Hydraulics & Distillation Column Performance

Influence of Hydraulics on Distillation Column Performance Abstract This report probes the fundamentals of column design with emphasis on impact of tray geometry or design on the column performance. In addition establish the fact that system efficiency cannot be influence by the designer as it depends solely on the fluid property or system. Guideline on appropriate contact device for various applications is provided. Furthermore, discussion on the hydraulic parameters highlighted the valid operating limits as a guide to ensure good column performance. Background The performance of distillation column depends heavily on the internal. The arrangement and construction of the internals is such that it promotes mass transfer. The type of columns encountered in the industry is packed and tray column. It is paramount to be able to quantify mass transfer for proper manufacture of column internals. In general, an expression used to determine mass transfer is: n_a = K_c×A×?C_A {1} Note from {1} it is important to have a large effective contact area and the main driving force for column operation can be concentration, pressure or temperature difference. Performance measurement based on efficiency in the industry involves system (or point) efficiency and column efficiency. Limitation on efficiency caused by nature of the system rather than physical design is referred as Point or System efficiency. On the other hand, when the efficiency of the column is limited by physical design it is referred as Column efficiency. A quick estimate of the column efficiency is obtained by taking the whole column as a section and then applying the formulae below: E_o=N_T/N_A {2} However, treating the whole column as a section is too crude and can be misleading on the required number of trays to effect a particular separation. More appropriate is to establish section efficiency which takes into account variation along the column. The section efficiency reflects the degree of contact on trays in the column, desirable is to have 100% as section efficiency. However, deviation from ideality on trays in the column makes it difficult to achieve such efficiency. It is common practice to estimate the point efficiency first and later modify calculations to obtain average tray efficiency. Contact Device Contact device available in the industry is valve tray, bubble cap and sieve tray. The selection of tray design suitable for a particular column usually depends on capacity and desired efficiency. It is important as a designer to recognise the opposing relationship between column efficiency and capacity. In addition, energy consumption is also an important factor to consider alongside capacity and efficiency during column design. More importantly in selection of appropriate contact device is consideration of the liquid to vapour flowrate ratio (i.e. L/G). For normal distillation column the ratio is close to 1 while for strippers the ratio is greater than 1. Finally, for absorbers the ratio is less than 1.

Sieve Tray Sieve trays are simply metal plates perforated with round holes. Vapour passes straight upward through liquid on the plate. The flow of vapour through tray deck to contact the liquid on the plate is controlled by the number and size of the perforations. They are the simplest contacting device available in the industry; however column efficiency is significantly reduced to a maximum of about 50 percent. The lowered efficiency is linked to available open area which is quite small in comparison to other available tray design. Typical sieve size ranges between 5 to 25mm. It is clear that a complete sieve tray specification include hole diameter, hole pitch, tray diameter and thickness, downcomer type and size, weir details and material of construction. Sieve trays are the most widely used mass transfer devices due to their simplicity, versatility, capacity and cost effectiveness. Figure 1: Sieve tray For efficient operation, the velocity of vapour through the plate holes must be sufficient to balance the head of liquid on the tray deck and thus prevent liquid from passing through the perforations to the tray below (otherwise known as weeping). On the otherhand, if the velocity of the vapour through the perforations is high then liquid from tray below will transfer to tray above (this phenomenon is better referred as liquid entrainment) therefore leading to lower column efficiency. Consequently, sieve trays have a narrow operating range (otherwise known as turndown), no more than 2:1. Valve Tray This type of contact device encompasses features of sieve and bubble cap tray design. Just like sieve tray design, valve tray have perforated tray decks and there exist two different type. One type of valve tray is known as fixed valve solely because the cap is not allowed to move due to the fixed valve leg. The other type has a moveable cap solely because the valve leg is allowed to vary and is known as dynamic valve. Figure 2: Fixed Valve Tray Figure 3: Dynamic Valve Tray In general, implementation of a valve tray lead to better column efficiency mainly because the cap ensures vapour flow horizontally into the liquid thereby enabling better mixing or contact with the liquid on the tray deck. Furthermore, a dynamic valve tray is capable of handling fluctuations in vapour flow rate than a fixed valve because of its ability to vary available open area. At very low vapour flowrates, the valve disc rest on the tray deck to almost close off completely the tray deck perforations thus minimising tray open area. As the vapour flowrate rises, the valve discs are lifted from the tray deck which increases the open area for vapour flow between the valve disc and tray deck. The effective operating range of valve trays is dependent on specific service condition as well as pressure drop limitations and can be as high as 10:1. It is important to recognise that for dirty service or polymerisation process the ideal type of tray design is fixed valve because it offers a fixed open area which is bigger than sieve and the likelihood of the dirt preventing the valve from rising renders dynamic valve unsuitable. Bubble Cap Tray It consists of bell shaped caps fixed to cylindrical risers through which the vapour passes the tray deck. The caps divert the vapour flow below the level of liquid on the tray deck where it is jetted into the liquid either through slots at the bottom of cap or else between the skirt of the cap and the tray deck. Figure 5: Bubble Cap Tray Sieve and valve tray have replaced bubble cap tray application in the industry because of their efficiency, wide operating range, ease of maintenance and cost factors. Flow Regime They are use as basis for fundamental modeling to provide a generalization of the bi-phase behaviour. On a distillation tray the flow regime could be continuous liquid - dispersed vapour, continuous vapour - dispersed liquid, or both. The two-phase dispersion on a tray is classified into five flow regimes: emulsion, foam, bubble, spray, and froth. However, regime encounter in industrial application include spray and froth only. In the spray regime, jets of vapour rises through the tray openings thereby turning liquid on the tray into droplets which projects to the intertray space. Consequently the liquid droplets are subject to drag, gravity and buoyancy forces. As a result the liquid droplets either fall back to the tray or entrain the tray above. Hence, the continuous phase is the vapour phase and the dispersed phase is the liquid phase. The spray regime is encounter at low liquid loads and high vapour velocities. Figure 6: Spray Regime

