selection of internals for distillation columns

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Process Engineering Guide: GBHE-PEG-MAS-610

Selection of Internals for 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: Selection of Internals for

Distillation Columns CONTENTS SECTION 0 INTRODUCTION/PURPOSE 3 1 SCOPE 3 2 FIELD OF APPLICATION 6 3 DEFINITIONS 6 4 GENERAL SELECTION CRITERIA 6 5 PERFORMANCE CHARACTERISTICS 6 5.1 Pressure Drop 6 5.2 Deep Vacuum 7 5.3 Hydraulic Capacity 8 5.4 Mass Transfer Efficiency 8 5.5 Turndown 8 5.6 Flow Regimes on Trays 9 5.7 Foaming 9 5.8 Two Liquid Phases 9 5.9 Scale Effects 9 5.10 Liquid Hold-up 10 5.11 Heat Transfer and Packed Columns 11



6 DISTILLATION TRAYS 11

6.1 Sieve Trays 11 6.2 Valve Trays 12 6.3 Dualflow Trays 12 6.4 Bubble Cap Trays 13 6.5 Baffle Trays 14 7 PACKINGS 14 7.1 Random Packings 14 7.2 Structured Packings 14 7.3 Grid Packings 15 8 EQUIPMENT MANUFACTURERS' PROPRIETARY

DEVICES 15

8.1 DisTech 15 8.2 Glitsch 15 8.3 Koch 16 8.4 Kuhni 16 8.5 Norton 16 8.6 Nutter 16 8.7 Sulzer 17 8.8 UOP 17 9 COLUMN REVAMP 17



TABLES 1 GENERAL SELECTION CRITERIA 7 FIGURES 1 DECISION TREE 4 1 EXAMPLES OF DISTILLATION TRAY TYPES 4 2 EXAMPLES OF PACKING TYPES 5 3 TWO-STAGE LIQUID DISTRIBUTOR 10 4 EFFECT OF HEAT TRANSFER ON LIQUID FILM STABILITY 11 5 DUALFLOW TRAYS 13 6 BUBBLE CAP DETAIL 13 7 BAFFLE TRAYS 14



0 INTRODUCTION/PURPOSE Distillation columns fall into two main categories with respect to their internals - trayed and packed columns. Taken simply the distinction between the two types of internals is: (a) Trays are horizontal and support a layer of liquid through which vapor is

blown via a series of holes in the tray floor, while (b) Packings provide more or less continuous surfaces down which the liquid

flows in films or streams; the vapor phase is continuous. Of the many types of trays and packings on the market, some are non-proprietary and may be obtained from almost any of the manufacturers. Others are proprietary, especially the more modern types of packings. The main types of trays and packings are: (1) Sieve tray, one of the two most common types. (2) Valve tray, the other common type, but marginally more expensive. (3) Bubble cap tray, the traditional type of tray, extremely expensive, but it

has its applications. (4) Random packing, the traditional type of packing. (5) Structured packing, made from folded metal sheets or gauze, the latter

being much the more expensive. (6) Grid packing, mainly developed for dirty services and not much used in

distillation as it generally has poor efficiency. Examples of these are shown in Figures 1 and 2. For most ordinary distillations both packings and trays will do the duty. The distillation tray and packing market is highly competitive. Even some proprietary devices which initially seem very expensive may be competitive when the total system costs are taken into account. This is because their high performance enables the duty to be achieved in a smaller vessel. Meaningful cost comparisons can be done only on the basis of specific quotations and total system costs.



1 SCOPE Clauses 4 to 9 of this Guide contain information to assist the process of selection by approaching the topic from different angles: (a) General selection criteria (Clause 4) - start here to produce a short list of

the options. (b) Performance characteristics of the generic types - look at the sub clauses

5.1 to 5.11 and read those paragraphs which are relevant to the process under consideration.

(c) Trays (Clause 6), listing and describing the features of both generic types

and the more common proprietary types - the choice will generally be between sieve and valve trays (6.1and 6.2).

(d) Packings (Clause 7), listing and describing the features of both generic

types and the more common proprietary types - except with high pressure (>10 bar), fouling or corrosive systems, structured packings should be considered, though detailed costs may determine the final choice.

