Product size limitation

Produced from Co l. Estimates of the cost of producing methanol from coal have been made by the U.S. Department of Energy (DOE) (12,17) and they are more uncertain than those using natural gas. Experience in coal-to-methanol faciUties of the type and size that would offer the most competitive product is limited. The projected costs of coal-derived methanol are considerably higher than those of methanol produced from natural gas. The cost of the production faciUty accounts for most of the increase (11). Coal-derived methanol is not expected to compete with gasoline unless oil prices exceed 0.31/L ( 50/bbl). Successful development of lower cost entrained gasification technologies could reduce the cost so as to make coal-derived methanol competitive at oil prices as low as 0.25/L ( 40/bbl) (17) (see Coal conversion processes). 

The molecular size of the product is limited insofar as the reaction is terrninated at the dimer or trimer stage. Thus the process is more properly termed oligomerization. The four- to twelve-carbon compounds required as the constituents of Hquid fuels are the prime products. 

In the suspension methods, agglomerate formation occurs by hardening of feed droplets into soHd particles, by layering of soHds deposited from the feed onto existing nuclei, and by adhesion of small particles into aggregates as binding soHds from the dispersed feed are deposited. The product size achievable in these methods is usually limited to ca 5 mm and is often much smaller (see Drying). 

Many attempts have been made to develop models which predict the behavior of materials undergoing size reduction. One proposal is that the energy expended in size reduction is proportional to the new surface formed (5). Another theory is that the energy required to produce a given reduction ratio (feed size product size) is constant, regardless of initial feed particle size (6). Practical results show, however, that both these theories are limited in their usehilness. 

The summation term is the mass broken into size interval / from all size intervals between j and /, and S is the mass broken from size internal i. Thus for a given feed material the product size distribution after a given time in a mill may be deterrnined. In practice however, both S and b are dependent on particle size, material, and the machine utilized. It is also expected that specific rate of breakage should decrease with decreasing particle size, and this is found to be tme. Such an approach has been shown to give reasonably accurate predictions when all conditions are known however, in practical appHcations severe limitations are met owing to inadequate data and scale-up uncertainties. Hence it is stiH the usual practice to carry out tests on equipment to be sure of predictions. 

A device is a product having all major parts are essentially made by a single manufacture. Thus, the device design and its manufacturing operation are under the complete control of the designer. This definition does not dictate a particular product size or complexity, but does infer that such a product will have limited size and complexity. 

Product size is limited to available equipment that can handle the size and pressure as well as other processing requirements. Also involved are factors such as packaging and shipment to the customer. The ability to achieve specific shapes and design details is dependent on the way the process operates. 

At 250 °C, the reaction rate was shown to be affected by pellet sizes as small as 0.2mm [31]. At 210 °C, the reaction rate was found to be by-product diffusion limited at a particle size >16-18mesh (ca. 1.3mm) [27], while at 160°C no effect of pellet size was seen for particle sizes <2.1 mm [29], Therefore, it can be seen that the influence of the pellet size on reaction rate becomes more pronounced as the temperature increases. Under normal industrial SSP conditions, where the pellet size is between 2 and 3 mm and temperatures are >200 °C, decreasing the pellet size will lead to an increasing of the reaction rate. 

Capacity is an inverse problem in that a typical 15-mm diameter tube reactor of 5-6 m length will process in the order of 20 1/h. This therefore sets a lower product volume limit to the usefulness of continuous reactors. Heterogeneous reactions are more difficult to handle in flow reactors, particularly as size is reduced. 

Three types of shape-selective catalysis are distinguished depending on whether pore size limits the entrance of reactant molecules, the departure of product molecules, or the formation of certain transition states [6]. The suitability of zeolite structure for the catalysis is essential for high shape-selectivity. Alkylation of biphenyl is also explained by sterlc control by pore size and shape of zeolite. HY, HL and HM have different pore structures... 

---