Ind. Eng. Chem. Res. 2009, 48, 2801–2809
2801
Influence of EDC Cracking Severity on the Marginal Costs of Vinyl Chloride Production Reinhard Schirmeister,*,† John Kahsnitz,‡ and Michael Tra¨ger† VESTOLIT GmbH, P.O. Box 102360, D-45753 Marl, Germany, and EVonic Degussa GmbH, D-45753 Marl, Germany
The selectivity of the EDC (1,2-dichloroethane) cracking process in vinyl chloride monomer (VCM) manufacturing strongly depends on the cracking severity and the EDC feed quality. A strong decline of selectivity is observed for EDC feed purities below 99.5 wt % or cracking rates above approximately 60%. The selectivity of the cracking process also deteriorates in the course of the plant’s operating time as a result of accelerated coke formation. Reduced selectivity in the process of EDC cracking results in increased marginal costs (additional costs per each additional tonne of VCM). This is especially relevant for a VCM plant running near full load. The marginal costs can be expressed as a function of the cracking rate and the resulting costs for processing the uncracked portion of EDC. With a new and simplified EDC cracking simulation model for the energy input and conversion, together with a simplified reaction model for the definition of the corresponding product spectrum, the cost impact of the aforementioned influences on the cracking process can be estimated in advance based on the current production rate and plant performance. 1. VCM Manufacturing Process To manufacture vinyl chloride monomers (VCMs) economically through the thermal cracking of 1,2-dichloroethane (EDC), hydrogen chloride (HCl) produced in the cracker must undergo further processing, normally by oxychlorination to yield EDC. Under practical conditions, the cracking process is run with a residence time of 20-30 s in the gaseous phase at 480-520 °C. It is typically conducted until an EDC cracking rate of about 55-63% is reached in order to avoid a strong increase of byproducts. The VCM yield after the cracking furnace is at about 98-99% based on cracked EDC. Volatile and high-boiling byproducts are subsequently removed from the residual amount of unconverted EDC as much as possible by distillation. The latter is recycled back to the cracker along with the EDC from other process steps (direct chlorination and oxychlorination units), resulting in a typical EDC feed purity of >99.5 wt %. The overall selectivity of the entire VCM manufacturing process including reprocessing is hence even lower. In an ethylene-based integrated balanced process (see ref 1, p 290) such as that used in VESTOLIT’s production plants (Figure 1), HCl production in the cracker and HCl consumption in other production units must be balanced. The “EDC mix” in Figure 1 represents the fresh feed to the crackers, resulting from EDC from direct chlorination, EDC from oxychlorination, and EDC from reprocessing. The byproduct formation in the EDC cracking process essentially determines the raw materials lost during distillation product reprocessing and, hence, the VCM yield and the resulting VCM manufacturing costs. A high EDC throughput at a concurrently high cracking rate can result in a high VCM production; however, it also increases the amounts of undesired byproducts. These are, in particular, chlorinated C1, C2, C4, and C6 hydrocarbons and unsaturated hydrocarbons such as acetylene and benzene. The availability of the plant is strongly reduced in the course of such a procedure as a result of coke deposits. * To whom correspondence should be addressed. E-mail:
[email protected]. † VESTOLIT GmbH. ‡ Evonic Degussa GmbH.
These deposits are visible on the walls of the ends of cracker tubes, both on the connecting tubes outside the radiation zone and in the subsequent heat recovery section. Lower cracking rates improve the yield but also result in lower VCM production, higher specific energy costs, and higher specific losses of raw material (amounts of residues and waste gas). Higher specific energy costs are due to higher residual amounts of EDC that require reprocessing steps. Experience shows that regular downtimes lasting over several days are inevitable after every 1-2 years of operation for cleaning of the cracker. To avoid expensive storage of intermediate products, it is important for the entire production line, including all associated units, that these downtimes are planned reliably in advance. The undesirable effects already mentioned Table 1. Compounds, Radical Species, and Short Forms Used in the Reaction Model in Figure 2 and Table 2 no.
