Should your ethanol plant invest in carbon dioxide?

Market conditions, capital equipment, operating costs determine whether CO2 recovery and purification plant investment makes sense
By William Bennett, Wittcold Systems | June 01, 2002
As with any investment, we try to maximize our returns at an ag-processing faciltiy. To this end, we look for valuable by-products that may result from the production of Ethanol.

Carbon dioxide is a natural by-product of an ethanol production plant. Is it a valuable by-product? Market conditions determine the demand and the price. Your capital equipment and operating costs will add the remaining pieces of the puzzle needed to determine whether an investment in a by-product CO2 recovery and purification plant provides an attractive return on your investment.

In determining the size of the plant, the method of operation of the ethanol production facility needs to be considered. The CO2 evolution from a Fermenter consists of a minimum and maximum flowrate. Typically, fermentation cycles will be completed in 18 to 24 hours. Unlike many other traditional CO2 sources, the flowrate from each fermentation vessel is not constant. A CO2 evolution chart could very well be described as a bell shaped curve. When your start at the base, the CO2 flowrate is minimal and gradually builds until you reach the maximum flowrate at the peak. During descent, the CO2 evolution decreases until you again reach the base.

When operating two or three Fermenter vessels, the CO2 evolution consists of staggered peaks and valleys. The design of the system is important to enable CO2 collection at the lower flowrates and peaks; to ensure that the average CO2 Recovery rate as required for the market is obtained. Generally, this means that a 250 STPD System must be capable of efficiently collecting at a low capacity, say 180 STPD, and again at the peak flowrate, say 280 STPD, to average a collection requirement of 250 STPD. This does increase the capital cost of the system but allows for the average recovery of liquid CO2 product that would be required in your area.

When you increase the number of Fermenters in your process, then there is more product in various stages of maturity that will minimize the peaks and valleys near 250 STPD. This means that the equipment does not have to be oversized to recover peak CO2 evolution. An added gain, the closer the compression and refrigeration equipment is operated to maximum design, the lower the kW per S-Ton of CO2 produced (i.e., lower cost of production).

Additionally, careful consideration has to be given to the ethanol production system employed. Many times, fermentation is enhanced with aeration of the first two or three Fermenters. This process will lower the inlet purity to the by-product CO2 recovery system. The CO2 system needs to be designed for this lower inlet purity or the amount of CO2 that can be collected is substantially reduced; only collecting the higher inlet vapor from the last two or three fermenters.

While only collecting the higher purity inlet vapor may be a good practice initially, the market potential and equipment design needs to be carefully evaluated so that, if required, collection of the lower purity CO2 vapor can be done as the market matures. Doing this initially, will allow for increased collection capacity later without additional capital expenditures or a change in the process.

Possibly the most important consideration is to ensure that the final product is acceptable, that is, it meets or exceeds International Society of Beverage Technologists [ISBT] Standards. This is a good starting point, but also the product must ultimately meet or exceed the various standards for the industrial gas marketer. The system has to be so designed as to protect the producer against off-spec product. Off-spec product is of no value and requires significant expense to rectify.

Safety in design as well as sufficient analytical equipment, constantly employed, to ensure that the delivered product always meets or exceeds what the market requires is the only way to ensure against an unacceptable liquid CO2 product.

A closing note on "CO2 credits" - a burgeoning market that may produce substantial revenue in the future - this needs to be evaluated as a separate revenue stream.

When considering whether or not to install a by-product CO2 recovery system, careful attention to the market conditions as well as the design of your equipment will ensure that when you do install a CO2 plant, that CO2 plant will maximize your investment. n