|
Separation - Science or Black Art? |
|
We have often heard the question asked: "Separation - is it a science, or a black art?" The answer is, we believe, a bit of both.
The science of separation is in fact very well developed, and good science should always underpin separator designs. However, although separation may at first sight be a simple unit operation, on deeper examination it becomes clear that there are many and varied factors influencing separator performance. This creates complexity, and typically requires the designer to adopt simplifying assumptions, which, unless they are fully understood, can easily invalidate a separator design and lead to performance failure.
It is probably true that gas/liquid separations can be fairly well predicted using close analysis of droplet behaviour and micro- and macro-fluid dynamics. However, experience shows that best practice is not always followed, and sometimes basic lessons can be forgotten.
Liquid/liquid (oil/water) separations are generally more problematic. As yet, there is no full analytical method of designing these separators, and until predictive
methods can reliably and quantitatively incorporate the effects of the greatest influences, i.e. dispersion behaviour and the action of demulsifier chemicals, that day is
still some way off.
There has been, and continues to be, a great deal of research carried out on separator performance prediction and eventually some further significant advances may be made. However, until that time we have to rely on our current level of understanding, with its combination of fundamental science and empirical design, and therefore there will always be an important place in the design process for experience.
We take a pragmatic approach
to separator design, incorporating a sound theoretical approach, whilst also recognising the need to satisfy
project and commercial needs. We will provide you with an independant and objective viewpoint
on separator and separation systems design, and the following list outlines some of the issues we would always consider.
Project and Commercial considerations
- To achieve effective separator design requires - first and
foremost - the correct Project, Commercial and Procurement environment! Do project methodologies and purchasing techniques
adversely affect separator performance? Yes, because conventional project environments often do not recognise the complexity and uncertainty inherent in sepration design. For further information, see our published paper here
Liquid - Liquid separationOil-Water Separation Theory
- When is "Stoke's Law" is applicable and when it is not, and what are the alternatives ?
- Can the "Residence Time" approach be used, and what are its limitations?
- How do you scale up from laboratory or pilot plant tests ?
- When are vertical vs. horizontal vessels appropriate ?
- How important the hydraulic (volumetric) efficiency of a separator is, and how to improve it
Fluid Characterisation
- What are characteristics of an oil/water/gas mixture ?
- The effect of the system upstream of the separator
- Why there can be up to 9 "phases" in a 3-phase oil/water/gas separator ?
- What is an "emulsion" layer, is it always an emulsion, and what can be done about it
- How foam forms and what to do about it
- How useful are bottle tests and what alternatives are there ?
- How do I run a pilot plant to obtain meaningful field data ?
Gas-Liquid Separation
- The effect of the system upstream of the separator
- When is each type of demister device appropriate ?
- How should the separator be divided into different functional areas to ensure correct operation of each part ?
General Separation ConsiderationsInternals Designs
- What is current best practice in separator design ?
- When plate packs and other extended surface internals are required
- What are the correct ways of designing internals ?
- Why baffles may not create uniform flow characteristics
The Importance of the Inlet Device- Why is an inlet device important and what are its desirable
characteristics?
- How do cyclonic inlet devices work, and when do they fail?
- What are the differences between the various cyclonic inlets
available, and when should each be used?
- What other inlet devices are available?
Performance Guarantees
- What is a process guarantee on a Separator, and what is it not?
- How to tell what is a valid estimate on likely separator
performance
- How to construct a meaningful process guarantee
System Engineering and Control
- How to integrate separators into a process system
- What are the correct level detectors to use for a separator?
- What are the best control configurations for separators?
|
Separation Science 