Determination effects of process parameters on CO 2 reactive absorption system by mathematical modeling

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Americn Journl of Chemicl Engineering 014; (): 8-13 Published online My 30 014 (http://www.sciencepublishinggroup.com/j/jche) doi: 10.11648/j.jche.01400.

1 Americn Journl of Chemicl Engineering 014; (): 8-13 Published online My ( doi: /j.jche Determintion effects of process prmeters on CO rective bsorption system by mthemticl modeling Adeyink Sikiru Yusuff Chrles Uliukhifo Omohimori Kyode Augustine Idowu Deprtment of Chemicl & Petroleum Engineering Afe Bblol University Km 8.5 AfeBblol Wy Ado-EkitiEkiti Stte Nigeri Emil ddress: (A. S. Yusuff) (C. U. Omohimori) (K. A. Idowu) To cite this rticle: Adeyink Sikiru Yusuff Chrles Uliukhifo Omohimori Kyode Augustine Idowu. Determintion Effects of Process Prmeters on CO Rective bsorption System by Mthemticl Modeling. Americn Journl of Chemicl Engineering. Vol. No. 014 pp doi: /j.jche Abstrct: A stedy stte model for CO - rective bsorption system ws developed bsed on principle of mss trnsfer nd chemicl rection. The pseudo-first order model is ssumed nd reversibility of chemicl rection ws lso neglected. The continuity model eqution in term of mteril blnce with chemicl rection cross n elementl stge K on CO ws developed. The model consists of system of liner simultneous equtions nd the equtions representing the composition of CO in both liquid nd gs phses were solved. The simultion studies were performed to investigte the effect of chnging vrious process vribles such s number of plte gs flow rte nd CO composition in the feed vpor. Keywords: Crbon dioxide Rective Absorption Column Process Vribles Modeling 1. Introduction Crbon dioxide hs been identified s one of the mjor cuses of greenhouse effect which results to globl wrming. It is produced in significnt mount from vrious industril processes including fossil fuel firing electric power genertion steel production chemicl nd petrochemicl mnufcturing col gsifiction etc. As men of mitigting the globl wrming removl of CO from industril flue gs is considered importnt (Prk et l 004). The method of removing CO gs mixture include bsorption (physicl or chemicl) cryogenic seprtion membrne seprtion dsorption nd so on. Among these methods CO bsorption by mine queous solution hs been considered s the most efficient wy nd vrious reserch ctivities were conducted by this method nd most commercil processes for the bulk removl of CO from gseous strems involved the use of mines (Mimur et l 1998; Srtori et l 1983; Piton et l 1996). Methyldiethnolmine (MDEA) is tertiry mine used extensively s solvent for removing CO nd H S from process gses [Astrit et l 1983]. The ddition of primry or secondry mine to tertiry mine hs found widespred ppliction in the bsorption nd removl of crbon dioxide from process gses. The success of these solvents is due to the high rte of rection of primry or secondry mine with CO combined with the low het of rection of tertiry mine (Bishnoi et l 000). The equilibrium solubility of CO in queous MDEA solutions hs been experimentlly studied by severl reserchers (Jou Mther & Otto ; Jou Croll Mther & Otto 1993; Bhir 1984; Ho nd Eguren 1988; Austgen Rochelle & Chen 1991). Mny reserchers hd lso recognized the importnce of using fundmentl rte-bsed pproch for modeling the het nd mss trnsfer processes present in seprtion systems (Pcheco et l 1998; Drton 199). Tomcej et l [1987] implemented n efficiency model bsed on the solution of the differentil mss blnce on try of given contctor. In the ppliction to the removl of CO using lknolmines were mde to estimte the enhncement fctor. Pcheco et l [1998] hd lso developed generl frmework to model the trnsport processes tht tke plce during rective bsorption. The work ws imed t developing better understnding of the rte processes present in rective bsorption systems. This present study is imed t studying the effects of operting prmeters on column performnce nd how its size depend on the feed concentrtion (mole frction) of crbon dioxide in the process gses.

