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Ind. Eng. Chem. Res. 2010, 49, 10046–10058
Process Intensification in HiGee Absorption and Distillation: Design Procedure and Applications Lava Agarwal, V. Pavani, D. P. Rao, and N. Kaistha* Department of Chemical Engineering, Indian Institute of Technology Kanpur, Kanpur 208016, India
HiGee technology is emerging as an alternative to conventional tray and packed towers for mass-transfer applications. To evaluate the process intensification (volume reduction) potential of the technology, a systematic design procedure is developed in this work. HiGee designs using split packing innovation of Chandra et al. [Ind. Eng. Chem. Res. 2005, 44, 4051-4060] are developed for four industrially relevant distillation and absorption systems. Comparison with conventional packed or tray tower designs shows that significant volume reduction is possible. For the systems studied, the process intensification achieved is particularly impressive when the gas side mass-transfer resistance is dominant. The results suggest that split packing design innovation may be particularly suitable for intensification of gas side mass-transfer resistance controlled processes. 1. Introduction 1
Ramshaw and Mallinson pioneered the concept of a rotating packed bed (RPB) to enhance mass-transfer rates by replacing gravitational field with centrifugal acceleration 100-1000 times gravity and aptly termed it HiGee, an acronym of high gravity. The high centrifugal acceleration allows the use of packing with significantly higher specific surface area and achieves order(s) of magnitude higher gas (vapor)-liquid throughputs and possibly mass-transfer rates. The combination of these factors can significantly reduce the size of conventional mass-transfer equipment such as absorption and distillation towers. Ramshaw2 speculated of achieving up to 100-fold reduction in equipment size using HiGee for distillation. Experimental studies by Kelleher and Fair3 and Lin et al.4 however showed only a 5-10-fold reduction in HETP. Rao et al.5 and Zheng et al.6 pointed out that the gas (vapor) experiences high frictional drag in the rotating bed because of the high surface area packing and acquires the angular velocity of the packing within a short span (∼millimeters) of entering into the packing. The gas then undergoes solid-body-like rotation with the packing so that the angular slip velocity between the liquid flowing over the packing and the gas is negligible. The enhancement in the gas side masstransfer coefficient compared to conventional towers is therefore likely to be negligible. To promote gas-liquid slip inside the packing, Chandra et al.7 proposed an innovative split packing rotating packed bed (RPB) design where the packing is split into concentric annular rings and attached to two different disks such that when the disks are brought together, a small gap remains between the adjacent rings (Figure 1). For the two disks rotating in opposite directions, large gas-liquid slip in the space between the rings is achieved with the liquid and gas flowing radially outward and inward, respectively (countercurrent flow). More recently, Ji et al.8 have come up with a zigzag HiGee, which permits multiple feeds and side gas/liquid draw offs. Its unique structure which can function without a liquid distributor and without a dynamic seal improves the safety and reliability of the equipment. Since the inception of the HiGee idea more than 2 decades ago, only a few commercial applications have been reported.9-11 One possible reason is that conventional HiGee with a continuous single block packing causes a significant reduction only in * To whom correspondence should be addressed. E-mail: nkaistha@ iitk.ac.in. Fax: +91-512-259-0104. Phone: +91-512-259-7513.
the liquid side mass-transfer resistance with little to no reduction in the gas side resistance. Substantial process intensification is thus achieved only in liquid side resistance controlled masstransfer applications. The recent HiGee design innovations are however very likely to expand its application scope to include gas side resistance controlled mass-transfer processes, distillation being one such process, and more commercial HiGee applications are expected to materialize in due course. Rao et al.5 present an appraisal of the potential of HiGee for distillation and absorption, the most commonly applied separation unit operations. One of the first steps in evaluating the potential of HiGee for a particular distillation/absorption application for a given production objective is to design the unit and compare the size reduction (or process intensification) with a conventional packed/ tray tower design. The design of a HiGee unit, however, differs significantly from a conventional tower due to the difference in flow geometry (radial flow in an RPB vs flow along the tower length) and RPB rotational speed as an additional degree of freedom as well as the need for proper design of the liquid/gas inlets and outlets in view of the significantly higher gas-liquid
Figure 1. Split packing rings mounted on disks.
