Acknowledgement: Paper presented by Prof. K. Chowdhury Cho wdhury at the 21st National Seminar on Industrial Gases, 29th-30th January 1999, Bangalore, Bangalo re, India.
ADVANCES IN CRYOGENIC AIR SEPARATION Kanchan Chowdhury Chowdhury Associate Professor Cryogenic Engineering Centre Indian Institute of Technology, Technology, Kharagpur Kharagpur 721 302(WB) ABSTRACT The use of atmospheric gases like oxygen, nitrogen, argon and rare gases has steadily increased during this century in a variety of commercial, defence, nuclear, space and power applications. Keeping in tune with the technological advancement in other areas, air separation technology too has improved in efficiency through improvement of process equipment and cycles as well as through the assimilation of advanced technology in other areas. Attempt has been made in this paper to highlight h ighlight some of the areas in air a ir separation technology, particularly in process cycles, where the advancement has taken place. After improving component efficiency to a large extent, the air separation industry is trying hard to reduce the power cost by decreasing the losses due to the Second Law irreversibility in heat and mass transfer. The constructions of large air separation plants have been made possible by extensive instrumentation and computer control. Some recent patents show that this industry is truly following the dictum "million drops make an ocean" and inching towards the reduction of energy at every level.
1.
INTRODUCTION
Air comprises of the constituents whose volume percentages and boiling points are given below[1]: Table 1. Composition of air and boiling points of components Gas
Volume or mole %
ppm
Boiling point (K)
N2 - Nitrogen
78.11
---
77.36
O2 -Oxygen
20.95
---
90.18
Ar - Argon
0.934
---
87.28
Ne -Neon
---
18.2
27.09
He -Helium
---
5.2
4.21
Kr -Krypton
---
1.14
119.83
Xe - Xenon
---
0.086
165.0
H2 - Hydrogen
---
0.5
20.3
N2O Nitrous --oxide Oxide
0.5
183.7
CO monoxide
Carbon ---
~0.1
81.6
SO2 dioxide
Sulphur ---
~0.04
263.2
---
~0.02
161.3
O3 - Ozone
Although pressure swing adsorption process and membrane separation process are increasingly being used for the production of atmospheric gases, cryogenic separation is likely to remain as the "workhorse" of the air separation industry in the foreseeable future, due largely to its flexibility in terms of purity, flow rate, pressure and state (liquid or gas) of the products. In the field of bulk production of cryogenic fluids and atmospheric gases, the cost of production remains the major factor in the spread of its use in a variety of application areas. Although, air separation technology has developed into a mature technology by 1935, it was around 1955 when the real need for the improvement of air separation technology was felt. The reason behind this need lay in the increasing use of oxygen in steel making, particularly the invention of L-D process in Austria. Even today, by the end of the century, after about 90 years since Prof. Carl von Linde introduced his concept of double column, technologists around the world are working towards the overall improvement of the Air Separation Technology through invention of new processes and assimilation of the developments in other related technologies. These efforts have lead to cheaper methods of production of atmospheric gases. As fibrereinforced plastics and aluminium are continuously replacing steel as manufacturing and construction materials, the cost of oxygen, which is supplied across the fence to the steel plants, need to be reduced continuously in order to enable steel to be produced at a cheaper price. Reduced power consumption enables the lower production costs and the resultant lower cost products can be used to stimulate the market growth [2]. Improvement of the efficiency of an air a ir separation plant, which forms an important objective of the industrial gas companies, can be achieved by development of more advanced and efficient equipment and optimisation of design of process cycles.
2.
