(55d) Mass Transfer Studies on a Novel Rotating Packed Bed (Higee)

Keywords: Rotating packed bed; interfacial area; liquid side Mass Transfer Coefficients;

Process Intensification (PI) is a design philosophy that can lead to significant energy, capital, environmental and safety benefits through dramatic reductions in the plant size for a given production objective. Distillation and absorption are two widely used unit operations in industry for separation. The design of these units is governed by gravity force which limits throughput and mass transfer rates resulting in the ubiquitous bulky towers. Ramshaw 1983, first proposed the use of rotating packed beds (RPB) for achieving higher ?g' through centrifugal acceleration with enhancement in mass transfer and throughput rates by 1-2 orders of magnitude and consequent drastic reduction in the column size for the same production objective. The process was aptly termed HIGEE (high gravity).

Research in HIGEE has shown that even as the liquid side volumetric mass transfer coefficient increases by an order-of-magnitude over conventional packed beds, there is little or no enhancement in the gas side volumetric mass transfer coefficient [2]. Rao 2004, proposed a novel RPB design with split-packing to enhance the gas slip velocity for higher gas side mass transfer coefficient. The design of the novel RPB can be seen in Chandra et al, 2005. A photograph of the split packing is shown in Figure 1. Concentric packing rings are attached to the two disks which in turn are attached to two different motor driven shafts. The two disks fit such that there is a small gap between adjacent rings and can thus co-rotate or counter-rotate. The high gas slip velocity in counter-rotation is speculated to result in enhancement in the gas side mass transfer coefficient.

Reddy et al, 2006 showed that the enhancement in volumetric transfer coefficients on the gas side (k_Ga) and liquid side (k_La), respectively, for the novel split packing were in the ranges 35-280 times and 25-250 times compared to packed columns. This work is an extension to separately quantify the interfacial area for mass transfer (a) and the liquid side mass transfer coefficient (k_L) for the novel split-packing RPB design. Danckwert's chemical method is applied for obtaining the interfacial area ?a'. Absorption of CO₂ in aqueous NaOH, a liquid side controlled system, constitutes the experimental system studied. Experiments were done on two types of metal foam packings (specific surface area = 1700 and 2500 m²/m³, porosity = 0.9), supplied by RECEMAT International B.V. Holland. Data for CO₂ absorption rates at high NaOH concentration (1 N), where the assumptions of a fast reaction (compared to mass transfer) and pseudo-first order kinetics are valid so that the rate of absorption in independent of k_L, is used for calculating the interfacial area ?a' for mass transfer. Using the values obtained for the interfacial area, the liquid side mass transfer coefficient is calculated from CO₂ absorption rate data at low NaOH concentration (0.05N) where the absorption rate depends on k_L.

The setup (Fig. 2) consists of a rotating packed bed (RPB), a blower, a liquid feed pump, motors and other auxiliary equipments. The blower is used to supply air in the RPB and the centrifugal pump circulates aqueous NaOH through the unit. In the discharge lines of both air and aqueous NaOH, bypass lines are provided to control the flow rate. The flow rates of the liquid are measured by calibrated rotameters. Two types of rotameters are used in the setup. For low liquid flow rates (25.7 and 45.7 ml/s), rotameter having range of 0-50 ml/s was used and for high liquid flow rates (82, 104.57 and 130 ml/s), rotameter having range of 0-300 ml/s is used. Gas flow rate is measured by an orifice meter. The packing RPM is varied from 500 to 2000. CO₂ concentration at the inlet and outlet is measured by ?INSPECT AIR CO₂ Meter, Model 8560? by TSI. The inlet and outlet gas CO₂ concentrations are measured to quantify the rate of absorption.

Preliminary experimental results show that the enhancement in liquid side mass transfer coefficient ?k_L' comes out to be 15-35 times that of conventional packed columns. The calculated interfacial area for mass transfer comes out to be about half of the manufacturer specified surface area. Even so, the calculated interfacial area ?a' is 20-40 times that in conventional beds. Detailed results on the variation in the interfacial area and the liquid side mass transfer coefficient with RPM, liquid and gas flow rates and co / counter rotation of the adjacent packing rings will be presented in the conference.

References:

1. Ramshaw, C. (1983), ?Higee distillation-an example of process intensification?, The Chemical Engineer, February, 13-14

2. D.P. Rao, A. Bhowal and P.S. Goswami, ?Process Intensification in Rotating Packed Beds (HIGEE): An Appraisal?, Ind.Eng.Chem.Res., 43, 1150-1162
3. 
   
   

   Gas velocity (m/s)

   A.Chandra, P.S. Goswami and D.P. Rao (2005), ?Characteristics of flow in a rotating packed bed (HIGEE) with
   Split Packing?,  Ind.Eng.Chem.Res., 44, 4051-4060

4. Reddy K.J., A. Gupta, D.P. Rao and O.P. Rama (2006), ?Process Intensification in a HIGEE with split packing?, Ind.Eng.Chem.Res., 45, 4270-4277

5. M. M. Sharma, P. V. Danckwerts (1970), ?Chemical methods of measuring interfacial area and mass transfer coefficients in two fluid systems? British Chemical Engineering., Vol. 15, No. 4, 522-528

Fig. 1: Photograph of metal foam packing arrangement on the rotor disks