Solvent extraction

1. In a process for the solvent extraction of butadiene from a mixture of C 4 compounds that boil close to butadiene and the recovery of butadiene as a product of the process, wherein at least one solvent extraction step is employed using at least one lean solvent that preferentially extracts butadiene from said mixture of C 4 'S to form a solvent extract stream that carries butadiene and tars that form during said butadiene extraction process, said extract being subjected to a stripping operation to separate butadiene from said solvent and tars, the butadiene separated from said solvent and tars being recovered as a product of the process, a separate primary mixture of solvent and tars being recovered for reuse in said at least one solvent extraction step, and wherein a secondary portion of said primary mixture of solvent and tars is separated from said primary mixture and subjected to a reclamation step in which substantial quantities of tars are removed from said secondary portion and from said extraction process thereby forming a secondary lean solvent stream that is essentially depleted in its content of tars, and reintroducing said secondary lean solvent stream into said extraction process to absorb additional butadiene, the improvement comprising operating said extraction process in a manner such that sufficient tars are removed from said extraction process to maintain the amount of tars present in said extraction process at a level no greater than about 1.6 wt. %, based on the total weight of solvent and tars in said extraction process.

2. The method of claim 1 wherein two solvents are employed in said extraction process, the first of said solvents being employed in a major amount and the second of said solvents being employed in a minor amount, said extraction process including said reclamation step is operated in a manner to maintain the total amount of secondary solvent and tars in said extraction process at a level of less than 5 wt. % based on the total weight of both solvents and tars in said extraction process.

3. The method of claim 1 wherein said reclamation step employs at least one reboiler with an overhead vapor condenser system in which solvent is boiled as vapor overhead from said tars and condensed to an essentially tar free liquid for reintroduction into said extraction process and tars are left in said reboiler for separate removal from said extraction process, said tars in said reboiler having an upper level in said reboiler above which is vapor space, said reboiler being operated in a manner such that said tar laden secondary portion solvent feed to said reboiler is at least one of 1) introduced into said reboiler below said tar level at a temperature sufficient to prevent foaming in said reboiler, and 2) introduced into said reboiler vapor space above said tar level.

4. The method of claim 3 wherein said tar laden solvent introduced below said tar level is sufficiently cool to essentially avoid flashing of said solvent while below said tar level.

5. The method of claim 3 wherein said tar laden solvent introduced above said tar level is heated to a temperature that can cause at least some flashing of said solvent while it is in said vapor space above said tar level.

6. The method of claim 2 wherein two solvent extractors are employed each associated with stripper units for separating tar laden solvent for reuse in said extraction process including said reclamation step, the primary mixture of solvent and tars from each of said associated strippers being combined to form a single primary mixture of solvent and tars, and a secondary portion of said single primary mixture is split off there from for tar separation in said reclamation step.

7. The method of claim 6 wherein said reclamation step employs at least one kettle reboiler with an overhead condenser system in which solvent is boiled as vapor overhead from said tars and condensed to liquid for reintroduction into said extraction process and tars are left in said reboiler for separate removal from said extraction process, said tars in said reboiler having an upper level above which is vapor space, said reboiler being operated in a manner such that tar laden secondary portion solvent feed to said reboiler is at least one of 1) introduced into said reboiler below said tar level at a temperature sufficient to prevent solvent/tar foaming in said reboiler and consequent carryover of tars into said overhead, and 2) introduced into said reboiler in said vapor space above said tar level.

8. The method of claim 2 wherein said solvents are dimethylformamide and furfural, said dimethylformamide being present in an amount greater than about 90 wt. %, and said furfural being present in an amount less than about 10 wt. %, both wt. % being based on the combined total weight of both solvents and tars, and essentially all tars are removed from said secondary portion of said primary mixture of solvents and tars before said reclaimed solvent is reintroduced into said extraction process.

9. The method of claim 8 wherein said secondary portion of said primary mixture of solvent and tars is from about 0.1 wt. % to about 1.6 wt. % of said primary mixture based on the total weight of said primary mixture.

10. The method of claim 9 wherein said extraction process including said reclamation step is operated so as to maintain in said extraction process both 1) the amount of tars at a level of less than 1.6 wt. %, and 2) the total amount of tars and secondary solvent at a level of less than 5 wt. %, all wt. % being based on the total weight of solvents and tars in said extraction process.

11. The method of claim 1 wherein said tars are polymers formed from said C 4 compounds, said polymers being not readily vaporizable under the operating conditions of temperature and pressure of said extraction process.