CRYOGENIC DISTILLATION OF AIR
Cryogenic separation of air is done in a distillation column following the same principle as in case of petroleum distillation. Crude petroleum is available in liquid form and a small part is vaporised by application of heat to create both vapour and liquid in the distillation column. The principle of separation is based on the differences of boiling points of different constituents in crude petroleum. In case of cryogenic air separation, however, the input air is available in the form of gas and one has to liquefy a small part of it to effect distillation. Table 1 shows the differences of boiling points of different constituents of air. Therefore, while petroleum distillation involves boiling of liquid, distillation of air involves the refrigeration and O liquefaction. While petroleum distillation is accomplished at a temperature 330 C higher than O that of ambient, cryogenic distillation involves a temperature of about 230 C below the ambient. Therefore, while heat has to be protected from being leaked out to atmosphere in case of petroleum distillation, the same has to be prevented from leaking in case of cryogenic distillation. Therefore, besides distillation columns, most of the equipment involved in air separation are dedicated to liquefaction processes which involves compressors, equipment for separation of moisture, CO2, hydrocarbons, heat exchangers, expansion devices, cryogenic pumps etc. The block diagram in Fig 1 depicts the processes involved in cryogenic air separation. 2.1
Process of compression
Compression process involves air filters, compressors, intercoolers and aftercoolers. There has been a lot of efforts by compressor manufacturers to improve the efficiency of compression during the past half a century. Moreover, with the improvement of air separation processes and increase of the plant-size, it has been possible to shift from the high pressure low capacity reciprocating compressors to the low pressure high capacity centrifugal compressors. The latter is inherently more efficient. A lot of improvement has been achieved in the performance of intercoolers and aftercoolers. Steep rise of cost of energy around the globe since 1970's oil crisis has motivated many companies to make more capital investment in intercoolers and aftercoolers resulting in a steady decline in the operating cost of compression process.
2.2
Pre-purification processes
Reversing regenerators were used for separation of moisture and CO2 till about 1960. The process involved using two alternating regenerators filled with high heat capacity stones which, in the steady state, acquired a temperature gradient of 300 K to 80 K. During one cycle, one regenerator is used to cool and purify the incoming stream while the other regenerator was used to warm the outgoing stream and simultaneously re-evaporate and remove the frozen impurities those were deposited during the cool down cycle.
First used around 1955, reversing heat exchangers, which are brazed aluminium heat exchangers, were used to remove CO2 and moisture from the air apart from the primary job of exchanging heat between the warm incoming air and the cold separated products. In around 1960, a plant of 300 TPD (tons per day) of oxygen used 20 cores in parallel of 0.43 m x 0.53 m x 2.7m long. By 1985, the size of brazed aluminium core reached around 1.2m x 1.3m x 6 m long [3]. A typical brazed aluminium heat exchanger is shown in Figure 2. With the appearance of molecular sieve in the market it has become possible to remove moisture O and CO2 by adsorption near ambient temperature (5 C). While the reversing heat exchangers were subject to failure by fatigue, by water freezing and by corrosion, with the front end purification, the operational reliability and down-time of the plant have considerably improved, particularly when the plants are located in heavily polluted areas. There have been continuous efforts by industry to increase the adsorption temperature (so as to decrease the load on refrigeration system), to increase its mechanical strength (so as to decrease its tendency to breakdown into dust) and to decrease the reactivation temperature (so as to decrease the energy consumption in regeneration heating and later cooling). The recently used molecular sieves are also capable of removing hydrocarbons, thus making it unnecessary to use silica gel adsorption of hydrocarbon from rich liquid and liquid oxygen. 2.3
Refrigeration systems
Refrigeration in an air separation plant is done for two reasons: (1) reduction of air temperature to facilitate front end purification and (2) to pre-cool air or nitrogen while they are being cooled to cryogenic temperature or are liquefied. With the advancement of the efficiency of compression, refrigeration and heat exchanger technologies, the energy requirement of refrigeration has decreased over the last few decades. Further, the recent air separation plants have made all out efforts to utilise evaporative cooling in a packed-bed direct contact heat exchanger utilizing bone-dry waste nitrogen and using the chilled water to cool the air before front end purification. This is a big energy saving device. 2.4
