Quickly design CO 2 amine absorber

Transcription

1 Indian Journal of Chemical Technology Vol. 13, January 006, pp Quickly design CO amine absorber Prakash D Vaidya & Vijaykumar V Mahajani* Chemical Engineering Division, Institute of Chemical Technology, University of Mumbai, Mumbai , India vvm@udct.org Received 1 December 004; revised received ugust 005; accepted 6 October 005 n integral rate-based model is used to find a quick and reliable estimate of the height of packing in a CO - alkanolamine absorber. Heat transfer in the gas phase is neglected. Due to the need to arrive at a quick estimate of the height of packing, this assumption is made. Starting from the bottom, the height of packing required for a small change in solute mole fraction in a differential section of the tower is calculated. n average of the enhancement factor calculated using film theory of mass transfer accompanied by chemical reaction for each boundary, is used. These calculations are repeated until at the top of the tower, the desired solute concentration is reached. The cumulative height gives total height of packing required. The method is illustrated. Keywords: CO, bsorption, lkanolamine IPC Code: Int. Cl. 7 C01B31/00; B01D15/00 bsorption of CO is of considerable importance in the manufacture of ammonia. fter the shift converters (high temperature shift, HTS and low temperature shift, LTS), the synthesis gas contains N, H, CO and CO at ppm level. In order to reduce load on methanator, and hydrogen consumption in totality, it is essential that CO is removed by absorption/ adsorption. The CO absorption is also important in natural gas and associated gas processing, particularly after selective desulfurization is done. The latest generation plants use chemically reactive absorbent for CO removal. n aqueous solution of alkanolamine [mainly monoethanolamine (ME), diethanolamine (DE) and methyl diethanolamine (MDE) promoted with stronger amines] being the most popular absorbent, the same is dealt with in the present study. The simplified methodology presented is applicable to any system in general. The process design of an absorber is based on hydrodynamic and mass transfer considerations. The hydrodynamics in column involves two phase flow, namely, gas phase to be treated and liquid phase as an absorbent. The column hydrodynamics through mass transfer coefficients k L a and k a has significant influence on process design based on mass transfer considerations. detailed discussion on hydrodynamic considerations is excluded from the scope of this work. Here, the aim is to decide the height of packing or mass transfer device, which is governed by mass transfer considerations. The CO -alkanolamine reaction is exothermic. Typically, in an industrial absorber, the liquid phase temperature at the top is 50 C and at the bottom is around 70 C. So in addition to mass balance, thermal effects have to be accounted for in process design. The heat generated in the liquid phase due to chemical reaction (ΔH R ) is dissipated in three ways, namely direct heat transfer between the gas and liquid phase, saturation of gas phase by vaporization of water and heat loss to the surroundings. In large-scale industrial absorbers, the heat loss to the surroundings is negligible. In ammonia plant, the reformed gas is at temperature near to absorption temperature say 50 to 70 C after the shift converter followed by heat exchange in a heat recovery system. The gas is saturated with water and hence practically very little water vaporization can take place. More or less the same is the case with the reformed syn-gas, CO from ethylene oxide plant and CO gas laden stream from desulfurization column. The rate of heat transfer to gas will be very low due to very low heat transfer coefficient. Therefore, it was thought desirable to get away with the complicated procedure of solving simultaneous mass and heat transfer rate expressions.

