Characterisation of High Performance Structured Packing

Column packings applicable to the field of rectification are mainly characterised by their number of theoretical stages per meter NTSM [1/m], specific pressure drop ∆p [mbar/m] and capacity limits. A set of dimensionless numbers are derived by means of a simple model of dry gas flow within parallel plates (slit flow). These numbers are dimensionless and allow the comparison of measured values from various packing types and their recently improved performance whilst only taking into account the specific surface. These numbers describe pressure drop, capacity and surface efficiency. Furthermore, a different heuristic approach is presented where the most important criteria describing packing performance are combined into one characteristic number which is plotted against a capacity factor. Characterisation of packings can thereby be concentrated within one single diagram. Examples of values from different packing types are presented. The recent developments of new generations of high performance packings are included. The investigations undertaken allow extrapolation and therefore performance prediction of further developments. STRUCTURED PACKING IN DISTILLATION APPLICATIONS Structured packings have been established in the field of distillation for several decades. They are preferred where a high separation performance is required and low pressure drop is of importance. A column design with several beds, liquid collectors and distributors is challenging and requires the consideration of many details. Driven by various needs for different applications during the recent decades several types of packing have been developed and are successfully used throughout process industry. They might be distinguished as follows: Gauze packing Grid type packing Corrugated sheet packing Random packing For each application, the different packing types have several advantages. For instance, random packing is generally the best choice for applications with very high liquid loads, whereas, gauze packing tends to be the best choice for operation at very low pressures and liquid loads. Metal sheet and grid type packings are suitable for a wide range of applications and are particularly suited to applications where the maximum allowable pressure drop and limitation in height, combined with a high separation requirement, are of importance. In addition to the degree of flexibility in terms of column height and diameter, the designer is confronted with different potential suppliers and their different packing types (leading to different column dimensions). Our contribution has the intention of presenting measurable criteria allowing a neutral and fair comparison of different types. They are best explained with the help of examples and for this reason we have chosen some “typical” and in the market well known packing types, although we know that this selection is not complete. Parameters Column packings applicable to the field of rectification are mainly characterised by their number of theoretical stages per meter NTSM [1/m], specific pressure drop ∆p [mbar/m] and capacity limits. The packing performance is mainly influenced by: vapour mass flow rate G& [kg/h] gas density ρg [kg/m] f-factor f Pa liquid mass flow rate L& [kg/h] liquid density ρl [kg/m] liquid load vl [m/m h] viscosities ηg, ηl, [Pa s] surface tension (g-l) σ [N/m] The packing might further be characterised by the description of its structure [1] like void fraction, grid angles, hydraulic diameter of channels (if present), and typically very often simply by its specific surface area a [m/m]. Reference geometry As a model geometry of reference we have chosen the infinite parallel plates (slit flow) with the same specific area a as the packing in consideration. Figure 1: Reference geometry parallel plates This reference geometry needs only to be characterised by the distance of the plates to each other; their spacing s. Deriving the hydraulic diameter dh for any arbitrary packing and for the infinite plates we apply the known ratio of cross section A to circumference U. U A dh ⋅ = 4 (1) resulting into a s dh 4 2 = ⋅ = (2) This hydraulic diameter might differ to that derived according to the specific geometry of certain packings (e.g. channel diameter for a corrugated sheet). The advantage of this definition is its universal applicability for any arbitrary geometry of a structured or random packing. Rombopak structure In the following sections data from different packing types will be presented (Mellapak, Rombopak, VSP, Nutter Ring, Pallring). Rombopak is the packing of Kühni [4], hence the focus of our development work is its further improvement. Due to this we are able to present recently measured data that can be compared with other packing type’s data. As not all readers might be familiar with the grid type packing Rombopak, it is depicted below. Figure 2: Rombopak structure Unlike other systematic column packings its surface is divided into a multitude of small lamellas. CHARACTERISTIC NUMBERS In designing a distillation column the engineer has typically establshed the process and is then confronted with the duty to choose the best column design. This is an interesting task and the engineer has the advantage (problem) of internals selection and the evaluation of their impact on column design and process (-costs). The well known, but still challenging objective, is to find the packing which provides a high NTSM [1/m], and produces the lowest possible pressure drop ∆p [mbar/m]. For new installations, the optimal packing leads to the smallest (most economic) column, whereas, for retrofits the highest possible capacity (besides improvement of ∆p and NTSM) is often of importance. Hence we detect a demand for objective and ideally simple criteria for packing comparison. Pressure drop, Np [-] Pressure drop becomes important at low absolute operating pressures. The demand for processes at very low operating pressures is increasing significantly. For instance, the development of new pharmaceutical products (active ingredients) results in more and more complicated molecules. These are generally temperature sensitive and can only withstand moderate temperatures. Another example is processes where natural ingredients are extracted by means of solvents which then need to be distilled off under very low pressure conditions. Due to the current trend of custom manufacturing in the fine chemicals industry nowadays more engineers are faced with such problems than before. ∆p for a given structure (here Rombopak) The specific pressure drop depends on structure type and specific surface area. In order to investigate its theoretical limit (to check the horizon) we compare measured values with calculated ones from parallel plates with identical specific surface. In the following diagram we have depicted measured and CFD calculated values of pressure drop versus f-factor for the structure Rombopak 4M. These are compared with the calculated values for the infinite parallel plates.