Comprehensive Filtration of Amine Liquid in Natural Gas Purification Treatment

As a kind of clean energy, natural gas has been widely used, but the natural gas extracted from the formation usually contains toxic and harmful acidic components such as h2s and co2, which need to be purified. 

methyl diethanolamine (MDEA) solution is commonly used in natural gas purification plants to remove acid gas components due to its advantages of selective desulfurization, high acid gas loading, low corrosion, and low degradation rate. 

In the process of removing acidic components from amine solution, the solution is often polluted to different degrees, resulting in poor operation effects. 

SOURCES AND HAZARDS OF AMINE LIQUID POLLUTANTS 

Pollutants in amine liquid come from a wide range of sources, some are impurities carried by natural gas from the bottom of the well, some are corrosion products produced in natural gas gathering, transportation, and purification, and some are organic substances produced by the degradation of amine liquid. 

The solid phase impurities in the amine liquid are mainly corrosion products of equipment pipelines brought in during the natural gas gathering and transportation process and corrosion products of equipment such as desulfurization towers and heat exchangers during the purification process. The main component is FeS. 

Liquid phase impurities mainly come from condensate oil carried by natural gas in gas well production, formation water with high salinity, and impurities such as wellbore corrosion inhibitor, foam discharge agent and methanol injected into gas wells. 

Another class of contaminants is the degradation products of amine liquors. After the amine solution is degraded, thermally stable salts and various organic substances will be produced, and the main degradation methods are thermal degradation, oxygen degradation, and chemical degradation. Among them, oxygen degradation has the greatest impact. 

The MDEA solution has good thermal stability, and thermal degradation has little effect on the amine solution. 

The MDEA solution has good thermal stability, and thermal degradation has little effect on the amine solution. 

Since MDEA is a tertiary amine, there is no active hydrogen atom directly connected to a carbon atom in the molecule, and it does not have the conditions for reacting with carbon dioxide, so the impact of chemical degradation on MDEA amine solution is also relatively small. 

  1. The operation of the device is unstable, the amine liquid pollution will cause the solution to foam, and in severe cases, the absorption tower will block the liquid, the differential pressure will increase, and the liquid level will fluctuate violently, and the stability of the device operation will be greatly reduced. 
  2. To aggravate the corrosion, the Jingbian Gas Field Natural Gas Purification Plant found during the maintenance and operation that the pipelines through which the amine liquid flows were severely corroded and perforated, and the heat exchanger tube bundles frequently leaked.  
  3. If the product does not meet the standard, the pollution of amine liquid will reduce the acid gas load of the solution and the absorption performance, which may lead to the unqualified natural gas of the product. 

COMMONLY USED AMINE CONTAMINATION PREVENTION MEASURES 

  • Strengthen the filtration and separation of raw natural gas, and reduce the impurities carried by the raw gas into the solution system. 
  • Strictly implement the operating procedures and parameter settings, operate finely and smoothly, and improve the storage protection measures of the amine solution to reduce the degradation of the amine solution. 
  • Strengthen the online filtration of amine liquid, and clean the filter in time to improve the filtration effect. 
  • According to the use of the solution, add new solution in time to replace or dilute the contaminated solution. 

COMMONLY USED FILTERING EQUIPMENT AND PROCESSES 

At present, organic wedge wire filter, bag filter, tubular filter cloth filter, and activated carbon filter are commonly used filters in natural gas amine liquid purification systems. 

Mechanical filter, bag filter, and tubular filter cloth filter adopt wedge wire filter element or filter bag installed in the equipment barrel for filtration, which can be regarded as a type of filter element filter equipment. 

The filter element type filtration equipment is to install a certain number of filter elements (bags) in the equipment barrel and use the characteristics of the filter element to filter. 

A mechanical filter is one of the typical equipment, which is mainly composed of the main body, support baffle, fixed rod, and filter element. Its working principle is: when the amine liquid flows through the filter, the particles with a certain particle size in the amine liquid are intercepted by the wedge wire filter element, and the solid particles smaller than the filter element (bag) precision are allowed to pass through, so as to achieve the purpose of purification.  

