Hydrocarbon By-Products

Successful petrochemical projects are characterised by either eliminating the production of by-products, or if they have to be produced, by maximising the value of by-products from the process. Here we are primarily concerned with maximising by-product credits. 

The basic value of a hydrocarbon by-product is its value as a fuel oil substitute for heating process furnace operations. Since fuel oil is generally the hydrocarbon of lowest value, degrading by-products to fuel value will result in a cost to the process - high value feedstock is 

A number of petrochemical processes produce significant volumes of hydrogen as a by-product, including pyrolysis cracking. This can be used as a fuel oil substitute, but this greatly undervalues hydrogen, and alternative use in other chemical processes is the better option and generally pursued by successful operations. 

Following is a discussion about the by-products from the various naphtha cracker streams  

The product of interest is ethylene and this is contaminated with ethane and acetylene. The most common practice is for acetylene to be selectively hydrogenated to ethylene using supported palladium catalysts  

---

C and 600 psig. Hydrocarbon by-products increase if the catalyst is reused and with increased temperature but decrease with increased pressure. Rhodium or palladium with rhenium also shows synergistic effects (27). A catalyst made from Re207 and Pd(N03)2-on-carbon gave a 97% yield of 1,6-hexanediol from adipic acid 10). 

Despite environmental concerns (Chapter 3), since 1980 MTBE has made a significant contribution to the lowering of VOC emissions from car exhausts. This is due to its clean bum properties, (producing fewer hydrocarbon by-products). MTBE is commonly produced in a fixed-bed reactor by passing a mixture of 2-methylpropene and excess methanol over... 

Polymeric hydrocarbon by-products accompany the products of the latter two reactions. The structures of the products are clear evidence of the occurrence of 1,2-alkyl shifts leading to more stable carbocationic intermediates. ... 

At temperatures between 900° and 2000°K. most hydrocarbons have a positive free energy of formation which, with the exception of acetylene, increases with increasing temperature (Figure 1). If coal carbonization could attain thermodynamic equilibrium over this temperature range, the hydrocarbon by-products would be decomposed mainly to carbon and hydrogen while any oxygen in the coal would be evolved as carbon monoxide. In... 

A less effective, but more economically viable method, would be to recycle all low-value hydrocarbon by-products to the cracker furnace. This particularly focuses on methane which within the confines of an operation is typically valued relative to the fuel oil price. However, this equally applies to ethane and propane which are generally recycled to the feedstock side of the cracking furnace. Depending on the relative value, it may be optimal for minimising carbon emissions in some operations to use ethane as a fuel rather than a feedstock. 

For the most part, we are concerned with hydrocarbon feedstock which is related to the prevailing crude oil price. For some feedstock and hydrocarbon by-product, this is a strong linear relationship. 

The liquid hydrocarbon by-product has a high cyclic content and so is useful as a petroleum refinery feedstock or as a source of aromatic organic chemicals. This material has a relatively high nitrogen content compared with the corresponding petroleum fraction. Its use as a refinery feedstock would require additional nitrogen removal processing by the refinery. 

The Chlorohydrin process involves the reaction of propylene with chlorine and water to produce propylene chlorohydrin. The propylene chlorohydrin is then dehydrochlorinated with lime or caustic to yield propylene oxide and a salt by-product. The chemistry is very similar to the chlorohydrin route from ethylene to ethylene oxide which was eventually replaced by the direct oxidation process. There are two major problems with the chlorohydrin route which provided the incentive for developing an improved process. There is a large water effluent stream containing about 5-6% calcium chloride or 5-10% sodium chloride (depending on whether lime or caustic is used for dehydrochlorination) and trace amounts of chlorinated hydrocarbon by-products that must be treated before disposal. Treatment of these by-products is expensive. The only practical way to handle it is to use caustic so that sodium chloride is produced and then integrate the effluent stream with a caustic-chlorine plant so that it can be recycled to the caustic plant. This, however, is also expensive because recovery of sodium chloride from this relatively dilute stream has a high energy cost. 

Two processes are currently used for the production of propylene oxide. About 50% is produced by the chlorohydrin process and the other 50% by the peroxidation process. The chlorohydrin process is the older technology and it is slowly being replaced by the more economical and environmentally acceptable peroxidation route. There are environmental issues associated with the large aqueous by-product stream of calcium chloride and chlorinated hydrocarbon by-products from the chlorohydrin process. The only producers that will continue to operate chlorohydrin plants are highly integrated caustic-chlorine producers who have chlorine production facilities which can handle the calcium chloride by-product and chlorinated hydrocarbons [9]. 

Selectivity of TBHP to both propylene oxide and rer/-butyl alcohol in the epoxidation reactor is 83%. The balance of the TBHP decomposes to hydrocarbon by-products and oxygen. For simplification, it is assumed that the decomposition is entirely to butane and oxygen. 

The oxygen required for the one-step process is calculated based on the reaction stoichiometry and catalyst selectivity. For simplification, it is assumed that the ethylene that does not form acetaldehyde is oxidized to carbon dioxide and water. Actually, several hydrocarbon by-products are also formed but the quantities are small and this simplification does not introduce appreciable error. 

The fluidized bed processes operate between 220-235 °C (430-455 F) and from 20-75 psig. The reaction is exothermic and the heat of reaction is removed by generating steam in internal coils in the reactor. Ethylene and HCl react quantitatively to EDC. A small amount of ethylene is oxidized to carbon oxides and some chlorinated hydrocarbon by-products are formed. About l-2< 7o of the ethylene feed to the reactor leaves unreacted in the vent gas from the system. A simplified process flow diagram depicting an air-based fluidized bed oxychlorination system is shown in Figure 14 [22]. 

The minimum feedstock energy is the value equivalent to the mass of hydrocarbon material that is needed to produce the finished polymer. Any unwanted hydrocarbon by-products can be used as a fuel that is, excess feedstock can be converted to a fuel. As a consequence, most polymer production units show a feedstock consumption that is very close to this expected minimum. Thus any attempts to make polymer production more energy efficient must concentrate on reducing the direct fuel consumption rather than the feedstock consumption. 

Typically, the copper-based famify of methanol synthesis catalysts are extremely selective. Methanol yields are hi relative to or nic byproducts, with generalfy over 99.5% of the converted CO + C02 present as methanol in the crude product stream. H20, of course, is normally a by-product, with a resultant concentration in the crude product that is influenced by the ratio of C02 to CO in the methanol synthesis reactor feed stream. Hydrocarbon by-products typically are present in concentrations of less than 5000 ppm(w) and consist of such compounds as the following ... 

Air is the usual oxidant and while natural gas is the most common source of hydrocarbons, by-product gases from other units such as an ethylene plant can also be used. 

One of the well-known phenomena in high-pressure polyethylene processes is a rapid ethylene decomposition reaction or thermal runaway, known as decomp. At 300°C, ethylene and even polyethylene decompose to carbon, methane, hydrogen, and other hydrocarbon by-products. When the decomposition reaction takes place, the reactor pressure builds up quickly and the reactor must be vented, shut down, and flushed for a long period of time before a new startup is initiated. The resulting economic loss will be quite... 

---