Clean Technol., Vol. 22, No. 4, December 2016, pp. 225-231 청정생산공정기술 증기재압축을활용한증발공정에서스팀절감에대한연구 노상균 * 동양대학교화공생명공학과 750-711 경북영주시풍기읍동양대로 145 (2016 년 8 월 8 일접수 ; 2016 년 8 월 17 일수정본접수 ; 2016 년 8 월 17 일채택 ) The Reduced Steam Consumptions in the Evaporation Process Using a Vapor Recompression Sang Gyun Noh* Department of Chemical & Biomolecular Engineering 145 Dongyangdaero, Punggi-eup, Yeongju-si, Gyeongbuk 750-711, Korea (Received for review August 8, 2016; Revision received August 17, 2016; Accepted August 17, 2016) 요 약 본연구에서는 2 중효용증발관을이용하여 21.0 wt% 의 NaCl 수용액에서고형물의 NaCl 을 1,524.58 kg h -1 만큼석출시키는공정에대해서증기재압축을활용하여스팀소모량을 3,139 kg h -1 에서 496 kg h -1 로줄이는공정에대한전산모사및최적화작업을수행하였다. 디에틸렌글리콜 (diethylene glycol) 을포함한 NaCl 수용액을농축시키기위한증발농축공정의전산모사를위해서는 AspenTech 사의 Aspen Plus V8.8 을활용하였으며, 중간에냉각기를가지는증기재압축공정의전산모사를위해서는 Schneider Electric 사의 PRO/II with PROVISION V9.4 를이용하였다. 증기재압축공정에대해서는 1 기의압축기를사용한공정과중간에냉각기를가지는 2 단압축공정을상호비교하였다. 주제어 : 증발, 단일효용, 다중효용, 증기재압축, 최적화 Abstract : In this study, modeling and optimization study have been performed to obtain 1,524.58 kg h -1 of a solidified NaCl by evaporating a 21.0 wt% of NaCl aqueous solution in order to reduce the steam consumption from 3,139 kg h -1 to 496 kg h -1 using a two-stage evaporation and a vapor recompression processes. Aspen Plus release 8.8 at AspenTech was utilized for the modeling of two stage evaporation process and PRO/II with PROVISION release 9.4 at Schneider Electric was also used for the simulation of two-stage vapor recompression process with an inter-cooler. For the simulation of the evaporation process containing NaCl aqueous solution, Aspen Plus release 8.8 at AspenTech Inc. was utilized and for the modeling of vapor recompression process PRO/II with PROVISION release at Schneider Electric Inc. For the vapor recompression process, single stage compression and two-stage compression system was compared. Keywords : Evaporation, Single-effect, Multi-effect, Vapor recompression, Optimization 1. 서론 증발농축공정은해수담수화 [1], 오렌지쥬스의농축 [2], 디에틸렌글리콜중에포함되어있는수분제거 [3] 등에폭넓게활용되고있다. 증발농축공정은단일효용증발관을사용하기도하지만압력을점차낮추어서진행하는다중효용증발 관을사용하기도한다. 다음의 Figure 1과 2에는단일효용증발관과이중효용증발관의공정개요도를나타내었다. Figure 2에의하면증발하고자하는성분이스팀인경우에는첫번째증발관에서증발하는스팀을두번째증발관의열원으로사용할수있으며, 두번째증발관에서증발하는스팀은다시세번째증발관의열원으로사용할수있다. 따라서 * To whom correspondence should be addressed. E-mail: sgnoh@dyu.ac.kr; Tel: +82-10-9779-1946; Fax: +82-54-630-1275 doi: 10.7464/ksct.2016.22.4.225 pissn 1598-9712 eissn 2288-0690 This is an Open-Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licences/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 225
226 노상균 증발시키는성분이스팀인경우에는단일효용증발관에서소모되는스팀사용량에비해서상당히많은양의스팀소모량을절감할수있다는장점이있다. 한편증기재압축공정을위해서는다음의 Figure 3과같은 1단압축공정과 Figure 4와같이중간에냉각기를가지는 2단압축공정사이에서스팀소모량을비교하였다. 또한 2단압축공정에대해서는압축기의총소요동력을최소화시켜주는최적의중간압력을결정하였다. Figure 1. A schematic diagram of a sinle-stage evaporator. Figure 3. A schematic diagram of a single-stage compression process with a desuperheater. Figure 2. A schematic diagram of two-stage evaporators. Figure 4. A schematic diagram of two-stage compression process with an inter-cooler and a desuperheater.
