Carbon dioxide process recovery description for typical by-product streams

March 23, 2010 |

By Biofuels Digest special correspondent Sam A. Rushing

In many developed world markets, carbon dioxide, in recent years, until the last year or two enjoyed a surge in new sourcing from ethanol projects, until some of this corn and grain based dry (and wet) mill fermentation industry collapsed due to oversupply of ethanol, the inability to make this fuel grade ethanol available for blending purposes into gasoline. Also utilization by the gasoline industry has lagged behind – and commodity prices for corn made many of the plants take bankruptcy, consolidate or close.

However, the biofuels and ethanol industry will revive via acquisitions by oil companies (such as Valero’s bold move into the ethanol industry); and the development of secondary and tertiary biofuels – cellulosic and algae based, for example, will carry the biofuels and fermentation industry far into the future, as will the availability of by-product carbon dioxide from these sources. States such as Florida may be the next focus for new second and third generation biofuels projects, cellulosic, algae, and allied new technologies.

The future is in the post-grain based ethanol world, however, lest we not forget the grain based projects which have always operated in a successful manner, as well as those which will now operate well under the guise of reorganization – the post bankruptcy and often improved companies, after the fallout during the collapse of the old style ethanol business, thus leading to more efficient grain based; and of greatest focus, the second generation biofuels and beyond – all of which will be key to the future in making many world markets more self reliant and environmentally friendly due to less dependence on fossil fuels.

Cellulosic technologies and algae have some way to go in terms of refinement; however, much of this is ready for commercialization. Should we have legislation passed within the climate bill, then I feel capital should then flow in favor of these ever vital projects – that being vital for the sake of new energy options, improved efficiencies, and environmentally friendly projects; which in the end, yield new job opportunities, new jobs so vital to the growth of the economy – this helping replace jobs which are long lost and will never return.

The ethanol industry has represented over 30% of the US (and many developed global) market’s raw gas availability; and to follow would probably be natural well sources, often high pressure, and relatively high purity; plus reformer operations in the oil refining sector, as well as the anhydrous ammonia industry, which has gained some level of resurgence, in part due to natural gas prices and availability – this order of dominance and availability varies from country to country; however, these are the lion’s share of sourcing today in the developed world. To follow, in terms of CO2 supply, would be a smaller number of sources from processes such as flue gas recovery (using MEA solvents in most cases in order to concentrate the lean flue gas into a high yield raw product); as well as some sources derived from ethylene oxide (which usually represents expensive metallurgy in the plant, thus a higher cost), and there is also titanium dioxide manufacturing as a raw CO2 source. In developing countries, unlike the developed countries, some of these sources exist, such as sugar mills with affiliated fermentation plants, oil refineries, and self-contained carbon dioxide generating combustion plants; which use MEA solvents for recovery of a combusted CO2 by-product when burning fossil fuels, and then downstream liquefying and purifying the product. All such combustion plants, or any flue gas plant does not have proven commercialized, full scale operations which produce CO2 from a lean flue gas stream into a finished refined liquid product, sufficient for service to the food, beverage and industrial sectors.

With the U.S. push to prove viable means of recovering & sequestering flue gas from power plant raw gas streams, generally using DOE grants and stimulus funding for technologies provided by companies to include Alstrom (a refrigerated process with a high energy demand). One such Alstrom project is underway in West Virginia for sequestration of CO2 from power plant based flue gas. Other companies are offering their version of a proprietary flue gas recovery process such as Fluor Daniel (which built and licensed via contractors numerous plants over the many years), UOP’s membrane systems, Mitsubishi Heavy Industries, and Air Products and Chemicals – to name a few. These test, pilot, and demo facilities by those trying to commercialize their technology such as Alstrom plans to have a focus on CO2 sequestration.

In summary, with respect to merchant CO2 raw gas sources, traditionally flue gas is not an option, short of remote, high priced, generally developing economies. On the other hand, sources are often over 98 – 99% CO2 content when derived from chemical and natural well sources, such as fermentation, ammonia, wells, and reformer, for example. The flue gas CO2 recovery interest today, generally surrounding the electric power (usually coal fired) projects – have sequestration targets in mind. On the other hand, some of these sequestration targets when defining enhanced oil recovery (EOR) as a sequestration target is not necessarily viable, since the entire CO2 is not left underground as enhanced oil recovery occurs; so this sector concerning CO2 sequestration has a long way to go in terms of providing a viable solution to the problem.

