(123d) PILOT Plant Design and Operation for Carbon Capture and Energy Conservation

Phaneswararao Damaraju^*, Devika Wattal¹, Rushikesh V. Janai², Rajat Agarwal³, Amit Agarwal⁴

^*Department of Chemical Engineering, Indian Institute of Technology, Hauz Khas, New Delhi , India

¹Battelle science and technology, pune, india

Toyo engineering limited, Mumbai,India

³CFA,Canada

⁴TechnipFMC,Gautam Buddha Nagar, UP, India

ABSTRACT

Carbon capture using chemical solvents is widely adopted in fertilizer, petroleum and petrochemical industries, and is presently is of utmost importance on account of its application in tackling global warming. In the present work, a pilot plant has been designed to conduct carbon dioxide removal operation on a continuous basis using 30 cm dia 3 m tall columns packed with Mellapak packings for absorption and stripping, besides other equipment such as heat exchangers, coolers, condensers, pumps , utilities, control systems, etc.,An attempt has been made in the present work, by incorporating the concept of energy conservation by mechanical vapor recompression of steam-carbon dioxide mixture leaving the desorption column from which the latent heat of steam available in the gas can be recovered by compressing the gas and condensing the gas mixture in the coils inserted in the regenerator or a separate steam generator. The pilot plant has been designed such that absorption of carbon dioxide can be conducted at atmospheric pressure which is relevant to carbon capture and sequestration, as well as at high pressures around 30 bar which is relevant to fertilizer , gas, and petrochemical industries.The plant design facilitates total recycle of unabsorbed gases as well as stripped carbon dioxide with the help of booster compressor to minimize energy and raw material consumption.The plant has been designed such that the plant can be run fully in automatic control mode, or manually. The absorption column is designed to handle 1000 lpm of gas feed at 130 C and a maximum pressure of 30 bar(g). The desorption column is designed to operate at 1 bar(g). The two stage metal diaphragm compressor of Fluitron(USA) suitably designed to stand corrosion can handle 1000 lpm of 50-50% steam carbon dioxide mixture at 1 bar(g) suction pressure and deliver gas at 30 bar(g) and 230 C temperature. The reciprocating booster compressor is designed to recycle unabsorbed gases at 30 bar(g) to the absorption column. The reboiler that is used to generate steam from the lean solvent is fitted with separate condensing coils to allow condensation of pure steam as well as compressed steam carbon dioxide mixture . A 125 kg/hr steam boiler has been used to supply steam required for the experiments. Besides the above, compressed air facilities and cooling water facilities required for running the control systems, compressors, coolers etc., were installed .A DASYLAB online computer control system was installed to facilitate online computer control. Control loops were tested individually at first. The plant was operated manually with activated MDEA solvent at atmospheric pressure using air-carbon dioxide gas mixture The gases required for feeding the pilot plant were supplied from a cylinder tank system provided separately for air, nitrogen and carbon dioxide such that a gas of desired feed composition can be obtained. The pumps are designed to circulate solvent through the columns at a rate of about 60 lpm. Pneumatically operated 1” Industrial control valves made by RK control instruments ltd were used at several locations as final control elements. All pipe lines and columns, tanks, vessels were insulated to minimize heat losses. The compressors were fully instrumented and fitted with safety valves, and automatic trip systems. The P&I diagram of the plant is shown in Figure 1.