compound
short form
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
1,2-dichloroethane vinylchlorid hydrogen chloride trichloromethane tetrachloromethane ethylchloride 1,1-dichloroethane 1,1,2-trichloroethane 1,1,1,2-/1,1,2,2-tetrachloroethane 1,1-/cis-/trans-dichloroethylene trichloroethylene 1-/2-chloroprene acetylene benzene 3,4-dichlorobutene soot/coke
EDC VCM HCl CHCl3 CCl4 EC 1,1 1,1,2 1,1,1,2 Di Tri CP C2H2 C6H6 C4H6Cl2 C
1 2 3 4 5 6 7 8
Cl• CH2ClsCH2•/CH3sCHCl• CH2ClsCHCl• CHCl2sCH2• CHCldCH•/CH2dCCl• CH2ClsCCl2•/CHCl2sCHCl• CHCl2sCCl2•/CCl3sCHCl• CCl3•
Radical
10.1021/ie8006903 CCC: $40.75 2009 American Chemical Society Published on Web 02/13/2009
R1 R2 R3 R4 R5 R6 R7 R8
2802 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009
Figure 1. EDC/VCM production at VESTOLIT.
can counteract this schedule with a dramatic increase toward the end of the operating time. It is practically impossible to monitor changes in cracked products by means of online analytics (e.g., gas chromatography or mass spectroscopy) at the outlet of the cracker coils because of the extreme conditions encountered there (approximately 12 bar pressure and 500 °C). Solids (soot and coke) and the condensation of high-boiling substances do not permit a correct withdrawal of samples. For this reason, a VCM production plant was modeled with all of the information from the upstream and downstream process and all available analytics. Simplified kinetic/reactor models were designed to calculate the effects of changes in the overall performance rate. 2. Kinetics and Cracking Model The radical chain mechanism of thermal cracking is wellknown in principle. Experimental studies and rigorous mathematical models of the complex EDC cracking reaction can be found in the literature (e.g., refs 2-7). Up to 135 compounds and radical species and more than 800 reactions have been reported for comprehensive calculations.4 In view of the accuracy of the data and optimization of the expenditure for model adjustments, simplifications must be made. In our model, this resulted in 31 reactions describing all relevant products, intermediates, and byproducts (cf. Tables 1 and 2). Benzene formation and the very complex soot and coke formation via acetylene, benzene, chloroprene, and polycyclic aromatics (cf. refs 8-11) are considered by reactions 30 and 31, which represent the summed pathways of model chain reactions. We also found more than 15 unknown high-boiling species in the GC analysis but neglected them in our model because they amounted to less than 0.1 wt % of the total mass flow. The component 3,4dichlorobutene was used as a key component for high boilers with close boiling points.
2.1. Reaction Model. Concerning the endothermic reaction 1,2-EDC f VCM + HCl ∆HoR ) 71 kJ/mol ) 0.717 kJ/kg (1) the conversion inside the tubular reactor depends on the local energy input by means of heat radiation on the heating side and local heat transfer. The local energy input is consumed for heating and for the reactions. For the calculation of the EDC conversion along the nonisothermal and nonisobaric tubular reactor, additional information about the axial temperature profiles of the cracker coil is needed. Depending on the cracking conditions (feed, conversion, heating) and operating time, these temperature profiles change with locally accumulating soot/coke layers, which reduce the local heat-transfer coefficient. As a consequence, a rising temperature level on the heating side compensates for the related heat flux reduction. Otherwise, the total conversion would decrease. This temperature is limited by material constraints of the tube material (e.g., Incoloy 800H with a working temperature up to 700 °C). To simulate the spectrum of byproducts analyzed in the technical plant, a simplified reaction model was designed on the basis of the published radical chain mechanisms, including data on bond energies, collision coefficients, and activation energies of each partial reaction step.2-6,12–18 If no data were available, analogous data of the same reaction type were applied for the parameters of the reaction rate constants. Table 1 lists all 16 “stable” substances and 8 radicals and their brief descriptions used in this kinetic model (cf. Table 2). The following assumptions were made for this model: (1) The reaction start by radicals via homolytic CsCl bond cleavage from EDC and CCl4 (reactions 1 and 2). (2) The chain reactions proceeds (a) by H abstraction from the stable compound to form a new radical (reactions 3-9, 11, and 19-23); (b) by decay of radicals, forming Cl• and unsaturated stable compounds (reac-
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 2803 Table 2. Kinetic Model Reactions Kinetic Data (Ahrrenius Law Expression Constants from the Literature and Estimated) no.
