- 2 days ago
This Video is about
1. What are the laws of separation
2. how to derive it
3. how separation happen which law to be used
4. Guideline for design
1. What are the laws of separation
2. how to derive it
3. how separation happen which law to be used
4. Guideline for design
Category
📚
LearningTranscript
00:00and when you put this value in your terminal velocity equation for the cd then this if your
00:07vapor phase is present and two liquid phases are present and low then it comes as a intermediate
00:14law by the drag or resistance of the object in the fluid so it is added to 1000 the micron
00:23hi friends welcome to our channel concept engineering flux focus on fundamentals we are
00:28in model 3 on separators we are discussing and this is the second lecture on separator introduction
00:34in today's session we will understand the three different laws of three different settling laws
00:39for the separation also the droplet size and other things we will discuss in today's session
00:47so just a quick recap on the lecture 1 introduction to the separators i request if you are not watched
00:54that earlier session or earlier video please go through it because without that this video will
00:59not be able to understand properly so in first session we have completed the method of separation
01:06separator zones what are the movement transfer and how how the three different ways of
01:12transfer or separation movement transfer gravity and the bulk transfer and then the terminal velocity and
01:20we have calculated the terminal velocity and we come to this equation of terminal velocity
01:26and in this equation we have not covered the cd which is the drag coefficient so drag coefficient so before
01:37to understand the drag coefficient we we have this equation of terminal velocity in this there are two
01:44unknown one is the diameter of the particle and the second is the drag coefficient so basically drag
01:50coefficient is the dimensionless quantity used to quantify the drag or resistance of the object in the fluid
01:58so it is it represents the basically the resistance in the fluid
02:07so drop the basically the drag coefficient is the function of shape and particle shape and the reynolds number
02:16so you know the reynolds number is nr is equal to dp rho u by mu this u you can
02:25define as the ut also
02:27which is your terminal velocity so at terminal velocity you will get the nre for that case but as a
02:34as we
02:35don't know the cd and cd is the function of shape and the reynolds number so to solve this terminal
02:43velocity
02:43equation we have to do some trial and error so instead of that uh there is a the drag coefficient
02:51presents the function of the product of drag coefficient and the reynolds number that eliminates if if you
02:59multiply the cd with nrv which is this function this function will because ut
03:08uh there are two unknowns like uh terminal velocity here also coming here also coming so when you
03:14multiply this the terminal velocity part will go and you can get a correlation so based on this
03:21correlation you can find the size uh or you can say cd or the drag coefficient for the different
03:27shapes of the particle like cylindrical spherical and other shapes and based on this equation
03:36the cd by cd by cd by reynolds number curve can be derived and this will be used for the
03:43uh finding out
03:45the drag coefficient for the various uh variation in reynolds number or the for the various uh number uh
03:55based on the reynolds number you can find out the uh cd
04:00in us unit or uh for the spherical particle this reynolds number is re comes as this and uh your
04:11cd into
04:11reynolds uh nre will be this so you will find out you have to just see the properly the uh
04:18unit of unit
04:19conversion this is for the us units and this is the mks or si units based on the equation of
04:26cd into
04:27reynolds square of reynolds number this equation or this graph has been plotted and this when uh we
04:35get the value of drag coefficient or cd from this graph based on the value of reynolds number
04:42and when you put this value in your terminal velocity equation for the cd then this
04:50graph will be separated into three sections which will be the three settling laws can be obtained
04:56for the based on the reynolds numbers the first settling law is the stroke's law so stroke's law
05:08follows when your reynolds number or nre will be less than two and based on this graph if you plot
05:17us
05:18take a slope the for the less than two nrv the slope comes out to be cd is equal to
05:2324 by nre
05:26so when nre is less than two that time the reynolds number and the terminal velocity equation
05:37you can find out based on the nre and the graph you know that the cd value is cd is
05:42equal to 24 by nre
05:44if you put this cd value into this equation of the terminal velocity then you will get the
05:50ut equal to rho p minus rho by g into dp square by 18 mu this will be the this
05:57is called as the stroke's law
06:00for the settling and if you use the us unit and for the spherical particle this terminal velocity
06:08for the stroke's law comes is 1488 g into dp square by rho 1 minus rho g upon u 18
06:15mu so this is
06:16basically based on the uh us equation us units and to find out this dp or you can say particle
06:27diameter
06:28gps gps has given some guideline or the formula which is dp is equal to kcr into
06:35mu square by g into rho g by rho 1 minus rho g to the power of 0.33 so
06:44if if for the nre less than 2
06:47if you put the kcr value as 0.25 then all these others are known to you so you can
06:55get the particle
06:56diameter as well so when this stroke's law is used this stroke's law is used when the vapor separation
07:03from continuous fluid flow is required or dispersed liquid separation from the continuous liquid phase
