From the second edition of Practical Process Research and Development by our mentor-by-literature Neal Anderson, a little tidbit about charging solids and gases:
In general charging liquids and solutions is easier and preferred over charging solids. One concerns with charging solids is that not all of the solids may be transferred to the reactor. Charging of liquids is usually completed by chasing residual liquids with a small charge of solvent; for instance, a solution maybe prepared using 95% of the volume specified in the laboratory process description, with 5% of the latter solvent volume being used to complete the transfer.
More specialized equipment is used for charging solids on scale; such equipment may be known as alpha-beta valves, butterfly valves, shot valves, star valves or other names.... Glove boxes have also been erected on top of reactors for charging moisture-sensitive or oxygen-sensitive materials.
Specialized equipment is used to charged gases on scale, with special considerations to minimize any risk of releasing gases. To protect personnel and equipment from leaks of hazardous material such as H2 or F2, the lines used to charge such gases may be encased inside sealed lines or pipe. Charging 37% aqueous HCl is more convenient than charging HCl from cylinders.I've had some experience transferring solids into a solvent-filled reactor recently; although the material was as close to ideal as you could possibly imagine (non-toxic, a nice round shape), it was an incredibly painful process. (I fear that I may be waylaid by an operator for suggesting this particular order of addition, but it (I think) will save us a good bit of time.
In general, I think it is to be avoided -- the specialized equipment above is expensive and does not necessarily guarantee that all the solid starting material will actually make it into the reactor.
There are a number of things a new process chemist has to learn for plant operation coming from bench scale research and this is one that can be under appreciated. How many reactions did one do in school where simply opened a stopper and scooped in small portion of a solid "in controlled" manner. This can not be duplicated at scale. There are Archimedes wheel/cork screw set ups that can partly handle phased solid additions but if need precise control then can forget about it.
ReplyDeleteHowever one of the coolest things I did was add large premeasured bags of LAH and NaH and then the bag was dissolved in the solvent (THF) in the reactor. Much neater and safer than handling in the lab.
CJ,
ReplyDeleteYour statements are all quite true, but for a limited context - traditional batch reactors. Any kind of continuous reactor will have proper equipment to make sure the solids are added at a controlled, continuous rate.
My experience has been with reactive polymerization in a twin-screw extruder. You use mass-loss feeders (the feeder is continually massed to help ensure the proper feed rate), often with some form of throat auger to prevent bridging. If very small quantities are needed, these are usually masterbatched - premixed with one of the components at a concentrated degree so that you are not trying to feed one powder piece at a time.