Expanding Ethanol Dehydration

A number of alternative methods are available for debottlenecking cooling water systems and the molecular sieves that are used for ethanol dehydration.
By Felipe Tavares, Jansley Pascoal and Bruno Maia | March 16, 2010
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Ethanol is gaining wide popularity as an alternative fuel as the value of diminishing crude oil reserves increases and research is directed toward the possibilities of employing biomass materials for fuel. Current U.S. policies are motivating ethanol dehydration companies in Central American and Caribbean countries to increase production capacity either by starting new plants or expanding existing ones. Expansion is often preferable as it usually involves a better ratio between throughput expansion and capital costs than new construction. Besides that, a plant upgrade will take less time than building an entirely new unit.

Central American and Caribbean countries import relatively high quantities of ethanol, normally from Brazil, although little is destined for domestic consumption. These countries reprocess the product, usually converting hydrated ethanol into anhydrous ethanol and then exporting it to the U.S. Not only is value added to the product through dehydration, but the stopover and further processing means the ethanol being exported to the U.S. avoids the 2.5 percent duty and 54 cent-per-gallon tariff on Brazilian ethanol, thanks to the trade agreements and benefits granted by the Caribbean Basin Initiative and the Central American Free Trade Agreement. The CBI limits the total amount of ethanol allowed to be exported to the U.S. to just 7 percent of the previous year's entire domestic consumption. To date, all CAFTA nations combined with CBI countries have not come even remotely close to meeting this gap. Nevertheless the percentage of U.S. imports from CBI and CAFTA nations have rapidly increased in recent years (see Figure 1).

Ethanol is readily produced through fermentation yielding a dilute aqueous solution of ethanol, normally 8 to 10 percent by weight. Further concentration of the ethanol by traditional distillation processes produces an azeotrope containing about 5 percent water by weight. Azeotropes are a mixture of two or more liquids that cannot easily be separated by distillation. To prevent phase separation during storage, ethanol-blended gasoline should contain at most only small amounts of water, generally 1 percent or less by volume. Thus, ethanol used in the U.S. for blending must be substantially anhydrous.
The energy requirement for final azeotropic distillation to achieve anhydrous ethanol is very high, and various means for a more energy-efficient dehydration process have been reported. Most current ethanol dehydration facilities rely on hydrophilic zeolite molecular sieves. Zeolites can be both size- and sorption-selective for water, thereby achieving a high selectivity for water, particularly 3A zeolites that exclude ethanol but not water.

Cooling Water System Issues
Debottlenecking improves profitability if one is aware of specific issues and targets the right improvements. In many projects the cooling water system is the production-limiting factor. A number of design options for debottlenecking cooling water systems are outlined below. Picking the best alternative is not a trivial task.

Upgrade cooling tower capacity: The obvious way that comes to everyone's mind for improving the heat load removal in a process is to upgrade the cooling tower (CT). This can be made by installing a new tower cell or by replacing the entire tower with a new one with higher capacity and/or efficiency.

Improve cooling tower maintenance procedures: Proper maintenance and operating procedures may increase CT performance without major capital expenditures. The performance of the CT depends not only on maintaining the proper water/air ratio, but it is also based on the assumption that the water and air are thoroughly mixed and properly distributed, the most significant factors affecting the thermal performance of the CT.

Installation of air cooler: Air coolers can be installed in the process or in the water system before the CT to reduce the heat load of the CT, which in turn reduces water recirculation and thus increases the tower efficiency by reducing cold and hot water temperatures. Special care should be taken with layout and installation space, operation noise and electric energy consumption.

Installation of seawater heat exchanger: For facilities located near coastal areas, the use of cold seawater is yet another way to decrease the return temperature of the hot cooling water, with the cooling tower being replaced with a cooler system using cold seawater. Advantages in using seawater include a nearly constant quality in its chemical analysis, a nearly constant temperature and ample availability. The drawback of this alternative is the environmental impact related to the hot water discharged to sea.

Extraction of hot blowdown: If the cold blowdown is changed to hot blowdown, the heat load of the cooling tower is reduced because the flow rate to the CT is decreased.

Retubing of condensers: The overall heat transmission coefficient for stainless steel is 9.4 Btu per hour per foot per degree Fahrenheit (at 212F), which is much smaller when compared with copper's coefficient - 218 Btu/hr/ft/F at 212F. In this way, the replacement of stainless steel tubes by copper tubes would increase the condenser heat transfer efficiency.

Replacement of condenser tube bundles: The change of the condenser's tube bundle with another using a different geometry with more tubes would increase the heat transfer area surface, increasing the heat exchange capacity and decreasing the cooling water consumption considerably if the condensers are the bottleneck.

Consider Heat Integration
In general, plant modifications and revamps that increase capacity and introduce other improvements will also change equipment design conditions which, in turn, lead to an offset between optimal and actual energy consumption. An optimization study can point out ways to reduce energy consumption through improving process integration, often through simpler and more elegant heat recovery networks that require less steam and cooling water. Heat integration usually incorporates one of three methods:

1. Direct heat integration without mechanical compression: A heat exchanger can be installed to totally condense a part of the dry-ethanol vapor stream leaving the molecular sieves by exchanging heat with the hydrated-ethanol feed entering the process.

2. Direct heat integration with mechanical compression: The aforementioned direct heat integration can be boosted by the use of mechanical compressors. In this case, the dry-ethanol vapor stream leaving the molecular sieves is compressed, raising its temperature and allowing a higher energy exchange with the hydrated-ethanol feed entering the molecular sieves. The process creates room for the condensation of more dry-ethanol vapor, increasing the debottlenecking potential. Furthermore, that compressed dry-ethanol vapor stream could be used, in part, to vaporize the regenerated ethanol from the molecular sieves.

3. Indirect heat integration: A heat exchanger can be installed to totally condense a part of the dry-ethanol vapor stream. The condensation process will generate low-pressure steam that will be boosted by the high-pressure steam from boilers using an ejector-generating, medium-pressure steam that, in turn, will be used to vaporize the regenerated ethanol from molecular sieves and, optionally, part of the hydrated-ethanol entering the process.

Increasing production from ethanol dehydration facilities to accommodate growing dry ethanol needs can be accomplished by building new units or by debottlenecking existing capacity. Even when new plant construction is the ultimate answer, debottlenecking of existing facilities is usually examined as an option and often is an attractive method for increasing production with minimal risk. The accompanying table presents a comparative analysis of each alternative reported here. (See Figure 2) EP

Felipe Tavares is president and CEO of Intratec Solutions LLC. Reach him at felipe.tavares@intratec.us or (713) 821-1745. Jansley Pascoal and Bruno Maia are technical managers at Intratec Solutions. Reach them at jansley.pascoal@intratec.us and bruno.maia@intratec.us.