reaction
frequency factor ko [(cm3/mol)n-1 s-1]
n
activation energy Ea (kJ/mol)
ref
15
1 2
EDC f R1 + R2 CCl4 f R1 + R8
5.9 × 10 2.2 × 1012
1 1
342 230
3, 5 12
3 4 5 6 7 8 9 10 11 12 13 14
EDC + R1 f HCl + R3 EDC + R5 f VCM + R3 EDC + R2 f EC + R3 EDC + R4 f 1,1 + R3 EDC + R6 f 1,1,2 + R3 EDC + R7 f 1,1,1,2 + R3 EDC + R8 f CHCl3 +R3 VCM + R1 f R4 VCM + R1 f HCl + R5 VCM + R5 f CP + R1 VCM + R4 f C4H6Cl2 + R1 VCM + R2 f EC + R5
1.3 × 1013 1.2 × 1013 1.0 × 1012 5.0 × 1011 2.0 × 1011 1.0 × 1011 1.0 × 1012 9.1 × 1010 1.2 × 1014 5.0 × 1011 2.0 × 1010 3.0 × 1011
2 2 2 2 2 2 2 2 2 2 2 2
7 34 42 45 48 56 63 0 56 31 30 61
4, 5 4 4 est est est 4 5 13 4 est 4
15 16 17 18
R3 R5 R6 R7
2.1 × 1014 5.0 × 1014 2.0 × 1013 2.5 × 1013
1 1 1 1
84 90 70 70
5, 14 est est 15
19 20 21 22 23
EC + R1 f HCl + R2 1,1 + R1 f HCl + R4 1,1,2 + R1 f HCl + R6 1,1,1,2 + R1 f HCl + R7 CHCl3 + R1 f HCl + R8
1.7 × 1013 1.2 × 1013 1.7 × 1013 1.7 × 1013 1.6 × 1013
2 2 2 2 2
4 6 15 17 14
16 16 17 17 18
24 25 26
CCl4 + R5 f Di + R8 CCl4 + R4 f 112 + R8 CCl4 + R6 T 1112 + R8
5.0 × 1011 1.0 × 1012 5.0 × 1011
2 2 2
33 33 33
est 4 est
27 28 29
R2 + R1 f VCM + HCl R3 + R1 f Di + HCl R6 + R8 f Di + CCl4
1.0 × 1013 1.0 × 1013 1.0 × 1013
2 2 2
13 12 13
4 est est
30 31
2C2H2 + R5 f C6H6 + R1 C2H2 +2 R1 f 2C + 2 HCl
1.0 × 1014 1.6 × 1014
2 2
20 70
est 4
T T T T
VCM + R1 C2H2 + R1 Di +R1 Tri + R1
tions 15-18 and final step of reaction 30); (c) by addition of Cl• (back-reaction of radical decay) or other radicals to double bonds (reactions 10 and 12-14 and initial step of reaction 30); and (d) by Cl• abstraction from CCl4 (reactions 24-26). (3) Chain terminations and decreases of the total radical concentration are considered to occur (a) by H abstraction from a radical, forming two stable compounds (reactions 27-29) and (b) by carbon formation (soot/coke) from acetylene (reaction 31). Not considered in our model are (1) HCl split-off reaction from chlorinated hydrocarbon in a monomolecular reaction [which is unlikely to occur concurrently with fast chain reactions because of its high activation energy (>200 kJ/mol)], (2) catalytic wall reactions, and (3) complex synthetic pathways from possible carbon precursors such as chloroprene and benzene via polycyclic aromatic hydrocarbons (PAHs). In Table 2, the partial reactions and applied kinetic parameters of the reaction rate constants are given. In the cases of benzene formation (reaction 30) and complex carbon formation (reaction 31), the parameters of the rate-controlling steps are used. Figure 2 visualizes these reaction pathways to all relevant “stable” components found in the feed or product streams after cracking. Byproducts showing a high change in concentration are shown in bold. The radicals are represented in ellipses. Reaction 1 (Cl dissociation from EDC) and reaction 2 (CCl4 dissociation) are the initiation reactions. At 500 °C, the rate constant for reaction 2 is approximately 104 times higher than the rate constant for reaction 1. However, with CCl4 from reprocessing, the initial CCl4 concentration in the EDC feed stream can give a rate of radical formation that is of the same order of magnitude as that of reaction 1. Reaction 2 becomes
relevant below 500 °C if the initial CCl4 feed concentration is >100 ppm or if in situ formation of CCl4 (for example, from CHCl3) occurs. The role of CCl4 in the EDC feed as a promoter has been described in the literature.4,19 The role of CCl4 in our reaction model (chain reactions 24-26) is similar to that of Cl2 in chlorination models. The bond dissociation energies for the two molecules are nearly the same as well. The main reaction pathways from EDC to the desired VCM are H-abstraction reactions 3-9 followed by decay of radical R3 in reaction 15. With decreasing EDC and increasing VCM concentration the formation of radical R5 becomes relevant, which can yield the desired VCM (reaction 4) or unwanted chloroprene and acetylene (reactions 12 and 16, respectively). For the byproducts acetylene, chloroprene, and 1,1-dichloroethane, a high net formation is observed. The 1,1,2-trichloroethane in the EDC feed is found to be mostly consumed. In the present model, a decrease of the total radical concentration (“chain termination”) is considered only by formation of carbon (reaction 30) and H abstraction between radicals as in reactions 27-29. Normally, chain termination by radical recombination needs a solid wall or a third molecule (as a third collision partner) to absorb the high energy released by recombination. The surface-to-volume ratio in laboratory studies5,19 is normally much higher than that in technical plants, or the pressure is low. In our case, with higher pressure, we assume that those wall collisions are negligible relative to bimolecular collisions in the gas and that terminations on the wall can be neglected. This assumption is also based on the observation that no significant amounts of C3 compounds from recombination of C1 and C2 compounds are found in the
