Heat pumps in distillation O.S.L. Bruinsma S. Spoelstra

Transcription

1 Heat pumps in distillation O.S.L. ruinsma S. Spoelstra Presented at te istillation & Absorption Conference, September 2010, Eindoven, e Neterlands ECN-M November 2010

2 istillation & Absorption Conference September 2010, Eindoven Heat pumps in distillation HEA PUMPS IN ISILLAON olf ruinsma and Simon Spoelstra Energy Researc Centre of te Neterlands (ECN), Westerduinweg 3, 1755 LE Petten, e Neterlands bruinsma@ecn.nl, spoelstra@ecn.nl Abstract Vapor recompression as become te standard eat pump tecnology in distillation and substantial energy savings in te order of 50% ave been acieved. Economic applications of VRC are limited to column temperature differences of about 30 0 C, wic is only one fift of te across te pinc columns in operation. 2 nd generation eat pump systems based on furter eat integration and novel eat pump equipment do not only increase te potential energy savings but also extend te application range to columns wit a larger temperature difference. Keywords: eat pump, eat integration, distillation, energy savings 1. Introduction istillation is te main separation tecnology in refineries and te cemical process industry, because of te attractive purification caracteristics, te ig production capacity and turndown ratio, and te straigtforward design procedures. More sopisticated tecniques ave become state of te art to andle streams wit less favourable termodynamic properties, in particular small relative volatilities and azeotropic mixtures. e ig energy demand in bulk distillation columns (1-100 MW) and te low termodynamic efficiency (5-10%) remain te major drawbacks. A number of improvements ave been developed over te years directed at reducing bot operating and capital cost. In extractive distillation (E) a solvent or separating agent is added in order to increase te relative volatility of te components to be separated. In azeotropic extractive distillation te separating agent is used to break te azeotrope. As a consequence te reflux ratio, column diameter and reboiler duty can be reduced and/or te column eigt can be lower. Commercial low volatility solvents include sulfolane, trietylene glycol (EG), NMP and NM. e recovery cost of te solvent is an integral part of te economy of extractive distillation processes. E is particularly effective for relative volatilities below 1.2. Industrial examples of E processes are purification of aromatics in petrocemistry, butadiene recovery in napta cracking and separation of cycloparaffins from napta 1,2,3. Instead of affecting te termodynamics of te system also selection of te column internals is a way to increase distillation efficiency. Random and structured packings wit specific surface areas from 250 up to 900 m 2 /m 3 are continuously being improved wit te objective to optimize stage eigt, pressure drop, liquid load, and turn down ratio. e main recent advancements in tray columns focus on ig-capacity trays wit centrifugal devices or structured packing demisters altoug at te cost of an increased pressure drop 4. Since te 1980 s dividing wall columns (WC s) ave been introduced wic allow te separation of tree component feeds in a single column leading to interesting reductions in bot energy consumption and investment cost. Recently even more complex WC s ave been constructed to separate four component mixtures in pure products 4. In contrast to improvements of te VLE or te column internals, bot inside te column, a number of energy reducing measures can be considered outside te column by addressing te reboiler and condenser. ese include side reboilers 5, deplegmators 6,7 and eat pumps. Side reboilers use waste eat at a lower temperature tan te bottom reboiler and tus increase te exergetic efficiency. eplegmators or reflux condensers are compact eat excangers, suc as PHE s, used to reduce energy consumption in low temperature gas separations. Heat pumps lift te temperature level of te top vapour in order to use tis as te eat source for te reboiler. is paper deals wit 1 st and 2 nd generation eat pumps for distillation energy savings.

