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Process Engineering Guide: GBHE-PEG-MAS-612

Design and Rating of Packed Distillation Columns Information contained in this publication or as otherwise supplied to Users is believed to be accurate and correct at time of going to press, and is given in good faith, but it is for the User to satisfy itself of the suitability of the information for its own particular purpose. GBHE gives no warranty as to the fitness of this information for any particular purpose and any implied warranty or condition (statutory or otherwise) is excluded except to the extent that exclusion is prevented by law. GBHE accepts no liability resulting from reliance on this information. Freedom under Patent, Copyright and Designs cannot be assumed.



Process Engineering Guide: Design and Rating of Packed Distillation Columns

CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 4 3 DEFINITIONS 4 4 DESIGN PHILOSOPHY 4 5 PERFORMANCE GUARANTEES 5 6 DESCRIPTION OF PACKED COLUMN INTERNALS 6

7 DESIGN CALCULATIONS 7 7.1 Selection of Packing Size 7 7.2 Rough Design 7 7.3 Detailed Design and Rating 9



8 LIQUID DISTRIBUTION AND REDISTRIBUTION 12 8.1 Basic Concepts 12 8.2 Pour Point Density 13 8.3 Peripheral Irrigation - the Wall Zone 13 8.4 Distributor Levelness 13 8.5 Maximum Bed Height and Liquid Redistribution 14 9 PRACTICAL ASPECTS OF PACKED COLUMN DESIGN 14 9.1 Packing 14 9.2 Support Grid 15 9.3 Liquid Collector 16 9.4 Liquid Distributor or Redistributor 17 9.5 Packing Hold-down Grid 21 9.6 Reflux or Feed Pipe 21 9.7 Reboil Return Pipe 23 9.8 Liquid Draw-offs 23 9.9 Vapor Draw-offs 23

10 BIBLIOGRAPHY 25 APPENDICES A DEFINITIONS 26 A.1 INTRODUCTION 26 A.2 MECHANICAL DEFINITIONS 26 A.3 PERFORMANCE DEFINITIONS 26 B PACKING HYDRAULICS - THE NORTON METHOD 30



TABLES

1 PACKING FACTORS FOR THE MORE COMMON

RANDOM PACKINGS 32



FIGURES 1 PACKED COLUMN - GENERAL ARRANGEMENT 6 2 LIQUID AND VAPOUR LOAD CONSTANTS, KL & KV,

FOR TRIAL DIAMETER CALCULATION 8

3 PLATE EFFICIENCY CORRELATION OF O'CONNE 10 4 DIAGRAMMATIC ARRANGEMENT OF GAS-INJECTION

SUPPORT 15

5 LIQUID COLLECTORS - VANE AND CHIMNEY TYPES 16 6 MULTI-SPRAY DISTRIBUTORS 17 7 LADDER OR PIPE LATERAL 18 8 ORIFICE DRIP PANS 18 9 NOTCHED TROUGH DISTRIBUTOR 19 10 ORIFICE TROUGH DISTRIBUTOR 20 11 TWO PHASE FEEDS 22 12 VAPOR SPARGER 24 13 GENERALIZED PRESSURE DROP CORRELATION FOR

RANDOM DUMPED PACKINGS 30 DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE 45



0 INTRODUCTION/PURPOSE GBHE does not manufacture packings or the associated column internals - they are purchased from specialist manufacturers. The detailed design of the packings and internals is undertaken by the manufacturers. In most cases, competitive bids are sought from various manufacturers. The role of the Process Engineer is to: (a) specify the process requirements, in the form of data sheets; (b) ensure that what is offered by the bidding manufacturers will meet these

requirements; and (c) compare the designs offered on technical merit; outstanding features of a

particular design may outweigh any additional cost which may be incurred. On existing plant there is often the need to assess the performance of packed columns for several reasons: (d) to assess the reasons for any shortfall in performance, compared with

design or earlier operation; (e) to assess uprating capability of the existing packings and liquid

distributors, from high rate plant trials; or (f) to explore modifications for operation at higher or lower rates. Packings are made in metal, ceramic and plastic; the vast majority of distillation applications use metal packings. Ceramic packings tend to give problems of breakage in service. They are used in corrosive services. Few plastics can tolerate the temperatures encountered in distillation. Three families of packings exist: random, structured and grid packings. Random packings are the traditional ones and various non-proprietary types are available; newer high performance random packings have been developed, but all are proprietary. Structured packings are the newest ones and most of these are proprietary. Grid packings are not generally used for distillation, because of poor efficiency, but can be useful in fouling services.



The design for proprietary packings and especially structured packings depends on the design methods supplied by their manufacturers. These design methods are sometimes unreliable and expert advice should be sought when considering proprietary packings. This Guide has been prepared for GBH Enterprises. 1 SCOPE This Guide deals with the design and rating of packed distillation columns. It covers neither guidance on the selection of trays and packings nor some aspects of their performance characteristics; advice on both of these is given in GBHE-PEG-MAS-610 - Selection of Internals for Distillation Columns. Guidance is also given on the assessment of liquid distributors - the detailed design of distributors requires specialist knowledge and experience not available in GBHE, though we are able to check some of the more critical aspects of design. Also, some of the more important practical issues are discussed. The scope of the guide is summarized in its clause headings: 4 Design Philosophy 5 Performance Guarantees 6 Description of Packed Column Internals 7 Design Calculations 8 Liquid Distribution and Redistribution 9 Practical Aspects of Packed Column Design In addition, Appendices give examples of design calculations by various methods for both random and structured packings, list packing factors for many of the available random packings, provide definitions of terminology used for packed columns, and the topics relevant to packings in the FRI Design Practices Manual (Ref. [1]). 2 FIELD OF APPLICATION This Process Engineering Guide applies to the design of packed distillation columns by process engineers in GBH Enterprises world-wide.