2804 Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009
Figure 2. EDC cracking reaction model (measured compounds from large-scale plant with a significant increase or decrease shown in bold).
cracked gas of our technical plant. This is in agreement with exit gas composition data from Borsa.4 2.2. Coke Formation. The formation of solid carbon particles transported by the fluid (so-called “soft coke”) and coke layers (so-called “hard coke”) and the possible role of tube material or iron chloride are discussed in detail elsewhere.9-11,20 In agreement with other studies,4 we observed that, in technical plants, deposition of coke is not uniform. One important factor for local coke formation is the net flux of local radiation energy and the resulting temperature profile. Another factor is the local concentration of coke precursors in the reaction mixture. To date, local hot spots have not been observed in our plant. However, the heat input through the burners needs to be adjusted to optimize the yield and minimize the coke formation. In both cracking units, we found coke deposits on the wall of each coil outlet close to the radiation zone after 1 year in operation. Typical deposits in the coil outlet showed more or less uniform layers of about 1-5 mm. Visible coke deposits of 1-5-mm thickness were also on the walls of the insulated transfer pipes connecting each coil with its heat recovery (two heat exchangers in line) from cracked gas. The strongest deposition (>10-mm thickness) was observed at the inlet of the heat recovery of cracking unit 2. Coke particles were found in the condensed raw product as well. Deposits on the coils and especially in the heat recovery sections cause an increasing pressure loss over time and higher external wall temperatures. This pressure loss finally limits the EDC throughput. 2.3. Reactor and Furnace Model. Similar design principles of noncatalytic crackers are found in the patent literature (e.g. ref 343 in our ref 1) and undisclosed company reports. Apart from the more recent, very complex models of the heating side,22,23 only empirical information is available with regard to
the overall process in technical plants. Apart from confidentiality, one reason for this dearth of information might be that the energy input varies among VCM producers as a result of the various construction details of the cracking and convection zones. Moreover, the different numbers or arrangements of heating gas burners can be crucial. Both of our EDC crackers consist of a cracking furnace with two coils with a length of approximately 330 m, each with horizontal straight segments of approximately 15 m. In the furnace, there are five horizontal burner lines mounted on each vertical furnace wall plane to the staggered coils (cf. Figure 3). A constant ratio of air to fuel in the feed to the burner lines ensures a uniform adiabatic combustion temperature. The heat produced by the burner lines decreases stepwise from the EDC inlet to the cracked-gas outlet. Our model calculation starts with an overall heat balance of the complete furnace considering heat recovery from flue gas and stack losses. The total heat input to the coils by radiation (subscript rad) and convection (subscript conv) is equal to the heat produced by combustion of the fuel minus the heat (Qfurnace gas) leaving the cracking zone (compartments 1 and 2 in Figure 3) with the flue gas Qrad total+Qconv total ) m ˙ fuelHcomb - Qfurnace gas ˙ fuel + m ˙ air) Qfurnace gas ) (m
∫
Tfurnace gas
Tgas
Cp,flue gas(T) dT
(2) (3)
˙ air are the total mass flows (kg/s) of heating where m ˙ fuel and m gas and air, respectively; Hcomb is the combustion heat of the fuel (kJ/kg); Cp,flue gas is the specific heat of the flue gas [kJ/(kg K)]; Tfurnace gas is the temperature of the flue gas leaving the cracking zone (°C); and Tair is the temperature of the air feed to the burners (°C).
