Acid Gas Dehydration

As has been stated earlier, dehydration is not commonly used for acid gas. However, a couple of processes have been discussed in the literature. They are also discussed here. 

---

When compression and cooling alone is inefficient to achieve the desired level of dehydration, then some additional dehydration is required in order to reduce the water content to the desired level. The commonly employed dehydration methods are applicable to acid gas dehydration, but there are several important considerations that apply when dehydrating a stream like this. 

The pressure of the rich glycol stream is dropped to near atmospheric and is sent to the regenerator system. Heat is added to the bottom of the column, and the water is driven out of solution to produce the lean glycol stream, which is returned to the contactor. The water exits as the offgas from the top of the regenerator still. In the case of an acid gas dehydration unit, this offgas steam contains H2S and C02 for reasons that will be discussed later, and cannot be vented to the atmosphere. In an acid gas dehydration... 

Table 7.5 Operating data for an acid gas dehydration unit from HO et al. (1996).

The UCB collection and refining technology (owned by BP Chemicals (122,153—155)) also depends on partial condensation of maleic anhydride and scmbbing with water to recover the maleic anhydride present in the reaction off-gas. The UCB process departs significantly from the Scientific Design process when the maleic acid is dehydrated to maleic anhydride. In the UCB process the water in the maleic acid solution is evaporated to concentrate the acid solution. The concentrated acid solution and condensed cmde maleic anhydride is converted to maleic anhydride by a thermal process in a specially designed reactor. The resulting cmde maleic anhydride is then purified by distillation. 

Sulfur Compounds. Various gas streams are treated by molecular sieves to remove sulfur contaminants. In the desulfurization of wellhead natural gas, the unit is designed to remove sulfur compounds selectively, but not carbon dioxide, which would occur in Hquid scmbbing processes. Molecular sieve treatment offers advantages over Hquid scmbbing processes in reduced equipment size because the acid gas load is smaller in production economics because there is no gas shrinkage (leaving CO2 in the residue gas) and in the fact that the gas is also fliUy dehydrated, alleviating the need for downstream dehydration. 

Dehydration to dew points below the temperature to which the gas will be subjected will prevent hydrate formation and corrosion from condensed water. The latter consideration is especially important in gas streams containing CO2 or H2S where the acid gas components will form an acid with the condensed water. 

In practice, one proceeds as follows. The value of bh >s determined for the reaction with a series of acids of similar structure, that is, for carboxylic acids or ammonium ions, etc. Limiting the data to a single catalyst type improves the fit. since the inclusion of data for a second ype of acid catalyst might define a close but not identical line. This means that Ga may be somewhat different for each catalyst type. A plot of log(kBH/p) versus log(A BH(7//i) is then constructed. This procedure most often results in a straight line, within the usual —10-15 percent precision found for LFERs. One straightforward example is provided by the acid-catalyzed dehydration of acetaldehyde hydrate,... 

Aluminum fluoride is produced when partially dehydrated alumina hydrate reacts with hydrofluoric acids gas. The solid aluminum fluoride produced is cooled with noncontact cooling water prior to further processing, while the gases from the reactor are scrubbed with water to remove unreacted hydrofluoric acid from the gas stream. Aluminum fluoride is mainly used in the production of... 

During gas-liquid chromatographic-mass spectrometric analysis of the acid-catalyzed dehydration reaction of D-mannitol, 1,4 3,6-dianhydro-2-chloro-2-deoxy-D-mannitol was found among the reaction products.35 Contrary to the postulated Sn2 mechanism, according to which 2,5-di-endo oriented leaving-groups are substituted by different nucleophiles, resulting... 

Grouped according to duty the typical elements of a base-load plant are as shown in Figure 3 reception, acid gas removal, C02 cleaning, dehydration and mercury removal, liquefaction and fractionation, nitrogen removal, LNG storage and loading system for shipment as shown in [13,10],... 

Alcohols can undergo acid-catalyzed dehydration to give either the corresponding alkenes or the corresponding ethers. The product distribution of the dehydration of alcohols over Nafion-H catalyst shows temperature dependence187 (Table 5.40). Alcohols are thus efficiently dehydrated in the gas phase over Nafion-H under relatively mild conditions with no evidence for any side reactions such as dehydrogenation or decomposition. At higher temperature, olefin formation predominates. 

Gas Dehydration. It has been found that water vapor permeates cellulose acetate membranes at a rate approximately 500 times that of methane (Ref. 2). This exceptionally high selectivity for water vapor make cellulose acetate membrane systems attractive for dehydration of hydrocarbon gas streams to pipeline specifications on either a pure gas stream or while simultaneously removing contaminating acid gases. For these applications the small size, low weight and low maintenance of the SEPAREX system is particularly advantageous for offshore installations. 

Most of the energy is consumed in the distillation section, namely for VAM recovery and purification. The reboiler duty for the azeotropic distillation of VAM is particularly high, of about 30 MW. It can be observed that this is due to the large recycle of VAM necessary to carry out the water formed by reaction (3 mole VAM per mol water). Thus, any measure is welcome that can reduce the water content in the crude VAM/acetic acid mixture. Figure 10.8 shows an ingenious method known as gas dehydration [1]. The reactor outlet, cooled up to the dew point,... 

Most industrial acid plants have three flows of sulfuric acid - one gas-dehydration flow and two H2S04-making flows. These flows are connected through automatic control... 

Table 6.2. Equilibrium H20(g) pressures over sulfuric acid at temperatures around those of industrial acid plant gas dehydration (Perry and Green, 1997). The pressures are translated into volume% H20(g) in gas and milligrams H20(g) per Nm3 of gas. An industrial dehydration target of 50 milligrams H20(g) per Nm3 of gas is chosen to avoid downstream corrosion.

Strong acid, -98.5 mass% H2S04, is an excellent choice for air and gas dehydration. It has a low equilibrium H20(g) vapor pressure. It removes H20(g) from air/gas very efficiently. 

Table 6.5. Details of zinc roaster and spent acid regeneration gas dehydration plants.

S02 oxidation requires 02. Sulfur burning furnace offgas already contains 02, Table 7.1. None needs to be added. Metallurgical and waste acid decomposition furnace offgases often contain little 02, so some must be added before catalytic S02 oxidation. It is added in air or ventilation gas before gas dehydration. 

Industrial H2S04 making is similar to the air/gas dehydration process described in Chapter 6. In both cases, gas is passed upwards through descending strong sulfuric acid, Figs. 6.3 and 9.1. The difference is that ... 

Gas dehydration dilutes the input acid while H2S04 making strengthens it. Both reactions are exothermic. Both heat up their circulating acid so that it must be cooled before being recycled or sent to storage. 

Air/gas dehydration (Chapter 6) produces fine spray of 10-250 pm diameter acid droplets. The droplets are removed from the dehydrated gas to prevent downstream corrosion. They are usually captured in a knitted Teflon /stainless steel pad at the gas exit, Fig. 6.3. 

Fig. 23.1. Simplified single contact sulfuric acid production flowsheet. Its inputs are moist feed gas and water. Its outputs are 98 mass% H2S04, 2 mass% H20 sulfuric acid and dilute S02, 02, N2 gas. The acid output combines gas dehydration tower acid, H2S04 making tower acid and liquid water. The equivalent sulfur burning acid plant sends moist air (rather than moist feed gas) to dehydration. Appendix V gives an example sulfur burning calculation.

Both a pump and a dehydration unit are required in only a few cases. A pump may be required if the injection pressure cannot be achieved by compression alone. As will be demonstrated in chapter 6, it is often possible to dehydrate the acid gas using compression and cooling alone. 

---