Systems generally are limited either by overhead condensing capability or tower capacity. If the overhead system can control pressure within the operating margin, it isn’t a limit. A quick review of control parameter ranges (for example, valve position) in the overhead system will rapidly identify if you have an overhead system limit.
In general, most towers will eventually flood as pressure drops. However this isn’t always true, so be careful. In the unusual case where flooding isn’t a concern, you can safely reduce pressure. In the more common situation where tower flooding becomes more of a problem at lower pressure, you need to know how to keep out of flood. This requires monitoring pressure drop across the tower’s limiting section, so you can operate as close as possible to tower flood (and at the lowest and cheapest pressure). Direct measurement of pressure drop is most common, but the flow of the vapor  or liquid by internal restrictions also can provide such data.
Are you controlling energy consumption at all?
Figure 4 shows part of a distillation scheme with heat integration to provide reboiler heat from two sources — recovery from reactor effluent and 600-psig steam.
Figure 4. In this scheme no process value has an impact on the heat input to the tower.
Taking advantage of the reactor effluent certainly reduces unit energy costs. However, the apparent complexity of the control scheme hides one major factor: no process value affects the heat input into the tower. The total heat input is set by the hand control valve (HC) on the reactor effluent split and the flow controller on the 600-psig steam. Essentially, the tower operates at constant duty. Constant duty operation, either in the condenser or the reboiler, nearly always is a sign of wasted energy.
A survey of this large plant showed that 42% of the reboiled towers used some form of constant energy control. While such control is necessary in some situations, it’s unlikely that nearly half the towers in a typical processing plant require it.
Check your control configurations for constant energy control in either the condenser or reboiler. Odds are you can change many of them to reduce energy use.
Are you effectively using stripping steam?
Steam stripping often is chosen for light-ends removal or recovery. It reduces the partial pressure of other components in a system, allowing them to vaporize. The heat of vaporization comes from the process fluid, not the steam. So, frequently far less steam is required compared to using a reboiler. This saves energy — but only if stripping is effective.
Tray towers. The relatively low steam rate can pose difficult equipment-design problems, especially for trays. Many stripping trays provide nearly 0% efficiency because their design doesn’t suit the service requirements. Even under the best conditions stripping tray efficiency rarely rises above 35%. Still the difference between 0% and 35% is huge.
A common design miscue is not accounting for the large changes in vapor rate that often occur across the stripping section. For trays to work effectively, they must have a minimum pressure drop to prevent liquid bypass and to provide good vapor distribution. Typically, a sieve tray needs a minimum of 0.05-psi pressure drop. When vapor rate varies, the number of sieve holes required to get the minimum pressure drop varies. If the vapor rate changes a lot, different trays may have to have significantly different hole layouts.
Table 1 summarizes stripping vapor loads for a service with eight real trays at three different efficiencies. In all cases the product (overhead) yield is constant. Zero percent efficiency shows the steam required for the stripping using only a steam flash.
Table 1. Ignoring vapor rates can take a severe toll on stripping efficiency and thus energy use. (Click to enlarge.)
Raising this plant from essentially 0% efficiency on the stripping trays to 25% percent saved 81,500 pounds per day of steam, which translates to $200,000 per year.
Improving efficiency demands accounting for the vapor profile through the trays. Figure 5 plots the vapor rate across the eight trays (assuming a constant 25% efficiency per tray). Initially, the vapor rate changes rapidly across each tray then levels out.
Figure 5. A single tray design generally can’t effectively handle wide differences in vapor rate.
The rate on Tray 8 is more than 1.5 times the rate on Tray 2. Using a tray design suitable for the maximum rate means that the design for the bottom tray is nearly completely ineffective. Because the bottom tray doesn’t do any stripping, the vapor rate remains the same and Tray 2 doesn’t work either. This cascades up the stripping section and none of the trays work well. Average efficiency is close to zero.
Stripping sections with rapid changes in vapor rate must use multiple tray designs. Our example calls for four designs with different hole area for Tray 1, Trays 2 and 3, Trays 4 and 5, and Trays 6 to 8. This results in 25% efficiency instead of 0% efficiency and reduces energy costs by 75%.
Packed towers. The same problems occur with high-efficiency packed stripping sections. Here the major problem is steam distribution. Natural vapor distribution across a packed bed depends upon pressure drop: the higher the pressure drop, the better the distribution. However, if you design for low vapor rates, you have low pressure drops. The low pressure drop results in poor vapor flow distribution to the packed bed. You lose the benefit that you might get from a highly efficient stripping section.