9 replies to this topic

[Ahmed Al-Amry]

Hi,

I am designing an Acrylonitrile plant facility with help of my team. I am responsible for designing the front-end which include feedstock storage and handling and production of the desired products and the rest of the team are responsible for purification, waste treatment and further utilities. I have several issues and would like you to give me your technical advice, which will be highly appreciated.


A- One of the technical obstacles I am encountring is storing propylene in liquid form. The idea is to pump liquified propylene from tanker to an onsite liquified propylene storage tanks and preheating or vaporize the propylene before injecting the propylene to the reactor. My questions are;

1- What are the usual pumping conditions for liquified Propylene through pipelines and what are the storage conditions for liquified propylene in a given storage tank.
2- How can I decide the amount of refrigerant required to maintain the storage conditions of liquified propylene?
3- The proposed feed conditions of propylene to the reactor is 25oC and 2 atm. Is it advantageous to use a vaporizor over a preheater? If yes, how do vaporizors work?

B- I have a huge amount of Air to be supplied to the Ammoxidation reactor, which is approximately 181,000 m3/hr. I am heating the air from ambient conditions to approximately 200 oC by steam generated from the reactor at 600 psig. My questions are;

1- Is it possible to heat the air using steam in an air heater?
2- What is the best approach for such a process?
3- a) Before entrainment of the air to the heater, the air is filtered. To pump the air into the heater, shall I use a blower or an air compressor?
I was thinking to place the blowers before the filters and blow air into the Air heater and than the hot air is further pumped to a header tank were the required hot air is withdrawn using air compressors? Let me know if this is applicable.

Sorry for the long questions, but your feedback will impact my final year design. I am in the PFD stage and trying to select the proper equipments to do the job.

Thank you
AHmed Al-Amry

[Art Montemayor]

Ahmed:

You’ve given us a lot of details and information on what you are assigned to do. However, you have missed out on giving us the real important basic data.

The critical properties of Propylene are 667 psia and 197 ^oF. We can all find this out if we refer to the GPSA Engineering Databook – as I would hope you have already. If you have, then you will also see the Mollier Diagram for Propylene in the same Databook, in the Thermodynamics Section. There you can appreciate that Propylene can exist in the liquid state in a variety of saturated temperatures and pressures – all of them under the Critical Point.

It is your job to determine (or set) the conditions under which you will receive the ocean vessel cargo. Therefore, what are the conditions that you will receive the Propylene? Next, what are the conditions under which you will locally store the Propylene feed? It is your job to define these conditions as the owner/designer of the facilities. There is nobody else. So please decide and tell us. This is probably the most important first step to take in your project. If you can’t or don’t know how to resolve this problem, then tell us and ask for help in resolving it. There are answers, but you can’t just let such an important issue just “hang there” for someone to take care of it.

Next, tell us the important process flow rates and the Storage capacities you are basing yourself on. I suspect that if you don’t know the storage conditions, you also don’t know the storage capacities of the raw materials – and probably also of the finished product. Is that true? If so, then also admit it and ask for help. These are very important issues that are the underlying basis for your design and will have a large impact on the final design – both in monies and in engineering effort.

If you have generated a PFD, then furnish it to us so that we can understand what you are describing. That is the real, accurate, and efficient way that engineers communicate. You say you are in the PFD stage at present. I don’t know how to interpret this except that I take it to mean that you haven’t finished the PFD yet. If you haven’t finished the PFD, then you haven’t finished the mass and heat balance. If you haven’t finished any of these basic and necessary data, then we haven’t much to base ourselves on. When you have finished the PFD – complete with a CHECKED heat and mass balance – furnish us a copy in Excel spreadsheet format and then we can start to really help you out. In the meantime, I believe you have a priority problem: establish the handling and storage conditions of your #1 feedstock item, Propylene. We can help you on this, but you have to give us BASIC DATA like: what are the reception conditions of the shipped Propylene and how much are the quantities and flow rates?

Await your reply with more meaningful information.

[Ahmed Al-Amry]

Hi Art,

I highly appreciate your prompt response, and I am lucky as a junior chemical engineer to have an access to highly experienced Chemical Engineers like yourself.

I apologize for the missing information and the way I addressed my questions. Truely, I don't have the conditions that, I am going to receive the liquid propylene feedstock from the ocean cargo.

