Enhancing Capacity of Existing Column
- Sameer Mehta, Manager - Process, Kevin Enterprises Private Limited

Energy conservation is the absolute need of the hour in today’s world. Engineers thus look for ways and means to get the best out of the given systems. So, when it comes to a Packed Tower then a Liquid Distributor and Feed Device play an indispensable role to get an optimised solution.

We are in times where energy conservation is the absolute need of the hour as much for mother earth's survival, as our very own. One thus looks for ways and means to get the best out of the given systems, without changing much and with least possible investment. So, when it comes down to more for less, in a packed tower system, the first thing which comes to the mind of any engineer seeking to increase capacity of the plant is to go for a larger diametre column.

However, when the user is introduced to high capacity packing (be it random or structured), which saves one from the need to change the tower itself, the user is delighted beyond expectation and starts believing that the problem is solved. Therein lies the fallacy, as while high capacity packing is a good solution, it is definitely not a complete one. While selecting mass transfer equipment, one needs to be correct, and often the cumulative accuracy lies in the finer details.

Let us examine the typical sequence adopted by the designer of packed tower system. Once the simulation of the system is done and the stages required for carrying out the separation or absorption is ascertained, the next focus is on selection of the packing. One assumes that once the packing is selected, the task is done and the results are assured. In particular, the confidence gets compounded especially when one plans to employ the high capacity packing over the conventional type. But, for the best results, high capacity packing also demands deployment of complementary well designed high performance internals for achieving the optimum results. Inside a packed tower, all hardware other than packing are known as internals and some of the key ones are - feed devices that introduce the fluids inside the tower, distributor for distribution of fluids, bed limiters as an insurance to retain the packing within its intended location, support plate for supporting the packing, collector trays to take partial or total draw offs, etc. (See Figure 1)

Here, we shall examine at length the most critical internals to bring out their salient features, namely the feed devices and the distributors.

Liquid and Gaseous Feed Devices (Predistributor)
The first and foremost are the feed devices that inject the liquid at the very top of the tower or the gas/vapour at the very bottom of it. These have to be designed carefully so that they are not only compatible with the mouth of the external pipelines and their orientation for ease of installation but also the next tower internal to which they introduce the fluid to.

The top liquid feed device feeds the top distributor. It must be so sized that the optimum pressure is maintained in it to prevent any back flow, while at the same time ensuring the liquid reaches the far end and is released at the optimum velocity. They must further be designed in conjunction with the distributor so that the liquid introduced from the feed pipe does not fall atop the gas risers located on the distributor below.

Similarly, we have gas or vapour feed device at the bottom of the tower. Though these are relatively less sensitive to operation than liquid feed device, nevertheless, they can create tower malfunction if poorly designed. These are placed below the support plates or support grids that hold up the packing or sometimes below the collector trays. Care must be taken in their design to ensure that the gas/vapour do not enter with such a velocity that they blow away or dislocate the tower internal above the feed device. Also, they must be so located from the elevation point of view, that they allow some time for the gas/vapour exiting it, to adequately spread before encountering the equipment next in line above.

Liquid Distributors (Main Distributor)
The liquid distributor is extremely critical and difficult to design. The distributor, as the name suggests, distributes the liquid and irrigates the packed section below it through delicately placed and sized openings from which the liquid exits out and on to the packing below. Before the same is done, the distributor collects the liquid from the liquid feed pipe above and typically allows (not always) the liquid to build up into a reservoir on its deck or in its troughs. This provides a calming zone of sorts for the liquid, before allowing it to exit from the openings. The liquid which was until now constantly moving (horizontally) at a higher velocity, is somewhat made stationary in the distributor, relatively speaking. It also ensures the liquid, as far as possible, moves slowly downwards (vertically) only, as it exits the distributor under the hydrostatic head of the built up reservoir. Thus an attempt is made to ensure a steady state flow downwards and no liquid gradient is formed on the distributor. The said liquid reservoir also cushions the flow from the exit openings, from the possible vigorous flow variations in the feed lines and the feed device above the distributor. The idea is to distribute the liquid evenly across the cross section of the tower, with as far as possible, each exit opening of the distributor receiving the same amount of liquid stream. To prevent entrainment or upward carry-over of the liquid by the force of the gas/vapour, separate channels are provided on the distributor for gas/vapour flowing upwards.