Heat exchangers
The heat exchanger played the most crucial role in the establishment of commercial cryogenics and bulk production of oxygen, when in 1902, Brin's Oxygen Company (the forerunner of the British Oxygen Company) acquired the sole rights to Dr. Hampson's patent on coiled tube heat exchanger[4]. This heat exchanger served as a main heat exchanger in air liquefaction for more than half a century. The shell and tube configuration used to be used as the condenser-reboiler. Both these applications of heat exchangers have now been replaced by plate-fin-heat exchangers (PFHE), shown in figure 2. PFHEs have the advantages over other forms of heat exchangers, such as: 1) Very close Temperature approaches and high thermal effectiveness. PFHE can O O achieve temperature approaches as low as 1 C between single phase streams and 3 C between multiphase streams. 2) Large heat transfer surface per unit volume is available in PFHE . About 2 2 1000 m heat transfer surface area per cubic meter volume is available compared with 300 m per m3 in a conventional shell and tube heat exchangers. 3) low weight per unit heat transfer. For a given volume the PFHE weighs one-third compared to the conventional heat exchangers. 4) The possibility of heat exchange between many process streams has made PFHE very welcome addition as a vital equipment for air separation plant. However, PFHE is very expensive and because of its small flow passages blockage may arise from solids carried by process streams. The use of plate-fin heat exchangers has greatly improved the efficiency of air separation plants. The improvement of efficiency of an air separation plant due solely to the improvement of heat exchangers occurs primarily for three reasons[3]: 1. A closer temperature approach at the outlet of the PFHE(when used as the main heat exchanger) minimises the cold loss. 2. Lower pressure drop in the PFHE (when used as the main heat exchanger) results in a lower pressure in the low pressure column, which meant a lower saturation pressure and consequently a lower saturation (evaporation) temperature in the reboiler in the condenser-reboiler. This results in the fall of required pressure at the high pressure column and reduces the compressor discharge pressure. 3. Availability of higher surface with the PFHE (when used as condenser-reboiler) area leads to a better heat exchange in the condenser-reboiler, which results in a lower ∆ T. This results in the requirement of a lower saturation (condensation) temperature required in the condenser side of the condenser-reboiler, which means a lower pressure requirement in the high pressure column and a lower discharge pressure from the compressor. Due to the above-mentioned advantages, the last few decades have seen the feed compressor
pressure decreasing from 7 bar(abs) to 5 bar(abs), which has resulted in a large improvement of plant efficiency.
3.
PROCESS CYCLES FOR CRYOGENIC AIR SEPARATION
At this point, we discuss some of the conventional air separation plants, which have been sold in the market during the last few decades. Fig 3 shows a high pressure air separation plant producing LN2, LOX and liquid argon. These liquid plants, built around 1965-1970 used "high pressure" (140bar) reciprocating air compressor, expansion engine and liquefied waste nitrogen recycling. WN2 recycling helped in a good argon recovery. There has been plants built around 1975, based on medium pressure (40 to 50 bara) which used (either or both) recycling of liquefied low pressure pure nitrogen and medium pressure pure nitrogen (Fig. 4). By this time, reliability of expansion turbines had largely been improved. Compared with the high pressure plants, the medium pressure plants 3 brought down the power consumption from about 1.7 units to about 1.5 units per Nm of LOX. With the development of high efficiency low pressure turbines, the compressor pressure has been successfully brought down to the bottom column pressure at about 6 bara (1990 decade) (Fig. 5), which are called "low pressure plants". These plants can produce liquid with a specific power consumption of about 1.3 units. During these 20 years (1970 to 1990) there have been large improvements in the process equipment, which are primarily confined to compressor efficiency, heat exchanger efficiency, quality of molecular sieve and turbine efficiency. But the most impressive development of the 80's decade has been the development of structured packing by Sulzer for the distillation columns[5, 6]. Fig 6 shows a primarily gaseous O2 plant, which uses structured packing for the columns. The reduction of price, easier availability and enhancement of performance-related knowledge of the designers have lead to the widespread use of structured packing in this decade (1990), which has resulted in the elimination of H2 dosification for the production of pure argon. Thanks to the use of structured packing, argon can now be produced at 5 ppm impurity without going through the expenditure and hazards of using H2.