The liquid propylene will be stored on a ground-level and with weather conditions of max 36 oC and min -30 oC. The amount of propylene required is 36,170 kg/hr. The storage capacity will be based on 30 days. In mind that I will be storing liquid ammonia in the same way. The amount of ammonia required is 21,960 kg/hr wit similar storage capacity as propylene.

As I mentioned earlier, I am in a PFD stage and I have carried out a preliminary mass and energy balance on a block diagram. I have attached my block diagram and excel file for the mass and energy balance.

I am working out my PFD and trying to re-establish my mass and energy balances on the PFD at this moment (That's the way the assignments are laid out from the Instructor). Starting to get all the missing info such as the storage conditions, to see how much utilities and what transport equipment I need.

My request about the Air Heater, I am requiring an air feed of 215,500 kg/hr to the process and need to heat it up from ambient temperature to 200 oC and than pump it to the reactor at 2 atm. I am thinking of using steam generated by the reactor to vaporize or preheat the propylene and ammonia.

When these questions are answered, I will be able to carry on further with my PFD. I have an incomplete sketch of my PFD on papers at this moment. I will be completing the PFD within the coming days.

Please, let me know if you need further information. Once again Thank you very much!

Ahmed Al-Amry

Attached Files

•  Visio_Ammoxidation_Unit_Block_Diagram_Rev.06.pdf   32.86KB   104 downloads

[Art Montemayor]

Ahmed:

Thank you for the rapid transfer of information. This helps out tremendously.

Now we need the corresponding PFD with the related stream numbers so that we can start to relate the quantities being moved about and handled.

Do no fail to heed what I have dropped to you as a hint: The method and conditions of Propylene shipping must be first defined in order to fix the reception terminal design conditions. In real life, you normally do not have a choice in this matter because you are driven to employ the existing, state-of-the-art shipping vessels available to you. Therefore, you normally find yourself accepting what the market sends you and designing around those conditions.

You could conceivably receive ambient temperature, compressed liquid Propylene; however, I seriously doubt that is what is going to happen. I see by your mass balance that you are already planning to storing at semi-cryogenic temperatures, so I gather that your storage pressure is going to be low - possibly around 10 - 15 psig (& refrigerated). Am I correct?

If so, then I assume the matter of shipping and storage conditions has been settled. I will await your PFD - which should not take very long to draw up in Excel.

[Ahmed Al-Amry]

Hi Art,

Thanks for the prompt reply. I dont have any information on the receiving condition in mind as you have already noticed this. Where will I be able to get some information about the recieving conditions. I was thinking if I can't find enough information about the receiving conditions. I am assuming that propylene will be received at ambient condition 25oC and 1150 kPa.


In my preliminary calculations, i proposed to store the propylene under cryogenic conditions. However, I am looking at storing the propylene liquid in a pressure dome-like vessel and hence this will reduce the need of refrigeration.

What is the normal pressure at which the liquid propylene is stored at in a pressure vessel. I will send you the PFD as soon as I am finished with it.

Thank you

[Art Montemayor]

Ahmed:

I am posting this response to your vaporizer query here in your basic, original thread because there is no need to create multiple threads for one project. All your questions now and in the future will be related to the same, common project. Multiple threads create chaos and confusion. You don’t need that.

I have directly helped, consulted, and guided over 5 Chem Engineering graduates in achieving successful organization and completion of their end-of-year design project. I have seen it all before and I’m seeing it again. Allow me to try to help you by giving you some experienced advice that I’m repeating and hoping that you will give importance to.

I’m pretty sure that I am correct in suspecting that you are at an impasse in your project and are presently suffering because of a lack of direction or positive belief in what you should be doing next and directly afterwards. This is pretty common in ChE design project assignments and you should not let this bother you to the point that you consume a lot of time needlessly and later are forced to “panic design” because of a lack of project time. Having done so many projects in the past and participating with students in their assignments has given me the ability to know beforehand just what is happening here. You need to sit back and re-organize your project in a reasoned, logical, and practical manner in order to meet your schedule on time with a successful and professional-grade design product. Doing so will almost always ensure you and your colleagues of a top academic grade and a first-class learning experience (plus having fun doing it). If you are attending the University of New Brunswick Chemical Engineering Department and taking CHE 4225 - Plant Design, then you have a pretty good start in life and should have a solid understanding of what I can add or contribute.

