Amine Circulation Rates

The circulation rates for amine systems can be determined from the acid gas flow rates by selecting a solution concentration and an acid gas loading. 

MF = total acid-gas fraction in inlet gas, moles acid gas/mole inlet gas 

For design, the following solution strengths and loadings are recommended to provide an effective system without an excess of corrosion  

Using these design limits, Equations 7-24 and 7-25 can be simplified to  

The circulation rate determined with these equations should be increased 

---

For most field gas units it is not necessary to specify a stripper size. Vendors have standard design amine circulation packages for a given amine circulation rate, acid-gas loading, and reboiler. These concepts can be used in a preliminary check of the vendor s design. However, lor detailed design and specification of large units, a process simulation computer model should be used. 

The large stripping heat supplied was insufScient to adequately strip H2S fiom a rich amine solution due to excessive amine circulation rate. An eightfold cut in circulation permitted an eightfold cut in stripping heat si-miiltaneaus wiHi a m or improvement... 

Acid gas loading of Amine is a function primarily based on two variables such as amine circulation rate and the contact time. As the inlet raw gas feed composition remains constant, total volume of acid gas picked up by the amine can easily be reduced by changing amine circulation rate. Whereas any amount of reduction in amine circulation rate has a direct negative impact on the treated gas H2S content and hence reducing amine circulation rate has been ruled out. 

Your regenerator reboiler duty should be controlled on the basis of pounds of steam per gallon of lean amine circulated (Ib/gas) proceed by cutting reboiler steam 0.1 Ib/gal. Note Scrubber sweet gas purity is controlled by the regenerator stripping steam rate, not by the amine circulation rate to the scrubber. 

Amine with a high concentration of absorbed H2S is more difficult to separate from FCCU Cj and Qs than leaner amine. This means that reducing amine circulation may promote amine carry-over. The reason for this is unknown to the author. However, many times amine carry-over has been stopped by increasing amine circulation rates. 

The first indication that the two phases are starting to separate more easily is a drop in the interface level and a loss in amine flow from the bottom of the column. Unfortunately, many operators misinterpret this initial response as an increase in amine carry-over and cut back on the lean amine circulation rate. 

The simplified design procedure is based on the approach to equilibrium method (described earlier) to determine the amine circulation rate, plus a series of enthalpy balances to determine temperatures and heat duties. The calculation procedure involves four steps ... 

If Tit calculated by equation 2-38 is less than the assumed in Step 6, the lean amine circulation rate can be reduced. Assume a new between Tg calculated and assumed, and repeat Steps 6 through 12 until a match is obtained. 

L = low amine circulation rate (I pump), I = intermediate amine circulation rate (2pumps), H = high amine circulation rate (3 pumps). Actual flow rates not reported... 

Primary alkanolamine solutions require a relatively high heat of regeneration. Also excessive temperatures or localized overheating in reboilers cause the MEA to decompose and form corrosive compounds. An inhibitor system, such as the Amine Guard system developed by Union Carbide, is an effective method of corrosion control (52). Inhibitors permit the use of higher (25—35%) concentration MEA solutions, thus allowing lower circulation rates and subsequendy lower regeneration duty. 

DGA systems typically circulate a solution of 50-70% DGA by weight in water. At these solution strengths and a loading of up to 0.3 mole of acid gas per mole of DGA, corrosion in DGA systems is slightly less than in MEA systems, and the advantages of a DGA system are that the low vapor pressure decreases amine losses, and the high solution strength decreases circulation rates and heat required. 

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. 

A corrosion inhibitor is used with this process. With the Amine Guard inhibitor, the MEA concentration in the circulating solution can be increased to 30% from a normal 20%. Hence, the circulating rate can be decreased by 33% and the heat requirements are decreased by 43%. 

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. 