3 DEFINITIONS For the purposes of this Guide the following definitions apply: Fractionation Research A co-operative research company. Many of the

definitions they use Inc. for packed column design are given in Appendix A of this Guide.

With the exception of proper nouns, terms with initial capital letters which appear in this document and are not defined above, are defined in the Glossary of Engineering Terms. 4 DESIGN PHILOSOPHY It is assumed that GBHE-PEG-MAS-610 “Selection of Internals for Distillation Columns” has been consulted and that, even if a firm decision to use trays has been made, the selection criteria have been checked to ensure that important factors have not been overlooked. GBHE does not manufacture packings or the associated internals for distillation columns - they are purchased from specialist manufacturers. Generally speaking, the packing manufacturers have more experience in packed column design than GBHE process engineers. It is therefore preferable for the manufacturer to take responsibility for the design. The role of a GBHE Process Engineer then becomes one of: (a) comparing the technical merits of designs proposed by manufacturers in a

competitive bid situation; (b) ensuring that what is proposed will work; and (c) seeking modifications where proposals appear unsatisfactory. If modifications are required, the aim should be to agree changes with the manufacturer which do not diminish his design contingencies, while removing the risk of poor performance. This is generally achievable since we will usually be seeking extra contingency in the design. Until recently, manufacturers' expertise has been mainly in the hydraulic design (flooding, weeping, pressure drop, etc.), but this is now changing. Most manufacturers will enter into discussions and share their experience on efficiency as it relates to our application.



Especially when they have prior experience with the same system or have done tests on our application (sometimes at their cost, but more usually at ours), they will be able to predict efficiency. It is important to distinguish between internals design and column design. Almost certainly, the vessel will be on a much longer delivery than the packing and other internals - typically about 1 year compared with 12 weeks. We often need to decide the column diameter and approximate height long before we want to talk to packing manufacturers. Usually this can be done using our own design methods and ensuring sufficient contingency for the manufacturer's final design. The recommended practice for packed column design is dependent on the purpose for which the design is required: (d) A rough column sizing is usually all that is needed for initial flowsheeting

studies; (e) Approximate designs are required for pre-sanction flowsheeting, where

cost estimating is the main requirement; (f) Detailed designs are required for:

(1) in-house column sizing, prior to enquiry on tray manufacturers, to determine diameter and approximate height;

(2) assessment of existing column capacity, either with the existing

internals or by considering modified designs; and

(3) checking manufacturers proposals, either for a new column or for debottlenecking an existing column.

(g) For a sanction estimate, enquiry on manufacturers is strongly

recommended. In-house designs are likely to require modification when manufacturers are consulted, unless generous contingencies have been incorporated. Such contingencies are likely to give scope for future uprating, but they may add significantly to the cost.



5 PERFORMANCE GUARANTEES The question arises of whether to seek guarantees from manufacturers. This needs to be decided by each project team, because there are pros and cons. If a guarantee is obtained, the manufacturer is bound to do all he can to resolve any shortcoming in his design, up to the limit of his liability (which will be stated in the guarantee). In practice, it may not be clear whether a particular problem has been caused by the design, the installation, or the subsequent operation. Since the manufacturer is bound by a guarantee, he will be concerned not to say or do anything which could be construed as an admission of liability. Experience shows that, even where there is no guarantee, most manufacturers are very willing to help us resolve the problem - after all, they stand to learn from a detailed knowledge of what went wrong, no matter whose fault it is. Guarantees do not cover consequential losses. If a failure occurs and the plant is shut down, the consequential loss will far exceed any sum in the manufacturer's guarantee - in cash terms a guarantee is of little value. Furthermore, the guarantee will generally contain clauses defining requirements of access for testing, dismantling and examination in the event of failure. The guarantee may be unenforceable if we do not meet these requirements. Before seeking a guarantee, it is important to be clear that it will be helpful to us when a problem arises and that it can be enforced. An efficiency guarantee is generally the most difficult to enforce. For example, if we have specified the number of trays on the basis of our own vapor-liquid equilibrium (VLE) model and the manufacturer has specified efficiency, he may claim that a failure to meet design is due to an error in our VLE model. Such a claim is very difficult to refute. 6 DESCRIPTION OF PACKED COLUMN INTERNALS Most designs of packed column internals are essentially similar and their main features are described in this Section. In describing internals, customary terminology is introduced. A list of definitions is given in Appendix A. Most of the definitions are those used by Fractionation Research Inc and are understood by the majority of packing manufacturers.



FIGURE 1 PACKED COLUMN - GENERAL ARRANGEMENT

Distillation columns are always cylindrical vessels. Figure 1 shows the general arrangement of a packed column, with two packed beds and feed between them. Liquid is fed to the top of a bed through a liquid distributor. A bed limiter is placed on top of the packing to restrict packing movement in the event of large surges in vapor flow. The packing rests on a support grid.



