2 replies to this topic

[Diederik Zwart]

Does anybody have any experience in compressing acetylene. From Material Safety Datasheets I see that it is unstable at pressures above 1 barg, I assume that is when mixed with air/oxygen and that you can safely go to higher pressures for pure acetylene. Furthermore I saw that it is shock sensitive and that it forms explosive compounds with copper, brass, mercury and silver. Beyond this I haven't found a lot yet, so if anybody can help, or point me into the right direction I'd appreciate it.

What I'd like to know is what are the basic design guidelines when specifying a compressor suitable for acetylene (e.g. type of compressor, maximum compression ratio, material of construction) and what are the safety issues involved? I don't have any specifics yet with regard to required pressure, so for now I'm just trying to gather some basic information.

Thanks,
Diederik Zwart

[Art Montemayor]

Diederik:

I've compressed pure Acetylene under routine process plant conditions, filling Acetylene cylinders at a pressure of 370 psig (25 Bars). I did this for a number of years and in various plants located in the USA and in Latin America. I never had an incident or an employee hurt; yet there are strict and serious rules and precautions that I would pass on to you.

The first thing I would emphasize to you is to be careful of what you read about Acetylene and its properties. Some people have a serious problem in communicating important information regarding potentially hazardous items – especially MSDSs. This must be because so-called “Technical Writers” are used to put out this information instead of experienced engineers. For example, you are correct about Acetylene being stable up to 1 barg. This is in the pure state. However, in the same pure state and above 1 barg, it has a tendency to self-decompose with extreme detonation. It does not require any air, oxygen or other reactant to do this. It will do it on its own, and sometimes will do it when catalyzed by other compounds or metals, shocks, or turbulence. This is the really dangerous part of dealing with free Acetylene. You don’t want to go there. I believe you are very familiar with the unique and much stressed molecular structure of the molecule: the triple bond is enough to advise you to be very careful with this compound that is armed with a lethal fuse (the triple bond), ready to go off at any convenient time. Compression aggravates the problem by introducing additional internal energy and heat, together with turbulence and friction. You definitely cannot assume that you can safely go to pressures beyond 25 bargs without having a potential internal explosion. This information on Acetylene has been gathered through the years by empirical experience working with the gas in the field and laboratories. There are many bad and tragic incidents over this characteristic of Acetylene and engineers have adopted an attitude of staying away from higher pressures on the free gas. Notice that I am saying “free” gas, not dissolved gas. What you have in an Acetylene cylinder is a porous medium inside that is saturated with pure Acetone and it is the Acetone that holds the compressed Acetylene in solution. This has proved to be the only practical, safe way to transport and handle Acetylene gas for industrial use. No one in his right mind would dare to compressed free Acetylene into a standard, void gas cylinder under pressure or much less try to liquefy the gas by taking it to its Critical Point. The results would be disastrous.

Secondly, you are precisely correct in stating that it forms explosive compounds with copper, brass, mercury, and silver. These compounds are beautiful, crystalline salts that explode with tremendous force when impacted by thermal or mechanical means. This means that ultra-pure and scrutinized materials of construction that come in contact with the gas must be employed. Cast Iron and steel are the materials of choice in Acetylene service – the thicker, the better.

I have employed two reciprocating type of compressors in the past with marked success and no mechanical or process problems:

1) a small, 3-stage, multi-cylinder compressor sold by Rexarc and, I believe, a sub-contracted Ingersoll-Rand design that was custom modified to be compatible with the Acetylene service. This unit is totally water-submerged (as are all Acetylene compressors that I know) and has inter-stage coils (also submerged) and water separators as well. The unit is characteristically driven at a much reduced speed (as are all Acetylene compressors that I know) to reduce the piston speed and valve operation. This unit can handle only small capacities (approximately up to 25 acfm).

2) A larger, 3-stage, horizontal, in-tandem, Norwalk compressor used to be the industrial standard for Acetylene compression up to the mid-1960s. Since then, this excellent and pioneering compressor company out of Norwalk, Connecticut has gone out of business (like most other companies). You may still pick a used unit in the market place and easily revamp it back into working order. It also was very slowly driven at 50 rpm and was submerged on all cylinders and interstage cooling coils. This was a noble and beautiful machine and it was a shame to see the company disappear. They started making reciprocating compressors in the 1800’s in the USA.

You will note that the compression ratio of approximately 25 is done in 3 stages. This is to keep the shocks and the discharge temperatures in each stage down. The suction pressure was always maintained at 3 psig as a minimum to eliminate the possibility of sucking a vacuum and air into the Acetylene (which would be a disaster). The common sense reason for this is that if that should EVER occur (& it can in most compressors) the operation would be in the very hazardous position of potentially sucking in atmospheric air (with 21% O2) into one of the most potentially explosive gas mixtures one could imagine and (and the following is the worst part!) compressing the subsequent mixture under increased pressure AND TEMPERATURE. A low pressure alarm and shutdown was put on the suction.

I didn't have major problems with pure Acetylene because I was well aware of what I was limited to do via compression and fluid transport. In other words, we know that you are in a progressively more and more dangerous zone when you have free, pure Acetylene at pressures above 25 barg. And even then, I limited all piping to 3/4", schedule 80. All separator vessels like interstage and final discharge pots were filled with steel chain in order to reduce the total free volume within the system. Only iron or steel was allowed as material of construction and potential reactants and catalysts such as silver, copper, etc. were kept out of the plant. Only wooden or non-sparking tools were allowed into the Acetylene equipment area. The Acetylene area was safely located at an isolated distance away from other process units and ran automatically and monitored by instrumentation in a remote control room. Routine operator visits to the area were done religiously every hour.

If you are embarking on a serious project, I recommend you contact people who are experienced and expert in handling Acetylene. You can contact the Rexarc Company at:

http://www.rexarc.com/history.cfm

Talk or write to their people at

Phone: (937)839-4604 Fax: (937)839-5897
35 East 3rd Street, Ohio 45381, USA
P.O. Box 7 West Alexandria, Ohio 45381, USA

I’m sure they can provide you with even more information about what you can and can’t do with Acetylene compression.

I would also recommend you obtain a copy of the CGA (Compressed Gas Association) booklet on Acetylene and its handling. It contains more useful practical engineering information than any MSDS.

I realize I have not given you all the help I would like to give you, but I hope I have identified some areas of potential concern that merit a high degree of precaution and planning. Some companies have already run into this sort of dilemma in the past. I seem to recall Union Carbide's attempt to dominate Reppe chemistry (synthesis starting with Acetylene) in the 1960's in a rather large Brazilian plant; I believe the results were a complete failure in trying to control the process of handling Acetylene. I have never heard of any other venture that has succeeded in conquering the Acetylene problem. I would welcome any news to the contrary from any other forum contributor, because this may be a sign of improved technology that has been developed with regards to safely handling Acetylene.

I believe this is a serious challenge that merits your continued, further attention, and study and I suspect that you are well aware of what you have in front of you and are capable of handling it with professional safety and care. I wish you luck and success and I hope that you may awaken some experienced know-how and ideas from other contributors to this forum.

Art Montemayor
Spring, Texas

[Diederik Zwart]

Thanks Art for your (as always) great response. I will certainly get the CGA booklet as soon as possible and I will contact the Rexarc Company if and when the project kicks off.

Diederik Zwart