(e) Equipment manufacturers' proprietary devices (Clause 8), listing the

better-known and more successful types - new devices should not be chosen without specialist advice.

(f) Column revamp (Clause 9) - increasing column performance in terms of

either capacity, efficiency requires special considerations. Various manufacturers have developed devices which are particularly useful for column revamp; seek specialist advice on their use.



FIGURE 1 EXAMPLES OF DISTILLATION TRAY TYPES



FIGURE 2 EXAMPLES OF PACKING TYPES



2 FIELD OF APPLICATION This Guide applies to the process engineering community in GBH Enterprises worldwide. 3 DEFINITIONS For the purposes of this Guide no specific definitions apply. 4 GENERAL SELECTION CRITERIA This Clause outlines general criteria on which to base an initial selection of the type of internals required. Although selection on the basis of cost has to be done with specific quotations and including vessel costs, a few general points can be helpful: (a) In general, a column of given height and diameter will have a higher

capacity for the same separation with packing than with trays. (b) The cheapest installation is likely to be a trayed column at pressures near

atmospheric - more metal is required for packings and associated internals than for trays.

(c) At high pressures a packed column may be cheaper, but beware of

structured packing - some failures at high pressures (above about 10 bar) have been reported.

(d) Under vacuum conditions, a packed column is likely to be cheaper. Also

packings have a much lower pressure drop than trays, which can be useful in vacuum services.

(e) Significant technical risks, especially with proprietary devices, should not

be taken to achieve only a small reduction in costs. Table 1 lists a variety of selection criteria and conditions under which the various types of device should be considered. In trying to keep this table simple, reference to proprietary devices has been avoided. These are dealt with in Clause 8.



5 PERFORMANCE CHARACTERISTICS This Clause describes the performance of the various types of internals with respect to a number of parameters which frequently influence the choice of internals. Some of these are included in Table 1. 5.1 Pressure Drop This is the biggest difference between packings and trays. The pressure drop of a trayed column may be up to 10 times that of a packed column, though some proprietary designs come fairly close. Pressure drop can be particularly important in vacuum distillation because a high value increases reboil temperature and usually reduces relative volatility in the lower part of the column. Typical pressure drops for one tray or equivalent in packing are given below:



TABLE 1 GENERAL SELECTION CRITERIA



5.2 Deep Vacuum In deep vacuum services (<0.1 bar) gauze packings are often required. The reason for this is that at very low pressure, the vapor density is very low and hence a large column diameter is required to avoid flooding. Thus the liquid volume rate per unit cross section of column is very low and it is difficult to ensure that all the packing is properly wetted. Gauze packings achieve this through the capillary action between the wires of the gauze. High efficiency is often achieved – at least 5 theoretical stages per meter for an ideal system with a relative volatility less than about 1.5. 5.3 Hydraulic Capacity In most cases a packed column will have a higher hydraulic capacity than a trayed column of the same diameter. This is because a significant proportion (often 20-30%) of the column cross sectional area is lost to downcomers in a trayed column. Bubble cap trays have a lower capacity than most other trays. Dualflow trays (which have no downcomers) have capacities similar to packings, though they are prone to unstable operation, low efficiency and poor turndown. Some proprietary designs have higher capacities - see UOP and Glitsch, Clause 8. 5.4 Mass Transfer Efficiency The parameters usually used to characterize efficiency in distillation are: (a) For trays: overall efficiency, defined as

100 x (No. of theoretical trays) / (No. of actual trays) (b) For packings:

Height equivalent to a theoretical plate (HETP)



Note: The words 'tray' and 'plate' are equivalent. The word 'stage' is sometimes used to mean 'theoretical tray' and is sometimes preceded by 'theoretical'. Both overall efficiency and HETP vary with system properties and hydraulic conditions, and are not easy to predict accurately. For the comparison of trays with packings a useful parameter is the height of packing equivalent to an actual plate (HEAP). The efficiency of packings and trays tend to vary similarly with system properties and hydraulic conditions such that HEAP is approximately constant. Approximate values relating Pall rings to sieve or valve trays are given below:

Modern proprietary packings should be able to do 10 - 20% better than this. A technique for achieving up to 30% improvement in tray efficiency is incorporated in some proprietary devices, notably the UOP Slotted Sieve Tray (see Clause 8). 5.5 Turndown All types of device have potentially good turndown, up to 2:1 or better. In most cases turndown is limited by loss of efficiency, which has different causes for different devices. In all cases the basic problem is inadequate liquid/vapor contact: (a) Sieve trays - excessive weeping allows liquid to fall through the column

without traversing the trays. (b) Valve trays - at low rates, large patches of valves on the trays are shut

and a significant proportion of liquid traverses each tray with little vapor contact.

