Distillation Columns – Internal Reflux Control

A view from the trenches considering one of the sources of distillation column instability that is often overlooked.

08/21/2012


Distillation columns are unit operations most often used for separation and purification in process industries. They can also be some of the most complex to operate and control, because they involve two-phase, multi-stage, counter-current mass and heat transfer, with each tray or segment of packing representing a theoretical equilibrium stage). The greater the number of trays, the longer the time constants related to composition changes.

For a two-product distillation column (top and bottom product), there are typically five degrees of control freedom (control valves):

• Reflux flow
• Top product flow
• Reboiler heat input flow
• Bottom product flow, and
• Pressure control valve,  the specific location of which depends upon how the pressure is controlled.


Three of these valves are needed for inventory control (reflux drum, column bottom, and vapor inventory or pressure control). That leaves two valves for achieving the primary operating and control objective, namely product composition control. These two valves are normally the reflux flow and the reboiler heat source flow. For many columns, the P&ID’s will specify a top or upper tray temperature controller that adjusts the reflux flow in a straightforward cascade for top product composition control.

Unfortunately, this type of cascade does not always perform very well, and often operators will end up breaking the cascade and using the reflux flow control in AUTO mode rather than CASC. There are several reasons for poor control loop performance – this discussion addresses one of the less recognized and often over-looked sources of process disturbance.

There are at least seven or eight different ways to control pressure on a distillation column, and several of these will result in sub-cooled reflux. Sub-cooled means that the temperature of the reflux exiting the overhead condenser is below its bubble point, the temperature at which the first bubble of vapor boils off the liquid. From a process and control standpoint, what are the implications of returning sub-cooled reflux to the column?

The purpose of reflux is to provide down-flowing liquid throughout the rectification section to contact with the up-flowing vapor in order to achieve stage-by-stage equilibrium heat and mass transfer and, hence, purification of the top product. When sub-cooled reflux is introduced to the top tray, it must be heated up to its bubble point before the lighter components will vaporize. Where does the heat come from? The only place it can come from is from condensing vapor that is approaching the top tray from below. When this vapor condenses, it adds to the total liquid flowing from tray 1 down the column. In other words, a sub-cooled reflux introduces a greater volume (or mass or molar) flow of reflux than is delivered to the column by the external reflux flow controller.

If the degree of sub-cooling was constant, then this wouldn’t be such a big source of disturbance; however, this is usually not the case. The amount of sub-cooling will vary with the temperature of the cooling medium (ambient air, cooling water, another process stream, etc.), rainstorms, and so on. To achieve satisfactory composition control, the most common approach is to employ an advanced regulatory control (ARC) technique referred to as internal reflux control.  The internal reflux, that is, the actual flow of liquid from tray 1 to tray 2, can be calculated as follows:

IR = R * (1+Cp * (TOTR) / Λ)

Where:

R    =   External reflux flow

Cp =    Heat capacity of the reflux (e.g., BTU/lb-°F)

TO =    Overhead vapor temperature (entering the condenser)

TR =    Reflux temperature

Λ    =    Heat of vaporization of the reflux (e.g., BTU/lb)

An internal reflux controller simply uses this equation to solve for the external reflux flow required to maintain a constant internal reflux at each control execution. In effect, this controller compensates for changes in the sub-cooled reflux temperature at each control execution.

The final step is to rebuild the cascade for composition control, namely, to re-introduce the temperature-to-internal reflux cascade, with the likelihood that this cascade will be more stable, will control composition better, and will enjoy greater operator acceptance.

This post was written by Dr. Jim Ford, PE. Jim is a process control consultant at MAVERICK Technologies, a leading system integrator providing industrial automation, operational support and control systems engineering services in the manufacturing and process industries. MAVERICK delivers expertise and consulting in a wide variety of areas including industrial automation controls, distributed control systems, manufacturing execution systems, operational strategy, and business process optimization. The company provides a full range of automation and controls services – ranging from PID controller tuning and HMI programming to serving as a main automation contractor. Additionally MAVERICK offers industrial and technical staffing services, placing on-site automation, instrumentation and controls engineers.



No comments
The Top Plant program honors outstanding manufacturing facilities in North America. View the 2015 Top Plant.
The Product of the Year program recognizes products newly released in the manufacturing industries.
The Engineering Leaders Under 40 program identifies and gives recognition to young engineers who...
Your leaks start here: Take a disciplined approach with your hydraulic system; U.S. presence at Hannover Messe a rousing success
Hannover Messe 2016: Taking hold of the future - Partner Country status spotlights U.S. manufacturing; Honoring manufacturing excellence: The 2015 Product of the Year Winners
Inside IIoT: How technology, strategy can improve your operation; Dry media or web scrubber?; Six steps to design a PM program
Getting to the bottom of subsea repairs: Older pipelines need more attention, and operators need a repair strategy; OTC preview; Offshore production difficult - and crucial
Digital oilfields: Integrated HMI/SCADA systems enable smarter data acquisition; Real-world impact of simulation; Electric actuator technology prospers in production fields
Special report: U.S. natural gas; LNG transport technologies evolve to meet market demand; Understanding new methane regulations; Predictive maintenance for gas pipeline compressors
Warehouse winter comfort: The HTHV solution; Cooling with natural gas; Plastics industry booming
Managing automation upgrades, retrofits; Making technical, business sense; Ensuring network cyber security
Designing generator systems; Using online commissioning tools; Selective coordination best practices

Annual Salary Survey

Before the calendar turned, 2016 already had the makings of a pivotal year for manufacturing, and for the world.

There were the big events for the year, including the United States as Partner Country at Hannover Messe in April and the 2016 International Manufacturing Technology Show in Chicago in September. There's also the matter of the U.S. presidential elections in November, which promise to shape policy in manufacturing for years to come.

But the year started with global economic turmoil, as a slowdown in Chinese manufacturing triggered a worldwide stock hiccup that sent values plummeting. The continued plunge in world oil prices has resulted in a slowdown in exploration and, by extension, the manufacture of exploration equipment.

Read more: 2015 Salary Survey

Maintenance and reliability tips and best practices from the maintenance and reliability coaches at Allied Reliability Group.
The One Voice for Manufacturing blog reports on federal public policy issues impacting the manufacturing sector. One Voice is a joint effort by the National Tooling and Machining...
The Society for Maintenance and Reliability Professionals an organization devoted...
Join this ongoing discussion of machine guarding topics, including solutions assessments, regulatory compliance, gap analysis...
IMS Research, recently acquired by IHS Inc., is a leading independent supplier of market research and consultancy to the global electronics industry.
Maintenance is not optional in manufacturing. It’s a profit center, driving productivity and uptime while reducing overall repair costs.
The Lachance on CMMS blog is about current maintenance topics. Blogger Paul Lachance is president and chief technology officer for Smartware Group.
This article collection contains several articles on the vital role that compressed air plays in manufacturing plants.
This article collection contains several articles on the Industrial Internet of Things (IIoT) and how it is transforming manufacturing.
This article collection contains several articles on strategic maintenance and understanding all the parts of your plant.
click me