증기재압축을활용한증발공정에서스팀절감에대한연구 227 2. 열역학이론 2.1. 증발농축공정을위한 Electrolyte NRTL 모델물에녹아서양이온과음이온의전해질을형성하는혼합물에대한열역학적인해석을위해서 Aspen Plus에는 Electrolyte NRTL 모델식 [4] 을사용하는데, 그 Gibbs 자유에너지표현식은다음의 Equation (1) 에나타내었다. Table 2. Coefficients in Twu s alpha function for water Coefficients Value C 1 0.38464 C 2 0.86999 C 3 1.9367 ln (1) (7) 또한본공정의전산모사를위해서전해질반응은다음의 3개의반응을고려하였다. (8) (2) (3) 그리고 α는알파함수로써순수성분의온도에따른증기압을잘추산하기위해서 Twu et al. [6] 이제안한새로운모델식으로이는아래의 Equation (9) 에나타내었다. (4) 위의 (2) 번반응은고체의 NaCl이 Na + 이온과 Cl - 이온으로의반응평형에대한평형상수를나타내었으며, (3) 번반응은평형반응이아니라해리되는반응이므로평형상수를표시할수없다. 그리고 (4) 번반응은전해질인물이 H + 이온과 OH - 이온으로해리하는반응이므로다음의 Equation (5) 와같이이온반응평형상수를온도의함수로다음과같이나타낼수있으며, 각계수들은다음의 Table 1에각각나타내었다. ln ln (5) 2.2. 증기재압축공정을위한 Peng-Robinson 상태방정식 Peng-Robinson 상태방정식 [5] 은다음의 Equation (6) 에나타내었다. (6) exp (9) 여기에서계수들인 C 1, C 2 및 C 3 값은물질마다고유한값들이며물에대한값들은위의 Table 2에나타내었다. 3. 전산모사 3.1. 2중효용증발공정의전산모사다음의 Table 3에는증발농축공정에주입되는원료의조성을나타내었다. 다음의 Figure 5에는본공정에서적용한 2중효용증발공정에대한개요도와주요스트림에대한스트림넘버를표기하였다. Figure 5에의하면첫번째증발기인 EV-101로원료인 1번스트림과 24번스트림이주입되고, 7번스트림으로 133.2 의저압의포화스팀이 HE-101 열교환기로주입되어서증발관하부로부터열교환기로주입되는액상류의일부를증발시켜서 4번스트림을다시 EV-101 증발관으로환류시킨 위의 Equation (6) 에서 a는에너지매개변수이며, b는크기매개변수이다. 이는임계온도와임계압력의함수로표현할수있으며다음의 Equation (7) 과 (8) 에각각나타내었다. Table 1. Coefficients in ionic equilibrium constant as functions of temperature Coefficients Equation (2) Equation (4) A -203.588 132.899 B 4,381.18-13445.9 C 35.8752-22.4773 D -0.0672161 Table 3. Feedstock condition Component Flow (kg h -1 ) Weight (%) Water 6,103 74 NaCl 1,732 21 Diethylene glycol 412 5 Total 8,247 100 Temperature ( ) 45 Pressure atmospheric Density (kg/m 3 ) 1,160 Viscosity (cp) 9.0 Specific heat 0.83
228 노상균 Figure 5. A schematic diagram for two-stage evaporation process in this study. Figure 6. Process flow sheet for two-stage evaporation process using Aspen Plus. 다. EV-101 증발관에서는 6번스트림이증기류로발생하는데대부분이스팀이다. EV-101 증발관의운전압력이 332 torr이므로 6번에서발생하는스팀은포화조건으로써그온도는 85.7 이다. 한편 EV-102 증발관의운전압력은 92 torr이므로포화온도는 56.3 이다. 따라서 6번스트림은 EV-102 증발관의열원으로사용하기위해서 HE-102 열교환기로주입된다. 