Oscillatory baffled reactor
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There is a growing need in the chemical and pharmaceutical industries for the more efficient use of reagents, solvents and energy while minimising the production of waste materials. These needs have led to a number of continuous reactors being developed in recent years, and oscillatory baffled reactor (OBR) is one such a reactor. The OBR technology generally consists of a cylindrical column or tube containing equally spaced orifice baffles and superimposing with fluid oscillation. Vortices are generated when fluid flow past through the baffles as shown in Figure 1, enabling significant radial motions where events at the wall are of the same magnitude as these at the centre.[1]
The generation and cessation of eddies creates uniform mixing in each baffled cell, collectively along the column or tube. The fluid mechanical conditions in an OBR are generally governed by two dimensionless numbers: the oscillatory Reynolds number (Reo = 2πfDxo/μ) and the Strouhal number (St = D/(4πxo)), where D is the column diameter (m), ρ the fluid density (kg/m3), μ the fluid viscosity (kg/ms), xo the oscillation amplitude (m) and f the oscillation frequency (Hz). The oscillatory Reynolds number describes the intensity of mixing applied to the column, while the Strouhal number is the ratio of column diameter to stroke length, measuring the effective eddy propagation.[2] The OBR is related to batch and fed-batch operations, and generally operated vertically. In OBR, the oscillation can be achieved by either using a piston or bellows arrangement at the base of the column or moving a set of baffles up and down the column at the top.
By connecting baffled cells in series, a continuous oscillatory baffled reactor (COBRTM) is created, as shown in Figure 2. As each baffled cell acts as a continuous stirred tank reactor (CSTR), with a large number of baffled cells plug flow condition is achieved under laminar flows (low flow rates). Figure 3 shows the characteristics of the residence time distribution (RTD) obtained in a COBR.
The COBR is for the continuous processing and operations, and can be operated horizontally, vertically or at any angle. The key difference between the COBR and other tubular devices on the market is that the mixing in the COBR is governed by the oscillation, not the net flow, allowing plug flow conditions under laminar flows (low flow rates). This enables much more compact reactor design and configurations with significant reduction of reactor volume/space; and accommodates much longer residence time (in terms of hours) while generates much less pressure drops in comparison to other tubular devices. Under plug flow conditions, the significantly enhanced mass and heat transfers can also be realised in COBR.
When solid particles are present in a liquid phase, uniform and efficient suspension and transportation of solids is obtained along the length of COBR. When a gas phase is involved, very good mixing and dispersion of the gas into the liquid occurs to give an exceptionally high surface contact area and a substantial increase in gas hold up and gas residence time, enabling enhanced mass transport characteristics. Due to plug flow conditions achievable in COBRTM, it also offers superior heat transfer rate. Note that the black boxes as shown in Figure 2 can be used for inputting, sampling, monitoring and outputting. The jackets along the COBRTM provide heating/cooling in either an individual or integrated manner so that different or same thermal profiles can be established along the length of the COBRTM.
[edit] References
- ^ M.R. Mackley and X. Ni, “Mixing and dispersion in a baffled tube for steady laminar and pulsatile flow”. Chemical Engineering Science, Vol. 46, No. 12, 1991, 3139-3151.
- ^ X. Ni and P. Gough, “On the discussion of the dimensionless groups governing oscillatory flow in a baffled tube”. Chemical Engineering Science, Vol. 52, No. 18, 1997, 3209-3212.
