Absorber-stripper combination

In most plant design situations of practical interest, however, the several pieces of equipment interact with each other, the output of one unit being the input to another that in turn may recycle part of its output to the inputter. Common examples are an absorber-stripper combination in which the performance of the absorber depends on the quality of the absorbent being returned from the stripper, or a catalytic cracker-catalyst regenerator system whose two parts interact closely. 

The H2S formed in the hydro-desulphurisation process can be removed from the product stream in a variety of ways. Commonly used methods are chemical reaction with, for example, zinc oxide or iron oxide, caustic scrubbing and absorption processes. For the H2S decomposition processes treated in this chapter, only the absorption/desorption methods are of importance. Most used are absorber/stripper combination units with an alkanolamine as absorbing compound [2],... 

The SCOT process provides an efficient way of removing sulphur-containing compounds from the tail gas of a conventional Claus reactor. The tail gas is heated to about 300°C and fed to a hydrogenation reactor, where all sulphur compounds in the gas are converted to H2S. Almost all H2S is removed in an absorber/stripper combination and fed back to the Claus plant. The off-gas from the absorber contains virtually no sulphur compounds (values as low as 500 ppm are reported [2]) and is incinerated in the Claus incinerator. A schematic diagram of the SCOT process is provided in Figure 2. 

Hydraulic turbines are used for recovering energy from high-pressure liquid streams. A common process application is an absorber-stripper combination. In this apphcation, a gas is absorbed in a solvent at a high pressure, where absorption is favored. Then, the solvent is stripped of the absorbed components at a low pressure, where stripping is favored, to recover the solvent. Thus, the energy of the high-pressure solvent stream from an absorber can be partially recovered by a hydraulic turbine. There are three types of hydraulic turbines, the Pelton-wheel turbine, the Francis turbine, and the propeller reaction turbine, an axial type turbine. The propeller reaction turbine is used in hydroelectric applications and will not be considered further. The Pelton-wheel and Francis tur-... 

Figure 8-9 shows an absorber-stripper combination. Note how both the liquid from the bottom of the absorber and the stripper overhead vapor... 

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. 

Process boundaries and external disturbances. Several process units, such as the desulphurization with an absorber-stripper combination and the separation section, consisting of multiple distillation colunms, have been omitted, since they operate more or less independently. 

The third key section of the process deals with ethylene oxide purification. In this section of the process, a variety of column sequences have been practiced. The scheme shown in Figure 2 is typical. The ethylene oxide-rich water streams from both the main and purge absorbers are combined, and after heat exchange are fed to the top section of a desorber where the absorbate is steam stripped. The lean water from the lower section of the desorber is virtually free of oxide, and is recirculated to the main and purge absorbers. The concentrated ethylene oxide vapor overhead is fed to the ensuing stripper for further purification. If the desorber is operated under vacuum, a compressor is required. 

When the feed is desulfurized, the Hydrofining unit is commonly integrated with the Powerformer to conserve heat. The stripper on the hydrofiner product which removes HjS is combined with the Powerformer absorber used to recover C4+ from the tail gas. This tower is termed an absorber stripper. Powerformer tail gas is used to strip HjS from the hydrofiner product. The hydrofiner product in turn serves to absorb the C4 + from this tail gas and return it to the Powerformer. 

The mass transfer equations discussed above are now combined with a material balance on the transferred component to calculate the column or packing height required for a given separation. The column cross-sectional area A is assumed known at this point although in a complete column design A must be determined based on pressure drop considerations. The column, which is in countercurrent flow with only liquid feed and vapor product at the top, and vapor feed and liquid product at the bottom (absorber, stripper, column section), is deflned as follows ... 

The block of inter-linked columns offers robust simulation of a combination of complex distillation columns, as heat-integrated columns, air separation system, absorber/stripper devices, extractive distillation with solvent recycle, fractionator/quench tower, etc. Because sequential solution of inter-linked columns could arise convergence problems, a more robust solution is obtained by the simultaneous solution of the assembly of modelling equations of different columns. 

Equations for absorber, stripper, enricher, and exhauster cascades plus the feed stage are summarized in Table 12.3. The equations are readily combined to obtain product distribution equations for the types of separators in Fig. 12.24. For a fractionator, we combine (12-98), (12-100), and (12-103) to eliminate lp and Vp, noting that r +i = vp and lu+i = If- The result is... 

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 Absorption factor or Stripping factor chart, as it is sometimes known, is shown in Fig. 50.5. It provides simple calculation methods for hydrocarbons when considering either an Absorber tower or a Stripper tower in hydrocarbon service, but for combination Absorber-Stripper towers the calculation procedures become iterative and a lot more complex. So for a combination Absorber-Stripper tower it is best not to attempt to use this chart. It is actually better to resort to a computer simulation. In fact, as a point of historical interest, Norman tells me that it was when the idea of combined Absorber-Stripper towers was first invented that brought about the use of computer simulations so as to handle all the intricate and cumbersome calculations needed to design them. 

The UCBSRP process is claimed to have significantly lower capital and operating costs than the combination of an ethanolamine absorber/stripper unit plus a Claus plant plus a SCOT tail gas unit. Most of the energy consumed by the process is connected with the recovery and fractionation of propane and heavier hydrocarbons. Approximately 92% of the electrical power usage and 89% of the cooling requirements are associated with hydrocarbon recovery and separation (Sciamanna et al., 1988). The heat generated by the sulfur furnace more than offsets the heat demand required by the desulfurization of the natural gas stream. 

Distillation Columns. Distillation is by far the most common separation technique in the chemical process industries. Tray and packed columns are employed as strippers, absorbers, and their combinations in a wide range of diverse appHcations. Although the components to be separated and distillation equipment may be different, the mathematical model of the material and energy balances and of the vapor—Hquid equiUbria are similar and equally appHcable to all distillation operations. Computation of multicomponent systems are extremely complex. Computers, right from their eadiest avadabihties, have been used for making plate-to-plate calculations. 

The acid-rich potassium carbonate solution from the bottom of the absorber is flashed to a flash drum, where much of the acid gas is removed. The solution then proceeds to the stripping column, which operates at approximately 245 °F and near-atmospheric pressure. The low pressure, combined with a small amount of heat input, drives off the remaining acid gases. The lean potassium carbonate from the stripper is pumped back to the absorber. The lean solution may or may not be cooled slightly before entering the absorber. The heat of reaction from the absorption of the acid gases causes a slight temperature rise in the absorber. 

An example given in Fig. 9.1 illustrates this combination of two processes. In an absorber, one or several gas components are absorbed by a lean solvent, either physically or chemically. A rich solvent, after preheating in heat exchangers Hi and H3, is transported to the top of a desorption unit which usually operates under a pressure lower than in absorber. Part of the gas absorbed by the rich solvent is desorbed due to flashing and heating. The other part has to be desorbed in the stripper with the... 

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