Below the support grid is a liquid collector. For some types of liquid distributor, this may not be necessary, but is recommended for large columns (> 1 m ID) to ensure that liquid from the bed above and the liquid feed are fully mixed. Liquid from the collector is then taken to the distributor at the top of the next bed. Below the bottom bed there is often some form of vapor distributor. It is much easier to achieve good vapor distribution than good liquid distribution. The reboil return pipe shown in Figure 1 is a design commonly used; it has a bottom aperture at the column centre line which directs the vapor and associated liquid downwards. Liquid falls to the sump and vapor turns round and flows up to the packing. This type of design also prevents vapor/liquid mixture from the reboiler impinging on the column wall opposite the inlet nozzle - otherwise erosion may cause vessel weakening in this area. 7 DESIGN CALCULATIONS Three categories of design can be identified which fall roughly in line with the stages of estimate in a project for a new plant: (a) Class D - budget - rough design. (b) Class C - pre-sanction - preliminary design, still mainly for costing. (c) Class B - sanction - detail design. For the assessment of existing packed columns or for plant modifications (except where a new column is required) all calculations will be in the detailed category. This Section is concerned with random and structured packings (grid packings are not generally used in distillation service). They are considered as two separate families since their performance differs significantly and cannot be represented by one method. Generally speaking, the design methods we have are not sufficiently reliable for final design except for duties where we have prior experience, either on the full scale or in a pilot unit. This is true both of hydraulic and efficiency correlations. For proprietary packings we generally have to rely on manufacturers data which varies from simple charts in sales brochures to computer programs which contain the manufacturers own correlations, with some know-how not disclosed.



7.1 Selection of Packing Size One of the first considerations is the size of packing which should be used. For random packings, For random packings general practice is to use 38 or 50 mm rings for all but small columns (< 500 mm diameter). By analogy, for sheet structured packings, the common sizes are in the range of 200 to 350 m2/m3 specific surface area. Gauze structured packings tend to be in the region of 500 m2/m3. 7.2 Rough Design For Class D purposes, it is rarely necessary to distinguish between random and structured packings. Two calculations are required: (a) hydraulic capacity to estimate column diameter; and (b) packing efficiency to estimate column height. The following equation provides an estimate of column diameter, assuming metal random packings, given a packing factor:



FIGURE 2 LIQUID AND VAPOUR LOAD CONSTANTS, KL & KV, FOR TRIAL DIAMETER CALCULATION



To a first approximation, packing efficiency (expressed as height equivalent to a theoretical plate, HETP) may be estimated using the plate efficiency correlation of O'Connell (Ref [2]), Figure 3, since packings and trays respond similarly to the VLE and transport properties of systems. O'Connell correlated efficiency in terms of relative volatility and liquid viscosity. This may be used to estimate packing HETP:

HETP = H/(plate efficiency %), m where H is a function of packing size: Size, mm 15 25 38 51 89 H 11 11 13 15 26 7.3 Detailed Design and Rating When the detailed design stage has been reached it is strongly recommended to gather the design data on the appropriate data sheet proformas. This should be done as a separate step in the design process, to avoid confusion and errors arising from ad hoc reference.



Detailed design and rating calculations are required in the following circumstances: (a) column sizing, to determine diameter and height, prior to enquiry on

packing manufacturers; (b) performance assessment of existing columns; (c) exploring design options for uprating an existing column or changing its

duty; (d) comparing manufacturers' proposals to establish their suitability and rank

them on technical merit. Manufacturers' quotations should contain enough detail for assessment. If they do not, we should request the missing information - the packing and internals manufacturing business is a highly competitive one and we can choose to go to another supplier, so we usually get what we ask for, provided our request is reasonable. Care is needed in using plant data for either the design of a new column or the assessment of an existing column for a new duty. For example, plant data may show, for a given system with a given column design (including packing, distributors, etc.), that flooding occurs at 105% of the predicted flood point. Beware: (a) How accurate were the plant data? It should be possible to obtain plant

data which show a mass balance accurate to within 5% and a heat balance within 10%, but this is not often achieved in practice.



(b) The same system may not perform so well on a different packing type or column diameter - it may flood at a lower predicted % flood if the correlation does not properly account for the effects of mechanical features in the tray design.

(c) A different system may not perform so well on the same equipment

design - it may flood at a lower predicted % flood if the correlation does not properly account for the effects of system physical properties.

7.3.1 Random Packings The methods described in this Section have been implemented as a Calculation in some commercially available programs. Details of the methods are given in Appendix B and may be used for manual calculation. The four main calculations to be performed are: (a) Flood point at constant liquid rate L/V ratio; (b) Load point at constant liquid rate or constant L/V ratio; (c) Packing pressure drop; (d) Packing efficiency, as HETP (height equivalent to a theoretical plate). For the first three, a generalized pressure drop correlation published by the Norton Company (Ref [3]) is used - see Appendix B. It is the latest in a long series of pressure drop correlations based on the original work of Sherwood (Ref [4]). Many variants have been developed and published in the open literature and in manufacturers' sales literature. The flood point is taken to be coincident with a frictional pressure gradient of 1.5 in.H2O/ft (12.3 mbar/m) of packing. In some cases, packings will operate at higher pressure gradient, but mass transfer is almost certain to be poor. The load point is defined as the combination of vapor and liquid loads at which the mass transfer efficiency is a maximum and above which it declines sharply. The load point may be taken to be 90% of the flood point. Design for 90% of the load point is generally recommended for well behaved systems.



