Yes, I am retired from Rohm and Haas. I am working part time for Chilworth Technologies as a consultant, and also with AIChE/CCPS.

With regard to the question, I am hesitant to recommend a specific procedure because I have never had any experience with sulfur trioxide, so my knowledge of the hazards is limited to what I can read on line. There is enough there to concern me, though.

From an ILO safety sheet at

"When the alpha form melts it takes the gamma form, and vapor pressure rises dramatically with a hazard of explosion. Melting point is 62, 33 and 17°C for alpha, beta and gamma forms. Vapour pressure is 9.7, 45.9 and 57.7 kPa at 25°C for alpha, beta and gamma forms."

Some other sources for what to do if you receive lab quantities of sulfur trioxide with solid present is to heat slowly over a long period of time, with the temperature never to exceed 35 C, and that it could take a week to melt the material. For example, the Sigma-Aldrich technical bulletin for lab material (attached) says:

"If material is not completely liquid ( it may contain low melting solid polymer), place bottle in a glass beaker or metal pail and warm under heat lamp or in oven at 35°C (95°F). DO NOT OVERHEAT!! Sulfur trioxide, stabilized, boils at 44.7°C 112.6°F). Melting may take up to one week. When the material is liquid, it is ready for use. Traces of non-melting polymer should not interfere with the performance of the liquid material."

But, this is for lab quantities. You would need to think about how to translate this procedure to a specific production scale storage tank.

When faced with a question like this, my advice is always to immediately contact the supplier, or other suppliers or manufacturers of the material. They have a much more extensive experience with dealing with issues such as this, both in their own facilities, and working with other customers. They may have a standard written procedure for dealing with this kind of an issue if it comes up often enough. And they certainly have access to experts with a lot of experience with handling the material for many years. For example, Rohm and Haas and other manufacturers of acrylic acid have very specific procedures for safely melting frozen material. I have found that manufacturers are usually very cooperative in helping avoid incidents.