Start with basics first, and get those items set in concrete to build your design on that solid basis. As I understand your description, you are assigned to use imported foreign liquefied propylene shipped in bulk quantities and use this as your project’s main raw material.

First, establish what I originally told you: identify the ocean transport conditions of the imported Liquefied Propylene. This is critical and must be done quickly and accurately – based on the best, existing, factual information that you can obtain. For starters, try calling up LPG or Liquefied Propylene shippers or shipping companies. I know that these sources will probably not bother with ChE students and give you the go-around or hang up. But at least try! If that fails, go to the Internet, where you have a wonderful tool to gather useful information on which to base yourself and your project. For example, to to Google’s search engine and type in LPG Shipping or Propylene Shipping. You will come up with sites such as:

http://www.ship-find.com/descriptions.htm

where you find the availability of a liquid propylene ship – complete with photos and where to get further information. Or, try

http://www.gasandoil.com/goc/features/fex83823.htm

Do you see where I’m going and what I’m setting up? Once you find the existing normal shipping capacities of these carriers and their design, you have identified how you would normally be receiving the liquid propylene and start to plan the reception terminal storage facilities based on that information. You can also design the wharfs, docking and off-loading capabilities as well – together with the quantity of ships required and their delivery schedule. And you do this all based on published and referenced information that can quickly and efficiently be looked up and confirmed by any supervising professor. This is the correct, engineering way to start and implement a project and your professor will see and credit this immediately. You do not just say you saw that some patent claims that the propylene storage pressure should be a certain value.

If you are receiving saturated liquid propylene at 35 psia and -16 ^oF, that is great; that means you logically would try to preserve the existing refrigeration in the received cargo and store it as such – or close to those conditions. But if you are receiving saturated propylene at 150 psia and +69 ^oF, then you must decide if you want to justify building refrigerated, low pressure storage tanks to refrigerate and store the product refrigerated to keep the saturated pressure low and the pressure in the tanks low as well. You should have process and/or economic reasons driving this decision forward. If you don’t have the driver, you simply store the propylene in the condition received. An economic driver would be an optimum design that ensures better profit in the overall operations. This requires additional, heavy engineering calculations and optimization – something you want to avoid if possible.

It would be a great and advantageous thing if the storage pressure is higher than the process pressure in the reactor. This would allow much simpler methods for feeding the raw material into the reactor – just a control valve. However, you have to arrive at this through engineering logical and accurate decisions based on fact and hard evidence. That’s why the shipping conditions are so vital to identify. And you have to do that; no one else can do it for you.

When you have the storage size and conditions identified, you merely apply the enthalpy difference between the liquid propylene entering the vaporizer and the propylene vapor exiting the same. You obtain the enthalpy of pure propylene by using a recognized and respected database:

http://webbook.nist.gov/chemistry/fluid/

See how easy it can get? It all depends on you “setting up your ducks all in a row”. You don’t need any "fancy" HySys simulation program to do this simple exercise in logic and planning. It’s quick, accurate, and perfectly sound engineering design. What you do need later, is a mechanical / process design of the needed vaporizer. When you set the storage conditions I can easily show you the shape and method of designing the vaporizer such that it can easily be controlled and operated. I will furnish you with a typical P&ID sketch.

[Ahmed Al-Amry]

Hi Art,

You are right, I am lacking guidance and direction at this time. There are things happening which are out of my hand and no much help or guidance is provided. I dont want to get in details about that. I want to say that I highly appreciate your support.

At this stage I am considered a little ahead of many of my colleagues. When I tried to put the block diagram into a PFD, there were I found things that I didnt take into consideration, that is the storage and handling of my Feedstock (Propylene and Ammonia). I have succefully completed my mass balance and the energy balance at the end of my unit is accomplished, but the I am finding problems with vaporizing or preheating the feedstock. I spent much of my time researching about my ammoxidation reactor and the quenching system and pretty much didn't spend enough time for the feedstock, I just looked at prices from the supplier in the region and the way of transporting the feedstock.

I had a hard time finding the receiving conditions of the propylene. However, the pressure at which propylene is stored as a liquid in tankers is approximately 250 psia. I decided to use this number as a receiving condition. The storage condition in the bullets would be 200 psia. For Ammonia, I found information stating the conditions at which the tankers store ammonia and conditions at which liquid ammonia stored in tanks.

