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Advanced chemical process for treating sour gas

01.01.2011  |  Ortega Peralta, C. A.,  Neuga SA, ArgentinaOrtega Castelan, M. J.,  Neuga SA, Argentina

Technology avoids huge capital investments while speeding up results

Keywords: [H2S] [gas processing] [LPG] [CO2] [SOx] [mercaptans]

Current gas-sweetening technology requires plants to treat at least 1 million standard cubic meters per day (MMm3/d) of gas to be economically feasible. This article presents a versatile technology that can be profitably applied to any gas flow from 10 thousand standard cubic meters per day (Mm3/d) up to several million with the benefit of reducing setup periods. This technology (Fig. 1) has been proven over five years in a gas-compression plant where sulfur and mercaptans (RSH) contamination unexpectedly rose from 150 ppm to 1,000 ppm. This allowed the company to comply with an existing commercial agreement for 100 Mm3/d.

 
  Fig. 1. The technology and conceptual
  design.      


The problem.

In the proven installation, gas-flow production is collected along 50 km in an 8-in.-diameter pipeline. Gas-flow temperature is generally around 60°F, and pressure is as low as 3 kg/cm2. The contaminants to be scavenged are hydrogen sulfide (H2S), RSH and carbon dioxide (CO2). Plant process is completed by a set of alternative compressors, a distillation plant and dew-point adjustment for high pressure. Afterward, sweetened natural gas is dispatched through a transport pipeline injection (95%) and liquefied petroleum gas (LPG) to trucks (5%). The original sweetening processing plant was designed through seven towers containing a solid chemical scavenger-based in OxFey.

Over time, H2S and CO2 levels strongly rose, and mercaptans appeared as contaminants.

Scavenger products used were specific for H2S acid treatments, since contamination was prevalent in the beginning. However, the scenario changed to have lower yields, requiring using different liquid chemical injections downstream to obtain the required output specification. Operating costs increased to a point where the alternative to interrupt supply needed to be evaluated, despite the reputation impact in the market.

Due to this problem, it was proposed to test the chemical process, changing the “chemical scavenger,” in a pilot plant. Amine technology was quickly discarded since the gas standard flow being treated was heavy in contamination, implying an expensive installation and high operative costs. As a consequence, testing began on a process using a strong alkali that reacts with weak acid contaminants forming a buffer solution, which means the possibility of obtaining a controlled pH in the effluents. Moreover, it is possible to add commercial value to all the salts obtained in the reaction.

The design.

Plant capacity defined what to consider in the selected process and included:
• Caustic recuperation and recycling
• Byproducts concentration
• Both, depending on the commercial alternatives.

A specific study must include a gas-contaminant assessment to define the technical application in function of the treatments to byproducts. In the opposite extreme, it is possible to simply install modular plants just near well-outs. An important advantage is the protection of pipeline transporting only sweet gas.

New process chemistry. Case with H2S (plus or without RSH), as contaminant:
H2S + H2O > SH + (H3O)+ Ka = 4 x 10–7
RSH + Na+ > SHNa + R+
S HNa + (OH) + (H3O)+ + H2O <=> SH + 3 H2O + Na+
pH = 7.1 (buffer), pH control is required according to Henderson-Hasselbach to monitor log ((SH)/(H2S)).

Such control is related to the tower and packed design, drain and make-up of the scavenger, with continual checking of pH values. The solubility of H2S in water is important in industrial practice, especially in these environmental-awareness times. The fate of such an unfriendly component such as H2S is important to track. H2S is highly toxic, has a noxious odor and can form harmful-reaction products [such as sulfur dioxide (SOx)].

The treatment for the solubility in strong bases is the same as that in acids, but the effect is dramatically different. As with the case of strong acids, the amount of the molecular species is dictated by the partial pressure. However, in this case, the reactions are shifted to the right producing more of the ionic sulfide species, thus dramatically increasing the total H2S concentration.

 
  Fig. 2. The solution.      


Case with CO2, as the contaminant.

CO2 + H2O > H2CO3
H2CO3 + H2O <=> H CO3+ (H3O)+
Ka = 6 x 10–7
NaCO3H + H2O <=> H2CO3 + Na+ + (OH)
buffer pH = 9/10
Na2CO3 + 2 H2O <=> H2CO3 + 2Na+ + 2 (OH)
buffer pH = 11/13

CO2 is a gas easily soluble in water and the solubility equilibrium is maintained in agreement with Raoult Law. Part of dissolved CO2 is converted to carbonic acid, which reacts as weak acid forming buffer salts. Both salts obtained have important industrial uses. On the other hand, the sodium carbonate can react, giving back the caustic alkali in agreement with the following:

Na2CO3 + CaO > CaCO3 + NaOH

Case with H2S and CO2, as the contaminant.

In cases where both contaminants are present, plant design must consider that an H2S acid in spite of having similar Ka as H3CO2, is more powerful as a reducer. Thus, H2S is the first priority as a reactive in the presence of a strong caustic like the scavenger.

When the designer defines basic engineering, spacial velocity of sour gas and scavenger flow are dependent on the quantities of acid to be neutralized. In the case presented in Table 1, the tower design has the capability of neutralizing, at first, 30% of the H2S being present. Afterward, it neutralizes up to 70% of H2S and 60% of the total CO2 present in the original sour gas. Designers can define the capability of the strong caustic, while waste solution pH will vary with the byproduct salts present in the overall solution. HP

 




The authors 

 
  Carlos Alberto Ortega Peralta is the executive president at NEUGA SA. He has over 42 years of experience in process design and plant processes. Mr. Ortega Peralta invented the Titular Process.

 
  Maria Jose Ortega Castelán is a manager NEUGA SA and has 10 years of experience in project management. She received a degree in industrial engineering and an MBA. Ms. Ortega Castelán is CFA certified.




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