2 Americn Journl of Chemicl Engineering 014; (): Mthemticl Model In order to develop model ble to represent chemicl bsorption of crbon dioxide in process gses the following ssumptions re mde: (1) Gs nd liquid flow rtes in ech of the plte re equl i.e... The Mteril Blnce Considering n elementl volume K in rective bsorption column G G G G 1 L L L L 1 N N () Every stge is in equilibrium stge or perfect stge.i.e. yk Mxk k. (3) The bsorption process is ssumed to be stedy stte nd counter-current. (4) There is negligible het effect in the column thus the column opertes with constnt temperture. (5) Liquid holdup is uniform throughout the column. (6) The bsorbent is of dilute concentrtion. (7) A pseudo-first order model is ssumed nd the reversibility of the chemicl rection is neglected (Tomcej et l 1987)..1. Kinetics of Rection In the bsorption process model the stoichiometry eqution of rection between trce component (CO ) nd bsorbent [Aqueous mine (MDEA)] is written s: CO H O MDEA MDEAH HCO (1) k + k 3 The rte of rection of CO with MDEA is represented s: R kc C kc C () MDEA 1 CO MDEA + MDEAH HCO k m kmols x T (Pcheco et l (. ) exp( 607 ) 1998) (3) The mthemticl model is bsed on the continuity eqution: Rte of mteril in Rte of mteril out + Rte of mteril generted by rection + Rte of Accumultion Subject to those forementioned ssumptions nd continuity eqution Mteril blnce cross the K on CO stge L x + G y L x + G y + K xh (4) k 1 k 1 k + 1 k + 1 k k k k R k L All the terms re expressed in kmol/co per hour. The liquid holdup per try (h L ) is pproximted by using correltion found by Miyhr nd Tkhshi (1979 & 199) h 1.53x10 d + 1.4x10 ε µ z d x10 C M L L p p rg µ HO dp (5) Every stge is n equilibrium stge or perfect stge y mx k k k (6) Put eqution (6) into eqution (4) we obtined mg x ( L + mg + k h ) x + L x 0 (7) k + 1 R L k k 1 Eqution (7) bove cn be written s follows: Where: γx + β + α (8) k + 1 xk k 1 0 γ mg (9) β ( L + mg + k h ) (10) R L α L (11) Eqution (8) is second order liner constnt coefficient difference eqution. Since it is second order two boundry conditions re required for specific solution. Here they re: x0 x (1) y mx y x y m N + 1 N + 1 N + 1 (13)

3 10 Adeyink Sikiru Yusuff et l.: Determintion Effects of Process Prmeters on CO Rective bsorption System by Mthemticl Modeling Eqution (8) hs n nlyticl solution; however it cn be expnded s follows: k 1: γx + βx1 + αx 0 (14) k : γx + βx + αx 0 (15) 3 1 k 3: γx + βx + αx 0 (16) 4 3 k N 1: γx + βx + αx 0 (17) N N 1 N k N : γx + βx + αx 0 (18) N + 1 N N 1 This is the system of liner equtions; In the n-unknownsx where Ax b (19) 3. Method of Numericl Solution The method dopted for the solution of system of liner simultneous equtions which resulted to tri-digonl mtrix for the model of chemicl bsorption of crbon dioxide (CO ) in methyldiethnolmine solution is the Gussin elimintion method. A mthemticl lgorithm to solve these systems of liner simultneous equtions ws developed nd implemented into progrm using Microsoft Excel. The prmeters reported by Pcheco et l; 1998 Muhmmd et l; 008 nd literture were used for simultion in this study which re given below CO- Rective Absorption System The used prmeters in simulting the developed model re summrized in Tb 1 & 3. Tble 1: Used prmeters in simultion of CO -Rective Absorption System. Prmeter Rective Solvent eed Vpor MDEA 50 wt% - H S CO kmolhs/kmol MDEA KmolCO/kmol MDEA 0.6mol % *& 17. * mol % H O Blnce Sturted CH mol%. low rte (kmol/h) *& 500* Temperture ( 0 C) 3 3 Pressure (tm) 0 0 Source: Pcheco et l Chrcteristics Tble : Chrcteristics of the Pcked Column Column dimeter (d) 1.68m Column height (h) nd Prking height (z) Corresponding Prmeter 8.0m nd 0.533/segment respectively Number of Plte (N) 4* 5* & 6 Type of contctor Pll rings m (d p) Surfce Tension of pcking (dyne/cm) 75.0 Void frction (ε ) of the pcking 0.95 Source: Pcheco et l Tble 3: Physicl properties of the MDEA nd eed Vpor This is N * N tri-digonl mtrix. Computing the degree of bsorption η ttinble by the column mthemticl expression written below is employed: y y η 1 EED EXIT 0 yeed (0) Property MDEA (Absorbent) eed Vpor Density ( ρ ) (kg/m3) Dynmic Viscosity( µ ) (mp.s) Surfce Tension ( η ) (mn/m) Moleculr Mss (M) Superficil Velocity (u) (m/s) * Source: Muhmmd et l 008 & literture.