(c) Packings - at low rates, the liquid distributors can no longer achieve an

even distribution because the liquid head above the orifices is too small and too sensitive to any out-of-levelness.



Generally speaking, packings have better turndown than trays. For liquids which wet the packing surface, turndown in excess of 10:1 may be achievable, but this is unlikely with non-wetting liquids. Valve trays have better turndown than sieve trays, generally at least 3:1. Packings are attractive for batch distillations because they have a lower liquid inventory than trays, but beware the distributor design - liquid hold-up in the distributor may be so high that the advantage is lost. For a high turndown, a two or three stage distributor may be required to avoid high liquid inventory. 5.6 Flow Regimes on Trays In the normal operation of trays, much of the two phase region on a tray is liquid phase continuous with bubbles of gas; this is known as the froth or mixed froth regime. In vacuum conditions, say below 0.5 bar, trays often operate in the spray regime, where much of the two phase region is vapor phase continuous with droplets of liquid. This can lead to high entrainment and low efficiency. Sieve trays are more prone to this than valve trays. At high pressures, say above 10 bar, trays often operate in the emulsion flow regime, in which the froth on the tray consists of large numbers of small bubbles (<5 mm dia.) dispersed in the liquid. When this froth enters the downcomer, the bubble rise velocities and coalescence rates are so low that little phase separation occurs. Hence a large tray spacing and/or low tray pressure drop are required to accommodate low density froth without flooding the downcomer. 5.7 Foaming Foaming can arise from several causes but in effect is similar to the high pressure phenomenon noted in 5.6, and is highly dependent on flow regimes. It rarely causes problems on the active areas of trays. The usual problem is flooding of the downcomer caused by the back-up of a low density froth. Foaming problems in the downcomer are less likely if the tray active area operates in the spray regime, since the material entering the downcomer is then not an established froth, but a shower of droplets. Packings are rarely affected by foaming at normal design rates since they generally operate in a totally different flow regime - a continuous gas phase with streams or films of liquid cascading over the packing. However, they are likely to flood at rates substantially below that calculated for a non-foaming system and may be at, or close to, flood at the normal design rate - say within 10%.



5.8 Two Liquid Phases Some distillations involve the intimate contacting of two liquid phases as well as the contacting between vapor and liquid. Trays are known to give good contacting of the three phases and are recommended for this type of duty. Success has also been had with random packings, but there is less experience and there is still some doubt whether adequate contacting can be achieved in all cases. Also, distributor design is likely to be complicated. Structured packings are to be avoided. These rely on the spreading of liquid over a more or less continuous sheet. The surface tension differences between the two liquids will ensure that the one with the higher surface tension will have very little surface area for contact with either the vapor or the other liquid phase. Poor performance can be expected. 5.9 Scale Effects Packings are used in columns ranging from laboratory scale of 25 - 50 mm up to at least15 m diameter. Mass transfer efficiency is generally better at small diameters (less than 500 mm or so), probably because of better liquid distributor performance and beneficial effects of liquid crossmixing. From 1 m upwards, efficiency should not change but distributor design is critical. Trayed columns are rarely used below 1 m diameter because of difficulties of installation and maintenance. For column diameters less than 1 m, trays may be assembled cartridges for insertion into the column. Typically each cartridge will consist of up to 12 trays. Not surprisingly, these are expensive and not worth considering if packings will do the duty. At diameters above 2 m, capacity may be improved by the use of multi-pass designs. Two pass designs are preferred as no loss of efficiency is expected. With more passes it becomes difficult to ensure that liquid and vapor distribution between the passes are equal so that the L/V ratio is always the same. Failure to achieve this can result in an unexpected and largely unpredictable loss of efficiency. Such designs therefore are not generally recommended. Seek specialist advice if necessary. In vacuum service, where liquid load is relatively small, single pass trays are used up to large diameters (at least 7 m) but special care is needed to maintain efficiency above about 3 m (see 8.8 - UOP).



