Refinery Feedstocks

2.1 Introduction

A refinery is a complex network of vessels (process units), varying types of support equipment, and pipes (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al, 2007; Speight, 2014a). The total scheme can be divided into a number of unit processes and it is the refined products establish the order in which each refining unit will be used. While product specifications are used to explain the purpose of each unit, there are choices among several types of units. The choice of the processing is dictated by the market demand as well as product specification for a designated use. However, every refinery feedstock is a complex mixture of different constituents and the two tasks of a refinery are: (1) to separate the usable products and (2) to convert the less desirable hydrocarbons into more valuable ones.

Thus, the key to petroleum refining, as in any industrial process, is the character of the feedstock. Unprocessed crude oil is not generally useful in industrial applications, although light, sweet crude oil (low viscosity, low-sulfur crude oil) has, in the past, been used directly as a burner fuel to produce steam for the propulsion of seagoing vessels. In the modern refinery, crude oil is processed to a variety of products: (LPG (liquefied petroleum gas), naphtha (solvents), gasoline, and kerosene light fuel oil), diesel fuel, jet fuel, fuel oil (various grades), lubricating oil, asphalt, and coke (Speight and Ozum, 2002; Parkash, 2003; Hsu and Robinson, 2006; Gary et al, 2007; Speight, 2014a).

Recall for clarification, the term fouling as used in this book refers to deposit formation, encrustation, deposition, scaling, scale formation, slagging, and sludge formation, as it pertains to petroleum refineries, is the accumulation of unwanted material within a processing unit or on the solid surfaces of the unit to the detriment of function. For example, when it does occur during refinery operations, the major effects include: (1) loss of heat transfer as indicated by charge outlet temperature decrease and pressure drop increase, (2) blocked process pipes, (3) under-deposit corrosion and pollution, and (4) localized hot spots in reactors, all of which culminate in production losses and increased maintenance costs. In addition, the term macrofouling if often used to generally describe the blockage of tubes and pipes while, on the other hand, microfouling is generally iced to describe scaling on the walls of tubes and pipes. Again, the outcome is a loss of efficiency and output to the refinery.

Fouling during refining can occur in a variety of processes, either inadvertently when the separation is detrimental to the process or by intent (such as in the deasphalting process or in the dewaxing process). Thus, separation of solids occurs whenever the solvent characteristics of the liquid phase are no longer adequate to maintain polar and/or high-molecular-weight constituents in solution. Examples of such occurrences are: (1) separation of asphaltene constituents, which occurs when the paraffin nature of the liquid medium increases, (2) wax separation which occurs when there is a drop in temperature or the aromaticity of the liquid medium increases, and (3) sludge/sediment formation in a reactor which occurs when the solvent characteristics of the liquid medium change so that asphaltic or wax materials separate, coke formation which occurs at high temperatures and commences when the solvent power of the liquid phase is not sufficient to maintain the coke precursors in solution, and sludge/sediment formation in fuel products which occurs because of the interplay of several chemical and physical factors.

Typically, the fouling material consists of organic and/or inorganic materials deposited by the feedstock that is deposited by the occurrence of instability or incompatibility of the feedstock (one crude oil) with another during and shortly after a blending operation (Speight, 2014a). However, fouling can also be a consequence of corrosion in a unit when deposits of inorganic solids become evident (Speight, 2014b). With the influx of opportunity crudes, high-acid crudes, heavier crude oils, extra heavy crude oils, and tar sand bitumen into refineries (Chapter 1) fouling phenomena are more common and diverse (Speight, 2005, 2008, 2009, 2013a,b,c,d,2014a).

Fouling can be classified into two broad categories: (1) microfouling and (2) microfouling. Common types of microfouling are: (1) biofouling, which is caused by microorganisms, (2) chemical reaction fouling, (3) precipitation fouling, (4) corrosion fouling, and (5) composite fouling, which is caused by more than one fouling mechanism or foulant. Marine fouling is a typical of the composite fouling category and occurs due to seaweed, bacteria, and other living organisms in the waters, which adhere to immersed surfaces such as ship hulls resulting in the formation of a layer that covers the surface, attracting and trapping more material. In any case, the extent and severity of fouling is dependent on variables such as process parameter and the immediate environment.

On the other hand, macrofouling is caused by matter (or constituents) of either inorganic or organic origin, such as animals and plants. An example is the occurrence of fouling that occurs in heat transfer components in heat exchangers which can cause blockages or fretting damage. By way of explanation, fretting refers to wear damage as well as corrosion damage at the uneven (or rough) areas of metal surfaces and such damage is induced under load and in the presence of repeated relative surface motion, as induced, for example, by vibration. The contact movement causes mechanical wear and material transfer at the surface, often followed by oxidation of both the metallic debris and the freshly exposed metallic surfaces. Because the oxidized debris is usually much harder than the surfaces from which it came, it often acts as an abrasive agent that increases the rate of both fretting and a mechanical wear (brinelling—the permanent indentation of a hard surface).

Whatever the cause, fouling is a serious problem in the petroleum industry and is dependent on the properties of the feedstock (Table 2.1) and the properties are indicative of refinery performance and profitability (Ohmes, 2014). Therefore, understanding the properties and contaminants of various crude oils as well as the intermediate streams and final products is critical to selecting the crude slate for the refinery. In fact, the occurrence of fouling in reactors during processing has become more common with the influx of heavier feedstocks (such as heavy oil, extra heavy oil, and tar sand bitumen—tar sand bitumen is an exception insofar as it is not classed as a member of the petroleum family as defined by the United States Department of Energy) and the requirement of more complex processing units to convert such feedstocks into saleable products (Speight, 2009, 2011a, 2013a, 2014a).