Absorber-stripper control

T vo carbon-capture processes have been studied in this chapter. Both use a two-column absorber/stripper flowsheet. The low-pressure amine system presents more problems in dynamic simulation than does the high-pressure physical absorption system. The plantwide control structures that are effective for the two systems are quite sunilar. 

Figure 33.2 shows an absorber-stripper combination. The absorption hquid circulates in a closed-loop system, consisting of absorber and stripper. Control of one of the liquid levels also determines the other liquid level. It is therefore no longer possible to control the latter hquid level. One of the intermediate flows can be used for level control, for example, the flow from the stripper to the absorber. This also determines the division of the hquid between the two towers. The other intermediate flow can be used to determine the circulation speed of the liquid, for example, the flow from the absorber to the stripper. This flow can be used for quality control. To compensate for hquid losses, a separate supply flow can be used, handled by the process operator. 

In many units, the light cycle oil (LCO) is the only sidecut that leaves the unit as a product. LCO is withdrawn from the main column and routed to a side stripper for flash control. LCO is sometimes treated for sulfur removal prior to being blended into the heating oil pool. In some units, a slipstream of LCO, either stripped or unstripped, is sent to the sponge oil absorber in the gas plant. In other units, sponge oil is the cooled, unstripped LCO. 

In these systems, the interface between two phases is located at the high-throughput membrane porous matrix level. Physicochemical, structural and geometrical properties of porous meso- and microporous membranes are exploited to facilitate mass transfer between two contacting immiscible phases, e.g., gas-liquid, vapor-liquid, liquid-liquid, liquid-supercritical fluid, etc., without dispersing one phase in the other (except for membrane emulsification, where two phases are contacted and then dispersed drop by drop one into another under precise controlled conditions). Separation depends primarily on phase equilibrium. Membrane-based absorbers and strippers, extractors and back extractors, supported gas membrane-based processes and osmotic distillation are examples of such processes that have already been in some cases commercialized. Membrane distillation, membrane... 

Simple process control large usage of stripper/absorbers towers (single specification) instead of distillation tower (antagonistic top bottom specifications). 

The number of independent variables required to define the operation of an absorber or stripper may also be determined by applying the description rule, stated in Section 5.2.1. The number of trays or the column height is set by construction and may, in the design phase, be used as design variables. Since, by definition, the feeds are introduced at the top and bottom of the column, the feed locations are not variable. The feed compositions and thermal conditions are set outside the column region and are therefore beyond the operator s control. The operator can, however, control the valves on the two feeds and the two products. One of these four valves, usually the bottoms product valve, cannot be controlled independently since it must be set at steady state such as to maintain the required liquid level in the bottom of the column. The overhead valve is usually used to control the column pressure. The two feed valves may be controlled independently one controls the main process stream rate and the other controls the solvent or stripping gas flow rate. 

The parameter used as the control variable to achieve performance specifications is the absorbent rate in absorbers or the stripping gas rate in strippers. With one feed rate allowed to vary, this type of column has one degree of freedom hence, one specification would be required to define its operation. 

Carbon dioxide removal in ammonia plants is usually accomplished by organic or inorganic solvents with suitable activators and corrosion inhibitors. In a few circumstances, C02 is removed by pressure swing adsorption (PSA) (see Chapter 3). The removed C02 is sometimes vented to the atmosphere, but in many instances it is recovered for the production of urea and dry ice. Urea is the primary use of carbon dioxide and, in case of a natural gas feed, all of the C02 is consumed by the urea plant. This practice is especially significant since C02 is a proven greenhouse gas. Typically, 1.3 tons of C02/ton of NH3 is produced in a natural gas-based ammonia plant. The C02 vented to the atmosphere usually contains water vapor, dissolved gases from the absorber (e.g., H2, N2, CH4, CO, Ar), traces of hydrocarbons, and traces of solvent. Water wash trays in the top of the stripper and double condensation of the overhead help to minimize the amount of entrained solvent. The solvent reclaimer contents are neutralized with caustic before disposal. Waste may be burned in an incinerator with an afterburner and a scrubber to control NOx emissions. 

