Acetaldehyde a colourless, flammable liquid, with a characteristic fruity order having a boiling point of 21 0C is an important chemical intermediate for many chemical industries. It is utilized principally for the commercial production of acetic acid, to such an extent that in 1976 approximately 60 % of the acetaldehyde demand arose due to its use in the production of acetic acid. In addition to this acetaldehyde has a history for being utilized in the production of other chemicals such as: pyridine and pyridine containing bases, butylene glycol, chloral, peracetic acid, acetic anhydride, cellulose acetate, vinyl acetate resins, terephthalic acid, pentaerythritol, glyoxan and various other alkylamines.
For any entrepreneur hoping to engage in this industry several commercial production methods are available, with commonly utilized feedstock's being ethylene, ethanol and acetylene. The major factor that influences the production route is often the price of the feedstock utilized in the manufacturing process. The major processes employed in the commercial production to-date are; Wacker- Chemie (two stage) and Farbwerke Hoechst (one stage): both employing oxidation of ethylene, Catalytic oxidation process: utilizing ethyl alcohol (480 0C temperature and silver catalyst), Hydration process and direct oxidation: employing acetylene ( 15 psi pressure, mercury catalyst, 70-90 0C and Synthesis gas process: utilizing carbon monoxide and hydrogen (single step, 5% rhodium SiO2 catalyst, 300 0C and 4000-6000 psi).
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For the purpose of this assignment the two stage Wacker- Chemie process will be discussed, and any relevant operating conditions/ process details will be presented with the motive of aiding the material selection and specification for a plant hoping to utilize the Wacker- Chemie process. A brief insight into the economics (i.e operating costs, utilities etc) and a summary of the recommendations will be presented in the concluding sections of this document.
Before proceeding any further the author of this document wishes to state the following to the reader/assessor: After carrying out the literature search it was seen that examples of acetaldehyde plants described in literary sources was almost at every instance incorporated with an acetic acid plant. Any inferences/ recommendations made by the respective authors in literature with regards to the designs aspects of the plants described collectively for both processes; acetaldehyde production and conversion to acetic acid, therefore hard to justify every minute detail for material selection. Another obstacle was faced in finding the exact operating conditions, other than that specified in generalized process descriptions sourced from literature. However, a modest attempt was to fulfil this deficiency by presenting the "Real-time Optimization of Acetaldehyde Production Process" described by Shao Zhijiang and colleagues.
Chemistry of Wacker-Chemie Process
A publication by Reinhard Jira in the journal Angewandte Chemie in 2009 to commemorate the fiftieth anniversary of this process describes it as being "One of the most important and successful processes developed by the chemical industry after World War II". The chemistry of this process involves the direct liquid phase oxidation of ethylene in the presence of a catalyst, which is an aqueous solution of palladium chloride (PdCl2) and copper chloride (CuCl2). The major reactions that forms the basis of this aqueous phase oxidation process is presented below:
C2H4 + PdCl2 + H2O ïƒ CH3CHO + Pd + 2HCl (Eq. 1)
Pd + 2CuCl2 ïƒ PdCl2 + 2CuCl (Eq. 2)
2CuCl + Â½O2 + 2HCl ïƒ 2CuCl2 + H2O (Eq. 3)
As seen from (Eq. 1) palladium is re-oxidized to form PdCl2 with CuCl2 and copper (i) chloride formed is simultaneously re-oxidized. The overall process occurring, which is the continuous oxidation of ethylene via an array of oxidation-reduction reactions can be summed up as in the equation below:
C2H4 + Â½O2 ïƒ CH3CHO Î”H = -244 kJ (Eq. 4)
Wacker-Chemie Process Description
In the Wacker- Chemie process (Fig. 1- block flow diagram, Fig 2- detailed process flow diagram) ethylene is oxidized by the oxygen present in air to form acetaldehyde in a tubular plug-flow type reactor (Fig. 2 [a]). Almost complete oxidation is achieved in a single pass
Fig. 1. Block flow diagram of two stage acetaldehyde production process
through this reactor. The oxidation process is carried out at 125-130 0C and 1.135 MPa (150 psig) in the presence of palladium and cupric (Cu2+ oxidation state) catalysts. The product (acetaldehyde) that is produced in the reactor vessel is removed from the loop by adiabatic flashing. This process also helps to remove the heat generated during the reaction. In the next vessel, oxidation reactor (d) the catalyst solution recycled from flash-tower base is introduced (pumped using pump (c); Fig. 2). Here, the CuCl is oxidized to CuCl2 utilizing air. The off-gas from this reactor (major composition being nitrogen and having a high pressure) is separated from the aqueous catalytic solution utilizing separator (e), scrubbed to remove any traces of acetaldehyde and vented to atmosphere. To destroy any undesirable copper oxalate produced during the series of reactions, a small portion of the catalyst stream is heated in a catalyst regenerator. In the distillation system, to which the flasher overhead is fed; water is removed and then recycled back to the reactor system. Any organic impurities such as chlorinated aldehydes present are separated, yielding a pure acetaldehyde product.
