Managing Trace Contaminants in Cryogenic Air Separation

W.P. Schmidt, K.W. Kovak, W.R. Licht, and S.L. Feldman Air Products and Chemicals, Inc.

7201 Hamilton Blvd. Allentown, PA 18195

Prepared for Presentation at the

12th Intersociety Cryogenic Symposium at 2000 AIChE Spring Meeting

Atlanta, GA March 5-9, 2000 Session T8001

Copyright Authors/Employers

AIChE shall not be responsible for statements or opinions

contained in papers or printed in its publications. ABSTRACT Atmospheric air contains many trace components which are process contaminants or equipment fouling agents in a cryogenic air separation process unit (ASU). Based on the potential problems that they may cause, the trace contaminants are categorized as corrosive, plugging or reactive species. This paper discusses a three step process to manage the trace contaminants as they move through the ASU: identify the hazard, abate the hazard and verify the abatement. Guidelines that contribute to safe and efficient operation of ASU’s are presented and summarized. Focus is placed on the reboiler sump, where high boiling components concentrate in high purity oxygen. The special needs of oxygen vaporizers, where the oxygen is boiled to dryness, are also discussed. Nitrous oxide (N2O) is one of the trace contaminants present in the air and it has received special attention in the past few years. It has a low solubility at cryogenic temperatures, and can be present in sufficient quantities to plug equipment. Various methods used to abate N2O accumulation are discussed, including a novel adsorbent, together with relevant plant operating data. INTRODUCTION Technology for separating air into its primary components (oxygen, nitrogen & argon) by cryogenic distillation has been practiced for over 100 years. A high degree of thermal integration is required for efficient production. Figure 1 is a basic flow diagram for the separation of air by cryogenic distillation. Air is compressed in the Main Air Compressor (MAC) to between 4 and 10 atm. It is then cooled to ambient temperature and passed through the Pre-Purification Unit (PPU). This consists of a pair of vessels containing a fixed bed of adsorbent, typically either or both activated alumina or molecular sieve. As the air passes over the adsorbent, many of the trace contaminants are removed, especially water, carbon dioxide, and the heavy hydrocarbons. The purified air then enters the main heat exchanger, where it is cooled to near its liquefaction temperature (approximately 100°K) before entering the distillation system. The products are produced from the Low Pressure (LP) column (the top column in Figure 1). The high pressure column’s main function is to allow thermal integration by producing the boil-up and reflux for the low pressure column.

Oxygen is the highest boiling of the three main components, so it is taken from the bottom of the low pressure column. Nitrogen is taken from the top of the low pressure column. (Argon splits between the oxygen and nitrogen, and can be recovered as a pure product by adding a third distillation column.) The product streams are warmed to ambient temperature against incoming air to recover the refrigeration. It is also possible to remove the products from the distillation system as liquid if sufficient refrigeration is provided. Liquid may be retained (for back-up or merchant sales). There are two primary configurations of the air separation process (See Figure 1). In the “GOX process”, oxygen is taken as a vapor from the bottom of the low pressure column, and warmed against incoming air. If a high pressure product is needed, this oxygen can be further compressed. As will be discussed in detail later, a liquid purge stream must be taken from the sump of the reboiler, to prevent high boiling components from concentrating above allowable limits. In the “Pumped LOX Process”, the oxygen is taken as a liquid from the bottom of the LP column, pumped to the product pressure, and vaporized against incoming air in the main exchanger. This eliminates the need for product oxygen compression, and the LOX purge stream may be eliminated from the LP column sump, because the product oxygen stream ensures an adequate purge rate. The reboiler/condenser which thermally integrates the distillation system is typically a brazed aluminum heat exchanger. This type of heat exchanger provides a large amount of surface area which increases the plant efficiency by allowing the heat to be transferred with a small temperature difference. Both thermosyphon and downflow reboilers are used for this service. The thermosyphon type is submerged in a pool of liquid oxygen which circulates naturally when heat is provided by way of the condensing nitrogen circuit. The downflow reboiler vaporizes oxygen as it flows downward through the reboiler. The downflow reboiler requires more detailed piping and distribution systems to introduce the liquid oxygen and may require additional equipment for start-up and operation. However, the downflow system allows for higher heat transfer coefficients associated with the vaporization of thin liquid films and hence tighter temperature approaches and a more efficient plant. Most of the problem components in the air separation process boil at temperatures above oxygen. Therefore, these concentrate in the oxygen product at the bottom of the low pressure column. The trace contaminants in the oxygen cause problems ranging from operability issues (fouling, plugging and contamination) to equipment damage and safety incidents, caused by the reaction of hydrocarbon compounds with high purity oxygen. For these reasons, care must be taken in both the design and operation of an air separation unit to prevent the accumulation of trace components. CONCENTRATION OF TRACE COMPONENTS Trace components approach the solubility limit five ways in a cryogenic air separation unit: 1) Increasing the amount of contaminant fed to the system. 2) Lowering the solubility by changing the operating conditions (such as temperature or pressure). 3) “Dry boiling” occurs if heat is applied to a pool or puddle of liquid, to which no more liquid is added. The heat causes

