the proportion of water in the vapor increases. The result is that as more water is carried in the vapor, the volume of distillate condensing in the product condenser will be drastically reduced. Constant volume liquid sampling with Liquid Management will result in a rapidly decreasing reflux ratio, just at the time you want it to remain constant, or even be increased! Cooling Management maintains a constant reflux ratio if the flow of cooling water is constant, no matter what the composition of the vapor might be, and Vapor Management accomplishes this with little or no control of the cooling water supply. In both of these last two methods, the volume of the product will dwindle towards the end of a run.
With both Cooling and Vapor Management you will therefore have not just one indication of the onset of "tails", but two: rising temperature, and rapidly decreasing rate of production. If you miss these signals, the lower rate of production means that you will get less contamination than with Liquid Management techniques. That can be a huge advantage!
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