3 . ruinsma and S. Spoelstra 2. Industrial eat tecnology A systematic approac in improving te energy efficiency of industrial processes is te onion-model developed in industrial eat tecnology. is model is visualized in igure 1. Utilities HP HEX Process igure 1. Onion model for energy efficiency improvement In te first sell te processes occurring in reactors and separators (Process) are optimized wit respect to energy consumption. In practice tis is done by an economic optimization in wic energy and oter operating cost are balanced wit annualized investment cost for te equipment. In distillation Process refers to molecular improvements suc as extractive distillation as well as optimization of internals, trays and column compartments. Energy consumption can be reduced furter by eat integration using eat excangers (HEX). As eat excangers need a driving force tere is a limit to wat can be acieved by eat integration. Optimization of te eat excanger networks is done using pinc tecnology leading to te rule of tumb: o not transfer eat across te pinc temperature. In addition te grand composite curve (entalpy flow rate versus temperature) provides te minimum total cooling and eating power required for te plant 8. Now te temperature difference at te pinc temperature, Δ pinc, is optimized by te economy: a iger value leads to smaller investment cost in eat excanger area but also to increased utility cost. Since te 1980 s eat integration as become a standard tool in optimizing process designs based on pinc tecnology. Wit te introduction of compact eat excangers in te 1990 s wit excange areas in te range of m 2 /m 3 te optimum temperature difference gradually decreased 9. Currently multi-effect evaporators are in operation aving aluminum compact eat excangers wit a temperature difference as low as 1-2 K. e standard eat integration in stand-alone distillation columns is pre-eating te feed wit te bottom stream. urter energy savings can be realized between condensers and reboilers of different distillation columns and applying side reboilers. After eat integration as been optimized, furter reduction of energy consumption can be acieved in te tird sell: te eat pump (HP). A eat pump is a device tat upgrades eat from a lower temperature source to a iger temperature. Originally eat pumps were only used for refrigeration as eating was done by burning ceap fossil fuels 10. Interest to use eat pumps also for eating purposes increased wit global awareness of te limited availability of fossil fuels in combination wit te greenouse effect. In fact modern air-conditioning equipment can be used eiter as a eater in wintertime, pumping up te low temperature outside eat, or as a cooler in summertime pumping te eat inside te building into te atmospere. is is acieved by reversing te flow of te working fluid. or a eat pump to be effective tere are a number issues to be considered: e pinc temperature and te flexibility of te plant e termodynamic cycle and te eat pump efficiency e temperature lift required e entalpy balance e selection and constraints of eat pump equipment e configuration of te system e available utilities e economy or te annualized capital cost versus te utility cost

4 Heat pumps in distillation 3. istillation column eat pump configurations e objective of a eat pump in distillation is to use te eat of condensation released at te condenser for evaporation in te reboiler. As te temperature at te reboiler is iger a eat pump is required. ere are essentially two conventional ways to integrate a eat pump and a distillation column: te vapour compression column (VC) and te vapour recompression column (VRC) as sown in igure 2, togeter wit te conventional column (CC). CC VC VRC igure 2. e conventional column (CC), te vapour compression column (VC) and te vapour recompression column (VRC) In a CC eat is added to te reboiler and extracted in te condenser, wile te column is adiabatic. e reboiler and condenser duties are usually in te same order of magnitude 11. or close boiling compounds iger reflux ratios are required, wic lead to increased duties. As eat pumps are more efficient for smaller temperature lifts, larger energy savings can be obtained for close boiling systems. is implies tat in te economic analysis tere is a critical temperature lift above wic eat pumps are no longer beneficial. In te VC a working fluid is evaporated at te condenser, compressed to a iger (saturation) temperature, condensed in te reboiler and cooled down by expansion over a trottle valve to a (saturation) temperature below te condenser temperature. Selection of te proper working fluid is an important degree of freedom in te design. An industrial example of a VC is te etylene-etane separation using propylene as working fluid 12. In cases were te distillate vapour can not be compressed or in some novel eat pump systems (see par. 5) te VC is te only option. In te VRC te working fluid is te vapour leaving at te top of te column, wic is compressed, condensed in te reboiler and partially refluxed to te top of te column after pressure reduction over a valve. A small trim condenser is needed to balance te eat input, mainly generated by te compressor. An interesting alternative for te VRC is te bottom flas column (C) 13. e advantage over a VC is tat te condenser in a VRC is smaller and tat te temperature lift is about 5 K lower because eat is excanged only once. is results in a iger termodynamic efficiency. ecause of tese advantages VRC as become te standard tecnology. e compression ratio in a VRC depends on te saturation p-curves at top and bottom composition, te temperature difference over te column, te pressure drop over te column and te required temperature difference for te eat excanger. How te eat pump is embedded in te distillation column is scematically sown in igure 3. e two saturation curves for te distillate and bottoms composition, x and x, are based on te termodynamic model. As in most column designs te pressure at te top, p top, is selected first. or te required distillate purity, x, te top temperature ten is on te x saturation curve. e bottom pressure is determined by te column pressure drop and bottom is on te x saturation curve. Now te temperature after te trim condenser, E, is determined by te optimized temperature difference over te eat excanger, Δ HEX (typically 5K). Assuming tat te trim-condenser is used to de-supereat te compressed vapour E is again found at te x saturation curve. inally te temperature lift, pressure ratio and compressor saft work can be calculated: = = Δ + Δ (1) c top E column HEX