4 Americn Journl of Chemicl Engineering 014; (): Simultion Results nd Discussion Rective bsorption of CO in n bsorption column ws studied nd model eqution which described the mss trnsfer nd chemicl rection processes in the column ws developed bsed on continuity eqution. In the modeling process vrious ssumptions hd to be mde in order to simplify the model eqution. A pseudofirst order model ws ssumed nd the reversibility of the chemicl rection ws s well neglected. A mteril blnce eqution ws obtined which result into system of liner equtions which describe the concentrtion (mole frction) of CO in both gs nd liquid phses. The first step tken in the solution of these systems of liner eqution ws the determintion of the initil nd boundry conditions of the system. The finl equtions obtined were then solved using Excel softwre. The results of the nlyticl solution of the model re presented in Tbles 4 to 8. Since no experimentl studies were crried out the input dt used for the model simultion were obtined from Pcheco et l (1998); Muhmmd et l (008) work nd literture however few prmeters were ssumed nd chnged to suit the purpose of this work. The simultion of the model gives concentrtions (mole frctions) of CO in liquid phse nd corresponding mole frctions of CO in gs phse were determined for different operting conditions. The degree of dsorption ws lso obtined using eqution (0). Tble 4: Compositions of CO in Liquid nd Gs Phses t G 300kmol hr L 700kmol hr y 15 mol% m 1.15 nd N 4 phse (x) 5.5 * * * * * * * * 10-3 Tble 5: nd Gs Phse t G 300kmol hr L 700 kmol hr y 15 mol % m 1.15 nd N 5 Phse (x) 5.5 * * * * * * * * * *10-3 Tble 6: nd Gs Phses t G 300kmol hr L 700kmol hr y 15 mol% m 1.15 ndn 6. Phse (x) 5.5 * * * * * * * * * * * * Effect of Number of Plte on the Absorption Process Tble 4 5 nd 6 show the effect of number of plte on rective bsorption of CO t constnt gs flow rte liquid flow rte CO concentrtion (mole frction) in feed vpor nd equilibrium slope. Three different numbers of pltes (N) which re 4 5 nd 6 were considered it ws observed tht the concentrtion of CO in the exit gs strem ws constnt. The degree of bsorption ws then determined using eqution 0 nd ws obtined to be 98.4%. This phenomenon is due to the fct tht gs nd liquid flow rte re lrge which result in low vlue of x (mole frction of CO in liquid phse) for the liquid leving t the bottom. Tble 7: nd Gs Phses t G 490kmol hr L 700kmol hr y 16.3 mol% m 1.15 nd N 5 Phse (x) 5.47* * * * * * *10-7.3* * *10-3 Tble 8: nd Gs Phses t G 500kmol hr L 700kmol hr y 17. mol % m 1.15 nd N 5 Phse (x) 5.36* *10-7.4* * * * *10-7.7* * * Effect of Gs low Rte nd Mole rction of CO on Absorption Process Tbles 5 7 nd 8 show the effect of gs flow rte nd mole frction of the trce component (CO ) on bsorption process it ws observed tht t different vlues of gs flow rte ( nd 500 kmol/hr) nd corresponding