Foaming systems rarely affect packed columns seriously, but a strong foaming tendency can reduce capacity. It is believed that the flood point declines towards the load point in foaming systems and therefore design should be for less than 90% load. Only experience on the same or similar systems can give a firm indication of a design value, but a value of 70% load should cover most cases. Packing efficiency (HETP) is predicted using an FRI correlation (Ref [1], Vol 2, Section 8.4) which was fitted to data on low relative volatility hydrocarbon systems from 250 mbar to 15 bar, and methanol/water at atmospheric pressure. Its performance with other types of system, especially with relative volatilities greater than 5, is not proven. 7.3.2 Detailed Design and Rating - Structured Packings Structured packings are all proprietary devices. We have no non-proprietary methods for the design of structured packing columns. We do not know what methods are used in these programs and although it is clear that the correlations used are very similar to (or the same as) the ones used by the manufacturers. We do not know how accurate they are. We therefore rely to a greater extent on manufacturers in the supply of structured packings than is the case for random packings. This is particularly true in two circumstances: (a) the prediction of efficiency for all systems; and (b) the prediction of capacity for high pressure systems (> 15 bar). If neither we nor the manufacturer have prior experience with a similar system we should not proceed without carrying out tests to measure efficiency, at least at total reflux. Most manufacturers have access to small scale test units in which such tests can be performed, but these may not be able to operate under all the conditions we require. This is a potentially difficult area where expert advice should be sought.



8 LIQUID DISTRIBUTION AND REDISTRIBUTION The performance of a packed distillation column is critically dependent on the quality of liquid distribution to each of the packed beds. Poor liquid distribution is the most common cause of unexpected poor separation. A full discussion of the problems is presented in the FRI Design Practices Manual (Ref [1], Vol 5, Section 2.02). The following is a summary. Liquid distribution is discussed in relation to random dumped packings, where there has been thorough research by FRI. The behavior of structured packings is believed to be similar. 8.1 Basic Concepts A perfect distributor may be considered as one which lays down liquid uniformly over the entire cross section of the column on the top of the packed bed. In practice this is not achievable: (a) the liquid is distributed from holes or slots and these must be large enough

not to block, (b) the distributor will not be perfectly level, (c) space must be allowed for the passage of vapor, and (d) process flexibility (turndown) is generally required. Indeed, perfect distribution is not necessary. Research results and mathematical simulation both show that packings have a characteristic "natural" distribution which is less than perfect, and that an excellent initial distribution is quickly degraded to the natural distribution - it takes only a few layers of packing. On the other hand, if the initial liquid distribution is not as good as the packing's natural distribution, it takes a substantial bed depth (meters in extreme cases) before the natural distribution is achieved. Although an over-simplification, it is helpful to think of liquid flowing over a packing as a series of discrete streams rather than a uniform film. The smaller the packing the greater the number of streams and the better the natural distribution. Small packings can therefore be expected to be more sensitive to poor distributor design and this indeed is so.



Clearly, the liquid streams do not flow vertically through the packed bed but are diverted laterally, in a random manner, by the packing elements. The extent of this lateral diversion may be expressed as a "spreading coefficient" which has the characteristics of a diffusion coefficient. The spread factor is easily determined experimentally. Large packings have higher spread factors than small packings. 8.2 Pour Point Density With the stream concept of natural distribution, it is important that the number of streams leaving the liquid distributor should be at least equal to the number of streams in the natural distribution. Based upon measurements of the natural liquid distribution of Pall rings, minimum values of pour point density for random packings are recommended: Ring size, mm 15 25 38 50 89 Pour points per m2 100 60 40 35 30 Distributors for sheet metal structured packings tend to have 60 to 100 pour points per m2, while distributors for gauze structured packings may well have 200 or more. Pour point density and peripheral irrigation (see 8.3 below) need to be considered together; in small columns it may be that the requirement for peripheral irrigation can be met only with a higher pour point density than given above. The holes may be on either a triangular or square pitch, but the pattern should be complete across the entire tray - vapor risers and support beams should fit within the pattern without eliminating any pour points. 8.3 Peripheral Irrigation - the Wall Zone It has been thought for many years that a common cause of poor performance in packed beds (high HETP) was the development of wall flow - liquid reaching the wall, flowing down it, bypassing the packing and not contacting the vapor effectively. With this in mind, many commercial distributors were designed so that a small zone along the column periphery was not irrigated.



FRI research has showed that a peripheral zone with insufficient liquid could double the effective HETP - this was obtained with 15 mm packing in a 1.2 m dia column with a 75 mm peripheral annulus which was not irrigated with liquid from the distributor. Larger packings, with higher spreading coefficients are not so severely affected by a 75 mm wall zone. The recommended maximum distance between the pour points in a distributor and the column wall, as a function of packing size (random packings) is: Ring size, mm 15 25 38 51 89 Max distance, mm 25 25 38 51 76 Given that the pour points are usually arranged on a triangular or square pitch, the criterion for the wall zone may require a larger number of pour points than the table in 8.2, above. This is most likely with small columns. Some manufacturers produce designs for small columns with the pour points in concentric rings. Such a design is not generally recommended since it is difficult to get an even distribution and efficiency may well suffer. Seek expert advice for an assessment of such a design. 8.4 Distributor Levelness Ideally, the liquid flow through all the pour points in a distributor should be exactly the same. Random variations, due to minor differences in hole finish (burrs, etc.) do not cause problems, but serious problems can arise if the distributor is not level. A key factor in the consideration of out-of-levelness is the shape of the orifices which control the liquid flow. Three shapes are in common use and they have different head/flow characteristics: (a) for a V notch, flow is proportional to head to the power of 2.5; (b) for a rectangular notch, flow is proportional to head to the power of 1.5; (c) for a submerged orifice, flow is proportional to head to the power of 0.5. The V notch is therefore the most sensitive while the orifice is the least sensitive to out-of levelness. Out-of-levelness may be tackled by calculation and specification, but is usually dealt with by distributor testing - see 9.4.4.



