Packaging printing industry

In the packaging printing industry solvents like ethyl acetate, ethanol, ketones, tetrahydrofuran (THF), hexane and toluene are used in printing. The solvent laden air generally contains between 2 and 15 g/m3 of solvent and, depending on the season, 5-18 g/m3 of water.

Adsorptive recovery with steam desorption was widely used for many years but its disadvantage is that the solvents are recovered in various mixtures containing large amounts of water. The recovery of solvents is complicated because most solvents produce azeo-tropes with water which are not easily separated and, consequently, the waste water fails to meet the more stringent environmental regulations. In the last 10 years the adsorbent regen eration with hot inert gas has became the more favored process in the packaging printing industry.

22.1.5.2.1 Fixed bed adsorption with circulating hot gas desorption31'32 Activated carbon preferentially adsorbs non-polar organic solvents, while adsorbing relatively little water. Thus activated carbon allows the passage of 95-98 % of the water while retaining the solvents from the gas stream. During the adsorption step solvent laden air or gas is delivered to the bed through the appropriate valves. As adsorption starts on a fresh bed, the effluent gas contains traces of solvent. Through time, the solvent level increases and when it reaches a predetermined value, the adsorption is stopped by closing the feed gas valves. The adsorption step usually lasts for 4-16 hours. An example of a solvent recovery plant with 5 activated carbon beds and hot gas desorption is shown in Figure 22.1.19.

Ifnecessary, feed air is cooled to 35°C and sent by the blower V-1 to the mainheater6 from which it enters 4 beds simultaneously, while one bed is being regenerated. The cleaned air is collected in header 7 and vented.

After the adsorption step, the air is displaced from the bed by an inert gas such as nitrogen. A circulating inert gas at 120-240°C serves for heating and stripping the solvent from the carbon. The solvent is recovered by cooling and chilling the circulating gas. The optimum chilling temperature depends on the boiling point of the solvent. Generally, chilling temperatures are between +10 and -30°C.

During the heating step, the temperature of the effluent gas gradually increases until a predetermined value (for example, 150°C) is reached, after which the heater is bypassed and the bed is cooled down by cold gas. Typical solvent recovery plants will have from 2 to 8 beds. All beds go through the same adsorption, inertization, heating and cooling steps but each at different time.

Referring to the flow sheet in Figure 22.1.19 the regeneration gas is circulated by blower V-2 to the gas heater, the header 8 to the bed being regenerated. The effluent gas is passed to the header 9, cooler, molecular sieves (or water condenser), to the chiller for sol

Vent Gas Chiller

Figure 22.1.19. Fixed bed adsorption with circulating hot gas desorption (After references 31,32).

Figure 22.1.19. Fixed bed adsorption with circulating hot gas desorption (After references 31,32).

vent condensation and back to the blower V-2. At the end of the adsorption step the bed contains 10-30% solvent and 1-2% water.

There are various ways to remove the water separately and to recover a solvent containing between 0.1 and 1.5 % water (Table 22.1.10).

Table 22.1.10. Process conditions and water content in the solvent (After references 32,33)

Water in the raw solvent

Investment cost, %

Heat needed, kWh/kg S

Power, kWh/kg S

Basic process with molsieve beds

0.1

100

2.2

1.0

Chiller instead of molsieve

1.5

91

1.5

1.0

Chiller and 1 carbon bed

0.4- 1.0

93

1.1

1.0

Chiller, and 1 carbon bed molsieve

0.1

96

1.1

1.0

Using molecular sieves

Beds of molecular sieves are used to recover a solvent with 0.1%. This process requires around 2-2.5 kWh heat/kg of recovered solvent. Regeneration loop pressure drops are high, because of the additional molsieve beds and valves.

Separate condensation of water

Initially, when regenerative heating of the bed starts, very little solvent is desorbed, but much of the water (about 87%) is desorbed. The circulating gas is first passed through a cooler and then a separate chiller for the condensation or freezing of the water. Subsequently the gas is passed to the chiller where the water is condensed or frozen. From 1 to 1.8% water is recovered. This process is simpler than the molecular sieves process.

Two separate chillers

Two separate chillers are used in another adsorption process which offers further substantial improvements. A third activated carbon bed is added so that while two of the beds are being regenerated one of these is being cooled and transferring its heat to the other bed which is being heated. This not only gives excellent heat recovery but also provides a means of reducing the water content of the recovered solvent. The cooled bed absorbs the water vapor from the gas stream coming from chiller. This dry gas stream is transported to the bed which is being heated and the desorbed solvent from the heated bed remains dry. Solvents recovered at this stage contain only 0.4 to 0.5% water. Solvent which is removed from the bed being cooled is readsorbed on the bed being heated.

