Jul-2016

Enhancing fractionator efficiency

Properly designed vapour and liquid distribution devices enable more effective distillation column efficiency.

Soun Ho Lee
GTC Technology

Article Summary

Number of trays and packed bed height are both primary parameters in determining the degree of separation of a distillation column. However, this factor alone cannot guarantee targeted distillation column efficiency. Vapour and liquid distribution devices play a vital role and overlooking these critical items often brings inferior results. The importance of vapour and liquid distribution design is more emphasised for a packed column where column efficiency is more sensitive to liquid and vapour distribution.

This article will discuss commonly overlooked items in vapour and liquid distribution devices. Actual retrofits for packed fractionators, which examine how column efficiency enhancements are achieved through careful analysis and design procedures, are demonstrated through two case studies: a crude atmospheric column in a petroleum refinery and a demethaniser in a natural gas processing plant. Useful methodologies for evaluating vapour and liquid devices and for remedies are also illustrated.

Liquid and vapour distribution for a packed column
Numerous publications have discussed the importance of liquid and vapour distribution in packed columns and extensive design criteria/guidelines. Nevertheless, liquid and vapour distributions are still ranked highly as causes of distillation column malfunctions.¹ The following parameters are often overlooked in liquid and vapour distribution device design.

Liquid distribution devices
The correct selection of a liquid distributor type is the first step for reliable liquid distributor design. The selection of the liquid distributor type requires extensive application know-how and should be customised for each service section. A refinery vacuum column is a good example that emphasises the importance of a liquid distributor type.

A refinery vacuum column is commonly configured with structured and/or grid packing to handle high amounts of vapour traffic with a minimum column pressure drop. In past designs, many equipment suppliers have selected a spray nozzle distributor coupled with a mist eliminator in this section as it is primarily utilised for heat transfer. However, spray nozzle distributors may cause significant liquid entrainment and a loss of top distillate yield. Switching the distributor type with a gravity flow trough-style liquid distributor can reduce the chances of liquid entrainment. A mist eliminator is unnecessary in this configuration; in fact, additional pressure drop can be eliminated by removing the mist eliminator. Meanwhile, the refinery vacuum tower wash section’s flow regime is characterised with much lower liquid traffic compared to the neighbouring pumparound section. Selection of a gravity flow trough-style liquid distributor for this low liquid traffic zone requires a small hole size, which increases the chance of distributor coking/fouling. Moreover, stagnated liquid fluid inside the trough is constantly exposed to hotter vapour traffic, and also increases the chance of coking.

Fouling tendency is often ignored in liquid distributor design. The basic equation used to size gravity liquid distributors is:

H = liquid height (‘head’) above distributor orifices
Lv = liquid volumetric flow
N = number of orifices
HA = hole area
K = orifice coefficient

A minimum liquid height (H) must be maintained to ensure uniform liquid distribution. In addition, a minimum number of orifices (N) should also be maintained to ensure good distribution (usually referred to as ‘drip-point-density’ in the number of holes per unit cross-sectional area). These concerns would tend to push the designer to use a greater quantity of small orifices. However, this design approach opens the risk of fouling the liquid distributor. A device designer’s application knowledge and experience are critical for the correct hole size against liquid distributor fouling.
The pattern of the liquid distribution points is also critical to achieve a good packed bed efficiency. A distribution quality rating index was developed to gauge the quality of liquid distribution.² A lower rating index of distribution quality can downgrade packed bed efficiency. Recent pilot plant testing and analysis verifies that packed bed efficiency is noticeably influenced by the pattern of liquid distribution points.³
The importance of initial distribution before entering the liquid distributor device cannot be over-emphasised. Improper pipe distributor or gravity down pipe/box design can affect liquid distributor performance adversely. It is observed that initial liquid maldistribution to a primary parting box of a gravity flow trough type distributor causes liquid splashing and poor liquid distribution performance.

The unnecessarily wide operating range requirements of a liquid distributor can downgrade liquid distribution quality. The multi-stage orifice hole design concept is commonly selected for wide operating range liquid distributor design. Flow instability between the orifice hole stage can cause high flow deviation, which can downgrade liquid distribution quality. The wide operating range requirement of the liquid distributor also affects pre-distribution quality. The perforated pipe distributor or gravity downpipe/box is usually arranged as a pre-distribution device. A liquid phase perforated inlet pipe distributor has been traditionally designed with maximum liquid velocity criteria in order to avoid mist formation. Meanwhile, lower velocity issues are often overlooked during turndown operations. Insufficient perforated hole liquid velocity does not maintain enough pressure drop for uniform liquid distribution and can result in poor initial liquid distribution.⁴

It is often perceived that the operating range of distillation equipment should be matched to distillation column feed/product ranges in the industry. This misunderstanding requires a wide operating range of the liquid distribution device and results in poor liquid distribution performance in the target operating point. As the traffic range of the liquid distribution device does not have to be matched to column feed/product ranges, adjusting the reflux/boil-up ratio or heat balance using pumparound shifting at an extremely low level of operating mode can allow normal operating range selection of the liquid distributor. This strategy increases energy consumption at an extremely low level of operating mode but can secure efficient performance within the operating range. Unless unit operating ranges are significantly swung in a short time span, this strategy provides better overall plant performance/economics.

Vapour distribution devices
A collector tray is a common device for an intermediate stream draw-out of a distillation column, such as a refinery multi-product fractionator. It is well known that a collector tray is needed for the successful function of liquid collection. On the other hand, it is not well recognised that collector trays affect vapour distribution throughout the column. Vapour distribution is an important factor to ensure desired column efficiency. Ignoring this factor can cause vapour maldistribution and result in downgrading column performance. Vapour maldistribution can also cause liquid maldistribution due to an uneven packing void area. Eventually packed bed performance can be downgraded.