5 1 Adeyink Sikiru Yusuff et l.: Determintion Effects of Process Prmeters on CO Rective bsorption System by Mthemticl Modeling vlues of inlet mole frction of CO ( nd 17. mol%) respectively holding other prmeters constnt the degree of bsorption decreses. When 300kmol/hr of gs flow rte nd mole frction of 15mol% were considered the degree of bsorption ws obtined to be 98.4% t 490kmol/hr nd 16.3mol% it ws 97.8% nd lso it ws 97.5% t gs flow rte nd mole frction of 500kmol/hr nd 17.mol% respectively. This phenomenon ws due to the fct tht liquid flow rte remined unchnged when gs flow rte nd mole frction of trce component (CO ) incresed. This problem could be solved to some extent by incresing the liquid flow rte or recirculting the liquid over the tower. 5. Conclusion In this present study mthemticl model ble to represent both mss trnsfer nd chemicl rection processes t given operting conditions for the rective bsorption of CO in n bsorption tower ws developed nd solved nlyticlly by Mtrix method vi Excel. Prmetric study crried out on the model shown tht the concentrtion of CO in the exit gs strem ws constnt using different numbers of plte; this ws due to lrge vlues of gs nd liquid flow rtes which result in low vlue of mole frction of CO in liquid phse. Also the degree of bsorption decreses s gs flow rte nd trce component s composition increses holding other prmeters constnt. Nottion G gs flow rte kmol/hr. L liquid flow rte kmol/hr y mole frction (composition) of CO in gs phse. x mole frction (composition) of CO in liquid phse. R MDEA rte of rection of methyldiethnolmine mol/m 3 s C concentrtion of specie mol/m 3 T temperture K h L liquid hold up per try kmol.m 3 /m 3 m equilibrium slope N number of plte d column dimeter m h column height m P pressure tm z prking height m 4 gµ L M Morton number of liquid 3 ρ σ C L L µ U L L σl RG U L U G U gd G Cpillry number of liquid roude number of gs superficil velocity of liquid m/s superficil velocity of gs m/s g ccelertion due to grvity m/s Greek Letter ε ρ density kg/m 3 µ viscosity Ns/m η surfce tension mn/m η L void frction of the prking degree of Absorption α liquid flow rte kmol/hr. β ( L + mg + k h ) kmol/hr. γ G Subscript R L gs flow rte kmol/hr. bsorption feed N number of plte G gs L liquid K stge P prking R rection 0 initil vlues Superscript *(Asterisk) References ssumed Vlue [1] Astrit G. Svge D.W. Bisio A. (1983). Gs Treting with Chemicl Solvents; John Wiley & Sons: New York. [] Austgen D.M. Rochelle G.T. &Chen C.C (1991). A Model of Vpor-Liquid Equilibrium for Aqueous Acid gslknolmine Systems. Industril nd Engineering Chemistry Reserch [3] Bhn A. (1984). Experimentl equilibrium between cid gses nd ethnolmine solutions. Ph.D disserttion Oklhom Stte University. [4] Drton R.C. (199). Distilltion nd Absorption Technology: Current Mrket nd New Developments Trns. Inst. Chem. Eng. 70 (Prt A) 435. [5] Ho B. nd Eguren R. (1988). Solubility of Acidic Gses in Aqueous DEA nd MDEA solutions. Presented t the 1988 AICHE Spring Ntionl Meeting Mrch [6] Jou.Y Croll J.J. Mther A.E. & Otto.D (1993). The solubility of Crbon dioxide nd Hydrogen Sulphide in 35 wt% Aqueous Solution of Methyldiethnolmine. The Cndin Journl of Chemicl Engr [7] Jou.Y Mther A.E & Otto.D (198). Solubility of HO nd CO in Aqueous Methlydiethnolmine Solutions. Industril nd Engineering Chemistry Process Design nd Development

6 Americn Journl of Chemicl Engineering 014; (): [8] Jou.Y Mther A.E &Otto.D (1984). Vpor-Liquid Equilibrium of Crbon dioxide in Aqueous Mixture of Monoethnolmine nd Methyldiethnolmine. Industril Engineering Chemistry nd Engineering Dt [9] Mimur T. Sud T. Iwki I. Hond A. Kmzw. H (1998). Chemicl Engineering Comm [10] Miyhr T. Ogw K. Hirde A. Tkhshi T. (199). luid Dynmics in Low Height Pcked Columns Hving Lrge rctionl Void Spce. Chemicl Engineering Science 47 (13/14) [11] Muhmmd A. Abdul Mutlib M.I. Wilfred C.D Murugesn T. nd Shfeeq A (008). Viscosity Refrctive Index Surfce Tension nd Therml Decomposition of Aqueous N-methyl diethnolmine solutions from (98.15 to ) K. Journl of Chemicl Engineering Dt [1] Pcheco M.A nd Rochelle G.T. (1998). Rte- Bsed Modeling of Rective Absorption of CO nd H S into Aqueous Methyldiethnolmine. Industril nd Engineering Chemistry Reserch [13] Pcheco M.A. Kginoi S. nd Rochelle G.T. (000). CO Absorption into Aqueous mixtures of Diglycolmine nd Methyl-dietholmine Chemicl Engineering Sciences 55(1) [14] Snjy B nd Rochelle G.T (000). Absorption of crbon dioxide into queous piperzine: Rection Kinetics Mss Trnsfer nd Solubility. Chemicl Engineering Science [15] Srtori G. Svge D.W. (1983). Ind. Eng. Chem. undm [16] Sung Y.P Byoung M.M. Jong S.L& Sung C.N (004). Absorption Chrcteristic of Continuous CO Absorption Process. Prepr. Pp-Am. Chem. Soc. Div. uel Chem 49(1) [17] Tkhshi T. Akgi Y. Ueyem K. (1979). A New Correltion for Pressure Drop in Pcked Columns. Journl of Chemicl Engineering of Jpn 1(5) [18] Tomcej R. A. Otto.D. Rngwl H.A. Mrrel B.R (1987). Try Design for Selective Absorption Gs Conditioning Conference Normn Oklhom.

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