diagram 1: Flow sheet of a solvent recovery system

Solvent recovery by adsorption is usually a batch operation involving multiple beds. At least one activated carbon bed remains online while the other is being regenerated.

The adsorber inlet gas stream is pre-treated to remove solids (dust), liquids (droplets or aerosols) or high-boiling components since they can hamper performance. Most of the systems pass the solvent-laden air stream upward through a fixed carbon bed.Spent carbon is usually regenerated with downward flowing low-pressure steam. This removes the adsorbed solvent, which is recovered by condensing the vapours and separating the solvent from water by either decantation or distillation.

The counter-current pattern of adsorption and desorption favours high removal efficiencies.

After steam regeneration the hot wet carbon bed is being dried by a hot air stream. Before starting the next adsorption cycle the activated carbon bed is being cooled by an air stream at ambient temperature.Although the method is proven from a technical point of view some operators report problems when recovering solvents of the ketone group.

During the adsorptive removal of methylethylketone and cyclohexanone a reduced adsorption capacity has been measured which went along with corrosion problems or, in some cases, even with spontaneous ignition of the activated carbon.

It may be of interest that spontaneous ignitions were mostly observed after an extended shut-off time, e.g. after weekends. The use of virgin activated carbon or fresh carbon make-up also caused adsorber fires in some cases.

Experiments:

We ran thermogravimetric tests in our laboratories in order to find out about the cause of the problems which are obviously due to undesirable chemical reactions of the ketones.

Our activities were to give an answer to the following questions:

• Is the activated carbon affected by undesirable chemical reactions during ketone adsorption or do these reactions happen only in the higher temperature ranges of the desorption steps?
• Which reaction products are generated by the ketones during reactions taking place on the inner surface of the activated carbon?
• Do the raw materials of the activated carbon have any influence on these undesirable reactions?
• Has ash content or ash composition any influence on the reactions of ketones, or are certain other properties of the activated carbon more important?
• What precautions should the operators of adsorption plants take for ketone recovery in order to optimise the process?
  The results we expected from our experiments were to contribute to the solution of these questions.

  The first thing we examined was whether the ketones, methylethylketones and cyclohexanone, underwent some chemical reaction as early as during adsorption or only during the subsequent desorption step.

  To this end we started to measure, at different temperatures, the adsorption equilibria of cyclohexanone during adsorption from inert helium and from air.

  Diagram 2 will show you, at first sight, a surprising result:

   

  

  diagram 2: Adsorption of cyclohexanone as a function of temperature

  If cyclohexanone is adsorbed from helium excluding oxygen, the equilibrium loads go down as increasing adsorption temperature increases, which was expected for physical sorption mechanisms. The loads measured at various temperatures comply with the equilibrium levels calculated in accordance to the theory of Dubinin.

  Whenever cyclohexanone is adsorbed from an air flow, however, influence of adsorption temperature is no longer perceived. Between 40°C and 60°C, loads markedly above the equilibrium are measured. The difference between the two curves stands for the amount of weight percentages of cyclohexanone having undergone reactive alterations already during adsorption.

  Activated carbon therefore acts as a catalyst already during adsorption and we were able to measure, indeed, catalytic surface reactions of the cyclohexanone. The diagram also illustrates the influence exerted by adsorption temperature. The lesson taught by the above results is that, for the sake of keeping catalytically induced reactions low, ketone adsorption should happen at low temperatures between 20°C and 30°C.

  The next important question is what chemical compounds may be formed during surface reactions?

  To find out if this activated carbon was loaded first by cyclohexanone or methylethylketone from an air flow at 40°C. Then the desorption of the sample was carried out in the temperature range of 40°C to 900°C. The desorption gas was then examined by mass spectrometry.

  As diagram 3 demonstrates, the catalytic reactions occurring for cyclohexanone are predominantly of the oxidation type. First adipic acid is formed from the cyclohexanone. Adipic acid has a relatively high boiling point of 213°C, at a 13 mbar vacuum.

   

  

  diagram 3: Surface reaction of cyclohexanone on activated carbon

  Further products identified were: cyclopentanone as a degradation product from adipic acid, phenol, toluene, dibenzofurane, aliphatic hydrocarbons and carbon dioxide.

  The high-boiling adipic acid cannot be desorbed by activated carbon in a usual steam desorption. So, due to such irreversible adsorption, the life of the activated carbon becomes further reduced after each adsorption/desorption cycle.

  This declining additional load, along with a progressing number of adsorption cycles, is illustrated by diagram 4:

   

  

  diagram 4: Additional load of cyclohexanone as a function of the adsorptive cycle.

   

  The reaction products are acetic acid, di-acetyl, and presumably also di-acetyl peroxide. Di-acetyl has an intensive green coloration and distinctive odour. Important for the heat balance of the adsorber is that all of the reactions are strongly exothermic.