When the distributor is located between two beds for collecting the liquid raining from the packed bed above and then re-distributing the liquid to the packed bed below, it is called a 'redistributor'.(See different types of distributors in Figure 3)

Importance of Liquid Distribution: Packed tower design is based on the fundamental concept of equal liquid and gas superficial velocity across the column section. The pressure drop across the packing provides an impetus for the upward flowing gas to become uniformly distributed across the column area. The liquid flows down the packed bed by gravity and unlike a gas, the liquid has poorer cross-mixing tendencies. It is therefore imperative to manage and ensure very uniform liquid distribution at the top of the bed. Distributors are internals installed above a packed bed, which perform the job of providing a finite liquid distribution over the packed bed. A distributor allows liquid to be distributed over the packed bed in discrete streams. This can be done either through orifices or V-weirs located on/in the distributor. Distributors also provide a separate passage for the upward flowing gas.

Once liquid enters the packed bed, the packing tends to redistribute the liquid by virtue of dispersion and after some height, the liquid profile adapts to the natural distribution tendency of the packings, which generally , is worse than the initial liquid distribution provided by the distributor. Because of this, liquid distribution in packed beds tends to break down after fixed heights and liquid redistributors are provided to collect all the down flowing liquid and redirect it uniformly into next packed bed.

A packed bed irrigated by a very good distributor allows one to realise the full separation potential(number of stages) of the packed bed.

Categories of Liquid Distributors
The liquid distributors are divided into two broad categories - gravity type and pressure type. The models under gravity type feed are pan type, deck type, trough type, V-weir type,etc. The models under pressure type feed are spray nozzle type, pipe arm distributor type, etc.

Distributor Testing
  • Liquid distributor testing is also done to ensure the proper distribution. It is found that the tested distributors at shop facility also perform very well in actual service condition. To carry out the distributor testing appropriate test facility shall be available with sufficient size for entire distributor.
  • The test facility should comprise of the following features:
    -Feed system with different pumps
    -Freshwater inlet and recirculation system
    -Flowmeter system
    -Support structure for mounting and levelling of distributors
    -Collecting funnel, measuring vessels, scale, stop watch, etc
    -For testing purpose, water is used as testing fluid
Pre Arrangements for Distributor Testing
After liquid distributor is mounted on test ring, it should be levelled properly. The test instrument shall be calibrated. After starting the test system, keeps the system running for a minimum of 15 minutes to ensure steady flow from feed pipe or pre distributor.

Testing Methods
  • For good performance of liquid distributor within given operating range, it is tested for maximum and minimum liquid loads. Checking of the liquid head is done across the distributor/different channels of distributors.
  • When specific guarantees are to be maintained, then actual outflow from multiple point is collected using funnels and measuring vessels and the coefficient of variation is checked.
  • The large distributors can be checked partly considering the size of distributor. A distributor test facility with upto 500 m3/hr (17,500 ft3/ hr) and upto 8 metre (26 ft) diametre testing is not uncommon.
Design of Liquid Distributor/Redistributor
  • Select suitable type of distributor/redistributor: based on various parametres like column diameter, service, flowrates, range of flowrates to be handled, installation feasibility, fouling service etc suitable type eg, deck, pan, trough, flow multipliers, weir, with/without drip tubes etc is selected.
  • Once the type is finalised number of irrigation points are decided based on the type of the packing. More numbers of points are provided for packings with higher surface area. Moreover, traditional packings require comparatively lesser number of distribution points than modern/third generation packing. Selection also depends upon type of packing viz, random or structured.