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Fig. 2. Detailed process flow diagram of two stage acetaldehyde production process
As previously mentioned in the introduction to source the specific operating conditions for a typical Wacker-Chemie process was a difficult task. The process flow chart as presented in Zhijiang et al (Fig. 2) is shown not as a repetition to that depicted in Fig. 1, but rather to show in detail the specific process parameters e.g. condensed water, desalted water, oxygen stream etc obtained from the ASPEN software, which its model is based upon.
Fig. 3. Process Flow Chart for Acetaldehyde Production process described in Zhijiang et al.
The data generated by this model, which theoretically closely mimic an industrial process is presented in Fig. 3. In the upcoming section, any justification for the selection of the respective materials are based mainly on this data and that described in section 3 (Wacker-Chemie process description)
Fig. 4. Operating conditions for Acetaldehyde Production process described in Zhijiang et al.
Material Selection: Major Plant Components
In the subsections presented below the suitability of several materials of construction are discussed in detail in the context of their usability in the Wacker-Chemie process. The process conditions are inferred from Fig. 3.
Many easily sourced stainless steel types, especially those containing molybdenum can be utilized for the construction of the plant, except the tubular plug-flow reactor which can be subjected to severe corrosion due to the aggressive catalyst solution. However, in stating this it is not impossible to eliminate by careful coupling of material, possibilities discussed bellow and upcoming sections.
Type 304 S30400 (UNS) - This type of stainless steel can be utilized in regions of the plant where there is no-aeration and rapid agitation. This material therefore has good potential to be utilized for the construction of any long-term storage tanks/vessels where such conditions are a requirement. According to test wok studies in 100 % acetaldehyde solution at 61 0C the corrosion rate was 0.003 mm/yr.
Type 316 S31600 (UNS) - From literature it seen that this type is the most prevalent type of stainless steel utilized for acetaldehyde production plant. Under slight- moderate aeration conditions and in the presence of impurities (acetic acid: 12-8%, other low boiling organics: 3%), together with acetaldehyde: 50-70% has a corrosion rate of 0.003 mm/yr. At temperatures between 92-104 0C under the pre-described conditions corrosion resistance was also observed. However, at 97-100 % acetaldehyde concentration, 61-66 0C temperature, no agitation and slight to moderate aeration conditions a corrosion rate of 0.003 mm/yr. This makes 316 S31600 (UNS) type also a candidate for storage tanks, distillation unit components, waste gas vent stream and scrubbing stream components.
Type 317 S317700 (UNS)- This specific type at acetaldehyde concentrations of between 50-98% , 92- 118 0C temperature range, 12 % acetic acid and 3% low boiling organic impurities exhibits a maximum corrosion rate of 0.003 mm/yr. If it is possible to source this material it is slight better than type 316 due to its ability to tolerate a slightly temperature though it has an identical corrosion rate with the former. It is ideal for utilization in any pre-heaters (together with Remaint 1710S [Cr, 17, plus Mo] tubes), distilling unit components and degassing columns.
