CO 2 RECOVERY FROM CO 2 REMOVAL UNIT AT GL1Z PLANT

Size: px
Start display at page:

Download "CO 2 RECOVERY FROM CO 2 REMOVAL UNIT AT GL1Z PLANT"

Transcription

1 CO 2 RECOVERY FROM CO 2 REMOVAL UNIT AT GL1Z PLANT Hocine Friha Chemical Engineer Technical Department GL1Z/ Sonatrach Bethioua, Oran, Algeria ABSTRACT Algeria which has ratified the Kyoto Protocol on the reduction of greenhouse effect gases to the atmosphere is making major efforts for the environment protection such as preventing the carbon dioxide CO 2 emissions from industrial plants. The liquefied natural gas plant GL1Z was designed to transform about 10.5 billions cubic meter per year of natural gas containing a considerable amount of carbon dioxide (about 50,000 Tons per year). Ahead of the liquefaction process, carbon dioxide is removed by absorption using an MEA solution in order to avoid its freezing in the cryogenic sections. The regeneration of the MEA solution is carried out in the regeneration column where CO 2 is sent to the atmosphere. This paper will present in detail a new process for recovering the actual carbon dioxide CO 2 sent to the atmosphere from the GL1Z plant in order to be used in the neighboring plants. These plants will use this product either in the synthesis of the urea, the production of methanol or in the food industry as an alimentary agent. The implementation of this process will contribute directly to the reduction of greenhouse effect gases in addition to the significant economic benefits for the GL1Z plant. PS7-4.1

2 INTRODUCTION The GL1Z plant started production in February It consists of six identical process trains, using the APCI propane pre-cooled-mr process to liquefy the natural gas. The design capacity of this plant is 10.5 billion cubic meters of natural gas per year. The plant feed gas is composed mainly of hydrocarbon components with presence of variable quantities of carbon dioxide, water, mercury (traces) and heavy hydrocarbon. All impurities are removed before liquefaction respectively in Acid gas removal, dehydration, mercury removal and Heavy hydrocarbon separation. The GL1Z plant has installed during a renovation project in 1995 respectively a new LPG recovery exchanger which has contributed to reduced the start up time and a new gasoline tank for export purposes, to avoid its combustion. These modifications have allowed reduction of carbon dioxide emission. This study will focus on the acid gas removal section where the carbon dioxide is removed from feed gas in order to avoid its freezing in the cryogenic sections throughout the liquefaction process. The carbon dioxide is removed in an absorption column using MEA amine solution as the absorbing medium. The regeneration of the MEA solution is carried out in the regeneration column where the CO 2 is sent to the atmosphere. This emission taken place since the start-up of the plant. The aim of this document is to show that there is a possibility to recover the carbon dioxide sent to the atmosphere from MEA- CO 2 removal section of each train. The study consist mainly in a process of CO 2 recovery from the acid gas removal section and to route it to the neighboring plants that use the carbon dioxide as primary materials in their process such as methanol, urea and food carbon dioxide. This will allow these plants to enhance their capacity of production. This process allows the protection of environment by reducing the emission of carbon dioxide from the acid gas removal. CO2 REMOVAL UNIT DESCRIPTION Introduction In order to locate the source of feed gas of the process proposed in this paper it is useful to give a general overview of the CO 2 removal unit. PS7-4.2

3 CO2 absorber column (Refer to figure 1) The CO 2 removal unit is located in each individual LNG train. It is the first section that the feed gas enters in the LNG process. Content of carbon dioxide contained in feed gas and leaving the Absorber Column are indicated in table 1. The purpose of the MEA system is to remove the Carbon Dioxide (CO 2 ) from the feed gas before it enters the Cryogenic section of the process. CO 2 removal from the feed gas using MEA solution as an absorbing medium is based on the principle that at high pressure and low temperature the CO 2 molecules are attracted to the MEA molecules (RNH 2 ) and combine with them according to the following chemical reaction (moving left to right). 42 bar; 41 C 2(HO CH 2 CH 2 NH 2 ) + CO 2 + H 2 O (OH CH 2 CH 2 NH 3 ) 2 CO 3 + Q 1, 7 bar;117 C During the regeneration, process the chemical reaction is reversed, (from right to left). A 15-20% Monoethanolamine, (MEA) solution is utilized to absorb the Carbon Dioxide (CO 2 ) in the incoming natural gas stream. A high concentration of CO 2 in the outlet of the MEA section could result in freezing problems in the downstream cryogenic equipment. Table 1. CO 2 Natural gas content Sample Description Design Specifications Present Value Light Feed Heavy Feed Average Feed Stream CO 2 absorber (mol %) Product Stream CO 2 absorber (ppm) < 90 < 90 < 10 MEA Regeneration column (Refer to figure 1) The function of the MEA regenerator is to remove the CO 2 from the rich MEA solution. This is achieved by reversing the chemical reaction, which caused the CO 2 in the feed gas to be absorbed by the MEA, moving from right to left. 42 bar; 41 C 2(HO CH 2 CH 2 NH 2 ) + CO 2 + H 2 O (OH CH 2 CH 2 NH 3 ) 2CO 3 + Q 1,7 bar ; 117 C PS7-4.3

