Return to TOM Report Table of Contents

L-53

December 22, 1947

Letdown in Stages of the CO₂ Scrubbing Water

By E. E. Donath

Summary

During discussions with Dr. R. C. Dressler, the possibility was mentioned of recovering CO /- H₂ by letdown in stages of CO₂ scrubbing water. It was thought that this principle might have application in the proposed Gas Synthesis Demonstration Plant. One of the principal costs of pressure water scrubbing of CO₂ from gases is involved in the losses of CO /- H₂. When the scrubbing water is aerated prior to recirculation, this loss is usually quoted to be from 3 to 5% of the gas scrubbed. Since the cost of manufacture of CO H₂ is a major contributing factor in the cost of manufacturing gasoline by the Fischer-Tropsch process, any saving of gas losses is desirable.

The idea of partial letdown and subsequent recovery of CO /- H₂ for recycle is not new. Mention is made of this principle by C. C. Wright and R. F. Michell. ¹⁾ However, no reference has been noted to the separation of H₂S and CO₂ by means of letdown in stages during the removal off these two constituents from gas streams.

In the present paper, calculations are shown for three cases of stepwise letdown:

1. Letdown to one intermediate stage at 64 psig resulting in a recovery of 81% of the CO /- H₂ which otherwise would have been lost in the scrubbing water.

2. Letdoen to 3 psig whereby 505 of the CO2 and 98% of the CO /- H₂ is recovered for recycle to the gas generator.

3. Letdown in two intermediate stages (64 psig and 10-21 psia) to recover 81% of the CO /- H₂ and a second stream representing 50% of the CO₂.

It is to be noted that figures, tables and graphs given are the result of calculations. The data, which are the basis for the calculations, were taken mainly from actual plant operation of a CO₂ pressure water scrubbing operation at the M.O.W. Plant, Louisiana, Missouri, and published solubility data. Although the calculations indicate the general feasibility of such an operation, it is believed that it would be desirable to conduct laboratory trials to confirm the results given. Also, it would be desirable to make a brief economic study of the proposed operation. It appears that the saving may amount to about 3-4% of the CO /- H₂, corresponding to 0.2-0.3¢/1000 cu.ft. assuming the CO₂ saturation calculated from the M.O.W. data.

The data from the M.O.W. CO₂ scrubbing plant, given in Appendix 1, show 51% CO₂ saturation. With these figures for a gas of the analysis

0.7%	H2S
10.0%	CO2
44.7%	H2
44.6%	CO

the following figures were obtained by calculation, assuming for 100 cu.m. gas at 20°C and 25 ata. water circulation rate of 89 cu.m., corresponding to 0.67 gal/cu.ft.:

ST% of the CO H₂ can be recovered by partial letdown of the scrubbing water and the CO H₂ loss reduced from 4.3% to 0.9%. The 5.6 sta. letdown gas can be recycled to the water scrubber, its CO₂ content (10%) being the same as that of the original gas, and its H₂S content (0.5%) being even lower.

b) Letdown to 3 psig (1.2 atm.)

50% of the originally dissolved CO₂ is recovered together with 98% of the CO H₂. The H₂ loss in the scrubbing water is 0.1% of that originally present. The 1.2 ata. gas with 54% CO₂ content could be recycled into the gasification.

c) Letdown in 2 intermediate stages

Cu.ft. per 1000 cu.ft. scrubbed gas

In the gas of the first release stage 81% of the dissolved CO H₂ can be recovered and recycled to the CO₂ scrubbing. In the second release stage 50% of the originally dissolved CO₂ is recovered in a gas with 84% CO₂. This gas could be recycled to the gasification zone to increase the CO/H₂ ratio. In this case about 25% of the originally present H₂S returns to the gasification zone.

The detailed calculations are given in the following part of the report.

Water Scrubbing of Synthesis Gas

From these figures a diagram, Fig. 1, is prepared. It shows under a) which amount of the originally dissolved total gas and of the individual components is released at various letdown pressures. Under b) the analysis of the released gas at various pressures can be soon.

Assuming that by partial release a gas with the same CO₂ content (10%) as the originally scrubbed gas shall be obtained, a letdown pressure of 5.6 ata. is required. Then the following figures are obtained from Fig. 1.

This means that a recovery of 80% of the originally dissolved CO and H₂ is possible. The CO and H₂ loss of 4.6% based on CO /- H₂ contained in the scrubbed gas can be reduced by 3.7% to 0.9%. The gas released at 5.5 ata. after compression can immediately be recycled to the CO₂ scrubbing plant.

In the gasification, assuming that CO₂ from the water scrubbing is required, the water can be let down either in one step, to obtain a CO₂ rich gas, or in two steps to obtain in the first letdown stage a CO /- H₂ fraction and in the second a CO₂ rich fraction. For the one stage letdown and the recovery of 50% of the originally dissolved CO₂ the following data are obtained from Fig. 1:

Recycling of 50% of the CO₂ originally dissolved means a recovery of 98% of the CO /- H₂ and recycling of 25% of the H₂S.

For the two stage letdown in the first stage a pressure of 5.5 ata. may be assumed. Then the following figures are obtained: