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Vacuum System Selection and Pumparound Control

Pumparound control with a total draw, including product and pumpdown control

Fatty acid distillation system

To: asloley@distillationgroup.com
Subject: Vacuum source and pumparound system control
Date: Sat, 01 Jun 2001 AM +0800

Dear Mr. Sloley:

I have a problem and I want to seek some realistic answer from you. Below is my question:

The distillation train is mainly used for fractionating fatty acids and operates at high vacuum. The distillation section includes a pumparound system and packed column with the pumparound used to cool down the top column vapor. Pumparound is taken off to give a reflux stream, a pumparound stream and a distillate steam from a trap-out or so-called chimney tray. The bottom stream is the same like normal distillation. The pumparound stream that returns to the column will first be cooled down to a specific temperature.

A) What is the best pressure control system or loop for this distillation setup rather than steam ejector?

B) What is the best way to simulate the process using commercial simulator: I mean that we want to control the level on chimney tray, and what type of configuration can you suggest for the simulation in steady-state and dynamic state?

C) How about revamping this process?

D) Can you send some material regarding this type of column configuration?

E) And how about overall control configuration for this column that you can suggest?


Thanks so much for your help.

Regards,

D.
Southeast Asian Chemical Plant
(edited for clarity)

To: D.
Subject: Vacuum source and pumparound system control
Date: Sat, 01 Aug 2001 11:31 -0500

You have described a common system where a direct-contact heat-transfer zone in the column coupled with a forced-convection heat exchanger replaces an overhead condensing system. The pumparound system configuration is very useful in systems where the overhead condenser might produce multiple liquid phases. Separation of the liquid phases to produce a single-phase reflux can present problems. Commonly this is seen is systems that produce both an aqueous condensate and a non-aqueous condensate.

In reply to your specific questions:

A) What is the best pressure control system or loop for this distillation setup rather than steam ejector?

From the context of your question, it's not clear if you want to know about pressure control systems for towers operating under vacuum or about how to select between different vacuum generating devices. Since the pressure control system configuration depends upon the type of vacuum selected, we'll discuss the question of selection of the device type.

For commercial systems, the major types of vacuum devices are ejectors, liquid-ring pumps, and dry pumps. Ejectors are most often steam driven, but also include systems that use compressed gases, other vapors, and even liquid as motive energy sources. The major advantages of ejectors includes relative low to purchase, simple maintenance, and extremely reliable operation. Their major disadvantages include adding a new stream to the process and potential energy costs (for gas compression or steam generation). Multiple-stage ejector systems routinely operate at vacuum pressures down to 10-15 torr (10-15 mm Hg absolute). Some large industrial systems have operated at pressures as low as 2 torr (2 mm Hg absolute). For moderate vacuums down to 50 torr (50 mm Hg absolute), liquid-ring vacuum pumps are often useful choices. They minimize energy consumption. However, the have higher maintenance costs. Our personal experience is that relatively few liquid-ring pumps are used in petroleum refineries or petrochemical plants due to the maintenance problems. These problems seem to persist in spite of manufacturer reliability claims. Liquid-ring pumps appear to work well in systems where the operating temperatures are lower and the main source of gas load is leakage into the system rather than a process stream. Dry pumps come in various mechanical configurations. They are rarely used in large process systems. In smaller systems, often fine chemicals or other operations, they are more common. They have essentially the same benefits and problems as liquid-ring pumps except they dispense with the liquid sealing fluid. Dry vacuum pumps may also operate at much lower pressures than liquid-ring pumps.

B) What is the best way to simulate the process using commercial simulator: I mean that we want to control the level on chimney tray, and what type of configuration can you suggest for the simulation in steady-state and dynamic state?

Controlling level on the chimney tray is a common operation for a total draw tray. Steady-state simulation posses no particular problem. You ignore the level on the tray as you would ignore inventory considerations in most parts of the process. Dead-time inventories for storage in tanks or vessels are typically ignored in steady-state simulation.
In dynamic modeling, any standard commercial dynamic simulator package will handle problem if set up to reflect the unit configuration correctly. As a note, situations requiring dynamic modeling of chimney tray levels as part of the process are exceptionally rare. The only conceivable situations are where feed-forward or multiple-variable control is being used and either the heat shift in the pumparound section (requiring large flow rate changes) or the product rate changes are severe and rapid may require such analysis. Typically liquid level control is achieved with a SISO control loop that may include cascade control for particular applications.

C) How about revamping this process?

How you revamp a process depends very strongly on your specific revamp objectives. If you would care to discuss this in more detail, please forward detailed information on your specific revamp objectives and the limitations that you wish to improve.

D) Can you send some material regarding this type of column configuration?

Figure 1 shows a typical configuration for the pumparound section and level control for this type of column. An external (or manual reset) sets the pumparound flow rate needed for the required heat removal. External reflux is set by a manually adjusted flow regulating controller to meet process requirements. Of course, this may be reset by an advanced control system in cascade as well. Product draw is used to control inventory on the collector tray. The configuration shown is a basic configuration that works very well for most applications. Other variations are used to meet specific process needs.

Figure 1
Control configuration for a pumparound condensing section with product draw

E) And how about overall control configuration for this column that you can suggest?

Overall column configuration for control depends so much upon the service and process requirements that general rules cannot be made. Control configuration issues include selection of mass-balance versus material balance control, use of direct or inferred process measures, selection of control points, type of controller action, and use of advanced control, among others. Control configurations are very specific to the service, process, and equipment being used. You need to weigh all these factors in selecting your control configuration. No one 'best' control configuration that fits every column exists.

Andrew Sloley
DGI

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This page updated 01 August 2001.
© 2001 Andrew W. Sloley. All rights reserved.