Perspectives: Plant InSites

Distillation Columns: Get Some Inside Information

A gravity-flow loop can enable measuring internal liquid flow in a column

By Andrew Sloley

Throughput or fractionation performance in specific systems may vary dramatically depending upon how precisely the internal liquid rate can be controlled close to its limit. Four basic methods can determine the liquid rate within a column:

• calculating heat and material balances using heat input, heat output and external stream rates and properties;
• taking a total draw and then returning liquid through a metered pumped-flow loop;
• taking a total draw and then returning liquid through a metered gravity-flow loop; and
• measuring a column liquid level to infer flow.

Each of these has pros and cons; several or all four might work for a given system. Physical layout and data precision impose particular requirements for each option. Specific systems tend to favor one option or another. Getting the details right counts.

Here, let’s specifically look at taking a total draw and then returning the liquid through a metered gravity-flow loop.

Figure 1 is a partial elevation sketch of a 42-in.-diameter distillation tower. Feed enters the tower between Bed 2 and Bed 3. Beneath Bed 2, a collector tray takes liquid out as a total draw. A gravity-fed loop is used to measure the flow rate of liquid from that collector tray.

I first applied this configuration nearly 25 years ago in a specialty lubricants distillation unit as part of a larger revamp. Discussing that job after the fact, another senior engineer emphatically stated that this type of flow measurement system never could work. He had seen many attempts but none had succeeded. This surprised me because the client was very pleased with its performance. So, we sat down together to review what was built.

Figure 1 highlights seven key features:

1. Collector tray. This serves to gather the liquid. For a packed tower, it’s a default choice. Most applications of this configuration will have a low liquid rate. (A future column will cover the “why.”) Therefore, the tray must be fully welded. A collector tray is a good idea even in a tower with trays. However, with the correct modifications, a special tray configuration may suffice.

2. Nozzle sized for self-vented flow. Vapor bubbles entrained in the liquid reduce system capacity and give incorrect flow-meter readings. Self-vented nozzles have a low enough liquid velocity that vapor can escape and return to the tower. (See: “Assess the Gravity of the Situation.")

3. Sufficient static head. H1 is the minimum distance required to generate enough static head for pressure drop to drive flow through the system. This sets the minimum distance between the draw nozzle and return nozzle elevations. Pipe diameter often is reduced downstream of the self-venting nozzle to cut costs. In that case, H1 must be based on the elevation of the pipe diameter reduction, as shown in Figure 1. In an effort to minimize this height, high-β-value orifice plates often are used to decrease pressure drop.

4. Adequate upstream pipe run. H2 is the minimum upstream pipe run to get well-conditioned flow needed for accurate metering. With orifice flow meters, the higher the β value, the longer the pipe run required. (See: “Think Straight about Orifice Plates.”)

5. Flow instrument in liquid-filled location. The meter can be installed in either a vertical (as shown) or horizontal position. In all cases, the piping configuration must ensure the line is filled with liquid.

6. Long-enough downstream pipe run. H3 is the minimum downstream pipe run necessary for well-conditioned flow for meter accuracy.

7. Suitable distance between the return and lowest-pressure points. H4 is the minimum interval from the return elevation to the lowest-pressure point (vena contracta) to prevent vaporization downstream of the orifice plate. The vena contracta occurs some distance after the orifice plate. Most times, it’s close enough that the orifice plate location can be used to set elevations. Nevertheless, engineers with a good grasp of fundamentals never forget the difference. Avoiding vaporization is important to getting accurate flow measurements. Either H3 or H4 may set the minimum distance from the flow meter to the return nozzle.

One extra point not shown — insulation — is important. In hot systems, a little heat loss makes H4 less likely to set the system. In cold systems, heat gain easily may make H4 dominant. More insulation helps. Vaporization in the system also dramatically may boost pressure drop, increasing H1 as well.

These points apply equally to both packed and trayed towers.

Going back to the discussion with my colleague: After reviewing the system, he asked: “Just how tall was this loop?” The final system with its vertically mounted meter was nearly 30-ft high. Gravity-feed flow-measurement systems require ample height. A horizontal mounting may make H2 and H3 unimportant but H1 and H4 never go away.


ANDREW SLOLEY is a Chemical Processing contributing editor. You can email him at ASloley@putman.net

More from this perspective...

Title

Stripper Bares All

Analysis reveals how operating practices compounded design mistakes

11/21/2008

Steam Systems: Simple Solutions Can Prompt Complex Problems

Steam systems are especially susceptible to developing difficulties

12/23/2008

Spot problems with adsorbents

The longer-than-expected life of an adsorbent points up the need to always assess the consequences of system additions. While sometimes this may involve detailed calculations, simply looking to the laws of physics can eliminate potential headaches.

08/14/2006

Software: Show Some Skepticism

Calculated results require critical analysis.

06/14/2010

Sidestep side-draw control surprises

The simplest approach to control side-draw distillation columns uses flow control on the side product. This solution, however, is not ideal for all side-draw control situations.

07/01/2004

Shell-and-Tube Heat Exchanger: Pick the Right Side

Allocating fluids in a tubular exchanger demands care.

10/23/2013

Select the Right Reboiler

Understand the strengths and weaknesses of various options.

06/04/2012

Save a bundle solving pressure-drop problems

Pressure drop in compressor-suction and interstage coolers often creates problems. In some cases, just a few pounds of extra pressure drop make a revamp unworkable.

11/04/2004

Right Vacuum Control Choice Takes the Pressure Off

Choosing the proper recycle stream for regulation is crucial

08/07/2008

Rethink Reactor Temperature Control

Cascade strategy offers simplicity and fast response.

07/13/2010

Reliability and Maintenance: Don’t Slight Strainers

These simple but often essential devices deserve adequate attention

05/08/2014

Put V-cone Meters on Your Short List

06/10/2013

Properly Trigger Standby Pumps

A pressure signal works for many but not all situations.

05/06/2010

Properly protect centrifugal pumps

Consider various factors when selecting how to guard against low flow, Andrew Sloley says in this month's Plant Insites column.

07/11/2007

Properly Position Interlock Valves

The obvious location may not be the most cost effective

10/16/2012

Properly Assess Energy Recovery Projects

Impact on other operations and transfer prices may alter the economics.

03/09/2011

Process Engineering: Poor compressor design puts pressure on pumps

When a poor compressor design put too much pressure on the pumps, a new solution had to be constructed.  Sometimes the ideal solution is just an intricate compromise.

01/18/2005

Process Engineering: Inspect with your mind, not just your eyes

A thorough inspection of plant equipment following tiny installations can prevent a lot of future problems. Go beyond just checking for compliance with drawings when looking at hardware and physically look at the equipment itself.

03/11/2005

Preheater points out the value of cooling off

Lack of attention to detail in conceptual design can hide many sins. Put the problem aside and then ponder it again. A fresh look may lead to an even better solution.

05/12/2004

Plant InSites: Look Beyond the Lore

Don’t rely on recollections about how a unit had performed

10/10/2008