8.5 Maximum Bed Height and Liquid Redistribution Although FRI research in the early 1980s overturned traditional thinking about the influence of wall flow in packed beds, the phenomenon still exists. At the top of a packed bed, a modern distributor design is intended to put liquid evenly on to the packing. As the liquid passes through the bed it spreads and on reaching the wall some of it runs preferentially down the wall instead of returning to the packing. The build-up of wall flow in this manner eventually causes sufficient maldistribution to lower the efficiency of the packed bed. Packed beds with modern random packings have, in some cases, operated successfully with beds up to 13 m deep, but in other cases they have failed - for reasons that are not obvious (or have not been made public). Experience with beds up to 8 m deep has proved satisfactory and this is suggested as a working maximum, subject to the condition that the number of theoretical stages in the bed should not exceed 20. As liquid travels down a long bed and wall flow begins to develop, it results in variable liquid/vapor flow ratios across the column at any particular elevation. Thus a radial composition profile develops in both the vapor and liquid flows. The purpose of redistribution is to even out both the radial liquid flow profile and also the radial composition profile. Thus the redistributor needs to fully mix the liquid leaving a bed before distributing it to the bed below. Some commercial designs make specific provision for this mixing to take place; others do not. The same is in principle also true of the vapor, but specific provision is not made for either mixing or redistribution. It is probable that the tortuous passages through the risers, and the resultant turbulent eddies at their exit, provide good mixing and the pressure drop of the bed above ensures good vapor distribution. 9 PRACTICAL ASPECTS OF PACKED COLUMN DESIGN The FRI Fractionation Tray Design Handbook Vol 5 contains much invaluable information on the practical aspects of packed column design. A brief description of packed column internals is given in Clause 6. This Clause gives more detail and recommendations on some of the more common problem areas.



9.1 Packing Structured packing is installed in layers, usually about 250 mm high. For small columns (< 1 m) each layer may be supplied as one or two pieces. For larger columns, the pieces are typically 250 mm wide and high by up to 2 m long, shaped to fit the column. Manufacturers differ in how the packing relates to the column wall. One approach requires a gap between packing and wall with wall wiper strips fitted to the packing to bridge the gap. The other approach is to make the packing a tight fit against the wall. Both approaches are intended to avoid problems with wall flow, but the former approach is preferred. Random packing (except in ceramics) is usually tipped into a column from the bags in which it is supplied through a sock so that the maximum packing fall is no greater than 2 - 3 m and the packing is not seriously damaged. For ceramic packings, it is usual to fill the column with water first to minimize breakage. Breakage is often a problem in the supply of ceramic packings, a significant proportion being damaged on delivery. 9.2 Support Grid The support grid for random packings usually takes the form of illustrated in Figure 4 and known as a 'gas injection' or 'multi-beam' type. The requirement which leads to such a design is that the apertures in the support plate should be small enough to prevent packing pieces falling through, but the area for vapor flow must not be severely restricted by the area taken up by the ligaments between the apertures. The typical gas injection support plate has an area for vapor flow equal to the cross sectional area of the column. The design is also said to segregate the vapor and liquid flow; the vapor flows through the slots in the risers while liquid falls through orifices at the bottom.



FIGURE 4 DIAGRAMMATIC ARRANGEMENT OF GAS-INJECTION SUPPORT



9.3 Liquid Collector As explained in 8.5 there is a practical limit to bed height. When more stages are required, additional beds must be added. Between these beds, liquid from the bed above has to be collected, mixed and redistributed to the bed below. Also, if there is a liquid feed, this too must be mixed with the liquid coming from the upper bed. The two basic types are a chimney tray and a vane collector, examples of which are shown in Figure 5. The collector discharges into an annular and/or a central channel, where the liquid is mixed before feeding to the liquid redistributor. Liquid feed, if any, is fed into the collector or the channel. A liquid sidestream may be taken from the collector or its channel, but this usually entails special design to ensure that the liquid is de-aerated before withdrawal. FIGURE 5 LIQUID COLLECTORS - VANE AND CHIMNEY TYPES



9.4 Liquid Distributor or Redistributor There are four basic types which are in common use: pipe lateral, pan, trough and spray. Figures 6-9 show simple illustrations of the general principles. The spray type distributor (see Figure 6) is NOT RECOMMENDED for mass transfer duties - its usual application is in pumparounds where the main duty is direct contact heat transfer. A redistributor differs from a distributor in that it takes liquid from a packed bed above it and distributes it to the bed below. The design of a redistributor is essentially the same (and often actually the same) as a distributor, but the main difference is likely to be the provision of "hats" over the vapor risers to prevent liquid from the bed above falling through the risers to the bed below. A few general comments follow.



FIGURE 6 MULTI-SPRAY DISTRIBUTORS

The distributor should be located above the bed of packing (typically 150 mm) to provide sufficient free vapor space in the column to ensure liquid disengagement before reaching the confined vapor passages through the distributor. The vapor passages should be evenly distributed over the column cross section. Designs should be avoided where the vapor is required to move horizontally for any considerable distance at high velocity so that it causes the descending liquid to be displaced laterally. A common error in distributor design is to provide so many distribution points that the minimum liquid head at which the holes operate is too low. The minimum liquid head should be such that the maximum out-of-levelness can be tolerated. The minimum acceptable hole size depends on: - the presence of or tendency to form solids; - feed filtration facilities, if any; - corrosion or erosion, if any (small holes are affected to a greater extent

than large holes by corrosion and erosion). For non-fouling, non-corrosive systems with a high quality filtration (to say, one fifth to one tenth of the distributor hole size), a hole size of 2-3 mm may be considered acceptable. More usually, hole sizes of 5-8 mm are considered the minimum acceptable for clean non-corrosive systems with no feed filtration. For fouling systems, corrosive systems, or where solids are present in the feed, larger holes are required and in some cases notched troughs may be the



only practical choice. Especially in new equipment, beware of rust, fouling, mill scale, etc. from the shell material or upstream equipment. 9.4.1 Pipe Lateral Distributor (Figure 7) Primarily, this is designed for use where the feed liquid is available under pressure and where the turndown does not exceed 2.5:1. It has a high free area for vapor flow which gives a low risk of local flooding and usually allows the distributor to be placed close to the packing. It should be used only with clean or filtered liquids. The maximum liquid rate is in the region of 0.01 m 2/s. FIGURE 7 LADDER OR PIPE LATERAL