5.10 Liquid Hold-up Liquid hold-up in the mass transfer zone of a distillation column can be important in batch distillation; increased hold-up increases batch times. Packings have a much lower hold-up than trays but, unfortunately, the requirements for good liquid distribution result in significant hold-up in the distributors. Unless care is taken, the hold-up of a packed column may be as large as that of a trayed column. Steps to consider to reduce hold-up: (a) avoid batch sequences which result in high turndown ratio of liquid rates, (b) consider two stage distributor designs (such as in Figure 3) which reduce

the liquid head range for a given turndown ratio, and (c) consider trough type distributors which are likely to have lower hold-up

than pan types. FIGURE 3 TWO-STAGE LIQUID DISTRIBUTOR



5.11 Heat Transfer and Packed Columns Heat transfer between liquid and vapor is generally much more efficient than mass transfer and rarely causes problems in trayed columns, but beware of highly superheated vapors and highly sub-cooled liquids introduced into packed beds. If, at the point of introduction, the temperature difference between a superheated vapor feed and the column liquid, or between a sub-cooled liquid feed and the column vapor, exceeds about 50°C, the packing performance is likely to suffer badly. This needs explaining. Figure 4 illustrates a solid vertical flat surface with a liquid film flowing down it. The film thickness will naturally vary, as shown. Heat transfer to the film will be proportional to the surface area. In unit surface area of thick film the rate of temperature rise will be less than in unit surface area of thin film. Thus, the surface tension in thin areas will fall relative to that in thick areas, the film will tear apart and the surface area for heat transfer between liquid and vapor will be much reduced. Dealing with the problem of a superheated vapor is the easier of the two. In most cases a sieve, a valve or dualflow tray below the packed bed will remove the superheat sufficiently. It will also provide good vapor distribution for the packed bed. With a sub-cooled liquid, the best solution is to preheat the liquid before it enters the column. Sometimes, additional packed height is allowed, but success cannot be guaranteed with this approach.



FIGURE 4 EFFECT OF HEAT TRANSFER ON LIQUID FILM STABILITY

6 DISTILLATION TRAYS In this Clause the various generic types of tray are briefly described, concentrating on features which differ and are significant considerations in the selection process. Figures 1 and 4 illustrate the types discussed. Where relevant, some of the more common proprietary devices are mentioned. 6.1 Sieve Trays (Figure 1) The sieve tray is one of the simplest designs and is entirely adequate for a large range of duties. Its distinguishing feature is that the vapor flows through simple holes in the tray floor. The holes are generally in the range of 6 - 12 mm dia. Clearly, if the vapor flow rate is too low, liquid will flow down through these holes. This is called 'weeping' and it reduces liquid/vapor contact and efficiency. Even so, it is often possible to design for a turndown ratio of 2:1, or better. Another problem is the converse of weeping. The high vertical velocity of the vapor as it comes through the holes can entrain liquid droplets into the vapor space above the froth on the tray.



At high rates, the liquid droplets are entrained up to the tray above, causing a loss of efficiency. This is a serious problem only in vacuum service. It can be alleviated by using small holes (4 - 6 mm) and is one of the features of the proprietary UOP Slotted Sieve tray (see 8.8). 6.2 Valve Trays (Figure 1) The valve tray aims to overcome the weeping problem of sieve trays. The holes in the tray floor are large (almost always 39 mm dia) and are covered by a cap - or valve, as it is called - which is free to move from resting on the tray floor to lifting about 12 mm. Its movement is constrained either by a cage in which it sits or by legs which are an integral part of the valve and hook outwards underneath the tray floor. At high rates, all valves are fully open. At reduced rates, some of the valves shut while others remain open - the valves do not float in a midway position. Furthermore, the valves that are shut tend to cluster in patches and at very low rates this can allow a proportion of the liquid to flow across the tray with insufficient vapor contact. This is the main constraint on turndown on a valve tray. Its turndown is generally better than a sieve tray and will often achieve 3:1, or maybe 4:1. Valve trays are generally regarded as proprietary devices, but they have been on the market for many years. Most manufacturers have their own types, but they are all more or less the same - with a few exceptions. One of these is the Nutter valve tray - this uses rectangular valves. Another type is the Stahl Varioflex valve tray which is designed so that the valves will float at intermediate positions, hence better turndown is possible (12:1 turndown is claimed). Valve trays tend to be more expensive than sieve trays, but the difference is rarely sufficient to be a major factor in selection. 6.3 Dualflow Trays (Figure 5) These are similar to sieve trays, but they have no downcomers and are, therefore, cheaper. The hole diameter is often larger than that used on sieve trays; 25 mm is not uncommon and 12 mm is about the smallest used. Since they have no downcomers and tend to use larger holes, dualflow trays are often used in moderate to severe fouling duties.