5 . ruinsma and S. Spoelstra p sat. curve at x sat. curve at x p E E top p bottom p top top bottom E Δ column Δ HEX top bottom E igure 3. p-diagram of te VRC cycle In tis paragrap only compressors were considered as eat pumps. ese and oter eat pump equipment options will be discussed below. 4. Heat pump equipment Heat pumps are macines tat pump eat from a lower to a iger temperature (see igure 4). rom te first law of termodynamics, te amount of eat delivered to te ot reservoir (Q ) at te iger temperature ( ) is related to te amount of eat extracted (Q c ) from te cold reservoir at te low temperature ( c ) and te external work by te following equation: Q = Q W (2) c + Heat sink (Hot reservoir) Q, W HEA PUMP Δ lift = - c Q c, c Heat source (Cold reservoir) igure 4. Sceme of a eat pump e measure of te eat pump performance is te coefficient of performance (COP). or eating applications tis is te ratio of eat rejected at ig temperature to te work input: COP Q W = (3) e upper teoretical value of COP obtainable in a eat pump is COP C, related to te Carnot cycle: COP c = (4) c Were te temperature lift, - c, is te sum of te temperature difference over te column and te temperature difference(s) over te eat excanger(s), as sown in equation (1). e ratio between te two COP values is te exergetic efficiency of te eat pump, η e :

6 Heat pumps in distillation W C η e = (5) WVRC is is visualized in igure 5 for te Carnot and te VRC cycle in a S-diagram for te same Q =Q reboiler. liquid. critical point Q 3 2 W c gas liquid. critical point Q 3 2 W VRC 2 gas c 4 1 c S S igure 5. S-diagram of te Carnot cycle and te reversed Rankin cycle for te VRC It sould be noted tat te VRC is an open cycle and te S-diagrams are terefore a simplification. Weter a eat pump system actually leads to energy savings depends on te primary energy consumption of bot options, PE CC and PE HP. is not only depends on te efficiency of te eat pump but also on te efficiency of te steam boiler, η boiler, and of te power plant tat provides te electricity to drive te compressor, η el. e primary energy consumption for te conventional column equals: PE Q reboiler CC = (6) ηboiler or te vapour recompression column tis is: PE W Q Q ( Δ + Δ ) reboiler reboiler column HEX VRC = = = (7) ηel COP ηel ηe ηel e primary energy savings by introducing te VRC for te CC ten equals: 1 Δ column + Δ HEX PES = PE = CC PEVRC Qreboiler ηboiler ηe ηel is equation sows tat primary energy savings reduce wit increasing column temperature difference. At a certain Δ column te primary energy savings will not be sufficient to balance te compressor investment cost. At low Δ column, tat is wen te column temperature difference is in te order of te eat excanger temperature difference, it becomes more interesting to invest in compact eat excangers (as part of te eat pump system) tat can operate wit a low Δ HEX. (8)

