HDPE and other plastics used only as Storage tanks? 1

[ChemEng4Life]

Hi, I am wondering about the use of HDPE and other plastics (namely PVC and CPVC) in the chemical plant environment. Are these only used mainly for storage purposes, or do we find reactors made out of these materials?

I understand that it may not be the best idea to construct a reactor out of plastic material, esp. if the process was designed to be continuous and operated from a computerized environment because, at least to my knowledge, there'd be no way to send an electric current to the reactor and control the apparatus properly being that plastic does not conduct electricity.

If I'm wrong, please correct me.

Thanks

 

[Chemitofreak]

Selection of Material of Construction (MOC) for equipment is dependent on various criteria. Some of them are as follows:
1. Design life of equipment/plant
2. Design temperature and design pressure of the equipment
3. Corrosion resistance and corrosion allowance to be provided
4. Cost of MOC
5. Ease of fabrication
6. Compatibility between MOC of equipment and the material stored in the equipment
7. Material properties (ductility, malleability, crushing strength, load bearing capacity etc.)
8. Stability to ambient conditions etc.

HDPE, PVC and other plastics are generally used for storing water and acids in chemical industries. These tanks are generally atmospheric and their design temperature is around 70-80 deg C.

Reactors are generally/mostly operated in conditions (mainly temperature and pressure) which are not suitable for the MOCs in discussion, hence it is not used. Also, the weight bearing capacity of plastics is not as good the metals, and since the production capacity of a manufacturing plant is in tons plastics is not a workable option as a MOC.

If the design conditions are in favour of plastics, we can very well manufacture a PVC/HDPE reactor.

I did not understand you query regarding sending current to the reactor to control the apparatus. Please elaborate.

Revert in case of any query.

Dinesh S SHELATKAR.
Process Engineer

 

[EmmanuelTop]

I have never heard of plastic reactors, at least not in the Oil and Gas industry. "Plastic reactor" sounds a bit like an oxymoron to me.

On the other hand, Plastic is commonly used in O&G industry for piping and pipelines transferring water (produced water, injection water, fully treated water, etc.). In combination with steel (known as "composite"), plastic is widely used for transferring almost any kind of fluid. See

http://www.flexsteelpipe.com/

http://plasticpipe.org/

http://www.ppfahome.org/

Dejan IVANOVIC
Process Engineer, MSChE

 

[moltenmetal]

Plastics are widely used in strongly corrosive aqueous environments. You will find plastic tanks, both as storage tanks and as reactors used in many mining and extractive metallurgy applications which can take place at or near atmospheric pressure. Beyond a certain size though, FRP construction is preferred for structural reasons.

You will also find thermoplastic-lined steel pipe and vessels used in many corrosive applications, where corrosion immunity against a particular kind of corrosion is preferred to mere corrosion resistance. Such materials can be successfully applied in services where any metallic material would fail rapidly.

As to the absence of conductivity: as long as you're not handling flammable materials, it doesn't matter at all. In services where plastic tanks are used, the materials being handled are usually very conductive, though there are a few exceptions. It's a property you design around. You can compound polymers with sufficient conductive additives to make them capable of dissipating static electrical charge if that is required.

 

[ChemEng4Life]

Thanks everyone for the swift replies.

@Chemitofreak It would seem that the design conditions are indeed in favor of a plastic medium to withstand the high temp. and very caustic environment. Just thru Google alone, I do not see any reactors made out of HDPE or PVC, but perhaps you are aware of a firm that manufactures custom reactors.

Also, I want to clarify what I mean by "reactor". Currently, the material that is being synthesized can be made using large plastic bottles in an oven. However to scale this up for industrial purposes, it seems that an actual reactor(meaning something whose parameters (pressure, temperature, etc.) can be controlled electronically would be a better suit as opposed to a series of large tanks.

This is what I meant by conducting electricity to the plastic. I want to be able to control the operating conditions electronically (not actually send a current thru the reactants) but am not sure if this can be done in a plastic medium.

 

[moltenmetal]

You can mount the same pressure and temperature measurement devices on a plastic tank that you can on a metallic one. If they also need all their wetted components to be plastic for corrosion resistance reasons, this may dictate which technology you use for particular measurement instruments etc., but will likely not prevent you from making the measurements required by some means or another.

 

[bimr]

Most plastics are not capable of handling high temperatures because the plastics will melt.

It is more common to use a lined steel reactor than one fabricated from plastic.

 

[Chemitofreak]

@ ChemEng4Life:

Please note that the temperature increases (in the range of 80 to 120 deg C) will lower the properties like load bearing capacity of polymeric material in exponential terms. Also the polymeric material will melt.

I have worked in O&G (Oil and Gas) and Petrochemical projects where the reactors are operated in conditions (mainly temperature, pressure, tonnage etc.) which are not suitable for the MOCs in discussion, hence it is not used.

I would like to re-iterate that if the design conditions are in favour of plastics, we can very well manufacture a PVC/HDPE reactor, although I have not seen one in my career.

Refer post from moltenmetal in which he states that 'You will find plastic tanks, both as storage tanks and as reactors used in many mining and extractive metallurgy applications which can take place at or near atmospheric pressure'.

As far as instrumentation is concerned, it is not at all a problem, it can be provided to any system whether it's MOC (Material of Construction) is PVC or SS.

Revert in case of any query.




Dinesh S SHELATKAR.
Process Engineer

 

[bimr]

Discussion of plastics:

Polyolefines
Low-density polyethylene (polythene) is a relatively cheap, tough, flexible plastic. It has a low softening point and is not suitable for use above about 60° C. The higher density polymer (950 kg/m3 ) is stiffer, and can be used at higher temperatures. Polypropylene is a stronger material than the polyethylenes and can be used at temperatures up to 120° C. The chemical resistance of the polyolefines is similar to that of PVC.


Poly-vinyl chloride (PVC)
PVC is probably the most commonly used thermoplastic material in the chemical industry. Of the available grades, rigid (unplasticised) PVC is the most widely used. It is resistant to most inorganic acids, except strong sulphuric and nitric, and inorganic salt solutions. It is unsuitable, due to swelling, for use with most organic solvents. The maximum operating temperature for PVC is low, 60° C. The use of PVC as a material of construction in chemical engineering is discussed in a series of articles by Mottram and Lever (1957).

 

[StoneCold]

One reason that you never see plastic reactors would be poor heat transfer. Most reactions require some heat transfer, either in or out of the reactor to make the desired product. The combination of thick plastic for reactor integrity and the need for heat transfer don't go well together. Even the use of lined steels, which use much thinner plastic have relatively poor heat transfer properties and cause most users to use exotic metals.
My two cents.

Regards
StoneCold