  Unlike the side products of cyclohexanone, acetic acid as a reaction product of the MEK will be removed from the activated carbon during steam desorption. Adsorption performance thus is maintained even after a number of cycles. May we use the example of methylethylketone to illustrate what influence different raw materials of the activated carbon can have on the reactions mentioned.

   

  

  diagram 5: Surface reaction of methylethylketone on activated carbon.

  We started by examining three different activated carbons produced from lignite, peat and hard coal.

  On diagram 6, the acetic acid to methylethylketone ratio by the desorption as a function of adsorption temperature has been plotted.

   

  

  diagram 6: Ratio of acetic acid/methylethylketone as a funtion of the adsorption temperature.

  Here again you may recognise, above all, the influence of adsorption temperature: at 25°C none of the three activated carbons will form acetic acid.

  At adsorption temperatures beyond 40°C all of the three activated carbons produce some acetic acid. The acetic acid to methylethylketone ratio increases. Whilst there are gradual differences between the three activated carbons, one may still say that acetic acid formation goes up in the higher heat range.

  Whenever such surface reactions are to be minimised, a low adsorption temperature is more important than the raw material of the activated carbon. When we examine the influence of the mineral matter of ash constituents on the mechanism of undesirable reactions, we obtained some interesting results. In the literature, ash content or the constitution of activated carbon is mentioned as one of the possible causes of catalytic side-reactions.

  Quantity and constitution of the mineral matter contained in the three activated carbons were significantly altered by means of water and hydrochloric acid extraction. We were surprised to find during our experiments that more cyclohexanone and methylethylketone reacted on the extracted activated carbon than on the three untreated carbons.

  Investigations into the causes of such increased activity showed that the extraction process promoted the formation of surface oxides. It can be concluded from the micro-crystalline structure of activated carbon that the carbonaceous layers or clusters, at least at their edges, contain chemically unsaturated carbon bonds. It is at these high-energy "active centres" that oxygen and hydrogen are bound.

   

  Diagram 7

  

  diagram 7: Structure of acid surface oxides. [Garten and Weiss/Mattson and Mank]

  Extensive examinations by different scientists also explained the chemical structure of these so-called surface oxides. This type of alteration of polarity at the carbon surface may influence the adsorptive capacities and on the mechanisms of catalytic reactions.

  The influence of surface oxides may be illustrated by way of the methylethylketone adsorption example.

  On diagram 8, the acetic to methylethylketone ratio as a function of the surface oxides of the various samples has been plotted.

   

  

  diagram 8: Ratio of acetic acid/methylethylketone as a function of the surface oxide content.

  The methylethlyketone was adsorbed from an air flow at 60°C. The diagram contains the three hard coal, peat and lignite based activated carbon types and six samples obtained by extraction with water and hydrochloric acid. The activated carbon with higher contents of surface oxides were created deliberately by oxidative treatment with nitric acid.

  The measuring results obtained clearly support that there is a linear dependence between the proportion of acetic acid created during the adsorption of methylethylketone on the one hand and the quantity of surface oxides on the other hand. These results supply the producers of activated carbon with the proper approaches to develop modified activated carbons for ketone removal.

  Finally we should like to give some comments to answer the fifth important question.
  What can the operator of an adsorption plant contribute to improve the recovery of those problematic ketones?

  Now, operators should adhere to certain rules and also take some precautions:

  • Adsorption temperature should not be beyond 30 ° C since the surface reactions increase exponentially with higher temperature.
  • The above requirement of low-adsorption temperatures includes:
    1. adequate cooling after the desorption step;
    2. a flow velocity of at least 0.2 m/s in order to deal adequately with the heat of adsorption.
  Since the ketones will, even in an adsorbed state, oxidise on the activated carbon due to the presence of oxygen, the three following recommendations should also be adhered to:
  • short adsorption/desorption cycles,
  • desorb loaded activated carbon immediately at lowest possible temperatures,
  • desorb, cool and inertise the plant before extended shut-off time.
  If "hot spots" still occur in an activated carbon bed they can be identified best by the presence of CO and CO₂ in the waste gas. Apart from the possibility of heat measurement at different points in the adsorber one should also provide measuring instruments for CO and CO₂.

  Some of these recommendations have been established empirically by constructors and operators and are being observed. We are sure that the consistent application of all improvements proposed can enhance the safety and efficiency of activated carbon plants for the recovery of problematic solvents.

  Please do not hesitate to contact us via Carbon Link with any of your queries related to this subject matter or any other activated carbon issue.

  
  During adsorption of methylethylketone in the presence of oxygen there are likewise some catalytic surface reactions going on.