As with most of the gases, CO2 is produced, and stored at the plant site as a liquid. Then delivered and stored at the customer’s site as a liquid product again, with temperatures often ~ zero degrees F. This is a working range within the delivery and some of the bulk plant storage as well. The US is using the English system, selling by the short ton (2,000 pounds per ton); where in most other world markets, the metric system applies. Despite the form of measurement, the requirements for purity have been more strongly scrutinized over time as incidents including contamination with aromatic hydrocarbons, sulfur compounds, and hydrogen cyanide have taken place worldwide. Some of this has been heavily placed under the spotlight by the major soft drink bottlers and beer manufacturers, who have implemented stringent QA requirements due to such events. For example, in the US, some years ago, a southern source was producing and selling liquid to all forms of industry via gas companies for many years, and it was discovered, practically by accident, that HCN was contained in this source gas from coal gasification by-product. As a product of this, the plant disappeared from the merchant sector, and one global beverage concern discounted further coal gasification sourcing for the time being. Other incidents with another global beverage giant became involved in complaints surrounding contamination in Belgium and Poland; this is history of course, however today, once again, there are more stringent QC demands placed upon all beverage based CO2 sold to industry today. The concerns surrounding a few incidents of radon gas, and benzene have played a difficult role at times in sourcing from new and existing cheaper natural (underground wells); however, catalytic oxidation has alleviated part of this problem to be resolved. Radon gas is another story, often unwilling by those who even attempt to quantify safe levels, and adequate means of removal are often undefined. Much of this hardship concerning processing has been driven by consumer perception and acceptance; again most of these complaints stem from soft drink industry complaints, well beyond that which is acceptable in prior gas processing and quality terms.

With respect to QC and today’s expertise, the world’s gas majors, and many minor players in the industry have installed such strict QC standards used to fulfill the demands placed by the major beverage company requirements, that such events should not occur readily again. The so-called food grade product is technically defined and written to meet such (beverage like) standards, however the stringent testing of most food processors on a world scale do not meet with the beverage quality standards, at least technically, as written and performed.

When the term food and beverage grade quality (merchant CO2) product is defined, it is the benchmark of quality, particularly when meeting the stringent testing and related methodology of the soft drink product v. the ‘food grade’ product. The exception to this, is essentially a rare demand for a merchant CO2 would be USP grade; that being what more or less a medical grade product would consist of. USP essentially includes a few strategically located plants set aside by the majors, which involves a methodical record keeping of values surrounding CO2 plant production.

Beyond the liquid segment of the CO2 industry, would be dry ice, as you know. Dry ice in some markets, has become a strong money making niche, if sublimation, distribution, and pricing is handled properly. Over my experience during some 30 years, I felt the dry ice industry was slowly disappearing and converting to liquid cryogens (CO2 and Nitrogen); plus other forms of refrigeration such as more efficient mechanical units. On the other hand, some US gas concerns are aggressively promoting dry ice into existing markets, and essentially growing the business in an impressive way; therefore,  I can only say the long term prospects for dry ice, particularly on a global scale may be far brighter these days. One example of this bright spot has bee the use of so-called ‘rice dry ice’, for blasting for cleaning v. the use of sand blasting, etc. This has been a clean, efficient, environmentally friendly and feasible means of achieving such an end.



With respect to the processing of a raw CO2 gas, there is a relatively wide range of techniques for sourcing from a lean (power plant) flue gas; being the most expensive to produce a viable food and beverage grade product, to the more traditional sources of CO2 from source types such as by-product gas from the production of anhydrous ammonia, ethanol, and (hydrogen) reformer sources. This last ‘group of 3 types’ of raw gas are relatively clean and easy to yield a food and beverage product for industry. Separately, natural wells often have a head pressure in excess of 2,000 psig, so the feed compression component of the process described below would not be needed, thus saving money, and making this source, assuming it is otherwise ‘clean’; however many natural well sources require a catalytic oxidizer. When considering CO2 from fermentation, reformer, and ammonia sources types, a (water) saturated raw stream typically has a CO2 content over 98% by volume, and a raw gas pressure from atmospheric to about 15 psia, and a temperature of about 95 degrees F. Please refer to the typical CO2 liquefaction plant process flow diagram.