9.4.2 Pan Distributor (Figure 8) The pan distributor is adaptable to a wide range of liquid flow rates by varying the size and number of liquid orifices in the pan. A turndown of 3:1 is achievable and higher values may be obtained with special designs. Pans for small columns (up to 1.2 m) often take the form illustrated in Figure 8 (a). The pan is mounted on lugs fixed to the column wall. Gas flows up through the gas risers and the peripheral gap around the pan. In larger columns the pan (Figure 8 (b)) has no outer wall and is mounted on a support ring. The gas risers may be of circular, square, or rectangular cross section. They must be sized and located so that they do not interfere with the distribution pattern of the liquid pour points.



FIGURE 8 ORIFICE DRIP PANS

9.4.3 Trough Distributors There are two main types of trough distributor: the notched trough and the orifice trough – see Figures 9 and 10. FIGURE 9 NOTCHED TROUGH DISTRIBUTOR

For many years the notched trough was the most popular type for large columns - above about 1.5 m dia. However, because of the head/flow relationship mentioned earlier, it is very sensitive to out-of-levelness. Also, the liquid is discharged into the restricted gas flow area between troughs and at high liquid rates this may result in premature flooding and maldistribution.



In spite of these problems, it may be the best practical choice for a fouling service since the build-up of solids cannot totally block the notches as it can the orifices in other types of distributor. A turndown ratio of 4:1 is generally achievable and some manufacturers claim 10:1. Orifice trough distributors are less sensitive to the problems of notched troughs and are becoming the industry standard for large columns (above 1.5 m dia) with high liquid loads. In the simplest designs the orifices are in the base of the trough, but this gives only a limited turndown (< 3:1) and the orifices are prone to blockage. Often the orifices are either in the sides of the trough or in the sides of tubes fitted through the floor of the trough. This reduces the susceptibility to blockage and also allows design for higher turndown by having extra distribution holes at a higher elevation - see Figure 10. Orifices in the sides of the trough are fitted with guide tubes to lead the liquid down to the packing and avoid entrainment in the restricted space between troughs. Provision is sometimes made for leveling each trough individually. A major aspect of trough distributor design is the need for predistribution. If liquid comes through one nozzle (e.g. reflux to the top distributor) it may be fed into a single large trough (usually known as a parting box) which distributes the liquid accordingly to the final distribution troughs through large orifices either in the base or sides of the parting box. In many cases there is a second stage of parting boxes. The hydraulics of liquid flow in parting boxes and the final distribution troughs can cause problems. High lateral velocities occur where the liquid is introduced, the flow characteristics of orifices with a high transverse velocity upstream are not well understood, and standing waves tend to develop at particular flow rates. It is important therefore that trough type distributors should be tested before installation and preferably at the manufacturers site.



FIGURE 10 ORIFICE TROUGH DISTRIBUTOR

- AT LOW RATES, ONLY THE BOTTOM ROW OF HOLES IS IN OPERATION - AT HIGH RATES, BOTH ROWS ARE IN OPERATION 9.4.4 Testing of Liquid Distributors Commercial distributor design technology has improved in recent years as suppliers and end users have begun to appreciate its importance. However many problem areas remain, such as the minimum acceptable liquid head, feed velocity dissipation, and turbulence caused by liquid cascading from a parting box into the troughs. There are also several mechanical factors which can cause maldistribution, such as levelness and fit-up tolerances, drill or punch reproducibility, and liquid tight seams. Therefore distributor testing is often warranted to ensure proper operation. Detailed recommendations and procedure are given in Ref [1], Vol 5, Section 2.02.1. Most manufacturers are able to provide distributor tests. A distributor should be tested when one or more of the following criteria apply: (a) Critical services where poor performance has a major impact on plant

profitability, safety, etc.. Services where the loss of fractionation will cause a product to go off-spec.



(b) Low liquid rate services (< 1.4 l/s m2) since uniformity of liquid distribution is more difficult to achieve under these conditions.

(c) Large diameter towers (> 2.5 m dia). (d) Extremely high liquid rate services (> 45 l/s m2). The tests are done with water on an open test stand where the operation of the distributor can be observed and samples taken of liquid rate at many locations under the distributor. As such the test does not cover all possible problems. Cold water has different physical properties to the liquids in most distillations. The actual process fluids may for example exhibit greater froth or foam heights than water. Nevertheless, the tests can be used to highlight potential operating difficulties and take preventative measures before installation. Specifically, tests can be used to: (e) permit visual observation of the distributor operation. Typical points of

interest include: overflow of troughs, excessive aeration, wave formation, vertical discharge of liquid;

(f) evaluate the turn-up and turn-down capabilities of the distributor; (g) expose leaks due to inadequate fit-up of internal flanges or poor

fabrication (welding), (h) check final assembly for dimensional tolerances; and (j) identify gross maldistribution caused by turbulence, excessive liquid

velocities, or errors in orifice sizing. See Ref [1], Vol 5, Section 2.02.1. For details. 9.5 Packing Hold-down Grid In the event of a packed column being run flooded or subjected to sudden vapor surges, it is likely that the packed bed will be disturbed. This is particularly true with random packings, but structured packings also can suffer. The purpose of a hold-down grid is to hold the packing in place.



