Technology

The ElectroSep™ Process

Electrodialysis is a separation process in which ion permeable membranes (sometimes called ion selective or ion exchange membranes) are placed in an electric field to facilitate the removal of substances that ionize in solution. These semi-permeable membranes contain electrically charged functional sites chosen such that they are selective and allow the passage of either anions or cations, but not both. By correct sequencing, anions and/or cations can be extracted from one solution into another. This process has been adapted to remove heat stable anions from amine solutions. The process does not remove non-ionic impurities from amines, such as HEP and THEED.

Two variations of this process have been developed. ElectroSep’s™ original process (i.e., TL2™) removed both aions and cations from the treated amine solution. In a modified process developed by ElectroSep™ (i.e., TL3™) only anions are removed from the amine solution. The use of one or the other process depends on the nature and type of salt incursion in the amine process.

In the TL2™ process salts (anions and cations) are separated from the amine solution and concentrated in an aqueous "brine" stream for disposal. Anion and cation permeable membranes, separated by specially designed spacers, are assembled in alternating fashion between anode and cathode end plates and operated in a "sheet flow" arrangement as shown in Fig 1. The spacers serve to promote good flow distribution between the membranes and direct the amine and brine solutions to the appropriate channels. The membranes are sequenced such that when the amine solution enters the channel between an anion and cation permeable membrane the anions move towards the anode through the anion permeable membrane (A) and the cations move towards the cathode through the cation permeable membrane (B). On the other side of both membranes an aqueous brine (waste) solution flows and the ions are collected and swept out of the system. The ions are prevented from further migrating out of the brine stream towards their respective electrodes by the alternating sequence of ion exchange membranes: the anion on passing through the anion permeable membrane into the brine stream is prevented from migrating further (into a solvent channel) since the next membrane encountered is a cation permeable membrane, which will not allow the passage of the anion. Similarly the cation migrating through the opposite side of the solvent channel will next encounter an anion permeable membrane. In Fig. 1 caustic is assumed to be the base utilized to neutralize the amine solution.

 Fig. 1 - Principles of UCARSEP Electrodialysis Proces 

Fig. 1 - Principles of ElectroSep™ TL2™ Electrodialysis Process

The TL2™ (Fig. 2) process can be operated in two distinct configurations. In the first, the amine charge can be neutralized as the HSAS build-up over time. Once the level of neutralized salts reach a desired maximum level the salt are removed with the electrodialysis unit. This method is generally utilized at sites were there is no permanent electrodialysis unit. In these case the ElectroSep™ mobile unit (Fig. 3) is used to remove the salts. As the capacity of this unit is far higher than the rate of accumulation of salt the system salt concentration can be rapidly reduced. The amine can also be neutralilzed at the inlet of the ElectroSep™ unit, if additional neutralization is required.

 Fig. 2 - UCARSEP Process Flow Arrangement 

Fig. 2 - ElectroSep™ TL2™ Process Flow Arrangement

  Fig 3. – UCARSEP Mobile Electrodialysis Unit  \

Fig 3. – ElectroSep™ TL2™ Mobile Electrodialysis Unit

Electrodialysis based salt removal process has a number of advantages over competing salt removal processes. These are:

  • The process is continuous, thus easier to control.
  • There is no dilution of the process stream being treated.
  • Waste volume is minimized, and can in most cases be sent without further treatment to the facilities wastewater treatment system
  • Neutralization requirements are essentially stoichiometric. No other chemicals are required.
  • There is no impact on the operation of the host amine or glycol unit. No operating adjustment have to be made will the electrodialysis unit is in operation nor during start-up or shut-down.
  • The process is inherently safe. In over 20 years of operating experience there have been no safety incidents or chemical releases.

The ElectroSep Process

In situations were no cations are to be removed the ElectroSep™ process (Fig. 1) can be utilized. In this process base is added directly to the electrodialysis stack. Base anions are transferred to the amine solution by passing through a membrane. Base cations are transferred directly to the waste stream through a distinct membrane. No cations are added or removed from the amine stream. As there is no neutralization of the amine this process is only practical for on site permanent units. This process has the advantage of lower amine losses. 

Amine losses in this process are essentially negligible, making it especially attractive for permanent units having large salt removal requirements.

      

Amine Neutralization
When the salts accumulating in an amine system are the amine salt, that is a HSAS, resulting from the incursion of acids into the system rather than the incursion of inorganic salts, such as KCl, it has been shown that they can be effectively controlled by the addition of a stronger base such as NaOH. The addition of the stronger base raises the pH of the system and converts the amine salt to the inorganic salt, with the benefit of deprotonating the amine and making it available for acid gas removal purposes again. The overall effect is shown in equation 3:

R3NH + + OH- <-------> R 3N + H2O (3)

The result is the same as if the acid had entered the system as the inorganic salt and therefore, if the level of the "neutralized" salt builds to a sufficiently high level, it can still be detrimental, but obviously much less so than the equivalent HSAS. Depending upon the relative rates of incursion and amine loss from the system it may well turn out that the equilibrium inorganic salt level in the system is acceptable and no further remedial action is required, other than regular solution monitoring and the addition of strong base as necessary. However, if the equilibrium level is high, action is required to control the level salts. Two basic options are available: "purge and makeup", with the purged amine being either reclaimed off-site or disposed of, or amine reclamation such as electrodialysis.

Control of Heat Stable Amine Salt (HSAS) anions is an ongoing issue with amine plants that have low inventory losses. To avoid any confusion we define HSAS content of the amine solution as the amount of amine that is needed to neutralize the non-heat regenerable acids present in the amine solution. When stronger bases, e.g. NaOH, are added to the amine solution, the amount of amine required to neutralize the non-heat regenerable acids is reduced.

The concept of neutralizing an amine solution with a strong base has been practiced for over 25 years. Neutralization provides several benefits. First the percentage of amine available in the solution for acid gas removal is increased since the non-heat regenerable acid content has been neutralized by another base. A second benefit is the risk of corrosion does not increase as the level of most of the usually encountered anions increases; an exception is chloride. Third, the amount of contaminant lost per unit volume of solution by natural routes, liquid loss, increases. Occasionally this equals the rate of ingress/formation and no further remedial action other than addition of strong base is required.

Many of the reported problems with neutralization, stress cracked flow lines, flow lines plugged, were most likely due to how the concept was put into practice rather than the concept itself. Proper design of the neutralizing process is essential as it is more difficult than “just adding a shovel full or two of flake sodium hydroxide to the sump”. Briefly, slow addition of suitably dilute base to a flowing stream with good mixing is all that is needed. Most plants can implement a neutralization program at minimal cost with a minimum amount of engineering provided by either plant or vendor personnel.

Additonal neutralization can take place when the ElectroSep™ unit is on site. In this case base is added to the amine bleed stream that is fed to the ElectroSep™ unit, which is equipped with a caustic metering pump and in line mixer for this purpose.