2 48 INDIN J. CHEM. TECHNOL., JNURY 006 simple practical approach of adiabatic mode (only liquid phase) of operation is presented here. The heat liberated due to chemical absorption is thus assumed to result in the rise of temperature of liquid phase only. Process design The CO -alkanolamine absorber operating in countercurrent fashion in an adiabatic mode of operation is shown in Fig. 1. It is assumed that the gas and liquid phases move in a plug flow manner and that the operation is isobaric. From a mass balance for CO over a small section of differential height dz at the bottom of the tower, the height of packing is given by Z = s dy Ra S (1- y ) (1) y The volumetric rate of absorption R a can be expressed as R a = k a (P P i ) () = k L a { ( i *) ( 0 ) } E = k L a H {P i P 0 } E (3) From Eqs () and (3), eliminating unknown ( i *), ( P P0 ) Ra = (4) (1/ k a + 1/ k a HE) L (P P 0 )=P T (y y 0 ) (5) Let β be defined by β =(1/ k + 1/ k L H E) -1 (6) Substituting in Eq. (4), R a = β a P T (y y 0 ) (7) The value of β remains practically constant throughout the length of the column in case of physical absorption. However, when absorption is accompanied by a chemical reaction, the rate of absorption varies along the column height due to change in the enhancement factor E and hence the value of β is not constant. From Eqs (1) and (7), one has s dy (8) T β y 0 Z = Sa P ( y y )(1 y ) Eq. (8) is the general design equation for calculating the height of packing. The reaction between CO and alkanolamine is, in true sense, reversible. This always results in equilibrium partial pressures of CO, also Fig. 1 Countercurrent packed absorption tower known as backpressure. t the tower bottom, the solute CO exerts equilibrium backpressure. The mole fraction in gas being very high the difference (y y 0 ) is almost the same as y. Therefore, the effect of reversible reaction can be neglected. In other words reaction is pseudo irreversible. In the control volume or height under consideration, the change in mole fraction of CO is assumed such that the variation in enhancement factor E and hence β at the top and bottom of this control volume is very small. Therefore, an average value of β can be safely used for finding the height of packing. Eq. (8) then takes the form s dy Tβ (9) y Z = Sa P y (1 y ) On the other hand, at the top of the column, the solute CO exerts very low but finite backpressure over the absorbent. lso the mole fraction y of the gas being absorbed is very small. Therefore, the difference (y y 0 ) is different from y and hence the effect of y O (backpressure) is significant. Thus, the effect of backpressure (equilibrium) becomes dominant and here, the reversible nature of the reaction needs to be considered. The value of y 0 is taken as the mean value of the equilibrium mole fractions calculated at the top and bottom of the control volume. The equation used for the top section of the tower is s dy Tβ (10) y 0 Z = Sa P ( y y )(1 y )

3 VIDY & MHJNI: QUICKLY DESIN CO -MINE BSORBER 49 The design algorithm The following rate consideration approach is based on the film theory of mass transfer. The mole fraction of CO at inlet depends upon the performance of shift converters and feedstock (syngas originating from natural gas, naphtha or coal). The mole fraction of CO at exit is decided by overall optimization including downstream methanator. The CO content in lean and rich amine is decided by energy requirement in the regenerator and corrosion, respectively. It is assumed that mole fractions of CO at inlet and exit and gas and liquid flow rates are known. s discussed earlier, the heat released is assumed to be absorbed by the absorbent liquid. The inlet temperature of absorbent is known. Therefore temperature, T, at the bottom of the column is fixed by energy balance. By and large, 0-30 C temperature rise is observed in liquid phase depending upon the heat of reaction (ΔH R ) and liquid flow rate. Some values of heat of reaction are depicted in Table 1. Thus, terminal temperatures are fixed by adiabatic mode. The absorption of CO in alkanolamine under industrial operating conditions (strength of amine and temperature) exhibits first order behaviour with respect to CO and alkanolamine, thus making overall second order reaction. The enhancement factors under various conditions of operation are listed in Table. It is further assumed that the pressure drop across the column is negligible. Therefore, the column operates at constant pressure. The logic block diagram for the proposed design procedure is shown in Fig.. The controlled volume, dv, where CO absorption takes place also needs consideration. The calculation algorithm starts from the bottom of the tower (gas inlet and absorbent exit conditions). 1 The column diameter is obtained via flooding considerations. part from flooding, consideration is Table Enhancement factor E under various conditions of operation (Reaction: + w B Products, First order with respect to and B and hence overall second order, Reaction takes place in B phase, Film theory of mass transfer holds true) Case No. M E m = 1, n = 1 1 M = 1 or M < 3 M << q 3 << M << q M 3 M >> 3 M q 4 M > 5 M >> q m 1 mn( *) ( 0) (m+1) Dk B M = kl q= ( B0) DB w ( *) D E i = 1 + q n 1/ 1/ M ME i M 1 i i i E = + + 4( E 1) ( E 1) ( E 1) De Coursey 1 (M + 1) E E i Table 1 Heats of reaction for absorption of CO in alkanolamine solutions lkanolamine Exothermic heat of reaction, kcal kg -1 ME DE TE D Kohl and Risenfield 11 Fig. Logic block diagram for proposed design procedure