7 . ruinsma and S. Spoelstra 4. Market analysis of eat pumps in distillation An analysis was made of distillation eat pump potential in te Neterlands, leaving out columns tat do not cross te pinc and oil refinery columns. e data sow tat te total eat pump potential is in te order of 2.4 GW and tat te average temperature lift over te column is 59 0 C. ese data are given in able 1. able 1. Across te pinc distillation in te Neterlands 11 istillation in NL otal Q reboiler (GW) 2.36 otal Q condenser (GW) 2.39 Average reboiler ( 0 C) 128 Average condenser ( 0 C) 69 Average Δ column ( 0 C) Novel developments in eat pumps in distillation Conventional eat pump cycles are driven by compressors or blowers depending on te required volumetric capacity and pressure ratio or temperature lift. e economic range for te VRC configuration driven by a compression eat pump is limited to columns wit a temperature difference of about 30 0 C. New developments in distillation eat pump tecnology are terefore aimed at novel eat pumps wit a iger economic range and at new eat integrated configurations. 5.1 Novel eat pumps An extensive overview of different eat pumps is given by inçer 14, mainly described from a refrigeration perspective. Here only tree versions will be discussed as tey ave te potential capacity of interest for distillation: te termoacoustic, te compression-resorption and te adsorption eat pump. e ermoacoustic eat pump An electrically driven ermoacoustic (A) eat pump 15 consists of a linear motor tat generates an acoustic wave, a resonator tat togeter wit te working gas (usually elium) determines te resonance frequency and ouses all equipment and two eat excangers on bot sides of te porous regenerator, as sown in igure 6. gas-side vapor-liquid side igure 6. e electrically driven A eat pump (left) and eat excanger details (middle and rigt) ot compact eat excangers are flat and located at one end of te resonator, igure 6 (left). At te A gas-side a open fin structure is used, igure 6 (middle). or te vapour-liquid side microcannels connected wit a manifold are brazed in between te fins, igure 6 (rigt). e A eat pump can also be driven by a A-engine instead of a linear motor. e A engine is driven by steam or a burner. e Compression-Resorption eat pump Until now te top and bottom product were considered as pure compounds wit fixed condensation temperatures at te operating pressure. In reality one or bot products are often mixtures wit a condensation trajectory between dew and bubble point; te glide. e temperature difference over te

8 Heat pumps in distillation glide leads to an extra exergy loss over te eat excanger, unless te working fluid as te same glide. is principle is applied in te Compression Resorption (CR) eat pump. In te CR eat pump te working fluid is a zeotropic mixture, usually ammonia-water. e composition of tis mixture is adjusted until te glide of te working fluid optimally matces te glide at te condenser or te reboiler. e adsorption eat pump In an adsorption eat pump waste eat is upgraded in a cycle based on adsorption and desorption of a working fluid onto a low-temperature salt (LS) and a ig-temperature salt (HS). An example is te system NH 3 -LiCl 2 -MgCl 2 wit te cycle sown in igure 7. LS HS Condenser iscarge Carge: LiCl.1NH NH 3 LiCl.3NH 3 + eat ( ambient ) MgCl 2.6NH 3 + eat ( waste ) MgCl 2.2NH NH 3 HS Carge LS Reboiler iscarge: LiCl.3NH 3 + eat ( waste ) LiCl.1NH 3 + 2NH 3 MgCl 2.2NH NH 3 MgCl 2.6NH 3 + eat ( reboiler ) Cooling water igure 7. e adsorption eat pump for te NH 3 -LiCl 2 -MgCl 2 cycle 16. e adsorption eat pump is essentially a eat transformer wit four adsorption columns were waste eat is upgraded to te temperature above te pinc required for te reboiler by adsorption of NH 3 onto te ig-temperature salt MgCl 2.2NH 3. e eat balance sows tat for most distillation columns an adsorption eat pump will need an additional eat source. 5.2 Heat integrated distillation columns A eat integrated distillation column (HIiC) is diabatic wit eat being excanged in te column from te ig pressure rectifier to te low pressure stripper, as sown in igure 8. S Heat R igure 8. e HIiC principle Vapour from te top of te stripping section is compressed and directed to te rectifier. In te rectifier te vapour condenses, creating an internal reflux tat is returned to te top of te stripper. e eat of condensation is used to evaporate te liquid at te stripper side. Usually te reboiler duty can be close to zero and a small external reflux is required at te top of te rectifier in order to produce te required distillate purity. Optimization of te pressure ratio for a constant separation task is based on te balance between te compressor power cost and investment cost for compressor and HIiC column. e HIiC configuration can reduce te utility cost compared wit te VRC wit an additional 25-35% and te total annualized cost wit 10-20% 17,18,19.