The process, which is patented, brings important benefits of regeneration gas flow rates. Such low flow rates mean that the process requires less heat, less refrigeration and a lower cost regeneration loop. The overall efficiency is high because the low residual solvent loading of the regenerated beds leads to increased solvent recovery.

Two separate chillers plus molecular sieve bed

A two chiller system with a molecular sieve bed has to be regenerated only about once per week, since most of the water is removed in the chiller and readsorbed on the cooled carbon bed. This process offers a high solvent recovery rate giving a solvent with only 0.1 % water at greatly reduced heat consumption, lower investment, and higher solvent recovery rate.

Key plants with all the above mentioned processes are offered by special engineering companies.31'32 The investment cost for a typical recovery plant with hot gas regeneration can be estimated by the following equation:

where:

A feed air rate in Nm3/h

S solvent flow rate in kg/h

22.1.5.2.2 Solvent recovery with adsorption wheels

Adsorption wheels33 are used for the continuous purification of large volumes of exhaust air containing relatively low solvent concentrations.

The adsorption wheel consists of a number of identical chambers arranged axially around a vertical axis. All chambers contain adsorbent. The wheel rotates and each chamber passes in sequence over an exhaust air duct and solvent molecules are adsorbed. As wheel continues to rotate the chamber in which adsorption had occurred now moves into a desorption position in which hot air is passed through the chamber and over the adsorbent. This removes the adsorbed solvent. The hot air flow in the desorption sector is at relatively low flow rate compared to that of contaminated gas stream. The number of chambers in the desorption zone is much greater than the number in the absorption zone so the desorption stream is many times more concentrated in solvent than was the exhaust stream. At this higher concentration, the desorption stream can now be economically treated in one of the ways:

  • The adsorption stream is purified by means of recuperative or regenerative oxidation. This approach is advantageous in painting applications with solvent mixtures that cannot be reused in production.
  • For solvent recovery by condensation, the desorption stream is cooled down in a cooling aggregate and the liquid solvent is recovered for reuse. Water contents below 1% can be achieved without further purification.

Example:

A manufacturer who specializes in flexible packaging, i.e., for confectionary, always adds the same solvent mixture (ethanol, ethyl acetate, ethoxypropanol) to his printing ink.

Adsorptive solvent removal of the solvent-mixture by use of an adsorption wheel (Figure 22.1.20) and solvent recovery via condensation proved the technically and cost effective. 10,000 to 65,000 Nm3/h of exhaust air from printing machines and washing plants at a temperature maximum of45°C and a maximum solvent loading of 4.6 g/m3 is to be cleaned. Depending on the air volume two adsorption wheels with a capacity of 26,000 Nm3/h and 39,000 Nm3/h and separate desorption circuits are used either alternatively or together. The total desorption air of max. 11,000 Nm3/h is being concentrated to max. 27 g/m3, which corresponds to 50% of the lower explosion limit. The condensation unit for the solvent recovery process is gradually adjusted to small, medium or large exhaust air volumes. The recovered solvents are stored and returned to the production process. 22.1.5.3 Viscose industry

In plants which produce viscose fibre (stable fibre), viscose filament yarn (rayon) and viscose film (cellophane), large volumes of exhaust air contaminated with carbon disulfide

H2s Production Process
Figure 22.1.20. Adsorption wheel for solvent recovery in packaging printing (After reference 33).

(CS2) and hydrogen sulphide (H2S) have to be cleaned. Typical waste gas volumes and concentration of CS2 and H2S are given in Table 22.1.11.

Different cleaning/recovery processes are available for the removal of each of the two sulfur containing compounds:2'17'34'35 Hydrogen sulfide

  • Absorption of H2S in a NaOH-scrubber
  • Catalytic oxidation of H2S to elemental sulphur on iodine impregnated activated carbon (Sulfosorbon-process)
The sulphur is adsorbed at the internal surface at the carbon-catalyst. Table 22.1.11. Waste gas in the viscose industry (After reference 35)

Product

Spec. waste gas volume, m3/t

Concentration (mg/m3)

H2S

CS2

Rayon

400,000 - 700,000

60 -130

300 - 800

Staple fibre

50,000 - 90,000

700 -1800

2300 - 4000

Viscose fibre

100,000 -150,000

280 - 400

1000 - 3000

Carbon disulfide

Adsorptive removal on activated carbon and recovery by steam desorption. For simultaneous H2S and CS2 removal the Sulfosorbon-process uses adsorbers packed with two different activated carbon types

  • Iodine impregnated wide-pore activated carbon for H2S oxidation in the bottom part (gas inlet) of the adsorber
  • Medium-pore activated carbon for the adsorption of CS2 in the upper layer of the fixed bed.

