Absorbers, intercooling

Distillation columns and other multistage columns generally require heat addition and/or removal, which normally take place at the reboiler and/or condenser. For a variety of reasons, it may be desirable in certain types of columns to add or remove heat at intermediate trays aside from the condenser and reboiler. Examples of such columns include demethanizers that utilize a side reboiler alongside the bottom reboiler, multi-product columns where intermediate condensers are associated with some of the side products, and absorber intercoolers used for partial removal of the heat of absorption. The method most commonly employed for exchanging heat between a column tray and a heat source or sink is the pumparound, where a fluid is drawn from the tray, sent to a heat exchanger, then pumped back to the column. The following sections pertain to the various applications of side heaters and coolers and the different types of pumparounds. 

Approximately 0.016 (kg-mol)/s [126 (lb-mol)/li] of vapor is absorbed with an energy liberation of about 198,000 W (670,000 Btii/b), 20 percent of which is removed by the intercooler on stage 7. The temperature profile departs from a smooth curve at stages 4 and 7, where secondary oil enters and heat is removed respectively... 

A - ABSORBER FD - FLASH DRUMS C - COMPRESSORS 1C - INTERCOOLED COMPRESSORS P- PUMP T - HYDRAULIC TURBINE D - DEHYDRATOR AC - AFTER COOLERS X- THROTTLE G GENERATOR... 

Most absorbers require some intercooling between some stages or trays to remove heat of absorption and to provide internal conditions compatible with proper or required absorption. Some temperature rise (10-30°F) is usually designed into the initial conditions. The rise above this must be handled with intercoolers. 

The total intercooler duty is the difference between the total heat in of the rich gas and lean oil and the total heat out of the off gas and rich oil all at the terminal calculated or design conditions. The total duty is often divided between several coolers placed to re-cool the oil as it passes down the column. If intercoolers are not used, then the absorption cannot meet the design terminal outlet conditions and the quantity of material absorbed will be reduced. If the intercooling is too great so as to sub-cool, then greater absorption may be achieved, but this can be controlled by the intercooler operation. 

Reducing the operating temperature. Consider adding an intercooler on the absorber. Minimize lean oil temperature. Consider the use of a chiller. Each 10°F reduction in lean oil temperature will increase C3 s recovery about 0.8% (Figure 9-11). 

The chilled water (7°C) for the absorber is supplied by an absorption refrigeration unit, using heat recovered from the compressor intercooler as the energy source. Heat for ammonia vaporization, as previously noted, is available at 35°C and is recovered from the secondary gas cooler. The system uses circulating condensate as the energy transfer medium. 

Kremser-Brown-Sherwood Method — No Heat of Absoip-tion, 108 Absorption — Determine Component Absorption in Fixed Tray Tower, 108 Absorption — Determine Number of Trays for Specified Product Absorption, 109 Stripping— Determine Theoretical Trays and Stripping or Gas Rate for a Component Recovery, 110 Stripping — Determine Stripping-Medium Rate for Fixed Recovery, 111 Absorption — Edmister Method, 112 Example 833 Absorption of Hydrocarbons with Lean Oil, 114 Intercooling for Absorbers, 116 Absorption and Stripping Efficiency, 118 Example 834 Determine Number of Trays for Specified Product Absorption, 118 Example 835 Determine Component Absorption in Fixed-Tray Tower, 119 Nomenclature for Part 2, 121... 

FIGURE 13.1 Typical flow diagram for a coupled absorption-stripping unit. Because the solvent flows in a closed circuit, allowance is made for losses ( makeup ) as well as removal of accumulated impurities ( cleanup ). An intercooler is shown to suggest the need to remove heat of solution in the absorber. 

Each equipment symbol shown in Table 1.1 corresponds to the simplest configuration for the represented operation. More complex versions are possible and frequently desirable. For example, a more complex version of the reboiled absorber, item (5) in Table 1.1, is shown in Fig. 1.7. This reboiled absorber has two feeds, an intercooler, a side stream, and both an interreboiler and a bottoms reboiler. Acceptable design procedures must handle such complex situations. 

Use the Naphtali-Sandholm SC method, with the Chao-Seader correlation for thermodynamic properties, to determine product compositions, stage temperatures, interstage flow rates and compositions, and reboiler duty for the reboiled absorber shown below. Repeat the calculations without the interreboiler and compare both sets of results. Is the interreboiler worthwhile Should an intercooler in the top section of the column be considered ... 

THAHRA is designed for the assessment of solvents for application in high pressure CO2 capture processes. Intercooling of the solvent in the absorber column is able to increase solvent capacity when using reactive solvents. Within the operating window of THAH, one is able to mimic the treatment of different feeds. These include shifted syngas for pre-combustion, typically 30 bar, 40%-60% CO2-H2, as well as natural gas for pipe line and LNG, typically 45 bar, 20%-80% CO2-CH4. The maximum feed flow is 5 Nm /hr, with a liquid flow of up to 25 kg/hr. The solvent inventory is approximately 4.5 1. Due to the small liquid inventory, non-commercially available solvents can easily be tested. 

This rather nebulous concept can best be appreciated by performing a tray-to-tray computer simulation of the column. In the field I first observed this phenomenon when the operators noticed that shutting down the intercooler shown in Figure 8-11 did not affect the propylene content of the absorber off-gas. 

The system pressure drop is defined as the sum of the individual component fiactional pressure drops. The component fractional pressure drop is defined as the actual pressure loss divided by the absolute working pressure of the component. This system fiactional pressure drop determines how much of the turbine output is absorbed as parasitic pressure loss. A reasonable and realistic value of 5% is chosen here, where 1 % is budgeted for the core heat transfer fins and 4% for the remainder of the closed Brayton cycle. To keep the system as imcomplicated as possible, die compressor will not be intercooled. 

Estimate of Average (L/V). The temperature of the gas leaving (he top tray will be determined principally by the temperature of the lean oil. The temperature approach at the top of the tower can be anywhere from 5 to 50 degrees F and will depend somewhat on the presence of components lighter than the keys in the exit gas. It is generally true that the average temperature in the absorption section will be no more than 5 degrees F hi er than this exit gas temperature. In the case of reboiled absorbers which have liquid intercoolers, the above statement can almost be considered a rule. 

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