Defining the most cost effective
and reliable production and distribution system for nitrogen, oxygen, argon and
other industrial gas products requires optimizing a number of supply system
The basic technology choice between cryogenic or non-cryogenic technology, is largely determined by the number of products that must be supplied (e.g. nitrogen or oxygen or both), the required production rates for each gas and/or liquid product, and required product purities.
The need to accommodate user demand patterns (flow rate fluctuations) leads to additional system optimizations.
Demand for nitrogen or oxygen will always have some degree of fluctuation. Instantaneous demand rates are typically met by a combination of potential product sources:
When gases are used at a number of distinct pressure levels, and each level has its own usage patterns, defining the optimal gas storage and liquid vaporization system can be complex.
Normally, in an optimal on-site gas supply system, the local gas production plant is sized to cost-effectively provide the maximum possible percentage of on-site gas demand, and the amount of liquid vaporization required to support day-to-day operations is minimized.
|Factors favoring or necessitating cryogenic plants:|
|Cryogenic processes are required to generate
liquefied product – either for plant backup or for use in very low temperature applications such as food freezing.
Cryogenic distillation is necessary for producing significant amounts of oxygen at greater than about 96% purity. Argon boils at a temperature very close to oxygen. Separating oxygen plus argon from nitrogen is relatively easy, but that results in 95 to 96% purity oxygen. To make higher purity oxygen requires removing the argon, and the only commercially viable technology for making this separation is distillation.
Cryogenic distillation is also necessary to produce argon. Argon is only 1% of air, so it can only be produced economically as a co-product: usually from of a high purity oxygen plant, but sometimes from an ammonia plant.
Cryogenic separations are most cost effective at higher production rates. Cryogenic processes are often preferred above about 50 tons per day. They are used almost exclusively when gaseous product requirements exceed about 100 tons per day.
The lowest unit cost for products is typically attained when a "piggyback" plant can be installed to meet the needs of a users having upwards of 50 tons per day of nitrogen or oxygen demand, and the user is located in an area which does not have adequate local production of bulk liquid products. "Piggyback" plants serve one or more onsite gas customers while co-producing bulk liquid products which are then delivered to remote liquid nitrogen and/ or liquid oxygen users. These plants offer economies of scale and provide excellent production backup to the onsite user due to the large amount of liquid that will be stored on site to support merchant liquid deliveries.
When merchant liquid production is desirable, several factors often argue in favor of installing liquefiers that have more than the minimum required capacity. On a per-unit-of-production basis, larger plants are more capital efficient and often more energy efficient as well. If power costs in a particular area are lower than those in surrounding areas, supplying customers in outlying areas can sometimes be done more economically with product made in the low-cost area; as the production cost savings can more than offset increased transportation costs.
LIN-assist plants (also known as LIN-injection plants) are a special type of cryogenic nitrogen plant that uses "imported" refrigeration derived by vaporization of a small amount of purchased LIN to drive the cryogenic separation process. Because this type of plant has no mechanical refrigeration components, their capital cost is reduced versus a "complete" cryogenic plant. They are particularly cost effective for applications where users require high-purity nitrogen but have relatively low usage rates (less than about 30 tpd). They are most likely to have favorable economics when demand is relatively steady and close to the plant's maximum design capacity.
|Factors affecting selection of non-cryogenic plants:|
|Non-cryogenic processes are most likely to be economical when it is not necessary to produce very high purity product (e.g.
when oxygen of 90 to 95% purity, or nitrogen that is 95 to 99.5% oxygen-free is acceptable).
Non-cryogenic processes are capable of producing relatively high purity nitrogen (up to 99.9%) but capital and operating costs go up with purity, and climb rapidly above 99.5%. In some cases it can be cost effective to make 99.9% or higher purity nitrogen by first making 99.5% purity nitrogen in a PSA and then using a de-oxo unit to eliminate the residual oxygen.
Non-cryogenic oxygen purity is generally produced at less than 95%, with 90 - 93% the most common purity target.
Non-cryogenic separation systems are generally most cost-effective at relatively low production rates. Individual non-cryogenic units rarely produce much more than 60 tons per day. Some locations use multiple non-cryogenic units to attain higher total production rates (e.g. 120 tons per day). This can be an attractive production strategy when demand varies widely, but in discrete steps, as in a facility with multiple oxygen-enriched furnaces.
|Required delivery pressure affects selection of the optimal plant:|
Lower pressure product is less expensive in almost all plants. Do not over-specify the required
Different process cycles can produce gaseous oxygen or nitrogen at quite different pressures - from just above atmospheric pressure to about 100 psig (9 barg) - without a product compressor. It is worth investigating the trade-offs among capital cost, process simplicity and operating cost for various configurations.
Oxygen compression equipment can be expensive. Plants using cycles that produce product at the required delivery pressure (e.g. an oxygen PSA or pumped LOX cycle vs. a VSA or conventional low product pressure processes) may be a better choice than those that require a product compressor.
|Summary of available technologies and regions of applicability:|
|The following charts present some general relationships applicable to selection of new plants producing oxygen and/or nitrogen.|
|Demand fluctuations affect plant sizing and may necessitate additional equipment:|
plant “load following” is difficult and energy intensive when changes are frequent, rapid
or of large magnitude.
Moderate demand fluctuations may be handled with "line packing" and/ or a gas receiver (a.k.a. an accumulator or buffer vessel) that allows accumulation of temporarily excess product at somewhat elevated pressure and its subsequent release when demand increases. Accumulators can be simple in-line devices, but are more often coupled with a throttling (and venting) or on/off control scheme that keeps the receiver vessel and the distribution pipeline pressures within predetermined ranges. The more severe the expected demand swings, the more useable storage capacity must be provided in the accumulator. Higher storage capacity is achieved by using a larger vessel or increasing the peak storage pressure (or both).
Demand fluctuations that cannot be handled with production rate load following and accumulator action are usually accommodated by adding vaporized liquid product to the distribution system when demand increases quickly or significantly and by venting temporarily excess production when demand drops rapidly or severely.
Some plants have usage patterns that exhibit a relatively constant base load with severe intermittent demand peaks. The most cost effective approach with this type of load is to use vaporized liquid (usually from an outside source) to meet the peaks while the base load is supplied by an onsite production plant. This allows the local production facility to operate at an efficient, trouble-free, close-to-constant rate while minimizing the use of relatively expensive liquid.
|Effect of plant location on backup liquid storage requirements:|
|Customers located in
heavily industrialized areas may need only small amounts of on site liquid storage
capacity since trucked in liquid
for back up to the plant can be obtained relatively quickly and economically.
Customers in remote locations may need to install redundant production components (e.g. dual air compressors) and more liquid storage (larger storage tanks and/ or multiple storage tanks). These customers will often prefer an onsite plant that is capable of producing 2 to 10% of its separation capacity as liquid to allow maintenance and rebuilding of backup storage levels with local liquid production.
Universal Industrial Gases, Inc.
Universal Cryo Gas, LLC
3001 Emrick Blvd., Suite 320
Bethlehem, Pennsylvania 18020 USA
Phone (610) 559-7967 Fax (610) 515-0945
All material contained herein Copyright 2003 / 2017 UIG.