I have attached my sketch for the PFD. Please, let me know what you think about the flow of the storage and handling of the feedstock.

Thank you

Amry

Attached Files

•  Ammoxidation_Unit.pdf   41.24KB   82 downloads

[Art Montemayor]

Ahmed:

Your PFD needs a lot of work and thought. Please read my comments included in your workbook in the added worksheets.

I'm in a busy situation at work and can't spare the time to dedicate to furnish you with a proper vaporizer design until next week - but you need to establish what I have already expounded: set your reception terminal thermal and pressure conditions in accordance with proper engineering design as to max. pressures expected and the minimum required equipment.

Regards.
 Ahmed__s_Propylene_Vaporizers.zip   383.49KB   158 downloads

[Ahmed Al-Amry]

Hi Art,

I have set my receiving and handling condition of my feedstocks. I wasn't able to get numbers from vendors. However, my instructor agreed upon my assumption for now. The feedstocks propylene will be received at 250 psia and stored at 250 psia.

I have attached the PFD for propylene storage and handling. In addition, I have simulated the vaporizor using HYSYS. I have attached PDF of both with this reply.

Note: Stream indicated as AS means steam supply and AC is condensate. A stands for my section of the plant.

Will be waiting for your valuable comments

Amry

Attached Files

•  HYSYS_Propylene_Handling.pdf   12.57KB   94 downloads
•  Propylene_Feedstock.pdf   32.7KB   91 downloads

[Art Montemayor]

Ahmed:

I consider your sketches and presentation as bad quality – as I have inferred in my previous post, which I note you haven’t followed. This comment is not meant as a criticism of you or to make you feel bad. As engineers we are trained to be honest and straight-forward in our consulting and advice in order to pass on what can be valuable and profitable information. Please refer to my attached Excel worksheet – which I did in one hour of work. I am not bragging when I tell you that your HySys (or Visio) PFD sucks compared to mine. Allow me to tell you what you are doing wrong and can lead to a bad grade:

1. As I stated, label your major equipment in an orderly and simple manner. But most of all, identify the equipment by name and tag. Also state the design characteristics of the equipment such as MAWP, working temperature, pressure, etc.
2. Identify what you are labeling as flows; it is very difficult to read your mind when you are many miles away. You are creating a product for a client or a reader. Make it worth his/her time to try to read and understand all the work you did. Otherwise, you are wasting your time and everyone else’s.
3. Use flow arrows to assist the reader to understand the logic and direction of the flow(s). You don’t use arrow heads in your flow lines. That is probably because you did this in a hurry and just took the HySys print out (which is, like I state a recognized BAD PFD). HySys is a simulation company and they don’t really give a lot of importance on communicating through drawings and visuals. They really couldn’t care about the PFD presentation. That’s why their drawings SUCK. And when you adopt their drawing as yours, your work correspondingly sucks. Use your own devices and communicating skills (through Excel) or another program to create a meaningful PFD that communicates all the information needed.
4. You still seem to miss out on phase equilibria and thermodynamic states. You don’t need any HySys program to design this Propylene Storage and vaporization Unit. (I think I already stated this before). Study the simple method of receiving, storing and vaporizing the Liquid Propylene as I’ve shown it. Note that I don’t use even a single pump to transfer liquid Propylene. To size the vaporizer, all I need is enthalpy values which I can easily obtain from the NIST database (which I already gave you a hint on). I consider the HySys simulation of this part of the plant as useless and a waste of money and time that could be used on much better things
5. Note the flexibility and simplicity of the storage process. Your instructor really did you a big favor by granting you license to design on a 250 psig storage basis. This, as I stated previously, makes it much, much easier to design and build. You were very smart in going to him/her and asking for this design basis. It has saved you a lot of grief and work.
6. You still need to calculate the size and number of Propylene “bullet tanks” that you need for your month’s net storage. Do not count on the Day Tank as useful storage. It is a working tank and acts only to cushion any flow surges or process upsets downstream. Size the Day Tank as you see fit and practical. A ten-hour working capacity (on a manual basis) would be one logical basis. This gives you one 8-hour shift with an over-lap to make sure there are no upsets. However, you can also automate the Day Tank make-up fill from storage very easily. It all depends on your scope of work and how you want to operate.

I hope these frank comments and the attached sketches are of help to you.
 Ahmed__s_Propylene_Storage___Vaporization.xls   1013KB   238 downloads