It is always recommended for random packings, but may be optional for structured packings, except in vacuum operations, where large pressure surges seem surprisingly common. Ideally the hold-down should be attached separately to the column wall, but is often attached to the liquid distributor above. If the problem is one of massive surges, this latter option could result in disturbance of the distributor and loss of performance. 9.6 Reflux or Feed Pipe For a detailed discussion, see Ref [1], Vol 5, Section 2.02.2. Reflux may be introduced direct on to the top distributor, provided care is taken to ensure that its inlet velocity is dissipated before it comes near any of the distribution orifices. With a pan distributor, this may take the form of a ladder type predistributor. With a trough distributor, a ladder type predistributor may be used instead of a parting box. Two phase feeds can be much more difficult to deal with. It is important to avoid slug flow of liquid into the column, since slugs tend to be travelling at much higher velocities than normal liquid flow and can cause severe damage to the liquid collector and/or distributor. Ideally the phases should be separated outside the column, with each phase introduced to the column through a separate nozzle. Some manufacturers provide a "flash box" designed to separate the phases in the column. Similarly, a vane separator can be installed at the column entry. When the feeds are predominantly vapor or liquid, one of the devices shown in Figure 11 are often used, the "raceway" is good for a high vapor fraction (i.e. mist flow) and the baffle design is suitable for a low vapor fraction (i.e. bubbly flow). With two phase feeds a problem which needs to be considered is the possibility of slug flow either in the distributor or in the pipework to it. If slug flow exists at bends or orifices, equipment will be damaged due to severe vibration.



FIGURE 11 TWO PHASE FEEDS



9.7 Reboil Return Pipe For a detailed discussion, see Ref [1], Vol 5, Section 2.02.2. Most reboilers return a two phase mixture to the column. The flow regime will generally be mist flow. If this is simply fed through the inlet branch, impingement of the liquid on the column wall opposite will probably cause erosion and may lead to failure. The most common design of vapor distributor is shown in Figure 12. This directs the vapor/liquid mixture downwards, the liquid falling to the column base and the vapor turning round, losing much of its velocity and flowing up to the bottom packed bed. Sometimes further provision for vapor distribution is required. This can be assessed by comparing the stagnation pressure rise in the incoming vapor (neglecting the liquid content) with the pressure drop across the first meter of packing:

The provision for vapor distribution can then be determined according to the value of R: (a) if R < 2.5 no vapor distribution is required; an open nozzle will suffice if

there is no risk of erosion on the opposite column wall; (b) if 2.5 < R < 4.5 the simple vapor distributor described above will suffice;



(c) if R > 4.5 consideration should be given to a more sophisticated vapour distributor.

For cases where R > 4.5, a chimney tray may be specified and designed on the basis that its pressure drop should be at least 0.25 times the stagnation pressure rise. The space between the tray and the packing support plate should be at least 0.3 m. A conventional tray (e.g. sieve or valve) may be used, in which case the space between the tray and the support plate will probably be greater but the tray will achieve some mass transfer as well as vapor distribution. 9.8 Liquid Draw-offs Liquid removal at an intermediate position in a column may be taken from a liquid collector below a packed bed. The collector will be of the chimney tray type (see Figure 5) and will be designed to have a working liquid level to ensure that the draw-off is liquid as intended. The level can be maintained with a high weir over which liquid flows to the distributor below. 9.9 Vapor Draw-offs Vapor draw-off from an intermediate position in the column may be taken from between beds. Consideration must be given to the possibility of liquid being taken at the same time. Provision may be needed for removing this liquid and returning it to the column. The vapor draw-off nozzle should be located below a liquid collector to minimize the risk of liquid entrainment. Preferably the draw-off nozzle should not finish flush with the inside surface of the column shell but should protrude into the column, so that any liquid draining down the column does not get into the draw-off.



FIGURE 12 VAPOR SPARGER



10 BIBLIOGRAPHY [1] FRI Fractionation Tray Design Handbook, Reports Centre, C & P, Wilton Five volumes as follows:

Vol 1 - Sieve, bubble cap and dualflow trays

Vol 2 - Packings, proprietary tests, baffle trays, FRI report index

Vol 3 - Computer program listings

Vol 4 - FRI experimental data

Vol 5 - Design practices Volumes 1, 2 & 5 are the most generally useful. [2] 0'Connell; Trans AIChemE, Vol 42, p241, 1946. [3] Strigle R F; Random Packings and Packed Towers; Gulf Publishing

Company, 1987. [4] Sherwood T K; IEC, Vol 30, P 765, 1938. [5] GBH Enterprises; Absorption and Stripping Towers;



APPENDIX A DEFINITIONS A.1 INTRODUCTION The following definitions are in the main those used by FRI and are understood by most tray manufacturers; especially those who are members of FRI (see Appendix E). The definitions are given in two groups: (a) Those relating to the mechanical features of packed columns. (b) Those relating to the performance of the packing and internals. A.2 MECHANICAL DEFINITIONS A.2.1 Average Bulk Density

Weight divided by the volume of a packed bed. For random packing it is a function of container or bed size, shape and loading method, in addition to the size, shape and weight of the individual packing elements.