한편다음의 Figure 6에는 Figure 5에나타낸 2중효용증발공정에대해서 Aspen Plus를활용해서수행한전산모사플로우쉬트를나타내었으며, Table 4에는전산모사결과를요약하여정리하였다. Table 4. Simulation summary for two-stage evaporation process Item Result Unit EV-101 pressure 332 torr Stream 6 flow rate 2,558 kg h -1 HE-101 duty 1.6371 10 6 kcal h -1 Stream 7 flow rate 3,139 kg h -1 EV-102 pressure 92 torr Stream 13 flow rate 3,006.8 kg h -1 HE-102 duty 1.4243 10 6 kcal h -1 CON-101 duty 1.7204 10 6 kcal h -1
증기재압축을활용한증발공정에서스팀절감에대한연구 229 Table 5. Heat and material balance for two-stage evaporation process Stream Unit 1 7 8 12 14 15 22 23 24 25 Temp. 45.0 133.2 133.2 78.3 56.8 49.8 32.0 37.8 40.0 40.0 Press. bar 1.0 3.0 3.0 0.4 1.8 0.1 3.0 3.0 3.0 3.0 Vapor frac. 0.0 1.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 Mole flow kmol h -1 401.9 174.2 174.2 141.9 76.9 166.8 16,652.5 16,652.5 4.4 5.6 Mass flow kg h -1 8,247.0 3,139.0 3,139.0 2,557.6 2,862.7 3,006.8 300,000.0 300,000.0 80.0 100.0 H 2O kg h -1 6,103.0 3,139.0 3,139.0 2,556.2 719.2 3,003.5 300,000.0 300,000.0 80.0 100.0 NACL kg h -1 1,732.0 0.0 0.0 0.0 1,728.7 0.0 0.0 0.0 0.0 0.0 DEG kg h -1 412.0 0.0 0.0 1.4 414.8 3.3 0.0 0.0 0.0 0.0 H 3O + kg h -1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 OH - kg h -1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 NA + kg h -1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 NACL (S) kg h -1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 CL - kg h -1 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 0.0 다음의 Table 5에는 Figure 5에나타낸 2중효용증발공정에대한열및물질수지결과를요약정리하였다. 3.2. 단일압축기를활용한증기재압축공정의전산모사증발농축공정에증기재압축공정을적용하면다음의 Figure 7과같다. 위의 Figure 7에의하면첫번째증발기에서발생하는스팀은두번째증발기의열원으로사용하고, 두번째증발기에서발생하는저온, 저압의스팀은증기재압축공정을거친후에최초의스팀과합쳐서첫번째증발기의열원으로사용한다. 첫번째로는 Figure 3에나타낸바와같이단일압축기와 desuperheater를활용하여 92 torr, 56.3 의저온과저압상태의쓸모없는스팀의압력을가압하여 133.2 의스팀을만드는공정에대한전산모사를수행하였다. Figure 8에는 PRO/II with PROVISION을활용한 1기의압축기를활용한증기재 Figure 8. Process flow sheet for a single-stage evaporation process using PRO/II with PROVISION. 압축공정에대한플로우쉬트를나타내었다. 압축기의효율은 70% 를가정하였으면압축기후단의열교환기는 desuperheater 로써여기에서과포화상태의스팀을포화상태로만든다. 한편 controller의역할은 desuperheater 후단의스팀의온도가 Figure 7. A schematic diagram for two-stage evaporation process with a mechanical vapor recompression process.