the more volatile components to vaporize , leaving behind the less volatile components in a concentrated form. 4) “Pot-boiling” is similar to dry-boiling, but is distinguished by the continuing addition fresh liquid to the “pot”. Again

the less volatile components are concentrated as the more volatile components are vaporized. 5) In distillation the less volatile components are concentrated in a liquid, as it countercurrently contacts vapor. The less

volatile components of the vapor end up in the liquid. Heat exchangers are vulnerable to all of these mechanisms if care is not taken in design and operation. Process precipitation is prevented by removing compounds (typically in the PPU) that have freezing points greater than the dewpoint of liquid air. Concentration by dry boiling is unavoidable when the purpose of the equipment is to completely vaporize a liquid stream that contains high boiling point contaminants. However, the concentration effect can be limited by ensuring adequate velocities to carry the liquid through the unit to prevent pool boiling or distillation. Instrument lines and “dead-legs” are vulnerable to pool boiling if liquid is allowed to enter and boil without a mechanism to remove the contaminants. These circuits should be periodically flushed with fresh liquid to prevent this accumulation. Distillation is normally used to purify the desired products (oxygen, nitrogen, and argon). The high boiling components of air concentrate along with the oxygen at the bottom of the low pressure column. The degree of concentration of the most non-volatile components is controlled by the liquid oxygen production rate. To prevent the high boiling components from concentrating to unsafe concentrations a liquid purge must ALWAYS be taken from the low pressure column sump, even if

liquid production is not required for any other reason. Equation 1 describes the concentration of high boiling components in the liquid oxygen in terms of a concentration factor (γ).

γ Trace contaminateTrace contaminate, LOX

Trace contaminate, Feed Trace contaminate

xy

≡ =+

FF K F

feed

LOX GOX

(1)

where x & y are mole fractions of the indicated component in liquid and vapor phases, F is the molar flow rate and K = y/x for the indicated component at equilibrium. Taking all of the oxygen product as liquid gives a GOX flow of 0 and a LOX flow of about 20% of the air, so the concentration factor is about 5 for all species. However, when LOX is only produced for purge it is necessary to consider the species separately. The concentration factor for the most common contaminants of air are given in Table 1. MANAGING TRACE CONTAMINANTS The discussion of managing trace contaminants begins with identifying the types of contaminants that plague cryogenic air separation plants. The problem contaminants can be classified as belonging to one or more of the following hazard classifications: Corrosive, Reactive, or Plugging1 (1). Once the species is classified an abatement method is chosen and procedures are developed to verify the success of the abatement procedure. Table 1 classifies the major contaminants of air, along with typical concentrations and concentration factors. Experience has shown that in many cases problem trace contaminants are removed to levels low enough to allow safe, reliable operation of the cryogenic air separation plant if the PPU is designed and operated just to CO2 breakthrough. Table 1 shows the level to which the compound is removed when the PPU is operated to CO2 breakthrough based on standard adsorbent materials. In some cases, special materials have been developed to retain compounds that are not normally retained (see below). For those species which are removed by the PPU, the PPU is the abatement method. According to our philosophy, each abatement method must be accompanied by a mechanism for verifying that the system is working as designed. A continuous CO2 analyzer on the outlet of the PPU verifies that it is functioning properly. Additionally, we periodically switch the CO2 analyzer to the sump of the LP column to ensure that there is no trace CO2 breakthrough. In many plants, this continuous analysis is supplemented by periodic batch samples taken from the liquid oxygen at the bottom of the column. The batch sample is analyzed off-site using instruments capable of providing individual component analysis. This method provides direct verification of the abatement for individual species. Two general rules are followed in the management of trace compounds. First the total hydrocarbon concentration in the bulk liquid oxygen is limited to 450 ppm as “methane equivalent”. This methane equivalency accounts for carbon atoms present in the hydrocarbon molecules, so the limit imposed allows for more methane to be present than heavier organic molecules. This is about 1% of the Lower Explosive Limit (LEL) of hydrocarbons in oxygen, providing a margin of safety for any further concentration in local zones. The second general rule is to limit the concentration of plugging compounds in the bulk liquid or vapor to 50% of their solubility. This allows a margin for uncertainties in flow imbalances, the thermodynamic data, and any other non-idealities. Recently Air Products and Chemicals, Inc. (APCI) has reported that CO2 and N2O form a solid solution (2). This means that the CO2 solubility is lower when N2O is present in appreciable quantities and vice-versa. The computation of solubility must take this in to account, and the operating limits reduced accordingly. A special case on solubility limits is imposed when operating a downflow reboiler. The importance of liquid contaminant concentration in the safe operation of downflow reboilers has only recently been understood, so it is worth discussing further. Testing of both plant and pilot downflow reboilers has shown that N2O and CO2 accumulate to some degree in the passages of a downflow reboiler, even at concentrations several orders of magnitude below the liquid solubility (3). Air Products has done extensive testing which shows that at N2O concentrations typical of liquid plants, N2O accumulation is held to low levels and plant operability is not affected. More important, hydrocarbon accumulation, if any, is at trace levels, so operation is safe. Air Products’ experience is that for downflow reboilers, the accumulation rate is acceptable and safe if the CO2 is less 1 Two additional, non-hazardous classifications could be included as well. These are “Light” components such as helium, hydrogen, neon and carbon monoxide and “Inert” components such as krypton and xenon. Typically, no special measures are needed to manage these compounds. The Light components end up in the nitrogen or waste products. The Inerts end up in the oxygen product, but do not pose any safety or operating hazards. Carbon monoxide and hydrogen can be catalytically oxidized and removed as CO2 and H2O if low levels are required in the nitrogen product. The other species are sometimes separated because they are valuable co-products, but further discussion is outside the scope of this paper.

than 6% of its solubility and N2O is less than 1% of its solubility. Note that in addition to these limits, other specific design and operating features are required for downflow reboilers which are beyond the scope of this paper, including the need for uniform boiling flow, for uniform defrost flow, and for periodic warm defrosts. Methods of achieving removal of N2O to the low levels possibly required for safe operation of downflow reboilers are discuss in more detail later in this paper. Corrosive Compound Abatement. The hazard of corrosive compounds (see Table 1) is that they react with the materials comprising the pressure containment for the process. Over time, the equipment integrity can be compromised, leading to pressure and material release. In addition, many of these corrosive compounds are relatively insoluble in cryogenic liquids, and will precipitate out, leading to equipment plugging. These compounds are abated by removing them before they enter the cryogenic portion of the process. They are primarily removed by the adsorbent in the PPU. However, the acid gases can also be partially removed in a Direct Contact Aftercooler, if the pH of this liquid is controlled within certain limits. Removal is verified by proper operation of the PPU as discussed above. Reactive Compound Abatement. The hazard presented by reactive compounds is that if they reach the LEL, they can react with oxygen in an explosive manner. Even more dangerous is that if the oxygen/hydrocarbon reaction is sufficiently energetic, it can ignite the aluminum equipment in contact with liquid oxygen. The aluminum/oxygen reaction can be extremely violent, and compared with hydrocarbon-oxygen reactions, yields more energy on a weight basis. All hydrocarbons , when present in sufficient quantity and in a concentrated form, pose a hazard in that they can react with liquid oxygen. However, acetylene is a particularly dangerous hydrocarbon. It is only slightly soluble in LOX (8 ppm at 1.4 bara), and if the solubility limit is exceeded, large quantities of solid acetylene can accumulate without being detected by total hydrocarbon analysis. The solid is unstable, and decomposes violently. Ozone is a special case, where experiments have shown that it can increase the reactivity of hydrocarbons in LOX (4). NOx is another special case (5). As air cools, if NOx is present along with conjugated dienes, they will form a sticky gum which precipitates out in the main heat exchanger. If enough of this component collects, it can spontaneously react with the oxygen in air, leading to an explosion. Fortunately, effective and simple methods exist to abate the hazard associated with reactive compounds. Typically, these methods combine removal in the PPU with a purge of liquid oxygen. Acetylene, propylene, and all C4

