LIQUID RECYCLE

In the case of a liquid recycle, the cost of this pressure increase is usually small. Pumps usually have low capital and operating costs relative to other plant items. On the other hand, to increase the pressure of material in the vapor phase for recycle requires a compressor. Compressors tend to have a high capital cost and large power requirements giving higher operating costs. 

Sometimes it is extremely difficult to avoid vapor recycles without using very high pressures or very low levels of refrigeration, in which case we must accept the expense of a recycle compressor. However, when synthesizing the separation and recycle configuration, vapor recycles should be avoided, if possible, and liquid recycles used instead. 

In a single stage, without liquid recycle, the conversion can be optimized between 60 and 90%. The very paraffinic residue is used to make lubricant oil bases of high viscosity index in the range of 150 N to 350 N the residue can also be used as feedstock to steam cracking plants providing ethylene and propylene yields equal to those from paraffinic naphthas, or as additional feedstock to catalytic cracking units. 

In a single stage with liquid recycle, total conversion to products lighter than the feedstock is possible. The yield of kerosene plus diesel is between 70 and 73 weight %. 

Fig. 3. LP Oxo liquid recycle flow scheme A and B, reactors C, vaporizer D, catchpot E, stabilizer F, syngas cleanup and G, propylene cleanup.

These reactions occur with similar rates to those carried out in dipolar aprotic solvents such as DMF or DMSO. An advantage of using the room-temperature ionic liquid for this reaction is that the lower reaction temperatures result in higher selec-tivities for substitution on the oxygen or nitrogen atoms. The by-product (sodium or potassium halide) of the reaction can be extracted with water and the ionic liquid recycled. 

The influence of liquid recycle rate and liquid-feed space velocity upon desulfurization level is the subject of a brief theoretical discussion. 

Figure 5.4.77. Monolith reactor with external liquid recycle.

Metal Nanoparticles in Ionic Liquids Recyclable Multiphase Catalyst-Systems 13... 

Gas Recycle technology has been licensed worldwide by Union Carbide-Davy for the hydroformylation of propene.[9] It has also been operated by Union Carbide for ethene hydroformylation. Its use with butene is feasible, but at the margin of operability. Liquid Recycle, described below, is a better option for butene. 

In Liquid Recycle, the conditions for the reaction are decoupled from those for the separation system. [10] Distillation is a widely practiced and well-understood technology, so it is generally the first consideration for any homogeneous catalytic process. A typical Liquid Recycle system is shown in Figure 2.2. 

Figure 2.2. Block Flow Diagram for a Liquid Recycle Process...

Whereas in Gas Recycle the product must be removed at the same temperature and pressure at which it is formed, in Liquid Recycle the separation of product (and byproducts) from catalyst is independent of the conditions under which the products were formed. This added degree of control brings a variety of benefits. Since large gas flows are no longer required in the reactor, the liquid expansion due to gassing is reduced and more catalyst can be contained in a specific reaction vessel. Reactor temperature and reactant concentrations can be tuned for optimum catalyst performance. The conditions in the separation system can likewise be tuned for optimum performance. In particular, more severe conditions will permit better control over the concentration of heavies in the catalyst solution. 

Another advantage of Liquid Recycle is that multiple reactors may be arranged in series with the effluent from one passing on to the next. The alkene concentration is less in the downstream reactors, but reaction conditions can be adjusted to optimize each reactor s performance. In back mixed reactors in continuous operation, the effluent from the reactor is the same as the catalyst solution throughout the reactor. By placing reactors in series, the first reactor can be optimized for high rates and later reactors for high conversion. 

Liquid Recycle OK OK Product thermally unstable Product labiality... 

Induced Phase Separation would work technically, but would be uneconomic relative to Liquid Recycle because of additional unit processes and increased energy requirements. 

Liquid Recycle is practical for octene hydroformylation. 1-Octene is readily soluble in organic based catalyst solutions, and product aldehyde and its condensation products can be separated by vaporization. 

Induced Phase Separation is also a good choice for octene hydroformylation. Octene can easily dissolve in the organic based catalyst solution, and with addition of small amounts of water, nonanal and its condensation products will readily separate from the sodium salt of a monosulfonated phosphine. To choose between Liquid Recycle and Induced Phase Separation would require a detailed technical and economic study that is outside the scope of this chapter. 

NAPS is also a possibility for octene hydroformylation, but again a detailed technical and economic comparison would be required in order to chose among it, Liquid Recycle and Induced Phase Separation. 

In this example, we will consider asymmetric hydroformylation to give an aldehyde intermediate with a high ee. Gas Recycle is out of the question because of the low volatility of the product. Vaporization in a Liquid Recycle process is theoretically possible, but impractical if we wish to maintain the high enantioselectivity of the product. 

Rhodium precipitation in solubilized rhodium-phosphite complex catalyzed liquid recycle hydroformylation may be minimized or prevented by carrying out product recovery in the presence of an organic polymer containing polar functional groups such as amides, ketones, carbamates, ureas and carbonates.[20] Patent examples include the use of polyvinylpyrrolidone and vinylpyrrolidone-vinyl acetate copolymer with diorganophosphite-modified rhodium catalysts. 

Aldehyde dimer may undergo dehydration to give an a, -unsaturated carbonyl. From butanal, the conjugated carbonyl is ethylpropylacrolein (Equation 2.10). The conjugated system of this material competes for coordination sites on the rhodium catalyst so that hydroformylation inhibition is observed.[8] The formation of 2-ethylhex-2-enal can be limited by minimizing the concentration of dimers. Dimers are removed along with the product in a liquid recycle separation system. 

When a catalyst has sufficiently deactivated to justify taking some action is determined by economics. Both Gas and Liquid Recycle hydroformylation plants may be operated to give essentially constant production rates as the catalyst deactivates. Hydroformylation is approximately first order in both rhodium and alkene concentration. As the rhodium catalyst deactivates, the alkene concentration may be allowed to increase to compensate for the declining catalyst activity. Action is taken when the alkene efficiency declines to the point where it approximates or exceeds the cost of catalyst replacement or reactivation. 

A consequence of the value of the ligand is that one of the simplest ways to restore catalyst activity is simply to add fresh catalyst precursor. Unfortunately, there are practical limits as the rhodium concentration increases. First one must consider metal complex solubility, particularly in the recycle catalyst solution in a liquid recycle system. Secondly, higher rhodium concentrations favor formation of various types of rhodium clusters.[11] As rhodium increments are added to a partially deactivated cata-... 

Entrainment Separators. In any process in which the product is volatilized, including both Gas Recycle and Liquid Recycle, ppm or ppb levels of metal catalyst may be entrained in the vapors leaving the separation system. Entrainment separators (Figure 2.9) are often included to recover the metal. Vaporous product effluent from a gas recycle reactor may be sent to a separator where it is passed through a demisting pad to return some aldehyde and condensation product and particularly to prevent potential carryover of catalyst. [6]... 

Selective Condensation of Vaporized Organophosphorus Ligand. Certain phosphorus ligands have sufficient volatility that portions may be volatilized when aldehyde and higher boiling aldehyde condensation byproducts are separated from the catalyst solution in, for example, a liquid recycle vaporizer. The phosphorus ligand may be condensed, recovered and returned to the catalyst solution [35] according to the procedure disclosed in US 5,110,990. 

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