230 노상균 Table 6. Simulation summary for a single-stage evaporation process Item Result Unit Feed flow rate 2,558 kg h -1 Compressor power 747.24 kw Compressor efficiency 70 % Compressor discharge pressure 2,228.807 torr Desuperheater duty 0.5548 10 6 kcal h -1 Cooling water consumption at desuperheater 69.4 ton h -1 Cooling water supply temperature 32 Cooling water return temperature 40 Table 7. Simulation summary for two-stage vapor recompression process Item Result Unit Compressor 1 power 320.34 kw Compressor 1 efficiency 70 % Compressor 1 discharge pressure 550 torr Inter-cooler duty 0.2257 10 6 kcal h -1 Compressor 2 power 334.11 kw Compressor 2 efficiency 70 % Compressor 2 discharge pressure 2,228.81 torr Desuperheater duty 0.2493 10 6 kcal h -1 133.2 가되도록압축기후단의압력을결정해준다. Table 6에는단일압축기를활용한증기재압축공정에대한전산모사결과를요약하였다. 3.3. 2 단압축공정을활용한증기재압축공정의전산모사 2단압축공정을활용한증기재압축공정에대한 PRO/II를활용한플로우쉬트는다음의 Figure 9에나타내었다. 한편 2단압축공정에서는 2기의압축기의소요동력의합이최소가되는첫번째압축기의최적의도출압력을결정하기위한최적화작업을수행하였다. 다음의 Figure 10에는첫번째압축기도출압력의변화에따른총소요동력의변화를도시하였다. Figure 10. Determination of an optimal intermediate pressure which minimizes the compression power. Table 7에는 2단압축기를활용한증기재압축공정에대한전산모사결과를요약하였다. 4. 결론본연구에서는디에틸렌글리콜 (diethylene glycol) 이함유되어있는 NaCl 수용액을 2중증발농축공정을활용하여고형물의 NaCl을얻기위해서두번째증발관에서발생하는증기류를단일압축과 2단압축공정을활용한증기재압축공정을활용하여다음과같은결론을얻을수있었다. 첫째, NaCl 수용액과같은전해질이포함된 2중효용증발농축공정의전산모사를위해서는 Aspen Plus에내장되어있는 Electrolyte NRTL 모델식이가장적합함을알수있었다. 둘째, 두번째증발관에서증발하는저압의스팀을증기재압축공정을활용하여 133.2 의스팀을얻기위한공정의전산모사를위해서는 PRO/II with RPOVISION에내장되어있는새로운알파함수를적용한 Peng-Robinson 상태방정식이가장적합함을알수있었다. 셋째, 증기재압축공정을활용하지않았을경우에첫번째증발기에필요한스팀의유량은 3,139 kg h -1 임을알수있었으며, 증기재압축공정을적용할경우 2,558 kg h -1 의저압의스팀을활용할수있기때문에스팀소모량은 581 kg h -1 로줄어들었다. 넷째, 단일압축기를적용할경우압축기의소요동력은 747.27 kw임에비해서 2단압축공정을적용하고총소요동력을최소화시키는최적의첫번째압축기의도출압력을정했을경우총소요동력은 654.45 kw로줄어들었음을알수있었다. Figure 9. Process flow sheet for two-stage vapor recompression process using PRO/II with PROVISION.
증기재압축을활용한증발공정에서스팀절감에대한연구 231 감사 Subscripts 본연구는 2015년도동양대학교학술연구비의지원으로수행되었음. Nomenclature a : Equation of state attraction parameter b : Equation of state van der Waals co-volume parameter C 1 m P : Dimensionless alpha function coefficient : Characteristic constant of a substance : Pressure [kpa] R : Molar gas constant [J mol -1 K -1 ] T : Temperature [K] v : Relative volume [m 3 mol -1 ] Greek Symbols α : Dimensionless function of reduced temperature and acentric factor ω : Acentric factor Superscripts C 2 C 3 : Alpha function dimensionless parameter : Alpha function dimensionless parameter c : critical property r : reduced property mix : mixture References 1. Cipollina, A., Micale, G., and Rizzuti, L., Seawater Desalination: Conventional and Renewable Energy Processes, Springer, Heidelberg, 1-16 (2009). 2. Salunkhe, D. K., and Kadam, S. S., Handbook of Fruit Science and Technology: Production, Composition, Storage, and Processing, New York, New York: Marcel Dekker, 1-6 (1995). 3. Dan Laudal Christensen Gas Dehydration, Aalborg University Esbjerg, February (2009). 4. Chen, C.-C., Britt, H. I., Boston, J. F., and Evans, L. B., Local Composition Model for Excess Gibbs Energy of Electrolyte Systems. Part I: Single Solvent, Single Completely Dissociated Electrolyte Systems, AIChE J., 28(4), 588-596 (1982). 5. Peng, D.-Y., and Robinson, D. B., A New Two-Constant Equation of State, Ind. Eng. Chem. Fundam., 15(1), 59-64 (1976). 6. Twu, C. H., Coon, J. E., and Cunningham, J. R., A New Generalized Alpha Function for a Cubic Equation of State Part 1. Peng-Robinson Equation, Fluid Phase Equilib., 105 (1), 49-59 (1995).