{[ promptMessage ]}

Bookmark it

{[ promptMessage ]}

060522 Ensure Proper Design

060522 Ensure Proper Design - Distillation Ensure Proper...

Info iconThis preview shows pages 1–2. Sign up to view the full content.

View Full Document Right Arrow Icon
22 www.cepmagazine.org June 2005 CEP Distillation S tandard crossflow trays in distillation columns operate with gravity-driven liquid flowing downward in a ser- pentine pattern while contacting the vapors that are flowing upward. As the liquid flowrates increase, the friction- al resistance and corresponding liquid head increase on the tray deck and in the downcomers. Once the liquid resistance becomes substantial, the tray configuration is split into multi- ple streams or passes. Two-pass trays are quite common and generally don’t pose significant design problems, since the flow paths and tray designs are symmetric. Tray designs with more than two liquid passes are considerably more challenging because of their lack of symmetry, and need to be designed with care. Three-pass trays are rare because of their inherent asymmet- ric design and difficulty to balance. Four-pass trays are more common. Trays with more than four passes are rare as well. Unless noted otherwise, the multi-pass trays discussed in this article are four-pass trays. Hydraulic considerations One of the most critical operational aspects is the balance of the vapor and liquid streams on the tray deck. It is impera- tive that the vapor and liquid streams contact each other and flow evenly across the tray. When the vapor flow is disproportional to the liquid flow in different areas of the tray, a compositional pinch can occur and limit the efficiency. From a hydraulic standpoint, when the flows are imbalanced, the side that is more heavily loaded will prematurely limit the tray’s capacity. The effect of an imbalance varies depending upon the tray design and the application. For example, in applications that are heavily vapor-loaded, excessive vapor flow to one section will cause a flood in that section; in highly liquid-loaded applications, excessive liquid flow to a section may cause a localized flood there as well. A common term used to evaluate the vapor and liquid flows is V/L, the vapor/liquid ratio. When measured on a molar basis, this value is generally unity for a column operat- ing in a total reflux mode. Correspondingly, the V/L ratio should also be unity for a perfectly balanced tray operating at total reflux. Thus, V/L is the defining measure of balance between passes. Since most columns do not operate at total reflux, the V/L ratio may vary widely from unity. The absolute value of the number is really not important, but the consistency of this num- ber between the various passes of a multi-pass design is critical. Tray geometry and nomenclature In its simplest form, a standard crossflow tray consists of a tray deck and a downcomer. The tray deck is perforat- ed with either sieve holes or valves that allow the vapor to travel vertically upward through the deck to contact the liq- uid layer that is flowing horizontally across it. After con- tact, the vapor disengages from the liquid and travels to the next tray immediately above. The mixing of the vapor and liquid generally forms a froth or spray on or above the deck. As the frothy liquid mixture leaves the deck, it trav-
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full Document Right Arrow Icon
Image of page 2
This is the end of the preview. Sign up to access the rest of the document.

{[ snackBarMessage ]}