An overview into submarine CO2 scrubber development

Ocean Engng, Vol. 10, No. 4, pp. 227-233, 1983.

0029-8018/83 $3.00+ .00' Pergamon Press Ltd.

Printed in Great Britain.

A N O V E R V I E W INTO S U B M A R I N E C O 2 S C R U B B E R DEVELOPMENT R. CAREY Code 2831, DavidTaylorNavalR&D Center, Annapolis,MD 21402 A. GOMEZPLATA Chemical EngineeringDepartment, Universityof Maryland, College Park, MD 20740 and A. SARICH Naval SystemsEngineeringDepartment, U.S. Naval Academy,Annapolis, MD 21402, U.S.A. Abstract--An overview into the development of a carbon dioxide (CO2) removal plant for submarines to meet the Navy'sCO2 requirementsis presented. The monoethanolamine (MEA)-CO2 removal process and parametric studies to reduce atmospheric CO2-1evelsin submarines to 0.5% and possibly0.2% are discussed. INTRODUCTION THE DEVELOPMENT of the monoethanolamine (MEA) carbon dioxide removal plant (scrubber) has eliminated the need for a submarine to surface or snorkle periodically to replenish the submarine's atmosphere. All US Navy submarines have been equipped with scrubbers. Initially, each submarine was equipped with two plants: one forward and one aft. It was felt both scrubbers would have to be operated simultaneously to keep the CO2 level in the submarine below 1-1.3%, the maximum level then permitted by the Navy's Bureau of Medicine and Surgery (BUMED). However, it was s o o n found that one scrubber could do this job. Consequently, the other scrubber was kept in stand-by status. The installed MEA scrubbers performed well; they could maintain CO2 levels in the submarine atmosphere below 1.3%, and generally below 1%. All available medical evidence indicated that continuous exposure of humans to these CO2 levels was not harmful, a fact supported by more than two decades of nuclear submarine operations. Approximately five years ago, the results of a British research project changed this situation. These findings were that continuous exposure to 1% CO2 produced a discernible change in blood pH, and interfered with the body's ability to retain essential metallic salts. Upon learning this, BUMED took a tougher stand and imposed a maximum continuous CO2 exposure limit of 0.2% as a goal with an immediate required reduction to 0.5% (BUMED, 1972; 1974). Studies (Naval Submarine Medical Research Laboratory, 1982) just reported where humans were exposed to atmospheres containing up to 0.65% CO2, for up to 90 days, were safely conducted. Therefore, these exposures are considered safe at this time. These limits were completely unattainable with existing equipment, so the Navy began a scrubber development plan. All known CO2 removal 227

228

R. CAREVet al.

processes were considered. The two approaches having the lowest associated risk level were:

• improvement of the MEA plant, and • development of a suitable molecular sieve plant Improvement of the MEA scrubber was undertaken by the David Taylor Naval Ship R&D Center. This paper presents an overview into the development of improved MEA-CO2 scrubbers. MEA-CO2 SCRUBBER PROCESS DESCRIPTION The plant operation is based on the ability of cool amine solution to readily absorb carbon dioxide, while hot amine solution gives up most of its carbon dioxide content. The amine solution used in the scrubber consists of water, a chelating agent, and monoethanolamine, H O - C H 2 - C H 2 - N H 2. When carbon dioxide is absorbed or desorbed in aqueous M E A solution, two reactions (Astarita, 1967; Riesenfeld and Kohl, 1974) take place. CO 2 +

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(1)

heat

CO2 + R N H C O O - + 2H20 ~

RNH3 ÷ + 2HCO3

(2)

heat

where R is HOCHE-CH2. MEA solution higher in CO2 content is referred to as rich MEA, solution containing lower amounts of carbon dioxide is referred to as lean MEA. A MEA-CO2 scrubber (Fig. 1), is composed of five interrelated subsystems: air circulation, monoethanolamine solution circulation, carbon dioxide removal, chilled water cooling, electrical and instrumentation. All of these systems and their components form a single, integrated unit.

Air circulation subsystem Air containing CO2 is drawn into the plant by a blower. It enters the distributor head and passes through the packed bed absorber tower. The air mixes with lean MEA solution which is discharged downward from the top of the tower. The packing provides a large surface area for intimate contact between the air and solution. On reaching the bottom of the tower, the air is deflected upward through an exhaust duct and enters the air purifier for removal of mists and vapors before being discharged into the submarine. The air purifier consists of a cooler, a demister to remove entrained liquid and an ion exchange resin chamber to chemically remove amine vapors. The rich MEA solution falls to the sump. MEA solution circulation subsystem The MEA solution is drawn from the absorber sump and is pumped through the distributor head in the top of the tower. The solution contacts the air as described above. A small amount of the enriched MEA solution is continuously drawn from the pump discharge and passed through the MEA heat exchanger where it is preheated by lean M E A solution returning from the boiler-stripper. The rich M E A enters the boiler-stripper through a distributor imbedded in the stainless steel Raschig ring packing of the stripper. The packing provides contact area between the incoming MEA solution, steam and CO2 generated in the boiler. Heaters located in the bottom of the boiler

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provide the heat required for C O 2 removal. The lean M E A solution from the boiler is returned by way of the M E A heat exchanger to the top of the absorber.

C02 removal subsystem Steam and CO2 exit the top of the boiler-stripper tower and enter the CO2 cooler where the steam is condensed and returned to the boiler-stripper. The cooled CO2 is passed through a pressure regulating valve and into a compressor for discharge.

Chilled water cooling subsystem Chilled water enters the air cooler to cool the warm air from the absorber tower. Cooling water is also fed to the CO2 cooler.

