US2094612A - Removal of mercaptans from hydrocarbon mixtures - Google Patents

Description

Patented Oct. 5, 1937" CARBON MIXTURE S William M. Malisoif and James H; Boyd, Jr.,

Philadelphia; Pa., assignors to The Atlantic Refilling Company, Philadelphia, Pa., a corporation of Pennsylvania No Drawing. Application ltlovembert,1933,, I Serial No. 696,534

- Claims.

" Our invention relates to the fractional extraction of mercaptans from hydrocarbon mixtures,

and particularly from hydrocarbon mixtures from petroleum sources containing mercaptans of 5 various molecular weights.

Untreated fractions obtained from petroleum by distillation; "cracking or other processing usually contain mercaptans and may contain these compounds in quantities up to 0.5% or more. Since the lowest member of the mercaptan series, methyl mercaptan, has a boiling point of about 6 C. this compound occurs largely in the propane and butane fractions. The boiling points of the mercaptans progressively increase with increasing number of carbon atoms and for example, normal hexyl mercaptan has a boiling point of 147 C. and is therefore, present in the gasoline fraction. The normal refining practice is to free these fractions from mercaptans by processes which destroy these compounds, 'as for example, by treatment with sodium plumbite or doctor solution. The mercaptans are, however,

valuable chemical compounds and it is desirable to recover them in a fairly pure state.

In accordance with our invention we fractionally extract the desired 'mercaptans from the hydrocarbon fraction in recoverable form in such a way that the mercaptans are separated substantially one from the other. It is a well known fact 3 that the various members of the mercaptan series differ from one another in their acidity. We have found that if the hydrocarbon fraction be contacted with successive portions of an alkaline solution the more acidic members are concentrated in the solution first brought into contact with the fraction and the successively less acidic members progressively in the subsequent stages of extraction with alkali solution. We prefer to accomplish our results by continuous passage of the hydrocarbon mixture either as a gas or as a liquid through successive stages in each of which stages it is brought into contact with alkali solution, as for example, a 5% solution of sodium hydroxide in water. In carrying out the process F in this manner the most acidic mercaptan which is methyl mercaptan, will be almost completely absorbed in the first stage. This first stage will, however, also dissolve appreciable quantities of the less acidic mercaptans. However, as the fraction is continuously contacted with these successive stages .and the solution in the first stage approaches the saturation point of the alkali, i. e., the free alkali has been converted into sodium mercaptides, the less acidic mercaptans will be displaced from the solution by fresh incoming methyl mercaptan, so that the point is eventually reached where the concentration of methyl mercaptan relative to the other members of the series is very high. Similarly, by suitable arrangement of the successive stages it is possible to concentrate ethyl mercaptan in the secondstage, isopropyl mercaptan in the third stage and so on.. 1

After the solutions have reached the desired concentration they may be withdrawn, acidified to .liberate the mercaptans and the mercaptans recovered by distillation or other well known means.

We may modify our process by using alkali solutions of different strengths in the various stages, employing the weakest in the first stage and the strongest in the last stage, and having progressively increasing concentrations in the intermediate stages. For example, we may use an aqueous 1% sodium hydroxidesolution in the first stage, a 2% solution in the second stage, and progressively increasing thereafter up to a 15% solution in the last stage. Since the strength of the alkali is a matter of hydrogen ion concentration, in place of using various strengths of sodium hydroxide we may also use solutions of alkaline salts such as sodium carbonate, of lower hydroxyl ion concentration. By this means the less acidic mercaptans are brought into contact with stronger alkali solution in which they are more easily dissolved and our separation is thereby enhanced. We may also vary the temperatures in the different stages to accomplish a somewhat similar result. 7

Since the mercaptans of higher molecular weight and lower acidity than normal hexyl mercaptan are not dissolved to a substantial extent, even by relatively strong aqueous alkali solution, we may use, particularly in the last stages, a solution of alkali in other solvents, such as for example, methanol or other aliphatic alcohols. We may further modify the extracting power of the solution by using a mixture of solvents, such as for example, water and methanol as solvents for the caustic soda or other alkali.

While our process is particularly applicable to the extraction of mercaptans from such fractions as commercial butane or gasoline characterized by distilling not less than at 200 C. by A. S. T. M. method #D86-30, we do not intend to limit our process thereto. We may apply our process to kerosene or even higher boiling frac tions; Furthermore, thefraction may have been treated in such a way before subjecting it to our process that the mercaptan content of it is substantially increased or decreased. The fraction,

' particularly a fraction such as commercial butane, may be subjected to our process either in the gaseous or liquid phase.

What we claim is:

1. In a process for separating the mercaptan of lowest molecular weight from a hydrocarbon mixture containing mercaptans of different molecular weights, the step which comprises contacting said mixture with an alkaline solution until substantially all of. its alkaline content has been converted into the mercaptide correspond:

ing to said mercaptan of lowest molecular weight.

2. In a process for separating the mercaptan of lowest molecular weight from a hydrocarbon mixture containing mercaptans of different molecular weights, the step which comprises contacting said mixture with a solution of sodium hydroxide until substantially all of the alkali content thereof has been converted into the mercaptide corresponding to said mercaptan of lowest molecular weight.

3. In a process for separating a mixture of mercaptans of different molecular weights contained in a fluid into fractions of progressively increasing molecular weights, the step which comprises passing the mercaptan-containing fluid into contact with a series of. alkali solutions, and continuing such passage until substantially the entire alkali content of at least two of said solutions is converted into mercaptides of respectively higher molecular weights.

4. The process for separating a mixture of mercaptans of different molecular weights contained in a fluid into fractions of progressively increasing molecular weights, the step which comprises passing the mercaptan-containing fluid into contact with a series of alkali solutions of progressively increasing hydroxyl ion concentration, and continuing such passage until substantially all the alkali content of at least two of said solutions is converted into mercaptides of respectively higher molecular weights.

5. In a process for separating the mercaptan of lowest molecular weight from a hydrocarbon mixture containing mercaptans of different molecular weight, the steps which comprise contacting said mixture with an alkaline solution until substantially all of its alkaline content has been neutralized, and continuing said contacting until the mercaptan of lowest molecular weight has replaced substantially all of the mercaptans of higher molecular weight from said solution.

WILLIAM M. MALISOFF. JAlWES H. BOYD, JR.