The steady-state RadFrac model in Aspen Plus consisted of four-column sections one stripper, two parallel absorbers, and a rectifier. In reality, there is only one column, but these four fictitious vessels are used in the simulation to model the real physical equipment Before exporting the file into Aspen Dynamics, a number of important changes had to be made in order to obtain a pressure-driven dynamic simulation. Figure 12.21a gives the Aspen Dynamics process flow diagram with aU the real and fictitious elements shown. The lower part of Figure 12.21b shows the controller faceplates. Note that the two controllers with remote set points (RCl and RC2) are on cascade. 

Liquid Fiows. The liquid levels in the base of all three fictitious columns must be controlled. Fictitious pumps and control valves are installed at the base of each column. The base level in the stripper is controlled in the conventional way by manipulating bottoms flow rate. The liquid level in the base of each of the absorber columns (the prefractionator and sidestream side of the waU) are controlled by their corresponding control valves. 

A considerable amount of water is lost in both the absorber and the stripper gas product streams. Some solvent is also lost in these two streams. So the management of the water and solvent fresh makeup streams is one of the essential features of the plantwide control structure. 

The major control objective is to recover the specified fraction of the carbon dioxide in the feed gas. This is achieved by controlling the composition of CO2 in the off-gas at 1.3 mol% by manipulating the flow rate of the lean solvent to the absorber. The removal of CO2 from the fat solvent fed to the stripper is achieved by controlling the temperature in the stripper reflux drum at 363 K by manipulating the heat input to the stripper reboUer. 

Figure 14.11a gives results for a 20% step increase in the flow rate of feed gas. The solvent-to-feed ratio immediately increases the lean solvent flow rate to the absorber, which rapidly drops the level in the stripper base from 2.4 m down to a minimum level of 1.2 m before coming back up to a steady-state level of 1.8 m. Remember this level controller is proportional only. Makeup water is increased by the base level controller, lining out at a higher flow rate since more makeup water is required at the higher feed flow... 

Al. How can the direction of mass transfer be reversed as it is in a conplete gas plant What controls whether a column is a stripper or an absorber ... 

For absorbers and strippers f Section 16.41. the film model is often used. We again postulate a film of thickness 6, which has mass transfer by diffusion only. This is shown in Figure 15-6 for absorption with mass transfer of solute A in the gas-phase controlling. In absorption the carrier gas B often does not absorb thus, there is a stagnant layer of B, Ng = 0. Then, from Eq. tl5-19T... 

When operated in conjunction with an absorber, the product becomes the vapor leaving the condenser, while the bottom stream is recycled to the absorber. A typical absorber ripper combination for the separation of carbon dioxide and hydrogen is shown in Fig. 12.3. Monoethanola-mine (MEA) is used as the solvent. Control of CO2 content in the MEA leaving the stripper is only important for its influence on the equilibrium maintained with the gas leaving the top tray of the absorber-C02 is not lost. Cooling the lean MEA enhances absorption, although its control is not really warranted. In addition, the absorber usually operates at a higher pressure than the skipper. 

The solvent circulation rate is selected so as to keep the vapor pressure of the solute above the rich solvent below the partial pressure of that solute in the entering gas stream. Obviously, this liquid rate should be sufficient to provide good wetting of the packing based on the column cross-sectional area required to handle the gas flow. The concentration of solute in the lean solvent will control the loss of solute in the exit gas stream from the absorber. The solute concentration in the lean solvent is determined by an equilibrium flash in the stripper therefore, the stripper pressure should be fixed to provide the desired solute recovery efficiency. 

The basic Solinox process employs a typical absorption/desorption cycle with SO2 removed from the feed gas in a countercurrent absorber, and stripped from the physical solvent in a countercurrent reboiled stripper. In practice, the process is complicated somewhat by the need to water wash the feed gas before it is contacted with solvent to reduce the gas temperature and remove dust and some impurities and the need to water wash both the purified gas and the stripper off-gas to recover entrained or vaporized solvent. A distinctive feature of the process is its ability to remove hydrocarbons, such as benzene, which are present in some vent gas streams and may require removal to meet air pollution control requirements. Hydrocarbons are generally quite soluble in the solvent. They are absorbed and stripped with the SO2. The hydrocarbons can be removed from the SO2 byproduct by a fractionation step or can be destroyed by oxidation during subsequent processing. 

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