10 replies to this topic

[Ali-Jabbaran]

Dear Friends

 

I’m trying to liquefy an ethane stream by using available liquid ethylene as attached scheme.

Ethane liquid will be stored in an atmospheric tank and superheated ethylene will be delivered to battery limit at 20 degC and 24 barg.

I’ve considered a plate fin heat exchanger for this process.

 

My first question is, Could the shown scheme (using a plate fin heat exchanger on 1 step) practically leads to superheating the ethylene stream, or I should first produce a saturated ethylene vapor in one exchanger and then superheat it in another exchanger (by using a side stream of ethane inlet).

 

The second question is about the control scheme. Is the shown control scheme practical, and will it works for liquefying the ethane stream and delivering ethylene vapor at required condition?

 

Many thanks for your valuable replies

Attached Files

•  Ethane Condenser Scheme.jpg   51.38KB   6 downloads

[Bobby Strain]

You need to start at the beginning with proper heat balance. The duties for both must match.

 

Bobby

[Ali-Jabbaran]

Thanks
Reported values are based on heat balance using a simulation tool.
The question is,
Is it possible to reach an exact superheated values in one step, in a plate heat exchanger, while we have boiling liquid in it..

Edited by Ali-Jabbaran, 25 January 2017 - 12:32 PM.

[Pilesar]

I think you are asking too much from a single exchanger. If you are trying to meet an exact superheat temperature for the ethylene export you will need to add a temperature controller to the ethylene export. What variable will you manipulate for this controller? To cool the ethylene export, you can add an ethylene bypass to your process/process exchanger. To heat the ethylene export, you will likely need a separate utility exchanger to the ethylene export stream so that you can manipulate the hot utility flow.

[Bobby Strain]

You have too many controls! And, what is the LC? Pilesar has some good points, but you will have major problems without a heater on the ethylene from the interchanger. Design for a reasonable ethylene temperature, 0 C or less. Then finish heating the ethylene in another exchanger with a readily available source. A simple shell and tube exchanger would be sufficient, and you could probably use cooling water as a heating medium. Once you set this configuration, then have a look at materials of construction and controls. You need to consider various instrument failures, because you are dealing with some cold stuff at fairly high pressure. And the cold stuff can potentially enter carbon steel piping and equipment. So, start at the beginning with some more manageable conditions. I would avoid any cold ethylene bypass to the exchanger outlet. Make sure that 20 C is required, and not minimum temperature required. You also need to include the storage tank controls in your analysis. And whatever is upstream for the ethane. Unlimited ethylene is probably an exaggeration. Look at the ethylene source, too. The exchanger is more accurately referred to as a brazed aluminum core exchanger. Let us know when you come up with a new scheme.

 

Bobby

[Art Montemayor]

Ali:

 

You have received 2 excellent responses from experienced and capable engineers such as Bobby Strain and Pilesar.  Their suggestions and comments are, in my opinion, what you should be following.

 

I don't understand how you could be assigned to, or responsible for, the type of application that you have described if you haven't had prior experience in this type of heat transfer.  I am presuming that this is a real-life application and not an academic exercise.  You have made some basic decisions that don't apply to this type of heat transfer, in my opinion.

 

Basic answers to your 2 questions are:

1. No, trying to superheat the ethylene refrigerant in one heat exchanger is the wrong way to do it.  It practically won't work.  Even if it did, it would be very difficult to control.
2. Comments to your control scheme can't be made in a specific manner because you haven't furnished a detailed P&ID sketch.  You fail to show the entire control circuit(s).  Note Bobby's remark on the LC (level control?).

I have done both design and application of many refrigeration and evaporation projects in my professional career.  I have condensed a wide variety of vapors and gases as well and that is why I can attest to the value of Bobby and Pilesar's comments.  Please refer to the attached copy of the Ethylene Mollier Diagram and note that I show the path you are proposing.  You will appreciate that you are expanding a supercooled liquid ethylene fluid, sensibly heating the product supercooled liquid, vaporizing the saturated liquid ethylene, and finally superheating the ethylene saturated vapor.  To do all these steps will involve at least 2 heat transfer exchangers:

• The first one will sensibly heat and vaporize the refrigerant ethylene;
• The second will superheat the produced saturated ethylene vapor.

The first heat exchanger cannot, in my opinion, be of the type you are describing.  You should take into consideration that you are vaporizing a liquid and should design a vapor disengagement space within the exchanger itself - or attached to it.  The saturated vapor should be superheated in a separate, connected heat exchanger.  Why you have already selected a brazed aluminum core exchanger to multi-task this type of heat exchange is something you should explain.  As Bobby has pointed out, the second step can be done using a conventional shell & tube.  However, use your common sense when using common water as the heating fluid: the relatively cold ethylene vapor will have the potential of freezing the water to ice if you don't take care and design against this happening.  Carefully follow the steps shown on the Mollier and you will see that this is a possibility.

 

The above reinforces what I have always maintained: simply generating a process simulation does not solve a process design problem.  You have to apply the most important ingredients, which are practical common sense and a thorough understanding of the various steps that are taking place - no matter how simple the process may outwardly appear.

 

 Ethylene Refrigerant Evaporation.xlsx   222.39KB   21 downloads

[Bobby Strain]

Art provides sound advice. You should discuss your concept of using one brazed aluminum heat exchanger (BAHX) to warm the ethylene to ~ 0 C with the manufacturer. But normally when vaporizing a refrigerant like ethylene, one would use a core in kettle where ethylene is vaporized in the kettle. If the manufacturer can't guarantee performance in the single exchanger application, then you would likely need to supply 4 separate heat exchangers.The coldest BAHX would heat ethylene to near bubble point with the ethane subcooling to storage temperature. Then ethylene would feed a core in kettle with ethylene boiling in the kettle and ethane condensing in the core. Then a BAHX would superheat the ethylene to ~ 0 C with the warm ethane. And finally, a shell and tube unit to superheat ethylene to the required temperature with some heating medium. The three cold exchangers can be shop fabricated in a coldbox. So, your best approach is to prepare a duty spec for the entire heat exchange train and let vendors do their best to meet your requirements.

 

Bobby

[Ali-Jabbaran]

Thank you very much
Art and Bobby

For your valuable comments
I will come up with new scheme based on your points.

[Ali-Jabbaran]

Thanks again for your kind replies

I prepared a new sketch based on your advice and will surely manage “Cold Box” section with vendors.
I think, it might be better to add Ethylene super heater (shell and tube exchanger) in “Cold Box” vendor scope of work.
I also think that a side stream of ethane inlet may be used to superheat the cold box outlet (ethylene stream).With ethane, I wouldn't have the problem of freezing. I should hydraulically check and update this scheme.
For now, just preparing the main control scheme in attached flow diagram, and then P&ID will be prepared in next stage.

Regards
Ali

Attached Files

•  Ethane Condensation - update scheme.jpg   59.22KB   4 downloads

Edited by Art Montemayor, 28 January 2017 - 10:51 AM.
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[Art Montemayor]

Your decision to use "a side stream of ethane inlet to superheat the cold box outlet (ethylene stream)" may not be the best or the most cost-effective design.  Remember that gas or vapor film coefficients in heat transfer are among the lowest and, as such, will require larger and bulkier heat exchangers to transfer the required heat from one gas stream to another.  Liquid coefficients are much more efficient.

[Ali-Jabbaran]

Thanks very much

I am very interested to know Bobby's viewpoints/comments about last scheme and control philosophy.