Question

Problem 1 An absorber-stripper process is used to remove carbon dioxide (CO2) and hydrogen sulfide (H2S) from a feed (stream 1) consisting of 25% CO2 and 15% H2S in nitrogen. In the absorber, a solvent selectively absorbs hydrogen sulfide and carbon dioxide. The absorber overhead (stream 2) contains only 2% CO, and the balance Nitrogen with no Hys. Ny is insoluble in the solvent. The rich solvent stream leaving the absorber (stream 3) is flashed in a flash separator: the overhead stream of the flash (stream 4) consists of 20% solvent, and contains 20% of the Co, and 10% of the H2S from the raw feed to the absorber. The liquid stream leaving the flash unit (stream 5) is split into equal portions, one (stream 6) being returned to the absorber. The other portion (stream 7), which contains 5% CO2, is fed to the stripper. The liquid stream leaving the stripper (stream 8) consists of pure solvent which is mixed with the makeup solvent (stream 9) and the resulting stream (stream 10) is returned to the absorber. The stripper overhead (stream 11) contains 30% solvent. 1. Give a brief description of the following units commonly used in chemical engineering processes: Absorber, Stripper and Flash separator. Taking as a basis the gas feed rate equals to 100 mol/h: 2. Draw and completely label a flowchart of this process. 3. Do the degree-of-freedom analysis for the overall process, absorber, mixer, stripper, flash and splitter. 4. Calculate the molar amounts and compositions of all streams (1-11). Summarize all your results in one table as below: Stream (i) (molin) YNZ n^2 ycoz üçoz y#25 #25 100 11

Answer 1

Here, I am providing the answer to the first sub-part.

SOLUTION:

(a) Absorption and stripping are gas-liquid contact operations.

ABSORBER

An absorber is a vessel in which liquid is absorbed during distillation. An absorber extracts a substance from a gas by absorbing the substance into a liquid. Absorbers are typically tall cylinders in which the liquid flows down and gas bubbles upward. The absorption of a gas in a liquid is generally conducted in packed columns, plate columns, centrifugal absorbers, spray columns or bubble columns.

A packed bed column uses gas and liquid streams running counter to each other in a column packed with loose packing materials, such as ceramics, metals, and plastics, or structured packing. The packed bed uses surface area created by the packing to create a maximum amount of efficient contact between the two phases. The systems require low maintenance and can handle corrosive materials with high mass transfer rates. Spray columns are another type of absorber, which uses constant direct contact between the two phases, with gas moving up and liquid being sprayed down into the gas flow. This system only has one stage and poor mass transfer rates, but is very effective for solutes with high liquid solubility

STRIPPER

Stripping column (stripper) is used to remove an impurity from a liquid feed by stripping the impurity into a gas stream. Stripping is mainly carried out in tray towers (plate columns) and packed columns, and less often in spray towers, bubble columns, and centrifugal contactors.

Tray towers consist of a vertical column with liquid flowing in the top and out the bottom. The vapor phase enters the bottom of the column and exits out of the top. Inside the column are trays or plates. These trays force the liquid to flow back and forth horizontally while the vapor bubbles move up through the holes in the trays. The purpose of these trays is to increase the contact area between the liquid and vapor phases.

Packed columns are similar to tray columns in that the liquid and vapor flows enter and exit in the same manner. The difference is that in packed towers there are no trays. Instead, packing is used to increase the contact area between the liquid and vapor phases. There are different types of packing used and each one has its own advantages and disadvantages.

FLASH SEPARATOR

A flash separator or vapor–liquid separator is a device used in several industrial applications to separate a vapor–liquid mixture. A vapor–liquid separator may also be referred to as a flash drum, breakpot, knock-out drum or knock-out pot, compressor suction drum or compressor inlet drum. When used to remove suspended water droplets from streams of air, it is often called a demister.

Most commonly, gravity is utilized in a vertical vessel to cause the liquid to settle to the bottom of the vessel, where it is withdrawn. In low gravity environments, a common liquid separator will not function because gravity is not usable as a separation mechanism. In this case, centrifugal force needs to be utilised in a spinning centrifugal separator to drive liquid towards the outer edge of the chamber for removal. Gaseous components migrate towards the center.

For both varieties of separator, the gas outlet may itself be surrounded by a spinning mesh screen or grating, so that any liquid that does approach the outlet strikes the grating, is accelerated, and thrown away from the outlet. The vapor travels through the gas outlet at a design velocity which minimises the entrainment of any liquid droplets in the vapor as it exits the vessel. The feed to a vapor–liquid separator may also be a liquid that is being partially or totally flashed into a vapor and liquid as it enters the separator.