+ hydrocarbons are removed to very low levels in the PPU. Periodic defrosts are required to remove the small quantities of these compounds that may pass through the PPU and accumulate over years of operation. Other species are at most partially removed by the PPU (see Table 1) and must be removed in the liquid product/purge as discussed in the previous section. Although the abatement methods are simple, they are also extremely important. For this reason it is necessary to verify that the systems are functioning as designed. Proper function of the PPU is verified with a continuous CO2 analyzer on the PPU exit. Adequate purge is verified by direct measurement with a flowmeter, or is inferred by level rise in a product tank. Air Products requires that the purge rate must be at least 0.2% of the air to ensure that all hydrocarbons stay within acceptable levels. Even though the abatement methods are always monitored for proper function, in most cases a measurement of the concentration of hydrocarbons in the liquid oxygen is also justified. The design purge rate is only high enough to remove the expected level of hydrocarbons present in the air. Without directly measuring the hydrocarbon concentration in the purge, there would be no way to tell if the actual inlet concentration was as expected or if the hydrocarbons were building up to dangerous levels. There is an additional requirement for thermosyphon reboilers to prevent excessive internal accumulation of reactive components. It is required that the thermosyphon reboiler be operated at full submergence (liquid at or above the top of the heat exchanger). This gives maximum circulation and minimizes the extent of internal accumulation. The reboiler liquid level is continually monitored with a level transmitter and low level alarm. In addition, full trycocks can be added to verify proper transmitter calibration (1). Plugging Compound Abatement. The plugging compounds can create problems if their concentration exceeds their solubility, in which case a solid precipitates. This solid can plug equipment or piping. In itself, plugging is not a safety concern, but it

may make operating the plant difficult or impossible. However, if the precipitation occurs in such a place as to cause dry boiling or pot boiling of LOX, a hazardous condition is created in which the reactive compounds can concentrate locally. Any exchanger where liquid oxygen is vaporized is susceptible to hazards associated with plugging compounds. The PPU has historically been designed to remove water, NOx (shorthand for NO and NO2) 2, and CO2 from the air feed. N2O, another plugging compound, is only partially removed in the PPU. Proper PPU operation is verified by monitoring the CO2 outlet concentration, since it is the controlling contaminant. Typically, there will be some slip of CO2 and N2O through the PPU, so the system must be designed such that the time averaged concentration out of the PPU is low enough so as not to cause plugging problems anywhere in the system. Most frequently, the limiting point in the coldbox is in the low pressure column sump, as described above. In the Pumped LOX Process, where LOX is taken from the reboiler sump, pumped to pressure, and then vaporized against incoming air, the limit on the maximum allowable CO2 and N2O may be set by the operating conditions in the LOX vaporizer. For a typical time average value of 100 ppb N2O in the coldbox feed, the boiling pressure must be above 2.8 bara for the N2O concentration to be less than 50% of the vapor phase solubility. If N2O is not present, and CO2 is removed to 5 ppb on a time averaged basis, the boiling pressure must be above 2.3 bara for the CO2 concentration to be less than 50% of the vapor phase solubility. For higher CO2 concentrations in the coldbox feed, the boiling pressure must be higher. Summary of Managing Trace Contaminants. In the description above, the approach is to examine each hazard, define procedures to abate the hazard and verify the abatement. As an aid to the reader, the specific actions are summarized below, along with the reason for the action:

ACTION REASON Analyze CO2 at PPU Exit (1) Verify Proper PPU Operation LOX Purge > 0.2% of the air, and it must be measured Ensure no contaminant buildup in the reboiler sump HC Analyzer < 450 ppm as C1 equivalent Ensure no hydrocarbon buildup in the reboiler sump Batch Analysis Verifies each component is present at acceptable levels. Periodic Defrost Remove trace components from low flow areas Full Submergence of Thermosyphon reboiler(1) Ensure adequate circulation to prevent locally high

concentrations

(1) Plant shutdown required after time delay HIGHER N2O REMOVAL REQUIREMENTS Higher N2O removal may be required than is typically available from a standard PPU in the following process applications: • Plants which have a low LOX purge rate (<0.2% of the air). An example of this is a plant producing Krypton and Xenon. • Plants which evaporate oxygen to dryness at low pressure (<2.8 bara), such as a low pressure Pumped LOX plant. • Some downflow reboilers, where N2O can precipitate even when the concentration is well below the solubility limit. (3) In these cases, the N2O to the coldbox must be less than the typical 0.1 to 0.2 ppm from conventional PPU’s. CURRENT N2O REMOVAL TECHNOLOGIES There are three technologies which are currently capable of removing N2O to greater levels. Liquid Phase Adsorbers. With this technology, a liquid stream containing the contaminant, is processed through a fixed bed of silica gel adsorbent. Some hydrocarbons, CO2, and N2O are adsorbed on the silica gel. After a few days of operation, the bed is taken off-line and heated to approximately 150°C, where the impurities are desorbed and vented. It is then cooled to cryogenic temperatures and placed back online. If continuous operation is needed, a pair of adsorbers must be installed. The 2 The removal of NOx over the molecular sieve absorbent is interesting. NO2 is absorbed, and NO reacts with oxygen in the air to form NO2 with the molecular sieve adsorbent acting as a catalyst.

