Is it possible to pagebreak aligned equations?

In relation to the comments I want to answer this question.

First of all to allow page breaks inside equations you have to set the command \allowdisplaybreaks. Without setting this one no page break occurs.

Using a simple align-environment. Of course as @DavidCarlisle suggested you can use the star variant to suppress the numbering and set a specific tag by \tag.

The environment split can't handle page breaks.

\documentclass{article}
\usepackage{amsmath}
\usepackage{kantlipsum}
\usepackage{showframe}
\allowdisplaybreaks

\begin{document}
\kant[1-3]
\begin{align*}
    a\\
    b\\
    c\\
    d\\
    e\tag{\stepcounter{equation}\theequation}\\
    f\\
    g\\
    h\\
    i
\end{align*}
\end{document}

Additionally to the above answers, as asked in comments, the command \allowdisplaybreaks from amsmath can be used locally do a display break by using the \begingroup and \endgroup commands:

\begingroup
\allowdisplaybreaks
\begin{align}
    ....
\end{align}
\endgroup

Just add the \allowdisplaybreaks command from the amsmath package to the preamble of your document. Works only with the math environments provided by this package. So you can use align.