Laboratory Columns Packed with Silicon Carbide J
HORIER J . HkLL' A ~ G. D B R 1 4 h T UiCHI\IX'4.2 The Ohio State Uni\ersity. Columbus, Ohio
Tests on distillation columns packed with ordinary lump silicon carbide indicate that this cheap, chemically inert material may properly be classed with the most efficient column pacltings now in use. h-o special care is required i n handling, and a fouled packing can be cleaned with
I
S A PREVIOUS article (8)the authors described a 3.15 X 150 em. laboratory tlistillation column packed x i t h 3- to &mesh silicon carbide, used for the successful isolation of 1pentene and 1-hexene as chemical raw materials from cracked gasoline. The tests described lielox indicate that this column compares favorably in efficiency Ti-ith the liest of tlie recently developed packed columns, and this serves to direct' further attention to this cheap, sturdy, and cheniically inert packing material. The chief advantage of silicon carbide oT-er these other packings, some of n-hich are equally efficient, lies in the fact that it inay be used for aliiiost any type of tlistillation. The use of 60-111esl1silicon carbide t o coat the 11-allsof an efficient laboratory colunin of special design was first noted by Alidgley ( 9 ) ,wlio also suggested its use as a column packing. L-nfortunately, hen-ever, its ralue as a packing material in the ordinary sense does not seein t o be i d e l y appreciated. Columns of this type have been used in t,liese lahoratories for a xide Tariety of distillations, including organic sulfides, polpieriaable or gum-forming hydrocarbons, aiitl rolatile inorganic halides. This particular coluinn n-as used for tlie vacuiini distillation of several large batches of crucle olefin tlibromicles, Jvliich inr-olred a certain amount of clecoinposition and tar formation due to t,he presence of sniall aiiiounts of t'etrabroinides as impurities; after this use the packing n-as rest'ored t o practically its original efficiency by siniple solvent refluxing. The same column Tvas also used for tlie separat'ion of alpha- and gamma-picolines and quinaldine from crude nitrogen bases ( 1 ) . I n general, simple solvenk are used to clean up the packing aft,er one of these general utility dist'illations. When pronounced decomposition occurs in a sniall batch distillat,ion, tlie packing is cheap enough to be discarded after use, alt,liough it may be boiled x i t h nitric acid or chromic acid cleaning solution if desired. There is no major difference between the various types of silicon carbide available on the market', although there is a slight difference in favor of a fine-grained crystalline form, with a fairly open or porous structure. Results discussed lielow also indicate that there niay be a distinct advantage in throughput when using 3- to 6-mesh material instead of straight 6-mesh, but this point is still suliject to direct experimental confirmation. Silicon carbide-packed colunins are found to operate most efficiently a t the iiiaxiniuiii t'hroughput,,just short of tlie flooding point.
Test Data The all-Pyrex laboratory coluiiiii mentioned above antl used for the tests was niodeleti after Peters and Baker ( I I ) , v-ith an insulating dead-air jacket and a second insulating 1 2
boiling acid cleaning solutions if necessary. Since distillations may be made with equal impunity using organic bases, corrosive sulfur compounds, or unstable halides, silicon carbide-paclied columns are particularly recommended for general laboratory use.
heated-air jacket. The test, data given in Table I were 01)tainecl by using a niixture of benzene and ethylene chloride (4) on t'otal reflux under substantially adiabatic conditions. Tlie test mixture used forins a nearly ideal solution with coniponents hoiling 3.39" apart (IR), and the molar coiiipositions of t'he still pot liquid, io,and distillate, T,,, analyzed hy refractive index may lie substituted in the formula of Beatty and Calingaert (2) to calculate tlie number of theoretical plates, 11: I2 =
(log (1 -
xu)
X " / ( l - xc) S n ) : ' log I