1 机械过滤器结构图

Generally speaking, the filter element type filter usually has a series of filter elements with different filtration precision to be used together. The higher the filter element precision, the better the filtration effect of the solution, but too high filtration precision will cause the differential pressure to rise rapidly. The amine liquid filtration experience believes that the solid particle size in the amine liquid system is mainly distributed in the range of 5 to 40 mm, and the removal of solid impurities in the amine liquid can effectively reduce the foaming probability of the amine liquid. At present, it is usually used with 10 groans and 5FW1 filter elements. 

When choosing a filter, it is also necessary to consider the processing capacity of the equipment. The application examples of filter element equipment show that wedge-shaped mesh filter and tubular filter cloth filter are suitable for use in occasions where the circulation volume of amine liquid is small, while mechanical filter has the advantages of high filtration efficiency and large processing capacity. It is suitable for occasions where the circulation of amine liquid is large.    

FILTRATION PROCESS OF AMINE LIQUID

In the development of natural gas desulfurization and purification, the filtration of amine liquid has also been continuously updated, and a variety of different filtration modes have been developed one after another. 

Lean liquid filtration In the early amine liquid filtration, the partial filtration mode is generally adopted in the lean liquid system, filtering 10% ~ 15% of the circulating amount of the amine liquid, and the filtration equipment is mostly diatomite pre-coating filter, coke packing filter and plate and frame filter, etc. 

This filtration mode has a certain effect in practical use, but it cannot prevent the impurities carried by the feed gas from entering the flash tower, heat exchanger, regeneration tower, and other equipment through the rich liquid. 

In addition, there are also shortcomings such as a small amount of filtration and a long filtration cycle, which are gradually replaced by other filtration methods, and equipment such as diatomite pre-coated filters are gradually replaced by new filters.

The mixed filtration of the filter element and activated carbon filter is to solve the problem of poor liquid filtration. The purification plant began to adopt the rich amine liquid filtration method, mainly using filter element filtration equipment and an activated carbon filter.  

Mechanical filters are used for full filtration, while activated carbon filters are used for partial filtration with a smaller amount of filtration. 

The filtration process is shown in Figure 4. A filter element filter is installed downstream of the rich liquid flash tower for full filtration of amine liquid, and an activated carbon filter is installed downstream to filter 10% of the circulating volume by partial filtration.  

This filtration mode alleviates the blockage and foaming problems caused by the introduction of solid particles into the heat exchange equipment and regeneration tower, and can also absorb some organic substances. 

However, the activated carbon powder will be washed downstream, causing blockage of the heat exchange equipment and foaming and blocking of the amine liquid in the regeneration tower.  

Mixed filtration of lean and rich liquid Some foreign natural gas purification plants generally believe that activated carbon filters are suitable for installation in the lean liquid process because the possibility of amine liquid producing degradation products in the regeneration process is greater than other equipment. 

Installing an activated carbon filter in the lean liquid process is beneficial to adsorb degradation products nearby and improve the absorption effect of amine liquid. 

The filtration model is: install a mechanical filter at the outlet of the rich liquid flash tower for full filtration and install an activated carbon filter at the inlet or outlet of the amine liquid circulating pump for partial filtration. 

Generally, it is 10% of the circulation amount. In order to prevent the activated carbon powder from being carried into the absorption tower by the solution device, a mechanical filter is connected in series downstream of the activated carbon filter.

APPLICATION STATUS OF FILTER EQUIPMENT  

The precision of the wedge wire filter element gradually decreases. As the operating life of the purification device increases, there are always some small-sized particles in the amine liquid that are not filtered and accumulated.

The solid impurities in the amine liquid increase and the load of the mechanical filter gradually increase. The use of the original wedge wire filter element will lead to a rapid rise in differential pressure and frequent cleaning and replacement. It is necessary to open the filter bypass for partial filtration or reduce the wedge wire filter element. Precision operation. 