3.1
Structured Packing
The use of structured packing instead of trays for the distillation of air provides a substantial reduction in the power required to drive the air compressor. Packing has a pressure drop which is about 10% to 20% that of trays. Fig 7 shows how the use of structured packing can reduce the feed compressor discharge pressure to 5.65 bara from 6.33 bara [7]. The resultant saving in power can be substantial. The distillation equipment is a mass transfer equipment required to produce a degree of mass transfer from liquid to vapor and reverse, which is achieved in a plate column by bubbling the vapor through a pool of liquid a number of times. Packed columns achieve this mass transfer by countercurrent flow of the liquid and vapor stress. Packing is a material of high surface area per unit volume and of high voidage that is used to fill the distillation column. Liquid runs the down the surface and vapor flows through the voids and mass transfer occurs at the liquid interface. Structured packing has a regular structure, as opposed to the randomness of dumped packing and it is supplied in section that fit snugly, directly into the column. In spite of its recent cost reduction, structured packing still costs more than trays. The additional cost of packing over trays has to be offset against the power savings and additional argon recovery. For structured packing, HETP or height equivalent to a theoretical plate is defined as the height difference between the points in a packed column where the gas composition is in equilibrium with the liquid composition. The salient points highlighting the advantage of structured packing over the plate is given below: 1. The pressure at the air compressor The pressure at the air compressor outlet is dictated by the pressure required to condense nitrogen in the condenser-reboiler within the main rectification columns. For every 0.1 bar drop in pressure on the reboiler side (upper column), there is approximately 0.3 bar advantage (reduction) in the pressure on the condenser side (lower column). Therefore, it makes sense to use structured packing in the upper column. 2. Turndown flexibility Two factors which determine the flexibility of load variation or the operating range in a plate column, has been dealt in detailed by Latimer [1]. These are: entrainment-cum-
flooding and weeping. At higher vapor rate, liquid droplets are entrained in the vapor stream and are returned to the plate above. This phenomenon is called liquid entrainment. It has a completely negative effect on the separation efficiency, because the enriched liquid is simply mixed back again with the leaner liquid. The reduction in plate efficiency is a function of the liquid entrained and beyond a few percent entrainment, there is a natural fall in efficiency. Further entrainment causes flooding. The operating range of the column is generally 80% of the physical flooding point because of the reduced plate efficiency. At low vapor rates, with most tray designs, the liquid cannot be supported on the plates and "weeping" occurs, whereby some of the liquid passes through the plates, bypassing vapor liquid contact and causing vapor instabilities. This causes a reduction in the overall plate efficiency, giving a minimum useful operating point for a plate [6]. Structured packing suffer from problems like flow maldistribution and inadequate wetting. Maldistribution occurs when the packing is large compared to the column diameter. At low liquid rates, packing may suffer from inadequate wetting. Nevertheless, it is claimed that structural packing, with the flexibility of operating at 30% to 110% of design, can be used in an ASU to run it at a very low partial load [5]. 3. Rapid Response to change Use of structured packing contributes to another factor of significant importance to the operation of the plant. The small hold-up in the packed column helps a plant with packed columns to have a better load-following characteristics and responds quickly to a change in oxygen and nitrogen demand. 4. Better recovery of argon Because of low absolute pressures which are obtained in both the upper and lower columns, the vapor-liquid equilibrium relationships become more favourable and the recovery of argon is increased. 3.2
Improvements in Process Cycles
The key to improvement in the efficiency of process cycles lies in the reduction of losses. These losses are of two kinds: 1) The loss due to First Law Of Thermodynamics and 2) The loss due to Second Law of Thermodynamics. 1) The first law of thermodynamics refers to the loss due to transfer of heat. For example, heat inleak into the system is a loss of First Law of Thermodynamics. Similarly, the exit temperature loss in the main heat exchanger causes a loss of cold (which had been produced after spending energy) and results in a loss of plant efficiency, which is of First Law of Thermodynamics in nature. Attempts have been made during the last decade to reduce these losses through the application of high efficiency heat exchangers and improved insulating materials. It is, however, very natural to reach a point of diminishing returns, when large capital investments in heat