They give a higher throughput for the same tower diameter than sieve or valve trays but may achieve only half to two thirds of their efficiency. Also they suffer from problems at large diameter, above about 2 m, when efficiency can be desperately low, say about half to two thirds of the efficiency of a 1 m diameter dualflow tray on the same system. Turndown is another problem with dualflow trays. This will be significantly less than that achievable with a sieve tray. A turndown of 1.4 (i.e. to 70% of design rates) will typically incur a 25% loss of efficiency. Heat transfer is generally much more efficient than mass transfer and dualflow trays will achieve entirely satisfactory performance. In summary, a dualflow tray is worth considering only for fouling duties in a column less than 2 m diameter, with a low turndown requirement. For large diameters a proprietary design, the Stone and Webster Ripple tray, is much more successful. It still has a fairly poor efficiency and is expensive due to complicated mechanical design. FIGURE 5 DUALFLOW TRAYS

6.4 Bubble Cap Trays (Figures 1 and 6) Bubble caps, and various proprietary variants, are the original type of tray design. The unique feature of the bubble cap design is illustrated in Figure 6. It consists of a vapor riser in the tray floor which ensures that there will be a liquid level on the tray through which the vapor has to bubble. Properly designed, they do not weep and can be useful where this is essential.



They are also used on trays which require high liquid inventory, either for chemical reaction or where heating or cooling coils are installed on the trays. In such cases the chimneys may be high, sometimes up to 300 mm. Apart from the situations just described, bubble cap trays are rarely used since they can be 10 times the price of a sieve tray, have lower hydraulic capacity and lower efficiency. FIGURE 6 BUBBLE CAP DETAIL

6.5 Baffle Trays (Figure 7) These are used only with fouling systems where even dualflow trays suffer blockage. They generally have good capacity, but poor efficiency. Seek expert advice before using them.



FIGURE 7 BAFFLE TRAYS

7 PACKINGS Packings fall into three general families: random, structured and grid. Figure 2 shows typical examples. While some types can be described as generic in that they have been around a long time, newer packings have been developed which offer advantages although they are proprietary and tend to be more expensive. There is a bewildering variety of packings on the market - inventing a new shape is a popular pastime. In the descriptions that follow, the main types (generic and proprietary) which are most commonly used in distillation are mentioned. Other types may well be appropriate for particular circumstances, but expert advice should be sought. 7.1 Random Packings Random packings, often described as rings or saddles although other shapes exist, are made in metals, plastics and ceramics. Metals are almost exclusively used for distillation because few plastics have sufficient strength at distillation temperatures and ceramics tend to suffer from breakage during service. They are easy to install in a column and can be removed and reinstalled for a full visual vessel inspection - say with a corrosive system. For a given hydraulic capacity in a given diameter they are usually less efficient than structured packings except at high pressure (>10 bar). Pall rings are the most common non-proprietary packing and generally the most suitable for distillation duties. The major proprietary ones are CMR (Glitsch), IMTP (Norton) and Nutter Ring (Nutter).