4 50 INDIN J. CHEM. TECHNOL., JNURY 006 also given to possible foaming due to degradation of alkanolamine. The superficial gas velocity, liquid velocity is determined and hence the values of k, k L and a are obtained from published data. 3 From an overall mass and energy balance, the exit temperature of the liquid at the tower bottom can be obtained. ΔH R =m C P dt (11) 4 The value of the enhancement factor E and hence β at the exit can be calculated with the help of formulae exhibited in Table. 5 value of y, i.e. mole fraction of CO at the top of the control volume is selected, as per the situation of the absorption system. 6 The temperature of the liquid stream entering this control volume is found from an energy balance (Heat released = Heat gained by absorbent liquid). 7 The free alkanolamine concentration in the liquid phase entering this control volume is found from a material balance over this volume. 8 s before, the values of E and β at the top of this control volume are calculated. If the variation in β at the top and bottom is less than 10%, then an average value of β is used to find the height of this control volume. It is to be noted that β accounts for change in H and E at the boundaries of control volume. revised value of y is to be assumed if this variation is more than 10%. 9 In the bottom section of the tower, absorption is not sensitive to equilibrium pressure of CO, so Eq. (9) can be used to find the height of packing. 10 The calculations proceed upward in similar fashion until the top section of the tower is reached where CO exerts a finite backpressure over the absorbent solution. If (y y 0 )/y < 0.9, the backpressure needs to be considered. The equilibrium condition for CO in liquid is obtained from the data published by Kent and Eisenberg 1. Eq. (10) is then used instead to find the height of packing. 11 The total packed height is the sum of the heights of all sections. This design procedure would enable the process design engineer to find a quick estimate of the height of packing. The results obtained from such an approach can serve as a preliminary estimate for a more elaborate rigorous design procedure. Such procedures have earlier been used by various other researchers -7. Illustration This calculation algorithm described above was used to find the height of packing for a worked example presented by Danckwerts and Sharma 8 on absorption of CO in ME. Here, the adiabatic approach was adopted. The relevant data are as follows: Inlet gas composition: N, 18.7, H, 56.3 and CO, 5.0%; Pressure 06.5 kpa; Liquid inlet temperature, 303 K; as flow 0.5 kmol s -1 ; bsorbent.5 M ME solution, flow rate 0.1 m 3 s -1 ; Carbonation ratio at inlet and outlet 0.15 and 0.4; CO in treated gas 10-3 %; and Packing 1½ inch Raschig rings. The gas temperature throughout the length of the column was assumed to be 303 K. The equilibrium pressure of CO along the entire length of the column was negligible when compared with the partial pressure of CO in the gas. For the sake of simplicity, the physical properties of the liquid such as density (1000 kg m -3 ) and specific heat were assumed to be constant. The diffusivities of the amine and CO in solution at 303 K were assumed to be and m s -1, respectively. 1 The column diameter was assumed to be known (.75 m). The effective gas-liquid interfacial area was assumed to be 140 m /m 3. The value of k L at 303 K was assumed to be m s -1. The variation in k L along the length of the column could be found using the relation k L α D. lso, it was further assumed that the value of k a at a point where the mass flow rate of the gas is (lb h -1 ft - ) is given by k a= ( /360) 0.7 (1) 3 From an overall energy balance, the exit temperature of the liquid was found to be K. The variation in the temperature of the liquid phase along the height of the column is shown in Fig The second order reaction rate constant for CO -ME system was found using the following correlation 9 log 10 k = (14.34/T) (13) 5 The temperature coefficient of diffusivity of amine D B in solution was assumed to be the same as

5 VIDY & MHJNI: QUICKLY DESIN CO -MINE BSORBER 51 Fig. 3 Variation in temperature of liquid along the height of the column that of CO in water. The diffusivity of CO in water was found using the following correlation given by Versteeg 10 D CO, H O= exp (-119 / T) (14) 6 Further, it was also assumed that the ratio of diffusivity of CO in.5 M ME to that in pure water at 98 K was 0.64 and that it remains unchanged throughout the length of the column. The ionic strength varies from to 1 g ion L -1 and hence H varies from 10 -( ) = 0.94 to 10 -(0.07 1) =0.85 times that in pure water. The solubility of CO in water was found using the following correlation given by Versteeg 10 H CO, H O= exp (044/T) (15) 7 In the lower part of the column, from a solute mole fraction of 0.07 to 0.5, the CO -alkanolamine reaction is instantaneous. Therefore, the height is independent of the solubility parameter H and instead varies with k L. lthough a large amount of solute is absorbed here (73.9%), this section accounts for only 0% of the total height. The rise in temperature of the liquid phase is about 10 C. However, the change in k L in this section is only 11%. 8 The reaction is in the depletion regime from a solute mole fraction of 0.03 to Thereafter, the system is confined to the fast reaction regime in the upper part of the column up to a solute mole fraction of Here, the height depends on the term H (D k ) 1/ and is independent of k L. The contribution of this section to the total height is maximum (73%) while the amount of solute absorbed in this section is only 1.5%. Here, the liquid phase temperature increases by around 3 C only. So the variation in H (D k ) 1/ in this section is only 1%. 10 The total height of packing was found to be 4.5 m, which compared well with the value reported by Danckwerts and Sharma 8 for isothermal operation (4.6 m). From the foregoing results, it can be concluded that the height of packing in adiabatic operation is relatively insensitive to changes in k L and H (D k ) 1/ with temperature. Instead, the height is more sensitive to the effective gas-liquid interfacial area. This, in turn, depends on the type of packing, which is supplied by the manufacturer, the properties of the absorbent liquid and the system under consideration. Therefore, the emphasis should be given on accurate determination of the interfacial area a for any particular system, which will enable the design engineer to find a reliable estimate of the height of packing for a commercial contactor. The mass-transfer description by rate-based models is now increasingly being used in process design of CO absorbers. These models are implemented into a commercial simulator (spen Custom Modeler is preferred) to carry out verification and sensitivity analysis. The simple procedure presented here gives the design engineer one more option of checking calculations of the process licensor in quick time, before using these commercial simulators. Thus, review of the basic engineering package is facilitated to make it cost-effective due to savings on engineering man-hours. Conclusion simplified rate-based approach for finding the height of packing in a CO -alkanolamine absorber is presented. s is often the case in industrial packed absorbers used in ammonia manufacture, the gas phase heat transfer is neglected. The procedure can be applied to any gas-liquid reactive absorption by appropriately confining to the assumptions made here. cknowledgement P D Vaidya wishes to thank University rants Commission, New Delhi, overnment of India, for the financial support provided during this study.