exchange and insulation systems are not likely to achieve a corresponding reduction in the operating costs. It is very important to mention here that this decision of diminishing return is a function of the prevailing cost of energy and, therefore, changes with time. For example, the pressure drop in a pipeline carrying liquid or gas, which were acceptable in 1960, should no more be acceptable today and there is nothing called a permanent design formula. Therefore, standing at the end of twentieth century, it is very easy to conclude that efficiency improvements in process equipment and the resultant reduction of First Law losses have almost reached a plateau. 2) During the last two decades there has been a continuous quest to reduce the losses due to Second Law of thermodynamics. Thermodynamic losses occur throughout the air separation plants. By understanding the nature of these losses, they can be minimised, resulting in increased overall efficiency of the process cycle[2]. Losses can be divided into 3 categories described below: a. Irreversible Expansion or Compression: When a gas flows though a pipe, there will be some pressure drop due to friction. Although the pressure falls, there is no recovery of energy and so the work, which was spent in increasing the pressure, is irreversibly lost. Similar losses also occur in machinery such as compressors and expanders. The minimum work required to compress a fluid from P1 to P2 isothermally is: WORK = R TO ln P2 / P1
…. …. …. ( eqn. 1)
b. Irreversible Heat Transfer: In heat exchangers, heat flows from a warm stream to a colder stream. This exchange takes place across a temperature difference of ∆ T = Tw − Tc Since energy would be required to transfer the heat back to the warm stream, this heat transfer represents a loss. Similar losses also occur in distillation where warm streams mix with colder streams on a tray. A reversible heat pump transfers heat Q from a temperature Tc to Tw, using: WORK = Q (Tw-Tc) / Tc
…. …. …. ( eqn. 2)
c. Irreversible Mass Transfer: In air separation plants, a waste N2 stream is produced and vented to atmosphere. The mixing of N2 with the air is a loss, since energy is required to recover the N2 stream from air. In addition to process mixtures such as this loss, losses can also occur when streams, which are not in thermodynamic equilibrium, are mixed in distillation trays. In a reversible separation device, using the perfectly selective properties of an ideal membrane to separate a gas into its components, the work required is given by the expression: WORK = R TO Σ xi ln xi ,
…. …. …. ( eqn. 3)
where xi's are the mole fraction of the separated components. Timmerhaus[8] has given the work involved in reversibly separating a specified gaseous component from other components in air at 300K (Table 2 )
Table 2 : Work needed for reversible separation
Gas
Mole %
Work per mole of Work per unit mass mixture (KJ/kgmol) of separated gas (KJ/kg)
Nitrogen
78.084
1311.6
60.0
Oxygen
20.946
1280.2
191.0
Argon
0.934
132.1
353.9
Carbon dioxide
0.033
7.42
511.1
Neon
1.818x10
0.54
1472.5
Krypton
1.14x10
0.0404
438.1
Xenon
8.6x10
-6
0.0037
328.1
Hydrogen deuteride
3.12x10
-2
7.06
7485
Deuterium
1.56x10
-2
3.80
6090
Helium
1.0x10
0.00427
14,220
-3
-4
-5
For a modern plant, the specific power needed to produce the gaseous O2 is of the order of 0.6 3 3 KWh/Nm , while for liquid O2 it is of the order of 1.3 KWh/Nm . It may be worthwhile to explain at this stage where this specific power is exactly utilized or lost in order to understand the relevance of present improvements in the process cycles that are taking place today. Figure 8 shows the energy consumed in producing gas at atmospheric pressure, pressurised gas and liquids. When air is separated into gaseous O2 and other constituents, it has to undergo the following processes with the corresponding energy consumption. 1) Work of separation and 2) Make-up work for heat inleak and heat exchanger losses. The latter can be accounted as de facto liquefaction. (3) For producing pressurised gases, work is to be done in compressor or in liquid pump. (4) Work is required for the production of liquid in an air separation plant.
By working out from the chart given in Table 2 for ideal separation work, the ideal power requirement for separating O2 works out to be 0.08 KWh/Nm3. However, column pressure drop, finite temperature difference and mixing of fluids with different compositions cause the deviation from ideality and thermodynamic efficiency of separation may be close to 0.25 to 0.30. Therefore the energy requirement for separation alone works out to be 0.08/0.25 = 0.32 3 KWh/Nm . If insulation loses and heat exchanger losses are taken equivalent to about 6 % liquefaction of the 3 feed air, it means a refrigeration load of 0.027 KWh/Nm . The ideal work requirement to get this refrigeration, according to equation (2), is 0.027 X (310K-77K) / 77K ≈ 0.081 units. If the thermodynamic efficiency of the actual refrigeration cycle is 30% of the ideal (Carnot Cycle), the actual power requirement is 0.081 / 0.3 = 0.27 units. So together, the specific power required 3 for producing gaseous O2 works out to be 0.32 + 0.27 = 0.59 KWh/Nm . In order to get a liquid product we have to add to it the power required for liquefaction. The heat exchanger does not have to handle oxygen and therefore the heat exchanger losses will decrease. Assuming that 18% 3 . O2 is liquefied, the power required will be 0.32 + 0.18 + 0.8 =1.3 KWh/Nm The recent improvements in process cycle attempt to improve the work of separation, which has been discussed in this paper with some detail. 3.2.1
Reduction of Separation Work
In order to reduce the net work consumption in a binary distillation, it is necessary to reduce the driving forces for heat and mass transfer within the individual stages. This reduces to a problem of making the operating and equilibrium curves nearly coincident. The point is illustrated in Fig.9. Fig 9(a) represents an ordinary distillation when the reflux is substantially greater than minimum. The driving forces for heat and mass transfer between the streams entering a stage can be reduced by moving the operating lines closer to the equilibrium curve. The minimum reflux condition shown in Fig 9(b) corresponds to the upper and lower operating lines having been moved as close as possible to the equilibrium curve. It can be seen that the