Ind. Eng. Chem. Res., Vol. 48, No. 6, 2009 2805
where εF and εP are the dimensionless emissivities of the furnace wall and pipe wall, respectively, and Ap is the cross-sectional area of the pipe wall (m2). The temperature, TF of the emitting furnace wall was about 780 °C. The outer wall temperatures, TPo j, in the coil sections were calculated in the simulations. As shown in Figure 4, at steady state, the local heat input on the coil is equal to the enthalpy consumed for both heating of the reaction fluid and conversion in the endothermic crack reaction. Tgas represents the mean gas temperature in the cross section of the pipe instead of a more realistic radial temperature profile at turbulent flow. The local heat balance for a coil section j with volume Vj gives Qrad j+Qconv j)Qcond j)Qreac j+Qheating j
(5)
Qcond j ) λp /spAp(TPo - TPi)j ) λcoke /scoke jAcoke(TPi - TWi)j )
∑ (-∆H ) r V
R k k j
+m ˙ EDC feedCp,gas j(Tgas j - Tgas j-1)
k
(6)
Figure 3. Vertical cut through the furnace and the coils.
The total heat input into the coils was about 60% of the heat produced by combustion. A typical value of Tfurnace gas was about 700 °C. To calculate the local radiation heat transferred on the coils, a heat flux profile in the furnace is required that considers the given geometric conditions, e.g., similar to calculations of the heat flux profiles in ethylene crackers.23 Then, the net local radiation energy, Qrad j, absorbed on a coil section can be calculated according to ref 24, Chapter K and with the normalized heat flux profile ψj and σS ) 5.67 × 10-8 W/(m2 K4) by Qrad j ) ψjεFεpσSAp(TF4 - TPo j4)
Figure 4. Local heat transfer with reaction inside the pipe.
(4)
where Qcond j is the heat transferred through the metallic wall and coke layer (kW); Qreac j is the heat consumed in chemical reactions (kW); Qheating j is the heat consumed for heating the reaction mixture (kW); TPo and TPi are the temperatures (°C) of the outer and inner walls, respectively, of the metallic pipe; TWi is the inner wall temperature (°C); λp is the heat conductivity of the Incoloy wall [16.3-24.7 W/(m K) at 300-800 °C]; λcoke is the heat conductivity of the coke [i.e., amorphous carbon, 1.6 W/(m K)]; Sp and Scoke are the thicknesses (mm) of the pipe wall and coke layer, respectively; Ap and Acoke are the areas (m2) of heat transfer surfaces in the coil section; rk is the rate of reaction k [mol/(m3/s)]; ∆HR is the reaction enthalpy (kJ/ mol); mEDC feed is the mass flow of EDC feed (kg/h); Cp,gas j is the mean specific heat of the reaction mixture in section j [kJ/ (kg/K)]; and Tgas j is the mean temperature of the reaction mixture in section j (°C). The local heat transfer by convection on the heating side (typical
