Save Energy In Distillation

Revamping may cut consumption considerably

By John Pendergast, Louisiana State University, Dennis Jewell and David Vickery, Dow Chemical Company, and Jose Bravo, Fractionation Research, Inc.

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Separations operations account for roughly 50–70% of the energy used in large-scale chemicals manufacturing [1]. Distillation — which we’ll use to encompass the related operations of absorption and stripping — dominates for such separations.

Distillation rules the separation landscape not because of any efficiency advantage but rather because of several other factors that favor it. For instance, distillation:

• scales well, generally to the 0.6 power;
• when performed by itself, doesn’t introduce an extra mass agent (solvent, sorbent, etc.) that needs subsequent recovery, as in the case of absorption or liquid extraction;
• allows heat integration within individual units and across facilities, which can foster effective heat utilization in many separations operations; and
• provides an inter-relationship between pressure and temperature.

Moreover, the technology is well understood and robust, leading to high confidence in designs.

However, its maturity means that distillation usually isn’t the focus of academic research, with several notable exceptions such as the work of Rakesh Agrawal at Purdue, Bruce Eldridge, Frank Seibert and Gary Rochelle at the University of Texas, and Ross Taylor at Clarkson. Fortunately, long-established and robust industrial consortia carry out investigations and continue to refine the practice of distillation. For instance, Fractionation Research, Inc. (FRI), which has more than 85corporate members, operates industrial-scale distillation columns and performs research on modern distillation devices.

Progress is taking place in decreasing distillation energy consumption. Facilities now being planned certainly can benefit. However, energy reductions alone generally can’t justify investment to replace an existing column.

Operating companies expect distillation columns to operate for decades; a life span of 30 years isn’t uncommon. So, sites over time may make modifications and upgrade instrumentation, and often keep the units in excellent physical condition.

Thus, the most-feasible approach for saving energy is via retrofitting. Here, we’ll focus on some ideas for energy reduction in existing columns.

Converting An Existing Sequence

If the infrastructure already exists to perform the separation by conventional methods, you may be able to convert the current sequence to a prefractionator or Petlyuk arrangement (Figure 1). This can save a significant amount of energy for the required separation; typically, it reduces energy consumption by 20–40%. Alternatively, you can produce more products from the facility for the same amount of energy.

Converting an existing facility from a conventional sequence to a prefractionation one certainly isn’t trivial; it requires careful study, including rigorous simulation, and then a detailed evaluation of the mechanical modifications necessary. You should assess, e.g., the internal tower hardware before and after the modifications, suitability of the pre-fractionator reboiler and condenser in the new service, turndown capability in the new service, and other details.

A further way to achieve or increase energy savings is by converting the sequence to sequences that employ side-rectifiers or side-strippers.

Opting For A Single Dividing Wall

A dividing wall column (DWC) may save a substantial amount of energy. (See: “Consider Dividing Wall Columns,”[2].)Depending on whether significant changes in product mix or capacity requirements have occurred over time, it may be possible to convert a two-tower system to a single dividing wall column (Figure 2). This is more likely when the original tower configuration was developed to remove relatively small fractions of light component (A) or heavy component (C) or both.

You must assess mechanical details such as tower hydraulics, feed locations and auxiliary equipment rating as well as other details to determine the feasibility of such a conversion. As with all complex column configurations, it’s essential to remember that the levels of energy consumption are an important consideration for the evaluation of the benefits of new separation configurations. While the single DWC will consume less energy from a First Law perspective, it maybe disadvantaged from a Second Law perspective. So, you must analyze the system individually, taking into account the utilities and economics for the particular operation or entity.

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