number of required theoretical plates has increased, which will increase the pressure drop in the distillation column. Therefore, the advantage in the decrease of the separation work required because of the decrease in driving forces for heat and mass transfer will somewhat will be offset by the pressure drop in the column. (It is exactly for this reason that structured packing gives higher efficiency, because increase of number of plates does not necessarily increase the column pressure drop). Even at minimum reflux, there are still substantial driving forces for heat and mass transfer at compositions in the tower removed from the feed stage in a binary distillation. Using different operating lines in those portions of the column, where the irreversibilities with the original operating lines were more severe, can reduce these irreversibilities. Such a situation is shown in Fig. 9(c), where there are two operating lines applied to different parts of stripping section and two operating lines belonging to the rectifying section [9]. The situation shown in Fig.9(c) is shown schematically in Fig.10. This knowledge of process improvement using
intermediate condenser and intermediate reboiler has been implemented as shown in Fig.11(a) and Fig. 11(b) [10]. In Fig. 11(a) and 11(b), the improvements in the high efficiency process cycles are obtained by providing intermediate height reflux to all three rectifying section of the high purity oxygen plant: the high-pressure column, the low pressure column and the argon rectifying section. An intermediate height reflux is defined as a reflux , which has a zone of counter-current vapourliquid rectification both above and below the point of introduction. In Fig 11(a), the argon sidearm intermediate height reflux is provided by evaporating partially depressurized liquid
nitrogen from the high-pressure column. The argon recovery increases to 80% and pressurised nitrogen pressure increases from 3.4 bara to 5.3 bara and the quantity increased from 5% to 11%. The primary difference between Fig
11(b) from Fig 11(a) is that the oxygen product is evaporated from the condensing nitrogen versus air i.e. at a lower pressure. Argon recovery is also marginally higher owing to the two sequential refluxes from boiling kettle liquid [10] The ultimate of reducing thermodynamic irreversibilities within a distillation column would be to introduce reflux to all the stages above the feed and to reboil at all the stages below the feed. In this way, the operating line at each stage will almost be coincident with the equilibrium curve as shown in Fig.9(d). A schematic diagram of a device for carrying out such a process is shown in Fig 12 [9]. There is no doubt that a considerable capital outlay is involved in altering an ordinary distillation to bring the process closer to reversible distillation. The number of stages required for a given separation becomes greater and the required heat duty will be split up between the terminal reboiler, terminal condenser and those reboilers and condensers that are necessary to generate intermediate boil up and intermediate reflux. Offsetting this need for considerable additional capital investment are two factors: 1)
The heat energy used in the distillation is degraded to a lesser extent. Much of the
reboiled heat are added at temperatures lower than the bottom temperature and much of the heat removal can be effected at temperatures warmer than the overhead temperature. These decrease the Second Law loss and improve the thermodynamic efficiency of the distillation column. 2) The reduced vapor and liquid flows towards the product ends may make it possible to use towers of different diameters at different stages. This may improve the capacity control of the distillation column. 3.2.2
Ultra High Purity Oxygen Production
One of the recent trends of air separation plant cycle is dedicated to the production of ultra high purity oxygen. Although ppm impurity concentration level is very common now a day, it has become a thing of the past for the semiconductor industry [11]. The acceptable impurity level has already dropped below 10 ppb (parts per billion) and likely to touch ppt (parts per trillion) level in the near future. This calls for removal of heavier hydrocarbons in distillation column. Air contains acetylene (0.1 to 1 ppm), ethylene (0.01 to 2 ppm), propylene (0 to 0.2 ppm) and other heavier hydrocarbons. Although these are adsorbed on molecular sieve bed, some ethane (0.02 to 0.1 ppm), propane(0 to 0.1 ppm) and methane (2.0 to 10 ppm) flow with air to the cryogenic feed process. Other heavier contaminants are krypton ( ≅ 1.1ppm), xenon (≅ 0.08ppm) and nitrous oxide(0.01 to 0.05 ppm). The conventional standard grade oxygen is composed of about 99.7% O2, 0.3% Ar, 10 ppm methane, 0.5ppm other hydrocarbons, 5 ppm krypton, 0.4 ppm Xenon, 0.1 ppm nitrous oxide and no nitrogen. Fig 13 is a schematic diagram for conventional process for gaseous O2 production. Fig 14 shows the concentration profile for O2 and methane in the liquid phase in the upper column for a conventional oxygen plant of figure 13. It can be easily seen from Fig.14 that methane concentration reduces to almost nil at a few plates above crude O2 feed location. Fig 15 and Fig 16 are two suggested air separation cycles from where ultra high purity O2 can be produced.