A.2.2 Pour Point Density

The number of irrigation points provided by a distributor divided by the cross sectional area of the column.

A.2.3 Specific Surface Area

Packing surface area divided by the volume of a packed bed. A.2.4 Void Fraction

Volume fraction available for vapor and liquid flow within a packed bed.



A.3 PERFORMANCE DEFINITIONS A.3.1 Dynamic Pressure Drop

See Frictional Pressure Gradient (A.3.5). A.3.2 F-Factor

A correlating parameter for hydraulic capacity and pressure drop. It is similar to the capacity factor, but does not include liquid density and is applicable primarily to vacuum conditions where pressure drop and capacity are essentially independent of liquid rate. It is defined as:

A.3.3 Foam

A mass of bubbles stabilized by surface effects. A.3.4 Flooding

Inoperability due excessive retention of liquid inside the column. A.3.5 Frictional Pressure Gradient

That part of the pressure gradient in a packed bed which is due to the frictional forces between the vapor flow, liquid flow, packing and wall of the column. It is sometimes called the dynamic pressure gradient. The total pressure gradient is the sum of the frictional and static pressure gradients.



A.3.6 Flow Parameter, X

A correlating parameter used in conjunction with Y (qv) for hydraulic capacity and pressure drop. Essentially it is the square root of the ratio of liquid to vapor kinetic energies, based on column superficial area:

where L and G are vapor and liquid mass rates. A.3.7 Hydraulic Transition

Loading above which the vapor rate influences the liquid rate contribution to pressure drop.

A.3.8 Lambda

A parameter in efficiency calculations, relating to the relative contributions of vapor and liquid film resistances. It is defined as:

where G and L are vapor and liquid molar flow rates, and m is the slope of the equilibrium curve. The slope may be obtained from commercially available computer programs - the vapor and liquid compositions on each stage are in equilibrium (by definition) and the gradient may be estimated from the composition differences of adjacent stages:

where y and x are vapor and liquid mole fractions respectively.



A.3.9 Load Point

The combination of vapor and liquid loads above which there is a sharp decline in mass transfer efficiency.

A.3.10 Packing Factor

A correlating parameter for hydraulic capacity and pressure drop, characterizing the packing. It is applicable to random packings only, has the units of reciprocal length and is most often quoted in 1/ft. Values for most of the commonly used packings are given in Table 1 of Appendix B.

A.3.11 Radial Spreading Coefficient

The radial spreading of liquid in a packed bed has been shown to follow the laws of diffusion. The following equation describes the spreading of liquid from a point source:

A.3.12 Static Pressure Gradient

That part of the pressure gradient in a packed bed which is due to the static head of the continuous phase, namely the vapor. The total pressure gradient is the sum of the frictional and static pressure gradients.

A.3.13 System Limit (Ultimate Capacity)

A limiting combination of vapor and liquid loads which is a function of system properties only. If exceeded, massive entrainment of liquid droplets will occur. It can only be overcome by increasing the column cross sectional area available for vapor flow.



A.3.14 Universal Packing Factor

See Packing factor. A.3.15 Wall Flow

Liquid flowing on the wall in a packed column. A.3.17 Wall Zone

In a packed bed, the peripheral annulus where the inner diameter is tangent to the outermost pour points in the liquid distributor.

A.3.18 Wetting Rate

Liquid flow over a packing surface is conveniently expressed as flow rate per unit periphery, known as the wetting rate. It is given by:

For absorption services, wetting rate so defined is regarded as an important design parameter (see Ref [5]). Below a minimum value (which depends on system and packing material) low efficiency is expected.

This concept does not apply to distillation.



A.3.19 Y - A Parameter Relating Capacity and Pressure Drop to Flow Parameter It is defined as:



APPENDIX B PACKING HYDRAULICS - THE NORTON METHOD This appendix details the Norton generalized pressure drop as published in Ref [3] and gives guidance on how to use it for distillation column sizing. This may be used as a "hand" calculation. FIGURE 13 GENERALISED PRESSURE DROP CORRELATION FOR

RANDOM DUMPED PACKINGS

The Norton correlation is expressed in graphical form, Figure 13, as:



Y, the ordinate is the gas velocity term, defined by:

X, the abscissa, is the flow parameter, defined by:

ΔP, the parameter in the graph, is the frictional pressure gradient in the packed bed, in. H2O / ft. To this must be added the vapor phase static head to give the total pressure gradient

The flood point is taken to be coincident with a frictional pressure gradient of 1.5 in. H2O/ft (12.3 mbar/m) of packing. In some cases, packings will operate at higher pressure gradient, but mass transfer is almost certain to be poor. The load point is defined as the combination of vapor and liquid loads at which the mass transfer efficiency is a maximum and above which it declines sharply. The load point may be taken to be 90% of the flood point. Design for 90% of the load point is generally recommended for well behaved, systems. Foaming systems rarely affect packed columns seriously, but a strong foaming tendency can reduce capacity It is believed that the flood point declines towards the load point in foaming systems and therefore design should be for less than 90% load. Only experience on the same or similar systems can give a firm indication of a design value, but a value of 70% load should cover most cases.



DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE This Process Engineering Guide makes reference to the following documents: GBHE ENGINEERING GUIDES GBH Enterprises Glossary of Engineering Terms

(referred to in Clause 3) GBH Enterprises Use of Process Data Sheets for Distillation and

Absorption Columns (referred to in 7.3) GBHE-PEG-MAS-610 Selection of Internals for Distribution Columns

(referred to in Clauses 1 and 4)

Download - Design and Rating of Packed Distillation Columns

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