silica gel has a relatively high equilibrium capacity for N2O, but the mass transfer is relatively poor. This results in a very long, diffuse breakthrough curve. There are two ways to install liquid phase adsorbers. In the most straightforward way, a liquid stream is sent through the adsorber, and the N2O is removed. The liquid is either the crude liquid oxygen stream from the high pressure column to the low pressure column, or the product oxygen stream in a Pumped LOX process. The alternative is to use a recirculating system, where a portion of the liquid in the reboiler sump is passed through the liquid phase adsorber and then returned to the reboiler sump. The once-through system can achieve a lower level of N2O in the product stream, but requires larger adsorbent beds. The recirculating system can achieve high fractional N2O removal levels when used in combination with a relatively small liquid purge, but the ultimate purity tends to be higher than for a once-through system. In either system, liquid phase adsorbers are relatively expensive, requiring many equipment items. Each time the adsorber is regenerated, heat leak is added to the process, reducing efficiency. They require a fair amount of operator attention and periodic maintenance Significantly Oversize the PPU. Since the PPU adsorbent has some N2O capacity, it is possible to use enough adsorbent to completely remove the N2O from the feed. However, with conventional PPU adsorbents, this requires approximately doubling the PPU size, which is typically not very attractive. Catalytic Reaction. Reports in the literature allow for the possibility of catalytically reacting N2O to N2 and O2 in the presence of oxygen (6). However, this requires very high operating temperatures and adds an additional unit operation. PPU INNOVATION FOR N2O REMOVAL Since all of the above methods for N2O removal have significant disadvantages, Air Products developed a new PPU adsorbent, which has a much higher N2O capacity. More importantly, it has approximately the same affinity for N2O as it does for CO2. This is typically expressed as selectivity:

α =

xy

xy

N ON O

COCO

22

22

(2)

where α is the selectivity for N2O relative to CO2, yi is the vapor phase mole fraction of component i, and xi is the adsorbed phase mole fraction of component i. For conventional PPU adsorbents, the selectivity for N2O is about 0.4, meaning that the adsorbent has a significantly stronger affinity for CO2 over N2O. The adsorbent developed by Air Products has a selectivity of about 1.0, indicating that CO2 and N2O are adsorbed approximately equally. This new adsorbent is placed within the conventional PPU system, so it requires no new equipment items or changes to the flowsheet. This makes it equally suitable for new plants and retrofits. This adsorbent, for which patents are pending, has been applied on a commercial scale and is now being used in two operating plants. Data from one such plant is shown in Figure 2, along with data from a plant with conventional adsorbents. Figure 2 shows that at normal online times, the conventional adsorbents slip 30 to 50% of the N2O in the feed air into the coldbox. The new adsorbent slips only 5% of the N2O when run to CO2 breakthrough. More importantly, if higher removals are needed, the new adsorbent system requires only a slight reduction in online time to slip only 1% of the N2O, achieving over 99% N2O removal. To approach 5% slip, the conventional systems must be run at 50% of the normal online time, which effectively doubles the PPU size, and even further increases in size are needed to achieve 1% slip. The doubling or more of the PPU is not economic. This new adsorbent is a significant step forward in high N2O removal. It is much easier to implement, requiring no new unit operations nor any more operator attention. The energy efficiency of the ASU is unaffected. While not needed for all cryogenic ASU’s, when it is needed, it is a very effective and attractive alternative to conventional removal methods.

Acknowledgments: Thanks to Chris Raiswell, Fred Taylor, Annemarie Weist, Nasim Malik, Greg Weyrich, Paul Elks, Georgeanna Cleary, and Steve Wieder for their comments and help in obtaining much of the data for this paper, along with many helpful comments and suggestions.