3.2.3
Some Recent Patents
(i)
Improving Condenser-Reboiler efficiency by Removing Hydrocarbons
A recent patent by Honda and Kishida[12] describes a process (Fig. 17) where a reduction power cost is effected by removing hydrocarbon in a cleaning section at a lower position in the low pressure rectification column. The development relates to an air liquefaction separation apparatus where a low pressure column of a double rectification column has a cleaning section at a lower position and a liquid oxygen is partly withdrawn from the space above the cleaning section so as to supply it to a main condenser-evaporator. The liquid oxygen supplied to the main condenserevaporator is subjected to heat exchange with a nitrogen gas separated at the head of a high-pressure column and gasified into an oxygen gas. This oxygen gas is introduced to the space under the cleaning section. Hydrocarbons contained in the oxygen gas ascending through the cleaning section are washed down by the reflux liquid oxygen descending though the cleaning section to provide a clean oxygen gas. The liquid oxygen passed through the cleaning section is withdrawn from the low-pressure column. Thus, since the hydrocarbons are prevented from being concentrated highly to or over critical levels in the liquid oxygen in the main condenser-evaporator, submergence in the main condenser-evaporator can be reduced to minimize the influence of the depth of the liquid, thus improving the efficiency of the condenser-reboiler. This will lower the pressure of the nitrogen gas and reduce the operating cost by reducing power of the compressor for compressing the feed air.
(ii) Variable Production Rate of LOX And LIN from the Same Plant A very useful patent has been taken by R. Agarwal et al.[13] where variable rate of production of liquid O2 and liquid N2 can be obtained from a single plant. This is likely to be a useful concept in case of fluctuating demand at a particular location (Fig. 18). The air separation plant uses a liquefier and a two stage distillation column capable of operating in two modes, namely a first mode of operation during which only liquid nitrogen is produced and a second mode of operation during which liquid nitrogen and liquid oxygen are produced. By adjusting the duration of operation in each mode, any ratio of liquid nitrogen to liquid oxygen greater than the ratio achieved during the second mode of operation can be achieved. In the first mode of operation, a condenser is used to condense the lower-pressure-stage gaseous nitrogen into lower-pressurestage nitrogen condensate. To condense the lower-pressurestage nitrogen, either at least a portion of the crude oxygen liquid from the higher-pressure-stage, a least a portion of the oxygen-enriched liquid from the lower-pressure-stage, at least a portion of the liquefied air, or mixture thereof, are introduced to the condenser. In the second mode of operation, the top condenser is not used. Instead, all of the crude oxygen liquid is introduced into the lower pressure stage, which produces a bottom liquid oxygen stream and a low pressure overhead waste stream containing nitrogen. The system includes fluid flow lines and valves for directing the flow of certain fluids, particularly the crude oxygen liquid and the oxygen enriched liquid, during the two modes of operation.
iii. Decreasing Separation Work A double column cryogenic rectification system for producing lower purity oxygen wherein a minor portion of the feed air is successively condensed in two vertically oriented stages within the lower pressure column before undergoing rectification. This concept is primarily aimed at reducing the work of separation (Fig. 19)[14].
4.