9 . ruinsma and S. Spoelstra 6. Conclusions igure 9 represents te distribution of te reboiler duties in te Neterlands for columns wit increasing temperature lift; only tose columns tat cross te pinc ave been included novel eat pumps Reboiler uty NL / MW HEX VRC HIiC >100 emperature lift / C igure 9. Reboiler duties for across te pinc columns in te Neterlands (2006) 11 In te grap four recommendation regions are identified: for temperature lifts below 20 0 C compact eat excangers wit small Δ HEX are crucial for te performance of te eat pump system, VRC s sould be applied below 30 0 C, wic covers about 23% of te across te pinc columns HIiC s are probably interesting for temperatures in te range C, about 29% of te across te pinc columns, partly overlapping wit VRC but wit a iger savings efficiency novel eat pumps, typically for temperature lifts of C, would contribute an additional 21% ased on tis analysis te combination of VRC, HIiC and novel eat pumps would lead to an estimated 820 MW savings, wic is almost 35% of te reboiler duties of all across te pinc columns in te Neterlands. References 1. M.. oerty and J.P. Knapp, istillation, azeotropic and extractive, In: Kirk-Otmer Encyclopedia of Cemical ecnology, Vol. 8, , Wiley (2004) 2..M. Lee, Extractive distillation, In: Encyclopedia of Separation ecnology, , Elsevier (2000) 3. S. Kossack et al, Cem. Eng. Res. es., 86(2008) Z. Olujic et al, Cem. Eng. Proc., 48(2009) S. andyopadyay, Cem. Eng. Res. es., 85(A1) (2007) K. akke, Experimental and teoretical study of reflux condensation, P tesis, NNU rondeim (1997) 7. J. Wang and R. Smit, Cem. Eng. Res. es., 83(A9) (2005) Linnoff et.al. Heat integration of distillation columns into overall processes, Cem. Eng. Sci. 38(8) (1983) J.E. Hesselgreaves, Compact eat excangers, Pergamon (2001) 10. M. Zogg, Proc. 9 t Int. IEA Heat Pump Conf., May 2008, Züric, pp J. Cot and O.S.L. ruinsma, Market survey, Heat pumps in bulk separation processes (2010), ECN report xx 12. Zimmerman, H., Walzl, R., Etylene, Ullmann's Encyclopedia of Industrial Cemistry 7t Ed., Jon Wiley & Sons, (2009) 13. Z. onyo and N. enko, rans ICemE, 76(Part A) (1998) I. inçer, Refrigeration systems and applications, Wiley (2003) 15. G.W. Swift, ermoacoustics, Acoustical Society of America (2002), ISN M. van der Pal et.al., Proceedings Heat Powered Cycles Conference, 7-9 September 2009, erlin 17. J.A. Hugill et.al., Proceedings 6 t Int. Conf. Enanced, Compact and Ultra-Compact Heat Excangers, September 2007, Potsdam, pp A. e Rijke, evelopment of a concentric internally eat integrated distillation column (HIiC), P tesis U elft, M. Nakaiwa, Cem. Eng. Res. es., 81(1) (2003)