Diameter Calculation of Sieve Tray Column

This blog is about, the diameter calculations of a sieve tray column. Software's can give diameter for each stage but to verify on our own,the below Sinnot Procedure will help you in comparing the results. We may not get the exact match becasue software equations might be much more detailed. The below procedure help a student on how text book methods are used for comparing the results and where exactly the basic knowledge will help in sizing a distillation column.

In a similar way, diameter is calculated for each stage and the largest diameter will be selected.

If you calculate for other stages the diameters will be different because properties will be different. Constructing a column with single diameter is cheaper. But how we accommodate the lesser diameter stages, is by simply blocking perforations of a tray so that total openings will reduce which is equivalent to lesser diameter.

Columns with different diameters also in use, but only thing is cost is higher.

In my next blog I will write a blog on pressure drop calculations.

   Approximate Column height is calculated as (Number of stages*Tray Spacing) + (15% of  Number of stages*Tray Spacing).

Questions:

How maximum vapor rate is decided? This is an initial guess which we need to mention for a coloumn to converge. What i have done is i have taken a vapor fraction of feed stream and multiplied with feed flow. Simulator will give slightly different answer. But for hand calculations this value is required so guess value using above procedure.

Topics you may like and will be covered in my future blogs:

Column Height/Number of stages is decided by relative volataility,lesser the value difficult will be separation and higher will be the stages/Height.

Example: C3 splitter, Separation of Xylenes.

Poynting correction factor can be ignored for low or medium pressure situations. This topic will be understood if you know gama-Phi Formulation.

References:

Guidance by Prof KV Rao

Introduction to chemical Engineering Thermodynamics by Smith, Vannes and Abbot.

Mass Transfer operations by Treybal.

Natural gas Hydrates in brief!

     So hello everyone I have been trying to learn on natural gas hydrates and problems on hydrate formation. So below content is only for academic point of view.

Hydrate formation is the major problem in offshore exploration which leads to flow blockage, loss of circulation and even abandonment of the well. This leads to lot of loss in form of dollars and time. The practical solution for preventing or delaying hydrate formations for a long pipeline subsea network is by adding inhibitors, any chemical which reduces the water potential can be used as an inhibitor but MEOH (Methyl alcohol is a universally accepted chemical inhibitor). Knowing the conditions (Pressure and Temperature) with and without inhibitors will help pipeline designing and pipeline operation. The most common hydrate forms structures (Structure I and Structure II) hydrates will be described.

This study also shows how to dehydrate a wet gas using glycols as a solvent which are generally available in offsite gas plant facilities. Dehydration is a solution for reducing the water content before it is sending to gas transportation.And Pressure has to be maintained with an objective of gas has to be in dew point conditions.

Basic substitute equations for calculating HFT.

·               1934, Hammerschmidt.

·               Makogon correlation.

·               1991, Motiee, 15 parameter T-Explicit Equation.

·               2005, Towler and Mokhatab and

·               Bahadori and Vuthaluru. Etc

In all the cases know the process pressure condition i.e. at what pressure it is transporting and then calculate the temperature.

How to describe Type1 and Type2 Hydrates.

Actually we can easily describe Type I and Type II (or Structure I and Structure II). Based on the below graph.

Graph -->Check Figure2.2

·         Less than 3.8 A it is difficult to form hydrate.

·         If the guest molecule are Ar,Kr,N2,O2 this is Type II.

·         From 4.4 A methane, H2s, Xe, ethane, propane will fall in Type I cages.

·         From 7A no Type I and Type II will form. There is another type called TypeH.

Definition of hydrates:

I am also going through the most common gas hydrate forms. According to Von Stackberg and Muller, each of the structure contains two types of cavities.

·         Structure I consists of 46 water molecules which constitute 2 small pentagonal dodechahedra cavities and six tetrdecahedral large cavities, having 2 opposite hexagonal faces and 12 pentagonal faces.

·         Structure II consists of 136 water molecules comprising  16  small pentagonal dodechahedra cavities and 8 hexadecahedra large cavities, having 4 symmetrically arranged hexagonal faces and 12 pentagonal faces.

The average cavity radiuses of 3 cavities are

3.9 A for pentagonal dodechahedra.

4.3 A for tetrdecahedral.

4.7 A for hexadecahedra.

Basically the reason why gas hydrates will form above the ice point of water is, gases will diffuse in these cavities which will alter the condition of formation of Ice. So this is the reason hydrates will form in ~10C.

References:

·         http://petrowiki.org/Predicting_hydrate_formation

·         http://www.gasprocessingnews.com/features/201508/predict-gas-hydrate-formation-temperature-with-a-simple-correlation.aspx

·         http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.623.6856&rep=rep1&type=pdf

·         Empirical expressions for gas hydrate stability law, its volume fraction and mass-density at temperatures 273.15 K to 290.15 KZhengquan Lu1, 2, * and Nabil Sultan1

·         PREDICTION OF GAS-HYDRATE FORMATION CONDITIONS IN PRODUCTION AND SURFACE FACILITIES A Thesis by SHARAREH AMERIPOUR