Solvent circulation rate

The actual liquid-to-gas ratio (solvent-circulation rate) normally will be greater than the minimum by as much as 25 to 100 percent and may be arrived at by economic considerations as well as by judgment and experience. For example, in some packed-tower applications involving veiy soluble gases or vacuum operation, the minimum quantity of solvent needed to dissolve the solute may be insufficient to keep the packing surface thoroughly wet, leading to poor distribution of the liquid stream. 

This process uses propylene carbonate as a physical solvent to remove CO2 and H2S. Propylene carbonate also removes C2+ hydrocarbons, COS, SO2, CS2, and H2O from the natural gas stream. Thus, in one step the natural gas can be sweetened and dehydrated to pipeline quality. In general, this process is used for bulk removal of CO2 and is not used to treat to less than 3% CO2, as may be required for pipeline quality gas. The system requires special design features, larger absorbers, and higher circulation rates to obtain pipeline quality and usually is not economically applicable for these outlet requirements. 

Solvent temperatures below ambient are usually used to increase the solubility of acid gas components and therefore decrease circulation rates. 

The reaction mass consists of two liquid phases and one solid phase no solvent is required. The major liquid phase is the crude amine product itself. The solid phase is promoted sponge nickel catalyst. Surrounding the catalyst is a second liquid phase consisting of concentrated caustic and water. Water and caustic are added continuously to make up for losses leaving in the crude product. The ratios of water, caustic, and catalyst in the reaction mass are controlled to produce high yields of product amine and very low catalyst usages. High catalyst concentrations are employed in the reaction mass to keep the concentration of unreacted nitriles very low the upper limit on the catalyst concentration is the point where the circulation rate is inhibited. 

Diethanolamine (PEA) has replaced MEA as the most widely used amine solvent. High load DEA technologies, such as that developed by Elf Aquitaine, permit the use of high (up to 40 wt % DEA) concentration solutions. The Elf Aquitaine—DFA process allows lower circulation rates, and has consequent reductions in capital and utility expenses. DEA tends to be more resistant to degradation by carbonyl sulfide and carbon disulfide than MEA. DEA is, however, susceptible to degradation by carbon dioxide. 

The diglycolamine (DGA) process also allows high (up to 60 wt % DGA) solvent concentrations for reduced circulation rate and energy requirements. High solvent costs and a higher tendency to absorb heavy hydrocarbons have limited the use of this solvent. 

The measured values of LH appreciably increased with increasing shear rate, and those at the highest shear rate were 3-6 times as large as the diameter of a free polymer in the bulk solution. To explain this result Gramain and Myard considered that there should exist a net circulation of solvent in the adsorbed layer to elongate adsorbed polymers axially. This effect reduces the number of trains to about 3 or 4 per polymer chain. 

The aMDEA solvents are non-corrosive due to their chemical structure. This allows a high sour gas loading and therefore reduced solvent circulation rates, which results in low investment and low operating costs. 

Capital costs for a Sulfinol unit are lower than alkanolamine units that have the same capacity. This is because the equipment is smaller as a result of less foaming and lower circulation rates. However, certain treating applications may not be ideal for the Sulfinol process. A reclaimer may be needed to remove DIPA degradation products when C02 partial pressure is high. Also, aromatic and hydrocarbon co-absorption occurs when solvent -to-gas ratios are high260-... 

In a potassium carbonate system, different additives can be used to increase the C02 absorption rate. In any wet scrubbing system, improved tower packing can be used. A change from random to structured packing can lead to higher solvent circulation rates and improved mass transfer86. 

Figure 6.14 provides an example of the type of decision that a process designer would face in flowsheet evaluation. In this simple example, absorption with a regenera-ble solvent is used to capture (and recycle or sell) toluene and ethyl acetate, which might otherwise be emitted into the atmosphere. To increase the fraction of the hydrocarbons that are absorbed, the circulation rate of the... 

The volatilities of the feed components relative to the solvent are generally high therefore, the design of the secondary column is primarily a function of the solvent circulation rate. The condenser duties depend on the reflux ratios in the two columns which are thus affected by the relative volatilities. The solvent cooler duty is a function of the solvent circulation rate and the recovery column reboiler temperature which is determined by the solvent volatility. The sum of the reboiler duties in... 

Solvent Loading. The solvent circulation rate is a function of the reflux ratio in the primary tower and the liquid-phase concentration of the solvent. For a given solvent selectivity, as the solvent concentration rises, the propane-propylene relative volatility increases and hence the required reflux rate falls. The increased relative volatility results in a decreased number of equilibrium stages required for the desired separation. Figure 4 shows the effect of solvent concentration on the number... 

A higher selectivity requires a thinner but taller tower. It also requires a smaller solvent cooler because of the lower solvent circulation rate and smaller cooler and reboiler heat duties. The net result is shown in Figure 7 where the solid lines indicate the annual costs as a function of selectivity they show a falling annual cost with increasing selectivity. 

Recently UOP has introduced a new activator, called ACT-1 [673] which is claimed to reduce C02 levels in the absorber exit gas in existing plants and should reduce the solvent circulation rate as well. For new installations the enhanced mass transfer achieved with the new activator translates into smaller towers and therefore to investment capital savings. The activator is an amine compound, but speculations on its nature are still going on. 

Table 4.15 compares common sulfur removal processes. Amine processes are based on the removal of an acid gas by virtue of a weak chemical bond between the acid gas component and the amine. Amine-based sulfur removal processes are generally regarded as a low capital cost option with part C02 coabsorption. However, amines do not chemically combine with COS. Only limited amounts of COS are absorbed with a physical solvent. COS can be physically removed only with very high solvent circulation rates. For syngases that contain appreciable quantities of COS, prior removal of the COS is usually required. In addition, for some of amine solvents, degradation and corrosion are also main disadvantages of the process. 

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