10.1021/ie101195k 2010 American Chemical Society Published on Web 09/07/2010
Ind. Eng. Chem. Res., Vol. 49, No. 20, 2010
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Figure 2. HiGee unit schematic: (a) continuous packing, (b) split packing. Legend: 1, liquid feed inlet; 2, liquid outlet; 3, vapor inlet; 4, vapor outlet; 5, liquid distributor; 6, motor; 7, packing; 8, supporting discs; 9, baffle; 10, liquid pool.
throughputs per unit area. Even as established procedures exist for designing a conventional tower, an examination of the literature reveals that a systematic design procedure for HiGee systems has not been reported. There exist experimental reports that quantify and correlate the limiting gas-liquid flows through an RPB for specific packing types.12-15 Some studies also quantify and correlate the volumetric mass-transfer coefficient for simple systems such as CO2 absorption in aqueous NaOH,15 VOC removal from air16 and water,13 SO2 absorption in aqueous NaOH,17 and cyclohexane/n-heptane distillation,3 etc. A systematic design procedure utilizing these flooding and masstransfer correlations along with a treatise on salient design considerations for HiGee absorption/distillation systems is however not available. Moreover, there are no studies that systematically evaluate the process intensification using HiGee over conventional towers. This work develops such a systematic design procedure and applies it to four industrially relevant absorption/distillation systems quantifying the process intensification potential of the technology. In the following, a brief description of HiGee units for absorption or distillation and the corresponding design task is provided. The considerations in fixing the various design parameters of a HiGee unit are briefly described and combined into a systematic step-by-step design procedure. The procedure is applied to the design of four industrially relevant systems, an n-butane/isobutane and a benzene/cumene distillation column, a natural gas dehydration system using triethylene glycol (TEG) as the absorbent, and a CO2 absorption unit using aqueous diethanol amine. The process intensification potential with respect to a conventional tower is quantified and discussed. The article ends with the conclusions from the work. 2. HiGee Unit Description Figure 2 shows a sketch of a HiGee unit for high-throughput countercurrent gas (vapor)-liquid contact. The packing, a single block (Figure 2a) or split annular rings (Figure 2b), is attached to side disks, which in turn are connected to a rotating shaft(s). The assembly of packing, disks and shafts is housed in a casing. Liquid flows radially outward from the inner periphery of the packing due to centrifugal force, and the gas, introduced into the casing, flows radially inward from the RPB outer periphery due to the imposed pressure gradient. Countercurrent gas-liquid contact is thus achieved inside the packing. Note that, in the split packing RPB design, the two disks onto which the alternate packing rings are attached are rotated by separate motors.
Rotating the two disks in counter direction causes significant gas-liquid slip in the small gap between the adjacent rings due to the opposite tangential fluid flow direction. The RPB assembly as in Figure 2 is suitable for gas-liquid absorption (desorption). The RPB unit may also be used as a stripping section or an enriching section. If the RPB size is sufficiently small, columnless distillation can be envisaged with the enriching and stripping RPBs fitted into the condenser and the reboiler, respectively (Figure 3a). Alternatively, where the separation is easy and the enriching/stripping RPBs are sufficiently small, a single RPB with off-center feed and central reflux may be fitted into the reboiler (Figure 3b) to replace the bulky column section. The packing section between the reflux and the feed then acts as the enriching section while the packing section beyond the feed until the outer periphery acts as the stripping section. Even as Figure 3 envisages the RPBs as housed inside the reboiler/condenser for columnless distillation, for practical reasons such as ease of fabrication and maintenance, the RPBs may be housed separately. 3. Basics of RPB Design Figure 4 depicts an RPB (split or conventional design) with its basic dimensions, namely, the inner and outer packing radii (ri and ro, respectively) and the axial width (h). These basic dimensions must be chosen for accomplishing the given separation task (absorption, stripping, or enriching). The separation task is typically specified in terms of the feed to be processed and the level of purification desired. For example, a typical absorber design problem would require designing the RPB (or conventional column) for processing a specified feed gas (given flow rate, composition, temperature, and pressure) such that the mole fraction of the absorbed component in the exit gas is below a small value (for example,