IMPROVEMENT IN RELATED TECHNOLOGICAL AREAS
No industry can be insulated from the technical changes which take place in other related areas and air separation industry is no exception. It has gained from the technical advancement of materials, production technology, transportation and, more importantly, from the advent of highspeed computers and sophisticated instrumentation. One of the reasons of reduction of energy requirement in an air separation plant is in increase of 3 its sizes from a few hundred Nm capacity during WWI, to a few tens of tons per day during WWII, to a few hundred tons per day during 1960, to a few thousand tons per day now. This increase of sizes of ASUs have been made possible by making large sizes of distillation columns, liquid pumps, apart from construction of large heat exchangers and compressors, which depended on the development of these industries and improvement of production processes. Large size also leads to reduction of losses, particularly insulation losses per unit product. The advancement of instrumentation in process industry has seen the changes from mechanical instrumentation, to pneumatic instrumentation, to Programmable Logic Controls (PLC) and finally to Distributed Control System (DCS). Accurate feed-forward and feed-back controls have now been possible with the availability of on-line data. Steady state and dynamic simulation programs, with the advancement of computers, now produce results that are far more accurate. Computer networking has now made it possible for someone to monitor and control the operation from a far-off place. The instruments, for the purpose of measurement, control and analysis have become more reliable with great improvement of accuracy, repeatability, sensitivity and response time. All these have made a large plant to be safe enough to be built and operate. The technology of building large-sized (5000 Tons) storage tanker and reliable transport tankers have made the large plants a commercial viability. The improvement in superinsulation has resulted in reduction of losses to a great extent. All these above advancement of technologies have contributed positively to the advancement of air separation technology.
1. CONCLUSIONS Unless there is a major conceptual breakthrough, the trend in the improvement of the efficiency of cryogenic air separation is likely to follow the same trend in the foreseeable future. As we have seen in this paper, the process improvement means a lower energy consumption with the associated penalty of higher capital investment. The new processes envisage structured packing, more column height and more number of heat exchangers for the plant. It is also important to note that the optimum level of capital outlay will depend on cost of energy prevailing at any instant of time.
2. ACKNOWLEDGEMENT The author is grateful to A. R. Singh and Pankaj Gupta, Consultant and Secretary of All India Industrial Gases Manufacturers' Association, New Delhi respectively, for suggesting the subject of the present paper.
7.
REFERENCES 1. R. E. Latimer, Distillation of Air, Chemical Engineering Progress, Vol. 63, no. 2, February, 1967.
2. T.D.Atkinson and T.Rathbone, Advances in Cryogenic Air Separation, in Separation of Gases, Proceedings of the fifth BOC Priestley Conference, Page 35-52, 1989. 3. M.A.Taylor, Plate-Fin Heat Exchangers: Guide to their Specification and Use, Amended October 1990, Published by HTFS, Harwell Laboratory, UK. 4. R.G.Scurlock, A Brief History of Cryogenics, Advances in Cryogenic Engineering, Vol. 37, part A, 1992. 5. E. Schoenpflug, W.Deinart and G.Rueckborn, Recent Advances in Air Separation Technology, Cryogenics, Vol. 30, Page 17-22, September Supplement, 1990. 6. L.M.Rose, Distillation Design in Practice, Elsevier, Chapter 7, Column Fundamentals, page 137-167 7. M.J.Lockett, Gas Separation by Distillation, in Separation of Gases, Proceedings of the fifth BOC Priestley Conference, Page 19-34, 1989. 8. K.D.Timmerhaus and T.M.Flynn, Cryogenic Process Engineering, Plenum Press, 1989. 9. C. J. King, Separation Processes, McGrawHill Book Company, 1971 10. R. Agarwal, D.C.Erickson and D.W.Woodward, High efficiency Processes for Cryogenic Air Separation, in Process and Equipment ⎯ 1989, The Seventh Intersociety Cryogenic Symposium. 11. R. Agarwal, Production of Ultra-high Purity Oxygen: A Distillation Method for the Co production of the Heavy Key Component Stream Free of Heavier Impurities, I and EC, 1994. 12. H. Honda and Y.Kishida, Air Liquefaction Separation Process and Apparatus Therefor, Patent no. 763301 (1996) 13. R. Agarwal et al., Method and Apparatus for Producing Liquid Products from Air in Various Proportions, Patent no.660311 (1996) 14. D.P.Bonaquist, Cryogenic Rectification System with Staged Feed Air Condensation, Patent no.617591 (1996)
This page has been developed by Mr. Uttam Bhunia & Prof. Kanchan Chowdhury © 1999, Cryogenic Engineering Centre, Indian Institute of Technology, Kharagpur 721302, India. Suggestions may please be forwarded to:
[email protected]