7.2 Structured Packings As with random packings, structured packings are made in metals, plastics and ceramics, but metals are almost always used for distillation. They are more difficult to install than random packings. They are much more difficult to remove and inevitably a proportion of the packing will be irretrievably damaged. They perform well at around atmospheric pressure down into the vacuum region. Gauze types are particularly useful below about 0.1 bar - although expensive, the efficiency (as number of stages per m) is around 3 × that of sheet types. However the gauze types are sensitive to solids in the feeds and reflux. All structured packings are proprietary and the most common ones of the sheet type are Mellapak (Sulzer), Flexipak (Koch - in the US, under licence from Sulzer), Gempak (Glitsch), Montz-Pak (Montz - under licence to Nutter in the US and to ETA in the UK) and Intalox Structured Packing (Norton). All these manufacturers have gauze versions for the high vacuum region and Montz has a version made from expanded metal. 7.3 Grid Packings Grid packings are primarily used for fouling systems, especially where heat transfer is the main requirement. They were developed for the oil industry and used in crude column quench sections, etc. Mass transfer efficiency is relatively poor. There is insufficient GBHE experience of grid packings to make any value judgments, but the major packing manufacturers include grid packings in their range. 8 EQUIPMENT MANUFACTURERS' PROPRIETARY DEVICES Worldwide there are many manufacturers of distillation column internals and many of them offer proprietary devices which claim advantages in particular circumstances. Some of these devices (e.g. structured packing) have proved so successful that other manufacturers are bringing similar products to the market which, nevertheless, avoid any patent problems. Other devices are eminently forgettable. The ones listed here are not of this latter category, though some are of limited value.



8.1 DisTech DisTech concentrate on small hole sieve trays, but will supply valve trays and packings. 8.2 Glitsch (a) Screen Trays - a high capacity tray which uses Johnson Vee-Wirescreen.

Efficiency may be a few % less than a sieve tray. The small width apertures in the screen may limit use to clean duties.

(b) Superfrac Trays - a new device designed for high capacity and high efficiency (at high cost) - seek specialist advice before choosing it. (c) CMR (random packing) - used to be known as Cascade Mini Ring. A large range of sizes, 8 in all, available. Claim higher efficiency for given capacity than a Pall ring. (d) Gempak (structured packing) - the nearest look-alike to Sulzer's Mellapak (see below), has been challenged by Sulzer and Koch (Sulzer's US licencee) in the courts. (e) Higee (ICI invention licensed to Glitsch) - Mass transfer is achieved in a rotating torus. May be worth considering for duties with no intermediate feed (absorption and stripping duties) especially where space is at a premium. 8.3 Koch (a) K-pac - essentially a modified Pall ring made from less metal. It has little

advantage over the Pall ring and is physically weaker. Norton has a similar product.

(b) Flexipak - a structured packing made under license from Sulzer. It is

essentially the same as Mellapak, but is made in different block sizes and has different surface area per unit volume.

(c) Fleximax - A random packing claimed to be of similar performance to

Norton's IMTP packing.



8.4 Kuhni Kuhni specialize in equipment for distillation and liquid-liquid extraction. For distillation they market a tray and a structured packing. (a) Slit-valve tray - this is not a valve tray in that the valves are fixed, similar to

the Nutter V>grid tray. The fixed 'valves' are much smaller than Nutter's and each has two slits roughly 25 mm long by 2 mm high. Tray configuration is unusual and there are two types. One type has a disc and doughnut arrangement with alternating central and peripheral downcomers. The other type has central downcomers only with pipes to lead liquid from the central downcomer to the periphery of the tray below. Both have radial liquid flow. The design is well suited to small to medium size columns - say up to 2 to 3 m diameter. For larger columns, conventional chordal downcomers and pass layouts are used

(b) Rombopak - a structured packing with a rather different design from

Sulzer Mellapak. It has a more open structure which has problems with high liquid loads

8.5 Norton (a) Hy-Pak - essentially a modified Pall ring made from less metal. It has little

advantage over the Pall ring and is physically weaker. Koch has a similar product. Hy-Pak was replaced with IMTP, giving better performance.

(b) IMTP - a metal saddle-type random packing developed for distillation.

Considerable commercial success since its introduction in 1977. Applications vary from high vacuum to high pressure. The "standard" against which other proprietary packings are judged.

(c) Intalox Structured Packing (ISP) - a recently introduced structured packing

rather different from Sulzer's Mellapak. Tests at low to moderate pressures, by University of Texas at Austin and FRI, gave good results. FRI tests above 15 bar were disappointing getting steadily worse as pressure was increased.



8.6 Nutter (a) Float Valve Tray - a unique rectangular valve design in two basic sizes

with various lift heights. Claimed to have less weeping at tray inlet than a circular valve.

