Improving Agility of Cryogenic Air Separation Plants
Jason Miller and William L. Luyben*
Department of Chemical Engineering, Lehigh UniVersity, Bethlehem, PennsylVania 18015
Paul Belanger, Stephane Blouin, and Larry Megan
Process and Systems R&D, Praxair Inc., Tonawanda, New York 14151
Cryogenic air separation plants that produce argon in addition to oxygen and nitrogen use two heat-integrateddistillation columns and a side rectifier. The dynamics of the side rectifier are very slow because of threefactors: the large number of separation stages, the small amount of argon in the air feed, and the high productpurity. Currently, when the plant shuts down, the liquid inventory in the argon column drains into the upper(low-pressure) column. A normal start-up takes about 10 h to achieve argon product purity. The start-up timecan be reduced significantly by using storage vessels to collect the liquid that drains from the argon columnupon shutdown and reintroducing the liquid during the subsequent start-up. Compared to a 10 h start-up timewith no collection points, the time required to achieve argon product purity is reduced to 3.35 h by using 2collection points and is reduced to 2.23 h by using 6 collection points.
Historically, the ability of a process to reject load disturbanceswas the main criterion on which its dynamic performance wasjudged. The ability to quickly transition between operatingconditions was of little importance because the operating condi-tions were changed rather infrequently. However, in recent years,there has been more emphasis placed on the ability of a processto rapidly transition between different operating conditions.
There are two paths that can be taken in improving processagility. The first path involves making improvements to thecontrol structure. The other path consists of the addition of, orthe modification of, process equipment. In most cases, improve-ments to the control structure is the method preferred becauseit is not capital-intensive and can be accomplished withoutshutting down the process. Unfortunately, this approach islimited by the dynamics of the present process equipment.
The ability to start up plants and transition between operatingstates efficiently is of particular interest in the area of cryogenicair separation. The only real costs encountered in the processare capital equipment costs and energy costs since the rawmaterial (air) is taken directly from the atmosphere. Thus, if aplant were capable of producing product in a more efficientmanner, energy cost and thus total costs could be reducedsignificantly. Also, the faster a plant can be started up, the moretime existing manpower will have to take care of other tasks.In an ideal scenario (instantaneous start-up), the process couldbe started up and shut down in response to fluctuating powerprices. The economic incentive for intermittent operation ofcryogenic air separation plants has been discussed previouslyby the authors.1
Typically, a significant amount of time is required to restarta cryogenic air separation plant (Figure 1) after an interruptionin operation or a scheduled shutdown. The shutdown orinterruption may be brought about by a power failure or byeconomic concerns over high power rates. If the interruption isnot planned, the time required to re-establish the desired productpurities can be quite costly. For example, product demands by
customers have to be satisfied by other means. During currentplant shutdown, liquid drains from the column trays or packingsections and collects in the upper (low-pressure) and lower(high-pressure) column sumps. Many times the collected liquidis drained before the next start-up, causing a loss of refrigerationto the plant. In a plant producing argon, a large amount of argoncan be lost in this draining process, and as argon makes up lessthan 1% of air, this loss can be significant. Thus, it has beenproposed that the collection of descending column liquid (frompacking sections or trays) upon shutdown and the reintroductionof the collected liquid during the subsequent start-up can helpreduce the time required to reach the desired product composi-tions in the plant. Although the methodology can be applied toall three distillation columns, the current investigation hasfocused on collection and redistribution from/to the argoncolumns only.
A number of patents have been issued that involve thecollection and re-distribution of process liquid for rapid start-up of air separation plants.2-6 A range of implementations arediscussed including the use of single or multiple storagelocations, internal or external liquid storage, recirculation ofliquid during start-up or one-way introduction, and collectionof liquid from the argon column only or collection of liquidfrom the upper (low-pressure) and lower (high-pressure) col-umns also. Methods for reintroducing the collected materialinclude pumping the stored liquid, pressurizing the storage vesselwith vapor from another portion of the plant, or including aheat exchanger in the storage vessel and vaporizing the collectedliquid. In the last scenario, some or all of the collected materialis returned to the column in the vapor phase.