07:10so means vapor separation from continuous liquid phase means if you have a lot of liquid and
07:16the bubbles or vapors has to be separate then this stroke's law will be used
07:20or dispersed separation from the continuous liquid phase so if if one of the phase has to be go
07:25from the continuous liquid phase liquid continuous liquid phase we understood in earlier session as
07:30well drag coefficient versus reynolds number this graph when you when when we talk about the reynolds
07:38number between 2 to 500 then it comes as a intermediate law between the 2 to 500
07:50and if you take a slope of this line and if you take a slope of this line you will
07:53find that cd is
07:54equal to 18.5 by nre to the power 0.6 so this is the slope typical slopes comes on
08:02this section
08:03for nre or you can say reynolds number 2 to 500 so when nre is between the 2 to 500
08:11then for
08:12uh terminal velocity formula and from the the graph we can get the value of cd which is 18.5
08:21by nre to
08:22the power 0.6 when you replace this with drag coefficient value in the terminal velocity equation
08:27you will get the equation terminal velocity is equal to 0.15 g to the power 0.7 dp by
08:341.14
08:36uh it within the bracket rho p minus rho 1 to the power 0.71 uh to the rho to
08:42the power 0.9 divided by
08:43rho to the power 0.9 to the mu to the power 0.43 which is the intermediate law equation
08:49which is used for
08:50the uh finding out the terminal velocity when your reynolds number is between 2 to 500 this is this this
08:58expression is for the us unit for the feet and the um feet per second or for this uh that
09:06equation that
09:07units similarly to find out the particle diameter for this intermediate law uh the kcr can be replaced
09:15with 0.334 then you can get the particle diameter and this intermediate law is used generally when the
09:23liquid separation from the continuous vapor phase so it can be used mostly in the vertical section you
09:30can say where the vapor flow rate is more and the liquid has to be separated or separate out
09:36what law is the newton law newton law is always when uh the reynolds number is between the 500 to
09:45or more than 500 the this graph will be segregated into third part which is the last part which is
09:52the
09:52newton newton newton's law and if you see this the slope on this section you will find the cd is
10:00equal
10:00to 0.44 so for the uh reynolds number value of between find uh 500 above 500 and up to
10:10the 2 lakhs
10:11newton's law will be applicable and this is terminal velocity equation which from the first way we
10:18understood and from the graph we uh we find it out that cd is equal to 0.44 if you
10:24put this cd value into
10:26this terminal velocity equation then you will get ut equal to 0.175 gp by dp uh to the uh
10:35within the
10:36bracket rho p by rho divided by rho to the power 0.5 this is the basically the newton's law
10:43equation for
10:43uh newton's uh settling law equation which will be used for the finding out the terminal velocity
10:51this is in the us unit and to find out the particle size or dp of the particle when we
11:00replace the kcr value
11:02by 18.13 then you will get the dp for the for this particular conditions of rho mu and your
11:10inlet conditions
11:12so basically generally this newton's law is not used for the separators because if you see the
11:19particle size and the uh cd value it comes like a bigger particle size so which will not generally
11:27will be the use in the refinery separators it will be for some uh bulk chemical chemical plants it
11:35this will be used for the settlements where the particle size or crystallization or something can happen
11:40but not generally in the hydrocarbon if you see this droplet size uh distribution which is taken from the
11:50gpsa guideline again so in this if you see uh you have to see make little bit efforts to read
11:58this this
11:58is the stroke's law this is intermediate law and this is the newton's law if you draw a line like
12:04this
12:05so you will find the stroke's law will be applicable when the micron size or the particle size is between
12:10around 2 to less than 100 and intermediate law is between 100 to 1000 the micron i am talking about
12:20the micron particle size and if you see the newton's law newton's law is more than thousand microns or
12:27uh and then it increases like one centimeter or 10 000 micron or one like micron which is around
12:3410 centimeter here here is a one inch so particle size is bigger as i said in earlier section the
12:41earliest
12:44slide that this size is very big so it will not be used for the uh general use for the
12:49uh separators
12:50the hydrocarbon suppressor and if you see the particle size starts around the 0.001 micron which is on the
12:56molecular side so if you see this uh distribution you can find it out that which part like which this
13:04is foreground and this based on the sizes this is the uh law and even this to find out the
13:12droplet size
13:14or dp size the kcr factor is also mentioned in this here which is put it here to read it
13:21properly
13:23if you do not know the droplet size and you are not able to have the even enough information to
13:27calculate it then uh standardly there is liquid particle size used for the gravity separations are
13:37basically the liquid uh for a liquid from vapor separation then you can take as a 250 micron
13:44as a particle size if the vapor from liquid separation then it is 175 micron
13:51and if it is a dispersed liquid from the continuous liquid then it is a 125 micron you can take
13:57it as a
13:59general basic guidelines in some other uh guidelines it is based on the specific gravity of the material
14:06so if you have a light phase which is having the specific gravity less than 0.850 or which means