It is reported that purification plants with a long operating time have gradually reduced the precision of the wedge wire filter element from 10 microns to 50 microns, and some even reduced them to 100 microns. 

PROBLEMS WITH SOLUTION FILTRATION 

The currently adopted amine liquid filtration mode alleviates some of the amine liquid pollution problems, but there are also certain drawbacks. 

  • The equipment is not completely filtered, and the quality of the solution continues to deteriorate. In the filtration of amine liquid, the filtering effect of solid particles in the amine liquid is certain due to the limitation of the accuracy of the filter element. 
    As a result, some tiny particles in the solution cannot be removed and continue to accumulate, which makes the quality of the amine liquid continue to deteriorate, and the deterioration of the quality of the amine liquid will lead to aggravated corrosion and further increase the solid impurities in the amine liquid. 
    The increasing amount of solid impurities can lead to frequent filter cleaning, more damaged and replaced filter elements, and increased filter operating costs. 
  • The thermally stable salt cannot be removed because the thermally stable salt dissolves in the solution. 
    It is difficult to remove it by the current filtration means, and the thermally stable salt has a strong influence on the foaming and corrosiveness of the solution, so it is necessary to adopt a new means to remove it. 

COMPREHENSIVE FILTERING MEASURES AND PROCESSES

The anion resin exchange process usually uses filaments with variable filtration precision as the filter material, which increases the specific surface area and surface free energy of the filter material, increasing the contact opportunity between impurity particles and the filter material, and the adsorption capacity of the filter material. 

Therefore, the filtration of tiny solids is improved; the anion exchange resin is used to remove the thermally stable salts and degradation products in the amine solution, and the latter is converted into free amine through ion exchange with the thermally stable salt-bound amine in the amine solution. Amine, improve the utilization rate of the amine solution.  

When the anion resin exchange equipment absorbs certain impurities and forms a certain differential pressure, the equipment automatically enters the backwashing process, uses desalinated water backwash to filter the solid impurities on the fiber, and uses the lye exchange to regenerate the anion resin for reuse. , thereby saving operating costs. 

Anion resin exchange equipment can be installed in the purification unit for in-line filtration, or it can be periodically filtered using mobile equipment. The filtration process is as follows: the amine liquid from the upstream first enters the fine filtration process to remove solid particles, then enters the resin exchange layer to remove the thermally stable salts therein, and the purified amine liquid enters the downstream. 

When the impurities in the equipment reach a certain level, the filter fiber is backwashed with regeneration liquid and the resin is regenerated.  

COMPREHENSIVE FILTRATION SELECTION OF AMINE LIQUID 

At present, the commonly used mechanical wedge wire filter has a good filtering effect on solid impurities, and the filtering capacity is large, but it cannot be filtered to remove thermally stable salts. 

The activated carbon filter can only adsorb organic matter and degradation products in the solution, but cannot filter solid impurities, and the adsorption effect is general. 

It is recommended to consider the use of mechanical filters combined with an anion resin exchange process (BLUSLOT resin trap filter) in the purification unit to filter the amine liquid.  

The mechanical filter installed in the rich liquid system can better remove the solid impurities brought into the amine liquid by the raw material gas and the corrosion products such as ferrous sulfide produced in the production process. Moreover, the equipment has a large processing capacity and is suitable for full filtration in rich liquids. 

The rich liquid filtration has higher requirements on the anti-corrosion performance of the equipment, and the loss of the anion resin exchange equipment increases when the amine liquid contains acid gas.  

Therefore, it is suitable for use in the lean liquid system, which is also conducive to the nearby filtration and regeneration of degradation products caused by high temperature. The equipment usually uses partial filtration to filter out tiny particles and heat-stable salts in the amine solution. 

To sum up, when the mechanical filter and anion exchange resin exchange equipment are selected for comprehensive filtration, the mechanical wedge wire filter should be installed in the rich liquid.