As with many other 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, at temperatures often -40oC. This is a working range within the delivery and some of the bulk plant storage as well. The US sells CO2 in short tons (2,000 lbs 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 hydrocarbons, sulphur 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 quality assurance (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. 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, and today, once again, there are many more stringent quality control (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. Much of this hardship concerning processing has been driven by consumer perception and acceptance; much of which stems from the 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 versus 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 been the use of so-called ‘rice dry ice’, for blasting for cleaning versus the use of sand blasting, etc. This has been a clean, efficient, environmentally friendly and feasible means of achieving such an end.
Production of merchant CO2 – The process
Please refer to the process diagram (figure 1)
for corresponding components during the process discussion ahead.
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, and 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. For example, the feed gas from one of the last three chemical source types would have a (water) saturated raw stream with a CO2 content averaging 98% , a raw gas pressure from atmospheric to about 15 psia, and a temperature of about 35oC. Please refer to the typical CO2 liquefaction plant process flow diagram.
In summary, 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 40oC with 35oC ammonia, as the typical refrigerant. Further cooling then takes place to ~ 10oC with 4.5oC ammonia. From this point, the CO2 vapour 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 ~ 10oC, using +4.5oC ammonia. The water condensate from E-4 is to be handled in the knockout (D-4).The CO2 is then superheated (E-5) to 15.5oC 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 sulphur 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 = 21oC 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; vapour 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 – 32oC evaporator temperature to condensate and sub-cool the CO2. Intermediate temperature ~ +4.5oC 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.
Quality assurance and
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. I am including specification data, typical for beverage grade.
In a subsequent article we can review other forms of CO2 supply which can hold less stringent standards, such as EOR and chemical feedstock grade. Please see the table showing limiting characteristics, and related data.
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.