A simplified schematic of the implementation that involvesthe use of a single, external storage vessel with the pressuredriving force provided by a vapor stream from another portionof the plant is shown in Figure 2. During normal operation, thevalve on the argon return would be open and that to the storagetank closed. Upon plant shutdown, the valve on the argon returnwould be closed and the valve to the storage tank opened.During the next start-up, vapor from the lower (high-pressure)column could be used to pressurize the storage tank to provideenough driving force to transfer the liquid from the storage tankto the argon column. Once all the collected liquid is introduced
* To whom correspondence should be addressed. Tel.: 610-758-4256. Fax: 610-758-5297. E-mail: email@example.com.
394 Ind. Eng. Chem. Res.2008,47, 394-404
10.1021/ie070975t CCC: $40.75 2008 American Chemical SocietyPublished on Web 12/13/2007
into the column, the tank vent could be opened to remove thevapor that was used to pressurize the tank. Because the vapor
contains a large quantity of nitrogen, it is undesirable for thevapor to enter the argon column.
Figure 1. Simplified schematic of cryogenic air separation plant.
Figure 2. Simplified schematic of design modification.
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Although there are several patents describing the collectionand redistribution of liquid from/to argon columns, there is verylittle mention of quantitative improvements in start-up achievedby implementing such designs. Smith and Espie6 discusssimulation results for an argon column that is split into twoshells because of height considerations in the cold box. Theirresults suggest that collecting liquid in the sump of the secondargon column and recirculating liquid in the column during start-up can reduce the time required to achieve product purity by50% over the base case where no liquid is collected orrecirculated. By collection and recirculation of liquid in bothargon columns, the time required to achieve argon purity isreduced by 64% over the base case. However, until now, therehas not been a study published that provides insights into theimpact of the number of collection vessels and liquid introduc-tion policy on the start-up of cryogenic air separation plants.
In this study, we have assumed a semi-cold start-up. Thismeans that the plant will have been shutdown for a brief periodof time, at most a few days, before the next start-up. Thus, liquidis present in the column sumps prior to the introduction of airto the plant. As the plant remains idle. heat will be transferredto the cold box, which contains the distillation columns, fromthe atmosphere and the liquid in the system will eventuallyevaporate. When a plant is started with essentially zero liquid,this is characterized as a warm start. The warm start scenariois not considered in this study.
2. Process Description
A typical super-staged argon cryogenic air separation plant(Figure 1) includes a double distillation column with a siderectifier to recover high-purity liquid argon. Feed air (from theatmosphere) is compressed and passed through an adsorbentbed of molecular sieves to remove water, carbon dioxide,acetylene, ethylene, butane, and other heavier hydrocarbons.This helps alleviate the potential dangers of hydrocarbon-oxygen mixtures and prevents the freezing of material in thesystem.
The feed air stream is split, with a good portion beingexpanded in the lower column turbine, after being cooled inthe primary heat exchanger against returning cold oxygen andnitrogen product streams, along with the waste nitrogen stream.The expansion of the air provides refrigeration for the plant.This stream provides vapor air feed to the high-pressure column.The air that is not expanded is also cooled in the primary heatexchanger and provides liquid air for both the high-pressureand low-pressure columns.