6 5 INDIN J. CHEM. TECHNOL., JNURY 006 Nomenclature a = gas-liquid interfacial area, m m -3 = solute gas, CO ( i *) = concentration of CO at gas liquid interface in equilibrium with the liquid, kmol m -3 ( 0 ) = concentration of CO in the bulk of the liquid, kmol m -3 B = reactant in liquid phase, ME (B 0 ) = free amine concentration, kmol m -3 (CO ) = concentration of CO in bulk liquid, kmol m -3 C P = specific heat of absorbent, kcal kg -1 K -1 dv = control volume D B = diffusivity of ME in.5 M ME solution, m s -1 D = diffusivity of CO in.5 M ME solution, m s -1 D CO, H O = diffusivity of CO in water, m s -1 E = enhancement factor due to chemical reaction = molar flow rate of gas phase, kmol s -1 s = molar flow rate of inerts, kmol s -1 = mass flow rate of gas phase, (lb h -1 ft - ) H = Henry s law constant, kmol m -3 kpa -1 H CO, H O = Henry s law constant for CO in water, kmol m -3 kpa -1 ΔH R = heat of reaction, kcal s -1 k = gas side mass transfer coefficient, kmol m - s -1 kpa -1 k L = true liquid side mass transfer coefficient, m s -1 k = reaction rate constant for the CO -ME reaction, m 3 kmol -1 s -1 m = order with respect to solute gas CO m = mass flow rate of liquid, kg s -1 n = order with respect to ME P = partial pressure of CO in bulk gas phase, kpa P i = partial pressure of CO at gas-liquid interface, kpa P 0 = equilibrium pressure of CO, kpa P T = total pressure, kpa P* CO = equilibrium pressure of CO, kpa R a = volumetric rate of absorption, kmol m -3 s -1 S = cross-sectional area of the column, m T = temperature of liquid phase, K w = stoichiometric coefficient of reaction y = mole fraction of CO in the gas phase y 1 = mole fraction of CO at the bottom of the control volume y = mole fraction of CO at the top of the control volume y 0 = mole fraction of CO in the bulk liquid Z = height of packing, m β = variable defined by Eq. (6) 1, = subscripts for values at bottom and top References 1 Kent R L & Eisenberg B, Hydrocarbon Processing, 55 (1976) 87. Kenig E Y, Kucka L & orak, Chem Eng Technol, 6 (003) Kenig E Y, Schneider R & orak, Chem Eng Sci, 56 (001) latiqi I, Sabri M F, Bouhamra W & lper E, as Sepn Purifn, 8(1) (1994) 3. 5 Thorat P, Tontiwachwuthikul P & Meisen, as Sepn Purifn, 7(1) (1993) Phaneswara Rao D, as Sepn Purifn, 5(3) (1991) Linek V, Sinkule J, Richter M & Pospisil J, Ind Eng Chem Res, 9 (1990) Danckwerts P V & Sharma M M, I Chem E Review Series No, (1966) CE Pandya J D, Chem Eng Commun, 19 (1983) Versteeg F & van Swaaij W P M, J Chem Eng Data, 33 (1988) Kohl L & Riesenfeld F C, as Sepn Purifn, 3 rd edn (ulf Publishing Company, Houston), De Coursey W J, Chem Eng Sci, 9 (1974) 1867.