4 The MEA regenerator column contains 21 single pass valve trays. Rich MEA exits exchanger tube side at 93 C and enters the MEA regenerator on tray 1 via a level valve. Initially a large portion of the CO 2 flashes due to the pressure drop across this valve from 8 Bar to 0.7 Bar and exits the column overheads to the Acid Gas separator. The flow from the Acid Gas Separator composed mainly of carbon dioxide as indicated in table 2 below is rejected to atmosphere. Table 2. Downstream Acid gas separator quality Components Formula Mole% Carbon Dioxide CO Water H Methane CH Ethane C 2 H Nitrogen N Total PS7-4.4

5 Figure 1 CO 2 Removal Unit PS7-4.5

6 Carbon dioxide flow Table 3 summarizes the quantity of carbon dioxide send to the atmosphere from the six acid gas drum of plant at 100 % and 110% production rate. Table 3. Carbon dioxide plant quantity Production Rate Complex 100 % 110 % Ton/day Ton/year 52,145 57,364 Nm3/h 3,207 3,528 Comments: As indicated in the table 3 above, the quantity of carbon dioxide to recover is approximately a hundred and seventy tons per day (170 ton/day), equivalent to sixty thousand tons per year (60,000). CO2 RECOVERY PROCESS DESCRIPTION (refer to figure 2) The carbon dioxide gas stream at the composition indicated above in table 2 from the six acid gas drum of liquefaction process trains joins the carbon dioxide common header. The carbon dioxide stream flows from the common header to a seawater heat exchanger where it is cooled from 51 C to 30 C. Before suction by compressor, the flow of CO 2 should pass in the 1st Stage Suction Drum to remove any liquid water which may have condensed prior to entering CO 2 Compressor. The carbon dioxide stream flows from the 1st Stage Suction Drum to the CO 2 Compressor where it is compressed from 1, 6 bar to an intermediate pressure (3 bar). The discharge from 1st Stage CO 2 Compressor is cooled to 43 C against a seawater stream flowing in the CO 2 Inter-Cooler tube side. The cooled CO 2 flows to the CO 2 2nd Stage Suction Drum to remove any liquid which may have condensed prior to entering 2nd Stage CO 2 Compressor. The carbon dioxide stream is compressed in the 2nd Stage CO 2 Compressor and the discharge stream is cooled to 43 C against seawater stream flowing in the CO 2 After- Cooler where water condenses. The CO 2 stream flows into a Separator where the two phases are separated. The liquid water flows out of the Separator to waste water. The gas of carbon dioxide stream flows out of the Separator to the users with a pressure of about 6 bars. PS7-4.6

7 Two compressors are provided, one in service and the other in "standby". In order to maintain the operating pressure in the regeneration columns, a line with a pressure control valve will be provided on the common header and it discharges to the atmosphere when the pressure increases in the regenerator columns. Table 4 Process specifications Feed Gas Acid Gas Flow Rate CO 2 Recovery Rate CO 2 product Purity Acid gas 4,000 Nm 3 /H 173 Ton/Day more than 97 mol % Figure 2 Proposed Carbon dioxide Recovery Process PS7-4.7

8 Main equipments 1. Common header: Collect the six flows of carbon dioxide from acid gas drum of each train 2. Pressure control valve: To control the pressure in header. 3. Seawater exchanger: To reduce the flow temperature. 4. Two stage compressor system: To increase the pressure as required by users 5. Inter-Coolers: To cool the carbon dioxide flow after the 1stage of compressor 6. An After-Cooler: To cool the carbon dioxide product after compression system 7. A CO 2 Separator: To remove any liquid condensed in the cooler 8. Pipe: To deliver carbon dioxide to users CONCLUSION The application of this process of carbon dioxide recovery from of the CO 2 removal units presents the following advantages, while having no negative impact on the LNG process operation: Contribution to the prevention of global warming (Reduction CO 2 Emission), Enhancement of production of the methanol plant, Enhancement of production of the urea plant, Enhancement of food CO 2 production capacity of the industrial Gas plant PS7-4.8