As soon as the CS2 concentration in the treated air approaches the emission limit, the exhaust air stream is directed to a regenerated adsorber. After an inert gas purge, the carbon disulfide is desorbed with steam at 110to 130°C. The resulting CS2/steam mixture is routed through a condenser and cooler, before entering a gravity separator where phase separation occurs.

At the usual CS2 and H2S concentrations in viscose production exhaust air, the CS2 adsorption/desorption cycles can be run for a prolonged periods before the first layer loaded with elemental sulphur has to be regenerated.

The regeneration process of the sulphur loaded activated carbon involves the following steps:

  • washing out of sulphuric acid with water
  • extraction of elemental sulphur with carbon disulfide
  • desorption of carbon disulfide with steam
  • air drying and cooling of activated carbon

The elemental sulphur present in the carbon disulfide in dissolved form can be separated by distillation and recovered as high-purity sulphur.

Design example34

In the production of viscose filament yarn a Supersorbon®-system combined with a NaOH-scrubber system (Figure 22.1.21) has been in use since 1997. A very high standard of safety engineering is implemented in the treatment system owing to the flammability of the CS2 and the toxic nature of the constituents to be removed from the exhaust air.

System concept

First treatment stage:

Absorption of H2S in two NaOH jet scrubbers and one water-operated centrifugal scrubber.

Second treatment stage:

Fixed-bed adsorption for purification of exhaust air and CS2 recovery.

Treated air stream

Exhaust air from viscose filament yarn plant.

Design data:

Exhaust air flow rate 12,000 Nm3/h

Solvent capacity 27 kg CS2/h and 7.5 kg H2S/h

Exhaust air temperature 15 - 30°C

Clean air solvent concentration

Schwefelwasserstoff Absorption
Figure 22.1.21. H2S and CS2 removal in the filament yam production (After reference 34).

HS-removal

The exhaust air, saturated with water vapor, is first treated in an absorption unit for removal of H2S by routing it through two successive jet scrubbers working with dilute caustic soda solution. Downstream of these, a centrifugal scrubber is installed as an entrainment separator. The sulphide-containing solution rejected from the scrubbers is used to precipitate zinc in the waste water treatment system of the production plant. By products adsorbed on the activated carbon, such as sulphuric acid and elemental sulphur, are removed periodically by water and alkaline extraction.

CS2-removal and recovery

The pre-cleaned airstream containing carbon disulfide vapors passes through two or three parallel adsorbers, in which the CS2 is absorbed on a bed of Supersorbon® activated carbon. As soon as the adsorbent is saturated it is regenerated by desorbing the solvent with a countercurrent flow of steam. The resulting mixture of water and CS2 vapors is condensed and separated in gravity settlers. The recovered CS2 is returned to the viscose production without further treatment. The condensed steam is stripped of residual CS2 in the centrifugal scrubber and used as dilution water for operation of the jet scrubbers.

Operating experience

The specified purity of the treated exhaust air is reliably achieved with the Supersorbon® process34 with respect to both CS2 and H2S. The CS2 recovery rate is about 95%. The purity of the recovered solvent meets viscose production specifications. A widely varying CS2 concentration in the exhaust air has not adversely affected operation of the adsorption system.

Was this article helpful?

0 0
Going Green For More Cash

Going Green For More Cash

Stop Wasting Resources And Money And Finnally Learn Easy Ideas For Recycling Even If You’ve Tried Everything Before! I Easily Found Easy Solutions For  Recycling Instead Of Buying New And Started Enjoying Savings As Well As Helping The Earth And I'll Show You How YOU Can, Too! Are you sick to death of living with the fact that you feel like you are wasting resources and money?

Get My Free Ebook


Responses

  • Elio Flocco
    Good Afternoon,<br /><br />we are very interested in this article (Packaging Printing Industry).<br />We would like to get further information about the plant of the Figure 22.1.20.<br />Do no know if there is any reference of a similar plant? <br />Thanking you for your attention.<br /><br />Best regards,<br /><br />Ing. Elio Flocco<br />BROFIND S.P.A.
    8 years ago
  • Temshe
    How recycle molecular sieve bed oxygen?
    8 years ago
  • Nicol
    How to produce rayon yarn?
    8 years ago

Post a comment