Jul-2020

Amine cleaning in gas treating

Effective amine cleaning requires a three-step filtration system

JAN HERMANS
Sulphurnet

Article Summary

Gas treating or sweetening is a term used to describe the various processes for removal of certain contaminants, primarily hydrogen sulphide (H2S) and carbon dioxide (CO2) from natural gas or hydrocarbon liquids. CO2 and H2S are also named ‘acid gases’ because when absorbed in water they form an acidic solution. Reasons to remove these contaminants include toxicity, corrosiveness, freezing problems, and to increase the overall heating value of the natural gas. As a consequence, amine absorption and regeneration equipment are exposed to corrosive and oxidising conditions. This has to be taken into account with the design of the equipment (see Figure 1).

Alkanolamine solvents are widely used to remove CO2 and H2S  from gas and lighter hydrocarbon products. Amine has a natural affinity for both CO2 and H2S, allowing this to be a very efficient and effective removal process. Crude oil with low sulphur content is becoming less prevalent and is costly on the market while on the other side environmental regulations concerning H2S and CO2 content in light hydrocarbon products (fuels) are becoming more stringent. Many processes for the removal of acid gases have been employed commercially and various amines are used, each of the amines offering distinct advantages to specific treating problems.
Depending on the required selectivity, CO2 or H2S  removal, various solutions of solvent can be used.

The most popular ones are MEA/DGA, MDEA/DIPA, and DEA. This article describes a three-step system for amine filtration.
In Figure 1:
-    Sour gas passes through an inlet separator and/or a gas-liquid filter/coalescer to  
-    Remove the majority of the hydrocarbons and solids
-    Sour gas flows through the absorber tower and rises through the descending amine
-    Purified gas flows from the top of the tower
-    The amine solution is now considered rich and is carrying absorbed acid gases
-    Rich amine is heated in the regeneration column. The steam rising through the stripper column regenerates the amine
-    Steam and acid gases separated from the rich amine are condensed and cooled
-    The condensed water is separated in the reflux accumulator and returned to the still

The installation and operation of an appropriate filtration system is one of the key components of all amine systems. The cleaner the amine, the better the amine system operates. Filtration has proven to be the most effective and only protection system for the removal of contaminants such as solids, liquid hydrocarbons, and heat stable salts. Removal of contaminants provides the following benefits:
-    Prevention of amine foaming
-    Reduced corrosion problems
-    Reduced fouling problems in the process
-    Increased lifetime of the carbon bed
-    Reduced amine consumption
-    Maintaining amine efficiency and plant capacity

If these filtration systems are not designed and operated correctly, then the amine units regularly suffer from several inefficiency problems, such as high operating costs, lean separation efficiency of H2S/CO2, amine losses, and excessive maintenance costs.

Sulphurnet’s approach to amine filtration
Adequate amine cleaning includes a three-step filtration system. It starts with a gas inlet filter, followed by a rich amine filtration system. Finally, a mechanical precoat type filter, in combination with an activated carbon filter and a particle filter, is installed in the lean amine side of the process.

Gas inlet filters
The first filter section is the gas inlet filter. Here the liquid hydrocarbons and solids that can upset plant operations are removed. Knockout drums, mist eliminators, filter vanes, or coalescing filters can be used for this process. A combination of various techniques in series is required to provide optimum removal of all levels of particles and liquids.

Rich amine stream
When the amine system is extremely contaminated, filtration of the rich amine may be required. This prevents the heat exchanger from plugging, along with the problem that iron sulphide in the rich amine can dissociate in the regenerator under certain conditions to form soluble iron compounds. Due to its reactivity, iron sulphide cannot be removed during filtration of the lean amine.

Due to the concentration of H2S in the system, self-cleaning filtration systems are preferred. Self-cleaning strainers are suitable for filtration in the range up to 30 microns. For finer filtration efficiency, and in order to prevent filter cloth from clogging, the concept of precoat filtration is used. Filter elements are protected by depositing a layer of precoat material on their surface, prior to filtration. In order to obtain a layer of precoat material, liquid with precoat material is introduced into the filter vessel, filtered and returned to the precoat tank. The precoat solution is prepared by uniformly mixing part of the amine solution with the precoat material. This can be cellulose or diatomaceous earth. Proper selection of the precoat media in combination with the filter cloth is a key factor for the successful operation and performance of the filtration unit.

Lean amine stream
In the lean amine stream, a combination of separation equipment is required. Mostly 10-20% of the stream is filtered. This is sufficient to obtain a stable amine system and control the amount of solids in the plant.
Here we have a sequence of three systems: a particle filter in combination with an activated carbon filter, and finally a polishing filter.

Particle filter
The particle filter (see Figure 2) is installed to remove fines at the inlet side of the carbon filter, preventing particles from blocking the activated carbon. The particle filter in the lean amine stream is a self-cleaning candle filter. This filter consists of a vertical cylindrical vessel with a conical bottom. The filter is filled with vertically positioned filter candles which are installed in horizontal collecting manifolds. The outside of the candle has supporting rods; inside, a central filtrate discharge pipe. The filter candles are covered with a filter cloth carefully selected for the application. Once the filtrate has passed through the filter cloth, it flows into the support candle down to the bottom of the candle, before rising again in the central filtrate discharge pipe toward the registers. The filter cake is formed on the outside of the filter cloth and over time the pressure differential slowly increases.

When the filtration cycle is completed, cake discharge is initiated by introducing gas in the reverse direction to the inner part of the candle. The quick-pulse gas blow-back lifts the filter cloth from the support candle and the filter cake is released, causing it to fall through the bottom discharge valve (see Figure 3). Filtration requirements depend on the micron size and quantity of the particulates. Generally, 5- to 10-micron filtration efficiency is suitable for particulate removal.

Activated carbon filters
Since the rich amine solution is heavily loaded with acid gas, it can form gas pockets in the carbon filter, resulting in a reduced or completely blocked flow. In terms of safety for the workers who dismantle, inspect, and clean out the filters, it is safer to install the carbon filters in the lean amine stream. Carbon filtration removes surface-active contaminants, hydrocarbon corrosion inhibitors, amine degradation products, and oils and reduces the tendency of the amine solution in the stripper to foam.