Two-phase feed distributors for distillation and absorption columns

There is no one solution that fits every scenario. What is feasible for a new column may not be possible for an existing one. Additionally, whether the column is packed or trayed will influence the decision making. Last but not least, the feed location – top, intermediate or at the bottom of the column – should also be considered.

Flow regimes

In horizontal pipes, the flow regimes that can exist for two-phase flow are shown in Figure 1 (in order of increasing vapour flow). The flow regime depends on the flow rates and the properties of the two fluids.

Figure 1. Flow regimes for two-phase flow in horizontal pipes.

Bubbly flow

At low vapour flow rates, the liquid will occupy the bulk of the cross-section of the pipe. Due to buoyancy, the vapour will flow in the form of bubbles, towards the top of the pipe.

Plug flow (intermittent flow)

As the vapour rate increases, the bubbles will coalesce, forming alternating plugs of liquid and vapour along the top of the pipe. Liquid will remain as the continuous phase along the bottom of the pipe.

Stratified smooth flow

As the vapour rate increases further, the vapour also becomes a continuous phase. Vapour will flow at the top of the pipe, whilst liquid occupies the bottom.

Stratified wavy flow

As the vapour rate increases even more, since vapour moves appreciably faster than the liquid, the resulting friction at the interface between the two forms liquid waves. The wave amplitude increases proportionally to the vapour flow.

Slug flow (intermittent flow)

At a certain critical value of vapour flow, the crests of the liquid waves reach the top of the pipe, forming alternating frothy liquid slugs and vapour slugs. The velocity of both slugs will be greater than the average liquid velocity. Where a vapour slug occurs, the liquid level will be depressed such that vapour occupies a large part of the cross-section of the pipe.

Annular flow

At high vapour rates, the liquid flows as an annular film of varying thickness along the wall, while the vapour flows as a high speed core down the middle. Under these conditions, there is substantial slip between the phases. Part of the liquid is sheared off from the film by the vapour, creating entrained liquid droplets along the core of the pipe. Simultaneously, turbulent eddies in the vapour will deposit droplets onto the liquid film. Due to gravity, the liquid film will be thicker at the bottom of the pipe than at the top.

Dispersed flow

Dispersed flow is also known as spray flow or mist flow. At very high vapour rates, the vapour velocity in annular flow becomes so high so that all the liquid film is torn away from the wall and is carried by the vapour as entrained droplets.

This article was originally published in the September 2023 issue of Hydrocarbon Engineering magazine. To read the full article, sign in or register for a free subscription.

Written by Aadam F. Aryan, Distillation Equipment Co. Ltd, UK.

Read the article online at: https://www.hydrocarbonengineering.com/special-reports/31082023/two-phase-feed-distributors-for-distillation-and-absorption-columns/