The enhancement of ozone injury in animals by activity during exposure to ozone has been the most striking demonstration of all. The lethal outcome of otherwise noninjurious concentrations of ozone results undoubtedly from a multiplicity of factors in addition to that of the more obvious reactions associated with activity such as increased respiration and circulation rates. Hormonal releases, for example, in the form of catechol amines, epinephrine, and norepinephrine, as well as in certain adrenocorticosteroids (compounds B and F) have been reported in tumbled rats (9), a condition not completely unrelated to that in cage-activated rats, especially during the terminal phases on the exposure. How much of a role these hormones play in these experiments, however, still remains to be determined. 

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. 

Traditionally, acid gas treatment has required two separate processes. One is to remove hydrogen sulfide and the other is to convert the concentrated hydrogen sulfide stream to sulfur. A new process called the SulFerox process, developed by Shell Oil Company and The Dow Chemical Company, now offers a single process that handles both steps. The process has high degree of flexibility and requires a smaller capital investment than the Amine/Claus process. Chelated iron compounds are the heart of the process and the chemistry allows an aqueous solution of iron in high concentrations. As a result, circulation rates are low and equipment size is very small for the capacity. Figure 5.1 shows a schematic of the SulFerox process [76]. 

Amine carry-over from such columns is common. Field observations have shown that the best way to stop the carry-over is to increase the rate of amine circulation. 

After the lean amine temperature and lean solution loading are determined, the designer should use the following steps to determine the required circulation rate. Refer to Figure 2-93 for information on the amine contactor calculation envelope and to Table 2-20 for a definition of the variables used in these calculations. 

Operating data for an aqueous DEA plant in high pressure natural gas service have been presented by Berthier (1959), and are summarized in Table 2-24 as plant A. The data were obtained in the early phases of development of the S.N.P.A.-DEA process at Lacq and are not quite representative of the current process which uses substantially lower solution circulation rates (see Table 2-2). Vaz et al. (1981) used the Berthier data as a check against a calculation algorithm called Amine Process Model (APM). Two of the calculated values are included in Table 2-24 to provide a more complete data summary. Many other operating data values were also calculated by the model, and in general, the calculated results agreed very weU with the actual data. 

An economic comparison of the Flexsotb HP Process with a conventional amine-promoted hot potassium carbonate process has been made by Goldstein et al. (1984). The basis of comparison was a 20 MM scfd hydrogen plant with a 250 psia absorber. Assumed gas compositions were 19.9 mole % CO2 for the feed and 0.2 mole % CO2 for the product. The calculated results showed a 19% higher solution circulation rate and 11% higher reboUer duty for the conventional amine-promoted hot potassium carbonate design. 

Preliminary information useful in prodrug design has been obtained with amino acids attached to model aromatic amines. Thus, N-(naphthalen-2-yl) amides of amino acids (6.1, R=side chain of amino acid, R =H) proved to be of interest as test compounds to monitor peptidase activity such as ami-nopeptidase M (membrane alanyl aminopeptidase, microsomal aminopepti-dase, EC 3.4.11.2) [16][17], In the presence of purified rabbit kidney aminopeptidase M or human cerebrospinal fluid (CSF) aminopeptidase activity, the rate of hydrolysis decreased in the order Ala-> Leu->Arg->Glu-2-naphthyl-amide. Ala-2-naphthylamide, in particular, proved to be a good test compound, as its rate of hydrolysis was influenced by experimental conditions (preparation, inhibitors, etc.), as was the hydrolysis of a number of low-molecular-weight opioid peptides and circulating vasoactive peptides. 

These are the agents which block the action of sympathetic nerve stimulation and circulating sympathomimetic amines on the beta adrenergic receptors. At the cellular level, they inhibit the activity of the membrane cAMP. The main effect is to reduce cardiac activity by diminishing (3 receptor stimulation in the heart. This decreases the rate and force of myocardial contraction of the heart, and decreases the rate of conduction of impulses through the conduction system. They are classified as in table 3.3.2. 

---