Rhodium recovery

Since butyraldehyde has a low boiling point (75 °C) separation of catalyst from both reactants and product is straightforward. Most of the rhodium remains in the reactor but prior to recovery of propene and distillation of crude product the gaseous effluents from the reactor are passed through a demister to remove trace amounts of catalyst carried over in the vapour. This ensures virtually complete rhodium recovery. 

Although rhodium recovery is efficient it is difficult to separate it from heavies that are formed in small amounts. Over time these heavies tend to result in some catalyst deactivation. One solution to this problem has been developed by Ruhrchemie/Rhone-Poulenc. In this process sulfonated triphenyl phosphine is used as the ligand, which imparts water solubility to the catalyst. The reaction is two-phase, a lower aqueous phase containing the catalyst and an upper organic phase. Fortunately the catalyst appears to sit at the interface enabling reaction to proceed efficiently. At the end of... 

Figure 2.9. Entrainment separator for trace rhodium recovery...

Trace Rhodium Recovery from Product or Byproduct Streams. As will be discussed later, there are what might be viewed as the ultimate rhodium recovery methods in which the organic matrix is burned, the rhodium recovered as an ash, then processed through a precious metal refinery before conversion into a catalyst precursor. Once rhodium is processed into an ash, there is significance expense associated with its conversion to a suitable catalyst precursor. Therefore, technologies which permit capture and reuse or reactivation and reuse are strongly preferred over more extreme procedures. 

The rhodium process is highly selective and operates under mild reaction pressure, 400 to 1000 psig. However, because of the high price of rhodium, an efficient recovery of the catalyst is essential. An expensive rhodium recovery section is an integral part of any new acetic acid plant (15-16). This can be a substantial financial burden, especially in a smaller plant. 

Solvent Pressure (atm) Ethylene glycol rate (hr- ) Methanol rate (hr-1) Rhodium recovery (%)f... 

Fig. II. Effects on rate and catalyst stability of using sulfolane-tetraglyme mixtures as solvent ( ) methanol ( ) ethylene glycol ( ) rhodium recovery. Reaction conditions 75 ml solvent, 3 mmol Rh, 0.65 mmol cesium benzoate, 544 atm, H2/CO = 1, 240 C, 4 hr (94).

The heavy end products of acetic anhydride processes are separated from catalyst streams and distillation residues. The high affinity of the heavy ends for rhodium components affords specified procedures to separate rhodium and recycle it to the reaction stage of the process. Different methods for rhodium recovery were tested during the process development, including extraction methods [65], precipitation [66], complexing, and electrochemical methods [67]. 

A number of methods were also developed for removing iodide impurities from acetic anhydride, such as syn-gas stripping [68], extraction in the presence of phenyl or alkyl phosphines [69], the use of lower fatty acids in combination with rhodium recovery [70], the use of silver-containing ion-exchangers [71], or oxidation with hydrogen peroxide [72]. 

Successive extraction of the catalytic mixture of XANTHAM affected the recovery of rhodium. Although the overall rhodium recovery was almost quantitative (97%), a retention of activity of 86% was observed when the acidic titration procedure was performed at pH 5-5.5. This result equals the success of PhP(C6H4CH2NEt2)2 [20]. 

Recovery of rhodium is an important component of the process. One example of the method to accomplish this, which was disclosed by Hembre and Cook [46], is described in Reference 44. The rhodium complex in methyl iodide is extracted into an aqueous phase using a 13% aqueous solution of hydrogen iodide. In the industrial operation, rhodium recovery of over 99.99% is achieved. 

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