4 replies to this topic

[fernandoz]

Hi!, this is my first time using this incredible resource!.

Well, I am doing an internship at a Plant that processes Wastewater, this company pumps out water from grease traps.

They are buying a new storage tank (6,000 gallon capacity) and they want to install an Immersion Heater into the Tank to separate grease from water.

I would like to know how much Energy would I use and how Hot should the Heat Exchanger be in order to have a good separation of grease/water? How can I calculate that?
Is it better to do the separation trough a shell & tube heat exchanger? What would it be cheaper?

I hope I've been clear with my question.

Thanks,

[Zauberberg]

Separation cannot be done inside the shell & tube heat exchanger, that's obvious; how would you separate two liquid phases?

Separation works better at elevated temperatures, somewhere in the range 100-120C which can be done by inline heater, or by applying the heat directly to the separator vessel (or tank). Configuration largely depends on hydrocarbon phase properties, and it may require additional chemical injection in order to enhance the separation between phases.

[smalawi]

Hi,

the way I see it, you may opt for a feed / effluent exchanger(s) then inline heater (steam ??) before your feed enters the separation vessel, depending on the quantity of water/grease you have and fouling tendency.

you did not put any process data so I cant quantify any numbers, also you did not indicate if you have any similar process or this is an entirly new process you are designing.

I totally agree with Zauberberg, the properties of grease is critical to identify the temperature of the process and size of the vessel, moreover the vessel internals. Grease is not liquid and may have a very flat viscosity index & chemical additives that makes water separation efficiency a process specific parameter. Process simulations results could be way off. The GPSA handbook has some general guideline that might be of help to you.

I suggest you do some lab experiments first to determine your separation temperature and efficiency to establish your process conditions.

For the equipment design you can use simulation software (requires experience) or simply call the equipment vendors and state conditions / duty / flows, they would give you an order of magnitude estimate for any feasibility study to decide best option,

cheers,

sm

[fernandoz]

Zauberberg & Smalawi: Thanks for the tips! I'll take a look at the GPSA handbook.

Well, there is already a "system" for the separation, but they are not using any heat to separate it, just gravitational force and "time" to let the water go down to the bottom of the vessel. Then the water is pumped out of the vessel "free" of grease.

The thing is that this process obviously takes a lot of time, and they are continuously filling the vessel with water and grease.

So, I tried to think in the most logical way to optimize this process.

Fill a first tank with this grease/water mix (about 30/70%), and in this vessel insert an immersion heater (i was looking for one of the chromolax company). We only need to get and keep the grease/water mix at a temperature of 90 F so that you can have two visible phases, and then pump from the bottom the water into a second vessel and stop it until it reaches the grease phase (using transparent piping). We can do the same again in this second vessel, just to get the water as pure as possible (well, at least free of grease).
I think this process could be the easiest way to get it.

But only as a personal challenge I would like to know, how can I calculate the flow of water needed , if I would to design a heat exchanger (shell & tube inside this vessel). I know it's kind of difficult to add tubes in to this vessel, but I would like to have an idea of how it could be done.

I was thinking to heat water and make a close system (recirculation of water, heated by a boiler, furnace, whatever). Hot water inside tubes, and in the shell (the vessel) the mix of water/grease.
I found an aprox of .54 Kcal/kg C of CP for grease (I know it depends of the kind of grease... but at least it's a number). and density of grease aprox 772 kg/m³

The mix of water/grase usually comes at a range of temperature of 55 - 70 F (room temperature). And we have to reach to aprox 90F in order to have two visible phases. The dimensions of the vessel is: 10 ft diameter and 16ft tall (about 9,400 gallons).

So how much process water should I need to reach the 90F. What should be the inlet temperature of the process water? Should I better use oil as the heating element? If I want a simple and cheap process, what kind of oil should I use for it?

I know this depends on how many tubes I am going to put inside the vessel, but I would like to have an idea of how to start with it.

Thanks!!

[smalawi]

Hi,

looks like a batch process to me. so this is what I think

heating by oils in stagnant fluid may yield very low heat transfer coefficients. You maybe better off with a steam or electrical coil around the vessel tank or covering more area rather than a tube dense bundle. also the agitation if the heater is intense will disturb the separation process.

you could also have a pump / exchanger circulation system to gradually heat up your process, you can share it with your existing system if your design temperature / pressure allows you to do so.

Have a look at Kern book for heat transfer calculations. for settling time needed I still recommenced you experiment with water / grease separation lab trails. the GPSA has good reference as well for water separation

cheers,

sm