Electrical and instrumentation subsystem Heaters, motor, and their control circuits operate on 440 V, 60 Hz a.c. power. A stepdown transformer reduces the 440V supply to 115V, which operates the temperature indicating alarm and shut down circuits. NAVY CO2 SCRUBBERS Presently, the Navy has six scrubber models. The physical characteristics of the most c o m m o n scrubber is presented in Table 1. This scrubber was the test platform for TABLE 1.

PHYSICAL CHARACTERISTICS OF SCRUBBERS

Small low-capacity scrubber

Large high-capacity scrubber

Weight

4000 lb (1.8 x 103 kg)

7200 lb (3.3 x 103 kg)

Size

80 f3 (2.27 m 3)

180 f3 (5.11 m3)

Power

15 kW

27 kW

Air circulation

250 fa/m (0.118 ma/sec)

700 f3/m (0.330 m3/sec)

MEA circulation

28 gal/min (1.8 x 10.3 m3/sec) 42 gal/min (2.7 x 10-3 m3/sec)

Packing material absorber tower

Raschig rings

Wire mesh

Boiler-stripper pressure heater power

35 lbf/in2 (2.4 x 105 Pa) 11 kW

25 lbf/in2 (1.7 x 105 Pa) 21 kW

Air purifier

Ion exchange resin

Ion exchange resin

Capacity 1% CO2 atm. 0.5% CO2 atm.

12 lb/hr (1.5 x 10-3 kg/sec) (outside operating range)

25 lb/hr (3.2 × 10-3 kg/sec) 16 lb/hr (2.0 x 10-3 kg/sec)

Developmentof a submarineCO2 scrubber

231

parametric studies to develop advanced high capacity systems. This scrubber is being upgraded during overhaul periods. For comparison purposes, the physical characteristics of the largest high capacity plant is also given in Table 1. HIGH CAPACITY SCRUBBER DEVELOPMENT An indepth parametric study to increase the capacity of scrubbers was performed to meet BUMED CO2 goals. The results of this study are summarized.

Absorber packing Parametric studies were made on a land-based shipboard scrubber and a reduced-scale system. Laboratory and theoretical findings were correlated. Results indicated: • increased air flow rate very significantly increases CO2 removal, • increased absorber pressure increases CO2 removal, • changes in liquid rate had no significant effect on CO2 removal, and • there was no apparent interaction among the variables. Stainless ~teel wire mesh packing was found to give a significant improvement in CO2 removal over other packings. The CO2 removal rate versus the degree of solution CO2-content for Raschig rings and wire mesh is compared in Fig. 2. Figure 2 shows that for the same degree 0~fsaturation (up to 80% saturation) the wire mesh packing provides significantly higher CO2 removal rates.

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Boiler stripper Parametric studies to identify the controlling parameters for stripping C02 from the MEA were performed. The results showed that the more fully saturated the MEA solution entering the boiler-stripper, the easier desorption of CO2 from the solution would be. The wire mesh absorber packing meets the criteria of providing a highly saturated solution to the boiler-stripper with minimal effect on CO2 absorption.

Air purifier Removing the trace amounts of MEA and ammonia from the air leaving the absorber has been a continuing problem due to pressure loss across the bags used to retain the ion

Development of a submarine CO2 scrubber

233

exchange resin. Numerous substitutes for resin have been evaluated. At the present time, the best method available for air cleanup is a combination of carbon filter treatment of the MEA solution and downflow air pattern at reduced velocity through the resin bags. Carbon filter

In Fig. 3, the effects of the carbon filter on airflow rate, blower pressure, air contamination and solution color are shown. A scrubber was operated with a degraded MEA solution. The air flow decreased (Fig. 3a), the blower back pressure increased (Fig. 3b), and the air contamination (Fig. 3c), was at a high level. After approximately 80 hr of operation, the carbon filter was placed on line. The air flow rate returned to normal, blower discharge pressure decreased, and air contamination became satisfactory. The solution color (Fig. 3d), corresponding to these events was recorded. A color of 100 units is amber and a color of 0 units is water clear. The carbon filter maintains the MEA solution in “like new” condition. As a result, air contamination levels are reduced to a minimum at the scrubber air discharge. SUMMARY In a submarine atmosphere the removal of carbon dioxide is accomplished by COz absorption in monoethanolamine solution followed by regeneration of the solution. The progressive development of the carbon dioxide removal plant has resulted in the present high-capacity scrubber system enabling Navy submarines to meet the immediate BUMED atmospheric requirements of 0.5% CO2 and the long term goal of 0.2% C02. Based on medical research completed3, it is anticipated that the requirement for a 0.2% CO* scrubber will be eliminated. REFERENCES ASTARITA,

G. 1967. Carbon dioxide absorption in amine solutions. Mass Transfer with Chemical Reaction, pp. 153-1.56. Elseviers, New York. NAVALSUBMARINE MEDICAL RESEARCH LABORATORY 1982. Position paper: the toxic effects of chronic exposure to low levels of carbon dioxide, Submarine Base, Graton, CT. RIESENFELD, F.C. and KOHL,A.L. 1974. Ethanolamines for hydrogen sulfide and carbon dioxide removal, Gus Purification, 2nd Edn. pp. 24-25. Gulf, Houston. U.S. NAVVBUREAUOFMEDICINE AND SURGERY(BUMED) 1972. Instruction 6270.3F, Washington, D.C., 15 August. U.S. NAW BUREAUOF MEDICINEAND SURGERY(BUMED) September.

1974. Letter 6421.1, Ser. 564, 9