Oil shale is a naturally occurring material which has a hydrocarbonaceous component referred to as kerogen that when heated decomposes to yield shale oil which may serve as a petroleum substitute. In some processes for recovering shale oil from oil shale, the shale is crushed and ground into a particulate solid prior to being introduced into a retort. In the retort the particulate shale is mixed with a hot particulate solid that serves as a heat transfer solid to raise the kerogen to a temperature capable of releasing shale oil vapors, usually a temperature of at least 800.degree. F. In some retorting processes of this type, a non-oxidizing gas is passed through the solids mixture. The velocity of the gas depends upon its function in the retorting scheme. For example, in some schemes the gas is used to fluidize or partially fluidize the shale/heat transfer solids mixture. See, for example, U.S. Pat. No. 4,199,432. In other schemes the gas merely serves a stripping function, i.e., to aid in carrying away shale oil vapors from the retort.
Steam has been suggested for use as a stripping gas or fluidizing gas in such processes for retorting oil shale. However, the use of steam presents some serious practical problems. Areas where oil shale is usually found and consequently processed are typically arid making recycle of water in order to minimize usage essential. However, oil shale contains a number of contaminants which accumulate in a water recycle system rapidly fouling and clogging the boiler and superheater. This results in the need to treat large volumes of foul water in a treatment section before recycling to the steam boiler. In addition, the use of steam in the retorting zone results in high heat losses in the condensation section. Hence, the practical utility of steam in commerical processing has been seriously questioned.
Applicant has discovered that despite its practical problems, the use of steam as a fluidizing or stripping gas gives significantly higher product oil yields than other non-oxidizing gases such as nitrogen or recycle retort gas. Therefore, the present invention is directed to a practical method for recycling steam in an oil shale retorting process.