(b) V-Grid Tray - similar to the float valve, but the 'valve' is fixed - it is simply

pressed out of the tray floor. (c) Nutter Ring - a random packing similar to IMTP. Two sizes, No 2 and No

2.5, have been tested by FRI and have performed well in comparison to Pall ring packing with up to 10% better efficiency and capacity.

(d) Montz Packing - licensed from Julius Montz in Germany specifically for the

US Market. It is a structured packing rather different from Sulzer Mellapak. FRI tests have shown disappointing performance with the type B1, but good performance with the newer type BSH at around atmospheric pressure.

8.7 Sulzer Sulzer was the first company to market structured packings. They started with gauze types (BX and CY) which were developed for high vacuum distillation. A sheet metal family (Mellapak) was then developed and has become so successful that other companies have developed their own variants. Structured packings are also available in plastics and ceramics, as are laboratory scale versions (DX and EX). In recent years, Sulzer have entered the tray market having acquired Metawa-Tray BV. More recently, they have decided to enter the random packing market. (a) BX and CY Gauze structured packings - developed for high vacuum. The

gauze structure greatly assists liquid distribution with the very low liquid rates in such duties. Feeds need to be clean and are usually filtered to better than 100 micron.

(b) Mellapak - a highly successful series of sheet packings. Used in vacuum,

atmospheric pressure and up to 10 - 15 bar. At higher pressures, there have been serious failures - unexpected poor efficiency, though Sulzer now believe they know how to solve the problem.

.



8.8 UOP Although an old name, UOP has changed. It is now a joint venture between the Union Carbide and Allied Signal Corporations, which handles the Union Carbide distillation technology. Union Carbide concentrated their interests in two types of tray, both being variants of the sieve type. With both types, small holes are generally used - typically 6 mm. (a) Multi Downcomer tray - designed for high liquid rate duties, it can give up

to 30% extra capacity from the same column diameter. Tray spacing is sometimes smaller than used with conventional sieve trays. Efficiency is often lower than conventional one or two pass trays, by 5 - 10%.

(b) Slotted Sieve tray - designed for low liquid rate duties (e.g. vacuum duties) in large diameter columns (greater than 2 m). Aims to give high tray efficiency. Efficiencies up to 100% can be achieved, possibly more. 9 COLUMN REVAMP During the life of a plant it is common for significant process changes to occur which change the demands on distillation columns. We are then faced with considering changes to the column internals to meet the new requirements, which will usually consist of one or more of: (a) Increased product purity. (b) Increased product rate(s). (c) Changed feed rate(s) and/or composition(s). (d) Increased recoveries. With trayed columns, standard tray designs may be replaced with special designs or with packings. With packed columns, there is generally less scope. Random packings can generally be replaced with structured packings. In the replacement of trays with packings, a perennial question is what about the support rings and downcomer bolting bars? It has in the past been common practice to leave these in place when installing random packings. FRI tests



indicate that this is probably a bad practice and is likely to result in a loss of capacity and efficiency. It is recommended that the rings and bolting bars be removed if at all possible. If time precludes this, it must be stressed that there is some chance that the resulting performance may be disappointing, but if this is seen as an acceptable risk, the job should proceed. The extent of the risk is difficult to judge but may be a 25 - 40% increase in HETP. Under no circumstances should structured packings be installed with tray support rings and downcomer bolting bars still in place - it is difficult to do anyway. Structured packing manufacturers often ask for the rings and bars to be cut back to within 5 - 10 mm of the column wall. At the beginning of a revamp exercise, it is strongly recommended that a careful plant trial be done to establish the performance of the existing internals. This should first of all be used to establish that there is nothing obviously wrong with the internals. It is then useful to check how the performance compares with prediction, because it is a truism that revamp involves the attempt to get the most that is available. Design correlations are never accurate and their accuracy is different for different systems. Test data at maximum operation can enable normal design margins to be reduced. Revamp studies should be undertaken with care and where it is clear that the objectives cannot be met, except with reduced design margins, specialist help should be sought.

selection of internals for distillation columns

Technology

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situ exsitu sulfiding

selection of internals

layer of liquid

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distillation trays116

particular purpose

tray floor