The utilized of alloys such as Worthite and Duriment for pumps and valves can be suggested, especially in the coarse acetaldehyde stream and distillation units. Hastelloy B and C are also widely utilized in the acetic acid and acetic anhydride industries with losses less than 1 mm/yr.
Craig et al states that aluminium alloy 1100 was resistant in acetaldehyde solutions of concentration range 0.1-100%. Though Lee et al states that if impurities such as acetic acid is present at higher concentrations in highly dilute environments at boiling point of the mixture, corrosion is a possibility. This aspect has to be accounted if aluminium is to be utilized for distillation unit components, stills and condensers. Nevertheless, it can be specified for the utilization in tubing components in heat exchangers, stills and especially in storage tanks and shipping drums. If the strength and other physical properties of the material is insufficient for the material to be used on its one e.g. for building storage tanks, aluminium lining can be used in components made of 304, 316 and 317 type stainless steel to give it extra protection.
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Titanium is the only stable metal that is able to withstand the aggressive catalyst solution, which other materials though utilized with protective lining of various rubber and resins are unable to withstand. The tubular plug-flow reactor should therefore be made out of titanium, though it is a costly material to specify the pre-said advantage with its long service life makes a compromise. The oxidation reactor too should be made of titanium since the catalyst solution is recycled from the base of the flash tower. Any pumps utilized for pumping this highly corrosive catalyst mixture should also be made of titanium for a long service life, though it is possible to offer high silicon iron pumps as an alternative (refer to section 5.4).
High Silicon Iron
It is important to state that the properties of this material is such that it is extremely resistant to the temperature, concentration, purity of acetaldehyde or other process conditions such as aeration. Therefore it is recommended to be utilized for piping, fittings and valves. The material also demonstrates good corrosion resistance therefore can be suggested as an attractive alternative to titanium pumps. However, if pumps are made with such materials it is a requirement that the pumps be pre-warmed before operation to prevent any damage occurring from heat shock. It is also recommended to utilize high silicon iron fittings where catalyst handling operations are involved.
In the days pre-dating to the utilization of titanium for tubular plug-flow reactor and oxidative reactor rubber linings were extensively utilized in the reactors, pipes and vessels. For the purpose of safety to the other stainless vessels (absorber, degasser and acetaldehyde tower) though they may be made of 316 or 317 type stainless, it is recommended that they be coated with rubber. Semi-hardened or hardened rubber generally suites this application, if softer rubber is required it can be made from natural rubber or GR-S type rubber. Caution should be taken before its use due to the possibility that colour from the rubber may be extracted into the acetaldehyde, though Lee et al suggests that this being invisible to the naked eye in typical acetic acid operations.
Due to its tendency to corrode at a higher rate than the pre-suggested materials of construction such as stainless and high silicon iron it is not recommended to utilize monel in a large scale. However, it is fitting to be utilized in heating coils and piping, especially that is involved in transferring the aldehyde product to storage facilities
The chemical resistivity of carbon makes it suitable for its application in certain piping fittings and globe valves. In the past it also has demonstrated a good service life without any noticeable corrosion for many years. It also shows no discoloration irrespective of any adverse process condition.
Teflon filler can be utilized for the gasket material of flanges. The exact filler material is chosen depending on the material flowing through and its temperature. It is recommended also that any O-rings and packing material be utilized having Teflon.
The following economic analysis adapted from Gunardson et al for a Wacker-Chemie process plant having a production capacity of 300 million pounds per annum is presented below in Tables 1-4.
Cost (Millions of Dollars)
Off-sites and tankage
Total fixed capital
Table 1. Acetaldehyde capital investment
Miscellaneous production costs
SG & A
Return of investment (25% per yr of TFC)
Table 2. Acetaldehyde production costs
Cost (Dollars per metric ton)
Aluminum 1100 alloy
Table 2. Typical costs of the major materials recommended for construction