14:13which
14:13is basically hydrocarbon then you can go for the 121 micron size and if you have a
14:21light phase whose density or specific gravity is more than 0.850 then you can go for the 89 micron
14:28size
14:30this is a general guidelines given in some books also the settling velocity should not exceed the
14:390.005 meter per second for all these two cases any either cases or either phase
14:48this light phase this light phase specific gravity means the other phase is a heavy phase like such as
14:54water so here we are almost completed our theoretical part of separators and we are moving for the
15:03separator sizing so before the separator start the separator sizing we should uh understand what it is
15:11depending on what separator sizing is depending on basically a few factors like the effect on the
15:19downstream equipment effluent and overall economics so if what you are giving to the downstream equipment
15:26like pump or compressor is there then for filling the fulfilling the requirement that is your basis for
15:32the separator design considering difficulty and cost of separating smaller particles if you have a very
15:40fine particles and if you go if you want to how much percentage of removal you required based on that
15:46your size will be increased so that economics or cost versus removing the smaller particles is it has to be
15:53considered similarly the plot space required because if you go for the very long length of the horizontal
16:02separators then you need a more space of you go vertical then you need a vertical some empty space should
16:09be
16:10available separators should not be too tall or if you are shifting it from fabrication yard to your plant
16:20then it will be again a problem because it will incur you the more cost in transportation enough
16:28interfaces available between three phases or not means the density differences are there are not properly
16:33that also has to be checked density difference or they find they will they should form a uh emulsion
16:40emulsions and other things now the how much surface area is available for decreasing or separating the liquid
16:52surge volumes in liquid to be handled without large change in liquid levels
16:57because there will be a fluctuation into the uh in your vessel so you it should be handle your uh
17:04that
17:05surge or liquid such coming the liquid because of the some sudden high flow rate larger liquid retention
17:13retention is required or not because somebody will not ask you design your um separator with the level of
17:20maybe very high level so when you put a high level then uh vapor separation in the space will be
17:26less
17:26then you will increase the diameter so it will finally increase your cost and the dimensions everything so
17:33these few factors also you should understand before or you should take care before finalizing your separator
17:40sizing so before start the design of the vessel we should know vessel configuration whether we have to go for
17:47the
17:47vertical uh separator or the horizontal separator so how we will find it out if the flow rate is uh
17:55if the uh
17:56your fluid contain or your stream contains high vapor flow rate and the low liquid rate and it is only
18:02one
18:02liquid then you can go for the vertical type of configuration if a low vapor rate high liquid rate and
18:10one phase
18:11only then you can go for horizontal preferably but still you can go for the vertical as well the second
18:16option
18:18based on the space requirement and this parameter you can decide that whether you want to go for vertical
18:25no vapor phase and two liquid phases then you should go for horizontal if your vapor phase is present and
18:30two
18:31liquid phases are present and low heavy liquid phase rates are there heavy phase means the heaviest are
18:38lighter then you should go for the not lighter lower then you should go for the horizontal type of separator
18:45vapor phase is present also two liquid phases and low light liquid phase rate so that the liquid phase
18:54rate of the like water is more and there is a gases and there is a little amount of oil
19:01that is a low liquid
19:03phase rate then you should go for horizontal with a baffled outlet so it will be overflowed and
19:09uh the lower or lighter low light liquid will be overflow from the baffle so vapor if your vapor phase
19:18is
19:18present two liquid phases and the liquid phase is equal rates then you can go you can go for the
19:24horizontal
19:24with the two baffles outlet if the low light phase rate and the vapor phase is present then you can
19:32go
19:32horizontal with the horizontal with a calming vapor as well and the small droplet size and load density
19:38difference then you can go with the horizontal and the coalescer coalescer will give you more surface
19:44to coalesce and the get the settle because of the small droplet size or the load density differences
19:51so this way we have uh we can we can uh generalized or you can understood that the vessel configuration
19:58which vertical horizontal to be done and in next session we'll uh start the design of uh vertical
20:04and the horizontal uh separator so first we'll start with the horizontal separator design
20:10all the steps and the guidelines for the design of horizontal separation
20:16thank you very much if you have any question you can write to us or comment on the comment box
20:23or you can
20:23also reach on to us at conceptengineering2025gmail.com link of the sessions uh all the sessions we
20:32completed till date is module 1 pressure relief valve module 2 heat exchangers and model 3 separators
20:37are going on so links for all this uh available in description box you can go through it thank you
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