Strong Acid System - Towers - Packing
December 12, 2002

Introduction Packing Comparisons Installation

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Introduction

Packing in an acid plant tower serves to bring into intimate contact, gas and acid to promote more efficient mass and heat transfer.  To achieve this requirement, the following properties are desired:

• High surface to volume ratio
• Ability of high liquid surface renewal
• Low cost
• Low weight to volume (i.e. bulk density)
• Resistance to sulphuric acid in all concentrations and at high temperatures

Early packings were simply randomly shaped ceramic pieces dumped in a tower.  Pressure drops were high and mass transfer efficiency relatively low.  Early plants were small so these drawbacks were not critical.  As plant size increased, the deficiencis of the early packings became more noticeable.  The design and operation of towers was revolutionized by the introduction of the Raschig ring.   The rings were uniform in shape so tower performance was predicatable and consistent. 

The next advancement in tower packings was the introduction of the saddle.  The saddle offered further improvements in mass transfer efficiency and lower pressure drop.  Today, saddles are still the standard for acid plant towers.   The term 'Intalox' is oftened used to reference saddles in general but the term 'Intalox' is actually a tradename belonging to Norton.  Other equivalent names are Novalox and Flexisaddle (Koch).  There are also generic type saddles available from various manufacturers.  Different saddles may appear similar but care should be taken to ensure the product will perform as anticipated.  The basic characteristics of the packing (i.e. pieces/volume, specific surface area, bulk density, etc. should be examined and check against the parameter assumed in the design of the tower.  As well, the mechanical properties and dimensions of the saddles should be checked against the standard.  Wall thicknesses may vary considerably resulting in a saddles that may appear similar to another manufacturer's saddle but is not as robust and may be subject to chipping nd breakage.

Small improvements and variations have been made to the basic saddle shape to further enhance the packing performance.  Norton developed Super Intalox saddles which incorporated scallops or ridges along the curved edge of the saddle.  As well, small holes were introduced in the body of the saddle.  These modifications were intended to improve mass transfer efficiency and reduce pressure drop.  The scallops creates points at which liquid droplets can form and fall to next piece of packing,   exposing new liquid surface for mass transfer.  The holes allow gas to more easily flow through the packing resulting in less pressure drop.  The packing has not been widely used in the industry even though the performance data shows an improvement over regular saddles. 

Another variation of the basic saddle shape is the CECEBE HP™ packing. The packing is characterized by its larger size, larger rectangular holes and scalloped edges.   These features are similar to the Super Intalox saddles but are magnified in terms of size.

 

 

A complete departure from random packing is structured packing.  The packing comes in 1 cubic foot block and is stack in the tower so that each layer is orientated 90 degrees to the layer above or below it.  At the edge of the tower the packing can be shaped to the contour of the inside curvature or the gap filled with smaller packing to prevent gas bypassing.   Structured packing exhibits extremely low pressure drops.  Liquid and gas divided and recombined by the packing thus providing the high mass transfer rates by continuously renewing the liquid surface.  The packing has not caught on for new tower installations because of its high cost but has found a niche in the plant upgrade market.  Installing structured packing can allow a tower to handle a high gas flow without the need to completely replace the tower.  In these cases, the packing cost is small compared to the cost of a new tower designed for a higher capacity.

Another relatively new packing is WavePak™ available from Monsanto.  The packing gets its name from the unique shape of the packing which is a complete departure from the traditional saddle shape.

 

 

The newest packing to hit the market is from Koch Knight LLC which already market a line of packing under the tradename Flexisaddle™.  The newest packing is Flexisaddle™ LPD (low pressure drop) (patent applied for) which is basically a saddle shaped random packing with holes in the surface of the saddle.  A significant difference is a rib in the middle of the saddle which appears to extend the surface area available for each piece of packing.  The added surface area combined with the numerous holes appears to give the packing the low pressure drop and high performance (i.e. high mass transfer rates) claimed by Koch Knight LLC.

Packing Comparisons

Random Packing

Type	Saddle	Saddle	WavePak™	Super Intalox	Rashig Ring	HP™ Saddle
Size	3"	2"	-	No. 2	75x75x9.5 mm	3"
Pieces per (ft³)	53	266	26	-	48	38
Pieces per (m³)	1870	9400	918	-	1700	1342
Bulk Density (lb/ft³)	36.8	47.4	38	-	368	26.6
Bulk Density (kg/m³)	590	760	608	-	498	426
Surface Area (ft²/ft³)	28	36	16.5	-	19	18.3
Surface Area (m²/m³)	92	118	54	-	62	60
Void (%)	80	79	-	-	75	81.5
Packing Factor (1/ft)	21	40	18	30	37	-
Packing Factor (1/m)	70	130	-	98	120	-

Structured Packing

Type	Type 88	Type 48	Type 28	-	-	-
Bulk Density (lb/ft³)	21	32	48	-	-	-
Bulk Density (kg/m³)	336	513	769	-	-	-
Surface Area (ft²/ft³)	31	48	86	-	-	-
Surface Area (m²/m³)	102	157	282	-	-	-
Void (%)	85	77	72	-	-	-
Packing Factor (1/ft)	-	-	-	-	-	-
Packing Factor (1/m)	-	-	-	-	-	-

Installation

Installation of random packing is important in order to prevent gas or liquid channelling, high pressure drops and broken or chipped packing.   To ensure a uniform density throughout the bed the packing should be installed evenly across the the entire cross-section of the tower.  Large piles or mounds of packing should be avoided.  If the density of the bed is different in one area, gas or liquid channelling may occur.

Packing should be carefully laid or poured out from the box or bag onto the surface of the packing already in the tower.  The packing should never be dumped from any great height.  Small chips created from improper handling have been known to plug pump strainers, acid coolers and acid distributors.

Installers should not stand directly on the packing but on plywood boards laid on top of the packing surface.  The plywood serves to spread out the weight which will avoid further chipping and breakage of the packing.

Ensure that all boxes and bags are removed from the tower as soon as they are empty.  Ensure that no plywood, rope, or other construction material gets buried in the packing all debris is removed from the tower.

The tower should always be packed up to the required level as shown on the drawings.  Usually extra packing is order to allow for settling and breakage.  The volume of the packaging will not equal the volume of the tower.   The wall effects of the smaller packaging results in a different volume when the packing is installed in the tower.

Standards

ASTM C515 provides a standard for ceramic random packing in terms of the chemical composition, physical size of the packing and the number of pieces in a given volume which ensures a minimum quality for the packing.  The standard does not cover properties that affect actual operational performance (i.e. pressure drop, mass transfer, etc.)