Figure 1. P&I diagram of the pilot plant setup for carbon capture

Enough instrumentation has been done to control most of the important process variables such as steam pressure, condensate drum level, solvent flow, reboiler temperature, solvent feed temperature, absorption column bottoms level, condensate temperature, feed gas temperature and composition by using an online computer. However, it was decided to operate the plant in manual mode to start with as some of the control loops or instruments that were procured did not function properly. In the present work, the above control loops could not be tested as the I/P transmitters of most of the control valves failed due to seepage of condensate water when compressed air was supplied to them during monsoon season as apparently the drier did not function properly. The absorption bottoms level transmission was found to be affected by the computer signals reaching the VFD. The control of cooling water flowing to the solvent cooler could not be done due to limitation in the heat transfer area available in the solvent cooler. The control of cooling water to the partial condenser could not be done as only one cooling water pump was used to pump water from the cooling tower to the partial condenser as well as the solvent cooler and thus variation in the flow of water by positioning one valve caused variation in flow through the other valve . Cascade control of reboiler temperature could not be accomplished due to very frequent malfunctioning of the boilers. Feed gas composition control could not be tested as the dynamics of the carbon dioxide analyzer was found to be very slow and also only one analyzer was available which had to be used to measure both inlet gas and outlet gas compositions. The control of solvent with the help of VFD could not be implemented as the response of VFD to signals from the computer was found to be not good. As it has been found out that inspite of providing insulation to piping and equipment, there were significant heat losses from the pilot plant, a heat loss experiment was conducted by circulating hot solvent from the reboiler to the desorption column and back and find the steam demand for maintaining the required temperatures.

The experimental data obtained are given below:

Rate of steam condensation in the re-boiler = 45 kg/hr

Rate of condensation of vapors in the steam-carbon dioxide condenser = 30 kg/hr

Rate of heat loss = 45- 30 =15 kg/hr

Temperature in the re-boiler = 118 C

Temperature at the top of the desorption column = 113

Solvent circulation rate = 22 -24 lpm

Heat transfer area of the re-boiler = 4.37 sq.m

Heat transfer area of the de-sorber = 4.9 sqm

Heat transfer area of piping from des. Col. To tank = 0.92 sq. m

Total area of the loop = 10.2 sq. m

From the above data, it appears that heat losses are about 30% of the heat supplied. However, when the plant is scaled up , the surface area to volume ratio will come down by a factor of 15 , and hence heat losses in a commercial plant would come down to 2%.

The preliminary experimental results on carbon capture obtained by using activated MDEA are given below :

Solvent compositions:

Run 1: MDEA :29.2%; Promoter : 2.1%; K₂CO3: 5.5%; KHCO3: 0.9%; water :55.5%

Runs 2&3: MDEA :28.0%; Promoter : 10.0%; K₂CO3: 5.7%; KHCO3: 0.8%; water :62.3%

The results obtained for the above runs are given below:

R-1 R-2 R-3

Temperatures

1. Absorption column outlet solvent : 55 58 49
2. Rich solution after the heat exchanger : 9 101.2
3. Rich solution inlet to desorption column: 2 98 88
4. Lean solution outlet from desorption column: 117 116 93
5. Reboiler temperature : 125 125 104
6. Solvent inlet to absorption column: 53 56 5
7. Gas outlet from desorption column : 102 99 88
8. Condensate outlet from the steam –CO₂

condenser : 29 36 36.5

9. Gas inlet to absorption column: 27 35 38
10. Gas outlet from absorption column: 51 55 48

Flow rates:

1. Steam condensate flow from Reboiler(kg/hr) : 84 81 45
2. Pure carbon dioxide flow rate: (lpm) 78 78 61
3. pressure(bar(g) and temperature of CO₂ ,C 0.6 , 24 0.8.35 1.2,38
4. Feed gas flow rate(air and carbon dioxide mixture),lpm: 626 676 733
5. Pressure of feed gas , bar(g) 0.55 0.8,30 1.1.33
6. Flow rate of stripped gases going out from the condenser,lpm 107 99 53

7. Solvent flow rate , lpm 5 18.7 17

8. Flow rate at absorber exit, lpm 950 1192

Pressures:

1. Steam condensate drum pressure, bar(g) 3 1 1.8
2. Desorption column pressure, bar(g) 8 0.8 0.2

Compositions:

1. Gas inlet to absorption column, CO₂% : 8 15.3 12
2. Absorption column inlet gas humidity, % 57 58 58
3. Gas outlet from absorption column, CO₂% 0 4.9 7.4
4. CO₂ loading of solvent in the Reboiler : 1076 0.177
5. CO₂ loading of solvent at the outlet of the

absorption column: 0.1629 0.223

6. Moles of total amines/L 15 3.15
7. Moles of k+/L 096 0.096

In the case of runs 2 and 3, the two heat exchangers were used in series connection.. Condensate from the condensate drum was recycled to boiler room to decrease boilers load. Liquid collected in gas liquid separated was pumped back to Tank T1 at the end of the run