REFERENCES (1) “Safe Operation of Reboilers/Condensers in Air Separation Units”, European Industrial Gases Association, IGC

Document Doc 65/99/EFD. (2) Miller, Edwin, S. Auvil, N. Giles, and G. Wilson, “The Solubility of Nitrous Oxide As a Pure Solute and in Mixtures with

Carbon Dioxide in Air Separation Liquids”, AIChE Meeting, Atlanta, GA, Session T8001, March 5-9, 2000. (3) Houghton, P.A., S. Sunder, A.O. Weist, and T.P. Trexler, “Trace Component Accumulation in Downflow Reboilers”,

AIChE Meeting, Atlanta, GA, Session T8001, March 5-9, 2000. (4) Karwat, E., “Some Aspects of Hydrocarbons in Air Separation Plants”, CEP, Vol. 54, No. 10, Oct 1958, pp. 96-101. (5) Haseba, S. T. Shimose, N, Kubo, T. Kitagawa, “Nitric Oxide Explosion”, CEP, Vol. 62, No. 4, Apr 1966, pp. 92-96. (6) Wenning, U., “Nitrous Oxide in Air Separation Plants”, International Institute of Refrigeration, Munich Meeting on Air

Separation Technology (MUST ’96), 10-12 October, 1996. (7) McKinley, C. and E.S.J. Wang, “Hydrocarbon-Oxygen Systems Solubility”, International Advances in Cryogenic

Engineering, Vol. 4, 1958, pp. 11-25. (8) S. Sunder, D. L. Bennett, D. M. Herron, K. A. Ludwig, E. C. Rogusky, "Boiling process and a heat exchanger for use in

the process", U.S. Patent 5,122,174 , 16 June 1992. (9) S. Sunder, P.A. Houghton, F. J. Riska, W. R. Licht, and M.R. Collyer, "Defrost and liquid distribution for plate-fin heat

exchangers", U.S. Patent 5,730,209, 24 March 1998. (10) Ball, W.L., “Safety in Air and Ammonia Plants”, vol. 8, 1966, pg. 12

Table 1 - Trace Contaminants in Air

Species Class

C=Corrosive R=Reactive P=Plugging

Typical Design Basis for

concentration air[1](ppm)

Typical % removal in

PPU[2]

Concentration factor, γ

(purge flow = 0.2% of air

flow)

Concentration in LOX sump, with

conventional PPU and 0.2% purge (ppm)

Pure Component Solubility in LOX at 1.4 bar[3] (ppm)

CO2 P 400 >99% 430 3[4] 7[5] SO2 C, P 0.1 100% 500 0 N/A HCl C 0.05 100% 490 0 N/A H2O P 10,000 (approx.) 100% 500 0 N/A N2O P 0.3 30-70% 470 40-100[4] 250[5]

NOx (NO+NO2) P, R 0.05 100% 500 0 N/A O3 R 0.2 100% 480 0 N/A

CH4 R 10 0% 16 160 soluble C2H6 R 0.1 0% 490 49 3 x 105 [6] C2H4 R 0.3 50% 350 53 4 x 104 [7] C2H2 R 1.0 100% 140 0 8[7] C3H8 R 0.05 67% 500 8 7 x 104 [8] C3H6 R 0.2 100% 500 0 1 x 104 [8] C4

+ R 1.0 100% 500 0 2 x 102 [9] Notes: [1] This is a typical design basis. For most components, the actual value will be lower. [2] Approximate breakthrough order for conventional PPU’s (first to last): CH4 > C2H6 > NO > C3H8 > N2O > C2H4 > CO2 >> C3H6, NO2, HCl, SO2, C2H2, C4

+ > H2O [3] 1.4 bara is a typical low pressure column sump pressure [4] If both CO2 and N2O are present, their co-solubility in the solid precipitate must be considered (ref. 2). [5] Solid phase solubility from APCI’s proprietary thermodynamic computer program. [6] Second liquid phase solubility (ref. 7). Solid phase does not form. [7] Solid phase solubility (ref. 7) [8] Second liquid phase forms (ref. 7). Other sources imply that ref. (7)’s C3H8 solubility may be high by a factor of

approximately 5 and that C3H6 solubility may be high by a factor of approximately 2. [9] Solid phase solubility for butane (ref. 10)

FIGURE 1 CRYOGENIC AIR SEPARATION PROCESS

FIGURE 2 N2O SLIP TO COLDBOX

0%

10%

20%

30%

40%

50%

0% 20% 40% 60% 80% 100% 120%

% Breakthrough time (1 ppm CO2)

% N

2O S

lip to

Col

dbox

Convetional PPU

PPU with New Adsorbent