For these source types and other traditional types in general, the raw gas enters the plant, and passes through the water knock out (D-O), and then enters a blower (B-1) to boost the pressure to >/+21.6 psia. The gas is then de-superheated in E-1 to about + 105 degrees F with +95 degree ammonia, as the typical refrigerant. Further cooling then takes place to ~ 50 degrees F with +40 degree ammonia. From this point, the CO2 vapor passes through a water knockout vessel (D-1) before being compressed to ~ 325 psia, with a compound screw compressor (C-1). Then the CO2 passes through a high temperature liquid ammonia cooled de-superheater (E-3), and into the water wash column(T-1), where water soluble impurities are removed/diluted, before further purification. The CO2 is then cooled in a refrigerated aftercooler (E-4) to ~ 50 degrees F, using +40 degree ammonia. The water condensate from E-4 is to be handled in the knockout (D-4).The CO2 is then superheated (E-5) to 60 degrees F before the final purification.

In the case where further purification is required, depending upon source types, again assuming the three mentioned relatively simple off gas types, the CO2 would pass through carbon beds (D-5/ A&B), which remove sulfur and HC compounds, which are not removed in the water wash column earlier in the process. The duel carbon beds are at the same time, one absorbing, and the other regenerating. After this process, a dew point should be dried to </= 70 degrees F. in the duel dryer towers D-6/A&B). The same as with the duel absorbers, one dryer is active, while the other is regenerating; and these processes recycle.

After this, the gas stream passes through the reboiler (E-6), and is sent to the CO2 condenser (E-7). The CO2 then travels to the stripper column (T-14), to achieve further purification; vapor off this system travels back to the reboiler. Non condensable constituents are vented to the atmosphere or used for regeneration. What liquid CO2 which flows over the internal weir in the reboiler (E-6) travels through the subcooler (E-9), and then on to the liquid storage tanks.

Typically the ammonia refrigeration system used in such a process is a closed loop system. This system consists of a compound ammonia compressor designed with a sufficient cooling capacity at – 25 degrees F evaporator temperature to condensate and sub-cool the CO2. Intermediate temperature ~ +40 degree F gas is returned to the inter-stage port of the compound compressor. The ammonia vapor is discharged from the compressor to an evaporative or water cooled condenser where it is condensed and drained into a high pressure receiver (D-11) and is elevated to provide refrigerant to supply the thermosyphon loop, which provides high temperature liquid ammonia to cool the compressor oil coolers and the CO2 desuperheaters (E-1 and (E-3). Today, both the CO2 and ammonia compressors are equipped with high efficiency oil coalescers to reduce lubricant carryover to < 1 ppm.


As was summarized in the earlier background section, outside of a methodical approach to record-keeping and quality records for the very small (maybe less than 3-5% of the total merchant market) as dedicated to USP grade; would be the stringent requirements of the soft drink industry. The ISBT (International Society of Beverage Technologists) standard for CO2 is what the major gas firms as well as the soft drink producer’s use as a standard. Other standards include those established by the Compressed Gas Association. Test methodology, purity specifications, and handling of the CO2 product from plant site, on site QA lab, and transfer to trucks; and ultimately testing a the customer’s site and unloading into their storage tanks are all criteria governed under such standards; as well as that which the major beverage firms have adopted as well.

In summary, if an ISBT quality CO2 is produced from a specific plant, this is suitable short of the negligible USP market. Given the complicated and often challenging demands behind distribution of CO2, particularly with a large multinational gas concern with many plants, service to all applications for CO2 are conceivably handled by such a high purity grade product via the same trucking fleet, in order to manage the distribution challenges..

In the rare case where the purely oilfield grade CO2 is delivered with a dedicated fleet of trailers, such a group of trailers cannot be mixed with the ISBT / beverage, food grade product, due to severe contamination results. Such so-called ‘frac’ trailers in the oil and gas patch generally are used for natural gas well stimulation jobs, and are dedicated separately for this job alone, but can also be served by the high quality ISBT CO2. In a few rare cases, so-called industrial or frac plants are simply available for this natural gas well stimulation (frac) market alone.

Purity is the hallmark of quality in the CO2 industry; to be closely followed by reliable and timely service, competitive pricing, and integrity.

About the author: Sam A. Rushing is president of Advanced Cryogenics, Ltd., USA, P.O. Box 419, Tavernier, FL Tel 305 852 2597, [email protected] ; web: . Mr. Rushing is a chemist with vast merchant and consulting expertise, of some 30 years; handling all technical, business, market, process, and applications areas of the industry. Mr. Rushing is a CO2 consultant to all types of projects worldwide.

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