The lower column (high-pressure column) operates at ap-proximately 85 psia (0.586 MPa) and separates the air into ahigh-purity nitrogen stream (top) and oxygen-enriched liquidstream (bottom). Nitrogen vapor at the top of the lower columnis condensed against boiling liquid oxygen in the bottom of theupper column by heat exchange in a reboiler-condenser. Thenitrogen stream from the top of the lower column is referred toas the shelf transfer and the enriched oxygen stream is calledthe kettle transfer. The upper (low-pressure) column operatesat approximately 20 psia (0.138 MPa) and produces high-puritynitrogen (top) and oxygen (bottom) product streams. The oxygenliquid product stream is pumped to a higher pressure. A portionis vaporized in the primary heat exchanger and provides high-pressure gaseous oxygen product, while the remainder goes tothe product oxygen liquid storage tank. Reflux for both columnsis generated at the top of the lower column (i.e., shelf transferacts as reflux for the upper column). Additional reflux for theupper column is provided by a liquid nitrogen-add stream,
which is combined with the shelf transfer. The liquid nitrogen-add stream is provided by drawing from the liquid nitrogenstorage tank or by recycling liquid from a nitrogen liquefier. Anitrogen liquefier includes a series of compression, expansion,and heat-exchange equipment.7
Argon boils between oxygen and nitrogen, which results ina peak argon composition in the lower portion of the uppercolumn. A vapor side stream is drawn from the upper columnnear the argon peak and is fed to the super-staged argon column.Physically, the super-staged argon column consists of twoseparate shells due to height restrictions in the cold box. Thevapor stream from the top of the first argon column is feddirectly to the bottom of the second argon column. A pump isutilized to produce the driving force for liquid transfer fromthe sump of the second argon column to the top of the first.The argon columns produce a liquid argon product that containsparts-per-million level impurities of oxygen and nitrogen. Theproduct stream is drawn several stages from the top of thecolumn to prevent too much nitrogen from entering the productargon stream. Reflux for the argon column is provided by heatexchange in the argon condenser between the vapor at the topof the argon column and the oxygen-enriched liquid (kettletransfer) from the lower column. This stream, after expansionto the lower pressure, has a lower boiling point than the argon.The liquid from the bottom of the first argon column is fed tothe upper column. The oxygen-enriched liquid and vapor fromthe cold (boiling) side of the argon condenser are also fed tothe upper column. The oxygen product from the bottom of theupper column typically contains 99.5% or greater oxygen withthe remainder being argon. The nitrogen product from the uppercolumn typically contains parts-per-million level impurities ofoxygen.
3. Plant Start-Up Model
3.1. Introduction. To analyze a variety of design modifica-tions and liquid feed trajectories, a detailed start-up model ofan existing cryogenic air separation plant has been developed,which is capable of reproducing historical plant start-up data.This model is based on a first-principles approach8 and wasdeveloped in gPROMS version 3.0 (Process Systems EnterpriseLimited, London, UK). The model has proven to be quiteefficient, with 17 h of plant operation being simulated in about20-30 min. This 17 h span follows one particular period ofplant operation from the initial introduction of air to the plant,through the start-up phase, followed by a period of steady-stateoperation, right up to the point of shutdown. The simulationswere run on a Dell OPTIPLEX GX620 with an Intel Pentium4HT processor.
3.2. General Modeling Equations.The model is based ona first-principles approach utilizing traditional component moleand stage energy balances as given by eqs 1 and 2.8 Note thatthe terms must be added for each feed and product stream,
whereM is the total liquid molar holdup (lbmol), N is the stagenumber,i is the component number (1) argon, 2) oxygen,and 3) nitrogen),x is the liquid mole fraction,y is the vapormole fraction,L is the liquid molar flow rate (lbmol/h), V is
dt) LN+1xi,N+1 - LNxi,N + VN-1yi,N-1 - VNyN (1)
dt) LN+1hn+1 - LNhN + VN-1HN-1 - VNHN (2)
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the vapor molar flow rate (lbmol/h), h is the liquid molarenthalpy (Btu/lbmol), andH is the vapor molar enthalpy (Btu/lbmol).
For all column sections containing trays, a Francis-weirrelationship was utilized to determine the liquid flow rates fromeach stage as shown in eqs 3 and 4.9 For all packing sections,the liquid flow rates leaving a given stage were determined asa function of the overall stage liquid holdup as described by eq5,
where lw is the weir length (ft),Fl is the liquid molar density(lbmol/ft3), how is the height of the liquid over weir (ft),Aact isthe available area on the tray (ft2), andhw is the weir height(ft).
Vapor flows in the columns were determined by solving thefull energy balance under a steady-state assumption. Both theenergy and mole balances were assumed to be at steady state.This allows one to calculate the vapor molar flow rate from agiven stage based on the liquid (L) and vapor (V) flows to thatstage along with the liquid (h) and vapor (H) enthalpies aroundthat stage as shown in eq 6.8 Note that the terms must be addedto the energy balance for each product and feed stream.
The rate of change of the pressure at the top of each column,along with the boiling side of the argon condenser, wasdetermined by performing a mole balance at the top of thecolumns as shown in eq 7.9 In general, the equation includes aterm for the vapor flow rate from the top stage of each column(Vtop), the product vapor flow rate from the top of the col...