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Apr-2012

Using physical solvent in multiple applications

A physical solvent’s properties and versatility in a variety of flow schemes for energy-efficient gas treating in a range of applications are discussed

Jack Mcjannett
Dow Chemical Canada
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Article Summary
Selexol solvent has been successfully used for many different applications, including acid gas (CO2, H2S, COS and mercaptans) removal from gas streams and hydrocarbon dewpoint control. This article describes the solvent’s properties and versatility in a variety of flow schemes, providing energy-efficient gas treating in a range of applications: ammonia, gasification, natural gas treating (CO2/H2S and organic sulphur) and hydrocarbon dew point control. Design guidelines and performance limitations are discussed. 

Most publications on Selexol solvent are focused on syngas treating from gasification. This article provides design and operations information for applications not typically found in earlier studies.

Selexol is sold by The Dow Chemical Company and its affiliates. It is a dimethyl ether of polyethyelene glycol of the general formula [CH3 - O - (CH2 - CH2 - O)n - CH3], where n ranges from 3 to 10. The solvent was developed by Allied Signal, and the name and technology have been owned by Norton, Union Carbide and, most recently, Dow. Its primary use is to remove acid gases and other contaminants from various gas streams by physical absorption, not chemical reaction typical of amine technology. The major benefit of this process is that the energy required to regenerate a physical solvent such as Selexol is significantly less than the energy needed to regenerate a chemical (amine) solvent. In addition, since the acid gas removal capacity of Selexol is only dependent on physical absorption, it is possible to achieve much higher amounts of acid gas absorbed in the solvent, as compared to amines.

As Table 1 shows, all gases are soluble to some extent in Selexol, but some gases are much more soluble than others. This difference can be exploited and makes the process very efficient for certain situations. For example, if there is a gas stream containing 40 mole % H2, 20 mole % CO and 40 mole % CO2, the difference between the 
solubility of H2 and CO compared to that of CO2 is quite large. As a result, Selexol is often used to remove CO2 from gas streams rich in H2 and CO (gasification, for example). Another example is that mercaptans are highly soluble in the solvent, relative to methane. This makes it a good choice for sweetening molecular sieve regeneration gas, where the molecular sieve was used to remove mercaptans initially from either a gas stream or a liquid hydrocarbon stream.

Physical solvents and their 
effectiveness are highly dependent on the partial pressures of the components to be removed. As a result, physical solvent systems typically operate best at higher pressures. In addition, most applications benefit from their use at lower temperatures. Typical pressures and temperatures that the contactors might operate at are >34 bara (493 psia) and <5°C (41°F). For removal of highly soluble compounds such as mercaptans, these systems might operate as high as 50°C (122°F) or at lower pressures.

Selexol properties
Selexol when manufactured has a pH of about 7.0 (neutral). It has no buffering capacity and will operate in the acid range with a pH as low as 4.0. It takes very little acid in the system (unregenerated acid gases such as CO2 or H2S or acids such as formic acid) to produce a pH of less than 7. Hence, proper selection of metallurgy for each piece of equipment is important. For example, the contactors are typically carbon steel. They usually operate at low temperature, so acid attack on the metal is minimised. If one needs to run a thermal regenerator, it is often suggested this unit be stainless clad to minimise corrosion.  Piping is nearly always carbon steel.

The solvent has a low viscosity (~7 cp at 20°C (68°F)). The viscosity increases gradually as the solution temperature is lowered, and also increases slightly with the addition of water. If a system has to operate with water in it, for a thermal regenerator for example, the water content is typically kept to <5 wt%.

The solvent also has a very low vapour pressure, which means that loss due to vapourisation is very low, especially from units operating at low temperature.

From a materials compatibility perspective, Selexol is similar to higher molecular weight glycol ethers. Hence, the choice of elastomers for seals, gaskets and so on requires care. Some elastomers will dissolve quickly in service. Proper selection of these materials is very important. In addition, the solvent is a very good degreaser, so pump and pipe joint specifications should minimise utilisation of lubricating oils. Also, it can be an excellent paint remover for certain types of coatings.

It is a very forgiving solvent in terms of thermal degradation. It is quite stable and is typically regenerated at temperatures around 150°C (302°F). It does not react with gases such as CO2 or CO. In contrast, amines can react with these gases to form degradation products that can be corrosive. The solvent will react with O2 and form weak acids such as formic acid, which, if thermal regeneration is used, can create the potential for corrosion. If this is anticipated, the corrosion issue is typically handled by specifying proper metallurgy.

Simulation capability
In order to design a facility using the solvent, it is necessary to have a simulation model to predict the optimal conditions necessary to achieve the goal of the treating facility. Dow (and previously Union Carbide) has continuously invested in upgraded simulation capabilities, whose results, when compared to actual results from operating units, are quite accurate.

Dow has undertaken limited study of other simulators that advertise the ability to simulate dimethyl ether of polyethylene glycol (DMEPEG). For the simulators studied, the results (depending on gas compositions, pressure, temperatures and so on) varied significantly from what Dow’s simulator would predict. This conclusion has been supported by comments from engineering companies, especially when predicting treated gas compositions for methane, carbon monoxide and hydrogen.

Guidelines on performance limitations
The general guideline for the removal of CO2 using Selexol is that the partial pressure of CO2 in the inlet gas must be >70 psia (4.8 bara) for the process to be competitive with an amine process. For CO2 removal, normally a specification of about 3% in the processed gas can be achieved with flashing to atmospheric pressure for regeneration.
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