In the froth regime, the continuous phase is the liquid while the dispersed phase is the vapour. However, it is made up of two sub-regimes which can be classified as bubbling froth and the mixed froth regime. The bubbling froth regime denote the region in which vapour transport is only by bubbles, on the other hand mixed froth regime denote region in which vapour transport is by both bubbles and jets. The froth regime in general is encounter at moderate to high liquid loads and low to moderate vapour velocities. Figure 7: Froth Regime Hydraulic Parameters The performance of a distillation column or columns in general is measured through the following parameters: Jet Flood Entrainment limit Tray Pressure drop Downcomer back-up Flood Downcomer Choking Flood Weir loadings Weeping Downcomer Inlet velocity Jet Flood: It is use to predict the point at which massive liquid carryover will occur due to the height of spray on tray deck exceeding the available tray spacing. It is normal practice to limit tray design to 10 - 85% of jet flood to allow safety margin on column control. Entrainment limit: The limit is reached when the velocity of vapour flow through the tray perforation is high enough to cause transport of liquid droplets to the tray above. The valid operating limits lies between 0 and 70%. Tray Pressure drop: The operating tray pressure drop is the sum of dry pressure drop and wet pressure drop. The dry pressure drop occur due to resistance to vapour flow through the tray open area while the wet pressure drop is a consequence of the liquid hold up on the tray deck. The head of the clear liquid on the tray deck (liquid hold up) is a function of the weir height and weir length (as well as liquid and vapour rates and physical properties). Hence, in high liquid rate services such as stripper the pressure drop may be reduced by increasing the number of flowpaths. Tray Pressure drop valid operating limit is 0.1 - 0.7kPa, beyond the maximum of 0.7kpa the tendency for flooding is very high. Downcomer Back-Up Flood: Terminology used to describe backward flow of liquid leading to accumulation of liquid in the tray above due to factors such as high liquid loading, insufficient downcomer area, and small intertray spacing e.t.c. Allowable range of downcomer back - up is 10 -85%. Downcomer Choking Flood: A consequence of vapour flowing through the downcomer is retardation in downward liquid flow, thereby resulting into accumulation of liquid in the tray above. From a mechanical point of view such situation can be attributed to things like small downcomer width, high vapour rate, highly viscous liquid, large downcomer clearance(in comparison to the weir height) e.t.c. It is normal practice to operate in the range of 10 -90% downcomer choking. Weir loadings: Simply refer to the volumetric flowrate of the liquid per unit horizontal length of the outlet weir. For normal distillation column operation the valid limit of weir loading is 1.49 - 32.29 m2/s. High weir loading result into large crests and high froths thereby result into excessive pressure drop. Further consequence of the excessive pressure drop created from the high weir loading could be downcomer back-up or choke flooding. Commonly employed procedure to deal with high weir loading is to use higher pass tray, however the price for such modification is reduced tray efficiencies, increased tray cost and installation time. Weeping: Terminology use to describe the flow of liquid through tray perforation or joints on the tray panel. Due to the fact trays are installed in panels which involve welding or joining to the column wall there will always be weeping during normal operation, however excessive weeping reduce the contact time between vapour and liquid thereby leading to loss in column efficiency. The effect of outlet weir side weeping on the column efficiency is minimal while that of the inlet weir side weeping is significant. Conclusion Based on earlier discussion, it is clear that contact device which maximize vapour-liquid contact while maximizing compositional approach between the vapour and liquid will also maximize the column or tower efficiency. Furthermore, the property of the fluid as pointed out in discussion on type of efficiency play a significant role. The tray efficiency reduces with increase in viscosity of the liquid. Also, of more relevance from earlier discussion is the need to check liquid to vapour ratio in order to appropriately decide on contact device to install in a column. For fouling or dirt service ideal choice of contact device is fixed valve and for high liquid service a dynamic valve is more suited.